Alternative Friction Mechanism for Amorphous Carbon Films Sliding against Alumina

Amorphous carbon films with low friction and wear become a hotspot of research and engineering application for meeting the requirements of energy savings and environmental benefits. However, the comprehensive effect of intrinsic microstructure, sp2/sp3 bonding ratio and hydrogen content, on the friction and wear of amorphous carbon film remains poorly understood. Here, basing on the experiments and first-principles calculations, the underlying mechanism between sp2 carbon lubricating and hydrogen passivation toward low friction is proposed. For the films with high sp2 carbon content, the friction coefficient increases with the hydrogen content, which demonstrates that the sp2 carbon acts as a dominant lubricating agent for low friction in vacuum. On the other hand, the decreased friction coefficient with high hydrogen content increasing is mainly attributed to hydrogen passivation. The studies will allow for significant insight into the friction mechanism in tribological contacts and provide guidance on designing high-performance tribo-systems for engineering applications