Reexamination of Damping in Sliding Friction.

Friction is responsible for about one-third of the primary energy consumption in the world. So far, a thorough atomistic understanding of the frictional energy dissipation mechanisms is still lacking. The Amontons' law states that kinetic friction is independent of the sliding velocity while the Prandtl-Tomlinson model suggests that damping is proportional to the relative sliding velocity between two contacting objects. Through careful analysis of the energy dissipation process in atomic force microscopy measurements, here we propose that damping force is proportional to the tip oscillation speed induced by friction. It is shown that a physically well-founded damping term can better reproduce the multiple peaks in the velocity-dependent friction force observed in both experiments and molecular dynamics simulations. Importantly, the analysis gives a clear physical picture of the dynamics of energy dissipation in different friction phases, which provides insight into long-standing puzzles in sliding friction, such as velocity weakening and spring-stiffness-dependent friction.

Resonance in Atomic-Scale Sliding Friction.

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.

Phonon dissipation in friction with commensurate-incommensurate transition between graphene membranes.

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.