Influence of hardness and roughness on the tribological performance of TiC/a-C nanocomposite coatings

The influence of the hardness of counterface materials on the tribological behavior of TiC/a-C nanocomposite coatings exhibiting various surface roughness was examined by using a ball on a disc configuration in humid air. While sliding against 100Cr6 steel balls, the steady state coefficient of friction (CoF) increased with increasing surface roughness of the coatings, but the CoF remained unaffected in the case of comparatively harder Si3N4 balls. It is shown that the ratio of hardness of the counterpart material and of the nanocomposite coating is crucial in determining the influence of roughness on the tribological properties of these coatings. In the case of rough coatings, the hardness ratio of the ball/coating tribo-pair determines mainly the wear behavior of the ball counterpart, which further influences the transfer film formation and hence the CoF. Detailed analyses of the transfer film, sliding surfaces of the ball counterparts, and wear surfaces of the coatings by optical microscopy, confocal microscopy, and atomic force microscopy are presented. The roughness evolution and the wear rates of the smooth and rough coatings sliding against steel and Si3N4 balls are discussed.