Contact of characterised metal surfaces at very low loads: Deformation and adhesion

Abstract

This paper describes a study of the contact between metal specimens in an UHV scanning electron microscope incorporating an Auger facility for characterising the surfaces in situ. One specimen is a flat single crystal of Ni: the other a tungsten “tip” of radius of curvature ca. 1 μm. Normal loads range from 10 −6 to 10 −3 N. The experiments fall into two groups. (a) Adhesion. The results show that with clean surfaces, the loading-unloading cycle is not reversible. Surface forces are very strong and in pulling apart the specimens the observed adhesive force is determined by ductile extension of the interfacial material. A monolayer of oxygen has little effect. By contrast, when a few layers of oxide (thickness ca. 5 nm) are present the loading-unloading cycle is almost reversible at low loads. Above a critical load the oxide is penetrated and the subsequent behaviour resembles that of clean surfaces. (b) Deformation. Although the deformed volume is extremely small the very high strength to be expected from the dislocation-free lattice is not observed with clean surfaces. However, in the presence of an oxide film the local contact pressure approaches the theoretical strength of the substrate before penetration of the oxide occurs.