Atomistics of friction

When two solid bodies contact and slide against each other, a frictional phenomenon occurs. There have been two models for the origin of the friction forces: the surface roughness model and Tomlinson's model. The surface roughness model explains the origin of the static friction force; contacting solid surfaces are so rough that surface asperities are mechanically locked against the gravitational force. From an atomistic point of view, Tomlinson explained a mechanism of the energy dissipation for the origin of the dynamic friction force. The atomistic mechanisms are described for the origin of the static and the dynamic friction forces, based on the theoretical conclusion that Tomlinson's mechanism is unlikely to occur in realistic frictional systems. The mechanism for the origin of the static friction force resembles the mechanical locking mechanism in a surface roughness model. The origin of the dynamic friction force is formulated as a problem of how the given translational kinetic energy dissipates into the internal relative motions of constituent atoms of bodies during sliding. From studying the available phase space volume of the translational motion becomes negligibly small for a large system size, compared with that of the internal motions, it is concluded that the energy dissipation occurs irreversibly from the translational motion to the internal motions. The comparison of the atomistic mechanisms with the surface roughness model and Tomlinson's model is discussed. A phenomenon of superlubricity, where two solid bodies move relatively with no resistance, is discussed.