Mechanism of Friction

The lack of observable adhesion between most metal surfaces in air has been explained in the past by invoking the effect of released elastic stresses. In this work, the effect of released elastic stresses on adhesion has been investigated experimentally by evaporating thin films of gold on substrates of different hardness and measuring the adhesion as a function of the load. It has been found that the magnitude of the released elastic stresses is insufficient to account for the lack of observable adhesion in air. In the case of copper, it was found that adhesion became observable when the oxide film on the surface was ruptured. It was concluded that a protective film formed in the atmosphere prevented adhesion. It is suggested, therefore, that a class of frictional phenomena exists which cannot be explained by the adhesion theory. For electropolished Cu surfaces in particular, it is found that a mechanism based on plastic deformation may account for the frictional force observed where the oxide remains in...