On the tribological performance of magnetron sputtered W-S-C coatings with conventional and graded composition

Abstract

Tungsten‑sulfur‑carbon coatings are coatings that have excellent lubrication capability, with an ability to adapt to different operating environments. Their performance as a function of the carbon content is still not clearly understood, and the usage of more complex coating architectures such as those with graded composition across the thickness is not still reported in the literature. In this paper, the deposition of three different tungsten‑sulfur‑carbon (W-S-C) coatings. Two coatings have a constant chemical composition of ∼35 at. % and ∼47 at. % of carbon and one coating has a graded composition with the bottom layers having a carbon content of ∼47 at. % and a gradual decrease in the carbon content to 0 at. % for the top-most layer. We assessed the morphological, structural and mechano-tribological properties of the coatings. The techniques used include scanning electron microscopy (SEM), X-ray diffraction, nanoindentation and reciprocating ball-on-disk testing, among others. The results indicate that the optimal tribological response, in all testing conditions, can be achieved with a coating with a moderate carbon content of ∼35 at. %. Using a coating with graded chemical composition containing softer transition metal dichalcogenide (TMD) layers as top-layers can be beneficial during sliding in inert conditions.