Tribological behavior of unlubricated sliding between a steel ball and Si-DLC deposited by ultra-high-speed coating employing an MVP method

Recently, with increasing demands for energy saving via friction reduction and lifetime extension via wear reduction, the applications of diamond-like carbon (DLC) are spreading gradually and steadily. It should be noted that the typical coating speed of DLC with such a conventional plasma chemical vapor deposition (CVD) is not very high, ~ 1 μm/h, due to the employment of low-density plasma. In our previous work, we proposed that the plasma CVD with MVP (Microwave-sheath Voltage combination Plasma) is capable of coating a Si-containing DLC (Si-DLC) film at larger than 100 μm/h. In this work, for achieving the design principle of Si-DLC by MVP, we studied the effects of process gas, which is a mixture of TMS, Ar, and one hydrocarbon gas, on the relationship between the film structure and the mechanical properties of Si-DLC films prepared by MVP. Si-DLC films were deposited by MVP with a hydrocarbon gas, CH4 or C2H2, where the TMS (tetramethylsilane)/hydrocarbon gas ratio was changed. The structure of DLC was estimated by Raman spectroscopy. The atomic composition of the films was evaluated by XPS for C, O, and Si, and RBS-ERDA for H/C. Friction tests were conducted using a ball-on-disk apparatus under dry condition. Si concentration in the films increased linearly with an increasing TMS/hydrocarbon ratio for both hydrocarbon gases, being the determining factor for the ID/IG ratio and SiC concentration of Si-DLC. In other words, we can determine the ID/IG ratio and SiC bond concentration in Si-DLC by controlling the TMS/hydrocarbon ratio. In addition, much higher deposition speed and higher H concentration are obtained by MVP using C2H2 than CH4. These findings, or the effect of gas source on the film structure of Si-DLC by MVP, would be an important design principle in designing Si-DLC for desired film structure and hardness, and better tribological performance.