Reactive Molecular Dynamics Simulations of Wear and Tribochemical Reactions of Diamond like Carbon Interfaces with Nanoscale Asperities under H2 Gas: Implications for Solid Lubricant Coatings

Diamond like carbon (DLC) is an excellent solid lubricant coating used in various nanoscale applications such as nanoelectromechanical systems and atomic force microscopy. Reducing the wear of DLC coatings is essential for improving their durability and reliability. Generally, the wear of DLC is strongly dependent on the working environment because of the tribochemical reactions with environmental gases; however, theoretical guidelines for wear reduction by controlling environments have not been established yet because the wear mechanisms arise from complex atomic-scale wear processes and tribochemical reactions. Here, we successfully use reactive molecular dynamics simulations to reveal the relations between wear and tribochemical reactions of DLC in a hydrogen gas environment, contributing to the wear reduction as a theoretical guideline. At the DLC friction interface, we find two different types of tribochemical reaction: one accelerates, whereas another reduces the wear. The reaction that accelerates wear is the emission of hydrocarbon molecules from the DLC surface. Hydrocarbon emission gradually depletes surface hydrogen terminations, accelerating surface adhesion and finally leading to severe adhesion-induced mechanical wear. In contrast, the reaction that reduces wear is the dissociative adsorption of hydrogen molecules from the environment onto the DLC surface. Hydrogen gas adsorption replenishes the depleted hydrogen terminations and suppresses surface adhesion, which directly reduces the mechanical wear of DLC. The reactions of losing and replenishing surface hydrogen terminations accelerate and suppress the wear of DLC, respectively. This work would contribute to establishing a guideline for wear reduction by controlling the tribochemical reactions.