The Contact Resistance and Mechanical Properties of Surface Films on Metals

The nature of the contact between sliding metal surfaces has been investigated by measuring the electrical contact resistance and the coefficient of friction between them. Observations were made over a wide range of conditions which extended down to very light loads of only a few milligrammes so that the influence of surface films might be studied. Many noble and base metals have been investigated in both clean and lubricated states. The contact between certain typical pairs of dissimilar metals has also been studied. It was found that at loads below a few grammes the oxide film on base metals prevented actual metal-to-metal contact almost entirely. At higher loads the film was disrupted, and metallic contact occurred. The change from oxide-on-oxide to metal-on-metal sliding was usually accompanied by changes in the coefficient of friction, the nature of the sliding, and the appearance of the track. This, however, was not invariable and occasionally changes in the nature of sliding occurred which were detectable only by measurements of the contact resistance. The experiments emphasize the importance of the measurement of contact resistance as a tool in the investigation of the behaviour of non-conducting surface films. The loads at which the oxide films on various metals, in particular platinum, disrupt have been compared; the manner in which the oxygen layer is attached to the metal and the physical factors controlling the break-through have been studied. The same techniques have been used to investigate the influence of the Beilby layer, by comparing the behaviour of annealed and electropolished surfaces with that of worked ones. The contact between lubricated metals has also been studied over a wide range of loads and the electrical contact measurements have shown that with monobasic fatty acids a change in the nature of contact occurred as the load was increased. It was found that at heavy loads the contact resistance between lubricated metals fell to a value comparable with that of clean surfaces although measurements of the coefficient of friction suggested that the area of actual metallic contact had not increased correspondingly. These experiments have shown that the behaviour of oxide, Beilby, and lubricant films on metal surfaces follow the same pattern, and that a single general picture, based on the theory of the sliding between clean metals at heavy loads, can be given which will describe their influence on the contact and sliding between metals.