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Size Effects on the Nanoindentation of Ni Nanowires using
Molecular Dynamics
Virginie Dupont
Los Alamos National Laboratory
PO BOX 1663, MS B268, Los Alamos, 87544, US
Phone: 505-665-5905, Email: vdupont@lanl.gov
Frederic Sansoz
The University of Vermont, Burlington, VT
Abstract:
There has been a recent scientific interest in one-dimensional metal nanowires because of their
potential use in nanotechnologies. Experimental evidence shows a strong influence of the sample
dimension on the mechanical properties of metals at nanometer scale. The relevant studied length scale
has a strong influence on the results of mechanical tests such as hardness, ductility, strength. In
macroscopic samples, the relevant length scale is often the grain size, which is much smaller than the
sample size, but as the sample size is reduced, the relevant length scale becomes the diameter or
volume of the sample, and the material strength significantly increases as the length scale decreases. A
micro-plasticity mechanism based on dislocation starvation has been proposed, which has been
confirmed experimentally on nanopillars of diameters larger than 150 nm. Atomistic simulations have
already been used to gain fundamental insight into the dislocation activity during metal nanopillar
compression, but little has been done on the radial nanoindentation of metal nanowires. We used
Molecular Dynamics simulations to study crystal plasticity and size effects in sub- 100 nm nanowires
deformed by spherical indentation. This work focused on &lt111>-oriented defect-free single crystals,
as well as nanocrystalline Nickel (Ni) nanowires. The indentation of thin films was also comparatively
studied to characterize the influence of free surfaces in the emission and absorption of lattice
dislocations in single crystal Ni. In single crystals, the nanoindentation response for both thin films and
nanowires in the elastic regime was found to obey a Hertzian behavior. The mean value of Young's
modulus for all samples for single crystals showed that the sample size had no noticeable effect on the
elastic properties of the nanowires during nanoindentation. No significant change at the onset of
plasticity (contact pressure and deformation mechanisms at yield point) were observed between the
samples. In the plastic regime, the hardness of Ni nanowires was found to be size-dependent, such that
the nanowire became softer as the wire diameter decreased. In contrast, no significant size effect of
hardness was revealed in thin films as a function of the film thickness. In nanocrystalline samples, grain
boundary sliding dominates the plastic deformation, and free surfaces are no longer significant due to
the accomodation of deformation by grain boundaries.
Society of Engineering Science ▪ 47 th Annual Technical Meeting 308