Adhesion and micromechanical properties of metal surfaces

Abstract

This paper is part of a long-standing attack on problems of friction and adhesion. It falls into two parts. The first discusses the general mechanism of adhesion and then describes an experimental study of the adhesion and deformation of a model microasperity. This consists of the contact formed between a fine pointed stylus of tungsten and a single crystal of a softer metal (nickel). The experiments are carried out in ultrahigh vacuum and the surfaces are characterized in situ using Auger spectroscopy. The loads range from 0.5 to 1000 μN and contact during loading and unloading is monitored using electrical resistance measurements. With clean surfaces the results suggest that surface forces alone are able to initiate plastic deformation. Oxide monolayers reduce the adhesion but the general behaviour is little changed. In contrast, oxide layers 5 nm thick greatly modify the adhesion and deformation behaviour. The second part describes microhardness measurements carried out over approximately the same load range. A Berkovitch triangular pyramid is used as the indenter. Some of the indents are studied in the electron microscope but the major hardness determinations are based on depth measurements of the indents formed. These measurements are extremely sensitive and may be conveniently monitored so that the plastic indentation process during loading and the elastic relaxation which occurs on unloading may be recorded. Both the adhesion measurements and the microindentation hardness measurements show that the plastic yield stress of very small volumes may be three or four times larger than the bulk values. The results also show that detailed contact models may be of limited value when dealing with contacts involving plastic deformation.