Influence of bias patterns on the tribological properties of highly hydrogenated PVD a-C:H films

In this study, different bias strategies were designed for the investigation of the influence of the bias patterns on the mechanical and tribological properties of highly hydrogenated a-C:H films. A series of a-C:H films were made using reactive magnetron sputtering, under several deliberately designed bias patterns. Film hardness and elastic modulus, structure and tribological properties were evaluated using nanoindentation, Raman spectroscopy and rotational ball-on-disk tribological tests. In particular, a quasi in-situ X-ray photoelectron spectroscopy (XPS) technique (vacuum tribo-test to XPS analysis without breaking vacuum) was used in this study. Results showed that low hardness and high CoF could co-exist with low wear rate for a-C:H films. Moreover, no clear relationship between the degree of graphitisation of the transfer film and the tribological properties of a-C:H films could be established. In addition, increased film hardness does not inevitably lead to better wear resistance. It may in fact lead to a much higher wear rate of the counterpart material, hence reducing the life of the tribo-pair as a combined system. The study also provides insights on the effects of biasing strategy on the tribological properties of highly hydrogenated a-C:H films, which would further improve the design quality of preparation parameters of such films.