The nature of engineering surfaces

No real engineering surface, no matter how carefully, or indeed expensively, prepared can possess perfect geometry. As well as errors in the form or shape of the component there will always be a roughness on the surface which is apparent when this is examined at a sufficiently high magnification. When two such surfaces are loaded together it is the tips of the surface roughnesses or asperities that must first carry the applied load: the geometry of individual contact spots and the way in which these islands of real contact are distributed throughout the nominal or apparent contact area is clearly of interest to tribologists in attempting to predict the overall performance, or likely life history, of the contact.

The geometric texture of an engineering surface reflects both its production route and the nature of the underlying material. It is possible to produce a truly smooth surface (for example, cleaving specimens of mica can produce a surface with roughness only on the atomic scale) and if two such surfaces are loaded together real and apparent areas are very nearly equal. The asperities on the surface of a very compliant surface, such as a soft rubber, may, if sufficiently small, be squeezed flat by quite modest contact loads, and in this way there can again be equality between real and apparent areas of contact. However, these are special cases; in general, useful metal surfaces exhibit a range of surface fluctuations which, although large compared to molecular dimensions, are small compared to the dimensions of most engineering components.