By Desmond F. Moore and D. W. Hopkins (Auth.)
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6. 13 Actual Contact Between Surfaces Single profiles of surfaces have been dealt with almost exclusively in this chapter. We will now consider briefly how pairs of surfaces interact when brought into contact under load. A distinction must be m a d e between solids of comparable hardness when brought into mutual contact, and solids of widely differing hardness. Metal-on-metal contact is a common example of the former classification, and elastomer-on-rigid-base typifies the latter, as shown in Fig.
In fact, the occurrence or absence of plastic flow depends t o a very large extent on the radius of curvature of asperity peaks, and bears n o relation t o the load W. (c) The mean elastic pressure ρ for elastic contact, or the mean plastic flow pressure p* (in the absence of work hardening), are virtually independent of load. (d) The average size of a micro-contact A/M remains constant with increasing load for b o t h elastic and plastic contact. 1 shows the n a t u r e of contact, the distribution of asperity heights, and the form o f the relevant equation as outlined in these last two sections.
A chemical reaction occurs a t the surface of the clean metal following the adsorption of molecules of oxygen a n d water vapour. T h e kinetics of growth of the film involve the diffusion of metal ions through the oxide film. It is interesting t o note that the topography of metal oxides depends on environmental conditions during their growth period. 50 Principles and Applications of Tribology Thus slowly grown oxides are relatively smooth, but at higher temperatures crystals a n d whiskers may appear and create a rough profile.