The influence of the addition of C and N on the wear behaviour of W–S–C/N coatings

WSx can be used as an alternative to MoSx as a solid self-lubricant material for applications at temperatures greater than room temperature, owing to its greater thermal stability. Moreover, good mechanical properties have been obtained when W-based sputtered coatings are alloyed with C and/or N. The aim of this study was to produce coatings of a W–S–C/N system combining good hardness with a low friction coefficient. The coatings show columnar cross-section morphology becoming featureless with increasing C/N content. The introduction of C/N into the W–S film leads to the deposition of amorphous structures. The W–S–C/N coatings are deficient in sulfur. The hardness of the coatings containing C/N reaches 5.5 GPa vs. 0.5 GPa for the W–S film. The wear coefficient calculated during reciprocal ball-on-disk testing, decreases significantly with the addition of C/N and, in some cases, the friction coefficient is also lower (<0.1) than for W–S films. However, on pin-on-disk testing W–S films behave better than W–S–C/N films. The analysis of the worn tracks showed that this behaviour can be attributed to a lack of adhesion and cohesion of the coatings, as is confirmed by the low critical loads (5–6 N) obtained by scratch-testing.