Fretting wear analysis of TiC/VC carbides multilayers: experiments and modelling approaches

TiC-VC multilayered hard coating were obtained using an original two step hard coating process. Xray analysis, micro-hardness measurements and GDMS’s chemical profiles have been conducted to characterise, respectively, crystallography, hardness and multilayer structure as a function of the sub-layer thickness. It shows that although well TiC and VC carbide textures are maintained, a decrease of the sublayer thickness promotes inter-diffusion phenomena which progressively erase the alternated structure. The micro-hardness variation as a function of sublayer thickness displays a parabolic evolution with maximum hardness for a critical thickness around 50 nm. Consistent with the dislocation moving force theories, these results confirm that the micro-hardness evolution is function of the carbide grain size. Wear and friction properties have been studied under gross slip fretting conditions. A “composite” wear model, which considers the diffusion inter-layer, has been developed. Unlike classical rules of mixtures, the modulation dependence of wear and friction is here considered. Reliable wear rates as a function the modulation thickness can be predicted. The friction evolution is less easily predicted due to the smoothing effect of third body. Finally, the opposite evolution between wear resistance and micro-hardness is discussed, outlining the necessity of avoiding any direct prediction of a given tribological response from a plain micro-hardness measurement.