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Dislocation Nucleation and Multiplication During Nanoindentation Testing

Published online by Cambridge University Press:  15 February 2011

A. B. Mann and
J. B. Pethica
A. B. Mann
Affiliation:
Oxford University, Dept. of Materials, Parks Road, Oxford, OXI 3PH, UK.
J. B. Pethica
Affiliation:
Oxford University, Dept. of Materials, Parks Road, Oxford, OXI 3PH, UK.
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Abstract

Small volumes of crystalline material can exhibit near theoretical lattice strengths due to the absence of pre-existing defects. We show here that careful control of experimental parameters and surface preparation are important in observing these increased stresses. In particular, tip impact kinetic energies at the initial contact of only a few eV have been found to influence events occurring at a much later stage in the loading cycle. This can be understood in terms of the energy required to deform a small number of atomic scale asperities and thus release sufficient surface energy to permit dislocation nucleation at impact. If the K.E. is not sufficient to overcome the asperities, dislocations must be nucleated at a later stage in the loading cycle, giving a apparently different mechanical property. We outline the implications for thin film systems where a hard surface layer may play a vital role, firstly, in preventing dislocation nucleation on impact and, secondly, in impeding the propagation of any such dislocations into the bulk below.

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