The persistence of roughness between surfaces in static contact

When unworn rough metal surfaces are lightly pressed together the asperities of the softer surface in contact with the harder are commonly deformed plastically and support a contact pressure equal to their hardness. However, in heavily loaded contact conditions, in which the real areas of contact occupy a large proportion of the apparent area of contact, asperities can support contact pressures greater than their hardness: they are said to persist. Experimenters have disagreed about the size of the persistence effect and its cause: it has been ascribed to work hardening and to interaction between asperity deformation fields, a review of previous work suggests it depends on the type of surface finish. In this paper, which is limited to dry contact conditions, it is shown that surface texture has no practical effect on persistence. For a non-hardening material the contact pressure supportable by an asperity is proportional to its hardness but the constant of proportionality increases with the degree of contact between the surfaces. The increase is caused by asperity interaction in a manner predictable by plasticity mechanics. The same holds for a work-hardening material provided allowance is made for the increase of asperity hardness caused by its deformation. Asperities on a sand blasted surface, for example, may be strained 5% during their generation and up to a further 8% when pressed against another surface. Elastic deformation beneath the asperities also influences the degree of contact. This has not received due consideration in some earlier work.

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