Connection Between Sliding Friction And Phonon Lifetimes: Thermostat-Induced Thermolubricity Effects In Molecular Dynamics Simulations

A typical nanotribology simulation setup is the semi-infinite substrate, featuring a sliding bead on top and with the lower substrate layers thermostatted to control temperature. A challenge is dealing with phonons that backreflect from the substrate lower boundary, as these will artificially reduce the friction F-fr acting on the sliding bead. One proposed solution is to use a Langevin thermostat, operating at temperature T-lan, and with the corresponding damping parameter gamma, optimally tuned such that F-fr is maximized [Benassi et al., Phys. Rev. B 82, 081401 (2010)]. In this paper, the method is revisited and related to the substrate phonon lifetime, the substrate temperature T-sub, and the sliding speed. At low sliding speed, where the time between stick-slip events is large compared to the phonon lifetime, we do not observe much dependence of F-fr on gamma, and here thermostat tuning is not required. At high sliding speed, upon varying gamma, we confirm the aforementioned friction maximum but also observe a pronounced minimum in T-sub, which here deviates from T-lan. For substrate particle interactions that are strongly anharmonic, the variation of F-fr with gamma can be understood as a manifestation of thermolubricity, backreflections being essentially unimportant. In contrast, for harmonic interactions, where phonon lifetimes become very long, F-fr is strongly affected by backreflecting phonons, though not enough to overturn thermolubricity.