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Incipient yielding behavior during indentation for gold thin films before and after annealing

Published online by Cambridge University Press:  03 March 2011

David C. Miller ,
Mellisa J. Talmage  and
Ken Gall
David C. Miller*
Affiliation:
Department of Mechanical Engineering, University of Colorado, Boulder, Colorado 80309-0427
Mellisa J. Talmage
Affiliation:
Department of Mechanical Engineering, University of Colorado, Boulder, Colorado 80309-0427
Ken Gall
Affiliation:
School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245
*
a) Address all correspondence to this author. e-mail: dcm@colorado.edu
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Abstract

We studied the deformation mechanisms and mechanics during indentation of polycrystalline gold thin films at depths below 100 nm. The measured material hardness decreased from 2.1 ± 0.1 to 1.7 ± 0.1 GPa after annealing for 4 h at 177 °C. Upon closer inspection, the hardness trends in the gold thin films were discovered to vary according to the indentation depth. At nanometer depths, the material hardness was quantified using multiple parameters, some of which were independent of the area calibration for the tip. The annealed specimen was very “hard” at low indentation depths, relatively soft at moderate indentation depths, and finally harder until the grain-size limit was reached. The as-deposited specimen demonstrated a relatively continuous harness trend as function of indentation depth, exhibiting monotonic convergence to Hall–Petch limited behavior. Discrete displacement jump events (excursions or “pop-ins”) were frequently observed for the annealed specimen but not for the as-deposited specimen. Variation in hardness, excursion activity, and displacement during the hold at maximum load was observed according to the applied loading, which was parametrically varied at constant strain rates. Hardness results are explained in terms of the population and evolution of defects present within the specimens. The population of point defects is also influential, and critical thermal fluctuations, as well as the thermally activated process of diffusion, are believed to influence hardness at the specimen’s free surface and further into its volume. After converging to a monotonic trend (proper tip engagement), the modulus of the gold was measured to be 106.0 ± 12.9 and 101.3 ± 6.0 GPa for the respective Au/Cr/Si specimens. These values exceeded predictions from the aggregate polycrystalline material theory, a representation used to estimate results for anisotropic single crystals. Exaggerated modulus measurements are explained as the result of the contribution of modulus mismatch with the substrate, pileup at the indentor tip, residual stress in the films, and crystallographic anisotropy of the gold.

Keywords

AuHardnessPolycrystal
Type
Articles
Information
Journal of Materials Research , Volume 21 , Issue 10 , October 2006 , pp. 2480 - 2492
Copyright
Copyright © Materials Research Society 2006

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