The adhesion of clean metals

Abstract

This paper describes a study of the adhesion between metallic surfaces which have been denuded of surface films by heating in a high vacuum (about 10$^{-5}$ $\mu $). Earlier experiments have shown that when two very soft metal specimens are pressed together they adhere strongly, but these results have in the past often been considered anomalous. The present work shows that harder metals also adhere on contact, although low values of normal adhesion are recorded after pure normal loading, even when the surfaces have been denuded. This apparent lack of adhesion is attributed to the influence of released elastic stresses, which may break many of the bridges formed between surface asperities as soon as the load is removed. If the conditions are such that the bridges can extend in a ductile manner and so accommodate the elastic recovery, the junction may retain its full strength. The measured adhesion is then equal to the original load. This is found for indium at room temperature and for other metals when the temperature is high enough to allow the bridges to anneal. When a tangential force is applied to the loaded specimens the area of contact can be increased by plastic flow under the influence of combined normal and tangential stresses, so that enhanced junction strength is obtained. Adhesion coefficients well over 10 have been measured at room temperature. A simple extension of the junction-growth theory proposed by McFarlane & Tabor for indium takes account of the influence of elastic recovery and is found to be applicable to metals generally. The close relationship between adhesion and friction is apparent.