Modification of magnetron sputter deposition of nc-WC/a-C(:H) coatings with an additional RF discharge

Abstract It is known that the degree of ionization of the coating precursors is one of the key factors in a control of the structure and properties of sputter deposited carbon-based coatings. Numerous modifications of a conventional magnetron sputtering technique were elaborated in order to increase the degree of plasma ionization. The most effective of them were based on an application of an additional source of plasma excitation, independent of magnetron discharge. This paper presents results of the investigations of nc-WC/a-C(:H) coatings, deposited by means of magnetron sputtering of graphite and tungsten targets, enhanced by an additional radio-frequency discharge of a maximum power of 150 W, limited to a volume directly adjacent to the specimens. In order to assess thickness, surface morphology and chemical composition of the coatings, scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) techniques were used, while Raman Shift Spectroscopy (RSS) together with XPS was applied to determine the types of bonds between carbon atoms. Tribological properties of the coatings were investigated using a ‘pin-on-disc’ method, with the friction couple comprising bearing steel counterbody. It has been found that additional RF discharge has a significant effect on the type of bonds of carbon atoms in the coating. In the case of non‑hydrogenated nc-WC/a-C coatings, an introduction of additional RF discharge caused an increase of a contribution of sp3 hybridized carbon atoms by 32%, as well as an increase of friction coefficient. The highest share of sp3 hybridization was obtained in the case of hydrogenated nc-WC/a-C:H coatings deposited in Ar + CH4 atmosphere with an enhancement of additional RF discharge maintained at 150 W of power. These conditions brought about a decreased friction coefficient (to a value of 0.05) and a decreased wear rate (to an approximate value of 10−17 m3 N−1 m−1).