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Dipartimento di Scienze Fisiche, Informatiche e Matematiche sede ex-Fisica
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Dipartimento di Scienze Fisiche, Informatiche e Matematiche sede ex-Fisica

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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.

2019 - Tribological Properties of High-Speed Uniform Femtosecond Laser Patterning on Stainless Steel [Articolo su rivista]
Gnilitskyi, Iaroslav; Rota, Alberto; Gualtieri, Enrico; Valeri, Sergio; Orazi, Leonardo

In this work, an analysis of the tribological performance of laser-induced periodic surface structures (LIPSS) treated X5CrNi1810 stainless steel was conducted. The approach followed by authors was to generate LIPSS-patterned circular tracks, composed of radial straight grooves with uniform angular periodicity. This permitted to measure the tribological properties in a pin-on-flat configuration, keeping fixed the orientation between the grooves and the sliding direction. A Stribeck curve was measured, as well as the consequent wear. A deep analysis of the sub-surface conditions after LIPSS generation was moreover performed using Focused Ion Beam (FIB) cross-section.

2017 - Multifunctional properties of high-speed highly uniform femtosecond laser patterning on stainless steel [Relazione in Atti di Convegno]
Gnilitskyi, Iaroslav; Rota, Alberto; Ctvrtlik, Radim; Serro, Ana Paula; Gualtieri, Enrico; Orazi, Leonardo

Highly-regular laser-induced periodic surface structures essentially change surface properties of treated surfaces. Here, effect of HR-LIPSS on optical, tribological, mechanical and wetting properties of stainless steel were systematically investigated.

2017 - The role of humidity and oxygen on MoS2 thin films deposited by RF PVD magnetron sputtering [Articolo su rivista]
Serpini, Elisabetta; Rota, Alberto; Ballestrazzi, Antonio; Marchetto, Diego; Gualtieri, Enrico; Valeri, Sergio

MoS2 is usually applied as thick films (1 μm and above) on sliding counterparts to decrease friction and wear. Thick films of MoS2 generally grow columnar-like and the bending or fracture of these columns during sliding are supposed to be the origin of the good tribological performances. In the present work, we studied the tribological behavior of 200 nm MoS2 films prepared by RF magnetron sputtering, with emphasis on the friction mechanisms. We performed ball-on-disc tests at different values of residual humidity, in pure oxygen atmosphere and at different temperatures, in order to disentangle the role of water and molecular oxygen during sliding. We found that, despite the fact that the inner structure of these thin films is not lamellar, the tribological behavior is similar to thicker ones. The friction in absence of humidity is well below that in standard conditions and the lifetime of the film is strongly enhanced. We observed similar performances in humid air while keeping the sample at 75 °C. Our results clearly demonstrated that absorbed water causes the deterioration of the lubricating properties of the films, while film oxidation plays only a marginal role.

2016 - Friction of rough surfaces on ice: Experiments and modeling [Articolo su rivista]
Spagni, Alberto; Alice, Berardo; Marchetto, Diego; Gualtieri, Enrico; Nicola, M. Pugno; Valeri, Sergio

Over a century of scientific research on the sliding friction of ice has not been enough to develop an exhaustive explanation for the tribological behavior of frozen water. It has been recognized that ice shows different friction regimes, but a detailed description of all the different phenomena and processes occurring at the interface, including the effect of surface roughness of both the ice and the antagonist material is still missing. In this work the effect of surface morphology on the friction of steel/ice interfaces is studied. Different degrees of random roughness on steel surfaces are introduced and the friction coefficient is measured over a wide range of temperature and sliding velocity. Correlation between the surface roughness and the lubrication regime and friction coefficient is discussed. A theoretical model is developed in order to explain this correlation, and to control the tribological behavior of the system by a proper selection of surface roughness parameters.

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

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.

2011 - Ag Surface Diffusion and Out-of-Bulk Segregation in CrN-Ag Nano-Composite Coatings [Articolo su rivista]
Incerti, Luca; Rota, Alberto; Ballestrazzi, Antonio; Gualtieri, Enrico; Valeri, Sergio

CrN-Ag nanocomposite coatings are deposited on Si(100) wafers and 20MnCr5 steel disks in amixed Ar+N2 atmosphere by reactive magnetron sputtering. Structure, composition and morphologywere investigated by Scanning Electron Microscopy (SEM), Auger Electron Spectroscopy (AES),X-ray Photoemission Spectroscopy (XPS), X-ray Diffraction (XRD) and Focused Ion Beam (FIB)cross sectional analysis. The as deposited film matrix is mainly composed by CrN phase (78%), buta relevant part (28%) is composed by Cr2N. Ag agglomerates in the CrN matrix forming elongatedgrains 200–400 nm wide and 50–100 nm high, which extends on the top of CrN columns. At thesurface Ag aggregates into two different structures: large tetrahedral crystalline clusters, with typicaldimension ranging from 200 to 500 nm, and smaller Ag nanoparticles with diameter of 15–25 nm.The annealing in N2 atmosphere up to 500 C does not affect size and distribution of the Aggrains in the sub-surface region, while it induces a size increase of the bigger Ag clusters onthe surface, mainly related to Ag surface diffusion and clusters coalescence. Annealing at highertemperature leads to an evident Ag out-of-bulk segregation, generating Ag depleted voids in thenear-surface region, and further increasing of the Ag clusters size at the surface. Tribological testson as deposited CrN-Ag film reveal a coefficient of friction against a steel ball reduced with respectto CrN film, probably related to the presence of Ag which acts as solid lubricant, but the coatingis removed after a very short sliding distance. The poor mechanical properties of the realized Agbasedcoatings are confirmed by lower hardness and Young modulus values with respect to pureCrN.

2011 - Role of Roughness Parameters on the Tribology of Randomly Nano-Textured Silicon Surface [Articolo su rivista]
Gualtieri, Enrico; N., Pugno; Rota, Alberto; Spagni, Alberto; E., Lepore; Valeri, Sergio

This experimental work is oriented to give a contribution to the knowledge of the relationship among surface roughness parameters and tribological properties of lubricated surfaces; it is well known that these surface properties are strictly related, but a complete comprehension of such correlations is still far to be reached. For this purpose, a mechanical polishing procedure was optimized in order to induce different, but well controlled, morphologies on Si(100) surfaces. The use of different abrasive papers and slurries enabled the formation of a wide spectrum of topographical irregularities (from the submicro- to the nano-scale) and a broad range of surface profiles. An AFM-based morphological and topographical campaign was carried out to characterize each silicon rough surface through a set of parameters. Samples were subsequently water lubricated and tribologically characterized through ball-on-disk tribometer measurements. Indeed, the wettability of each surface was investigated by measuring the water droplet contact angle, that revealed a hydrophilic character for all the surfaces, even if no clear correlation with roughness emerged. Nevertheless, this observation brings input to the purpose, as it allows to exclude that the differences in surface profile affect lubrication. So it is possible to link the dynamic friction coefficient of rough Si samples exclusively to the opportune set of surface roughness parameters that can exhaustively describe both height amplitude variations (Ra, Rdq) and profile periodicity (Rsk, Rku, lc) that influence asperity-asperity interactions and hydrodynamic lift in different ways. For this main reason they cannot be treated separately, but with dependent approach through which it was possible to explain even counter intuitive results: the unexpected decreasing of friction coefficient with increasing Ra is justifiable by a more consistent increasing of kurtosis Rku.

2011 - Size-dependence transition from nano-friction to micro-friction [Poster]
Rota, Alberto; Gualtieri, Enrico; Tripathi, Manoj; Valeri, Sergio

The interpretation of tribological processes is presently very different for nano-scale and micro-scale phenomena. This is mainly related to the presence of adhesive forces, which are not negligible at the nano-scale and, in some cases, dominate the tribological behaviour of the system. The morphological characteristics of sliding surfaces have been demonstrated to strongly influence both the nano-scale and the micro-scale tribology [1-4]. The phenomena which rule these properties are different depending on the dimensions of surface micro/nano-structures, on the contact area and on the applied load. At the micro-scale the presence of a regular array of micro-cavities could lead to the reduction of friction and wear, due to the enhancement of the hydrodynamic lift-off effect and to the role played by the micro-cavities as lubricant reservoir and debris pocket [1, 2]. At the nano-scale parallel nano-grooves induce a hydrophobic character to the surface, resulting in a reduction of the coefficient of friction and adhesion [3, 4]. The aim of the present study is to determine the critical size at which the transition from nano- to micro-scale regimes occurs, investigating the effect of surface patterning on the tribological properties of Si(100). With this purpose the Si flat surface have been patterned by means of Focused Ion Beam with regular arrays of parallel grooves 50nm wide and 4nm deep, varying the pitch from 125nm to 1m. Adhesion and friction have been studied as a function of the pitch by means of AFM in air and in vacuum, using a dedicated flat tip sensitive to the integrated effect of the nano-structures on the tribology of the system. The hydrophobicity of 125nm pitch array, already described in ref. 4, decreases between 400 and 500nm pitch, resulting almost absent for larger values. This finding identifies 400-500nm pitch interval as the threshold between nano- and micro-scale regimes. This research has been supported by PRRIITT (Regione Emilia Romagna) and Centro Interdipartimentale per la Ricerca Applicata e i Servizi nella Meccanica Avanzata e nella Motoristica (Università di Modena e Reggio Emilia). [1] A. Kovalchenko et al., Tribol. Int. 38 (2005) 219. [2] A. Borghi et al., Wear 265 7-8 (2008) 1046. [3] Y. Ando, Tribo. Lett. 19 (2005) 29. [4] D. Marchetto et al., Wear 268 (2010) 488.

2010 - Tribology and wettability of nano-machined silicon rough surfaces [Poster]
Incerti, Luca; Ballestrazzi, Antonio; Rota, Alberto; Gualtieri, Enrico; Valeri, Sergio

1 – Introduction The more and more pressing demand for eliminating liquid lubricants in mechanical devices, due to their environmental costs, represents an important challenge for the tribological community. Low-friction coatings, like layered materials (graphite, Mo2S), soft metals (Ag, Au), non-layered metal oxides (MoO3, V2O5), can substitute liquid lubricants, but presently they are characterized by poor mechanical properties. On the other hand, hard ceramic coatings, like CrN, TiN, TiCN, TiAlN, have been studied for the last three decades for their high hardness, chemical inertness, high oxidation resistance and low wear rate. The aim of our study is to couple the properties of hard ceramics with those of soft metals low friction coatings. The nanostructuring of CrN-Ag ternary coating represents an appealing solution. 2 – Main Body of Text We studied the tribo-mechanical properties of CrN-Ag ternary coatings, where Ag clusters are dispersed in a CrN thick film. Some preliminary tests were also performed on a similar system, composed by Ag/CrN multilayers. CrN-Ag composite coatings were deposited on Si(100) and on a steel (20MnCr5) using PVD Magnetron Sputtering in a mixed N2+Ar atmosphere (P=5mtorr), with temperature ranging from RT to 500°. Structure, composition and morphology of both systems were characterized by Auger Electron Spectroscopy, X-rays diffraction and cross sectional analysis (depth-profiling AES, FIB cross section). The co-deposition of CrN and Ag in N2 atmosphere leads to the formation of a composite nanostructured coating, where Ag clusters behave as solid lubricant in the hard CrN bulk. Tribo-mechanical properties were measured with a ball-on-disc tribometer and a Berckovich nanoindenter. 3 – Conclusion By tuning the preparation conditions we were able to control the size, the distribution and the self organization of Ag nanoclusters in the CrN matrix, thus controlling frictional behavior.

Bolelli, Giovanni; Gualtieri, Enrico; Lusvarghi, Luca; Valeri, Sergio

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 - Increasing nanohardness and reducing friction of nitride steel by surface laser texturing [Articolo su rivista]
Gualtieri, Enrico; Borghi, A; Calabri, L; Pugno, N; Valeri, Sergio

Laser surfacetexturingisoftenemployedtoimprovetribologicalperformancesofmatingsurfaces.Theprincipalaimofthepresentworkistostudylocalmodificationsinducedbylaserbeamduringthetexturingprocess(microdimpling),onthemechanicalpropertiesofa30NiCrMo12nitridesteel.A brieftribologicalcharacterizationwasinitiallymadeinordertoverifytheimprovementoftribologicalbehaviourensuredbymicrodimpling.Frictionmeasurementswerecarriedoutinlubricatedflat-on-flatandround-on-flatconfigurations.A sub-surfacecharacterizationwasperformedbyfocusedionbeamcross-sectionandimaging.Modifiedmateriallayerswerediscoveredincorrespondenceoflaser-affectedzonesarounddimples,where graindimensionswereobservedtobereduced.Toquantifythisphenomenon,anewtheoreticalenergetic modelwasdeveloped.Hardness measurementswereperformedbyatomicforcemicroscopenanoindentation.Thisapproach allowedtoobserveanhardeningeffectmovingfrombulkmaterialtodimpleedges.Therefore, thetheoreticalmodelwasintegratedwiththeHall–Petch’slawinordertoquantifytheobservedhardeningbehaviour.& 2008ElsevierLtd.Allrightsreserved.

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

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.

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

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 - Tribological effects of surface texturing on nitriding steel for high-performance engine applications [Articolo su rivista]
A., Borghi; Gualtieri, Enrico; Marchetto, Diego; L., Moretti; Valeri, Sergio

This study reports on the effects of surface modification by laser texturing on tribological performances of nitride steel for automotive engine applications. In particular, a comparison of tribological properties on untextured and textured 30NiCrMo12 nitride steel was made among hydrodynamic, mixed and boundary lubrication regimes.Ordered arrays of circular dimples were created on nitride steel surfaces by Laser Surface Texturing. Friction experiments were performed using a pin-on-disc apparatus at sliding speeds ranging from 1 to 12 cm∙s-1 and normal applied loads ranging from 1 to 10 N. Static counterpart was a custom pin of 100Cr6 steel. Two different configurations were studied: “single drop” and “dry contact”. Long sliding condition tests and Stribeck curves were obtained. Finally, Secondary Electron Microscopy imaging, Energy Dispersive Spectroscopy and Auger Electron Spectroscopy investigations were carried out to study morphology and chemistry at surfaces, before and after tribological tests.Maximum effects of laser surface texturing on friction coefficient and wear have been observed in single drop condition at high normal applied load and for long sliding time.

2006 - Focused ion beam induced swelling in MgO(0 0 1) [Articolo su rivista]
Rota, Alberto; Contri, Sara Federica; G. C., Gazzadi; S., Cottafava; Gualtieri, Enrico; Valeri, Sergio

The interplay between swelling and milling phenomena in determining the morphology of Focused Ion Beam (FIB)-processed MgO(001) was investigated by atomic force microscopy. At the early stages of ion irradiation.. before millinig erosion-is observed, MgO shows a relevant swelling behaviour with protrusion of the bombarded areas up to 6 nm for a dose of 5 x 10(16) ions cm(-2). The effect is mainly ascribed to subsurface defect accumulation, while the low Ga ions concentration, as measured by in-depth Auger analysis, seems to exclude a contribution from ion implantation. In order to explain and control the morphology of Fe/NiO FIB patterned sub-micron structures on MgO substrates, we have also investigated FIB effects on Fe(001) and NiO(001) single crystals. Absent or negligible swelling has been observed on these materials. (c) 2006 Elsevier B.V. All rights reserved.