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.

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To examine phonon transport during the friction process of commensurate–incommensurate transition, the vibrational density of states of contact surfaces is calculated based on molecular dynamics simulations. The results indicate that, compared with the static state, the relative sliding of the contact surfaces causes a blue shift in the interfacial phonon spectrum in or close to commensurate contact, whereas the contrast of the phonon spectrum in incommensurate contact is almost indiscernible. Further findings suggest that the cause of friction can be attributed to the excitation of new in-plane acoustic modes, which provide the most efficient energy dissipation channels in the friction process. In addition, when the tip and the substrate are subjected to a same biaxial compressive/tensile strain, fewer new acoustic modes are excited than in the no strain case. Thus, the friction can be controlled by applying in-plane strain even in commensurate contact. The contribution of the excited acoustic modes to friction at various frequency bands is also calculated, which provides theoretical guidance for controlling friction by adjusting excitation phonon modes.

Friction represents a major energy dissipation mode, yet the atomistic mechanism of how friction converts mechanical motion into heat remains elusive. It has been suggested that excess phonons are mainly excited at the washboard frequency, the fundamental frequency at which relative motion excites the interface atoms, and the subsequent thermalization of these nonequilibrium phonons completes the energy dissipation process. Through combined atomic force microscopy measurements and atomistic modeling, here we show that the nonlinear interactions between a sliding tip and the substrate can generate excess phonons at not only the washboard frequency but also its harmonics. These nonequilibrium phonons can induce resonant vibration of the tip and lead to multiple peaks in the friction force as the tip sliding velocity ramps up. These observations disclose previously unrecognized energy dissipation channels associated with tip vibration and provide insights into engineering friction force through adjusting the resonant frequency of the tip–substrate system.

Atomistic mechanisms of frictional energy dissipation have attracted significant attention. However, the dynamics of phonon excitation and dissipation remain elusive for many friction processes. Through systematic fast Fourier transform analyses of the frictional signals as a silicon tip sliding over a graphite surface at different angles and velocities, we experimentally demonstrate that friction mainly excites non-equilibrium phonons at the washboard frequency and its harmonics. Using molecular dynamics simulations, we further disclose the phononic origin of structural lubrication, i.e., the drastic reduction of friction force as the contact angle between two commensurate surfaces changes. In commensurate contacting states, friction excites a large amount of phonons at the washboard frequency and many orders of its harmonics that perfectly match each other in the sliding tip and substrate, while for incommensurate cases, only limited phonons are generated at mismatched washboard frequencies and few low order harmonics in the tip and substrate.