Fretting wear analysis of TiC/VC multilayered hard coatings: experiments and modelling approaches

TiC/VC multilayered hard coating were obtained using an original two-step hard coating process. X-ray analysis, GDOES chemical profiles and micro-hardness measurements have been performed to characterise the crystalline structure, the multilayer stacking and coatings hardness respectively as a function of the sub-layer thickness. It has been shown that although both the TiC and VC carbide phases are maintained, a decrease of the sublayer thickness promotes inter-diffusion phenomena which progressively erase the layered 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 driving force and the yield stress theory, these results confirm that the micro-hardness evolution is a function of the carbide’s grain size. Wear and friction properties have been studied under gross slip fretting conditions. A “composite” wear model has been developed, in which the diffusion interlayers between the adjacent sublayers have been taken into account. Reliable wear rates as a function of multilayer period have been deduced. However, the evolution of the friction coefficient has been shown to be less easily predictable due to the smoothing effect of the third body. Finally, the reverse evolution between wear resistance and micro-hardness of the TiC/VC multilayers as a function of the multilayer period is discussed and, in conclusion, the idleness of any prediction of the tribological response from the plain micro-hardness measurement is outlined in the work.