The effect of a tangential force on the contact of metallic bodies

Abstract

When two solid bodies are brought together, they are usually very heavily deformed in their region of contact. Bowden & Tabor (1954) have discussed very fully how a consideration of this deformation may be used to explain friction. They have shown that the interaction between the two bodies in the area of contact, where they come very close together on an atomic scale, is so strong that the application of a tangential force tending to slide two normally loaded bodies over each other nearly always produces deformation and rupture in the bodies themselves, rather than slip in the original interface. We have studied experimentally the tangential movement of two bodies relative to each other and the size of the area of contact between them, when they are first loaded normally and then subjected to tangential forces too small to cause sliding. We have examined the contact of like metallic specimens, using gold, platinum, tin, indium and mild steel. The experiments showed that both the relative displacement of the two bodies and the area of contact between them are smooth, increasing functions of the tangential force, as long as this is increased monotonically from zero. Any value of the tangential force, less than the force of static friction, gives rise to a certain equilibrium displacement. Such displacements cannot therefore be called ‘sliding’ in the ordinary sense. As the displacement increases the tangential force increases more and more slowly, tending asymptotically to the force of static friction. We found that the tangential force usually closely approaches the force of static friction while the displacement is still small compared with the diameter of the area of contact. The area of contact was found by measuring the electrical contact resistance. Only if there are no insulating layers between the bodies does this resistance give a direct measure of the contact area. The increase in contact area could therefore be clearly observed only with the noble metals. With these the behaviour can be described in terms of the simple analysis given by McFarlane & Tabor (1950) of the yielding in the area of contact under combined normal and tangential loading. With metals that bear an insulating oxide layer, the contact resistance gives information on the disruption of the layer. We found that purely normal loading causes very little mechanical breakdown; marked breakdown occurs only when the tangential force is increased to a large fraction of that needed to cause sliding. It was found that the changes both in displacement and area of contact produced by tangential loading are essentially irreversible. A reversible (elastic) component of the displacements exists, but it is a small part of the total except when the tangential force is small. Release and even reversal of the tangential force produces no further irreversible changes until the force is increased again, in either direction, to a numerical value as high as the highest it has previously reached. It was further found that a lubricant does not essentially affect the deformation process for tangential forces less than those required to cause slip. The lubricant acts by weakening the surface-interaction, so that slip occurs for a smaller value of tangential force. Displacements corresponding to forces less than those required to cause sliding are approximately equal for lubricated and unlubricated specimens. The experiments provide quantitative data on the micro-displacements before sliding. The results can be interpreted in terms of the adhesion theory of friction. They confirm the analysis of combined stresses given by McFarlane & Tabor and extend its applicability.