Molecular dynamics simulations on atomic friction between self-assembled monolayers: Commensurate and incommensurate sliding

Atomic friction between self-assembled monolayers (SAMs) on Au(1 1 1) has been studied through molecular dynamics simulations, with emphasis on the comparison of the performances of commensurate and incommensurate SAMs in relative sliding. Results show that the shear stress on commensurate SAMs exhibits a clean periodic pattern, manifesting the atomic stick-slip friction, while random fluctuations and a much lower average value of the shear stress are observed for incommensurate sliding. The different frictional behavior can be traced to the difference in molecule movements, especially in the collective nature of the motion. In commensurate sliding, all molecules move synchronously in each period and phase, but they swing randomly and independently for incommensurate monolayers. Simulations provide clear evidence that under the same working conditions friction forces on commensurate SAMs are always higher than those in incommensurate cases. The results also show a linear dependence of shear stress on normal pressure and logarithmic dependence on sliding velocity.