Friction—Effect of Oxide Films

The behavior in the presence of air is quite different. For example, Tingle [22] found that the friction between copper surfaces decreased from a fi value of 6.8 to one of 0.80 as progressive exposure of the clean surfaces led to increasingly thick oxide layers. As noted by Whitehead [23], several behavior patterns 

Bowden and Tabor [24] proposed, for such heterogeneous surfaces, that 

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