The friction and deformation of polymers

Abstract

Earlier work has suggested that the friction of polymers may be explained in terms of the adhesion mechanism originally developed for metals. The present paper confirms this view, but shows that the detailed behaviour differs from that of metals in two main ways. First, in marked contrast to metals, there appears to be little or no junction growth during sliding so that the area of junctions sheared is essentially the same as the area of contact formed under static loading conditions. Secondly, a study of the static indentation properties of polymers on a large scale shows that the deformation is neither purely elastic nor purely plastic but intermediate over a very wide range. The deformation behaviour may be expressed by a single law which may be extrapolated back to predict the area of deformation between crossed fibres at very small loads from deformation experiments carried out on large specimens at very heavy loads. Friction experiments between crossed fibres of circular section show that the friction is directly proportional to the area of deformation so deduced; this suggests that contact occurs at very few asperities, perhaps only one, so that the area of true contact is essentially the same as the area of deformation. This provides a satisfactory explanation of the observed dependence of friction on load and on fibre diameter. At heavy loads on bulk polymer the friction varies less markedly with load, presumably because there is multiple-point contact. Although a number of simplifications are involved, the work provides an effective physical picture of the frictional properties of polymers in terms of their deformation characteristics, assuming the formation of strong interfacial junctions at the regions of real contact. Reasonably good frictional correlation is obtained for a load range of 10$^{10}$ to 1 and for surfaces differing in radius of curvature by a factor of more than 300 to 1.