Effect of Surface Silanol Groups on Friction and Wear between Amorphous Silica Surfaces

Reactive molecular dynamics (ReaxFF) simulations are performed to explore the tribological behavior between fully hydroxylated amorphous silica (a-SiO2) surfaces as a function of surface silanol density. The results show that the interfacial friction and wear are greatly reduced by increasing surface silanol density, which originates from the suppression of the initial formation of interfacial Si-O-Si bridge bonds. Two different tribochemical reactions resulting in the formation of interfacial Si-O-Si bridge bonds are observed: i.e., one occurring between two silanol groups, which is insensitive to changes in silanol density, and the other occurring between a silanol group and a surface Si-O-Si bond, which is strongly suppressed with the increase of silanol density. We decouple the contributions of these two Si-O-Si bond formation mechanisms to the observed tribological behavior and find that the latter formation mechanism plays a dominant role. Furthermore, the changes in the geometry and structure of fully hydroxylated a-SiO2 surface caused by the increased surface silanol groups also play an important role in the tribochemical reactions and the tribological performance of the a-SiO2/a-SiO2 system. This work provides a deeper insight into the effect of surface silanol groups on the tribological behaviors of silicon-based materials.