Influence of surface topography of arc-deposited TiN and sputter-deposited WC/C coatings on the initial material transfer tendency and friction characteristics under dry sliding contact conditions

The influence of surface topography of PVD coatings on the initial material transfer tendency and friction characteristics in dry sliding contact conditions has been investigated. A modified scratch test was used to evaluate the material transfer tendency between ball bearing steel and two different PVD coatings, TiN and WC/C, under dry sliding contact conditions. Post test characterisation of the contact surfaces was performed using SEM/EDS and AES in order to map the initiation points and mechanisms for material transfer. The results show that the resulting topography of the PVD coated surfaces is strongly dependent on both the substrate material topography and the topography induced by the coating deposition process used. In sliding contact with a softer surface the coating topography results in a significant material pick-up tendency of the PVD coated surfaces. The material pick-up is mainly controlled by the abrasive action of hard coating asperities and as a result a polishing post treatment of the as-deposited PVD coatings significantly reduces the material pick-up tendency. For the WC/C coating, showing intrinsic low friction properties, the post treatment inhibits the material pick-up and results in a low and stable friction coefficient (μ ~ 0.1). For the TiN coating, that lacks intrinsic low friction properties, the post treatment reduces the material pick-up tendency but has no significant influence on the friction characteristics. This is mainly due to the presence of metallic Ti originating from the macroparticles on the TiN coating which results in a reactive surface that promotes a strong adhesion between the mating surfaces.