The Effect of Compressive and Shearing Forces on the Surface Films present in Metallic Contacts

Two electrical methods have been used to examine the extent to which the oxide or other natural surface films can prevent intermetallic contact when metal bodies are pressed together. One consisted of measuring the electrical resistance of the contact, the other of measuring the thermoelectric force which arises when there is a temperature gradient across the film; crossed cylinders were used for the contact members. Precautions were taken to minimize vibration and shearing forces when applying normal loads. The contact resistance was often as large as several ohms, showing that penetration of the films was negligible, and in some cases the variation of resistance and thermoelectric force with load was consistent with the assumption that the contact contained a uniform film. Friction forces up to a certain critical value could generally be exerted in the normally loaded contacts before any metallic contact appeared, and in certain cases the films remained intact even during sliding. When the film was penetrated, the area of metallic contact was small at first, but grew as sliding proceeded; there was a simultaneous growth in the force resisting sliding. Microscopic examination showed that the metal surfaces suffered severe damage only after an appreciable distance of sliding (of the order of 0.1 mm).