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Giovanni BOLELLI

Professore Associato
Dipartimento di Ingegneria "Enzo Ferrari"

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Pubblicazioni

2024 - Effect of Mo content on the microstructure and mechanical properties of CoCrFeNiMox HEA coatings deposited by high power impulse magnetron sputtering [Articolo su rivista]
Zin, V.; Montagner, F.; Miorin, E.; Mortalo, C.; Tinazzi, R.; Bolelli, G.; Lusvarghi, L.; Togni, A.; Frabboni, S.; Gazzadi, G.; Mescola, A.; Paolicelli, G.; Armelao, L.; Deambrosis, S. M.
abstract

In this work, CoCrFeNiMox high entropy alloy (HEA) films were deposited by High Power Impulse Magnetron Sputtering (HiPIMS) using pure Mo and equiatomic CoCrFeNi targets. The effect of Mo content on the microstructure, residual stress state, and mechanical properties of the films was investigated in the range of 0–20 at.%. All films exhibited a columnar growth morphology and a high density of planar defects. Increasing the Mo content promoted the formation of a fine-grained structure and induced the transformation from a single face-centered cubic (FCC) phase to a mixture of FCC and body-centered cubic (BCC) phases. All produced films displayed a compressive residual stress state regardless of the Mo concentration. In terms of mechanical properties, the hardness of the films increased with increasing Mo content due to solid solution and grain boundary strengthening, along with the formation of a hard BCC phase. On the other hand, the elastic modulus decreased, likely due to the formation of an amorphous phase at higher Mo concentrations.

2024 - The effect of ceramic YSZ powder morphology on coating performance for industrial TBCs [Articolo su rivista]
Bursich, S.; Morelli, S.; Bolelli, G.; Cavazzini, G.; Rossi, E.; Mecca, F. G.; Petruzzi, S.; Bemporad, E.; Lusvarghi, L.
abstract

The increasing turbine inlet temperatures in modern gas turbines have raised concerns about the corrosion of ceramic thermal barrier coatings (TBCs) caused by molten silicate deposits, commonly referred to as “CMAS” due to their main constituents (CaO-MgO-Al2O3-SiO2). The objective of this study was to investigate the combined influence of powder morphology and chemical composition on the CMAS resistance and thermal cycling resistance of ceramic monolayer and bi-layer coatings created through Atmospheric Plasma Spraying (APS). Three powder morphologies were examined: porous Agglomerated and Sintered (A&S) granules, Hollow Spherical (HOSP) powders, and dense, irregular Fused and Crushed (F&C) particles. Monolayer 7-8YSZ coatings with both porous and dense vertically cracked (DVC) microstructures, and bi-layer coatings consisting of a bottom layer of porous standard 7-8YSZ and a top layer composed of a porous high‑yttrium ZrO2–55 wt% Y2O3 were obtained using all three powder types (A&S, HOSP, or F&C). Furthermore, the bi-layer systems were deposited with different ratios between the individual layer thicknesses and/or different total thickness. FEG-SEM, EDX, and micro-Raman analyses, were conducted to assess the coatings' performance. Nanoindentation high-speed mapping and pillar splitting test were performed to evaluate the mechanical behaviour. The study on 8YSZ monolayers shows that coatings from a F&C feedstock exhibit higher density, reducing the CMAS penetration. However, these coatings demonstrate poorer thermal cycling performance due to increased stiffness and thermal stresses. Coatings from HOSP and A&S powders allow CMAS penetration but offer stress relief pathways, enhancing the coating's ability to withstand thermal stresses. Bi-layer coatings with a 55YSZ top coat show superior CMAS resistance compared to 7-8YSZ monolayer coatings, with limited penetration causing top coat peeling. The thickness ratio between the layers also affects thermal cycling resistance, where a thinner 55YSZ layer extends the TBC lifetime.

2024 - Using infrared imaging to measure the friction coefficient during pin-on-disk sliding wear tests [Articolo su rivista]
Ottani, F.; Lusvarghi, L.; Bolelli, G.; Amenta, F.; Pedrazzi, S.; Morselli, N.; Puglia, M.; Allesina, G.
abstract

In a standard pin on a disk tribometer, the friction coefficient under a sliding wear configuration is commonly obtained from the experimental measurement of the tangential force, which is usually achieved through either a load cell attached to the pin holder or a torque meter attached to the disk shaft. This paper presents an alternative method to measure the friction coefficient using a combination of thermal imaging and a reverse thermal model. The measurement of the temperature evolution of the pin, allow to solve the friction heat generation problem between the mating surfaces. Heat generation is influenced by the friction coefficient and the test parameters, whilst conduction is also significantly affected by the thermophysical properties of the materials. Experimental tests with PTFE pins with lamellar bronze filler and Cr2O3-coated stainless steel disks showed an average difference of the friction coefficient of about 12% between the new thermal and the standard measurement.

2023 - Adhesion of cold sprayed soft coatings: Effect of substrate roughness and hardness [Articolo su rivista]
Bruera, A.; Puddu, P.; Theimer, S.; Villa-Vidaller, M.; List, A.; Bolelli, G.; Gartner, F.; Klassen, T.; Lusvarghi, L.
abstract

2023 - Friction and wear of DLC films deposited on additive manufactured AlSi10Mg: the role of surface finishing [Articolo su rivista]
Salerno, E.; Casotti, D.; Paolicelli, G.; Gualtieri, E.; Ballestrazzi, A.; Gazzadi, G. C.; Bolelli, G.; Lusvarghi, L.; Valeri, S.; Rota, A.
abstract

2023 - High-Throughput Nanoindentation Mapping of aMicrosegregated CoCrFeNi Multi-Principal Element Alloy(MPEA): Challenges and Limitations [Articolo su rivista]
Gualtieri, Eva Magdalena; Colombini, Elena; Testa, Veronica; Bolelli, Giovanni; Giovanardi, Roberto; Veronesi, Paolo
abstract

2023 - Influence of heat treatment on fatigue resistance of two NiTi endodontic files [Articolo su rivista]
Generali, Luigi; Veneri, Federica; Barbieri, Marco; Bolelli, Giovanni; Lusvraghi, Luca; Cavani, Francesco; Matteucci, Tommaso; Pedullà, Eugenio
abstract

OBJECTIVES To evaluate the cyclic, torsional fatigue resistance and phase transformation of two heat-treated and non-heat-treated nickel-titanium reciprocating instruments. MATERIALS AND METHODS Twenty non-heat-treated (Procodile, Komet, Brasseler GmbH & Co., Lemgo, Germany) and 20 heat-treated (Procodile Q, Komet, Brasseler GmbH & Co., Lemgo, Germany) files (25 mm length, #25 apical diameter and 0.6 taper) were subjected to fatigue resistance tests. The dynamic cyclic fatigue was tested at 35 & PLUSMN;1 & DEG;C, using a dedicated patented device, in an artificial stainless-steel canal with a 60 & DEG; angle of curvature, the plate containing the artificial canal performing a controlled axial upand-down movement at 8 mm/s speed. The instruments were operated with a specific reciprocating motion (Reflex Dynamic & REG;, Komet, Breasseler GmbH & Co., Lemgo, Germany). Time to fracture (TtF) was recorded and the length of the fractured tips was measured. The torsional fatigue resistance was tested at room temperature (21 & PLUSMN;1 & DEG;C) using a custom-made device manufactured according to ISO 3630-1. The instruments were fixed 3 mm from the tip and their shafts were rotated counterclockwise at a speed of 2 rpm until fracture. The maximum torque load (Ncm) and corresponding rotation angle at fracture were recorded. All the results were statistically analysed (p <0.05). Fractographic analysis was performed using a field emission gun scanning electron microscope (FEG-SEM) to disclose the fractured surface characteristics. To assess the temperature range for phase transformations, differential scanning calorimetry (DSC) was performed on small segments of & AP;20 mg mass subjected to 2 heating and 2 cooling thermal cycles at rates of 5 & DEG;C min-1 in flowing N2 atmosphere over a temperature range of-40 & DEG;C to +110 & DEG;C. RESULTS The heat-treated instruments showed a greater resistance to dynamic cyclic fatigue as compared to the non-heat-treated sample (TtF 303 & PLUSMN;18.5 s vs 220 & PLUSMN;18.4 s; p <0.05) and a higher resistance to torsional fracture, bearing a greater maximum torque load (1.67 & PLUSMN;0.16 vs 0.82 & PLUSMN;0.07 Ncm; p <0.05). No significant differences were detected between heat-treated and non-heat-treated samples in mean angular rotation to fracture (298 & PLUSMN;25 & DEG; i,�312 & PLUSMN;32 & DEG;; p >0.05) and in the mean length of the fractured fragments (p >0.05). All instruments showed both ductile and brittle fracture patterns. According to the DSC, the direct (cooling) and reverse (heating) transformations of the non-heat-treated files occurred at lower temperatures (<25 & DEG;C) than those of the heat-treated files (& AP;50 & DEG;C), the latter thus not being austenitic at room and body temperatures at which they are operated. Also, the different transformation enthalpies suggest a multi-step transformation, likely involving R-phase formation, for heat-treat-ed files, against a direct transition between austenitic and martensi-tic phase for non-heat-treated files. CONCLUSIONS According to these findings, heat treatment of the tested files pro-vides them with microstructural properties more suited to the clin-ical operating conditions and im-proved performances in terms of torsional and flexural strength. CLINICA L SIGNIFICANCE Heat-treated files might be the best choice, over the traditional non-heat-treated files, when fac-ing challenging clinical condi-tions, such as curved and con-stricted canals.

2023 - Local wear resistance of Inconel 625 coatings processed by different thermal techniques: A comparative study [Articolo su rivista]
Cortés, R.; Garrido-Maneiro, M. Á.; Koivuluoto, H.; Bolelli, G.; Morelli, S.; Testa, V.; Lusvarghi, L.; Kondas, J.; Poza, P.
abstract

2023 - Sprayable Thermoset Nanocomposite Coatings Based on Silanized-PEG/ZnO to Prevent Microbial Infections of Titanium Implants [Articolo su rivista]
Morselli, D; Iseppi, R; Papadopoulou, El; Bolelli, G; Sabia, C; Degli Esposti, M; Fabbri, P
abstract

Post-surgery microbial infections are still one of the main reasons for implant failure, which results in very high physical and psychological pain for the patient and an increased cost for the healthcare system. A polymer nanocomposite antibacterial coating on titanium implants represents a valuable alternative to the more expensive and energy-consuming technological solutions nowadays used. In this regard, a sprayable thermoset nanocomposite composed of silanized-terminals polyethylene glycol (PEG)/ZnO nanoparticle is herein proposed. Initially, PEG's terminals' solvent-free silanization and curing are studied by Fourier Transform Infrared and mu Raman spectroscopies. Scanning Electron Microscope investigations and scratch tests have shown that the spraying procedure optimization and the oxidation treatment of the titanium substrate lead to a homogeneous coverage and improved adhesion of the coatings. The antibacterial activity is tested against not only both S. aureus and P. aeruginosa bacterial American Type Culture Collection strains, but also using very aggressive antibiotic-resistant clinical strains. Interestingly, antibacterial activity, evaluated by time-killing tests, is observed for all tested bacterial strains. Live/dead tests further confirm that 5 wt% of ZnO allows obtaining a bacteriostatic activity within 24 h, whereas a complete growth inhibition (bactericidal activity) of both tested strains is observed for coatings with 20 wt% of ZnO nanoparticles.

2023 - Tribological and electrochemical performances of HVOF sprayed NbC-NiCr coatings [Articolo su rivista]
Gehlen, L. R.; Bolelli, G.; Puddu, P.; Forlin, E.; Colella, A.; Pintaude, G.; Lusvarghi, L.
abstract

A wide range of mechanical parts are protected against wear and corrosion by thermally sprayed WC- or Cr3C2-based hardmetals. However, little is known about the potential of other hardmetal coating formulations. Since promising results were obtained with bulk NbC-based hardmetal cutting tools, in this work we investigated NbC-based coatings, which have not been reported yet in the literature. NbC particles were homogeneously dispersed with volume fractions of 25 % and 40 % in a Ni-20wt.%Cr matrix by mechanical alloying, and the resulting powders were sprayed onto stainless steel substrates by the High-Velocity Oxygen Fuel (HVOF) process.Irrespective of the process parameters, the coatings exhibit low porosity (<1 vol%) and hardness values of about 900 HV0.3 (NbC-25NiCr) and 1000 HV0.3 (NbC-40NiCr). The composition with higher amount of NbC phase is less hard because its greater brittleness results in cracking during indentation.The sliding wear resistance of the NbC-based coatings, tested by ball-on-disc tribometry from room temperature up to 600 degrees C, is generally intermediate between that of WC-CoCr and Cr3C2-NiCr references, and it is usually comparable to that of TiC-NiCr, another type of alternative hardmetal coating, tested in our previous work. Though the NbC-NiCr coatings exhibited limited three-body abrasion resistance due to the occurrence of some brittle fracture together with ductile grooving, they offer a particularly interesting performance at 400 degrees C, with lower wear rates than Cr3C2-NiCr and TiC-NiCr.Additionally, the NbC-NiCr coatings tested in this work, especially the one with 40 vol% matrix phase, exhibit excellent corrosion resistance. Corrosion current densities (similar to 0.1 mu A/cm(2)) and passive current densities (<1 mu A/cm(2)), both obtained by electrochemical polarization tests, were lower than the corresponding values found for all reference coatings (WC-CoCr, Cr3C2-NiCr and TiC-NiCr),Thus, the NbC-NiCr coatings are promising for applications where a good balance between corrosion resistance and wear resistance over a range of temperatures is required.

2023 - Wear resistance of HVOF- and HVAF-sprayed (Ti,Mo)(C,N)–Ni coatings from an agglomerated and sintered powder [Articolo su rivista]
Bolelli, G.; Lyphout, C.; Berger, L. -M.; Testa, V.; Myalska-Glowacka, H.; Puddu, P.; Sassatelli, P.; Lusvarghi, L.
abstract

2022 - Analisi morfologica e ultrastrutturale dei file Procodile Q 25.06 [Capitolo/Saggio]
Generali, L; Bolelli, G; Lusvarghi, L; Pedullà, E
abstract

2022 - CMAS corrosion and thermal cycling fatigue resistance of alternative thermal barrier coating materials and architectures: A comparative evaluation [Articolo su rivista]
Morelli, Stefania; Bursich, Simone; Testa, Veronica; Bolelli, Giovanni; Miccichè, Alessandro; Lusvarghi, Luca
abstract

The corrosion of ceramic thermal barrier coatings (TBCs) by molten silicate deposits, usually known as “CMAS” from their main constituents (CaO-MgO-Al2O3-SiO2), is an issue of increasing concern in modern gas turbines as the turbine inlet temperatures are increased to enhance thermodynamic efficiency. Because conventional ZrO2- 7wt%Y2O3 (7YSZ) dissolves quite readily in a CMAS melt, many alternative materials have been proposed, but there are not many comparative studies among them. Multi-layer architectures featuring a tougher 7YSZ bottom layer and a more brittle, but more CMAS corrosion-resistant top layer have also been proposed; therefore, a comparison among these architectures is also in order. In this paper we studied comparatively the resistance to CMAS corrosion and to thermal cycling fatigue (an essential pre-requisite for any TBC system) of Gd2Zr2O7, ZrO2–55wt%Y2O3 and Gd/Yb/Y co-doped ZrO2, both in the form of single, dense-vertically cracked (DVC) layers deposited by plasma spraying onto an MCrAlY bond coat, and as top layers with a bottom layer of either porous or DVC 7YSZ. It was found that Gd2Zr2O7 resists CMAS corrosion, without any grain-boundary dissolution, slightly better than does ZrO2–55wt%Y2O3. They both develop a solid Gd- or Y-apatite layer (respectively) at the interface with the CMAS melt, driven by the rather large difference in optical basicity between these compounds and CMAS itself, but the Y-apatite layer is less continuous and, therefore, a bit less protective. Gd/Yb/Y co-doped ZrO2, instead, suffers as much grain-boundary dissolution in contact with molten CMAS as does 7YSZ. A Gd2Zr2O7/porous 7YSZ system would therefore exhibit simultaneously high resistance to CMAS dissolution and to thermal cycling fatigue, although there is a risk that the CMAS melt might infiltrate the segmentation macro- cracks and the microcracks of the Gd2Zr2O7 layer and undermine the porous 7YSZ bottom layer.

2022 - Corrosion and wear performances of alternative TiC-based thermal spray coatings [Articolo su rivista]
Testa, V.; Morelli, S.; Bolelli, G.; Bosi, F.; Puddu, P.; Colella, A.; Manfredini, T.; Lusvarghi, L.
abstract

Thermal spray WC-Co based coatings are considered among the best solutions against wear and corrosion of industrial components, but it is necessary to reduce their usage due to sustainability and safety issues. In this respect, the paper is focused on the deposition and characterization of TiC-bases coating systems with cobalt-free matrices. Three alternative formulations, TiC-33 vol%NiCr, TiC-40 volþNiCr, TiC-18 vol%WC-33 vol%NiCr, manufactured by high-energy ball milling (HEBM) and deposited by HVOF, are employed in order to assess the effects of binder concentration, composition, and addition of some WC as second hard phase on their wear and corrosion resistance in comparison with WC-CoCr and Cr3C2-NiCr references. It was found that all coatings exhibit a combination of shallow abrasive grooving, surface fatigue (lamellar delamination) and tribo-oxidation under sliding conditions at room temperature. At 400 °C, surface fatigue is less relevant but abrasive grooving becomes more prevent and some adhesive wear also occurs. The performance of all coatings is intermediate between those of the WC-CoCr and Cr3C2-NiCr references. More specifically, the TiC-33 vol%NiCr composition exhibits similar performance as a TiC-25 vol% composition previously studied by the authors. The addition of WC led to the formation of a W-rich shell around the TiC particles through in-flight reactions during spraying. This limited in-flight oxidation of TiC but it could not prevent post-deposition oxidation of hot lamellae. Because interlamellar oxides formed at this stage have the most significant negative effect on wear resistance, overall the TiC-WC-NiCr composition did not exhibit any tribological advantage over pure TiC-NiCr. The Ti-FeNiCr composition is especially sensitive to surface fatigue but, to the contrary, provides good corrosion protection with no performance deficit compared to the same volume fraction of a NiCr matrix. All the TiC-based coatings are not suitable to protect against high-stress abrasion due to the overly negative influence of their interlamellar brittleness under these conditions.

2022 - Defect-Correlated Vickers Microhardness of Al-Si-Mg Alloy Manufactured by Laser Powder Bed Fusion with Post-process Heat Treatments [Articolo su rivista]
Cerri, E.; Ghio, E.; Bolelli, G.
abstract

Laser powder bed fusion is an additive manufacturing process characterized by different advantages like the manufacture of samples with complex geometry without the use of tools and/or molds. Generally, the manufactured samples are characterized by high tensile strengths which, however, can be affected by the presence of defects due to the unoptimized process parameters. In a large applications field, a low density of the as-built AlSi10Mg samples is a very important parameter to considered, e.g., due to both the loss of the tensile strengths correlated with a premature failure of the samples and the increase in time and costs associated with the manufacturing process. In addition, different post-process heat treatments can increase these effects leading to an ineffective manufacturing process. In this scenario, the present work shows the analysis of spherical and lack-of-fusion pores induced by the laser powder bed fusion process on the AlSi10Mg samples and their variations after different heat treatments (direct aging and T6). At the same time, the influence of pores on the Vickers microhardness and the tensile properties has been studied in the same AlSi10Mg samples (bars and billets) that were printed with single- and double-laser machine setup. Different process parameters were also analyzed and compared. The study was supported by the microstructural and pore analysis performed by optical microscopy along the XZ plane (build direction) and the XY plane. Finally, the greatest effects of pores were observed on the Vickers microhardness values; in fact, two different relationships between microhardness and density variation are discussed. The T6 heat treatment leads to a rounding of the pores already formed in the as-built samples and to a formation of new small pores. Graphical Abstract: [Figure not available: see fulltext.]

2022 - Effect of surface roughness and industrial heat treatments on the microstructure and mechanical properties of Ti6Al4V alloy manufactured by laser powder bed fusion in different built orientations [Articolo su rivista]
Cerri, E.; Ghio, E.; Bolelli, G.
abstract

The emergence of powder bed fusion in recent years has made it one of the most demanded additive manufacturing technologies for Ti6Al4V alloys in the biomedical and aerospace industries due to its ease of part fabrication with complex geometry. However, Ti6Al4V components require a post-processing treatment to optimize their mechanical properties for engineering applications. The present study offers an analysis of the effects of different industrial heat treatments (704 °C for 120 min, 740 °C for 130 min) on the microstructure and tensile properties of Ti6Al4V samples manufactured by laser powder bed fusion in different orientations (Z, 45°, XY and XZ). These heat treatments were selected to improve the mechanical properties of the as-built material and to obtain samples representative of real industrial applications. SEM observations illustrated that the α’ martensite grown in the columnar parent β-grains was converted into an α+β mixture after both heat treatments. EBSD showed that the newly formed α phase maintained, inside the parent β grains, the same orientation relationship as the as-built α′ martensite. However, on a macroscopic scale, the α phase exhibited no preferential orientation. The heat-treated samples, which exhibited approximately 10% lower ultimate tensile strength and yield strength than the as built samples but a 10–13% higher elongation, were practically isotropic in their mechanical response. What little anisotropy was left was mainly attributed to few elongated interlamellar pores. The sandblasting process was also investigated; it did not affect the static mechanical properties of the samples but it reduced the Ra values by 25%. Finally, Vickers microhardness vs. strength relationships were studied by considering the α-phase orientation.

2022 - Hard ultralight systems by thermal spray deposition of WC-CoCr onto AZ31 magnesium alloy [Articolo su rivista]
Morelli, S.; Rombola, G.; Bolelli, G.; Lopresti, M.; Puddu, P.; Boccaleri, E.; Seralessandri, L.; Palin, L.; Testa, V.; Milanesio, M.; Lusvarghi, L.
abstract

2022 - Pre-treatment of Selective Laser Melting (SLM) surfaces for thermal spray coating [Articolo su rivista]
Bolelli, G.; Bonilauri, M. F.; Sassatelli, P.; Bruno, F.; Franci, R.; Pulci, G.; Marra, F.; Paglia, L.; Gazzadi, G. C.; Frabboni, S.; Lusvarghi, L.
abstract

In this work, we investigated the deposition of thermal spray coatings onto additively manufactured parts obtained by Selective Laser Melting (SLM). SLM is indeed replacing conventional subtractive machining to produce diverse industrial parts; hence, it will become increasingly frequent to apply thermal spray coatings onto SLM surfaces. Whilst grit-blasting is the most widely accepted process to prepare a conventionally machined part for a thermal spray coating, the unique surface texture of SLM surfaces might open new possibilities. As a case study, we chose High Velocity Oxygen-Fuel (HVOF) spraying of WC-10%Co4%Cr onto AISI 316 L SLM surfaces in three different conditions: “as built”, pickled, or grit-blasted. We also examined SLM surfaces grown along different directions: parallel, perpendicular, or inclined with respect to the build platform. The references were a machined and grit-blasted SLM part, and a grit-blasted stainless steel bulk. Dense coatings were obtained on every surface regardless of its roughness and pre-treatment. Electrochemical corrosion testing confirmed the low porosity of the layers. Tensile adhesion/cohesion strength was >70 MPa on both SLM and reference surfaces, but the presence of an oxide scale on as-built surfaces degraded the adhesion strength to some extent. Although TEM analysis showed occasional evidence of diffusion bonding between the coating and the oxide scale, cyclic impact tests revealed that the scale itself broke. Particularly strong adhesion was achieved with pickled surfaces; indeed, failure was only cohesive (i.e. within the coating) in both tensile and impact tests. In addition to mechanical interlocking to the rough surface profile, TEM showed widespread diffusion bonding to the clean metal. An SLM part might therefore need chemical activation but no subtractive machining before application of a thermal spray coating. The coating also exerts a levelling action toward the SLM surface, i.e. the coated surface is smoother than the as built one.

2022 - Residual stress in laser-based directed energy deposition of aluminum alloy 2024: simulation and validation [Articolo su rivista]
Caiazzo, F.; Alfieri, V.; Bolelli, G.
abstract

Simulations of laser-based directed energy deposition of metals have received increasing interest aimed at reducing the experimental effort to select the proper processing condition for the repair or overhaul of actual components. One of the main issues to be addressed is the evaluation of the residual stress, which may lead to part failure under nominal loading. In this frame and specifically relating to aluminum alloys, few works have been developed and validated. This lack of knowledge is addressed in this paper: namely, the proper approach to simulate the activation of the deposited metal is discussed in case of single deposition and is shifted to a case of multiple depositions over a substrate. The validation of the predicted residual stress is made by comparison with the actual stress resulting from X-ray diffraction.

2022 - Ti6Al4V-ELI Alloy Manufactured via Laser Powder-Bed Fusion and Heat-Treated below and above the β-Transus: Effects of Sample Thickness and Sandblasting Post-Process [Articolo su rivista]
Cerri, E.; Ghio, E.; Bolelli, G.
abstract

Ti6Al4V-ELI is the most-used lightweight alloy in the aerospace industrial sector thanks to its high mechanical strength and corrosion resistance. The present paper aims, firstly, to evaluate the effects induced by different heat treatments, which were performed above and below the β-transus temperature on Ti6Al4V-ELI samples manufactured via Laser Powder-Bed Fusion in different orienta-tions (XZ, XY, Z and 45◦ ). The first set of tensile samples and bars were heat-treated at 1050◦ C × 1 h, while the second and third set were heat-treated at 704◦ C × 120′ following the AMS2801 standard specification, and at 740◦ C × 130′ . These heat treatments were chosen to improve the as-built mechanical properties according to the ASTM F3001 and also ASTM F2924-14 standard specifications. Optical and SEM measurements reveal primary, secondary and tertiary α-laths below the β-transus, while above this temperature, the microstructure varies in relation to the sample’s thickness. Secondly, this work analyzed the results obtained after a sandblasting process, which was performed on half of all the available heat-treated tensile samples, through XRD and Vickers microhardness measurements. XRD analysis also highlighted the presence of α2-Ti3 Al and TiAl3 precipitates and the microstructural change in terms of the α-phase.

2022 - Tribological Behavior of Reinforced PTFE Composites and Un-Reinforced Polyketone-Based Materials against Coated Steel [Articolo su rivista]
Amenta, F.; Bolelli, G.; De Lorenzis, S.; Bertarini, A.; Lusvarghi, L.
abstract

In this study, two polymeric materials were tested in a dry rotating “pin-on-disc” configu-ration against differently coated surfaces, to evaluate their tribological response under conditions, such as those of rotary lip seals, and to identify the wear mechanism of each coupling. A PTFE based material, reinforced with glass fibers and a solid lubricant, and unreinforced polyketone were tested against a chromium oxide coating deposited by plasma thermal spraying, a CrN/NbN superlattice coating deposited by Physical Vapor Deposition (PVD), and a Diamond-Like Carbon (DLC) coating obtained through a hybrid PVD/PECVD (Plasma-Enhanced Chemical Vapor Deposition) process. The PTFE matrix composite offers better overall performance, in terms of specific wear rates and friction coefficients than polyketone. Although the tribological behavior of this material is generally worse than that of the PTFE matrix composite, it can be used without reinforcing fillers. Our analysis demonstrates the importance of transfer-film formation on the counter-surfaces, which can prevent further wear of the polymer if it adheres well to the counterpart. However, the tribofilm has opposing effects on the friction coefficient for the two materials: its formation leads to lower friction for PTFE and higher friction for polyketone.

2022 - Tribological behavior of HVAF-sprayed WC-based coatings with alternative binders [Articolo su rivista]
Torkashvand, K.; Joshi, S.; Testa, V.; Ghisoni, F.; Morelli, S.; Bolelli, G.; Lusvarghi, L.; Marra, F.; Gupta, M.
abstract

The tribological performance of High Velocity Air-Fuel (HVAF) sprayed WC-based cermet coatings with binders containing no or very limited amount of cobalt was evaluated under dry sliding, erosion, and abrasion wear conditions. The wear and corrosion behaviors of WC-NiMoCrFeCo, WC-FeNiCrMoCu and WC-FeCrAl HVAF sprayed coatings were investigated and compared to standard WC-CoCr coatings as benchmark. Microstructure characterization along with XRD analysis was conducted on all powders as well as the corresponding coatings. Comprehensive post wear analysis was conducted on all coatings subjected to ball-on-disk, gas jet erosion and dry sand-rubber wheel abrasion tests. Moreover, all coatings were exposed to 3.5% (wt./vol.) NaCl aqueous solution to evaluate their corrosion performance through electrochemical testing. XRD results showed negligible phase transformation between the powders and the deposited coatings. The WC-NiMoCrFeCo coating exhibited the best sliding wear and electrochemical corrosion performance, with an average specific wear rate value of 3.1 × 10−8 (mm3·N−1·m−1) and a corrosion current density of 1.9 μA/cm2. This coating also showed comparable abrasive wear resistance to the WC-CoCr coating. Under erosive wear conditions, too, the WC-FeNiCrMoCu and WC-FeCrAl coatings showed a comparable performance to the benchmark. Dominant wear mechanisms for the reference WC-CoCr coating, under sliding wear conditions, were abrasion (deep grooving) and surface fatigue (crack propagation and pitting). On the contrary, no pitting was observed in WC-NiMoCrFeCo and WC-FeCrAl coatings during the sliding wear test. No considerable difference was identified in the wear mechanisms of the different coatings under abrasion and erosion wear conditions. The results highlight the promise of some of the environment friendly binders studied to replace Co.

2022 - Tribological behaviour of PTFE composites: Interplay between reinforcement type and counterface material [Articolo su rivista]
Amenta, F.; Bolelli, G.; D'Errico, F.; Ottani, F.; Pedrazzi, S.; Allesina, G.; Bertarini, A.; Puddu, P.; Lusvarghi, L.
abstract

In this paper, we studied the sliding wear behaviour of polytetrafluoroethylene (PTFE)-based composites under conditions relevant to rotary seals. We specifically aimed to investigate the interactions between different particle-based reinforcements (lamellar or spheroidal bronze particles, PEEK particles) and different counterfaces (uncoated or Cr2O3-coated stainless steel), building upon our own previous work on fibre reinforced-composites to make up for the paucity of literature papers on the role of counterfaces. Pin-on-disc tests were performed under different load and speed conditions using spherical-tipped composite pins against coated or uncoated stainless steel discs to mimic the (initially) non-conformal, unidirectional sliding contact of lip seals. Though all composites attained a steady-state regime controlled by a tribochemical wear mechanism, namely tribofilm formation, there were significant differences among the tribofilms produced by different tribo-systems. Systems that released fine, oxidized metal debris (bronze-filled PTFE and/or uncoated counterpart) developed >1 μm thick, continuous tribofilms on both mating surfaces. Thickness and continuity of the tribofilm decreased with a non-wearable Cr2O3-coated counterpart and/or PEEK as a filler. The tribofilm was conducive to lower steady-state friction, but not lower wear, against stainless steel, whereas all performances (friction coefficient, specific wear rate) were levelled out with a Cr2O3-coated counterface.

2021 - Effect of the Distance from Build Platform and Post-Heat Treatment of AlSi10Mg Alloy Manufactured by Single- and Multi-Laser Selective Laser Melting [Articolo su rivista]
Cerri, E.; Ghio, E.; Bolelli, G.
abstract

In the present study, AlSi10Mg samples produced by selective laser melting (SLM) were studied. Samples were machined from two types of bars obtained through different methods: either single laser (SL) or multiple laser (ML) machine setup. The bars were built perpendicular to the platform, which was pre-heated at 150 °C (working temperature), up to a height of 300 mm. The effect of the distance from the platform on the mechanical properties was investigated through tensile samples in as-built condition and after unconventional heat treatments (U-HT). Tensile strength changed by 80 MPa along the Z-axis (build direction) for SL case and by 100 MPa for ML case in the as-built samples. Vickers microhardness revealed an analogous gradient. This was correlated to a gradient in intra-granular precipitates' distribution along the Z-axis, as revealed by scanning electron microscopy (SEM). An unconventional heat treatment at 175 °C for 6h slightly improves the mechanical strength; higher temperature treatments at 200 and 225 °C for the same duration cause a progressive decrease in strength with an increase in elongation. The amount and size of the precipitates and the thickness of eutectic Si change with the heat treatment temperature, justifying the mechanical behavior.

2021 - In-depth metallurgical and microstructural analysis of oneshape and heat treated onecurve instruments [Articolo su rivista]
Azizi, A.; Prati, C.; Schiavon, R.; Fitzgibbon, R.; Pirani, C.; Iacono, F.; Pelliccioni, G. A.; Spinelli, A.; Zamparini, F.; Puddu, P.; Bolelli, G.; Generali, L.
abstract

Objective: To define surface, mechanical, microstructural and metallurgical features of conventional OneShape (OShape) and heat-treated OneCurve (OCurve) nickel-titanium instruments. Methods: Instruments were analysed by scanning electron microscopy (SEM) on new instruments and after simulated clinical use (SCU). Cyclic fatigue testing was performed and the number of cycles to fracture (NCF) and the length of the fractured instruments were measured (Mann-Whitney test). Fractured instruments during cyclic fatigue testing were then inspected by SEM fractographic analysis. Field emission gun scanning scanning electron microscopy (FEG-SEM), energy-dispersive X-ray spectroscopy (EDX) and micro-Raman spectroscopy were used to assess alloy surface chemistry. Focused ion beam (FIB) was performed to analyse the oxide layer on the surface of OCurve before and after SCU. X-Ray diffraction (XRD), metallographic evaluation and differential scanning calorimetry (DSC) were used to determine martensitic/austenitic phase transformation temperatures. Results: SEM observations on new instruments revealed a smooth regular surface with flattened milling grooves. No wear features were detected after SCU. OCurve exhibited a higher cyclic fatigue resistance (P<0.05), slower crack propagation and a surface layer of TiO2. Metallographic analysis and XRD showed the prevalence of martensitic grains on OCurve instruments that were stable at body temperature as confirmed by DSC analysis. Furthermore, DSC demonstrated a shift in the temperature transformation ranges suggesting an increase of martensite phase in autoclaved OCurve instruments. Conclusion: Heat treatment processes were confirmed as a valid enhancement of the properties of the new generation NiTi instruments. OCurve presented a significant improvement over OShape regarding both mechanical and metallurgical characteristics.

2021 - Micromechanical behaviour and wear resistance of hybrid plasma-sprayed TiC reinforced Tribaloy-400 [Articolo su rivista]
Testa, V.; Morelli, S.; Bolelli, G.; Lusvarghi, L.; Bjorklund, S.; Joshi, S.
abstract

Cobalt-based alloys such as Tribaloy-400 are known for their sliding wear resistance at room as well as elevated temperature. However, further enhancement in terms of hardness and wear resistance could be achieved by creating metal matrix composites reinforced by ceramic particles. For this purpose, Tribaloy-400 based coatings were deposited with the addition of different amounts of TiC reinforcement (≈25 vol%, ≈40 vol%, ≈60 vol%) through a “hybrid” plasma spray process, using a dry Tribaloy-400 powder with 10–45 μm particle size and a water-based TiC suspension (d50 = 2.2 μm). Pure Tribaloy and pure TiC coatings were employed for comparison. During spraying, TiC was partly oxidized but could nonetheless be embedded between the bigger Tribaloy-400 lamellae. Specifically, the coating containing ≈ 25 vol% TiC shows a homogeneous microstructure, whilst greater amounts of hard phase lead to some agglomeration. Accordingly, the coating with 25 vol% TiC exhibits a good combination of hardness and toughness (the latter being assessed by scratch testing) and low sliding wear rates of ≈10−5–10−6 mm3/(N·m) from room temperature up to 700 °C in ball-on-disc tests against an Al2O3 counterbody. At room temperature, the tribological behaviour of the coatings is controlled by abrasive grooving and spallation due to surface fatigue. With increasing temperature, tribochemical phenomena take on an ever-increasing role and, at 700 °C, a uniform, oxide-based tribofilm is developed, so that the wear rates are often lower than are recorded at 400 °C.

2021 - Sliding wear behaviour of fibre-reinforced PTFE composites against coated and uncoated steel [Articolo su rivista]
Amenta, F.; Bolelli, G.; Pedrazzi, S.; Allesina, G.; Santeramo, F.; Bertarini, A.; Sassatelli, P.; Lusvarghi, L.
abstract

The tribological response of PTFE-based composites for mechanical seals can be significantly affected by the nature of the counterface. In this work, glass fibre (GF) and carbon fibre (CF) reinforced PTFE-matrix composites were slid, in a pin-on-disc configuration, against AISI 304 stainless steel, either uncoated or coated with plasma sprayed Cr2O3. When mated with uncoated steel, GF-PTFE composites tended to wear down more, but also produced a thicker and more continuous tribofilm than did CF-PTFE ones. The pulverized glass debris adsorbed humidity and formed a cohesive “skeleton” to keep the PTFE debris in place, thus resulting in comparatively lower friction coefficients of ≈0.20–0.27. On the other hand, the polished Cr2O3 surface, with smooth plateaus and numerous valleys of various sizes, facilitated the retention of transfer material. Therefore, it produced less friction than uncoated steel, both against GF-PTFE and CF-PTFE, although the CF-PTFE pin showed a limited capacity to develop a tribofilm. CF-PTFE, however, suffered more severe wear than did GF-PTFE against Cr2O3, because the carbon fibres fractured more frequently. The bulk temperature of the composites never increased by more than 12 °C during the tests, and the rise in temperature depended more on the friction coefficient than on the thermal conductivity and diffusivity of the mated surfaces.

2021 - Tribological properties of plasma sprayed Cr2O3, Cr2O3–TiO2, Cr2O3–Al2O3 and Cr2O3–ZrO2 coatings [Articolo su rivista]
Bolelli, G.; Steduto, D.; Kiilakoski, J.; Varis, T.; Lusvarghi, L.; Vuoristo, P.
abstract

Plasma sprayed Cr2O3 is widely used to protect industrial components against wear. The present study seeks to clarify how its properties can be modified by alloying with other oxides. Therefore, pure Cr2O3 and Cr2O3–25%TiO2, Cr2O3–16%Al2O3, Cr2O3–35%Al2O3, Cr2O3–10%ZrO2 and Cr2O3–20%ZrO2 coatings were studied. All samples were obtained from pre-alloyed feedstock, resulting in rather homogeneous solid solutions. Compared with pure Cr2O3 and Cr2O3–Al2O3 coatings, the Cr2O3–25%TiO2 and Cr2O3–ZrO2 ones exhibit lower indentation hardness (HIT) but higher toughness, qualitatively assessed by scratch testing. Cr2O3 and Cr2O3–16%Al2O3 also exhibit higher hardness/elastic modulus ratios (HIT/E*, HIT3/E*2) than all other samples. The sliding wear resistance of the coatings against Al2O3 and ZrO2 balls is most closely correlated to indentation hardness and, secondarily, to the hardness/modulus ratios. Pure Cr2O3 is therefore the most sliding wear resistant of all samples, whilst Cr2O3–25%TiO2 suffers very severe wear. However, ZrO2 counterparts cause systematically more severe wear than do Al2O3 ones. Dry particles' abrasion, which proceeds through flake formation, is controlled by toughness. The resistance to abrasive wear is, therefore, predicted by scratch testing. The various coatings rank almost the opposite as they did in sliding wear tests, with comparatively lower wear losses for Cr2O3–25%TiO2 and (most of all) Cr2O3–ZrO2 samples.

2020 - Alternative metallic matrices for WC-based HVOF coatings [Articolo su rivista]
Testa, Veronica; Morelli, Stefania; Bolelli, Giovanni; Benedetti, Beatrice; Puddu, Pietro; Sassatelli, Paolo; Lusvarghi, Luca
abstract

Thermal spray coatings based on WC-CoCr are widely employed to protect industrial components against wear and corrosion. Although they provide good technical performances, it is necessary to replace Co-based hardmetal compositions to protect human health as well as for economic reasons. Alternative hardmetal formulations with little or no Cobalt were deposited by High Velocity Oxygen-Fuel (HVOF) spraying using agglomerated and sintered feedstock powders. They consist of WC as hard phase, but metallic matrices include iron- and nickel- based alloys (WC-NiMoCrFeCo, WC-FeNiCrMoCu, WC-FeCrAl). The characterization is focused on micro- structure, mechanical and tribological properties (sliding and abrasive wear behaviour) and corrosion resistance in 3.5% NaCl. Very low porosity, limited carbides' dissolution in the matrix, deposition efficiency ≥50%, and hardness values (≈1200 HV) comparable to conventional WC-CoCr mean that the studied coatings are viable for industrial applications. Both WC-NiMoCrFeCo and WC-FeNiCrMoCu coatings show intrinsically high sliding wear resistance (specific wear rates between 10−8 and 10−7 mm3/(N∙m) in a ball-on-disc test against Al2O3 counterpart), but they are less resistant to high-stress abrasion by coarse particles (wear rates ≈10−3 mm3/ (N∙m)). Their performances are anyway comparable to those of reference WC-CoCr samples. Additionally, WC- NiMoCrFeCo coatings show even better corrosion resistance in a 3.5% NaCl aqueous solution (ICorr ~ 10−6 A/ cm2). WC-FeCrAl coatings exhibit slightly poorer performances than the other samples: the formation of Al-based oxide inclusions during spraying cause higher wear losses, whilst the FeCrAl matrix has limited corrosion re- sistance.

2020 - Angular-dependent deposition of MoNbTaVW HEA thin films by three different physical vapor deposition methods [Articolo su rivista]
Xia, A.; Togni, A.; Hirn, S.; Bolelli, G.; Lusvarghi, L.; Franz, R.
abstract

Within this work, MoNbTaVW high entropy alloy thin films were synthesized by dc magnetron sputter deposition, high power impulse magnetron sputtering and cathodic arc deposition to study the influence of the growth conditions on structure and properties of the films. For deposition angles ranging from 0 to 90°, the deposition rate, chemical composition, morphology and crystal structure as well as the mechanical properties were analyzed. All films showed the formation of a solid solution with body centered cubic structure regardless of deposition angle and method, whereas higher energetic growth conditions were beneficial for improved mechanical properties.

2020 - Behavior of Two Resin Composites after Aging in Drink Solutions [Abstract in Rivista]
Generali, L; Vignudini, G; Maravic, T; Puddu, P; Bolelli, G; Lusvarghi, L; Breschi, L; Consolo, U; Checchi, V
abstract

Objectives: The aim of this study was to investigate the effects of artificial saliva and drink solutions on colorimetric variations and wear resistance of 2 composites: Herculite XRV Ultra (HXRVU) and Harmonize (HZ) (Kavo-Kerr), after one month aging Methods: Forty composite specimens were prepared in an oxygen-free environment and polymerized. Four specimens were immedialtely investigated (T0), while thirty-six specimens were soaked into different drink solutions (artificial saliva, cola, ethanol), sealed into PET bottles and incubated at 37°C. Control specimens were kept in air. Specimens evaluation was made at T0, after 1 week (T1) and 1 month (T2). Twenty-four samples (12 HXRVU, 12 HZ) were analyzed using a spectrophotometer to assay three color-identifying values: L, a and b. To evaluate wear resistance, the remaining sixteen samples (8 HXRVU, 8 HZ) were fitted into a rotary tribometer, placed into a food-like bolus-simulating slurry and wear rate was evaluated with a profilometer. Results: L values for all specimens of both groups did not show differences between T0 and T2. All a values showed an increase compared to control group. b values for HXRVU specimens did not show differences between T0 and T2, whereas b values for HZ specimens decreased, compared to control group. HXRVU specimens were more wear resistant than HZ group at T0 and seemed to be less susceptible to modifications in different drink solutions. Ethanol solution produced a marked increase of wear rate in HZ specimens at T2. Conclusions: Aging effects on chromatic variations of composites placed into different drink solutions were demonstrated only for a value, resulting in a slightly change into red color. Wear rate was composite related. Additional in vivo testing is needed to clarify the clinical behaviour of the tested composite materials in terms of color changes and wear rate.

2020 - CMAS corrosion of YSZ thermal barrier coatings obtained by different thermal spray processes [Articolo su rivista]
Morelli, S.; Testa, V.; Bolelli, G.; Ligabue, O.; Molinari, E.; Antolotti, N.; Lusvarghi, L.
abstract

Degradation of yttria-stabilized zirconia (YSZ) layers by molten CaO-MgO-Al2O3-SiO2 (CMAS)-based deposits is an important failure mode of thermal barrier coating (TBC) systems in modern gas turbines. The present work aimed to understand how the chemical purity and microstructure of plasma-sprayed YSZ layers affect their response to CMAS corrosion. To this end, isothermal corrosion tests (1 h at 1250 °C) were performed on four different kinds of YSZ coatings: atmospheric plasma-sprayed (APS) layers obtained from standard- and high-purity feedstock powders, a dense – vertically cracked (DVC) layer, and a suspension plasma sprayed (SPS) one. Characterization of corroded and non-corroded samples by FEG-SEM, EBSD and micro-Raman spectroscopy techniques reveals that, whilst all YSZ samples suffered grain-boundary corrosion by molten CMAS, its extent could vary considerably. High chemical purity limits the extent of grain-boundary dissolution by molten CMAS, whereas high porosity and/or fine crystalline grain structure lead to more severe degradation.

2020 - Corrosion Properties of Thermally Sprayed Bond Coatings Under Plasma-Sprayed Chromia Coating in Sulfuric Acid Solutions [Articolo su rivista]
Bolelli, G.; Meschini, D.; Varis, T.; Testa, V.; Morelli, S.; Lusvarghi, L.; Vuoristo, P.
abstract

Plasma-sprayed chromia coatings are known to have excellent corrosion and wear properties in highly acidic conditions at ambient and elevated temperatures, but are not watertight due to their intrinsic porosity. Therefore, in applications involving aggressive environments, the whole component is usually made of a corrosion-resistant alloy, to which the Cr2O3 coating imparts the necessary wear resistance. However, in such aggressive environments, the survival of thermal spray metallic bond layers becomes an issue. The present study deals with the performance in sulfuric acid solutions of coated systems consisting of a Hastelloy C-276 substrate and a plasma-sprayed Cr2O3 top coating with four different intermediate bond coatings. The bond coatings were HVOF-sprayed Ni-20Cr, Hastelloy C-276 and Ultimet alloys and plasma-sprayed tantalum. Open-circuit measurement, electrochemical polarization and electrochemical impedance spectroscopy tests were carried out at room temperature (RT) in solutions with various concentrations. Also, static immersion tests were performed at RT and 60 °C. The results revealed that the HVOF-sprayed Ni-20Cr and Ultimet alloy coatings were significantly attacked by the sulfuric acid electrolyte, especially at 60 °C, whereas the HVOF-sprayed Hastelloy C-276 and plasma-sprayed Ta coatings performed significantly better.

2020 - Influence of MAX-Phase Deformability on Coating Formation by Cold Spraying [Articolo su rivista]
Elsenberg, A.; Busato, M.; Gartner, F.; List, A.; Bruera, A.; Bolelli, G.; Lusvarghi, L.; Klassen, T.
abstract

As solid-state deposition technique avoiding oxidation, cold gas spraying is capable of retaining feedstock material properties in the coatings, but typically fails to build up coatings of brittle materials. Ceramic MAX phases show partial deformability in particular lattice directions and may thus successfully deposit in cold spraying. However, deformation mechanisms under high strain rate, as necessary for cohesion and adhesion, are not fully clear yet. A MAX-phase deposit only builds up, if the specific mechanical properties of the MAX phase allow for, and if suitable spray parameter sets get realized. To investigate the influence of material properties and deposition conditions on coating microstructure and quality, three MAX phases, Ti3SiC2, Ti2AlC and Cr2AlC, were selected. Up to ten passes under different spray parameters yielded Ti2AlC and Cr2AlC coatings with thicknesses of about 200-500 µm. In contrast, Ti3SiC2 only forms a monolayer, exhibiting brittle laminar failure of the impacting particles. In all cases, the crystallographic structure of the MAX-phase powders was retained in the coatings. Thicker coatings show rather low porosities (< 2%), but some laminar cracks. The deposition behavior is correlated with individual mechanical properties of the different MAX-phase compositions and is discussed regarding the particular, highly anisotropic deformation mechanisms.

2020 - Mechanical Properties and Metallurgical Features of New Green NiTi Reciprocating Instruments [Articolo su rivista]
Generali, Luigi; Malovo, Anastasiya; Bolelli, Giovanni; Borghi, Alessia; La Rosa, Giusy Rita Maria; Puddu, Pietro; Lusvarghi, Luca; Rota, Alberto; Consolo, Ugo; Pedullà, Eugenio
abstract

To evaluate the properties of two nickel-titanium (NiTi) reciprocating endodontic instruments (commercially known as Procodile and Reziflow), a total of 40 size 25 and 0.06 taper new Procodile and Reziflow instruments (n = 20) were subjected to cyclic fatigue tests (60° angle of curvature, 5-mm radius) at 20 °C and 37 °C and a torsional test based on ISO 3630-1. The fracture surface of each fragment was examined. The morphological, mechanical, chemical, thermal, and phase composition characteristics of the files were investigated by field-emission gun scanning electron microscopy (FEG-SEM) equipped with an energy-dispersive X-ray (EDX) detector, focused ion beam analysis (FIB), micro-Raman spectroscopy, X-ray diffraction (XRD), differential scanning calorimetry (DSC), and Auger electron spectroscopy (AES). Reziflow showed higher cyclic fatigue resistance than Procodile at 37 °C (p < 0.05). The maximum torsional strength of Procodile was lower than that of Reziflow (p < 0.05). No difference was found between their angular rotations to fracture (p > 0.05). SEM, FIB, Micro-Raman, and AES analyses revealed the presence of an Nb/Nb2O5 coating on the Procodile surface. DSC and XRD analysis confirmed that both files consist of an almost austenitic phase structure at 37 °C. The cyclic fatigue resistance of Procodile and Reziflow significantly decreases upon exposure to body temperature.

2020 - Mechanical properties and metallurgical features of new and ex vivo used reciproc blue and reciproc [Articolo su rivista]
Generali, L; Puddu, P; Borghi, A; Brancolini, S; Lusvarghi, L; Bolelli, G; Consolo, U; Pedullà, E
abstract

AIM: To compare the mechanical properties and metallurgic features of new and used Reciproc Blue and Reciproc instruments. METHODOLOGY: A total of 120 R25Reciproc Blue and R25 Reciproc instruments were used. The morphological, chemical, mechanical, thermal and phase composition characteristics of new and ex vivo used files were investigated by scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS), focused ions beam analysis (FIB), micro-Raman spectroscopy, optical and SEM metallography, X-ray diffraction (XRD), differential scanning calorimetry (DSC) and indentation tests. Usage-induce degradation was evaluated. Ten new and ten used instruments per type were run until fracture occurred in a stainless-steel artificial canal (60° angle of curvature, 4-mm radius). Time to fracture and the length of the fractured fragment were recorded. Torque and angle of rotation at failure of ten new and ten used instruments for each type were measured according to ISO 3630-1. The fracture surface of each fragment was examined. Analyses of variance and Bonferroni post hoc tests were used to analyse the data statistically (α-level 0.05). RESULTS: SEM analysis revealed microcracks near the tip on both files after ex vivo usage tests. FIB imaging and micro-Raman spectroscopy confirmed the presence of an oxide layer on the Reciproc Blue surface. There was no thinning of the coating after use. The area of grains of Reciproc detected by metallographic analysis was significantly wider than Reciproc Blue (P<0.05). XRD revealed a reduction of martensite and R-phase in Reciproc Blue after use. DSC analysis revealed different transformation temperatures for the instruments analysed. Reciproc Blue was significantly more flexible than Reciproc for both new and used samples (P<0.05) and they were significantly more resistant to cyclic fatigue than Reciproc (P<0.05). Ex vivo usage reduced the fatigue resistance of both files. Torsional resistance of Reciproc and Reciproc blue was not reduced by the simulated use (P>0.05). CONCLUSIONS: The thermal treatment of Reciproc Blue produced a finer structure with smaller grains than Reciproc, which increased its fracture resistance and was also responsible for its reduced hardness and lower elastic modulus. Both files were safe during ex vivo usage in severely curved canals

2020 - Metodo e dispositivo per la prova di file endodontici [Brevetto]
Barbieri, Marco; Bolelli, Giovanni; Generali, Luigi; Lusvarghi, Luca; Puddu, Pietro
abstract

RIASSUNTO Sono descritti un metodo, ed un dispositivo per l’attuazione di detto metodo, per realizzare prove di fatica, su file endodontici (2) rotanti o 5 reciprocanti in leghe Ni-Ti a memoria di forma. Il metodo è del tipo che prevede l’inserimento di detto file (2) in canali artificiali (3), ed è caratterizzato dal fatto di effettuare: - prove di fatica statica, ponendo in rotazione 10 detto file (2) e obbligandolo a deformarsi ciclicamente ad ogni rotazione del file (2) stesso; - prove di fatica dinamica, mediante un inserimento del file (2) in profondità in un canale (3), detta placchetta (4) venendo allontanata e 15 riavvicinata di una corsa predefinita. Il dispositivo è del tipo che comprende uno o più canali artificiali (3), ed è caratterizzato dal fatto di comprendere: - primi mezzi (5) atti a posizionare il file (2) 20 rispetto a detti canali artificiali (3); - secondi mezzi (6) atti a fare in modo tale che il file (2) entri ed esca da detti canali (3), per effettuare una prova di fatica dinamica; - terzi mezzi (7) per imprimere al file (2) un moto 25 rotatorio, per effettuare una prova di fatica statica. 1

2020 - Performance of wear resistant MCrAlY coatings with oxide dispersion strengthening [Articolo su rivista]
Bolelli, G.; Vorkotter, C.; Lusvarghi, L.; Morelli, S.; Testa, V.; Vassen, R.
abstract

Aiming to devise suitable materials for sliding wear protection at high temperature, aluminium oxide-dispersion strengthened (ODS) CoNiCrAlY coatings were manufactured by vacuum plasma spraying (VPS). Feedstock materials were ball-milled powders with 2, 10 and 30 wt% Al2O3 content. The ball-on-disc sliding wear behaviour of the coatings was tested at 750 °C against an Al2O3 counterpart, and compared to a pure CoNiCrAlY coating (obtained from a commercial feedstock not subjected to ball milling) and to an uncoated Ni-base superalloy. Sliding wear rates decrease from the uncoated superalloy (≈3 × 10−5 mm3/(N·m)) to the pure CoNiCrAlY coating (≈2 × 10−5 mm3/(N·m)) and to the ODS ones, with the notable exception of the 10 wt% Al2O3-containing sample. Analyses of worn samples indicate that pure CoNiCrAlY is subject to severe adhesive wear, mitigated by the formation of a thick (>1 μm) “glaze” layer via compaction and (probable) sintering of tribo-oxidized debris particles. Addition of Al2O3 particles to the CoNiCrAlY matrix can either enhance or worsen the “glaze” stability. Specifically, a coating strengthened with 30 wt% Al2O3 provides an especially good mechanical support to the “glaze”. This produces beneficial effects resulting in a particularly low wear rate of ≈3 × 10−6 mm3/(N·m).

2020 - TiC–NiCr thermal spray coatings as an alternative to WC-CoCr and Cr3C2–NiCr [Articolo su rivista]
Bolelli, Giovanni; Colella, Alberto; Lusvarghi, Luca; Morelli, Stefania; Puddu, Pietro; Righetti, Enrico; Sassatelli, Paolo; Testa, Veronica
abstract

TiC-based hardmetal coatings containing 25 or 40 vol% Ni-20 wt%Cr matrix (hereafter TiC–25NiCr and TiC–40NiCr) were obtained by High Velocity Oxygen-Fuel (HVOF) and High Velocity Air-Fuel (HVAF) spraying, starting from high-energy ball milled feedstock powders. These coatings are intended as critical raw materials-free solutions against wear and corrosion. HVOF-sprayed coatings contain some more oxide inclusions than do HVAF ones, but, irrespective of the deposition conditions, TiC–40NiCr coatings are usually somewhat harder (800–900 HV0.3) than TiC–25NiCr ones. They also exhibit lower wear rates in ball-on-disc sliding tests against Al2O3 at room temperature. A hard asperity can indeed penetrate slightly deeper into TiC–25NiCr, as it deforms inelastically through microcracking. Bigger abrasive grooves are thus produced. The wear resistance of TiC–40NiCr coatings compares favourably to that of a Cr3C2-25% (NiCr) reference, and even approaches that of WC-10 wt%Co-4wt.%Cr. TiC–40NiCr coatings are also more corrosion resistant than both reference materials when tested by electrochemical polarization in a 3.5% NaCl solution. At 400 °C, to the contrary, TiC–25NiCr coatings exhibit better sliding wear resistance, whilst more severe abrasive grooving and adhesive tearing affect TiC–40NiCr samples. TiC–NiCr coatings are also unaffected by the transverse macro-cracking that was found to compromise the usefulness of WC-CoCr at 400 °C.

2019 - Building up WC-Co coatings by cold spray: A finite element simulation [Articolo su rivista]
Bolelli, G.; Dosta, S.; Lusvarghi, L.; Manfredini, T.; Guilemany, J. M.; Cano, I. G.
abstract

The impact of porous WC-12wt%Co and WC-17wt%Co particles onto homologous, smooth surfaces is simulated through a coupled Eulerian-Lagrangian, three-dimensional finite element model, in order to study the build-up mechanisms of cold sprayed WC-Co coatings. Deformation of hardmetal particles upon impact involves compaction and spreading of a rim, whose bottom face experiences heating, close to or even above the melting point of the matrix, and develops large equivalent plastic strain. The rim therefore adheres strongly to the underlying hardmetal. SEM observations of cold sprayed WC-Co particles deposited onto a smooth hardmetal surface show metallurgic bonding of the heated matrix and mechanical fixation as WC grains from the particle indent and penetrate into the underlying material. Little deformation and heating are instead seen in the middle of the contact surface, where adhesion is limited. Deformation of the particle requires the criterion for ductile failure of the hardmetal to be exceeded. This means the deformed rim is also extensively failed and ejects fragments as it spreads. Increasing the momentum of the particle enhances the spreading of the rim and promotes its bonding to the underlying surface, but also causes ejection of larger fragments, as well as the elastic storage of a greater amount of rebound energy, which can result in eventual rebounding of the entire particle or its poorly bonded central portion. It is therefore inferred that the deposition efficiency of cold sprayed hardmetals cannot increase linearly with impact velocity, and it is further impaired with stiff, dense particles and/or with very hard materials (e.g. WC-12Co as compared to WC-17Co).

2019 - Comparison of shear bond strengths between a mica-based glass-ceramic and human dentin using three different resin cements [Articolo su rivista]
Srichumpong, T.; Suputtamongkol, K.; Thongpun, N.; Phokhinchatchanan, P.; Angkulpipat, S.; Prasertwong, S.; Bolelli, G.; Veronesi, P.; Leonelli, C.; Heness, G.; Chaysuwan, D.
abstract

Three resin cements, RelyX™ Ultimate, Panavia F2.0 and RelyX™ U200, were evaluated for their bond strengths between a mica glass-ceramic and human dentin. This research shows that a self-etch dental cement resin system, Panavia F2.0 provided the best bond strength. The glass-ceramic was produced and phases analysed. Biaxial flexural strength and hardness were measured and found to be comparable to that of human enamel and dentin. Bond strength was measured before and after thermocycling. Thermocycling was found to reduce the bond strength. Thermocycling reduced the shear bond strengths of all adhesives by 50–60%. The two-way ANOVA test was used to analyse the data (p = 0.05). Failure modes were analysed, showing failure predominately at the cement/glass-ceramic and dentin/resin cement interfaces. Failure modes changed after thermocycling. The resin cement bond between the dentin and mica glass-ceramics interfaces was examined for failure by optical microscopy (OM) and scanning electron microscopy (SEM).

2019 - Corrosion properties of thermally sprayed bond coatings under plasma sprayed chromia coating in sulfuric acid solutions [Relazione in Atti di Convegno]
Vuoristo, P.; Varis, T.; Meschini, D.; Bolelli, G.; Lusvarghi, L.
abstract

Plasma sprayed chromia coatings are known to have excellent corrosion and wear properties in highly acidic conditions at ambient and elevated temperatures. In applications requiring extremely good corrosion resistance, the whole components are usually made of a corrosion resistant alloy. For increased adhesion of the ceramic coating to the corrosion resistant substrate material, thermally sprayed metallic bond coatings are used. It is well known that the corrosion environment in such bond coatings between the ceramic top coating and the substrate can be extremely difficult due to the absence of dissolved oxygen, increased concentration of the corrosive electrolytes under the top coating, and galvanic and crevice corrosion mechanisms inside the coating structure. When bond coatings are used, it is of high importance to select the bond layer chemistry and method of production so that the bond coating can survive in such harsh conditions. In the present study, four different bond coatings were studied to evaluate their performance in corrosive acidic electrolytes. The coatings studied were HVOF sprayed Ni-20Cr, Hastelloy C-276 and Ultimet alloy coatings, and plasma sprayed tantalum coating. The substrate material was a solid Hastelloy C-276 metal alloy. The top coating used was plasma sprayed Cr2O3. Corrosion properties of various coating types were studied by electrochemical measurements in sulfuric acid solutions with various concentrations at RT, and by immersion tests at RT and at the temperature of 60°C. The coating microstructures were studied before and after the corrosion tests. The results showed that HVOF sprayed Ni-20Cr and Ultimet alloy coatings were significantly attacked by the sulfuric acid electrolyte, whereas HVOF sprayed Hastelloy C-276 and plasma sprayed Ta coatings performed significantly better.

2019 - Damage progression in thermal barrier coating systems during thermal cycling: A nano-mechanical assessment [Articolo su rivista]
Bolelli, Giovanni; Righi, Maria Grazia; Mughal, Muhammad Zeeshan; Moscatelli, Riccardo; Ligabue, Omar; Antolotti, Nelso; Sebastiani, Marco; Lusvarghi, Luca; Bemporad, Edoardo
abstract

This paper studies how the nano-mechanical properties of thermal barrier coatings (TBCs) vary during thermal cycling, as a way to shed new light on their failure mechanisms. In particular, high-throughput nanoindentation revealed the evolution of hardness and elastic modulus distributions of plasma-sprayed yttria-stabilized zirconia (YSZ) top layers. The evolution of fracture toughness of the YSZ layers and the thermally grown oxide (TGO) formed onto the vacuum plasma-sprayed NiCoCrAlY bond coat were investigated by nanoindentation micro-pillar splitting. The TGO fracture toughness increases up to ≈2.5–3.5 MPa√m at the early stages of thermal cycling, followed by a rapid decrease to ≈2.0 MPa√m after a critical TGO thickness of ≈5 μm is reached. Consequently, interface damage is initially limited to short cracks within the YSZ material. As TGO thickness exceeds the critical threshold, multiple cracks originate within the TGO and join through the YSZ to form long delamination cracks. Joining is favoured by a simultaneous loss in YSZ strength, testified by a decrease in the nanomechanical properties (hardness, elastic modulus) of both high- and low-porosity top coats. This is due to microstructural changes occurring because of the continuous interplay between sintering and thermal shock cracking in the YSZ layers.

2019 - Evaluation of the usage-induced degradation of Genius and Reciproc nickel–titanium reciprocating instruments [Articolo su rivista]
Generali, Luigi; Borghi, Alessia; Lusvarghi, Luca; Bolelli, Giovanni; Veronesi, Paolo; Vecchi, Andrea; Consolo, Ugo; Becce, Carlo; Bertoldi, Carlo; Sassatelli, Paolo
abstract

The aim of this study was to characterize the main features and the usage-induced degradation of the Genius file after four severely curved root canal instrumentations and to compare their properties to the Reciproc files. Brand new and ex vivo used files were analysed by scanning electron microscopy (SEM) with energy-dispersive X-ray spectrometry (EDS), differential scanning calorimetry (DSC), X-ray diffraction (XRD), optical metallography, and nano-indentation to disclose their morphological, chemical, mechanical, thermal, and phase composition features. Nano-indentation data were statistically analysed using the Student’s t test for normal distribution or the Kolmogorov–Smirnov test for not-normal distributions. SEM analysis showed the presence of micro-cracks near the tip on both files after ex vivo usage test. EDS analysis confirmed that both files are manufactured from an almost equiatomic NiTi alloy. DSC analysis revealed that the transition temperature of the Genius is below 20 °C, while that of the Reciproc is above 20 °C. XRD analysis of Genius files identified cubic B2 austenite with minor peaks of residual monoclinic B19 martensite, while the contemporaneous presence of martensite, austenite and hexagonal R-phase was observed in the Reciproc files. Significant differences in nanohardness and modulus of elasticity (P <.05) were observed in both Genius and Reciproc files before and after use. The collected results showed that both instruments can be safely used as single-use files.

2019 - Fatigue crack growth behavior of a selective laser melted AlSi10Mg [Articolo su rivista]
Di Giovanni, M. T.; de Menezes, J. T. O.; Bolelli, G.; Cerri, E.; Castrodeza, E. M.
abstract

Selective Laser Melting is an additive manufacturing technology capable of producing relatively dense parts from metallic and alloy powders using a high-power laser beam. The present study examines the high cycle fatigue crack growth behavior in the linear region of da/dN vs. ΔK diagrams of the AlSi10Mg alloy produced by SLM. Specimen building orientation and notch position have been combined to obtain 4 different crack growth orientations. Further, specimens were tested in both as-built (AB) and T6 heat treated (T6) conditions. The Fatigue Crack Growth (FCG) curves revealed the beneficial influence of the heat treatment on the FCG response, as compared to the as-built case. On the other hand, the fatigue crack growth behavior of the latter is found to be strongly dependent on material orientation. Fractographic analysis have been further performed to understand the mechanisms involved. Additionally, residual stress measurements were also performed to comprehend its contribution on the FCG curves.

2019 - Influence of hardmetal feedstock powder on the sliding wear and impact resistance of High Velocity Air-Fuel (HVAF)sprayed coatings [Articolo su rivista]
Lyphout, C.; Bolelli, G.; Smazalova, E.; Sato, K.; Yamada, J.; Houdkova, ; Lusvarghi, L.; Manfredini, T.
abstract

The present work aimed to clarify how the characteristics of WC-CoCr hardmetal feedstock powders, namely the grain size of the WC carbides and of the binder and the compressive strength of the sintered aggregates, affect the dry sliding wear and impact resistance of coatings deposited by High Velocity Air-Fuel (HVAF)spraying. Ball-on-Disc tests, which mimic a sliding wear process in the presence of hard asperities as it may occur e.g. in hydraulic seal joints or papermaking components, resulted in mild wear through near-surface microscale plastic flow, the exact nature of which was significantly affected by WC size. Finite element simulations of a single-asperity sliding process indeed showed that large WC grains concentrate contact stresses, thus undergoing very localised deformation. It is experimentally seen that repeated deformation of the carbide grains resulted in their cracking and pull-out. Uniformly distributed, fine carbides allowed the matrix to take on some stress, thus undergoing more homogeneous plastic flow. Block-on-Ring tests elicited adhesive wear as it may happen e.g. in metal-to-metal contacts (e.g. petrochemical valves). This could be effectively restrained by low matrix mean free path. Cyclic impact resistance of coarse-grained coatings was better than that of fine-grained ones, because of better large-scale cohesive strength.

2019 - Italian metallurgy [Articolo su rivista]
Brisotto, M.; Bolelli, G.
abstract

2019 - Properties of HVOF-sprayed TiC-FeCrAl coatings [Articolo su rivista]
Bolelli, Giovanni; Colella, Alberto; Lusvarghi, Luca; Puddu, Pietro; Rigon, Rinaldo; Sassatelli, Paolo; Testa, Veronica
abstract

As an alternative to WC-CoCr and Cr3C2-NiCr coatings for wear and corrosion protection, a TiC – 25 vol% (Fe-20 wt%Cr-5 wt%Al) powder, free from hazardous and/or supply-critical elements (Ni, Co, W), was produced by high-energy ball-milling and processed by High Velocity Oxygen-Fuel (HVOF) spraying, obtaining dense (<1 vol% porosity), hard (HIT > 12 GPa) layers with reasonably good deposition efficiency of ≈ 54%. Tribological testing revealed that the TiC-FeCrAl coatings are particularly promising for sliding contacts, as their ball-on-disc wear rates against an Al2O3 counterpart were lower than those of an HVOF-sprayed Cr3C2-NiCr reference, both at room temperature and at 400 °C, although they could not match the performance of WC-CoCr. At room temperature, brittle fracture along oxidized lamellar boundaries caused localized spallation, releasing debris in the contact region, but, in the incubation period before spallation cracks could propagate, remarkably low friction (≈0.27) was recorded. At 400 °C, spallation was largely suppressed by thermal softening, whilst coarser abrasive grooving became the dominant wear mechanism. TiC-FeCrAl coatings appeared less suited to high-stress abrasion, since extensive brittle fracture resulted in higher wear rates than HVOF-sprayed Cr3C2-NiCr, and to (acidic) corrosive environments. Electrochemical polarisation tests in 0.1 M HCl indeed revealed limited corrosion resistance of the FeCrAl matrix.

2019 - Tribological behavior of WC-Co HVAF-sprayed composite coatings modified by nano-sized TiC addition [Articolo su rivista]
Myalska, H.; Lusvarghi, L.; Bolelli, G.; Sassatelli, P.; Moskal, G.
abstract

The method of improving the properties of WC-Co thermally sprayed coatings by addition of nanostructured reinforcement for mechanical strengthening was explored in this research. Nanostructured WC-Co coatings were characterized by higher hardness than commercial counterparts, but wear tests in the literature have shown contrasting results, whereas bimodal coatings, where nanostructured components are mixed with conventional, micrometer-sized WC grains, exhibited enhanced abrasive and friction wear resistance, in comparison to coatings obtained exclusively from nano-sized powders. A mixed effect of matrix reinforcement by nanoparticles and strong fixing of the micron-sized WC grains was proposed as a possible reason for the enhanced wear resistance of bimodal coatings. A conventional, agglomerated and sintered WC-Co (83-17) feedstock powder was therefore blended with nano-sized TiC (40–100 nm) particles to obtain multimodal coatings containing a nanostructured reinforcement. The amount of TiC added into the powder mixture was in the range of 1–7 wt%. Powder mixtures were deposited onto carbon steel substrate using a High Velocity Air Fuel (HVAF) process. The dry sliding tribological behavior of the coatings was evaluated via ball-on-disk tests against Al2O3 counterparts. The tests were performed at room temperature as well as at 400 °C, with a sliding speed of 0.1 m/s, a sliding distance of 5000 m and a normal force of 10 N. The wear rates of the coatings and of the Al2O3 counterparts were measured separately, and friction coefficients were recorded during each test. The influence of TiC nano-particles on the sliding wear mechanisms was discussed in this work, based on an analysis of wear scars and wear debris through SEM + EDX and micro-Raman spectroscopy.

2018 - Bioactive Zn-doped hydroxyapatite coatings and their antibacterial efficacy against Escherichia coli and Staphylococcus aureus [Articolo su rivista]
Sergi, Rachele; Bellucci, Devis; Candidato, Rolando T.; Lusvarghi, Luca; Bolelli, Giovanni; Pawlowski, Lech; Candiani, Gabriele; Altomare, Lina; De Nardo, Luigi; Cannillo, Valeria
abstract

Zinc-doped hydroxyapatite (HA + Zn) has aroused increasing interest in bone contact applications, for its capacity to modulate osteoblast activity and for the antibacterial properties of zinc ions. In this study, HA + Zn coatings deposited by the Solution Precursor Plasma Spraying (SPPS) process were characterized. This method employs solution feedstocks providing simple and effective ion doping and resulting in the deposition of lamellae of smaller size than the conventional Atmospheric Plasma Spraying method. After a comprehensive microstructural characterization of the coatings, their bioactivity, cytotoxicity, and antibacterial efficacy were investigated. The coatings' bioactivity was confirmed by soaking them in Simulated Body Fluid (SBF), a commonly-used protocol to evaluate samples' bioactivity in vitro. Further, the coatings were non-cytotoxic against human osteoblast Saos-2-cells and, additionally, they showed antibacterial effects against Escherichia coli and Staphylococcus aureus. The HA + Zn coatings obtained showed higher efficacy against the gram-positive S. aureus than against the gram-negative Escherichia coli. These findings demonstrate the potential of the SPPS method for fabricating Zn-doped hydroxyapatite coatings, and show that Zn ions impart antibacterial properties against E. coli and S. aureus bacteria.

2018 - Deposition behavior of cold-sprayed metallic glass particles onto different substrates [Articolo su rivista]
Henao, J.; Bolelli, G.; Concustell, A.; Lusvarghi, L.; Dosta, S.; Cano, I. G.; Guilemany, J. M.
abstract

The deposition behavior of cold-sprayed metallic glass particles onto different metallic substrates was studied by numerical analysis and simulation using the ABAQUS/Explicit software. The mechanical response of a Vitreloy-1 particle was modeled accounting for the non-Newtonian and Newtonian regime of metallic glasses in the undercooled liquid state. The spreading, viscous dissipation and stress distribution of the metallic glass particle at impact showed a strong dependence on the substrate properties. By describing the rheological behavior of metallic glass particles according to the dynamics of viscous fluids, defining the impact Reynolds (Re) number, the Weissenberg (Wi) number and the Elasticity (El) number, the simulation results prove that shear thinning is the main deformation mechanism of metallic glass particles during impact, regardless of the substrate used. Specifically, a threshold value of Re exists, above which the MG particles undergo homogeneous flow, regardless of the substrate material. The generality of this finding is confirmed by its independence of the mathematical model used to describe substrate plasticity. However, the mechanical and thermal properties of the substrate have a strong influence on the shear thinning level experienced by particles impinging at Re values above the threshold. In this manner, the present study considers various aspects of relevant importance to build up metallic glass coatings by cold spray onto different metallic substrates.

2018 - High temperature corrosion properties of thermally sprayed ceramic oxide coatings [Relazione in Atti di Convegno]
Fantozzi, D.; Kiilakoski, J.; Koivuluoto, H.; Vuoristo, P.; Uusitalo, M.; Bolelli, G.; Testa, V.; Lusvarghi, L.
abstract

Oxides are chemically stable and wear resistant materials. Because of these properties, they are often applied as protective coatings in harsh environments. However, their chemical and mechanical stability at high temperature in chlorine containing environments is uncharted. These conditions are present in waste-to-energy and biomass boilers in which the currently available metallic and metal matrix composite coatings provide unsatisfactory protection. To be effective in these conditions the coatings should be chemically inert, erosion resistant and act as environmental barriers. For this purpose, this research studies the corrosion behavior and microstructural features of HVOF- and APS-sprayed Al2O3-, Cr2O3-, TiO2-based coatings. Their chemical stability was evaluated by high temperature corrosion testing of self-standing coatings under KC1 salt deposit at 550, 650 and 720 °C for the duration of 72 h.

2018 - Photocatalytic enamel/TiO2coatings developed by electrophoretic deposition for methyl orange decomposition [Articolo su rivista]
Morelli, Stefania; Pérez, Rosalina; Querejeta, Amaia; Muñoz, Josemari; Lusvarghi, Luca; Lassinantti Gualtieri, Magdalena; Bolelli, Giovanni; Grande, Hans-Jürgen
abstract

The aim of this study was to obtain photocatalytic coatings, capable to decompose organic pollutants, through Electrophoretic Deposition (EPD) of enamels containing respectively 0%, 5%, 10%, 15% (in wt%) of TiO2onto carbon steel substrates. High quality and homogeneous coatings were obtained by applying 12.5 V during 10 s, as the best EPD conditions. The layers were subsequently heat treated at 740 °C for 10 min, in order to obtain dense glazes. Rietveld refinement of XRD patterns and Raman results show that, after the heat treatment at 740 °C, TiO2mostly exists as anatase, responsible of the photocatalytic effect. Semi-quantitative chemical analysis indicate segregation of TiO2on the coatings surface, reaching saturation in the sample with 10 wt% TiO2. FEG-SEM observations reveal rod-like and spherical Ti-rich phases along the cross section of the coatings; some Ti was also dissolved into the enamel. 3D topographical mapping shows that, by adding TiO2, surface roughness increases significantly. Photocatalytic tests were carried out using a 2 × 10−5M aqueous solution of Methyl Orange (MO) as an organic pollutant. By comparing the decomposition rate of MO achieved with the pure enamel (0% of TiO2) and with the sample with 10% of TiO2, it was shown that the addition of 10% of TiO2results in 90% photocatalytic efficiency. Moreover, the permeation of organic compounds and their UV degradation were studied by measuring the water contact angle onto the enamel surface directly after dipping into oleic acid and after various UV irradiation times. The longer the UV irradiation time, the lower the contact angle, down to a minimum of 14.54° after 8 h of UV irradiation. This means, the compound was initially adsorbed on the enamel/TiO2coating surface (10 wt% TiO2) but was efficiently decomposed upon UV irradiation.

2018 - Properties of HVOF-sprayed Stellite-6 coatings [Articolo su rivista]
Sassatelli, Paolo; Bolelli, Giovanni; Lassinantti Gualtieri, Magdalena; Heinonen, Esa; Honkanen, Mari; Lusvarghi, Luca; Manfredini, Tiziano; Rigon, Rinaldo; Vippola, Minnamari
abstract

Stellite-6 coatings were deposited onto AISI 304 stainless steel substrate by gas-fueled HVOF spraying, systematically varying the process parameter settings. By operating the HVOF torch with a fuel-rich mixture, dense coatings (&lt;1% porosity) are produced, containing up to ≈3 vol% oxide inclusions. A substantial amount of a Cr-rich f.c.c. phase is found, mainly produced by quenching of molten lamellae, and distinct from the equilibrium, Co-based f.c.c. solid solution retained in unmelted particles. These coatings exhibit pseudo-passive behavior and survive 5 cycles (100 h) of the Corrodkote test (ASTM B380-97) with no substrate corrosion. Coatings obtained from oxygen-rich mixtures, on the other hand, contain fewer oxide inclusions but also greater porosity, and do not protect the substrate against corrosion. The wear behavior of the coatings is less influenced by deposition conditions. In ball-on-disk dry sliding tests, all coatings exhibit wear rates of 2–3 × 10−5mm3/(N·m), higher than those reported for bulk or clad Stellite, because of interlamellar delamination. Strain-induced, “martensitic” phase transformation from the f.c.c. structure to a h.c.p. one is observed over a 1–2 μm depth below the contact surface. Additional tribo-oxidation is onset when frictional heat dissipation has heated the wear debris enough to trigger its reaction with the environment. Correspondingly, a transition to a regime of higher friction occurs (from ≈0.6 to ≈0.8). At 400 °C, lamellar delamination is suppressed but wear rates rise to 5–8 × 10−5mm3/(N·m) because of abrasive and adhesive wear. At 800 °C, a dense “glaze” tribofilm is formed by sintered debris particles, firmly bonded to a thermally grown oxide scale on the underlying metal surface. The “glaze” protects the coating, lowering the wear rate to ≈1 × 10−5mm3/(N·m) and the friction coefficient to &lt;0.45. Under high-stress particle abrasion conditions, wear rates of ≈1 × 10−3mm3/(N·m) are found.

2018 - Suspension HVOF spraying of TiO2using a liquid-fueled torch [Articolo su rivista]
Puddu, Pietro; Popa, Septimiu; Bolelli, Giovanni; Krieg, Peter; Lassinantti Gualtieri, Magdalena; Lusvarghi, Luca; Killinger, Andreas; Gadow, Rainer
abstract

TiO2coatings were deposited by suspension spraying using, for the first time, a liquid (kerosene)-fueled torch. A ball-milled TiO2powder with d50= 5.67 μm was dispersed in isopropanol and, using a peristaltic pump, it was radially delivered in between the combustion chamber and the conical expansion barrel of the HVOF torch, replacing the conventional dry powder ports with adapted liquid injection nozzles. Coatings are made up of highly flattened splats, whose diameter varies reflecting the size distribution of the feedstock powder, and <2% porosity is obtained when the torch is operated using adequate kerosene and oxygen flow rates and low standoff distances. Substantial amounts of anatase were produced even though the feedstock consisted solely of rutile, due to the particularly rapid cooling of molten droplets impacting on the underlying surface at velocities that could be as high as 1000 m/s. Ball-on-disk dry sliding wear tests reveal relatively low wear rates that decrease from ≈10−4mm3/(N·m) to <10−6mm3/(N·m) when the temperature increases from ≈25 °C up to 450 °C. The denseness of the coatings allows them to produce micro-scale plastic deformation with limited brittle fracture.

2018 - Tribology of FeVCrC coatings deposited by HVOF and HVAF thermal spray processes [Articolo su rivista]
Bolelli, Giovanni; Bursi, Marcello; Lusvarghi, Luca; Manfredini, Tiziano; Matikainen, Ville; Rigon, Rinaldo; Sassatelli, Paolo; Vuoristo, Petri
abstract

This work studies FeVCrC-based coatings as potential alternatives to conventional Ni- and Co-based alloys for wear protection. Specifically, the microstructure and tribological properties of the coatings are characterized as a function of the particle size distribution of the feedstock powder, of the deposition technique – High Velocity Oxygen-Fuel (HVOF) or High Velocity Air-Fuel (HVAF) spraying – and of specific processing parameters. HVOF-sprayed coatings obtained from fine feedstock powder exhibit numerous oxide inclusions, which provide high hardness (≈ 900 HV0.3) but do not excessively impair fracture toughness, as determined through scratch testing techniques. HVAF-sprayed coatings obtained from the same feedstock powder contain much fewer oxide inclusions, and some of them possess simultaneously high hardness and high toughness. Defects (e.g. speckles) are instead formed in case unsuitable HVAF torch hardware is employed. A coarse feedstock powder always results in unmelted inclusions, which impair the cohesion of the coatings, particularly of the HVAF-sprayed ones. Most coatings anyway exhibit very low sliding wear rates &lt; 3 × 10−6mm3/(N m); abrasive grooving and surface fatigue-induced pitting are the main wear mechanisms. Oxide inclusions do not affect negatively the response of HVOF coatings, whereas too many unmolten particles increase pitting under severe test conditions. Rubber-wheel abrasion testing produces comparatively more severe grooving.

2017 - A Study of Cr3C2-Based HVOF- and HVAF-Sprayed Coatings: Microstructure and Carbide Retention [Articolo su rivista]
Matikainen, V.; Bolelli, G.; Koivuluoto, H.; Honkanen, M.; Vippola, M.; Lusvarghi, L.; Vuoristo, P.
abstract

The research on high-velocity air-fuel (HVAF)-sprayed Cr3C2-based materials has mostly focused on conventional Cr3C2-25NiCr composition. In this paper, two alternative compositions (Cr3C2-50NiCrMoNb and Cr3C2-37WC-18NiCoCrFe) were sprayed with high-velocity oxy-fuel (HVOF) and HVAF spray processes to evaluate the material behavior during spraying and to provide characterization of the microstructures and mechanical properties of the coatings. For comparison, coatings from the Cr3C2-25NiCr composition were sprayed with both processes. Spray diagnostics were carried out to obtain average particle velocity and temperature for each material and process combinations. The measured average in-flight particle data were 1800 °C and 700 m/s for HVOF process, and 1450 °C and 900 m/s for HVAF process. Characterization of the coating microstructures was carried out by scanning electron microscopy and X-ray diffraction. In addition, the carbon content of the feedstock powders and sprayed coatings was measured with carbon analyzer. The results show that carbide rebounding or selective deposition of particles with higher metal matrix content is the dominating reason for carbide loss during HVAF spraying, while carbide dissolution is an additional source for the HVOF spraying. Higher particle velocities and controlled temperature measured for the HVAF process produced dense coatings with improved toughness and more homogenous coating structure.

2017 - Deposition mechanisms of metallic glass particles by Cold Gas Spraying [Articolo su rivista]
Henao, J.; Concustell, A; Dosta, S.; Bolelli, Giovanni; Cano, I. G.; Lusvarghi, Luca; Guilemany, J. M.
abstract

The deposition mechanisms of metallic glass particles impacting a substrate at high velocity (385–485 m/s) and temperatures near and above the glass transition are studied using finite element modeling. The deformation mechanisms of the metallic glass particles in these conditions are extremely dependent on their Reynolds number only leading to deposition and bonding at high Reynolds number. Unlike early works, this study includes the homogenous flow deformation under Newtonian and non-Newtonian regime modeled using the constitutive equations of the free-volume model. The computed results are compared against experimental data of metallic glass coatings build-up by Cold Gas Spray. A critical value of the Reynolds number is found by both experiments and simulation, showing that it is a useful parameter to control the activation of viscoplastic deformation and bonding of metallic glass particles. Interestingly, this work demonstrates that deposition of metallic glass particles is governed by a cooperative movement of the liquid instead of a simple shear instability effect at the particle-substrate interface unlike polycrystalline metals.

2017 - Infrared thermography as a Non-destructive Testing solution for thermal spray metal coatings [Articolo su rivista]
Santangelo, Paolo Emilio; Allesina, Giulio; Bolelli, Giovanni; Lusvarghi, Luca; Matikainen, Ville; Vuoristo, Petri
abstract

In this work, an infrared (IR) thermographic procedure was evaluated as a non-destructive testing tool to detect damage in thermal spray metallic coatings. As model systems, polished HVOF- and HVAF-sprayed Fe-based layers deposited onto steel plates were employed. Damage by external-object impingement was simulated through a cyclic impact-test apparatus, which induced circumferential and radial cracks across all model systems, and interface cracks of different sizes in distinct samples. Damaged and undamaged plates were bulk-heated to above 100 °C using an IR lamp; their free-convection cooling was then recorded by an IR thermocamera. The intentionally induced defects were hardly detectable in IR thermograms, due to IR reflection and artificial ‘‘hot’’ spots induced by residuals of transfer material from the impacting counterbody. As a micrometer-thin layer of black paint was applied, surface emissivity got homogenized and any artifacts were effectively suppressed, so that failed coating areas clearly showed up as ‘‘cold spots.’’ This effect was more apparent when large interface cracks occurred. Finite-element modeling proved the physical significance of the IR-thermography approach, showing that failed coating areas are cooled by surrounding air faster than they are heated by conduction from the hot substrate, which is due to the insulating effect of cracks.

2017 - Plastic deformation phenomena during cold spray impact of WC-Co particles onto metal substrates [Articolo su rivista]
Dosta, S.; Bolelli, Giovanni; Candeli, Alessia; Lusvarghi, Luca; Cano, Irene Garcia; Guilemany, Josep Maria
abstract

This paper studies the plastic deformation phenomena in ductile substrates (Al7075 alloy, carbon steel) during cold spray-deposition of WC-Co. Metallographic inspection of etched cross-sections and through-thickness nano-hardness profiling indicate that plastic deformation of the substrate (into which undeformed WC-Co particles penetrate deep) is confined to a shallow area, below which the material appears unaltered. Adiabatic deformation phenomena in the substrate are revealed by finite element modelling of the impact process. Close to the surface, the temperature raises in less than 10&nbsp;ns, leading to metal jetting, whilst the underlying material, below a depth of few micrometres, is substantially unaffected.

2017 - Process for obtaining a dense superhydrophobic or hydrophobic, icephobic and wear resistant coating by means of Cold Gas Spray technique [Brevetto]
Garcia Cano, Irene; Dosta, Sergi; Guilemany, Josep Maria; Lusvarghi, Luca; Bolelli, Giovanni
abstract

Process for obtaining a dense superhydrophobic or hydrophobic, icephobic and wear resistant coating by means of Cold Gas Spray technique The invention relates to a process of obtainment of a dense superhydrophobic or hydrophobic, icephobic and wear resistant coating by means of Cold Gas Spray technique, to the coatings obtained by said process, its use as coating in wind turbine blades, to a wind turbine blade comprising said coatings. Furthermore, the invention relates to the uses of said coatings as anti-fouling coatings, as self-cleaning architecture and as aircraft coatings, as well as the uses in the manufacture of civil engineering or machinery pieces and car, train or truck parts.

2017 - Sliding wear behaviour of HVOF and HVAF sprayed Cr3C2-based coatings [Articolo su rivista]
Matikainen, V.; Bolelli, G.; Koivuluoto, H.; Sassatelli, P.; Lusvarghi, L.; Vuoristo, P.
abstract

Thermally sprayed tungsten carbide (WC) and chromium carbide (Cr3C2) based hard metal coatings are commonly applied on component surfaces as corrosion and wear resistant layers. Typically, WC-Co/Ni with optional Cr addition and Cr3C2-25NiCr powders are sprayed with high velocity oxy-fuel (HVOF) or high velocity air-fuel (HVAF) processes. Due to the poor oxidation resistance of the WC particles, Cr3C2-25NiCr composition is typically selected for high temperature environments, up to 800–900 °C. In this study, two distinct Cr3C2-based compositions of Cr3C2-50NiCrMoNb and Cr3C2-37WC-18NiCoCr were selected as interesting alternatives to conventional Cr3C2-25NiCr. Sliding wear behavior of the coatings sprayed with HVOF and HVAF processes were tested with a ball-on-disk configuration against an Al2O3ball at room temperature and at 700 °C. It was found that both alternative materials had comparable coefficients of friction with the Cr3C2-25NiCr coatings. The Cr3C2-37WC-18NiCoCr coatings provided improved wear resistance at room temperature conditions, but at 700 °C the wear rate was increased to the level of the Cr3C2-50NiCrMoNb coatings. Cr3C2-25NiCr coatings experienced the lowest wear rates at elevated temperatures, which was even lower than at room temperature.

2017 - Structural analysis of HyFlex EDM instruments [Articolo su rivista]
Iacono, F.; Pirani, C; Generali, Luigi; Bolelli, Giovanni; Sassatelli, Paolo; Lusvarghi, Luca; Gandolfi, M. G.; Giorgini, L.; Prati, C.
abstract

Aim To compare the phase transformation behaviour, the microstructure, the nano-hardness and the surface chemistry of electro-discharge machined HyFlex EDM instruments with conventionally manufactured HyFlex CM. Methodology New and laboratory used HyFlex EDM were examined by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). Nano-hard- ness and modulus of elasticity were also investigated using a maximum load of 20 mN with a minimum of 40 significant indentations for each sample. Raman spectroscopy and field emission-scanning electron microscope (FE-SEM) were used to assess the surface chemistry of HyFlex EDM. HyFlex CM were subjected to the same investigations and used as a comparison. Nano-indentation data were statistically analysed using the Student’s t-test.Results XRD analysis on HyFlex EDM revealed the presence of martensite and rhombohedral R-phase, while a mixture of martensite and austenite structure was identified in HyFlex CM. DSC analysis also dis- closed higher austenite finish (Af) temperatures for electro-discharge machining (EDM) instruments. Significant differences in nano-hardness and modulus of elasticity were found between EDM and CM files (P < 0.05). FE-SEM and EDS analyses confirmed that both new EDM and CM files were covered by an oxide layer. Micro-Raman spectroscopy assessed the presence of rutile-TiO2. Conclusions HyFlex EDM revealed peculiar structural properties, such as increased phase transformation temperatures and hardness. Present results corroborated previous findings and shed light on the enhanced mechanical behaviour of these instruments.

2017 - Structures and properties of laser-assisted cold-sprayed aluminum coatings [Relazione in Atti di Convegno]
Koivuluoto, Heli; Milanti, Andrea; Bolelli, Giovanni; Latokartano, Jyrki; Marra, Francesco; Pulci, Giovanni; Vihinen, Jorma; Lusvarghi, Luca; Vuoristo, Petri
abstract

In the cold spray process, solid particles impact on a surface with high kinetic energy, deform plastically and form a coating. This enables the formation of pure and dense coating structures. Even more, coating performance and deposition efficiency can be improved by assisting the process with a laser. Laser-assisted cold spraying (LACS) has shown its potential to improve coating properties compared with traditional cold spraying. In this study, coating quality improvement was obtained by using a co-axial laser spray (COLA) process which offers a new, cost-effective laser-assisted cold spray technique, for high-quality deposition and repair. In the COLA process, the sprayed surface is laser heated while particles hit the surface. This assists thebetter bonding between particles and substrate and leads to the formation of tight coating structures. This study focuses on the evaluation of the microstructural characteristics and mechanical properties (e.g., hardness and bond strength) of LACS metallic coatings.

2017 - “Hybrid” plasma spraying of NiCrAlY+Al2O3+h-BN composite coatings for sliding wear applications [Articolo su rivista]
Bolelli, Giovanni; Candeli, Alessia; Lusvarghi, Luca; Manfredini, Tiziano; Denoirjean, Alain; Valette, Stéphane; Ravaux, Alice; Meillot, Erick
abstract

A novel plasma-spray process, featuring simultaneous injections of dry powders and multiple liquid streams, was employed to produce composite coatings where sub-micrometric particles of Al2O3 and hexagonal BN (h-BN) are dispersed within a NiCrAlY metal matrix. Various coatings, containing up to ≈10 wt% Al2O3 and ≈9 wt% h-BN, were obtained. A co-deposition effect was noted whereby a higher h-BN feed also increases Al2O3 incorporation in the coating, even under a constant flow rate of Al2O3 suspension. Although the microhardness (≈600 HV0.3) seemed rather insensitive to the composition of the coatings, their sliding wear resistance (tested under ball-on-disk configuration against corundum spheres at various temperatures) improved with increasing contents of Al2O3 and h-BN. The improvement was more significant at room temperature, but some beneficial effect also emerged when testing at 400 °C and 700 °C. Al2O3 and h-BN indeed promote the formation and enhance the mechanical stability of an oxide-based tribofilm, protecting the coating surface from direct contact with the counterbody. Specific tribofilm formation mechanisms however vary with temperature. Overall, coatings containing ≥5 wt% of Al2O3 and h-BN keep a reasonably stable wear rate (&lt;5*10−4 mm3/(Nm)) over a wide temperature range.

2016 - Manufacturing and Properties of High-Velocity Oxygen Fuel (HVOF)-Sprayed FeVCrC Coatings [Articolo su rivista]
Sassatelli, Paolo; Bolelli, Giovanni; Lusvarghi, Luca; Manfredini, Tiziano; Rigon, Rinaldo
abstract

This paper studies the microstructure, sliding wear behavior and corrosion resistance of high-velocity oxygen fuel (HVOF)-sprayed FeVCrC-based coatings. Various process parameters were tested to evaluate their effects on the coating properties, which were also compared to those of HVOF-sprayed NiCrBSi and Stellite-6 coatings. The Fe alloy coatings are composed of flattened splats, originating from molten droplets and consisting of a super-saturated solid solution, together with rounded particles, coming from partially unmolten material and containing V- and Fe-based carbide precipitates. All process parameters, apart from “extreme” settings with excess comburent in the flame, produce dense coatings, indicating that the feedstock powder is quite easily processable by HVOF. These coatings, with a microhardness of 650-750 HV0.3, exhibit wear rates of ≈2 × 10−6 mm3/(Nm) in ball-on-disk tests against sintered Al2O3 spheres. They perform far better than the reference coatings, and better than other Fe- and Ni-based alloy coatings tested in previous research. On the other hand, the corrosion resistance of the coating material (tested by electrochemical polarization in 0.1 M HCl solution) is quite low. Even in the absence of interconnected porosity, this results in extensive, selective damage to the Fe-based matrix. This coating material is therefore unadvisable for severely corrosive environments.

2016 - Microstructure and Sliding Wear Behavior of Fe-Based Coatings Manufactured with HVOF and HVAF Thermal Spray Processes [Articolo su rivista]
Milanti, A; Matikainen, V.; Bolelli, Giovanni; Koivuluoto, H.; Lusvarghi, Luca; Vuoristo, P.
abstract

The microstructure and micromechanical behavior of thermally sprayed Fe-based coatings manufactured with high-velocity oxygen fuel (HVOF) and high-velocity air fuel (HVAF) processes were investigated. Fe-Cr-Ni-Si-B-C and Fe-Cr-Ni-Mo-Si-B-C powders were used as the feedstock materials. The coatings showed a highly dense microstructure with near-zero oxidation. The microstructure of the feedstock powders was better retained when sprayed with HVAF process. Differential scanning calorimetry revealed two small exothermic peaks at about 600 °C for the HVOF-sprayed coatings, without any increase in weight in thermogravimetric analysis. It suggested the re-precipitation of carbides that were dissolved during spraying due to the higher particle temperature reported by spray diagnostics system during the HVOF process (≈1800 °C) compared to the HVAF one (≈1400 °C). Micro- and nano-indentations helped to show the difference in inter-lamellar cohesive strength and, in turn, in the particle deposition mechanism. Coatings sprayed with Fe-Cr-Ni-Mo-Si-B-C composition possessed higher sliding wear resistance than that of Fe-Cr-Ni-Si-B-C due to higher nano-hardness. More specifically, HVOF-sprayed Fe-Cr-Ni-Mo-Si-B-C coating showed the largest intra-lamellar hardness, the largest elasticity, and high quality of particle interfaces which resulted in lower sliding wear rate.

2016 - Sliding and abrasive wear behaviour of HVOF- and HVAF-sprayed Cr3C2-NiCr hardmetal coatings [Articolo su rivista]
Bolelli, Giovanni; Berger, L. M.; Börner, T.; Koivuluoto, H.; Matikainen, V.; Lusvarghi, Luca; Lyphout, C.; Markocsan, N.; Nylén, P.; Sassatelli, Paolo; Trache, R.; Vuoristo, P.
abstract

This paper provides a comprehensive characterisation of HVOF- and HVAF-sprayed Cr3C2-25 wt.% NiCr hardmetal coatings. One commercial powder composition with two different particle size distributions was processed using five HVOF and HVAF thermal spray systems. All coatings contain less Cr3C2 than the feedstock powder, possibly due to the rebound of some Cr3C2-rich particles during high-velocity impact onto the substrate. Dry sand-rubber wheel abrasive wear testing causes both grooving and pull-out of splat fragments. Mass losses depend on inter- and intra-lamellar cohesion, being higher (≥70 mg after a wear distance of 5904 m) for the coatings deposited with the coarser feedstock powder or with one type of HVAF torch. Sliding wear at room temperature against alumina involves shallower abrasive grooving, small-scale delamination and carbide pull-outs, and it is controlled by intra-lamellar cohesion. The coatings obtained from the fine feedstock powder exhibit the lowest wear rates (≈5×10-6 mm3/(Nm)). At 400 °C, abrasive grooving dominates the sliding wear behaviour; wear rates increase by one order of magnitude but friction coefficients decrease from ≈0.7 to ≈0.5. The thermal expansion coefficient of the coatings (11.08×10-6 °C-1 in the 30-400 °C range) is sufficiently close to that of the steel substrate (14.23×10-6 °C-1) to avoid macro-cracking.

2016 - Tribological Properties of Hard Metal Coatings Sprayed by High-Velocity Air Fuel Process [Articolo su rivista]
Lyphout, C.; Sato, K.; Houdkova, S.; Smazalova, E.; Lusvarghi, Luca; Bolelli, Giovanni; Sassatelli, Paolo
abstract

Lowering the thermal energy and increasing the kinetic energyof hard metal particles sprayed by the newly developed HVAF systems can significantly reduce their decarburization, and increases the sliding wear and corrosion resistance of the resulting coatings, making the HVAF technique attractive, both economically and environmentally, over its HVOF predecessors. Two agglomerated and sintered feedstock powder chemistries, WC-Co (88/12) and WC-CoCr (86/10/4), respectively, with increasing primary carbides grain size from 0.2 to 4.0 microns, have been deposited by the latest HVAF-M3 process onto carbon steel substrates. Their dry sliding wear behaviors and friction coefficients were evaluated at room temperature via Ball-on-disk (ASTM G99-90) wear tests against Al2O3 counterparts, and via Pin-on-disk (ASTM G77-05) wear tests against modified martensitic steel counterparts in both dry and lubricated conditions. Sliding wear mechanisms, with the formation of wavy surface morphology and brittle cracking, are discussed regarding the distribution and size of primary carbides. Corrosion behaviors were evaluated via standard Neutral Salt Spray, Acetic Acid Salt Spray, accelerated corrosion test, and electrochemical polarization test at room temperature. The optimization of the tribological properties of the coatings is discussed, focusing on the suitable selection of primary carbide size for different working load applications.

2016 - Wear and impact behaviour of High Velocity Air-Fuel sprayed Fe-Cr-Ni-B-C alloy coatings [Articolo su rivista]
Bolelli, Giovanni; Milanti, Andrea; Lusvarghi, Luca; Trombi, Lorenzo; Koivuluoto, Heli; Vuoristo, Petri
abstract

The tribological properties of High Velocity Air-Fuel sprayed Fe-31Cr-12Ni-3.6B-0.6C (wt%) coatings are studied as a function of the deposition parameters. At room temperature, ball-on-disk sliding against Al2O3 is controlled by abrasive grooving and interlamellar cracking, with some tribo-oxidation. Interlamellar crack propagation also controls the coatings response to cavitation erosion and cyclic impact tests. Coatings deposited with higher powder feed rate exhibit poorer performance under all conditions, because of weaker interlamellar cohesion. At 700 °C, sliding wear rates are levelled out, and they are one order of magnitude higher than at room temperature, because of severe abrasive grooving.

2015 - Comparison between Suspension Plasma Sprayed and High Velocity Suspension Flame Sprayed bioactive coatings [Articolo su rivista]
Bolelli, Giovanni; Bellucci, Devis; Cannillo, Valeria; Gadow, Rainer; Killinger, Andreas; Lusvarghi, Luca; Müller, Philipp; Sola, Antonella
abstract

This paper assesses the diverse potentialities of two different suspension spraying processes, namely High Velocity Suspension Flame Spraying (HVSFS) and Suspension Plasma Spraying (SPS), for the deposition of bioactive coatings based on hydroxyapatite and on a new, custom-made K2O–Na2O–CaO–P2O5–SiO2 bioactive glass. With both feedstock types, the HVSFS process imparts high in-flight velocities to the particles and aggregates released after solvent vaporisation, resulting in well flattened, tightly bound lamellae. The coatings, b50 μm thick and very dense, have hardness and elastic modulus values close to those of the corresponding bulk materials. They can be employed as high-quality bioactive layers on metallic implantable devices. Few days of soaking in simulated body fluid (SBF) results in the re-precipitation of a surface hydroxyapatite layer, albeit through different mechanisms. In HVSFS bioactive glass coatings, ion leaching turns the surface into a silica gel, onto which hydroxyapatite subsequently deposits. In HVSFS hydroxyapatite, the amorphous fraction is progressively dissolved and microcrystalline hydroxyapatite precipitates onto the remaining coating layer. The SPS technique, due to the lower in-flight velocity of particles and agglomerates, always produces more po- rous, rougher layers with columnar-like growth. They are not mechanically strong, but their peculiar structure can be useful for specific, functional applications. The high surface area of porous SPS bioactive glass coatings favours ion leaching and fast dissolution in simulated body fluid (SBF); hence, it is suggested that SPS bioglass could be useful as a rapidly resorbable layer. SPS hydroxyapatite, by contrast, is more stable than the corresponding HVSFS layer, despite its porosity, because of the higher crystallinity. After the amorphous fraction is dissolved in SBF, newly formed hydroxyapatite does not constitute a surface layer but precipitates inside the pores, suggesting that a sealing pre-treatment in SBF could be a means to tune porosity and phase composition.

2015 - Effect of spraying parameters on the microstructural and corrosion properties of HVAF-sprayed Fe-Cr-Ni-B-C coatings [Articolo su rivista]
Milanti, A.; Matikainen, V.; Koivuluoto, H.; Bolelli, Giovanni; Lusvarghi, Luca; Vuoristo, P.
abstract

Thermally sprayed Fe-based coatings have been extensively studied as future solution in order to replace more expensive, harmful and environmentally dangerous Ni- and WC-based coatings for several industrial applications where high corrosion and wear resistance are required. The aim of the present study is to investigate the effect of spraying parameters on the microstructure and the corrosion resistance of Fe-based coatings manufactured with the High Velocity Air Fuel (HVAF) thermal spray process. Six sets of thermal spraying parameters have been chosen and their effect on the overall quality of coatings was investigated. All HVAF coatings showed comparably dense microstructure with near-zero oxidation, proving the high quality of the deposition process. However, higher anti-corrosion and mechanical properties were achieved by increasing the spraying air pressure and decreasing the particle feeding rate without altering the thickness and the overall deposition rate. Powder feeding rate was reported to have a remarkable effect on microstructure and corrosion properties. Coatings with beneficial compressive residual stresses were successfully obtained by increasing air pressure during spraying which resulted in improved microstructural and corrosion properties.

2015 - Microstructural analysis of high-pressure cold-sprayed Ni, NiCu and NiCu+Al2O3 coatings [Articolo su rivista]
Koivuluoto, Heli; Bolelli, Giovanni; Milanti, Andrea; Lusvarghi, Luca; Vuoristo, Petri
abstract

Cold spraying has shown its potential to produce metallic and composite coatings with high quality and performance. For instance, the impermeability of the coatings is the criterion for the corrosion resistance and thus, fully dense coatings can act as real corrosion barrier coatings. Our previous study has demonstrated the good corrosion properties of high-pressure cold-sprayed (HPCS) Ni and NiCu coatings whereas the present study focuses on the analysis of structural characteristics behind the dense coatings. Microstructures of as-sprayed and heat-treated HPCS Ni, Ni20Cu and Ni20Cu + Al2O3 coatings have been evaluated with FESEM from top-view direction. This revealed clearly particle deformation and particle boundaries whereas particle bonding was evaluated with cavitation–erosion test. Density was proven with Corrodkote test and impermeable HPCS NiCu + Al2O3 coating structures were achieved. Furthermore, cavitation–erosion resistance of NiCu coating was improved by heat treatment.

2015 - Microstructure-based thermo-mechanical modelling of thermal spray coatings [Articolo su rivista]
Bolelli, Giovanni; Candeli, Alessia; Koivuluoto, Heli; Lusvarghi, Luca; Manfredini, Tiziano; Vuoristo, Petri
abstract

This paper demonstrates how microstructure-based finite element (FE) modelling can be used to interpret and predict the thermo-mechanical behaviour of thermal spray coatings. Validation is obtained by comparison to experimental and/or literature data.Finite element meshes are therefore constructed on SEM micrographs of high velocity oxygen-fuel (HVOF)-sprayed hardmetals (WC-CoCr, WC-FeCrAl) and plasma-sprayed Cr2O3, employed as case studies. Uniaxial tensile tests simulated on high-magnification micrographs return micro-scale elastic modulus values in good agreement with depth-sensing Berkovich micro-indentation measurements. At the macro-scale, simulated and experimental three-point bending tests are also in good agreement, capturing the typical size-dependency of the mechanical properties of these materials. The models also predict the progressive stiffening of porous plasma-sprayed Cr2O3 due to crack closure under compressive loading, in agreement with literature reports.Refined models of hardmetal coatings, accounting for plastic behaviours and failure stresses, predict crack initiation locations as observed by indentation tests, highlighting the relevance of stress concentrations around microstructural defects (e.g. oxide inclusions).Sliding contact simulations between a hardmetal surface and a small spherical asperity reproduce the fundamental processes in tribological pairings. The experimentally observed "wavy" morphologies of actual wear surfaces are therefore explained by a mechanism of micro-scale plastic flow and matrix extrusion.

2015 - Properties of Al2O3 coatings by High Velocity Suspension Flame Spraying (HVSFS): Effects of injection systems and torch design [Articolo su rivista]
Bolelli, Giovanni; Cannillo, Valeria; Gadow, R.; Killinger, A.; Lusvarghi, Luca; Manfredini, Tiziano; Müller, P.
abstract

Through the High Velocity Suspension Flame Spraying (HVSFS) process, fine (sub-micrometric) oxide ceramic particles can be sprayed at high velocity to achieve denser and more finely structured layers, compared to those obtained by conventional thermal spraying. This paper explores the effect of some key HVSFS hardware components on the properties of Al2O3 coatings. Two feedstock powders with different particle size distributions, dispersed in isopropanol medium, were processed by setting up the HVSFS torch with combustion chambers of different lengths and with two distinct injection systems: a water-cooled mechanical injector, delivering a con- tinuous stream of suspension, or a gas-atomiser injector. The use of the atomising injection system and of longer combustion chambers, promoting better fragmentation of the liquid stream and better homogenisation with the gas flow, can suppress the formation of defects between coating layers, resulting in remarkably high tensile adhesive/cohesive strength values, up to ≈70 MPa. All coat- ings are under compressive residual stress, the magnitude of which varies between −150 MPa and −300 MPa depending on the maximum surface temperature attained during spraying, and have a microhardness of 1000–1200 HV0.1. Quite mild wear rates of ≈1 ∗ 10−6 mm3/(Nm) are obtained by ball-on-disk tests performed under sliding conditions severe enough to cause complete failure of reference HVOF-sprayed Al2O3. Wear mech- anisms involve the formation and fatigue delamination of a smooth surface tribofilm. Detailed analyses of both coatings and single splats also reveal that extremely fast quenching of very small mol- ten droplets caused the formation of ≈20 wt.% glassy Al2O3. Glass transition at 527 °C and three crystallisation reactions at about 800, 1000 and 1180 °C were revealed by differential thermal analysis (DTA).

2015 - Tribological and mechanical performance evaluation of metal prosthesis components manufactured via metal injection molding [Articolo su rivista]
Melli, Virginia; Juszczyk, Mateusz; Sandrini, Enrico; Bolelli, Giovanni; Bonferroni, Benedetta; Lusvarghi, Luca; Cigada, Alberto; Manfredini, Tiziano; De Nardo, Luigi
abstract

The increasing number of total joint replacements, in particular for the knee joint, has a growing impact on the healthcare system costs. New cost-saving manufacturing technologies are being explored nowadays. Metal injection molding (MIM) has already demonstrated its suitability for the production of CoCrMo alloy tibial trays, with a significant reduction in production costs, by holding both corrosion resistance and biocompatibility. In this work, mechanical and tribological properties were evaluated on tibial trays obtained via MIM and conventional investment casting. Surface hardness and wear properties were evaluated through Vickers hardness, scratch and pin on disk tests. The MIM and cast finished tibial trays were then subjected to a fatigue test campaign in order to obtain their fatigue load limit at 5 millions cycles following ISO 14879-1 directions. CoCrMo cast alloy exhibited 514 HV hardness compared to 335 HV of MIM alloy, furthermore it developed narrower scratches with a higher tendency towards microploughing than microcutting, in comparison to MIM CoCrMo. The observed fatigue limits were (1,766&nbsp;±&nbsp;52) N for cast tibial trays and (1,625&nbsp;±&nbsp;44) N for MIM ones. Fracture morphologies pointed out to a more brittle behavior of MIM microstructure. These aspects were attributed to the absence of a fine toughening and surface hardening carbide dispersion in MIM grains. Nevertheless, MIM tibial trays exhibited a fatigue limit far beyond the 900&nbsp;N of maximum load prescribed by ISO and ASTM standards for the clinical application of these devices.

2015 - Tribological properties of hard metal coatings sprayed by high velocity air fuel process [Relazione in Atti di Convegno]
Lyphout, C.; Sato, K.; Houdkova, S.; Smazalova, E.; Lusvarghi, L.; Bolelli, G.; Sassatelli, P.
abstract

Lowering the thermal energy and increasing the kinetic energy of sprayed particles by newly developed HVAF systems can significantly reduce material decarburization, and increases sliding wear and corrosion resistance of hard metal coatings, making HVAF coatings attractive both economically and environmentally over its HVOFs predecessors. Two agglomerated and sintered feedstock powder chemistries, respectively WC-Co (88/12) and WC-CoCr (86/10/4), with increasing primary carbides grain size from 0.2 to 4.0 microns, have been deposited by the latest HVAF-M3 process onto carbon steel substrates. Respective dry sliding wear behaviours and friction coefficients were evaluated at room temperature via Ball-on-disk (ASTM G99-90) wear tests against A1203 counterparts, and via Pin-on-disk (ASTM G77-05) wear tests against modified martensitic steel counterparts in both dry and lubricated conditions. Sliding wear mechanisms, with formation of wavy surface morphology and brittle cracking, are discussed regarding the distribution and size of primary carbides. Corrosion behaviours were evaluated via standard Neutral Salt Spray (NSS), Acetic Acid Salt Spray (AASS), accelerated corrosion test and electrochemical polarization test at room temperature. Optimization of coating tribological properties are discussed regarding the suitable selection of primary carbide size for different working load applications.

2015 - Tribology of HVOF- and HVAF-sprayed WC-10Co4Cr hardmetal coatings: A comparative assessment [Articolo su rivista]
Bolelli, Giovanni; Berger, L. M.; Börner, T.; Koivuluoto, H.; Lusvarghi, Luca; Lyphout, C.; Markocsan, N.; Matikainen, V.; Nylén, P.; Sassatelli, Paolo; Trache, R.; Vuoristo, P.
abstract

This paper provides a comprehensive assessment of the sliding and abrasive wear behaviour of WC-10Co4Cr hardmetal coatings, representative of the existing state-of-the-art. A commercial feedstock powder with two different particle size distributions was sprayed onto carbon steel substrates using two HVOF and two HVAF spray processes. Mild wear rates of <10-7mm3/(Nm) and friction coefficients of ≈0.5 were obtained for all samples in ball-on-disk sliding wear tests at room temperature against Al2O3 counterparts. WC-10Co4Cr coatings definitely outperform a reference electrolytic hard chromium coating under these test conditions. Their wear mechanisms include extrusion and removal of the binder matrix, with the formation of a wavy surface morphology, and brittle cracking. The balance of such phenomena is closely related to intra-lamellar features, and rather independent of those properties (e.g. indentation fracture toughness, elastic modulus) which mainly reflect large-scale inter-lamellar cohesion, as quantitatively confirmed by a principal component analysis. Intra-lamellar dissolution of WC into the matrix indeed increases the incidence of brittle cracking, resulting in slightly higher wear rates. At 400°C, some of the hardmetal coatings fail because of the superposition between tensile residual stresses and thermal expansion mismatch stresses (due to the difference between the thermal expansion coefficients of the steel substrate and of the hardmetal coating). Those which do not fail, on account of lower residual stresses, exhibit higher wear rates than at room temperature, due to oxidation of the WC grains.The resistance of the coatings against abrasive wear, assessed by dry sand-rubber wheel testing, is related to inter-lamellar cohesion, as proven by a principal component analysis of the collected dataset. Therefore, coatings deposited from coarse feedstock powders suffer higher wear loss than those obtained from fine powders, as brittle inter-lamellar detachment is caused by their weaker interparticle cohesion, witnessed by their systematically lower fracture toughness as well.

2015 - Tribology of NiCrAlY+Al2O3 composite coatings by plasma spraying with hybrid feeding of dry powder+suspension [Articolo su rivista]
Bolelli, Giovanni; Candeli, Alessia; Lusvarghi, Luca; Ravaux, A.; Cazes, K.; Denoirjean, A.; Valette, S.; Chazelas, C.; Meillot, E.; Bianchi, L.
abstract

NiCrAlY layers containing different amounts of Al2O3 (0, 3, 6, 12, 18 wt.%) were deposited onto stainless steel substrates by a “hybrid” plasma spray process whereby the NiCrAlY powder was fed in dry form whilst fine Al2O3 powder, dispersed in ethanol, was injected through a suspension feeding system. The Al2O3 reinforcement, consisting of fine, rounded particles interspersed within larger NiCrAlY lamellae, only causes marginal changes in hardness, due to the limited particles-matrix cohesion. Nonetheless, at room temperature, ball-on-disk dry sliding wear rates against sintered Al2O3 counterparts decrease from ≈5⁎10−4 mm3/(Nm) for pure NiCrAlY to ≈5⁎10−6 mm3/(Nm) with 18 wt.% Al2O3 addition. Pure NiCrAlY indeed suffers adhesive wear, whereas, on the composite coatings, the pull-out of some Al2O3 particles triggers the formation of a tribo-layer of smeared oxide fragments, which mediates the contact with the counterbody. At 400 °C and at 700 °C, all wear rates are levelled to ≈8⁎10−5 mm3/(Nm) and ≈2⁎10−5 mm3/(Nm), respectively. An oxide layer grows on the NiCrAlY matrix upon high-temperature exposure, resulting in a tribo-oxidation wear mechanism, which makes the addition of Al2O3 irrelevant. At 700 °C, coatings are further strengthened by partial healing of interlamellar defects and by fine-grained β-NiAl precipitating within the metal matrix.

2014 - Comparative study of the dry sliding wear behaviour of HVOF-sprayed WC-(W,Cr)2C-Ni and WC-CoCr hardmetal coatings [Articolo su rivista]
Bolelli, Giovanni; Lutz Michael, Berger; Matteo, Bonetti; Lusvarghi, Luca
abstract

Recent studies of HVOF-sprayed WC-(W,Cr)2C-Ni coatings indicated an applicability at temperatures above 600°C. Therefore, in the present paper the dry sliding wear behaviour of coatings prepared from one selected feedstock powder was studied up to 750°C and compared to a WC-10%Co-4%Cr coating as reference.Coatings were prepared onto stainless steel substrates by a liquid-fueled high velocity oxygen-fuel (HVOF) spray process. A Cr-W-Ni-C mixed matrix phase and chromium-rich large (W,Cr)2C grains were formed as a result of metallurgical reactions between the components WC, Cr3C2 and Ni during spraying; therefore, the resulting coating can be better designated as WC-(W,Cr)2C-Ni.The dry unidirectional sliding wear behaviour of the coatings, studied according to ASTM G99 against Al2O3 counterbodies at four different temperatures (25°C, 400°C, 600°C, 750°C) and at two sliding speeds, was interpreted on the basis of their microstructure, micromechanical properties and oxidation behaviour. The morphology and composition inside the wear scars were also studied by SEM and micro-Raman spectroscopy. The results show that the WC-(W,Cr)2C-Ni coating exhibits good wear resistance up to 750°C, whereas WC-CoCr, in spite of its very mild wear at low temperature, suffers from catastrophic oxidation at 750°C.

2014 - Comparing the deposition mechanisms in suspension plasma spray (SPS) and solution precursor plasma spray (SPPS) deposition of yttria-stabilised zirconia (YSZ) [Articolo su rivista]
Joulia, Aurélien; Bolelli, Giovanni; Gualtieri, Enrico; Lusvarghi, Luca; Valeri, Sergio; Vardelle, Michel; Rossignol, Sylvie; Vardelle, Armelle
abstract

Thermal spraying using liquid feedstock has emerged as a promising technology for the deposition of finely structured ceramic coatings. In order to provide a comparative assessment of the deposition mechanisms occurring when spraying suspension or solution feedstock, suspensions of 300 nm-sized ZrO2–4.5 mol.% Y2O3 particles dispersed in water and in ethanol and solutions of zirconium and yttrium salts, corresponding to ZrO2–4.5 mol.% Y2O3 and ZrO2–8 mol.% Y2O3 stoichiometries, were processed by plasma spraying using different parameter settings. In-flight diagnostics of sprayed droplets, together with the morphological, microstructural and phase analysis of individual lamellae collected onto polished substrates, performed by SEM, FIB, AFM and micro-Raman spectroscopy, led to the identification of deposition mechanisms, which were subsequently verified through the characterisation of complete coating layers.

2014 - Diffusion mechanisms and microstructure development in pack aluminizing of Ni-based alloys [Articolo su rivista]
Francesco, Bozza; Bolelli, Giovanni; Carlo, Giolli; Andrea, Giorgetti; Lusvarghi, Luca; Paolo, Sassatelli; Andrea, Scrivani; Candeli, Alessia; Martin, Thoma
abstract

Despite the large industrial use of pack aluminizing processes for the protection of parts (e.g. cooling channels) of gas turbine blades, systematic studies relating the formation mechanisms and the chemical composition of pack aluminized layers to important process parameters, including the nature of the base alloy and the temperature, are scarce. In this study, 4 different alloys (pure Ni, Ni-20Cr, Inconel 738 and directionally solidified CM247LC) were subjected to a pack aluminizing process at three different temperatures (950. °C, 1000. °C, 1040. °C), using a pack mix containing a fluorine-based activator; the results were compared to those obtained with a chemical vapor deposition (CVD) aluminizing process performed at 1040. °C with the same fluorine-based gaseous precursor.The microstructural characterization, performed by SEM+quantitative EDX analysis, XRD and nanoindentation testing, shows that, during the heating stage of the pack aluminizing process, Al is transported to the sample surface at temperatures too low to allow significant simultaneous diffusion of Ni; therefore, a δ-Ni2Al3 outer layer is formed by inward Al diffusion below the alloy surface, and its growth then continues during the isothermal stage as well. As a result, the chosen isothermal treatment temperature does not affect growth mechanisms, although it modifies the overall thickness of the aluminized layer. δ-Ni2Al3 is converted to β-NiAl after a subsequent vacuum heat treatment at 1120°C. In a CVD process, where gaseous precursor are introduced only after attaining the isothermal treatment stage, Al and Ni diffuse simultaneously from the very beginning of the aluminizing process and β-NiAl is directly developed.Less mobile species (heavy atoms, such as W) in the alloy composition hinder all diffusion phenomena, both during pack aluminizing and during subsequent vacuum heat treatment: after aluminizing, precipitates are developed within the δ-Ni2Al3 outer layer and, after vacuum heat treatment, the resulting β-NiAl layer exhibits a compositional gradient.

2014 - High-pressure cold-sprayed Ni and Ni-Cu coatings: Improved structures and corrosion properties [Articolo su rivista]
H., Koivuluoto; A., Milanti; Bolelli, Giovanni; Lusvarghi, Luca; P., Vuoristo
abstract

Cold spraying is a promising technique for the production of dense metallic coatings. In cold spraying, coating formation is through high velocity impacts of solid particles with high kinetic energy. During impact, particles deform plastically and adhere to the substrate, gradually building-up the coating. This makes it possible to form pure and dense coating structures. These impermeable coatings are advantageous in many applications such as those where corrosion protection is required. Nickel and nickel-copper alloys have good corrosion resistance and therefore, as dense coatings, have high potential for employment as corrosion barrier layers. In this study, the structural and corrosion properties of high-pressure cold-sprayed (HPCS) Ni and NiCu coatings are characterized. NiCu alloys are known to have good corrosion resistance in sulphuric and hydrochloric acids, whereas Ni is resistant to caustic soda and alkaline salt solutions. This study also shows the effect of heat treatments on coating properties. FESEM studies of cross-sectional samples reveal structural details of the HPCS coatings while corrosion properties are evaluated with polarization measurements. The corrosion behavior of both the bulk and substrate material is determined in order to assess the real corrosion protection potential of the coatings.

2014 - Microstructural characteristics and tribological behavior of HVOF-sprayed novel Fe-based alloy coatings [Articolo su rivista]
Milanti, A.; Koivuluoto, H.; Vuoristo, P.; Bolelli, G.; Bozza, F.; Lusvarghi, L.
abstract

Thermally-sprayed Fe-based coatings have shown their potential for use in wear applications due to their good tribological properties. In addition, these kinds of coatings have other advantages, e.g., cost efficiency and positive environmental aspects. In this study, the microstructural details and tribological performances of Fe-based coatings (Fe-Cr-Ni-B-C and Fe-Cr-Ni-B-Mo-C) manufactured by High Velocity Oxygen Fuel (HVOF) thermal spray process are evaluated. Traditional Ni-based (Ni-Cr-Fe-Si-B-C) and hard-metal (WC-CoCr) coatings were chosen as references. Microstructural investigation (field-emission scanning electron microscope FESEM and X-Ray diffractometry XRD) reveals a high density and low oxide content for HVOF Fe-based coatings. Particle melting and rapid solidification resulted in a metastable austenitic phase with precipitates of mixed carbides and borides of chromium and iron which lead to remarkably high nanohardness. Tribological performances were evaluated by means of the ball on-disk dry sliding wear test, the rubber-wheel dry particle abrasion test, and the cavitation erosion wear test. A higher wear resistance validates Fe-based coatings as a future alternative to the more expensive and less environmentally friendly Ni-based alloys.

2014 - Post-mortem evaluation of oxidized atmospheric plasma sprayed Mn–Co–Fe oxide spinel coatings on SOFC interconnectors [Articolo su rivista]
Puranen, Jouni; Pihlatie, Mikko; Lagerbom, Juha; Bolelli, Giovanni; Laakso, Jarmo; Hyvärinen, Leo; Kylmälahti, Mikko; Himanen, Olli; Kiviaho, Jari; Lusvarghi, Luca; Vuoristo, Petri
abstract

Interconnects employed in solid oxide fuel cells require electrically conductive protective coatings such as those based on manganese cobalt oxide spinels in order to prevent evaporation of volatile Cr(VI)-compounds and to minimize high temperature corrosion. MnCo2−xFexO4 based (where x = 0.1 and 0.3) oxide spinel protective coatings were manufactured by the atmospheric plasma spraying process on Crofer 22 APU substrates. The coated substrates were oxidized at 700 °C in air for 1000 h and post-mortem analyses were conducted to study the performance of the thermal sprayed coatings. During the high temperature oxidation, a four-point on-line measurement technique was used for area specific resistance studies. The MnCo1.7Fe0.3O4 coating was tested together with the La0.85Sr0.15Mn1.1O3-spacer. The atmospheric plasma sprayed MnCo2−xFexO4 showed excellent Cr-barrier properties and decreased oxidation of the substrate. In addition, resistance as low as 9.20 mΩ cm2 values was obtained after the 1000 h oxidation cycle. The MnCo1.9Fe0.1O4 coating manufactured by the atmospheric plasma spraying process is a promising candidate for SOFC interconnect applications.

2014 - Preparation of innovative metallic composite glazes for porcelainized stoneware tiles [Articolo su rivista]
Siligardi, Cristina; L., Tagliaferri; Lusvarghi, Luca; Bolelli, Giovanni; D., Venturelli
abstract

Innovative metallic glazes for porcelain stoneware tiles, containing stainless steel and a special Ni-based alloy (NiCoCrAlY, commonly used as bond coating material in thermal barrier coating systems on Turbogas superalloy components), have been studied. These new products, called “ metallic composite glazes ” (MCGs), showed different aesthetic properties than usual lustres or metallic glazes. After heating, the surfaces of these innovative glazes did not display any interesting behaviour, but, after polishing, a particular metallic aesthetic effect has been found. These new glazes can be described as composite materials made of a glass matrix reinforced with metal particulates. A deep characterisation of metallic powders and MCGs was performed by using several techniques. The glazes containing the NiCoCrAlY powders manifested the best aesthetic, microstructural, thermal and chemical properties.

2014 - Properties of WC-FeCrAl coatings manufactured by different high velocity thermal spray processes [Articolo su rivista]
Bolelli, Giovanni; Iosif, Hulka; Heli, Koivuluoto; Lusvarghi, Luca; Andrea, Milanti; Kari, Niemi; Petri, Vuoristo
abstract

WC-FeCrAl coatings deposited by High Velocity Air-Fuel (HVAF) spraying and by High Velocity Oxygen-Fuel (HVOF) spraying with a liquid-fuelled and a gas-fuelled torch were studied and compared to a conventional HVOF-sprayed WC-CoCr coating.During HVOF spraying, WC-FeCrAl had a tendency to oxidise and decarburise. The gas-fuelled torch, operated with excess fuel, could restrain these phenomena, compared to the liquid fuelled one. In the HVAF coating, oxidation and decarburisation were further reduced, but porosity was >. 1%. As a consequence, the HVAF-sprayed coating was less hard than the HVOF-sprayed ones.Of all the WC-FeCrAl coatings, the gas fuelled-HVOF sprayed one exhibited the best wear resistance both in dry sliding conditions (at room temperature as well as at 400°C) and in dry particle abrasion testing. It indeed achieves the best compromise between density, hardness and low oxidation, so that it can reduce the extent of the brittle cracking phenomena affecting all WC-FeCrAl samples. Its dry sliding wear rate of ≈3*10-8mm3/(Nm) at room temperature is even slightly lower than that of the WC-CoCr reference. Its performance under high temperature sliding and dry particle abrasion conditions is, however, somewhat poorer than that of the reference.

2014 - Suspension thermal spraying of hydroxyapatite: microstructure and in vitro behaviour [Articolo su rivista]
Bolelli, Giovanni; Bellucci, Devis; Cannillo, Valeria; Lusvarghi, Luca; Sola, Antonella; N., Stiegler; P., Müller; A., Killinger; R., Gadow; L., Altomare; L., De Nardo
abstract

In cementless fixation of metallic prostheses, bony ingrowth onto the implant surface is often promoted by osteoconductive plasma-sprayed hydroxyapatite coatings. The present work explores the use of the innovative High Velocity Suspension Flame Spraying (HVSFS) process to coat Ti substrates with thin homogeneous hydroxy- apatite coatings. The HVSFS hydroxyapatite coatings studied were dense, 27–37 μm thick, with some transverse microcracks. La- mellae were sintered together and nearly unidentifiable, unlike conventional plasma-sprayed hydroxyapatite. Crystallinities of 10%–70% were obtained, depending on the deposition parameters and the use of a TiO2 bond coat. The average hardness of layers with low (b24%) and high (70%) crystallinity was ≈3.5 GPa and ≈4.5 GPa respectively. The distributions of hardness values, all characterised by Weibull modulus in the 5–7 range, were narrower than that of conventional plasma-sprayed hydroxyapatite, with a Weibull modulus of ≈3.3. During soaking in simulated body fluid, glassy coatings were progressively resorbed and replaced by a new, pre- cipitated hydroxyapatite layer, whereas coatings with 70% crystallinity were stable up to 14 days of immersion. The interpretation of the precipitation behaviour was also assisted by surface charge assessments, performed through Z-potential measurements. During in vitro tests, HA coatings showed no cytotoxicity towards the SAOS-2 osteoblast cell line, and surface cell proliferation was comparable with proliferation on reference polystyrene culture plates.

2014 - Tribological behavior of HVOF- and HVAF-sprayed composite coatings based on Fe-Alloy + WC-12% Co [Articolo su rivista]
Bolelli, Giovanni; T., Börner; A., Milanti; Lusvarghi, Luca; J., Laurila; H., Koivuluoto; K., Niemi; P., Vuoristo
abstract

Fe-based coatings are promising alternatives to Ni-based ones, because of lower cost and lower toxicity. Following a previous research, where the sliding wear resistance of HVOF-sprayed Fe-Cr-Ni-Si-B-C alloy coatings was found to compare favorably with that of a Ni-Cr-B-Si-C alloy and of electroplated chromium, the present study investigates the wear resistance of Fe-Cr-Ni-Si-B-C. +. WC-Co composite coatings. The Fe-alloy feedstock powder was therefore blended with 0, 20 and 40. wt.% of a WC-12. wt.% Co powder and sprayed by HVOF and HVAF processes. HVAF-sprayed coatings exhibit less structural alteration than HVOF-sprayed ones, which results in lower intrinsic nanohardness of both Fe-alloy and WC-Co splats; however, HVOF- and HVAF-sprayed coatings exhibit similar Vickers microhardness. Somewhat poorer interlamellar bonding in HVAF-sprayed coatings results in a greater tendency to microcracking during dry sliding wear testing at room temperature; however, dry sliding wear rates of HVOF- and HVAF-sprayed samples never differ significantly. The reinforcing effect of WC-Co decreases the wear rate of composite coatings (≈10-6mm3/(Nm)) by more than order of magnitude, compared to unreinforced ones (≈1-2*10-5mm3/(Nm)).As the test temperature is increased to 400°C and 700°C, the dry sliding wear rates of all samples increase (up to 10-4mm3/(Nm) or greater). The greatest changes are observed when the WC-Co content is larger, as it suffers from oxidation and thermal alteration more than the Fe-alloy matrix. The abrasive wear resistance of the Fe-based coatings, evaluated by rubber-wheel testing, is also significantly improved by the addition of WC-Co.

2013 - Analysis of pack chromizing of Nickel superalloys for gas turbines [Articolo su rivista]
Bozza, F; Bolelli, G; Lusvarghi, L; Siligardi, C; Giolli, C; Giorgetti, A; Scrivani, A
abstract

The pack-cementation process is a widespread technique used to obtain protective coatings formed by diffusion mechanisms. The protection of the base component is achieved by the enrichment of the outer layer with scale formers, such as Al, Cr and Si which react with the substrate material to form new phases and intermetallics. During the working life these chemical species react with the pollutant elements to form stable oxides and compounds, increasing the oxidation and hot corrosion resistance of gas-turbine components. The component is embedded inside a mixture of powder and then exposed at high temperature. The pack-mix is based on a scale former source, an activator (usually halide salts) and an inert filler (Al2O3). The pack-cementation process consists of a heating step, an isothermal stage and a cooling step.During the thermal treatment the activator reacts with the metal source, forming a vapor phase that transports the metal source to the surface of the component, where chemical reactions lead to the inward diffusion of the selected element into the substrate, forming new phases and intermetallics.The present investigation deals with the study of formation mechanisms of diffusion coatings using chromium as scale former, in order to obtain hot corrosion resistant coating, for gas turbine applications. According to the pertinent literature aluminide coatings provide the protection of the component against oxidation, whereas chromium rich coating are more suitable against hot corrosion type II, that usually occurs at about 700 degrees C due to pollutant agentsThe main phases of these coatings depend on many parameters, such as temperature, duration of the isothermal stage and pack-mix composition. However a systematic investigation about the effect of the main process parameters on the formation mechanisms is not available in the pertinent literature.Therefore this work aims to realize an accurate investigation that leads to a better comprehension of the influence of time and pack-mix composition on the microstructure and thickness of these coatings.A Ni-base superalloy, Inconel 738, was used as substrate material in this work. The samples (approx. size 15x15x3mm) were polished with a 1000 SiC paper and the cleaned in acetone.In order to investigate the effect of the pack-mix composition on the final microstructure and on the coating thickness different formulations were selected, varying the amount of the chromium source, based on pure Cr powder (10, 25 wt.%) and the activator (NH4Cl, 1, 2 and 5 wt.%). Before each pack-chromizing cycle two samples were embedded inside a steel retort with the chosen pack-mix.Three different isothermal stage were used to evaluate the effect of the thermal cycle duration on the diffusion phenomena: 0, 12 and 24 hours at 1100 degrees C. A summary of the process parameters and the pack-mix formulations is listed in tab. 1.At the end of the pack-chromizing one sample for each deposition condition was heat treated in vacuum to evaluate the effect of the interdiffusion phenomena that occur during the vacuum cycle.The surface of the specimens were characterized by X-Ray diffraction (XRD), the cross-section of the coatings were embedded in an epoxy resin and analyzed by Scanning Electron Microscope (SEM) and Energy-Dispersion X-Ray Spectroscopy (EDS).The analysis on the coating C10A1t0 shows that the diffusion mechanism takes place even before the selected process temperature (1100 degrees C), leading to the formation of a thin Cr-rich layer (fig. 2). The dark region is based on Al2O3 rich particles, formed due to secondary reactions occurred between the salt activator and the inert filler that caused the oxidation of the aluminum originally present in the Inconel 738 composition.The comparison between the coating C10A1t0 and C10A1t12 reveals that during the isothermal stage the Cr atoms diffused inward and Ni outward. The coating is based on 3 different layer: a Cr-rich outer region, an alumina

2013 - High-pressure cold-sprayed Ni and Ni-Cu coatings - Improved structures and corrosion properties [Relazione in Atti di Convegno]
Koivuluoto, H.; Milanti, A.; Vuoristo, P.; Bolelli, G.; Lusvarghi, L.
abstract

Cold spraying is a promising technique for the production of dense metallic coatings. In cold spraying, coating formation is based on high velocity impacts of solid particles with high kinetic energy. During the impacts, particles deform plastically and adhere, building-up the coating. This makes it possible to form pure and dense coating structures. Full impermeability of the coatings is required e.g. in corrosion protection applications. Nickel and nickel-copper alloys have good corrosion resistance and therefore, as dense coatings, they have a high potential for employment as corrosion barrier layers. In this study, the structural and corrosion properties of high-pressure cold-sprayed (HPCS) Ni and NiCu coatings are characterized. NiCu alloys are known to have good corrosion resistance e.g. in sulphuric and hydrochloric acids whereas Ni is resistant to e.g. caustic soda and alkaline salt solutions. This study also shows the effect of heat treatments on the coating performances. FESEM studies of cross-sectional samples reveal structural details of dense coatings while corrosion properties are evaluated with polarization measurements. Corrosion behaviors are compared with corresponding bulk materials and substrate material in order to perceive the real corrosion protection potential of the coatings.

2013 - Metal injection molding as enabling technology for the production of metal prosthesis components: Electrochemical and in vitro characterization [Articolo su rivista]
Virginia, Melli; Gianni, Rondelli; Enrico, Sandrini; Lina, Altomare; Bolelli, Giovanni; Benedetta, Bonferroni; Lusvarghi, Luca; Alberto, Cigada; Luigi De, Nardo
abstract

Industrial manufacturing of prosthesis components could take significant advantage by the introduction of new, cost-effective manufacturing technologies with near net-shape capabilities, which have been developed during the last years to fulfill the needs of different technological sectors. Among them, metal injection molding (MIM) appears particularly promising for the production of orthopedic arthroplasty components with significant cost saving. These new manufacturing technologies, which have been developed, however, strongly affect the chemicophysical structure of processed materials and their resulting properties. In order to investigate this relationship, here we evaluated the effects on electrochemical properties, ion release, and in vitro response of medical grade CoCrMo alloy processed via MIM compared to conventional processes. MIM of the CoCrMo alloy resulted in coarser polygonal grains, with largely varying sizes; however, these microstructural differences between MIM and for- ged/cast CoCrMo alloys showed a negligible effect on electrochemical properties. Passive current densities values observed were 0.49 microA * cm-2 for MIM specimens and 0.51 microA cm-2 for forged CoCrMo specimens, with slightly lower transpassive potential in the MIM case; open circuit potential and Rp stationary values showed no significant differences. Moreover, in vitro biocompatibility tests resulted in cell viability levels not significantly different for MIM and conventionally processed alloys. Although preliminary, these results support the potential of MIM technology for the production of CoCrMo components of implantable devices.

2013 - Suspension plasma sprayed bioactive glass coatings: effects of processing on microstructure, mechanical properties and in-vitro behaviour [Articolo su rivista]
Cattini, Andrea; L., Łatka; Bellucci, Devis; Bolelli, Giovanni; Sola, Antonella; Lusvarghi, Luca; L., Pawłowski; Cannillo, Valeria
abstract

Bioactive glass coatings deposited via suspension plasma spraying were studied to improve the adhesion between orthopaedic implants and bone. Fine powders of a bioactive glass, named BG_Ca, having composition (in wt.%): 4.7 Na2O, 42.3 CaO, 6.1 P2O5, 46.9 SiO2, were produced and dispersed in ethanol to form a suspension used as a feedstock. Various sets of spray parameters were applied in order to define the influence of the deposition process on the final coating properties. Consequently, the coatings were characterized in as-sprayed state and after soaking in a simulated body fluid (SBF) for different periods ranging from 1 to 14 days. The microstructural investigations were carried out using environmental scanning electron microscope (ESEM) and X-ray diffraction (XRD). The coatings’ adhesion to the substrate was evaluated by means of scratch tests. Finally, hardness and elastic modulus were determined by means of depth-sensing indentation methods.

2013 - Wear and corrosion resistance of high-velocity oxygen-fuel sprayed iron-based composite coatings [Relazione in Atti di Convegno]
Milanti, A.; Koivuluoto, H.; Vuoristo, P.; Bolelli, G.; Bozza, F.; Lusvarghi, L.
abstract

Thermally sprayed iron-based coatings are being widely studied as alternative solution to conventional hardmetal (cermet) and Ni-based coatings for wear and corrosion applications in order to reduce costs, limit environmental impact and enhance the health safety. The aim of the present work is to study the cavitation erosion behaviour in distilled water and the corrosion properties in acidic solution of four high-velocity oxy-fuel (HVOF) sprayed Fe-based composite coatings. Fe-Cr-Ni-B-C powder was selected for its good sliding wear properties. In addition, a powder composition with an addition of Mo was studied in order to increase the corrosion resistance whereas additions of 20 wt. % and 40 wt. % WC-12Co as blended powder mixtures were investigated in order to increase wear resistance. Improvement of coating properties was significant with the advanced powder compositions. Dense coating structures with low porosity were detected with microstructural characterization. In addition, good cavitation wear resistance was achieved. The cavitation resistance of customized Fe-based coating with Mo addition was reported to be twice as high as that of conventional Ni-based and WCCoCr coatings. The corrosion properties of HVOF Fe-based coatings were also evaluated by studying electrochemical behaviour in order to analyse their potential to use as corrosion barrier coatings. Copyright © 2013 by ASME.

2012 - Cermet coatings with Fe-based matrix as alternative to WC-CoCr: mechanical and tribological behaviour [Articolo su rivista]
Bolelli, Giovanni; T., Börner; Bozza, Francesco; Cannillo, Valeria; G., Cirillo; Lusvarghi, Luca
abstract

Recently, cermet coatings with Fe-based metal matrix have emerged as a less hazardous and more environ- mentally friendly alternative to WC–Co-based ones, which have known inhalation toxicity problems. This study therefore aimed to validate WC-based cermet coatings with Fe-based matrix, obtained using a com- mercially available feedstock powder, as an alternative to WC–CoCr. HVOF-sprayed WC–15 wt.%FeCrAl layers were therefore obtained using different oxygen and fuel (kerosene) flow rates and powder feed rates; their mechanical and tribological properties were compared to HVOF- sprayed WC–10 wt.% Co–4 wt.%Cr. The WC–FeCrAl coatings always exhibited equi-biaxial compressive residual stress state and possessed dense microstructures, with homogeneous metal matrix, but they contained more oxide inclusions than WC–CoCr. Their characteristics were significantly affected by the normalised oxygen-fuel ratio (λ). Small but meaningful differences existed between the ball-on-disc sliding wear rates of the various WC– FeCrAl coatings, the best sample being that with the most favourable combination of compressive residual stress, low oxidation and high hardness/modulus (H/E) ratio. Its sliding wear resistance was comparable to that of WC–CoCr. The cyclic ball impact resistance of WC–FeCrAl layers was also comparable to that of WC–CoCr, but the dry particle abrasion resistance was inferior, because of the brittleness induced by the oxide inclusions.

2012 - Deposition mechanisms in high velocity suspension spraying: Case study for two bioactive materials [Articolo su rivista]
Bolelli, Giovanni; Stiegler, Nico; Bellucci, D.; Cannillo, Valeria; Gadow, Rainer; Killinger, A.; Lusvarghi, Luca; Luca, Sola; Antonella,
abstract

Two different oxide-based bioactive materials, namely tricalcium phosphate (TCP) and K2O–CaO–P2O5–SiO2 bioactive glass (“Bio-K”), were processed by the High Velocity Suspension Flame Spraying (HVSFS) technique. Single splats were deposited onto polished cold glass plates using a custom-built experimental set up. In addition, complete coatings were deposited onto pre-heated Ti plates. Deposited splats and coatings were examined by scanning and transmission electron microscope (SEM, TEM) techniques and their phase composition was investigated by micro-Raman spectroscopy. The 3-D profiles of single splats were also reconstructed through atomic force microscopy (AFM). Based on the experimental results, explanations of the deposition mechanisms were put forward, discussing the different heating and melting behaviour of particles and agglomerates. The critical particle sizes needed to obtain deposition of molten splats at impact, and their solidification behaviour onto cold vs. hot surfaces were studied as well. Most importantly, the splats can only be formed by molten droplets comprised within a given size range (“sprayability window”). Smaller droplets possess insufficient thermal and mechanical inertia to overcome the stagnation flow in front of the substrate; larger ones are mostly unmelted. This “sprayability window” depends on the deposition process itself and on the physical properties of sprayed materials.

2012 - Engineering the Microstructure of Solution Precursor Plasma-Sprayed Coatings [Articolo su rivista]
G., Bertolissi; C., Chazelas; Bolelli, Giovanni; Lusvarghi, Luca; M., Vardelle; A., Vardelle
abstract

This study examines the fundamental reactions that occur in-flight during the solution precursor plasma spraying (SPPS) of solutions containing Zr- and Y-based salts in water or ethanol solvent. The effect of plasma jet composition (pure Ar, Ar-H 2 and Ar-He-H 2 mixtures) on the mechanical break-up and thermal treatment of the solution, mechanically injected in the form of a liquid stream, was investigated. Observation of the size evolution of the solution droplets in the plasma flow by means of a laser shadowgraphy technique, showed that droplet break-up was more effective and solvent evaporation was faster when the ethanol-based solution was injected into binary or ternary plasma gas mixtures. In contrast with water-based solutions, residual liquid droplets were always detected at the substrate location. The morphology and structure of the material deposited onto stainless steel substrates during single-scan experiments were characterised by SEM, XRD and micro-Raman spectroscopy and were shown to be closely related to in-flight droplet behaviour. In-flight pyrolysis and melting of the precursor led to well-flattened splats, whereas residual liquid droplets at the substrate location turned into non pyrolysed inclusions. The latter, although subsequently pyrolysed by the plasma heat during the depo- sition of entire coatings, resulted in porous ‘‘sponge-like’’ structures in the deposit.

2012 - Functionally graded WC-Co/Ni-Al HVOF coatings for damage tolerance, wear and corrosion protection [Articolo su rivista]
Bolelli, Giovanni; Cannillo, Valeria; Lusvarghi, Luca; Rosa, Roberto; A., Valarezo; W. B., Choi; R., Dey; C., Weyant; S., Sampath
abstract

The manufacturing of a HVOF-sprayed functionally graded coating (FGC), consisting of two NiAl/WC–Co composite layers with increasing cermet content and a pure WC–Co topmost layer, is discussed in this paper. As the stress build-up during spraying, measured via curvature method, is reduced in the NiAl-rich layers, thick coatings can be deposited with lower risk of delamination, in comparison to a pure WC–Co thick layer. Thermal stresses within the layers and the stainless steel substrate, measured from low temper- ature thermal cycling tests, are also reduced. The mechanical behaviour of the top layer in the graded struc- ture at relatively low loads (i.e. pin-on-disk) is equivalent to the same layer without gradation, whereas at high loads (i.e. ball-drop impact test) the enhanced compliance with depth increases the load bearing capa- bility of the graded structure. Electrochemical corrosion tests demonstrate no significant interconnected porosity and therefore, no interaction of the graded layers with the topmost WC–Co layer. The deterioration of the deeper layers under wear and corrosive conditions are also tested to investigate their performance in the scenario that the top layer coating wears out exposing the deeper layers to the surface.

2012 - High Velocity Suspension Flame Sprayed (HVSFS) Hydroxyapatite Coatings for Biomedical Applications [Articolo su rivista]
N., Stiegler; Bellucci, Devis; Bolelli, Giovanni; Cannillo, Valeria; R., Gadow; A., Killinger; Lusvarghi, Luca; Sola, Antonella
abstract

In this study, hydroxyapatite (HAp) coatings were deposited on Ti plates by the high-velocity suspension flame spraying (HVSFS) technique. The process characteristic, the microstructure and phase composi- tion of the coatings are significantly influenced by the solvent and by the design of the combustion chamber (CC) of the HVSFS torch. Water-based suspensions always lead to fairly low surface tem- peratures ( 350 °C), deposition efficiencies <40%, and produce coatings with low amount of crystalline HAp, which tend to dissolve very rapidly in simulated body fluid (SBF) solutions. DEG-based suspen- sions, when sprayed with properly-designed CCs, produce deposition efficiencies of 45-55% and high surface temperatures (550-600 °C). In these coatings, the degree of crystallinity increases from the bottom layer to the top layer, probably because the increasingly large surface temperature can eventually favour re-crystallisation of individual lamellae during cooling. These coatings are much more stable in SBF solutions.

2012 - High Velocity Suspension Flame Sprayed (HVSFS) potassium-based bioactive glass coatings with and without TiO2 bond coat [Articolo su rivista]
Bellucci, Devis; Bolelli, Giovanni; Cannillo, Valeria; R., Gadow; A., Killinger; Lusvarghi, Luca; Sola, Antonella; N., Stiegler
abstract

Titanium plates were coated by high-velocity suspension flame spraying (HVSFS) technique using a novel bioactive glass composition based on the K2O–CaO–P2O5–SiO2 composition (“Bio-K”). On half of the samples, an atmospheric plasma sprayed (APS) TiO2 bond coat was preliminarily deposited; suspensions of attrition- milled micron-sized glass powders, dispersed in a water + isopropanol mixture, were then sprayed onto both bare and bond-coated plates using five different process parameter sets. The microstructure of the coatings is independent of the presence of the bond coat but is strongly influenced by the deposition parameters. If the latter result in surface temperatures larger than the glass transition tem- perature of the Bio-K composition, large-scale viscous flow allows the expansion of the air entrained in the porosities, developing large rounded pores. When this phenomenon is avoided, denser layers are obtained. In tensile adhesion tests, porous layers fail cohesively at low loads, whereas adhesive/cohesive failure occurs in denser layers. In this latter case, the adhesion strength is significantly improved by the bond coat, reaching maximum values of 17 MPa. When immersed in simulated body fluid (SBF), the coating surface is rapidly converted into a silica gel because of ion leaching. A hydroxyapatite layer starts precipitating on top of it after 3 days and grows into a uniform film (of ≈ 10 μm thickness) after 2 weeks.

2012 - High resolution residual stress measurement on amorphous and crystalline plasma-sprayed single-splats [Articolo su rivista]
M., Sebastiani; Bolelli, Giovanni; Lusvarghi, Luca; P. P., Bandyopadhyay; E., Bemporad
abstract

Residual stress was measured on plasma sprayed crystalline Ni\Al, Al2O3 and amorphous Al2O3\TiO2\ZrO2-CeO2 single splats, by using an incremental focused ion beam (FIB) micron-scale ring-core method (IμRCM).Tensile residual stress exists in polycrystalline Ni\Al splats, where the quenching stress is only partially relaxedby edge curling and through-thickness yielding. Significant compressive stress was observed for theamorphous Al2O3\TiO2\ZrO2\CeO2 splats, where viscous flow above the glass transition temperaturecompletely relaxed the quenching stresses without micro-cracking. Comparatively lower compressive stresswas measured on crystalline Al2O3 splats, where, in spite of extensive micro-cracking, not all of the tensilequenching stress was relaxed. Using stress data and micro-crack geometry, the intrinsic shear adhesionstrength of Al2O3 splats was calculated, giving insights into the role of (sub)micron-scale phenomena onadhesion/cohesion of thermally sprayed coatings. The proposed stress build-up mechanisms and relaxationphenomena are supported by a TEM microstructural analysis of the splats.The experimental methodology developed provided a unique way for the study of the residual stress buildupmechanisms in amorphous and crystalline single splats obtained by plasma spraying, and gave further insightsinto the actual micro-scale phenomena that give rise to adhesion and nano-mechanical behavior ofthermally sprayed coatings.The proposed approach is also expected to find a wide range of applications in materials science and engineering,as it allows for the residual stress measurement even on amorphous materials with micrometer spatialresolution.

2012 - Micromechanical properties and sliding wear behaviour of HVOF-sprayed Fe-based alloy coatings [Articolo su rivista]
Bolelli, Giovanni; Bonferroni, Benedetta; J., Laurila; Lusvarghi, Luca; A., Milanti; K., Niemi; P., Vuoristo
abstract

The tribological performance of two Fe–Cr–Ni–Si–B–C (Colferoloy) alloy coatings manufactured by HVOFthermal spraying was studied by rubber-wheel dry particle abrasion test and ball-on-disk sliding wear tests. The results were compared to those obtained on Ni–Cr–Fe–Si–B–C and Cr3C2–NiCr layers (also manufactured by HVOF-spraying), hard chromium electroplating and bulk tool steel. At room temperature, the sliding wear loss of the Colferoloy coatings against alumina counterpart,caused by a mix of mild abrasion, delamination and tribo-oxidation, was larger than that of Cr3C2–NiCrand tool steel but lower than that of Ni–Cr–Fe–Si–B–C and hard chromium plating. At 400 ◦C and 700 ◦C,Colferoloy coatings mainly suffered abrasive grooving: they were still superior to Ni–Cr–Fe–Si–B–C butinferior to Cr3C2–NiCr. Against steel, Colferoloy coatings, with limited delamination and negligible wearloss, were comparable to Cr3C2–NiCr and superior to Ni–Cr–Fe–Si–B–C, tool steel and electrolytic hardchromium, although they inflicted quite significant wear to the steel counterbody. Colferoloy coatings were therefore validated as alternatives to Ni-based alloys and electroplatedchromium under sliding wear conditions, but appeared unsuitable for particle abrasion resistance. The different sliding wear behaviours of HVOF-sprayed coatings could be explained by coupling micro and nano-hardness to scratch testing, which reflected cracking resistance and plastic deformability.

2012 - Valorization of MSWI bottom ash through ceramic glazing process: a new technology [Articolo su rivista]
Schabbach, Luciana; Bolelli, Giovanni; Andreola, Nora Maria; Barbieri, Luisa; Lancellotti, Isabella
abstract

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2011 - Characterisation of plasma-sprayed SrFe12O19 coatings for electromagnetic wave absorption [Articolo su rivista]
K., Bobzin; Bolelli, Giovanni; M., Bruehl; A., Hujanen; P., Lintunen; D., Lisjak; S., Gyergyek; Lusvarghi, Luca
abstract

SrFe12O19 coatings, intended as electromagnetic wave absorbers, were produced by atmospheric plasma spraying (APS) using two different kindsof feedstock powders: spray-dried agglomerates of micrometric SrFe12O19 particles (type-A) or spray-dried agglomerates of raw materials (SrCO3,Fe2O3), reactively sintered at 1100 ◦C (type-B).During spraying, type-A agglomerates either remain unmelted, producing porous coating regions where crystalline hexaferrite is retained, orare disrupted into smaller granules which melt completely, resulting in dense coating regions with no crystalline hexaferrite.The sintered type-B agglomerates possess higher cohesive strength and do not fall apart: the finer ones melt completely, whereas, in the largerones, the outer region melts and infiltrates the porous unmelted core which retains crystalline hexaferrite. Dense coatings can therefore be obtainedwhile preserving high amounts of crystalline hexaferrite even inside the dense areas. Such coatings show magnetic properties that are promisingfor electromagnetic wave absorption applications.

2011 - Deposizione di rivestimenti in Al2O3 mediante High Velocity Suspension Flame Spraying (HVSFS): caratteristiche dei riporti ed effetto dei parametri operativi [Articolo su rivista]
Bolelli, Giovanni; B., Bonferroni; Cannillo, Valeria; R., Gadow; A., Killinger; Lusvarghi, Luca; J., Rauch; N., Stiegler
abstract

The HVSFS (High Velocity Suspension Flame Spraying) technique is a novel thermal spray process which consists of a modification to a standard gas-fuelled HVOF system in order to enable feeding of the torch with a liquid suspension instead of a dry powder. The suspension consists of fine (micrometre- or nanometer-sized) particles dispersed in a liquid medium. This process enables the deposition of relatively thin (&lt;100 μm) and dense ceramic coatings, which could not be obtained by standard thermal spraying, since the flowability of very fine powders in dry form is too poor to allow proper feeding. These flowability problems are overcome by employing a liquid medium as carrier. Using such fine particles, the average lamella size is reduced compared to conventional thermal spray coatings, which is beneficial for improved smoothness (lower surface toughness in as-deposited condition) and density (better packing, smaller pores) and allows the deposition of homogeneous layers of reduced thickness. The properties of a HVSFS-deposited coating are critically affected by the properties of the suspension and particularly by the nature of the solid phase (primary particle size distribution, agglomeration behaviour); however, systematic studies on this relationship have not been performed yet. In this research, HVSFS Al2O3 coatings were therefore produced using three different suspensions, each containing a different fine alumina powder, dispersed in a water + isopropanol mixture: a micrometric powder (d50 ≈ 2 μm) and two nanometric ones with identical average size (d50 ≈ 70 nm) but different agglomeration behaviour. All suspensions contained 20 wt.% of solid phase. Each suspension was sprayed using two distinct parameter sets, so that a total of 6 different coatings were obtained. The results showed that, irrespective of the process parameters, the suspension containing micrometric particles yielded harder (HV0.05 = 1200 - 1300 kg/mm2) and smoother (Ra ≈ 1.3 μm) coatings. It was noted that micron-sized particles have low tendency to agglomeration; therefore, when the solvent (water + isopropanol mixture) evaporates in the thermal spray gas jet, the suspension releases many individual micrometric particles of 1 - 2 μm diameter. These particles are fully melted at impact and, on account of their high impact velocity, they result in a dense and homogeneous stacking of well-flattened lamellae. Although these coatings also exhibit the largest tensile residual stresses (between 50 MPa and 100 MPa), their excellent cohesive strength endows them with superior sliding wear resistance (tested under "ball-on-disk" configuration against sintered alumina spheres) in comparison to the other HVSFS-deposited coatings and to conventional HVOF-sprayed Al2O3 coatings. As nanometre-sized particles are always agglomerated within the suspension, the resulting lamella size does not depend on the size of the (nanometric) primary particles, but on that of the agglomerates, which are released in the gas jet when the liquid phase evaporates. When agglomerates are sufficiently small (0.5 - 2.5 μm), quite dense coatings can be achieved; excessively large agglomerates, by contrast, tend to remain unmelted, giving rise to defective coatings and impairing the deposition efficiency. In conclusion, the HVSFS process was shown to be able to produce high-quality coatings, superior to those obtained by conventional thermal spraying techniques; however, proper control over the behaviour of the solid phase must be acquired; in particular, individual particles or agglomerates should ideally be comprised between 0.5 and 2.5 μm in order to optimize density, hardness, smoothness and deposition efficiency.

2011 - Hexaferrite/Polyester Composite Coatings for Electromagnetic-Wave Absorbers [Articolo su rivista]
D., Lisjak; M., Bégard; M., Bruehl; K., Bobzin; A., Hujanen; P., Lintunen; Bolelli, Giovanni; Lusvarghi, Luca; S., Ovtar; M., Drofenik
abstract

Composite coatings, consisting of a ceramic barium hexaferrite and polyester, were manufactured byatmospheric plasma spraying. The crystalline structure of the barium hexaferrite was preserved duringthe spraying process while the polyester first (partly) melted and then resolidified. The high magneticlosses of the barium hexaferrite at mm-waves were coupled to the dielectric losses of the polyester in thecomposite coating and the superior electromagnetic absorption of the composite coating with respect tothe pure materials was determined

2011 - Hexaferrite/polyethylene composite coatings prepared with flame spraying [Articolo su rivista]
Darja, Lisjak; Pertti, Lintunen; Arto, Hujanen; Tommi, Varis; Bolelli, Giovanni; Lusvarghi, Luca; Marko, Jagodič; Miha, Drofenik
abstract

Composite coatings from different volume ratios of hexaferrite (BaFe12O19 or SrFe12O19) and polyethylenewere prepared, for the first time, with flame spraying. The hexaferrite phase retained its crystal structure andmicrostructure during the process, while the polyethylene melted and resolidified. The coatings showedmagnetic hysteresis loops with high coercivities. The measured electromagnetic behaviour proved that thestudied composite coatings would be suitable for electromagnetic wave absorbers in the U-band.

2011 - In situ Raman spectroscopic investigation of bioactive glass reactivity: Simulated Body Fluid solution VS TRIS-buffered solution [Articolo su rivista]
Bellucci, Devis; Bolelli, Giovanni; Cannillo, Valeria; Cattini, Andrea; Sola, Antonella
abstract

In the present contribution, the innovative in-situ Raman micro-spectroscopy was applied to investigate the in vitro reactivity of various bioactive glasses. All the investigated glasses belonged to the Na2O\K2O-CaO-P2O5-SiO2 system, but contained sensibly different percentages of network modifiers. The glasses were immersed for increasing times, up to 96 hours, in simulated body fluid (SBF) and in tris-buffered (TRIS) solution. In this way, two fundamental items were addressed, i.e. the effect of the glass composition and the nature of the soaking fluid on the overall reactivity. As regards the SBF, all the glasses were able to promote the formation of a hydroxyl-carbonate apatite (HCA) surface layer in very short times. The reaction rate was particularly quick for the 45S5 Bioglass® and for its potassium-based variant (BioK), however all the glasses could form a continuous HCA layer already after 96 hours. The observed difference in reaction kinetics may be due to the glass composition, since the glasses relatively poor in Na ions (BG_Ca) experience slower ion release in the first stages of the HCA formation, while the glasses relatively poor in Ca ions (BG_Na) undergo slower nucleation and growth of HCA. The development of HCA was also observed in TRIS, but the reaction rate was generally slower than in SBF. In fact, while the SBF is a complicated solution supersaturated in apatite, which favours the precipitation of HCA, the TRIS is a simple tris(hydroxymethyl)aminomethane solution in water, which does not provide the ions for the HCA formation. As a consequence, the aforementioned effects due to the glass composition were even more evident in TRIS than in SBF. Nevertheless the TRIS could represent a valuable alternative to the standard SBF whenever a slow reaction rate might be beneficial, such as, for example, in order to better observe the samples evolution.

2011 - Microstructure and in vitro behaviour of 45S5 bioglass coatings deposited by high velocity suspension flame spraying (HVSFS) [Articolo su rivista]
L., Altomare; Bellucci, Devis; Bolelli, Giovanni; Bonferroni, Benedetta; Cannillo, Valeria; L., De Nardo; R., Gadow; A., Killinger; Lusvarghi, Luca; Sola, Antonella; N., Stiegler
abstract

The high-velocity suspension flame spraying technique (HVSFS) was employed in order to deposit 45S5 bioactive glass coatings onto titanium substrates, using a suspension of micron-sized glass powders dispersed in a water ? isopropanol mixture as feedstock. By modifying the process parameters, five coatings with different thick- ness and porosity were obtained. The coatings were entirely glassy but exhibited a through-thickness micro- structural gradient, as the deposition mechanisms of the glass droplets changed at every torch cycle because of the increase in the system temperature during spraying. After soaking in simulated body fluid, all of the coatings were soon covered by a layer of hydroxyapatite; furthermore, the coatings exhibited no cytotoxicity and human osteosar- coma cells could adhere and proliferate well onto their surfaces. HVSFS-deposited 45S5 bioglass coatings are therefore highly bioactive and have potentials as replace- ment of conventional hydroxyapatite in order to favour osseointegration of dental and prosthetic implants.

2011 - Variation of splat shape with processing conditions in plasma sprayed alumina coatings [Articolo su rivista]
Kazi, Sabiruddin; P. P., Bandyopadhyay; Bolelli, Giovanni; Lusvarghi, Luca
abstract

This paper is on various splat shapes obtained using three alumina based powders sprayed on various substrates. The parameters considered were substrate preheating temperature, nozzle diameter, and secondary and primary gas flow rates. The splat shape was found to be strongly dependent on spraying conditions. The substrate preheating temperature determined the degree of substrate wetting by the splat. A change in either nozzle diameter or primary gas flow rate brought about a change in the particle momentum and subsequently, a change in splat shape. The splat shape differed widely on an as – sprayed bond coat as compared to a polished one, owing to splat confinement by surface asperities. Sub-microscale surface roughness of polished substrate surfaces showed an increase with the preheating temperature and this in turn, resulted in better substrate wetting by the splats.

2011 - Wear and corrosion behaviour of HVOF WC-CoCr / CVD DLC hybrid coating systems deposited onto aluminum substrate [Articolo su rivista]
Bolelli, Giovanni; Bonferroni, Benedetta; G., Coletta; Lusvarghi, Luca; F., Pitacco
abstract

Hybrid coating systems consisting of a HVOF-sprayed WC–CoCr interlayer (75 μm or 125 μm thick) and a thinDLC-based top layer (obtained by electron cyclotron wave resonance–chemical vapour deposition: ECWRCVD)were deposited onto AA6082-T6 substrates; their tribological and anti-corrosion performances wereevaluated and compared to those of the DLC film deposited directly onto the Al alloy substrate withoutinterlayer.In scratch adhesion testing, the interlayer delays the onset of cracking and spallation of the film, because itprovides better mechanical support than the bare Al substrate. As soon as cracking begins, however, completedelamination of the films deposited on cermet interlayers occurs, revealing limited interface adhesion.Accordingly, in ball-on-disk wear tests, the films deposited on cermet interlayers can stand more severecontact conditions without cracking, but, as the contact pressure increases, most of the film delaminates.If no cracking occurs, the film is very effective at producing low wear and low friction, but it is not as effectiveat preventing penetration of corrosive agents, as shown by corrosion tests in aqueous environments, becauseit contains some small defects.

2010 - Corrosion resistance of cold-sprayed Ta coatings in very aggressive conditions [Articolo su rivista]
Bolelli, Giovanni; Lusvarghi, Luca; H., Koivuluoto; P., Vuoristo; F., Casadei
abstract

This study deals with the corrosion behaviour of high-pressure cold-sprayed Ta coatings, compared with Tabulk material and inert-atmosphere plasma-sprayed Ta coatings. Electrochemical polarization andelectrochemical impedance spectroscopy measurements are carried out in 1 M KOH solution; immersiontests are performed in 1 wt.% HF solution, and the resistance to high-temperature oxidation and molten saltattack is evaluated by hot corrosion tests. Moreover, the effect of substrate material (steel, Al and Cu) on thecoating formation is investigated using FESEM.

2010 - Damage tolerant functionally graded WC-Co/Stainless Steel HVOF coatings [Articolo su rivista]
A., Valarezo; Bolelli, Giovanni; W. B., Choi; S., Sampath; Cannillo, Valeria; Lusvarghi, Luca; Rosa, Roberto
abstract

In this paper, effective damage tolerance of a functionally graded coating (FGC) deposited by high velocity oxygen fuel (HVOF) spraying is observed. The thick FGC (≈1.2 mm) consists of 6 layers with a stepwise change in composition from 100 vol.% ductile AISI316 stainless steel (bottom layer) to 100 vol.% hard WC-12Co (top layer) deposited onto an AISI316 stainless steel substrate. Damage tolerance is observed via 1) an increase in compliance with depth, and 2) an increase in fracture resistance by containment, arrest and deflection of cracks. A smooth gradation in the composition and hardness through the coating thickness is found by scanning electron microscopy and depth-sensing microindentation, respectively. The in-situ curvature measurement technique reveals that during the deposition of the FGC, compressive stresses exist in the lower, metallic layers owing to peening effect of successive impact, and these gradually evolve to high tensile, in the top layers. Tensile stresses appear to be due to quenching alone; thermal stresses are low because of the gradation. All of this is beneficial for the deposition of a thick coating.The FGC structure shows the ability to reduce cracking with increased compliance in the top layer during static and dynamic normal contact loading, while retaining excellent sliding wear resistance (ball-on-disk tests). Results are discussed in comparison to the behavior and properties of coatings of similar individual compositions and thicknesses, as well as a thick monolithic WC-12Co sprayed coating. Further improvements in the processing are proposed to enhance the adhesion strength and avoid coating delamination under high load contact-fatigue conditions.

2010 - Depth-sensing indentation for assessing the mechanical properties ofcold-sprayed Ta [Articolo su rivista]
Bolelli, Giovanni; Bonferroni, Benedetta; H., Koivuluoto; Lusvarghi, Luca; P., Vuoristo
abstract

The stress–strain behaviour of cold-sprayed Ta coatings, deposited onto Al and steel substrates, was studiedby depth-sensing spherical and sharp (Berkovich) indentation testing, and was compared to that of anannealed Ta sheet. The mechanical properties of the coatings, free of any scale-dependence, are insensitive tothe presence of a lamellar structure, indicating strong, tight bonding between cold-sprayed Ta particles.Accordingly, the coatings are isotropic. The stress–strain curves reveal that the coating material experiencedsome degree of work-hardening during cold-spraying; indeed, the sprayed particles were plasticallydeformed at impact. This was confirmed by line profile analysis on XRD patterns. The consistency between theresults from different indentation testing techniques provides a means to cross-validate the entire analysis.

2010 - Development of Ba-hexaferrite coatings for electromagnetic wave absorption applications [Articolo su rivista]
K., Bobzin; T., Schlaefer; M., Bégard; M., Bruehl; Bolelli, Giovanni; Lusvarghi, Luca; D., Lisjak; A., Hujanen; P., Lintunen; U., Kanerva; T., Varis; M., Pasquale
abstract

In this research, Ba-hexaferrite coatings for electromagnetic wave absorption applications were deposited bythermal spraying. A suitable powder feedstock was manufactured by blending a BaCO3+Fe2O3 mixture,which was then agglomerated by spray-drying. The agglomerates were processed by air plasma spraying(APS) without any further treatment or were heat-treated and reactively-sintered to stoichiometric Bahexaferriteprior to spraying. As-sprayed coatings were analyzed by Raman spectroscopy and X-raydiffraction (XRD). Whereas the deposition of untreated agglomerates did not result in adequate amounts ofcrystalline Ba-hexaferrite in the coatings, the APS processing of reactively-sintered agglomerates led to ahigh content of Ba-hexaferrite and similar magnetic properties to those of Ba-hexaferrite bulk materials.

2010 - Effect of the suspension composition on the microstructural properties of high velocity suspension flame sprayed (HVSFS) Al2O3 coatings [Articolo su rivista]
Bolelli, Giovanni; Cannillo, Valeria; R., Gadow; A., Killinger; Lusvarghi, Luca; J., Rauch; Romagnoli, Marcello
abstract

Seven different Al2O3-based suspensions were prepared by dispersing two nano-sized Al2O3 powders (having analogous size distribution and chemical composition but different surface chemistry), one micron-sized powder and their mixtures in a water+isopropanol solution. High velocity suspension flame sprayed (HVSFS) coatings were deposited using these suspensions as feedstock and adopting two different sets of spray parameters. The characteristics of the suspension, particularly its agglomeration behaviour, have a significant influence on the coating deposition mechanism and, hence, on its properties (microstructure, hardness, elastic modulus). Dense and very smooth (Ra ~ 1.3 μm) coatings, consisting of well- flattened lamellae having a homogeneous size distribution, are obtained when micron-sized (~1 -2 μm) powders with low tendency to agglomeration are employed. Spray parameters favouring the break-up of the few agglomerates present in the suspension enhance the deposition efficiency (up to &gt;50%), as no particle or agglomerate larger than ~2.5 μm can be fully melted. Nano-sized powders, by contrast, generally form stronger agglomerates, which cannot be significantly disrupted by adjusting the spray parameters. If the chosen nanopowder forms small agglomerates (up to few microns), the deposition efficiency is satisfactory and the coating porosity is limited, although the lamellae generally have a wider size distribution, so that roughness is somewhat higher. If the nanopowder forms large agglomerates (on account of its surfacechemistry), poor deposition efficiencies and porous layers are obtained. Although suspensions containing the pure micron-sized powder produce the densest coatings, the highest deposition efficiency (~70%) is obtained by suitable mixtures of micron-and nano-sized powders, on account of synergistic effects

2010 - FIB assisted study of plasma sprayed splat–substrate interfaces: NiAl–stainless steel and alumina–NiAl combinations [Articolo su rivista]
Bolelli, Giovanni; Kazi, Sabiruddin; Lusvarghi, Luca; E., Gualtieri; Valeri, Sergio; P. P., Bandyopadhyay
abstract

This paper deals with the splat–substrate and intersplat interfaces of splats produced from Ni–5 wt.% Al and alumina based ceramic powders. The cross section of the splat–substrate interface was studied using a focused ion beam (FIB) assisted Field Emission Scanning Electron Microscope (FE-SEM). The metallic splat showed good interfacial integrity apart from a few interfacial pores resulting from entrapped gases and edge curl resulting from quenching stresses. The ceramics showed generally well adherent interfaces for both single and multilayer splats. Good adherence of both alumina splats was attributed to the improved wettability provided by the bond coat. One kind of alumina splats, namely, those obtained from agglomerated nanostructured powder was found to be free from crack and edge curl. This was attributed to the presence of a glassy phase in this splat, which can absorb quenching stresses.

2010 - Interesting aspects of indentation and scratch methods for characterization of thermally-sprayed coatings [Articolo su rivista]
Bonferroni, Benedetta; Bolelli, Giovanni; Lusvarghi, Luca; J., Nohava
abstract

In order to improve the knowledge on the use and significance of instrumented indentation and scratchtesting on thermally-sprayed materials, a wide range of tests was performed on thermally-sprayed ceramic,cermet and metal coatings. A scale-dependent behavior of hardness was observed as a function ofindentation depth for all coatings: at low penetration depths, the hardness value depends on theintralamellar material properties, whereas at larger depths it reflects the long-range cohesive strength of thecoating. In all cases, hardness becomes independent of the indentation depth above a threshold value of~2000 nm. The elastic modulus is also scale-dependent, but it never stabilizes to a depth-independent value,probably on account of crack opening/closing mechanisms. Scratch test on the cross-section has been deeplyinvestigated and identified as a comparative method to quantify the cohesion of the coatings.

2010 - Magnetic phase formation in CoTi-substituted Ba hexaferrite coatings prepared with atmospheric plasma spraying [Articolo su rivista]
D., Lisjak; Bolelli, Giovanni; Lusvarghi, Luca; M., Bégard; M., Bruehl; K., Bobzin; P., Lintunen; U., Kanerva; M., Pasquale; M., Drofenik
abstract

Thick coatings of CoTi-substituted Ba hexaferrite with thenominal composition BaCoTiFe10O19 were prepared using atmosphericplasma-spraying technology. The coatings were preparedfrom prereacted powders of the desired composition. Theas-deposited coatings showed a high degree of crystallinity. Adetailed investigation of the coatings’ phase compositions wasconducted with micro-Raman spectroscopy and energy-dispersiveX-ray spectroscopy. Two magnetic ferrite phases, CoTisubstitutedBa hexaferrite and Co ferrite, were identified in thecoatings. An X-ray powder-diffraction analysis and a quantitativestructural analysis based on Rietveld refinement showed thatthe mass ratio of the two phases and their chemical compositionvaried with respect to the spraying parameters. Consequently,the static and microwave magnetic properties of the as-sprayedcoatings also varied. We were able to explain the formationof the two ferrite phases based on the interplay between thepartial melting of the feedstock powder and the recrystallizationkinetics.

2010 - Microstructure and in-vitro behaviour of a novel High Velocity Suspension Flame Sprayed (HVSFS) bioactive glass coating [Articolo su rivista]
Bolelli, Giovanni; Cannillo, Valeria; R., Gadow; A., Killinger; Lusvarghi, Luca; Sola, Antonella; N., Stiegler
abstract

Bioactive glass coatings based on a novel SiO2-P2O5-CaO-K2O system (“Bio-K”) were deposited by the High-Velocity Suspension Flame Spraying (HVSFS) technique. Attrition-milled micrometric powder particles, dispersed in water+isopropanol, were sprayed onto Ti plates. The coatings remained entirely glassy, but the glass underwent some structural alterations during processing. After soaking in simulated body fluid (SBF), a quite uniform hydroxyapatite layer was developed on the coatings, which indicates they may have the potential to favour osseointegration of prosthetic implants.

2010 - Processing and characterisation of High-Velocity Suspension Flame Sprayed (HVSFS) bioactive glass coatings [Articolo su rivista]
Bolelli, Giovanni; Cannillo, Valeria; R., Gadow; A., Killinger; Lusvarghi, Luca; J., Rauch
abstract

The High-Velocity Suspension Flame Spraying (HVSFS) technique was employed in order to deposit bioactive glass coatings onto titanium substrates. Two different glass compositions were examined: the classical 45S5 Bioglass and a newly-developed SiO2–CaO–K2O–P2O5 glass, labelled as “Bio-K”. Suitable raw materials were melted in a furnace and fritted by casting into water. The frit was dry-milled in a porcelain jar and subsequently attrition-milled in isopropanol. The resulting micron- sized powders were dispersed in a water+isopropanol mixture, in order to prepare suitable suspensions for the HVSFS process. The deposition parameters were varied; however, all coatings were obtained by performing three consecutive torch cycles in front of the substrate. The thickness and porosity of the coatings were significantly affected by the chosen set of deposition parameters; however, in all cases, the layer produced during the third torch cycle was thicker and denser than the one produced during the first cycle. As the system temperature increases during the spraying process, the particles sprayed during the last torch cycle remain at T &gt; Tg while they spread, so that interlamellar viscous flow sintering takes place, favouring the formation of such denser microstructure. Both coatings are entirely glassy; however, micro-Raman spectroscopy reveals that, whereas the 45S5 coating is structurally identical to the corresponding bulk glass, the “Bio-K” coating is somewhat different from the bulk one.

2010 - Structural characterisation of High Velocity Suspension Flame Sprayed (HVSFS) TiO2 coatings [Articolo su rivista]
E., Bemporad; Bolelli, Giovanni; Cannillo, Valeria; D., De Felicis; R., Gadow; A., Killinger; Lusvarghi, Luca; J., Rauch; M., Sebastiani
abstract

The microstructural features of TiO2 coatings, deposited by High Velocity Suspension Flame Spraying(HVSFS) from a suspension of titania nanoparticles, were investigated by Focused Ion Beam (FIB)+ScanningElectron Microscopy (SEM) techniques, by Transmission Electron Microscopy (TEM) and by micro-Ramanspectroscopy, and were compared to those of conventional HVOF-sprayed TiO2. Proper selection of theHVSFS deposition parameters results in coatings consisting of a dense matrix, made up by the efficientsuperposition of well-flattened micrometric lamellae, with homogeneously distributed porosity containingsub-micrometric re-solidified spherical particles. Unlike conventional HVOF coatings, lamella boundaries arehardly discernible, no intralamellar cracking occurs and equiaxed crystals appear instead of columnar ones. Ahomogeneous distribution of anatase and rutile is also found. Modifications to the spray parameters can giverise to large, unmelted agglomerates, scattered throughout the coating and having poor cohesion to thesurrounding material. These agglomerates retain the original phase composition of the nanopowder.

2010 - Wear and Corrosion Behavior of HVOF-Sprayed WC-CoCr Coatings on Al Alloys [Articolo su rivista]
M., Barletta; Bolelli, Giovanni; Bonferroni, Benedetta; Lusvarghi, Luca
abstract

WC-CoCr coatings were HVOF-sprayed onto an AA6082T6 substrate. Thickness values between 50 and150 lm were produced by stepwise increase of the number of torch scans. This increase made thecoatings not only thicker but also denser. This was due both to peening effects and by modifications tothe splat formation mechanism, investigated by focused ion beam technique. Thanks to such densification,the hardness, the wear and impact resistance, and the corrosion protectiveness of the layersincreased with the number of torch scans. The largest improvement occurred from 2 to 3 torch scans.These coatings were also compared to anodized films: cermets had superior wear and impact resistancebut offered less corrosion protection.

2010 - Wear behaviour of high velocity suspension flame sprayed (HVSFS) Al2O3 coatings produced using micron- and nano-sized powder suspensions [Articolo su rivista]
Bolelli, Giovanni; Bonferroni, Benedetta; Cannillo, Valeria; R., Gadow; A., Killinger; Lusvarghi, Luca; J., Rauch; N., Stiegler
abstract

Three Al2O3-based suspensions were prepared using two nano-sized Al2O3 powders (having analogous size distribution and chemical composition but different agglomeration behaviour) and a micron-sized one. High velocity suspension flame sprayed (HVSFS) coatings were produced using these suspensions as feedstock and adopting two different sets of deposition parameters.The coatings produced by the micrometric powder suspension are denser and more wear resistant (as revealed by ball-on-disk testing) than those deposited using the two nanopowder suspensions, irrespective of the deposition parameters. The tribological behaviour of all coatings is dictated by the formation and subsequent removal of surface tribofilms: these tribofilms are more stable in the coatings obtained by the micrometric powder suspension, although they exhibit the largest tensile residual stress (as determined by X-ray diffraction and hole drilling techniques).Even though the as-deposited coatings are quite smooth (Ra around 1.3–2 μm), polishing to Ra around 0.1 μm further improves the wear resistance in all cases, as it delays the onset of tribofilm delamination phenomena.

2009 - Advances in High Velocity Suspension Flame Spraying (HVSFS) [Articolo su rivista]
J., Rauch; Bolelli, Giovanni; A., Killinger; R., Gadow; Cannillo, Valeria; Lusvarghi, Luca
abstract

Three suspensions, containing oxide nanoparticle single phases, (Al2O3, 3YSZ and Cr2O3) were prepared and sprayed using High Velocity Suspension Flame Spraying (HVSFS) technique. The coatings were characterized concerning their mechanical properties by means of nano indenter hardness measurements (all coatings) and ball on disk tribometry (Al2O3, 3YSZ). APS and HVOF sprayed Al2O3 coatings were characterized under same conditions for comparison. X-ray diffraction analysis was performed on HVSF sprayed Al2O3 and Cr2O3 coatings and a plasma-sprayed Cr2O3 coating for comparison. A Williamson–Hall line profile analysis was performed to estimate and compare crystallite size in the coatings

2009 - Deposition of TiO2 coatings: Comparison between High Velocity Suspension Flame Spraying (HVSFS), atmospheric plasma spraying and HVOF-spraying [Relazione in Atti di Convegno]
Bolelli, Giovanni; Cannillo, Valeria; Lusvarghi, Luca; R., Gadow; A., Killinger; J., Rauch; E., Bemporad; M., Sebastiani
abstract

A TiO2 (anatase) nanopowder suspension was processed by High Velocity Suspension Flame Spraying (HVSFS). The resulting coatings were characterized and compared to conventional atmospheric plasma sprayed (APS) and HVOFsprayed ones. The HVSFS operating parameters can be adjusted in order to achieve dense coatings, with a fine (nearnanosized) structure and a homogeneous distribution of anatase and rutile: these coatings have lower pore interconnectivity and higher wear resistance than APS and HVOF ones. Alternatively, quite large unmelted agglomerates of anatase nanoparticles can be embedded in the coating, increasing the porosity and the anatase content for enhanced photocatalytic efficiency.

2009 - EFFETTO DELLO SPESSORE SULLA RESISTENZA AD USURAE CORROSIONE DI RIVESTIMENTI WC-CoCr DEPOSITATI SU LEGA DI Al TRAMITE TERMOSPRUZZATURA HVOF [Articolo su rivista]
Bolelli, Giovanni; Gualtieri, Enrico; Lusvarghi, Luca; Valeri, Sergio
abstract

Si sono esaminati gli effetti dello spessore e della presenza/assenza di un sottile bond coat in Ni sulla resistenzaad usura e a corrosione di rivestimenti in materiale cermet WC-CoCr, depositati tramite termospruzzaturaHVOF su piastre in lega di alluminio AA 6082T6. Riporti con quattro diversi spessori, compresi nell’intervallo50 μm – 150 μm, sono stati prodotti eseguendo, rispettivamente, un totale di 2, 3, 4 e 5 scansioni consecutivedella torcia davanti al substrato. All’aumentare del numero di scansioni, la porosità del rivestimento cermet(sia con, sia senza bond coat) diminuisce; il cambiamento più sensibile si verifica fra i riporti depositati con2 e 3 passaggi della torcia. Per spiegare questo fenomeno, si sono analizzati i meccanismi di deposizione disingole lamelle di WC-CoCr, esaminandole con tecnica Focused Ion Beam (FIB). All’aumento di densità delrivestimento corrispondono un incremento di microdurezza, un miglioramento della resistenza all’usura perstrisciamento (misurata mediante test ball-on-disk) ed una maggior protezione del substrato contro la corrosione(valutata mediante spettroscopia di impedenza elettrochimica e monitoraggio del potenziale di libera corrosione).Rispetto a strati anodizzati, prodotti sulle stesse piastre, tutti riporti WC-CoCr (indipendentemente dallospessore) sono molto più resistenti ad usura ma meno protettivi contro la corrosione del substrato.

2009 - Failure Mechanism for Thermal Fatigueof Thermal Barrier Coating Systems [Relazione in Atti di Convegno]
C., Giolli; A., Scrivani; G., Rizzi; F., Borgioli; Bolelli, Giovanni; Lusvarghi, Luca
abstract

Thick thermal barrier coatings (TBCs), consisting of a CoNiCrAlY bond coat and yttria-partiallystabilized zirconia top coat with different porosity values, were produced by air plasma spray (APS). Thethermal fatigue resistance limit of the TBCs was tested by furnace cycling tests (FCT) according to thespecifications of an original equipment manufacturer (OEM). The morphology, residual stresses, andmicromechanical properties (microhardness, indentation fracture toughness) of the TBC systems beforeand after FCT were analyzed. The thermal fatigue resistance increases with the amount of porosity in thetop coat. The compressive in-plane stresses increase in the TBC systems after thermal cycling; neverthelessthe increasing rate has a trend contrary to the porosity level of top coat. The data suggest that thespallation happens at the TGO/top coat interface. The failure mechanism of thick TBCs was found to besimilar to that of conventional thin TBC systems made by APS.

2009 - Fatigue behaviour of 2011-T6 aluminium alloy coated with PVD WC/C, PA-CVD DLC and PE-CVD SiOx coatings [Articolo su rivista]
S., Baragetti; Lusvarghi, Luca; Bolelli, Giovanni; F., Tordini
abstract

In this paper, the effect of three different thin hard coatings on the fatigue behaviour of 2011-T6 aluminiumalloy was investigated. The coatings were DLC, SiOx and WC/C deposited with CVD and PVD processes. Thefatigue limit at 10000000 load cycles was evaluated for both coated and uncoated specimens in a rotating bending machine (R=−1). The DLC and SiOx films did not significantly enhance the fatigue limit of the studied alloy, whereas a slight beneficial effect was produced by the WC/C film. Subsurface fatigue cracks nucleated in the WC/C-coated samples, as revealed by SEM micrographs of the fracture surfaces. It was therefore argued that the enhancement in the fatigue resistance of the WC/C-coated specimens could be due to residualcompressive stress distributions introduced in the surface layers of the base material by the depositionprocess. The interface delamination was investigated with scratch tests and WC/C showed the best adhesionto the aluminium alloy.

2009 - HVOF-sprayedWC–CoCr coatings on Al alloy: Effect of the coating thickness on the tribological properties [Articolo su rivista]
Bolelli, Giovanni; Lusvarghi, Luca; M., Barletta
abstract

The microstructure, the micromechanical properties, the wear behaviour and the impact resistance ofWC–CoCr cermet coatings, deposited onto an aluminium alloy substrate by High Velocity Oxygen-Fuel(HVOF) flame-spraying,were examined as a function of the coating thickness, whichwas varied between50mand 150mby performing different numbers of scans of the HVOF torch in front of the substrate.The coatings became denser and significantly harder as the number of torch scans increased: the analysisof singleWC–CoCr splats by combined SEM and Focused Ion Beam (FIB) techniques enabled the interpretationof the mechanisms underlying this phenomenon. In accordance to such densification, the slidingwear resistance increased with the number of torch scans, as abrasive grooving and brittle failure mechanisms were progressively suppressed. The resistance to cyclic impact was also enhanced. In comparison to anodised films, theWC–CoCr coatings appeared much more resistant against wear and cyclic impact; specifically, three torch scans seem enough to produce a coating having suitable characteristics.

2009 - Microstructural and Tribological Investigation of High-Velocity Suspension Flame Sprayed (HVSFS) Al2O3 Coatings [Articolo su rivista]
Bolelli, Giovanni; Johannes, Rauch; Cannillo, Valeria; Andreas, Killinger; Lusvarghi, Luca; Rainer, Gadow
abstract

Al2O3 coatings were manufactured by the high-velocity suspension flame spraying (HVSFS) technique using a nanopowder suspension. Their structural and microstructural characteristics, micromechanical behavior, and tribological properties were studied and compared to conventional atmospheric plasma sprayed and high-velocity oxygen-fuel-sprayed Al2O3 coatings manufactured using commercially avail- able feedstock. The HVSFS process enables near full melting of the nanopowder particles, resulting in very small and well flattened lamellae (thickness range 100 nm to 1 lm), almost free of transverse microcracking, with very few unmelted inclusions. Thus, porosity is much lower and pores are smaller than in conventional coatings. Moreover, few interlamellar or intralamellar cracks exist, resulting in reduced pore interconnectivity (evaluated by electrochemical impedance spectroscopy). Such strong interlamellar cohesion favors much better dry sliding wear resistance at room temperature and at 400 °C.

2009 - Microstructural and in vitro characterisation of High-Velocity Suspension Flame Sprayed (HVSFS) bioactive glass coating [Articolo su rivista]
Bolelli, Giovanni; Cannillo, Valeria; R., Gadow; A., Killinger; Lusvarghi, Luca; J., Rauch
abstract

The paper reports the first attempt at employing the innovative High-Velocity Suspension Flame Spraying (HVSFS) technique in order to deposit bioactive glass coatings. Fine (micrometric) glass particles having a composition similar to that of the A-W (apatite-wollastonite) bioactive glass-ceramic as proposed by Kokubo were dispersed into a 50% water + 50% isopropanol solvent mixture and the resulting suspension (containing 20 wt.% glass powder) was thermally sprayed onto Ti plates using a modified high velocity oxy-fuel torch.Each torch pass produces a dense coating layer, featuring strong cohesion between lamellae thanks to viscous flow sintering along the interlamellar boundary. However, some porosity exists between different layers deposited during successive torch passes.In-vitro bioactivity tests indicate that the coatings interact remarkably with the simulated body fluid (SBF), developing a thick silica-rich layer containing hydroxyapatite crystals.

2009 - Nanoceramic based High-Velocity Suspension Flame Sprayed (HVSFS) of hydroxylapatite coatings for biomedical applications [Capitolo/Saggio]
Bolelli, Giovanni; Cannillo, Valeria; R., Gadow; A., Killinger; Lusvarghi, Luca; N., Stiegler
abstract

2009 - Preparation of barium hexaferrite coatings using atmospheric plasma spraying [Articolo su rivista]
Darja, Lisjak; Kirsten, Bobzin; Katharina, Richardt; Marion, Bégard; Bolelli, Giovanni; Lusvarghi, Luca; Arto, Hujanen; Pertti, Lintunen; Massimo, Pasquale; Elena, Olivetti; Miha, Drofenik; Thomas, Schläfer
abstract

Thick coatings of barium hexaferrite with the compositions BaFe12O19 and BaCoTiFe10O19 were prepared using atmospheric plasma spraying (APS)technology. The coatings were prepared from pre-reacted powders of the desired composition. The as-deposited coatings were poorly crystallized,but their crystallinity was improved with a subsequent annealing. The crystallization mechanism of the sprayed hexaferrites was studied duringannealing up to 1300 ◦C, using X-ray powder diffraction combined with thermal analysis and with electron microscopy including microanalysis.Single-phase coatings were obtained after annealing treatments at 1100–1300°C. Their magnetic properties showed that they would be suitablefor absorbers at microwave and mm-wave frequencies, depending on the coating phase’s composition, the crystallinity and the thicknesses.

2009 - Processing and characterisation of High-Velocity Suspension Flame Sprayed (HVSFS) bioactive glass coatings [Capitolo/Saggio]
Bolelli, Giovanni; Cannillo, Valeria; R., Gadow; A., Killinger; Lusvarghi, Luca; J., Rauch; Sola, Antonella
abstract

The High-Velocity Suspension Flame Spraying (HVSFS) technique was employed in order to deposit bioactive glass coatings onto titanium substrates. Two different glass compositions were examined: the classical 45S5 Bioglass and a newly-developed SiO 2 –CaO–K 2 O–P 2 O 5 glass, labelled as "Bio-K". Suitable raw materials were melted in a furnace and fritted by casting into water. The frit was dry-milled in a porcelain jar and subsequently attrition-milled in isopropanol. The resulting micron-sized powders were dispersed in a water+isopropanol mixture, in order to prepare suitable suspensions for the HVSFS process. The deposition parameters were varied; however, all coatings were obtained by performing three consecutive torch cycles in front of the substrate. The thickness and porosity of the coatings were significantly affected by the chosen set of deposition parameters; however, in all cases, the layer produced during the third torch cycle was thicker and denser than the one produced during the first cycle. As the system temperature increases during the spraying process, the particles sprayed during the last torch cycle remain at T > Tg while they spread, so that interlamellar viscous flow sintering takes place, favouring the formation of such denser microstructure. Both coatings are entirely glassy; however, micro-Raman spectroscopy reveals that, whereas the 45S5 coating is structurally identical to the corresponding bulk glass, the "Bio-K" coating is somewhat different from the bulk one.

2009 - Properties of High Velocity Suspension Flame Sprayed (HVSFS) TiO2 coatings [Articolo su rivista]
Bolelli, Giovanni; Cannillo, Valeria; R., Gadow; A., Killinger; Lusvarghi, Luca; J., Rauch
abstract

TiO2 coatings were manufactured by the High Velocity Suspension Flame Spraying (HVSFS) technique using a nanopowder suspension. Their microstructure, nanohardness, tribological properties and photocatalytic activity were studied and compared to conventional atmospheric plasma sprayed (APS) and HVOF-sprayed TiO2 coatings manufactured using commercially available feedstock. The HVSFS process leaves a fairly large freedom to adjust coating properties (thickness, porosity, anatase content, hardness, etc…) according to the desired objective. Layers with higher anatase content and higher porosity can be produced to achieve higher photocatalytic efficiency, better than conventional APS and HVOF TiO2. Alternatively, dense protective layers can be deposited, possessing lower porosity and pore interconnectivity and better wear resistance than as- deposited APS and HVOF layers. In all cases, HVSFS-deposited layers are thinner (20 µm–60 µm) than those which can be obtained by conventional spraying processes

2009 - Replacement of hard chromium plating by thermal spraying - Problems, solutions and possible future approaches [Articolo su rivista]
Bolelli, G.
abstract

2009 - The Preparation of Barium Hexaferrite Coatings Using HVOF [Articolo su rivista]
D., Lisjak; D., Makovec; S., Gyrgyek; A., Hujanen; P., Lintunen; T., Varis; Bolelli, Giovanni; Lusvarghi, Luca; M., Drofenik
abstract

Thick coatings of barium hexaferrite with the compositions BaFe12O19 and BaCoTiFe10O19 were prepared using high velocity oxygen-fuel (HVOF) spraying technology. Nanocrystalline precursors embedded in an amorphous matrix were obtained on both Fe and glass–ceramic substrates. To promote the crystallization of the hexaferrites, the coatings were annealed at 800°–1000°C, and single-phase coatings were obtained at 1000°C. The crystallization process was studied with X-ray powder diffraction and with electron microscopy. The magnetic measurementsof the coatings were carried out in a static field and athigh frequencies. The magnetization of the coatings increased with the annealing temperature to above 50 emu/g for both compositions.The coercivity of BaFe12O19 increased with the annealing temperature to above 2400 Oe, whereas the coercivity of BaCoTiFe10O19 decreased from over 800 Oe, for the as-deposited sample, to 400 Oe for the sample annealed at 1000°C. A minimum 90% absorption was calculated for the BaFe12O19 coatings with thicknesses of 0.15–0.25 mm at around 47 GHz and for the 1–4-mm-thick coatings of BaCoTiFe10O19 at 3–9 GHz.

2009 - Thermal spraying of Co,Ti-substituted Ba-hexaferrite coatings for electromagnetic wave absorption applications [Articolo su rivista]
M., Bégard; K., Bobzin; Bolelli, Giovanni; A., Hujanen; P., Lintunen; D., Lisjak; S., Gyergyek; Lusvarghi, Luca; M., Pasquale; K., Richardt; T., Schläfer; T., Varis
abstract

Co,Ti-substituted Ba-hexaferrite (BaCoTiFe10O19) coatings for applications as microwave absorbers weredeposited by the APS and HVOF techniques, using BaCoTiFe10O19 powders manufactured by solid-statereaction followed by spray-drying. The SEM+EDX, XRD and micro-Raman analyses indicated that impactquenching of molten agglomerates on the substrate hindered the crystallisation of the hexaferrite structure,resulting in poor magnetic properties of the coatings. Adjusted processing conditions enabled the depositionof a coating retaining enough hexaferrite phase, whose magnetic properties, close to bulk BaCoTiFe10O19, arevery promising for electromagnetic wave absorption.

2009 - Thermally Sprayed Coatings as Interlayersfor DLC-Based Thin Films [Articolo su rivista]
Bolelli, Giovanni; Gualtieri, Enrico; Lusvarghi, Luca; F., Pighetti Mantini; F., Pitacco; Valeri, Sergio; H., Volz
abstract

This article examines the usefulness of a thick thermally sprayed interlayer (plasma-sprayed Ni-50%Cr,plasma-sprayed Al2O3-13%TiO2, or high-velocity oxygen-fuel-sprayed WC-17%Co) for enhancing thewear resistance and the corrosion protectiveness of a diamond-like carbon (DLC)-based thin filmdeposited onto a carbon steel substrate. Scratch tests indicate that the Al2O3-13%TiO2 and WC-17%Cointerlayers definitely increase the critical spallation load of the thin film, but the Al2O3-13%TiO2interlayer itself undergoes brittle fracture under high-contact loads. Accordingly, during ball-on-disktests at room temperature, no cracking and spallation occur in the DLC-based film deposited onto theWC-17%Co interlayer, whereas the one onto the Al2O3-13%TiO2 interlayer is rapidly removed becausethe interlayer itself is fractured. At 300°C, by contrast, the DLC-based film on the Al2O3-13%TiO2interlayer offers the best tribological performance, possibly thanks to the increased toughness of theceramic interlayer at this temperature. Electrochemical polarization tests indicate that the thin film/WC-Co systems possess the lowest corrosion current density.

2009 - Thermally-sprayed BaCoTiFe10O19 layers as microwave absorbers [Relazione in Atti di Convegno]
Bolelli, G.; Lusvarghi, L.; Lisjak, D.; Hujanen, A.; Lintunen, P.; Kanerva, U.; Varis, T.; Bégard, M.; Richardt, K.; Schläfer, T.; Bobzin, K.; Pasquale, M.
abstract

Microwave-absorbing layers, consisting of Co, Ti-substituted Ba-hexaferrite (BaCoTiFe10O19), were prepared by the APS and HVOF processes, using BaCoTiFe10O19 powders manufactured by solid-state reaction at 1100 °C followed by spray-drying. The SEM+EDX, XRD and micro-Raman analyses indicated that the melting and quenching of the agglomerates during spraying hindered the crystallisation of the hexaferrite structure, resulting in coatings with poor magnetic properties. Adjusted processing conditions, allowing the retention of a controlled amount of unmelted material, enabled the deposition of a coating having magnetic properties close to bulk BaCoTiFe10O19, suitable for electromagnetic wave absorption. Copyright © 2009 ASM International® All rights reserved.

2009 - Wear and corrosion behaviour of HVOF-sprayed WC-CoCr coatings on Al alloys [Relazione in Atti di Convegno]
Bolelli, G.; Lusvarghi, L.; Barletta, M.; Karhail, V.
abstract

WC-CoCr coatings were HVOF-sprayed onto an AA6082T6 substrate. Thickness values between 50 μn and 150 μn were produced by stepwise increase of the number of torch scans. This increase makes the coatings not only thicker, but also denser. This is due both to peening effects and by modifications to the splat formation mechanism, investigated by FIB. Thanks to such densification, the hardness, the wear and impact resistance and the corrosion protectiveness of the layers increase with the number of torch scans. The largest improvement occurs from 2 to 3 torch scans. These coatings were also compared to anodized films: cermets have superior wear and impact resistance but offer less corrosion protection. Copyright © 2009 ASM International® All rights reserved.

2008 - A FIB study of sharp indentation testing on plasma-sprayed TiO2 [Articolo su rivista]
Bolelli, Giovanni; Cannillo, Valeria; Lusvarghi, Luca; F., PIGHETTI MANTINI; Gualtieri, Enrico; C., Menozzi
abstract

Instrumented sharp indentation is often employed formicromechanical investigation of thermal-spray coatings.However, rather complex dependenciesof measured values on indentation load have been reported. To verify the possible influence of sub-surface cracking,Vickers and Berkovich indentations onplasma-spray TiO2 were sectioned by FIB. Remarkable sub-surface cracking was found for indentations performed on the coating cross-section, even forrather low loads (1 N).Much less sub-surface cracking occurs when indenting the top surface, indicatingmaterial anisotropy. This preliminary investigationhighlights the need for thorough studies of sub-surface cracking during instrumented sharp indentation.

2008 - A comparison between corrosion resistances of some HVOF-sprayed metal alloy coatings [Articolo su rivista]
Bolelli, G.; Lusvarghi, Luca; Giovanardi, Roberto
abstract

This study compares the corrosion resistance of one Co–based alloy coating, namely Co–28Mo–17Cr–3Si (similar to Tribaloy-800), four Ni-based alloy coatings, namely Ni–17Cr–4Fe–4Si–3.5B–1C (Diamalloy-2001), Ni–20Cr–10W–9Mo–4Cu–1C–1B–1Fe (Diamalloy-4006), Ni–22Cr–9Mo–4Nb–5Fe (similar to Inconel-625), Ni–32Mo–16Cr–3Si–2Co (similar to Tribaloy-700), and a (WC-12Co)–33Ni–9Cr–3.5Fe–2Si–2B–0.5C cermet–Ni alloy blend coating. They were produced by liquid-fuelled HVOF spraying onto AISI1040 steel plates. Electrolytic hard chrome (EHC) plating was characterised as a reference material, to verify whether some HVOF coatings are suitable as an EHC replacement. The microstructure of the coatings was examined by SEM and XRD. Electrochemical polarization tests and free corrosion tests were performed in 0.1 M HCl aqueous solution; the corrodkote test (ASTM B380-97R02) was also performed, to rank coatings qualitatively.The lowest corrosion current densities (Icorr) were recorded for EHC and Tribaloy-700. The latter coating contained few secondary phases and little porosity; the damage was mainly due to corrosion activation along lamellae boundaries. Diamalloy-2001 exhibited the highest Icorr and was significantly damaged after the polarization test, as its multi-phase microstructure had triggered severe galvanic corrosion. During free corrosion in 0.1 M HCl, Tribaloy-700 and Diamalloy-4006 retained rather stable polarization resistance (Rp), whereas the Rp of EHC decreased significantly. Tribaloy-700 survived 40 h of corrodkote test with no apparent damage and EHC underwent limited pitting corrosion. All other coatings had visible corrosion. The Inconel-625 coating failed to protect the substrate after 20 h of testing, due to inadequate processing conditions.

2008 - Electrochemical comparison between corrosion resistance of some thermally sprayed coatings [Articolo su rivista]
Bolelli, Giovanni; Cannillo, Valeria; Giovanardi, Roberto; Lusvarghi, Luca
abstract

Electrochemical polarisation and impedance spectroscopy tests in0.1 M HCl and 0.1 M H2SO4 solutions were performed on HVOF-sprayedcermet coatings (WC–17% Co, WC–10% Co–4% Cr) and AtmosphericPlasma-Sprayed (APS) ceramics (Al2O3, Al2O3–13 wt%TiO2, Cr2O3) with APSbond coat (Ni–Co–Cr–Al–Y). Reference tests were also performed on hardchrome electroplating. Plasma-sprayed coatings offer limited substrateprotection owing to interconnected porosity, although the bond coat promotessome forms of passivation in H2SO4. HVOF-sprayed cermets possess goodcorrosion resistance in both environments. Chrome electroplating showscomparatively better corrosion resistance in H2SO4, but worse in HCl.

2008 - Enhanced tribological properties of PECVD DLC coated thermally sprayed coatings [Articolo su rivista]
Bolelli, G.; Lusvarghi, Luca; PIGHETTI MANTINI, F.; Pitacco, F.; Volz, H.
abstract

This research investigates the enhancement of the tribological properties of various thermally-sprayed coatings (APS Ni–50Cr, APS Al2O3–13%TiO2 and HVOF WC–17Co) on steel substrate, achieved through the deposition of a thin DLC-based film. Higher adhesive strength betweenthin films and thermally-sprayed coatings compared to the simple thin film/carbon steel system was found by scratch testing. Dry sliding ball-ondisktests performed under lower contact pressure conditions (5 N normal load, 6 mm diameter alumina ball) indicated a significant decrease inwear rates and friction coefficients of thermally-sprayed coatings when the thin DLC-based film is employed; little differences exist between thetribological behaviour of the various thin film/thermal spray coating systems and that of DLC-based film on carbon steel. Under higher contactpressure conditions (10 N normal load, 3 mm diameter alumina ball), the thin film/WC–Co system exhibited the best wear performance. Theseresults indicate the superior tribological performance of DLC/thermal spray coating systems, especially under severe contact conditions.

2008 - Fast Regime - Fluidized Bed Machining (FR-FBM) of thermally sprayed coatings [Articolo su rivista]
M., Barletta; G., Rubino; Bolelli, Giovanni; Lusvarghi, Luca
abstract

Finishing of thermally sprayed metallic, ceramic, and cermet coatings is required to meet tolerances andrequirements on surface roughness in most industrial applications. Conventional machining is a costlyand time-consuming process, and is difficult to automate. Therefore, this study investigates and developsa new technique highly amenable for automation: fast regime—fluidized bed machining (FR-FBM).Atmospheric plasma sprayed TiO2, Cr2O3, and HVOF-sprayed WC-17%Co and Tribaloy-800 coatings,deposited on AISI 1040 steel substrates, were subjected to FR-FBM treatment. The effects of the leadingoperational parameters, namely, abrasive size, jet pressure, and processing time, were evaluated on allcoatings by using a two/three-levels full factorial design of experiments. The FR-FBM treated surfaceswere observed by FE-SEM and their surface finishing was evaluated by contact profilometry. Significantimprovements in surface finishing of all the machined thermally sprayed coatings can always be detected,with FR-FBM being able to guarantee the precision and to ensure the closest geometrical tolerances.

2008 - Fast regime – Fluidized Bed Machining (FR-FBM) of Atmospheric Plasma Spraying (APS) TiO2 coatings [Articolo su rivista]
M., Barletta; G., Rubino; S., Guarino; Bolelli, Giovanni; Lusvarghi, Luca; A., Gisario
abstract

This paper deals with the development and application of a new technique, Fast Regime-Fluidized BedMachining (FR-FBM), for the easy-to-automate finishing of as-deposited Atmospheric Plasma Spraying (APS)TiO2 coatings on AISI 1040 steel substrates. The effects of leading FBM operational parameters, namely,abrasive size, jet pressure and processing time, were evaluated by using a full factorial Design OfExperiments. Machinability of APS APS-TiO2 coatings was found to be highly dependent on jet pressure andon abrasive mesh size as they affect the contact conditions between the abrasive and machined surface as well as the finishing force. By modifying these conditions, FR-FBM allows 3 to 4-fold improvements inroughness parameters of as-deposited APS APS-TiO2 coatings and imparts minimal defects or alterations tothe machined surface, whilst maintaining its ability to guarantee the respect of the closest geometricaltolerances.

2008 - HVOF Sprayed WC-Co as Hard Interlayer for DLC Films [Articolo su rivista]
Bolelli, Giovanni; Lusvarghi, Luca; M., Montecchi; F., PIGHETTI MANTINI; F., Pitacco; H., Volz
abstract

This study evaluates the technical improvement achieved by applying a thick HVOF-sprayed WC-17Cointerlayer between a DLC thin film and an AISI1040 steel substrate. The chemical composition (from XPSanalysis) and hardness (from nano-indentation test) of the DLC films deposited on both bare and HVOFcoatedAISI1040 steel are analogous. Nevertheless, the better adhesion of the DLC film onWC-17Co interlayeris demonstrated by higher critical loads recorded in scratch testing. During ball-on-disk dry slidingtribological tests, the low hardness of the AISI1040 substrate causes delamination phenomena in the DLCfilm. When contact pressure is low, delamination is limited, both at room temperature and at 400 °C; instead, delamination under high contact pressure is severe at room temperature and complete at 300 °C. The hard WC-17Co interlayer significantly improves the tribological performance, although delamination eventually occurs, after long sliding distances, under high contact pressure at 300 °C.

2008 - Heat treatment effects on the corrosion resistance of some HVOF-sprayed metal alloy coatings [Articolo su rivista]
Bolelli, Giovanni; Lusvarghi, Luca; M., Barletta
abstract

The present study evaluates the effects of a 600 °C, 1 h heat treatment on the corrosion resistance of threeHigh Velocity Oxygen Fuel (HVOF) flame-sprayed alloy coatings: a Co–28Mo–17Cr–3Si (similar to Tribaloy-800) coating, a Ni–20Cr–10W–9Mo–4Cu–1C–1B–1Fe (Diamalloy-4006) coating and a Ni–32Mo–16Cr–3Si–2Co (similar to Tribaloy-700) coating. Electrochemical polarization tests and free corrosion tests wereperformed in 0.1 M HCl aqueous solution. The corrodkote test (ASTM B380-97R02) was also performed, toevaluate the coatings qualitatively. The heat treatment improves the corrosion resistance of the Co–28Mo–17Cr–3Si coating and of the Ni–20Cr–10W–9Mo–4Cu–1C–1B–1Fe coating by enhancing their passivationability. The precipitation of sub-micron sized secondary phases after the treatment may produce galvanicmicrocells at intralamellar scale, but the beneficial contribution provided by the healing of the very small but dangerous interlamellar defects (normally present in thermal spray coatings but not detectable usingordinary scanning electron microscopy) prevails. The effect on Ni–32Mo–16Cr–3Si–2Co coatings is moreambiguous: its sensitivity to crevice corrosion is worsened by the heat treatment.

2008 - Investigation of High-Velocity Suspension Flame Sprayed (HVSFS) glass coatings [Articolo su rivista]
Bolelli, Giovanni; J., Rauch; Cannillo, Valeria; A., Killinger; Lusvarghi, Luca; R., Gadow
abstract

High-Velocity Suspension Flame Spraying (HVSFS) has recently emerged as a potential alternative to conventional HVOF-spraying:employing liquid suspensions instead of dry powder feedstock enables the use of very fine grain-sized particles, resulting in small-sized lamellae.Thin, low-porosity coatings can thus be manufactured. This paper details the first attempt at manufacturing glass coatings using the HVSFStechnique: these coatings can have multiple applications (anti-corrosion coatings on metal and ceramic substrates, bio-compatible coatings, etc). ACaO–ZrO2–SiO2 glass frit was selected for this attempt. Excellent potentialities emerged (very low porosity), but some problems still existed (big“droplet-like” features on the coating surface), which have recently been largely overcome thanks to process modifications.© 2008 Elsevier B.V. All rights reserved.

2008 - Mechanical strength and wear resistance of protective coatings applied by Fluidized Bed (FB) [Articolo su rivista]
Barletta, M.; Bolelli, G.; Gisario, A.; Lusvarghi, Luca
abstract

This study deals with the interrelation between the thermo-rheological behaviour of an epoxy-based powder coating system and its mechanicalstrength and wear endurance.Matte-finish protective polymeric films deposited by electrostatic fluidized bed (EFB) and conventional hot dipping fluidized bed (CHDFB)on metal substrates were examined. First, the analysis of thermo-rheological behaviour of the epoxy-based powder coating system was detailed.Secondly, the adhesion strength and wear endurance of polymeric films was related to the thermo-rheological behaviour of the starting materialformulation. Finally, based on the experimental data, generalized scratch and wear map, in which the overall mechanical performance of thematte-finish polymeric films at different curing levels is reported, was usefully provided.The experimental findings lead to further advances in the understanding of the mechanisms involved in the establishment of the overall mechanicalperformances of fluidized bed (FB) deposited polymeric films. They also provide important indications for the settings of curing parameters orpreheating temperatures in FB coating processes as well as for the development of new powder coating formulations.

2008 - On the combined use of scratch tests and CLA profilometry for the characterization of polyester powder coatings: influence of scratch load and speed [Articolo su rivista]
M., Barletta; A., Gisario; Lusvarghi, Luca; Bolelli, Giovanni; G., Rubino
abstract

The prediction of the mechanical properties of polyester powder coatings is of paramount importance, asthey have to undergo a wide variety of forces and deformations during their service life. Determine theresponse of polymeric coatings to the actual loading conditions can be, however, very troublesome astheir properties are function of the material physical state, rate-dependence and yield and breakbehaviour. Further, the characterization of soft, contaminated organic surfaces such as polymers canoften cause severe problems with reliability and imaging accuracy due to instrumental artefacts.This is therefore the context in which this paper investigates the capability of multiple parametersscratch tests joined with non-contact CLA profilometry and FE-SEM to detect the scratch response ofpolyester powder coatings for protective and decorative outdoor applications. Scratch tests with bluntcontact geometry were used to evaluate the response of polyester coatings baked at different time–temperature programs and, so, at different stages of their curing process. In particular, being suchcoatings highly non-linear in their response tomechanical stress or strain, the influence from scratch load and speed was carefully taken into account. Analytical evaluations of the experimental results led to goodcorrelations between the extent of the deformed zone after scratch, scratch parameters and curingoperational settings. This allows mapping the scratch response of the polyester topcoats to broad rangesof both scratch parameters and curing conditions.

2008 - Residual stresses in HVOF sprayed ceramic coatings [Articolo su rivista]
Bolelli, Giovanni; Lusvarghi, Luca; M., Barletta; E., Turunen; T., Varis; C. L., AZANZA RICARDO; M., Leoni; P., Scardi
abstract

In this paper, the residual stress state of thermally sprayed ceramic coatings was examined by combiningdifferent experimental and analytical techniques, in order to provide a thorough characterisation of throughthickness stress profiles and a cross-verification of results. HVOF-sprayed ceramics, manufactured usingcommercial and nanostructured Al2O3 powders and commercial Cr2O3 powders, and atmospheric plasma sprayed (APS) ceramics, manufactured using commercial Al2O3 and Cr2O3 powders, were investigated. The near-surface stress was measured by X-ray diffraction. The through-thickness profile and the intrinsic quenching stresswere analytically computed by the Tsui–Clyne iterative model, using the X-ray measurement result as input, and results were validated by the substrate chemical removal method. Further verification was achieved by applying the in-situ curvature technique to the deposition of HVOF-sprayed Al2O3 coating. HVOF-sprayed Al2O3 coatings deposited using both conventional and nanostructured powders feature a similar, almost equibiaxial tensile stress on the top surface (116.5 MPa and 136.5 MPa, respectively) and amoderate through-thickness gradient (about 12 MPa and 20 MPa, respectively). Their intrinsic quenchingstresses were analytically estimated to be 184 MPa and 205 MPa, respectively. APS Al2O3 possesses higher topsurface stress (220 MPa) and quenching stress (311 MPa). However, it shows a less pronounced stress gradient (≈3 MPa) than HVOF-sprayed Al2O3-based coatings, because cracks, pores and weak lamella boundaries in the APS coating can accommodate the deformations induced by the bending moments arising both during coating deposition and during cooling.The model-derived quenching stress of the conventional HVOF Al2O3 coating was validated by the in-situ curvature measurement technique.Cr2O3-based coatings are significantly different. They display a lower residual stress in the near-surface region: 20 MPa in the APS coating, 27.5 MPa in the HVOF one. The HVOF coating also exhibits a very large stress gradient of ≈77 MPa. Machining and sliding processes (like polishing and dry sliding tribological testing) change their surface residual stresses to compressive ones.

2007 - BAS, CMAS and CZAS glass coatings deposited by plasma spraying [Articolo su rivista]
Bolelli, Giovanni; Lusvarghi, Luca; Manfredini, Tiziano; E., Parsini; Siligardi, Cristina
abstract

In this study, three different industrial frits BaO–Al2O3–SiO2 (BAS), CaO–MgO–Al2O3–SiO2 (CMAS), CaO–ZrO2–Al2O3–SiO2 (CZAS) havebeen deposited on porcelainized stoneware tiles by plasma spraying. In the as-sprayed conditions, the microstructure of the coatings is defectivebecause of pores, microcracks and low intersplat cohesion. Hot stage microscope and differential thermal analysis measurements made on the glasspowders allowed to characterize the frits thermal behaviour. Post process thermal treatments have been arranged, following these indications as wellas preliminary tests, in order to achieve the lowest porosity and the highest resistance to abrasion. At the chosen temperatures, a microstructuralimprovement has been induced, but in the BAS specimens, an optimal sintering has not been accomplished because of the unavoidable fulloverlapping of the sintering and crystallization processes.

2007 - Comparison between Plasma- and HVOF-sprayed ceramic coatings. Part 2: tribological behaviour [Articolo su rivista]
Bolelli, Giovanni; Lusvarghi, Luca; T., Manfredini; E., Turunen; T., Varis; S. P., Hannula
abstract

This is the second part of comparative study between High Velocity Oxygen-Fuel (HVOF) flame-sprayed and Atmospheric Plasma-Sprayed (APS) ceramics. Dry particles abrasion test and dry sliding wear test at room temperature, 400°C and 700°C are performed. In dry sliding against SiC, at room temperature stable tribofilms are formed and mild wear (<10–6 mm3/(Nm)) occurs for all coatings. When temperature and normal load increase, making brittle cracking a significant wear mechanism, HVOF coatings become superior to APS ones, thanks to higher toughness. In dry particles abrasion, brittle fracture prevails; therefore, the tougher HVOF coatings outperform APS ones.

2007 - Comparison between plasma- and HVOF-sprayed ceramic coatings. Part I: microstructure and mechanical properties [Articolo su rivista]
Bolelli, Giovanni; Lusvarghi, Luca; Manfredini, Tiziano; F., Pighetti Mantini; R., Polini; E., Turunen; T., Varis; S. P., Hannulla
abstract

Few papers deal with High Velocity Oxygen-Fuel (HVOF)sprayed ceramics. This two-part study thoroughly compares HVOFsprayed Al2O3, nanostructured Al2O3, Cr2O3 to Atmospheric Plasma Sprayed(APS) ones. The first part discusses microstructure and micromechanics.HVOF-sprayed ceramics achieve superior cohesion (lower porosity and lower average pore area). Size effects in Vickers microindentation tests are differentfor HVOF and APS ceramics. At 1N load, hardness is higher for HVOFcoatings (no cracking). Under 5 N and 10 N loads, hardness decreasesgradually for HVOF coatings; APS coatings are largely cracked at 5 N.HVOF-sprayed ceramics are tougher than APS ones and have higher elasticmodulus.

2007 - Development of matte finishes in electrostatic (EFB) and conventional hot dipping (CHDFB) fluidized bed coating process [Articolo su rivista]
M., Barletta; Bolelli, Giovanni; S., Guarino; Lusvarghi, Luca
abstract

This study focuses on the correlation between the thermo-rheological properties of a thermosetting powder coating system with its surfacestructure build-up. Epoxy powder coating systems, which displayed surface matting and surface wrinkling, were examined. Firstly, the evolutionof the complex viscosity was correlated with the cure kinetic. Secondly, the structure build-up on the surface of the coatings was investigatedwith a combined SEM-CLA profilometry analysis at different stages of curing process for both EFB and CHDFB coating processes. Differentfinishes were found to characterize the films applied by using EFB and CHDFB coating processes as a result of the different way the film is heatedby. Finally, a strict relationship of film morphology to the degree of conversion and to the evolution of the complex viscosity was found out forboth EFB and CHDFB coating processes. The surface structure is built up after gelation point and continues to evolve after gelation with a fulldevelopment of the film fine structure. Differences were observed in the surface structure build-up when different curing temperature was used,thereby indicating an influence of minimum viscosity on achievable finishing.These experimental results lead to further advances in a better understanding of the formation of surface topography and morphology of polymericfilms. They also provide important indications for the settings of curing parameters in both EFB and CHDFB coating processes as well as for thedevelopment of new powder coating formulations.

2007 - Devitrification behaviour of plasma-sprayed glass coatings [Articolo su rivista]
Bolelli, Giovanni; Lusvarghi, Luca; Siligardi, Cristina; Manfredini, Tiziano
abstract

Glass and glass-ceramic coatings on ceramic tiles have been manufactured by plasma-spraying high-performance CAS (in wt%-SiO2, 60%; Al2O3, 15%; CaO, 23%; others, traces) and CZS (in Wt%-SiO2, 50%; CaO, 31%; ZrO2, 16.5%; Al2O3, 2%; others, traces) glass frits. The CZS system has a surface crystallization at about 1050 degrees C. Such behaviour would not easily allow to obtain a fully crystalline bulk glass-ceramic, but the defectiveness of the plasma-sprayed coating supplies many nucleation sites. Thus, it becomes completely crystalline and well sintered after a 850 degrees C for 30 min + 1050 degrees C for 15 min treatment. The CAS frit, designed not to produce significant crystallization, is well sintered after a 850 degrees C for 30 min + 950 degrees C for 30 min thermal treatment, but remains too brittle due to its glassy nature. A 1050 degrees C treatment allows a few pseudowollastonite crystals to form in a glassy matrix; their formation also hinders sintering. Thus, mechanical properties are inferior to heat-treated plasma-sprayed CZS. (c) 2006 Elsevier Ltd. All rights reserved.

2007 - Microstructural and tribological characterisation of as sprayed and heat treated HVOF deposited Ni alloys [Articolo su rivista]
Bolelli, Giovanni; Lusvarghi, Luca; F., PIGHETTI MANTINI; F., Casadei; M., Barletta
abstract

The microstructural, micromechanical (Vickers microindentation, scratch testing) and tribological(pin on disk tests against steel and alumina spherical pins) properties of three High velocity oxy–fuel (HVOF) sprayed Ni based alloy coatings, namely Diamalloy 4006 (Ni–20Cr–10W–9Mo–4Cu–1B–1C–1Fe), Tribaloy-700 (Ni–32Mo–16Cr–4Si–2Co) and Inconel-625 (Ni–22Cr–9Mo–4Nb), werecharacterised, both in the as sprayed condition and after thermal treatments at 600uC and 800uC.As deposited Tribaloy-700 possesses a low degree of crystallinity and lower hardness; crystallineintermetallics are formed after heat treatments, definitely improving mechanical strength andtribological resistance against 100Cr6 steel counterpart, but not against alumina counterpart. TheInconel-625 and Diamalloy 4006 as sprayed coatings consist of supersaturated crystalline solidsolutions. The former is not much affected by heat treatments and has low hardness and poortribological properties. The latter, instead, displays precipitation of secondary phases after heattreatment. Particularly, the 600°C treatment improves coating strength and wear resistanceagainst steel, whereas the 800u°C one is less effective, probably because it causes excessivecrystal grain size increase.

2007 - Microstructural and tribological comparison of HVOF-sprayed and post-treated M-Mo-Cr-Si (M = Co, Ni) alloy coatings [Articolo su rivista]
Bolelli, Giovanni; Cannillo, Valeria; Lusvarghi, Luca; Montorsi, Monia; F., PIGHETTI MANTINI; M., Barletta
abstract

High velocity oxygen-fuel (HVOF)-spayed wear resistant Co-28%Mo-17%Cr3%Si and Ni-32%Mo-15%Cr-3%Si coating, both as sprayed and after heat treatments at 600°C for 1h, have benn studied.

2007 - Tribological properties of HVOF as-sprayed and heat treated Co–Mo–Cr–Si coatings [Articolo su rivista]
Bolelli, Giovanni; Lusvarghi, Luca
abstract

HVOF-sprayed Co–28%Mo–17%Cr–3% Si alloy tribological performance was tested in the as-sprayed condition and after thermal treatments at 200, 400, 600°C for 1 h. As-sprayed coating possesses low hardness, undergoes adhesive wear against 100Cr6steel and displays an high-friction coefficient causing relevant thermal effects. The 600°C-heat treatment increases microhardness, thus preventing adhesive wear and reducing friction.

2006 - Corrosion resistance of HVOF-sprayed coatings for hard chrome replacement [Articolo su rivista]
Bolelli, Giovanni; Giovanardi, Roberto; Lusvarghi, Luca; Manfredini, Tiziano
abstract

HVOF-sprayed coatings (WC-17Co, WC-10Co-4Cr, Co-28Mo-17Cr-3Si) and electrolytic hard chrome (EHC) coatings corrosion resistances have been compared through electrochemical polarization tests (0.1 N HCl, 0.1 N HNO3) and Corrodkote test. EHC coatings passivate in HNO3, but undergo pitting corrosion in HCl and in Corrodkote test too. HVOF coatings do not passivate, but possess more noble corrosion potentials. Both in HNO3 and HCl, they undergo more generalized corrosion, with similar i(corr); crevice corrosion along splat boundaries is sometimes detected after the HCl test. Their i(corr) in 0.1 N HCl solution is lower than in several of EHC coatings. No visible damage in the HVOF coatings has occurred after the Corrodkote test. (c) 2006 Elsevier Ltd. All rights reserved.

2006 - Glass alumina composite coatings by plasma spraying. Part II: microstructure-based modelling of mechanical properties [Articolo su rivista]
Bolelli, Giovanni; Cannillo, Valeria; Lusvarghi, Luca; Manfredini, Tiziano; Montorsi, Monia
abstract

The mechanical properties of composite glass-alumina coatings produced by plasma spraying, as described in Part 1, were numerically characterized with relation to the peculiar microstructure. Finite element meshes were created for the most significant coating typologies, starting from SEM acquired microstructures. The coatings elastic properties and fracture behaviour were characterized as a function of relevant microstructural features. The results confirm that the coatings are anisotropic, with a lower elastic modulus in the direction perpendicular to the substrate plane (spray direction), because of the lamellar microstructure; increasing the alumina volume fraction increases the elastic modulus value both in the spray and transverse direction. Moreover, it is found that cracks start from large, irregular pores, and propagate easily through the glass areas, but are stopped by alumina. Smaller individual glass areas hinder crack propagation. The post-deposition thermal treatment described in Part I produces tensile residual stresses in the glass and compressive ones in the alumina; thus, the arresting effect of the latter on cracks propagation is greatly enhanced. (c) 2005 Elsevier B.V. All rights reserved.

2006 - Glass-alumina composite coatings by plasma spraying. Part I: Microstructural and mechanical characterization [Articolo su rivista]
Bolelli, Giovanni; Cannillo, Valeria; Lusvarghi, Luca; Manfredini, Tiziano
abstract

Composite coatings on ceramic tiles have been produced by plasma-spraying waste glass powders together with alumina as reinforcing phase. Obtaining a tough coating, in which a high amount of waste glass can be recycled, is the aim of the study. Glass powders with different particle size distributions were tested; coatings with increasing glass volume fractions (0%, 20%, 40%, 60%, 80%, 100%) were sprayed. Furthermore, two different kinds of alumina feedstocks were employed, namely, a commercial plasma-spray powder and a more economical laboratory manufactured spray-dried powder. Vickers microhardness, fracture toughness and abrasion resistance have been measured. The coatings were tested both in as-sprayed condition and after a thermal treatment. An abrasion resistance comparable to common industrial glazes has been obtained with as much as 60% of glass volume fraction, employing fine glass powders (< 45 mu m). No significant decrease in mechanical proper-ties has been noticed replacing the commercial alumina powder with the spray-dried one. (c) 2005 Elsevier B.V. All rights reserved.

2006 - Heat Treatment Effects on the Tribological Performance of HVOF-Sprayed Co-Mo-Cr-Si Coatings [Articolo su rivista]
Bolelli, Giovanni; Lusvarghi, Luca
abstract

The tribological behavior of high-velocity oxyfuel sprayed Co-28%Mo-17%Cr-3%Si coatings, both as sprayed and after heat treatments at 200, 400, and 600 °C for 1 h, has been studied. The as-sprayed coating contains oxide stringers and is mostly amorphous. It has low hardness (∼6.7 GPa) and toughness and undergoes adhesive wear against 100Cr6 steel. The friction coefficient increases up to ∼0.9, so the flash temperature reaches a critical oxidation value; then, friction decreases and increases again. This phenomenon occurs periodically. Much adhesive wear occurs in the first stage. Abrasive wear prevails against alumina pin: the coating wear rate is lower because it possesses good plasticity. Thermal effects still occur. The 600 °C treatment causes formation of submicrometric crystals. Hardness increases (∼8.8 GPa), adhesive wear is prevented, the friction coefficient has no peaks. Against the alumina pin, wear rates remain similar to the as-sprayed case. Nevertheless, the friction coefficient has no peaks and its final value is lowered (from 0.84 to0.75).

2006 - Mechanical and tribological properties of electrolytic hard chrome and HVOF-sprayed coatings [Articolo su rivista]
Bolelli, Giovanni; Cannillo, Valeria; Lusvarghi, Luca; Ricco, S.
abstract

The microstructure, mechanical properties (Vickers microhardness, fracture toughness, elastic modulus) and tribological behaviour (abrasion and unlubricated sliding wear resistance) of various kinds of electrolitic hard chrome (EHC) coatings and of metallic and cermet HVOF-sprayed coatings (WC-17Co, WC-10Co-4Cr, Co-28Mo-17Cr-3Si) have been studied. EHC coatings are very tough, in fact no cracks could be produced with 10 N Vickers microindentations. However, their superficial morphology and Vickers microhardness are significantly affected by the substrate preparation technique, the deposition process characteristics, and by post-deposition treatments. This causes the coatings to show different behaviors in different wear conditions; in particular, harder coatings are more abrasion-resistant, but undergo a more severe mass loss under unlubricated sliding conditions. HVOF-sprayed cermet coatings are harder but less tough than EHC ones. Therefore, they undergo a comparable or even higher mass loss when subjected to three-body abrasion conditions. However, their two-body sliding resistance definitely overcomes that of EHC coatings, because they form a tough and uniform surface film protecting them from further damage.

2006 - Plasma-sprayed graded ceramic coatings on refractory materials for improved chemical resistance [Articolo su rivista]
Bolelli, Giovanni; Cannillo, Valeria; C., Lugli; Lusvarghi, Luca; Manfredini, Tiziano
abstract

Plasma-sprayed ceramic coatings were manufactured on sintered alumina-mullite refractory bricks to improve their chemical resistance to molten glass. Mullite and alumina powders were employed. Graded layered coatings were designed and produced, to reduce the thermal expansion mismatch with the substrate: in all cases, the upper layer consisted in pure alumina (very resistant to chemical attack); alumina-mullite intermediate layers were added to match the low thermal expansion of the porous substrates. Plasma-sprayed coatings definitely improved both the abrasion resistance and the chemical resistance to long-time (8 h at 1400 degrees C) contact with molten glass, since the coating preserved its original microstructure. However, in thermal shock tests, some transverse cracks appeared; thus, thermal cycling tests in presence of molten glass indicated that, after a few cycles, the glass can penetrate down to these cracks. The tested samples were studied by means of scanning electron microscopy (SEM) with energy dispersive spectrometry (EDS) and X-ray diffraction (XRD).

2006 - Wear behaviour of thermally sprayed ceramic oxide coatings [Articolo su rivista]
Bolelli, Giovanni; Cannillo, Valeria; Lusvarghi, Luca; Manfredini, Tiziano
abstract

The wear resistance of plasma-sprayed ceramic coatings (Al2O3, Al2O3-13%TiO2, Cr2O3) has been investigated through pin-on-disk and dry sand-steel wheel tests, has been correlated to microstructural and micromechanical characteristics (microhardness, fracture toughness) and has been compared to well-known platings (such as Cr electroplating and electroless Ni) and HVOF-sprayed cermets (WC-17%Co, WC-10%Co-4%Cr). Plasma-sprayed ceramics are hard but brittle: dry particles abrasion occurs through splats detachment. The toughest coating (Al2O3) displays the highest wear resistance, which in fact overcomes HVOF-sprayed cermets and Cr electroplating, when a low number of wheel revolutions are considered. In pin-on-disk tests, no coating undergoes wear loss against the 100Cr6 ball, that possess lower hardness. Against the alumina ball, Al2O3 and Al2O3-TiO2. coatings show high wear rates and friction coefficients (due to chemical affinity), while Cr2O3 possesses better wear resistance, lower friction coefficient and inflicts less wear on the counterpart. Cr2O3 wear scar consists in plastically deformed splats and debris forming a quite adherent protective tribofilm. (c) 2006 Elsevier B.V. All rights reserved.

2005 - Influence of the manufacturing process on the crystallization behavior of a CZS glass system [Articolo su rivista]
Lusvarghi, Luca; Manfredini, Tiziano; Siligardi, Cristina; Bolelli, Giovanni
abstract

A glass belonging to the CaO-ZrO2-SiO2 (CZS) System has been plasma-sprayed on ceramic tiles. The crystallization behavior of plasma-sprayed coatings has been compared to the devitrification processes of the bulk glass and sintered glass powders of the same composition. DTA experiments reveal a crystallization peak at about 1323 K, with a quite high activation energy, indicating the bulk crystallization process is difficult, because of the high viscosity of the glass. Bulk samples require higher treatment temperatures (approximate to 1423 K) to develop significant amounts of crystalline phases. Furthermore, being the crystallization mostly superficial, high residual stresses develop between the crystalline surface and the glassy core. Sintered samples are fully crystalline, because crystallization starts from the particle surface, the specimens show a considerable shrinkage. After an 1123 K 30´ + 1323 K 15´ thermal treatment, plasma-sprayed coatings, possessing numerous defects and a slight degree of devitrification induced by thermal cycling during plasma deposition, become very well sintered and completely crystalline, because the as-sprayed microstructural features promote heterogeneous nucleation. In this case, the shrinkage is quite negligible, since partial sintering occurs already during the deposition process. In the fully crystalline coatings, excellent mechanical properties (microhardness, fracture toughness and abrasion resistance) are achieved, overcoming those of traditional industrial glazes.

2005 - Plasma-sprayed glass-ceramic coatings on ceramic tiles: microstructure, chemical resistance and mechanical properties [Articolo su rivista]
Bolelli, Giovanni; Cannillo, Valeria; Lusvarghi, Luca; Manfredini, Tiziano; Siligardi, Cristina; C., Bartuli; A., Loreto; T., Valente
abstract

This article reports the characterisation and optimisation of glass-ceramic coatings plasina-sprayed on traditional ceramic substrates, dealing with microstructures, chemical resistance, and superficial mechanical properties. A CaO-ZrO2-SiO2 (CZS) frit, capable of complete crystallization after proper thermal treatment, has been employed: due to its refractory nature, its firing temperature in a traditional process would be unbearable for common substrates. The frit was plasma-sprayed onto ceramic tiles and a post-process thermal treatment has been developed in order to sinter and crystallize the coating, employing much lower temperatures than a traditional ceramic firing cycle. The microstructure of both as-sprayed and treated coatings has been evaluated with SEM and EDS, acid resistance tests have been performed. Vickers microhardness, superficial fracture toughness, deep abrasion resistance, elastic modulus and coating adhesion have been evaluated. Unglazed stoneware and an industrial glaze were also tested for comparison and an experiment was made to produce a CZS-based glaze to ascertain the inadequacy of traditional firing. As-sprayed coatings have a typical plasma-spraying defective microstructure, but a thermal treatment with 30´ isotherm at 850 degrees C plus 15´ isotherm at 1050 degrees C allows good densification, excellent adhesion and complete crystallization, with formation of wollastonite-2M and many small Ca2ZrSi4O12 grains. Thus, the coating outperforms stoneware by 50% in abrasion resistance, possessing higher fracture toughness, thanks to the continuous crack deflections due to the numerous crystals. Fracture toughness appears to be the property most correlated to abrasion resistance, because brittle fracture is the dominant abrasion mechanism. Hardness and elastic modulus reflect quite well the coating inner cohesion. Treated coatings porosity is lower than industrial glazes and stoneware.