Molecular asperity theory of boundary friction

Abstract

A computer investigation of the hypothesis that boundary friction is caused by molecular forces between the tails of long-chain molecules attached vertically to the sliding surfaces is reported. It has previously been shown that the interaction statements which apply to this system are the Slater and Bartell scattering centre potentials. The Slater scattering centre potential was less accurate but was considered to see how much it affects the calculated friction. The frictional force was calculated and shown to be a two-term relation. The first term is due to the interaction energy barrier encountered when moving one surface from one equilibrium position to the next, together with the work done against internal rotation barriers. The second term is due to the lifting of one end group over the opposite one as the chains move, a molecular “asperity” friction. The calculations show that the friction rises with normal load in a manner similar to that found experimentally. As with almost all theoretical predictions of strength calculated from intermolecular potentials, these forces are some ten times too large, a discrepancy which is usually attributed to dislocations.