Abstract
During the last decade considerable progress has been made in our understanding of the contact between solid surfaces. Earlier work emphasized the plastic deformation and more recent work the elastic deformation of the surface asperities. However, when sliding occurs combined normal and tangential stresses in the contact zone greatly enhance the plastic régime.
Broadly speaking, recent work supports the earlier view that friction is attributable to adhesion at the points of real contact and to the ploughing or tearing of one surface by asperities on the other. In high vacuum the surface films normally present can be removed either by heating or by repeated rubbing. As a result the adhesion, friction and wear can rise to very high values. However, even under very clean conditions some metallic pairs appear to show relatively weak adhesion. With non-metals similar processes occur but with brittle solids they are complicated by surface and sub-surface cracking. Here crystal orientation can be very important. With polymeric solids, where strength properties are rate and temperature dependent, marked variations in friction with speed and temperature are often observed. The more rigid the polymer the smaller the effect. In the field as a whole little work has been carried out on the interaction between the adhesion and the ploughing mechanisms.
In the field of lubrication it is now apparent that a considerable amount of what was formerly termed boundary lubrication was probably elastohydrodynamic. However, recent studies show that elastohydrodynamic films can operate under more severe conditions if small quantities of surfactants are added. These do not affect the viscous properties of the lubricant but form protective surface films of molecular dimensions. During the last decade little work has been carried out on the formation, structure and strength properties of these boundary films. It remains an area of great importance.
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