Part III. Boundary and extreme pressure lubrication - Mechanism of boundary lubrication

Abstract

This paper shows how the theory of metallic friction, discussed in earlier papers, is modified in the presence of boundary films. Over the regions of contact the load is mainly supported by the boundary film, but some metallic interaction occurs through it with the formation of minute metallic junctions. These junctions contribute to the frictional resistance and play a major part in the wear of lubricated surfaces. The effect of temperature shows that the most effective lubrication is provided by a solid boundary film which possesses a close-packed strongly oriented structure. When the film melts, a marked increase in friction and metallic interaction occurs. For this reason fatty acids are generally more effective than hydrocarbons or alcohols, since they can react with the surface to form metallic soaps of relatively high melting-points. This chemical attack generally occurs via the metal oxide film which is present on the surface. The soap formation and the difference between a physically adsorbed and a chemically formed layer have been investigated by radioactive methods. A recent study has been made of the metal transference from one surface to the other as sliding takes place, using radioactive metals. The results again show that at the melting-point of the lubricant film a marked increase in pick-up and friction occurs. A new observation is that at still higher temperatures a second deterioration in lubricating properties occurs, corresponding to the desorption of the lubricant film. Although the surface is flooded with a lubricant the friction and surface damage are comparable with that observed with unlubricated surfaces. These effects are reversible on cooling and correspond to changes in state of the lubricant film. Similar changes are observed in electron diffraction studies of the structure and orientation of boundary films. The paper concludes with a discussion, based on these results, of the properties which a good boundary lubricant should possess.