Friction and adhesion. Surface forces in friction and adhesion

Abstract

Atomic forces at the interface constitute the major factor responsible for friction and adhesion between unlubricated surfaces. For non-metals these are basically the van der Waals dispersion forces, though in special cases electrostatic forces may be involved. Some recent work on van der Waals forces at very small separations will be described. These forces provide a fairly direct measure of the adhesion between soft rubber-like materials and between polymeric solids. With polymers, such interfacial forces are sufficient to transfer portions of the polymer from one surface to the other when they are placed in contact or slid over one another. Frictional transfer occurs even for materials such as P.T.F.E.

With metal surfaces, van der Waals forces are overtaken by metallic bonding when the regions of contact are separated by distances comparable with atomic dimensions. This leads to very strong adhesion between metals which are atomically clean. By contrast, very small quantities of active vapours adsorbed at the interface can produce a drastic reduction in adhesion. Another factor which greatly influences the adhesion of a metal is its ductility: metals which have a limited number of slip planes usually show smaller adhesion.

The importance of ductility is also shown in the behaviour of very hard materials such as TiC or diamond in ultra high vacuum. Intimate contact is restricted to individual asperities, and the adhesion for very clean surfaces is far less than might be expected from calculations based on surface forces. This is partly because the area of real contact is very small; partly because of the brittleness of these materials. If one of the surfaces is soft or ductile adhesion may be quite strong. For example the adhesion of clean TiC to TiC is extremely small; of clean copper to TiC extremely large. Here again surface films can greatly reduce the adhesive strength.