Diamond indentation of a surface with a thin passive film require s 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 1- 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.
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Acknowledgements--The authors would like to thank Michael Baskes and Neville Moody of Sandia National Laboratories, Livermore, William Nix of Stanford University, John Pethica of Oxford University and Timothy Foecke and Robb Thomson of NIST for stimUlating discussions and feedback. This research was supported by the Center for Interfacial Engineering and the University of Minnesota under grant NSF/CDR-8721551 for W. W. Gerberich, S. K. Venkataraman, S. E. Harvey and D. L. Kohlstedt; and the University of Minnesota Corrosion Center with sponsorship of the U.S. Department of Energy, Basic Energy Science, Grant DE-FG02-88ER45337 for H. Huang.