Friction of Self-Lubricating W-S-C Sputtered Coatings Sliding Under Increasing Load

Abstract

W-S-C films were deposited by non-reactive magnetron sputtering from a carbon target with several pellets of WS2 incrusted in the zone of the preferential erosion. The number of the pellets was changed to modify the carbon content in the films, which varied from 26 up to 70 at.-%. Alloying W-S films with carbon led to a substantial increase in the hardness in the range of 4-10 GPa; the maximum hardness was obtained for the coatings with carbon contents close to 40 at.-%. XRD diffraction patterns showed that there was a loss of crystallinity with the increase of the carbon content in the film. C-contents in the range (37-66 at.-%) were selected for sliding tests (pin-on-disk, 100Cr6 steel ball as a counterpart) carried out in humid air under increasing contact load. The friction coefficient was observed to decrease continuously with increasing load. The wear tracks and wear debris were also analyzed by Raman spectroscopy to understand the structural transformations induced by the increasing load. The friction results were compared with existing models for pure transition metal dichalcogenides (TMD), and it could be concluded that the friction mechanisms of W-S-C coatings fundamentally differ from those of pure TMD.