Nanoindentation of thin films: Simulations and experiments
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M.J. Cordillb) Department of Chemical Engineering/Materials Science and Engineering, University of Minnesota, Minneapolis, Minnesota 55455
Diana Farkasc) Department of Materials Science and Engineering, Virginia Tech, Blacksburg, Virginia 24061
W.W. Gerberich Department of Chemical Engineering/Materials Science and Engineering, University of Minnesota, Minneapolis, Minnesota 55455 (Received 23 July 2008; accepted 20 November 2008)
Atomistic simulations of nanoindentation of a 20-nm-thick Ni thin film oriented in the [111] direction were carried out to study the effects of indenter velocity and radii, interatomic potentials, and the boundary conditions used to represent the substrate. The simulation results were compared directly with experimental results of Ni thin film of the same thickness and orientation. It was found that the high indenter velocity does not affect the hardness value significantly. Different radii used for indentation also have negligible effects on the hardness value. Two different interatomic potentials were tested, giving significantly different hardness values but both within 20% of the experimental result. Different boundary conditions used to represent the substrate have a significant effect for relatively deep indentation simulations.
I. INTRODUCTION
One of the most important applications of nanoindentation is the determination of the mechanical properties of thin films. In nanoindentation tests, the properties of the thin film may be measured without removing the film from the substrate as is done in other types of testing. The spatial distribution of properties, in both lateral and depth dimensions, may be measured. Apart from testing films in situ, nanoindentation techniques can also be used for films made as freestanding microbeams or membranes.1 Advances in experimental equipment allow the testing to be performed with the indenter radii in the tens of nanometer scale. Experimental studies2 have shown that nanoindentation is a reliable and widely used technique for probing mechanical properties of materials. At the same time, computing capabilities now allow for atomistic simulations in parallel architectures that can approach these length scales. These advances make it possible for the first time to actually compare simulation and experimental results in a direct a)
Present address: Cornell University, Ithaca, NY 14853. Present address: Erich Schmid Institute for Materials Science, Austrian Academy of Sciences, Leoben, Austria 8700. c) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0136 b)
J. Mater. Res., Vol. 24, No. 3, Mar 2009
manner. A simulated load or pressure versus depth curve allows comparisons to be made with actual experimental data. These direct comparisons can be used to shed light on basic unresolved questions such as the effects of the unrealistically fast indenter speeds typically used in simulations, and the uncertainties related to the reliability of the interatomic potentials. Simulations have been used extensively in
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