Phase transition of the dry friction between crystalline surfaces induced by normal load

A major source of energy dissipation and surface wear is the kinetic friction at the interfaces of sliding bodies. Traditionally, on a macroscopic scale, this undesirable effect is reduced with lubricating the surfaces by introducing oil into their interface. An interesting phenomenon, called superlubricity, has been reported on a nanometer scale where dry (without lubricant oil) fruition and wear become very low. In contrast, interlocking between the crystalline surfaces at such length scales may lead to a high frictional state called stick-slip motion. We study the transition between these two frictional states by modeling the surface of one object as a chain of particles coupled harmonically to each other and to the object body and interacting with the other object via a sinusoidal potential. The amplitude of the sinusoidal potential corresponds to the normal load at the interface. Our calculations show that the transition takes place at some critical amplitude of the potential which, like the average contribution of each particle to the kinetic friction force, is practically independent of the contact size.