Tribological behaviour of metal-DLC nanocomposite coatings: The critical role of tribofilm build-up

The tribological behavior of metal-diamond-like carbon (me-DLC) nanocomposite coatings was studied. The structure of the coatings exhibited pure metallic clusters embedded in a carbonaceous matrix. The size and distribution of clusters in such structure varied significantly with metal sputtering rate and DLC deposition rate, leading to various mechanical and electrical properties. For a soft metal like copper, wear increased with increased metal content, leading for the highest amounts to drastic wear, due to the low hardness of the coating. For coatings with sufficient hardness, wear of the coated plate was minimized, and friction coefficients were always < 0.3, significantly lower than for pure metal. For a hard metal like tungsten, the wear was mild for the coating even at high metal contents. However, the friction was increasing with increased metal content, reaching values > 0.3, with large wear of the ball. In the case of Ir, two types of tribological behavior were observed. The evolution of the friction was strongly influenced by the nature of the metal, but the lowest friction and electrical contact resistance were always observed when a metallic tribofilm was formed on the steel ball. In the case of Ir-DLC, the brittleness of this metal would account for the observed instability of its metallic tribofilm. This is an abstract of a paper presented at the Proceedings of World Tribology Congress (Kyoto, Japan 9/6-11/2009).