Modification of a-C:H films via nitrogen and silicon doping: The way to the superlubricity in moisture atmosphere

Both nitrogen and silicon-doped carbon (a-CNx:H and a-CSi:H) films show superlubrcity properties in distinguished sliding conditions, but most running machines are required to serve in humidity air atmosphere. Therefore, we aim to explore N and Si doping effects on tribology of a-C:H films and reveal the basic factors of superlubricity in open atmosphere with a humidity of 41%. In present work, three kinds of films (a-C:H, a-CNx:H and the a-CSi:H films) were prepared on n-Si(100) substrates by plasma-enhanced chemical vapor deposition (PECVD). The thickness, element distribution, bonding topolograph and nanostructure of all films before and after friction test, including wear debris, tracks and debris, were detected by means of XPS, Raman, HRTEM and SEM, etc.. Compared with hydrogenated carbon film (a-C:H), the highest COF (0.089) and wear rate (3.61 × 10−7 mm3/Nm) of a-CNx:H film are attributed to forming Fe3C, Fe2O3 and Cr2O3 on the slide surface due to the chemical reaction with moisture atmosphere, which caused by the emission of N element via the broken CN structure that induced more activity dangling bonds. However, the silicon-doped carbon film (a-CSi:H) exhibited extremely low COF (0.007) and extremely low wear rate (4.76 × 10−8 mm3/Nm), which can be ascribed to the Si which is more reactive with moisture atmosphere, prohibiting the oxidization of carbon matrix and helping to forming more curved graphene structures to low the friction force.