Phononic mechanism determining threshold speed of wearless sliding nanofriction clarified based on molecular dynamics simulations

Abstract

Wearless nanofriction between nonmetallic solids is investigated in terms of molecular dynamics. Sliding-velocity dependence of the frictional force density is characterized by the threshold phenomenon. Further insights into this phenomenon were gained by analyses of ensemble-averaged behavior under the steady sliding condition. An enormous number of simulation runs and frequent validation of their results were carried out so that the interfacial temperature and the pressure, which directly affect the heat generation processes, were made common at different sliding velocities. The dramatic increase in rate of the frictional-heat generation at sliding speeds above the threshold is triggered by excitation of certain vibrational modes. Such elastic waves hidden in random thermal motion of atoms near the sliding interface were separated by the sampling and averaging. The atomistic reason for the dependence of the threshold speed on the combination of the lattice constants along the sliding direction is discussed based on the patterns of the orderly atomic-displacement fields.