ICMCTF2016 Session E1-3: Friction, Wear, Lubrication Effects, and Modeling

Transition metal dichalcogenides (TMD) coatings have been deposited by magnetron sputtering for applications requiring low friction, due to their layered structure and weak inter-layer bonding. However, such a lower bonding energy induces also very low mechanical strength with the low loading bearing capacity in sliding contact. One of the possible ways to improve mechanically TMD coatings is through the modification of the chemical composition by adding a third element. Due to the easy processing based on a reactive approach, C and N have been widely studied as alloying elements to TMD coatings (TMD+C(N)) with excellent results concerning the improvement of the mechanical strength and, in some cases, with lower sensibility to moisture, another well-known problem of TMDs. Although extensive tribological characterization has been carried out on TMD+C(N) coatings, the contact with rubber was not explored neither at room nor at high temperatures.

The aim of this work was to study the thermo-mechanical and tribological behavior of the TMD+C(N) sputtering coatings, particularly for comparing the efficiency of C and N when the coatings are sliding against rubber from 0 up to 300 ºC. For these purposes, W-S thin films were deposited with different C or N contents using magnetron sputtering onto steel substrates. Initially, structural and mechanical characterization of the coatings was performed. The increase of C or N led to a progressive amorphization of the coatings. In crystalline films, N-containing TMD showed a (001) preferential orientation whereas TMD+C was not preferentially oriented. In both systems, hardness as high as 8 GPa was possible to be achieved.

For W-S-C coatings, at room temperature and 100ºC the friction levels of all C-containing coatings are very high and close to that of uncoated steel, higher than 0.7. Only pure W-S coatings showed a much lower friction close to 0.3. The increase of the testing temperature led to a general decrease of the friction in all the TMD+C coatings (down to 0.4 for higher C content) while pure W-S kept the value measured at room temperature. However, in one case and for a very long test, one of the C-containing coating could reach a very low value even lower than W-S coating. The different behaviours between the coatings could be attributed to the formation or absence of a W-S oriented tribolayer in the sliding contact.

The tribological behavior of NbN_x, NbSiN_x andTaSiN_x thin films was studied at room temperature (RT), 150, 300 and 450 °C using Pin-on-Disk with an Al₂O₃ ball as the counter body. The films were deposited on AISI 410 Stainless Steel substrates by reactive magnetron sputtering or co-sputtering of the appropriate single metal targets in an Ar+N₂ atmosphere. The mechanical properties of the layers were studied by scratch-test and nanoindentación. The chemical composition of the coatings and wear tracks were studied by EDS and Raman. Scanning electron microscopy (SEM) and stylus profilometer were used to investigate the wear mechanism in the wear tracks and the wear rates, respectively.

The adhesion critical load for all of the more than 2 μm thick films was approximately 40 N, and the hardness was 29 GPa NbN_x, 32 GPa NbSiN_x and33 GPa TaSiN_x. The temperature had a large influence on the surfaces properties and, in consequence, the tribological behavior of the films. The changes in the surfaces properties modified the wear mechanisms that determine the wear rate and the friction coefficient. For the substrate, the wear mechanism changed from plastic deformation and fatigue, at RT to adhesion and transferred material at 450 °C. In the tribological studies of the NbNx coated samples, at RT the wear was by abrasion with accumulation of debris in the centre of the wear track, at 450 ^oC the coating failed due to facture and partial spalling. For the NbSiNx films, at RT the wear characteristics were similar to the NbNx case, but at 450 ^oC the film totally failed and was removed. For the TaSiN coating, at RT the wear was by abrasion with the formation of fissures in the centre of the wear track. As the temperature was increased the size and frequency of the fissures increased until at 450 ^oC the whole of the wear track was covered by a fine map of fissures. The friction coefficient of all of the samples was similar, 0.8 – 1.0, and did not change significantly with temperature. The chemical analysis showed that even at RT there was oxidation of the coating in the wear track, and that the amount of oxide present increased with increasing testing temperature.

The present paper gives a detailed description of structural changes in three types of MoCN-Ag coatings (Mo₅₁C₁₅N₂₇Ag₇, Mo₄₀C₃₁N₂₃Ag₆, and Mo₄₃C₁₄N₄₀Ag₃) during dynamic temperature ramp tribological tests with particular emphasis on the analysis of wear products to identify adaptive friction mechanisms in the temperature range between 250 and 550^oC. Thorough structural characterization using high-temperature XRD, SEM, TEM, GDOES, and Raman spectroscopy provided evidence of various tribo-chemical reactions in the zone of tribological contact affecting lubrication. The coating lubrication in the temperature range between 100 and 400 °C was observed to be different. Unlike Mo₅₁C₁₅N₂₇Ag₇ coating whose friction coefficient monotonously increased with increasing temperature from 25 to 250^oC, the Mo₄₀C₃₁N₂₃Ag₆ coating demonstrated low values of friction coefficient up to 250^oC due to the tribo-activated formation of carbon-based fibers normal to the sliding direction. The good lubrication of the Mo₄₃C₁₄N₄₀Ag₃ coating at elevated temperatures was attributed to almost no wear due to its high hardness and to the formation of a thin tribo-activated MoO₃ film at 350^oC. Hovewer, complete oxidation of the wear track at 400^oC resulted in intensive abrasion wear and high friction. Above 400^oC, all coatings demostrated similar values of friction coefficient irrespective of phase composition (melt, Ag₆Mo₁₀O₃₃, or MoO₃+Ag).

Self-lubricant solids are an alternative to traditional oil-based lubricants. Moreover, they can be used in applications where oil lubrication is not possible, such as at high temperatures or vacuum. Graphite/graphene and transition metal dichalcogenides (TMD) exhibit the lowest friction on a nanoscale; the latter is frictionless even at macroscopic contacts under ultra-high vacuum conditions. In general, TMD-based coatings are known to form tribolayer, which significantly decreases friction and wear. We investigate key properties of such tribolayer and its impact on tribological properties. We have found that tribolayer is well ordered TMD with basal planes parallel to sliding direction; it is formed almost regardless on atmosphere (vacuum, dry/humid air) within wide range of contact conditions (sliding speed, contact pressure). We show some successful microstructural designs of TMD-based self-adaptive coatings doped with metals, carbon or nitrogen and their application in industry.

Thanks to extremely low thickness of such tribolayer, typically few nanometers, atomistic simulations are natural tool to investigate nanoscale friction. We presents recent advances in atomistic simulations and in situ nanoscale observations of sliding process related to 2D, thin film and bulk TMDs. We briefly review theoretical aspects of TMD sliding at nanoscale and shortcomings of actual nanoscale models, particularly ab initio and molecular dynamics simulations. Then we compare frictional characteristics of 2D TMD layers with those of bulk crystal and relate them to macroscopic. We demonstrate that friction is scalable, i.e. similar values of friction coefficient are obtained by nanoscale (FFM) and macroscale (pin-on-disc) methods provide contact pressure is identical. Finally, three features of TMD sliding will be discussed: i) decrease of friction with increasing contact pressure, ii) effect of humid vapour on friction, and iii) formation of tribolayer.

This work is an analysis of the number of cycles variation influence during fretting wear tests: influence on wear and friction of both PET and PTFE rubbing against a titanium-alloy (Ti-6Al-4V). The aim of this polymer-metal compounds use is to reduce wear and friction. Polyethylene terephthalate (PET) and polytetrafluoroethylene (PTFE) are among thermoplastic polymers which do have valuable tribological properties: PET mainly due to its high wear resistance and PTFE especially due to its self-lubrication and low coefficient of friction.

In order to perform this analysis, fretting wear loadings are carried out in a flat-on-cylinder configuration under dry sliding conditions at room temperature. The flat specimen is Ti-6Al-4V and the cylindrical one is PET or PTFE, alloying comparison of their respective wear behavior. These tests are performed at constant frequency, sliding amplitude and normal load (f=1Hz; δ=±500µm; P=200N) varying only the number of cycles: N ∈ [50 000; 1 000 000].The worn-out volume as well as the surface roughness are estimated by 3D laser profilometer. Scanning Electron Microscopy is carried out for microstructural characterization to analyze worn surface morphology of the Ti-6Al-4V, PET and PTFE samples. The elemental analysis by energy dispersive X-ray technic is also carried-out. In parallel, two other thermo-mechanical experiments: Dynamic Mechanical Analysis and Differential Scanning Calorimetry are performed on PET and PTFE samples in order to study their thermomechanical properties.

Results have shown that PTFE adhesion increases with the number of cycles of the fretting tests, presenting a high wear rate with a mechanism of wear debris loss as a series of laminae. However, its coefficient of friction remains low when increasing the number of cycles. On the other hand, PET presents high coefficient of friction but low wear rate abrasion with a “Lumpy transfer” process. Worn volumes are up to 10 times bigger in case of PTFE.These differences in mechanisms of film-transfer, formed by these two semi-crystalline polymers when sliding against titanium alloy are mainly due to their crystalline structure. DMA tests have characterized the viscoelastic behavior of both PET and PTFE with a phase change detected for PTFE within a temperature of 19°C. This fact is corroborated by DSC thermograms.

Ternary oxides have gained increasing attention due to their potential use as solid lubricants at elevated temperatures. In this work, the tribological properties of three ternary oxides - AgTaO₃, CuTaO₃, and CuTa₂O₆ - were studied using a combination of density-functional theory (DFT), molecular dynamics (MD) simulations with newly-developed empirical potential parameters, and experimental measurements. Our results show that the MD-predicted friction force follows the trend AgTaO₃ <CuTaO₃ <CuTa₂O₆, which is consistent with the experimentally-measured coefficients of friction (CoF). The wear performance from both MD and experiment exhibits the opposite trend, with CuTa₂O₆ providing the best resistance to wear. The sliding mechanisms are investigated using experimental characterization of the film composition after sliding, quantication of Ag or Cu cluster formation at the interface during the evolution of the film, and DFT energy barriers for atom migration on the material surface. Our hypothesis that the formation of metal (or metal oxide) clusters on the surface are responsible for the friction and wear behavior of these materials is consistent with all our observations.

Superduplex Stainless Steels are alloys used in the manufacturing of components for Oil and Gas production equipment. The machining of superduplex stainless steel presents a significant challenge. A built-up edge (BUE), formation is observed on the cutting tool surface, due to the intensive adhesive interaction during friction under atmospheric conditions, which drastically reduces tool life. In this work, three types of commercial coatings composed respectively of AlCrN – based, AlCrN and AlCrN/TiSiN were deposited by physical vapor deposition (PVD) on cemented carbide substrate (WC-Co) and evaluated in relation to their mechanical and tribological properties. The coatings’ mechanical properties were characterized through nano-indentation, critical load under a scratch test and roughness. The tribological performance of the coated systems was investigated by pin-on-disc method, with and without lubrication. The results obtained from tribological tests showed that the predominant wear mechanism was adhesion for all the coatings tested. It was revealed that the friction coefficient was reduced with the use of lubrication during the experiments. The results also showed that the friction coefficient and wear rate is associated to the coating chemical composition. Additionally, the AlCrN/TiSiN coating demonstrated the lowest interaction with the stainless steel resulting in the lower friction coefficient and specific wear rate.

Titanium Vanadium Nitride (TIVN) coatings were prepared on various substrates using magnetron sputtering technique. This paper is focused on exploring the effect of nitrogen:argon (N₂:Ar) gas ratio on tribological properties of TiVN coatings. TiVN coatings displayed only (220) peak at N₂:Ar gas ratio values of 08:00 and 08:02. The evolution of (111) and (222) peaks of TiVN coatings were observed only when the N₂:Ar gas ratio values were 10:10, 08:12 and 08:32. The surface topography of TiVN coatings was studied using Scanning Electron Microscope (SEM). The evolution of well crystalline structure of TiVN coatings is observed with increase in N₂:Ar gas ratio from 08:00 to 08:32. The tribological properties of TiVN coatings were tested by pin on disc tribometer at different values of load, speed and sliding distance.