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STEFANIA MORELLI

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INTERMECH Centro Interd. per la Ricerca Applicata e i Servizi nel settore della Meccanica Avanzata e della Motoristica

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Pubblicazioni

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.

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 - 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.

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.

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 - 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 - 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.

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.