Phonon energy dissipation in friction between graphene/graphene interface

The theory of phononic friction attributes that the multiphonon processes are the main cause of the mechanical energy dissipation in a wear-free friction process. Unfortunately, it is still impossible to set up a direct relationship between the phonons and the frictional force. In this study, a classical molecular dynamics simulation model is used to mimic a piece of graphene sliding over a supported graphene substrate. It is found that the lifetime of some phonons, especially the modes around the Γ point of the first Brillouin zone, gradually decreases with the increase of the sliding velocity. A phonon lifetime-based model is proposed to explain the variation of the frictional force as a function of the sliding velocity, i.e., the shorter phonon lifetime corresponding to a higher friction force under the same temperature. This model is consistent with the traditional Prandtl-Tomlinson model at a low sliding velocity range, which predicts that the friction force increases logarithmically with the sliding velocity. Once the sliding velocity exceeds a critical value, the lifetime of the excited phonons is far longer than the time for the tip sweeping a lattice constant. In this case, the excited phonons do not have enough time to dissipate the mechanical energy, which leads to the reduced friction force with the increase of the sliding velocity.