THE INJECTION OF PLASTICITY BY MILLINEWTON CONTACTS

Diamond indentation of a surface with a thin passive film requires loads 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 be reduced by two orders of magnitude by removing the 10 nm thick native oxide film. These phenomena can be explained by considering the equilibrium of forces associated with tip, image and friction stresses acting on dislocations emitted from the indenter tip. The key ingredient to this model is the nucleation and growth of dislocation loops at loads of only tens of micronewtons. Three types of critical contact experiments demonstrate that dislocations can be initiated at loads well below those previously thought to represent elastic loading only.