Lubrication of Metal Surfaces by Fatty Acids

Abstract

WHEN surfaces in relative motion are separated by a fluid layer of appreciable thickness, the resistance to motion is due to the viscosity of the interposed layer. This type of lubrication is essentially a problem in hydrodynamics; the friction is very small and there is no wear of the moving surfaces. It is clear, however, that in many practical cases fluid lubrication is impossible. In many sliding mechanisms, or at the beginning and end of a reciprocating stroke, it is difficult for a thick continuous film of lubricant to be maintained, and even in rotating parts the thick film may break down and only a thin film of lubricant may remain. In such cases the friction is influenced by the nature of the underlying surfaces as well as the chemical constitution of the lubricant, and Hardy referred to such a state as ‘boundary lubrication’. Boundary lubrication is of great importance in practice, and the nature of the surface film will determine whether serious wear or seizure will take place. The coefficient of friction for really clean metals which have been out-gassed in a good vacuum is high and may be¹ about μ = 6. For ordinary unlubricated metal surfaces which are exposed to the air the coefficient of friction is of the order of μ = 0·5–1. For metal surfaces lubricated with a boundary film μ is about 0·05–0·15. This is considerably less than for clean surfaces, but is much higher than for fluid lubrication.