Surface deformation and friction of metals at light loads

Abstract

A study has been made of the interaction of sliding metals when the normal load between them is very small. A new apparatus is described which enables friction to be measured, under controlled conditions of sliding, down to loads of a few milligrams. The deformation of the sliding surfaces has been studied by light and electron microscopy. These techniques have been used to investigate the validity of Amontons’s law (proportionality of frictional force to load) over a very wide range of loads, from a few milligrams to several kilograms. Experimental results on electrolytically polished copper reveal a departure from Amontons’s law when the normal load is less than a few grams. A corresponding change is observed in the deformation within the track of sliding. At light loads the friction is low and the dam age is slight. This is attributed to the effect of the thin film of oxide formed by contact with the atmosphere. A thicker film of oxide, formed by heating in air, reduces the friction at heavy loads. Results of observations on copper surfaces prepared in various ways show that the influence of surface roughness upon friction is not great. The results of further experiments show that the friction of silver on silver and of aluminium on aluminium is constant and Amontons’s law holds over the whole range of loads. It appears that the thin film of oxide on aluminium is penetrated, even at the lightest loads. The results are discussed in relation to those for copper, and the difference in behaviour of the various metals is attributed to the contrasting properties of their oxides. Isolated experiments on sapphire and diamond show that the coefficient of friction is low and constant for these non-metals, and a few experiments made on boundary lubricated metals are recorded. In spite of the deviations from Amontons’s law on some metals, the results of all the experiments on dry sliding emphasize the essential similarity of the frictional process and of the basic mechanism over an enormous range of loads.