Effects of Modulation Ratio on Microstructure and Tribological Properties of WS_x/a-C Multilayer Films

WS_x/a-C multilayer films with different modulation ratios (modulation period ~20 nm) were prepared on monocrystalline silicon substrate by magnetron sputtering at 200 ℃. The morphology, microstructure and composition of the films were characterized by using scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffractometer and X-ray photoelectron spectroscopy. The mechanical properties and tribological behavior in air of the films were investigated by using nano-indentation tester, residual stress tester, scratch tester and ball-on-disc tribotester. The results show that the WS_x/a-C multilayer films were of smooth and compact structure and the S/W ratio fluctuated in the range of 0.92~0.97. The WS_x sublayer was defined as crystallite and the WC phase was found at the interface between WS_x sublayer and a-C sublayer. As increasing the modulation ratio, the hardness of the film increased from 7.8 GPa to 9.0 GPa, the compressive stress in the film decreased first and then increased. In addition, the adhesion strength of film to the substrate decreased monotonically, the friction coefficient increased from 0.18 to 0.29 and the wear rate presented a significant increase. The film with modulation ratio of 1∶39 was of the best tribological performance and its wear rate was about 6.1×10^–15 m³/(N·m).