The delamination theory of wear and the wear of a composite surface

The delamination theory of wear postulates that there is a “nonworkhardening” soft surface layer which deforms continuously due to the instability of dislocations, and that the low speed sliding wear of metals is caused by the subsurface crack nucleation and propagation nearly parallel to the surface. A corollary of the theory is that when hard metal surfaces are plated with a soft metal to reduce the coefficient of friction and the wear rate, the soft metal layer must be thinner than a critical thickness so as to prevent the accumulation of dislocations in the plated layer and the formation of the delaminated layer. This corollary was investigated by plating annealed AISI 1018 steel with cadmium. The wear rate of the steel specimen plated with 0.1 μm cadmium on both of the contacting surfaces was three orders of magnitude smaller than the unplated specimen when they were tested in argon. In the case of thicker coatings, the cadmium layer wears by the delamination process which occurs within the plated layer. The very thin cadmium plate is also effective in reducing wear in inert oil, but not effective in an oxidizing atmosphere. The coefficient of friction of the 0.1 μm Cd plated steel was less than the unplated steel under all test conditions.