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Bilinear Behavior in the Indentation Size Effect: A Consequence of Strain Gradient Plasticity

Published online by Cambridge University Press:  11 February 2011

A. A. Elmustafa,
J. Lou,
O. Adewoye,
W. O. Soboyejo and
D. S. Stone
A. A. Elmustafa
Affiliation:
NASA Langley Research Center-ConITS, Hampton, VA, U.S.A
J. Lou
Affiliation:
Department of Mechanical and Aerospace Engineering and Princeton Materials Institute, Princeton, NJ, U.S.A
O. Adewoye
Affiliation:
Department of Mechanical and Aerospace Engineering and Princeton Materials Institute, Princeton, NJ, U.S.A
W. O. Soboyejo
Affiliation:
Department of Mechanical and Aerospace Engineering and Princeton Materials Institute, Princeton, NJ, U.S.A
D. S. Stone
Affiliation:
Materials Science & Engineering, University of Wisconsin-Madison, WI, U.S.A
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Abstract

This paper examines the effects of stacking fault energy on the micro- and nano-indentation behavior of face-centered-cubic thin films. These include: LIGA nickel MEMS structures, alpha brass, copper and high purity aluminum. The measured hardness are then fitted to a strain gradient plasticity model based on the Taylor dislocation hardening model. Hardness is shown to exhibit a size dependence with different characteristic slopes in the micron and nano-scale regimes. Deep indents are shown to exhibit classical linear behavior. However, shallow indents exhibit an abrupt decrease in slope (almost by a factor of 10), giving rise to a bi-linear behavior. Furthermore, as the gradients become less sharp, the trends in the nano-hardness data become similar to those of the microhardness data predicted by the strain gradient plasticity model. Finally, the effects of stacking fault energy are then discussed within the context of cross-slip and hardening associated with Shockly partials.

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References

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