The friction of clean metals and the influence of adsorbed gases. The temperature coefficient of friction

Abstract

Experiments on electron diffraction and electron emission, as well as optical and other methods, have made it abundantly clear that, when a solid is cleaned in air by any ordinary method, its surface is still covered by a film of oxide, water vapour, and other adsorbed impurities. The surface film is often several molecular layers in thickness. Any complete theory of friction must take this film into consideration. Most experiments on the friction of metals are not made with metallic surfaces at all, but with a metal coated with this surface film. If a lubricant is used it is not added to the metal directly, but is superimposed on the film of contaminant. Recent papers (Bowden and Leben 1939; Bowden and Tabor 1939) have described an investigation of the kinetic friction between unlubricated metal surfaces which had been cleaned in air. The experiments show that sliding may not be continuous. The motion may proceed in jerks and large fluctuations may occur in the friction and in the surface temperature. The real area of contact between the metals is very small and corresponding fluctuations occur in it during sliding. The results suggest that metallic contact occurs only locally at the points of the surface irregularities. These contacts are usually few in number and their size is large compared with the dimensions of a molecule. The exact nature of the metallic junction depends on the relative physical properties of the metals, and the frictional resistance may be due to the surface irregularities of one metal ploughing through the other or to a cohesion and welding at the points of contact. The molecular field of force at the surface of a solid may be profoundly modified by the addition of even a single adsorbed layer, so that, on the above view the presence of this surface film should have a large influence on the friction.