The contribution of micro/nano-tribology to the interpretation of dry friction

Fundamental studies of the mechanics and physics of dry sliding friction between solid surfaces have been impeded in the past by surface roughness and the difficulty of determining the real area of contact. The last decade has seen an explosive development of techniques to study friction at single-asperity contacts in which the real and apparent contact areas coincide. The contribution of these developments to interpreting dry friction is discussed in this paper. Experiments with smooth compliant rubber led to an appreciation of the role of adhesive forces and to including its effects in contact mechanics. Atomically smooth surfaces are obtained in the surface force apparatus (SFA) through the property of mica to cleave on atomic planes, and in the atomic force/friction microscope (AFM) by the use of nanometre size contacts on single crystals. Based on friction measurements in the SFA and AFM, a hypothesis is advanced that sliding of atomically smooth contacts takes place by the nucleation and propagation of dislocation-like defects through the interface. An analysis by Hurtado and Kim [29] using dislocation mechanics suggests a 'scale effect' in which friction is governed by resistance to nucleation in very small contacts and by resistance to propagation in larger contacts.