MECHANISM OF METALLIC FRICTION

Abstract

AN experimental investigation of the kinetic friction of non-lubricated metals has shown that, in general, friction cannot be regarded as a surface effect. Penetration and distortion occur to some depth beneath the surface, and the frictional force and the nature of sliding are both influenced by the bulk properties of the metals. The physical properties of the metals, such as their relative hardness and melting point, play an important part. In fact, it has been shown that the nature of the sliding and the value of the friction are largely determined by these factors, and it has been suggested^{1, 2, 3, 4} that the frictional resistance between unlubricated metals is due primarily to the shearing of the metallic junctions formed by adhesion and welding at the points of contact, and to the work of dragging or ploughing the surface irregularities of the harder metal through the softer one. Even if the metals are carefully polished and are made as flat as possible, hills and valleys which are large compared with the dimensions of a molecule will still be present on the surface, and contact will occur only locally at the summits of these surface irregularities. The formation of metallic junctions between the surfaces will, of course, take place with cold metals. It has been shown that when the speed is slow or when the surfaces are stationary, so that there is no question of temperature rise, the high pressures at the points of contact will readily cause local adhesion and welding.