Development of a friction energy capacity approach to predict the surface coating endurance under complex oscillating sliding conditions

In the case of surface coatings application it is crucial to establish when the substrate is reached to prevent catastrophic consequences. In this study, a model based on local dissipated energy is developed and related to the friction process. Indeed, the friction dissipated energy is a unique parameter that takes into account the major loading variables which are the pressure, sliding distance and the friction coefficient. To illustrate the approach a sphere/plane (Alumina/TiC) contact is studied under gross slip fretting regime. Considering the contact area extension, the wear depth evolution can be predicted from the cumulated dissipated energy density. Nevertheless, some difference is observed between the predicted and detected surface coating endurance. This has been explained by a coating spalling phenomenon observed below a critical residual coating thickness. Introducing an effective wear coating parameter, the coating endurance is better quantified and finally an effective energy density threshold, associated to a friction energy capacity approach, is introduced to rationalize the coating endurance prediction. The surface treatment lifetime is then simply deduced from an energy ratio between this specific energy capacity and a mean energy density dissipated per fretting cycle. The stability of this approach has been validated under constant and variable sliding conditions and illustrated through an Energy Density–Coating Endurance char

Behaviour of shot peening combined with WC-Co HVOF coating under complex fretting wear and fretting fatigue loading conditions

This study investigated the fretting and fretting fatigue performance of tungsten carbide–cobalt (WC–Co) HVOF spray coating systems. Fretting wear and fretting fatigue tests of specimens with shot peening and WC–Co coatings on 30NiCrMo substrates were conducted. The WC-Co coating presents very good wear resistance and decreases by more than 9 times the energy wear coefficient (α) under fretting conditions. The tested coating reduces crack nucleation under both fretting and fretting fatigue studied situations. Finally the crack arrest conditions are evaluated by the combined fretting and fretting fatigue investigation. It is shown and explained how and why this combined surface treatment (shot peening and WC–Co) presents a very good compromise against wear and cracking fretting damage

Development of a Wöhler-like approach to quantify the Ti(CxNy) coatings durability under oscillating sliding conditions

The selection of a proper material for the particular engineering application is a complex problem, as different materials offer unique properties and it is not possible to gather all useful characteristics in a single one. Hence, employment of different surface treatment processes is a widely used alternative solution. In many industrial applications, coating failure may be conducive to catastrophic consequences. Thus, to prevent the component damage it is essential to establish the coating endurance and indicate the safe running time of coated system. To this study PVD TiC, TiN and TiCN hard coatings have been selected and tested against polycrystalline alumina smooth ball. The series of fretting tests with reciprocating sliding at the frequency 5Hz have been carried out under 50-150N normal loads and under wide rage of constant as well as variable displacement amplitudes from 50µm to 200µm at a constant value of relative humidity of 50% at 296K temperature. To quantify the loss of material a dissipated energy approach has been applied where the wear depth evolution is referred to the cumulative density of friction work dissipated during the test. Different dominant damage mechanisms have been indicated for the investigated hard coatings, which is debris formation and ejection in case of TiC coating and progressive wear accelerated by cracking phenomena in case of TiN and TiCN coatings. Energy-Wöhler wear chart has been introduced, in which the critical 1 dissipated energy density corresponds to the moment when the substrate is reached after a given number of fretting cycles. Two different methods to determine the critical dissipated energy density are introduced and compared. The Energy-Wöhler approach has been employed not only to compare the global endurance of the investigated systems but also to compare the intrinsic wear properties of the coatings. It has been shown that the fretting wear process is accelerated by the stress-controlled spalling phenomenon below a critical residual thickness and a severe decohesion mechanism is activated. Finally the applicability of the investigated method to other coated systems subjected to wear under sliding conditions is discussed and analyzed. The perspectives of this new approach are elucidated

On the use of statistical analysis for tribological evaluation of a solid lubricant

La dégradation des contacts tribologiques sous une sollicitation de fretting est un phénomène complexe lié à l interaction entre deux corps sous une faible amplitude de débattement, ce qui limite la durée de vie des pièces de manière significative. La lubrification solide est de plus en plus utilisée dans les applications tribologiques pour réduire les frottements et protéger la surface du substrat contre l'usure par fretting. La performance du revêtement dépend de nombreux facteurs tels que les conditions de sollicitation, les propriétés du substrat, du contre-corps et du revêtement et l'adhérence entre le revêtement et le substrat. Les objectifs de cette thèse sont de discuter le frottement et le comportement en usure du revêtement sous différentes conditions et d'analyser l'effet des paramètres d essai sur le coefficient de frottement et la durée de vie du revêtement, ce qui peut être effectivement utile pour l'évaluation de la qualité des revêtements. Les tests de fretting sont menés pour comprendre les relations entre le comportement tribologique d'un vernis à base de MoS2 et les conditions d essai. Le revêtement est étudié pour différentes valeurs de force de contact, amplitude de déplacement, configuration de contact, position du revêtement, nature du substrat et épaisseur. L'effet de chaque facteur est évalué par l'analyse de variance. L'analyse de régression est utilisée pour prévoir la performance du revêtement. L'observation de l'évolution de la trace d usure est réalisée pour expliquer l importance des facteurs et les coefficients dans les équations de prédiction. En outre, l'analyse de fiabilité est utilisée pour décrire et prévoir le taux de survie du revêtement sous certaines conditions d essai. Enfin, un modèle linéaire de durée de vie est évalué afin de prendre en compte l'amplitude de déplacement variable.Fretting wear is considered as a complex wear phenomenon related to interaction between two sliding bodies under very low displacement amplitude, which limits the lifetime of elements significantly. Solid lubricant is more and more applied in tribological applications to reduce friction and protect the substrate surface from fretting wear. The performance of coating depends on many factors such as running conditions, properties of substrate, counterbody and coating, and adhesion between coating and substrate. The objectives of this thesis are to discuss the friction and wear behavior of the coating under different running conditions, and to analyze the effect of test parameters on the friction coefficient and lifetime of the coating, which can be effectively helpful for the evaluation of quality of the coating. Fretting experiments are carried out to understand the relationships between the tribological behavior of a MoS2 based varnish coating and running conditions. The coating is investigated under different contact forces, different displacement amplitudes, different contact configurations, different coating positions, different substrates and different thicknesses. The rank of effect of factors is evaluated by the analysis of variance. Regression analysis is used to predict the performance of coatings under certain running conditions. The observation of evolution of wear scar is applied to explain the rank of factor and coefficients in the prediction equations. Furthermore, the reliability analysis is used to describe and predict the survival rate of coatings under certain running conditions. Finally, a linear model for lifetime is assessed in order to take into account variable displacement amplitude.LYON-Ecole Centrale (690812301) / SudocSudocFranceF

Evolution of damage in MoS2-based dry film lubricants (DFLs) in fretting wear – the effect of DFL thickness and contact geometry

Dry film lubricant (DFL) coatings are widely used to reduce coefficients of friction and damage in highly loaded contacts. This work investigates the behaviour of a commercially available MoS2-based DFL in a cylinder-on-flat contact geometry with a fretting amplitude of 300 µm. Cylinders with radii of 15 mm, 80 mm and 160 mm along with DFL thicknesses between 12 µm and 86 µm were utilized. Three stages in the lifetime of the system were investigated; in the first stage, the DFL wore with a volumetric rate that was proportional to the number of cycles (under a given load), until the thickness of the DFL was reached. In the second stage, a thin low friction surface layer existed, with its durability being proportional to the contact pressure. In the third stage, metal-metal contact resulted in short-term instabilities in the coefficient of friction and the DFL system was said to have failed

Wear resistant multilayer nanocomposite WC1−x/C coating on Ti–6Al–4V titanium alloy

A significant improvement of tribological properties on Ti–6Al–4V has been achieved by developed in this study multilayer treatment method for the titanium alloys. This treatment consists of an intermediate 2 μm thick TiCxNy layer which has been deposited by the reactive arc evaporation onto a diffusion hardened material with interstitial O or N atoms by glow discharge plasma in the atmosphere of Ar+O2 or Ar+N2. Subsequently, an external 0.3 μm thin nanocomposite carbon-based WC1−x/C coating has been deposited by a reactive magnetron sputtering of graphite and tungsten targets. The morphology, microstructure, chemical and phase compositions of the substrate material after treatment and coating deposition have been investigated with use of AFM, SEM, EDX, XRD, 3D profilometry and followed by tribological investigation of wear and friction analysis. An increase of hardness in the diffusion treated near-surface zone of the Ti–6Al–4V substrate has been achieved. In addition, a good adhesion between the intermediate gradient TiCxNy coating and the Ti–6Al–4V substrate as well as with the external nanocomposite coating has been obtained. Significant increase in wear resistance of up to 94% when compared to uncoated Ti–6Al–4V was reported. The proposed multilayer system deposited on the Ti–6Al–4V substrate is a promising method to significantly increase wear resistance of titanium alloys

The variable nature of cognitive control in a university sample of young adult drinkers

The current study investigates the effect of task-irrelevant alcohol distractors on cognitive control and its interaction with heavy/light drinking in a group of young adult drinkers. It was hypothesised that alcohol distractors would result in a reduction of proactive control (reduced conflict adaptation) especially in heavy drinkers. 60 participants took part in a face-word version of the Stroop task preceded by an alcohol or neutral image. Light drinkers only showed a congruency effect which indicated a greater level of proactive control. Heavy drinkers showed a greater level of reactive control in which the conflict adaptation effect occurred with neutral images but not with alcohol images. Possible explanations are discussed