Contrasting Roles of Water at Sliding Interfaces between Silicon-Based Materials: First-Principles Molecular Dynamics Sliding Simulations

It is known that the wear of silicon-based materials is due to the tribochemical reaction with water at the sliding interface, but the detailed mechanisms remain under debate. In this study, we used a first-principles molecular dynamics method to investigate the tribochemical wear mechanism. When a small amount of water was present at the sliding interface, the formation of interfacial bridge bonds connecting the two surfaces was observed. These bonds transmitted shear force to the surfaces that induced strain therein. The strained surface Si-O bonds subsequently reacted with water (Si-O-Si + H₂O → Si-OH + Si-OH), that is, the hydrolysis reaction occurred. Because the hydrolysis reaction resulted in dissociation of the surface Si-O bonds, water promoted tribochemical wear. However, when a large amount of water was present, it separated the two surfaces. The water thereby suppressed the formation of interfacial bridge bonds and in turn the hydrolysis of Si-O bonds and thus tribochemical wear. Our results indicate that water could either promote or suppress tribochemical wear, depending on how much was present. We suggest that the previously reported humidity dependence of the tribochemical wear of silicon-based materials can be explained in terms of these contrasting roles of water.