Tribological aspects related to the morphology of PVD hard coatings

In this paper the tribological behavior of different PVD hard coatings (TiN, CrN, TiAlN), prepared by thermionic arc ion plating and unbalanced magnetron sputtering, were investigated during the running-in period of a sliding test. We will present some experimental results that show why growth defects and other coating-surface irregularities must be considered in all the tribological tests of such coatings. All surface asperities, particularity the nodular defects, form the first contact spots between the sliding surfaces. Due to the high contact pressure at these spots they collapse. Thus nodular defects represent the primary source of abrasive particles in the sliding contact. These nodular defects and other protrusions are also the anchoring points for the counter-body material transfer. In this work the running-in behavior of different hard coatings was evaluated with respect to the type of substrate material, the substrate surface topography and the type of surrounding environment. The coatings with different topographies show different friction development during the running-in period and also a different level of friction in the steady-state period. We showed that post-polishing of the coating surface has a significant influence on both the coating wear and friction. The composition of tribofilm and roll-like debris formed in the wear track during sliding test were analyzed by nano-AES spectrometry and we found that both have similar composition. We also found that the highest friction coefficient and wear rate of all coatings were obtained in an oxygen atmosphere, while they are the lowest in nitrogen.