Effects of the normal load on the excited phonons in atomic friction

Abstract

Although the quantitative relationship between the normal load and the induced friction force has been established from the phenomenological laws of friction, it does not answer the question of how mechanical energy is dissipated into heat by exciting phonons. In this study, the effects of normal load on the friction force of a relatively sliding graphene film are investigated by molecular dynamics simulations. The results show that the excited phonon modes couple with the resonant frequency of the entire frictional system, and the enhancement of the normal load is equivalent to the increase of the resonant frequency of the frictional system. It is also found that the relative intensity of the resonant peaks in the vibrational density of states is a key factor affecting the friction force, which can explain the variation of friction force with the normal load. Moreover, under a certain normal load, the friction force can reach its maximum value when the washboard frequency equals the resonant frequency of the frictional system. Our work establishes the relationship between the normal load and the frictional force from the phonon level, which provides a method for regulating atomic friction and energy dissipation by considering both washboard frequency and contact resonant frequency.