Integration of contact size dependence and thermal activation in atomic friction

Nano-asperity friction force has been found to depend on both the sliding velocity and the contact size. The former is a consequence of stress-assisted, thermally activated process, which is often explained by the one-degree-of-freedom Tomlinson model that assumes a virtual point contact sliding over a periodic tip–surface interaction potential. However, this model cannot provide an accurate description of the rate-determining process when coupled with the contact size dependence. In this synergistic experimental/modeling study, atomic friction measurements were conducted on cleaved mica and HOPG surfaces with varying normal force and sliding velocity, and the spatially-nonuniform saddle-point configurations were calculated from a Peierls framework. The agreement between experiments and theory suggests that the detailed characteristics of the interface slip field govern the atomic friction.