Introduction to the discussion: the mechanism of friction

The application of the electron microscope, and of metallurgical, interferometric and other physical methods, shows that even carefully polished or cleaved surfaces have irregularities on them which are large compared with molecular dimensions. When two solids are placed together, the real area of contact is very small, so that the local pressure is high and, in general, exceeds the yield pressure, p m , of the metal. Plastic flow of the solid occurs at the summits of the irregularities so that the real area of contact A is proportional to the applied load W , i.e. A = W / p m . There is adhesion at these local regions of contact and the friction is, in a large measure, the force required to shear them. As a rough approximation F = As , where s is the shear strength of the junctions. Since A is proportional to W and independent of the size of the surfaces, this can explain the classical laws of friction. The mechanism of this process has been studied by optical and metallurgical methods and by the use of artificially radioactive metals. The strength and nature of the adhesion between metals is profoundly influenced by the oxide films which are normally present on them. It is the presence of these films which enables sliding to occur. If these surface films are removed in high vacuo and naked metals are placed in contact, there is strong adhesion—an attempt to slide them may cause further plastic deformation with consequent increase in the area of contact—and a large-scale ‘cold welding’ of the metals. If the sliding speed is appreciable, the temperature at the local points of rubbing contact will be raised. These surface temperatures have been measured by a thermo-electric method, by a visual or photographic method and by the use of an infra-red cell. All three methods give similar results and show that even at moderate speeds the local surface temperature may easily exceed 1000° C. It is limited by the melting of the metal. These high temperatures play an important part in polishing, the formation of the Beilby layer, the machining of metals and a number of other physical processes. Some low-friction materials are also described.