THE INJECTION OF PLASTICITY BY MILLINEWTON CONTACTS

Diamond indentation of a surface with a thin passive film requires loa ds an order of magnitude smaller for Ni[100] crystals than for Fe-3 wt % Si[100] crystals. The load bearing capacity of the Fe-3 wt% Si can b e reduced by two orders of magnitude by removing the 10 nm thick nativ e oxide film. These phenomena can be explained by considering the equi librium of forces associated with tip, image and friction stresses act ing on dislocations emitted from the indenter tip. The key ingredient to this model is the nucleation and growth of dislocation loops at loa ds of only tens of micronewtons. Three types of critical contact exper iments demonstrate that dislocations can be initiated at loads well be low those previously thought to represent elastic loading only.