A tribological study of PVD coatings with carbon-rich outer layers

Multilayered PVD coatings, about 2 mu m thick, based on nitrides and carbides of titanium or chromium have been deposited on high speed steel with a multi-are PVD equipment. In particular, the effect of the carbon content in the top layer of the coatings on the friction has been evaluated. The purpose of the coating structures was to obtain low friction, while maintaining high hardness and wear resistance and good substrate adhesion. The coatings have been characterised regarding hardness, adhesion to the substrate, friction properties against steel and residual film stresses. The structure and composition were evaluated with Raman, X-ray and glow discharge spectroscopy techniques. The coatings have also been studied with both scanning electron and optical microscopy. It was found that an increased carbon content consistently reduced the friction against steel. Nitrogen in the outer coating layers increased the friction in all cases. In particular, chromium carbide layers gave very low friction values. (C) 1997 Elsevier Science S.A.