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Dupont, Virginie

Mechanical Engineering

2008

PhD

ABSTRACT Nanocrystalline thin films are materials with a grain size less than 100 nm which are commonly used to fabricate microscale electro-mechanical devices. At such small scale, nanoindentation is the only standard experimental technique to study the mechanical properties of thin films. However, it is unclear if the continuum laws commonly used in nanoindentation analysis of polycrystalline materials are still valid for nano-grained metals. It is therefore critical to better understand the behavior of nanocrystalline materials under nanoscale contact. This dissertation summarizes the results of atomistic simulations aimed at modeling the nanoindentation of nanocrystalline metal thin films for which the grain size is smaller than the indenter diameter. The nanoindentation of aluminum thin films was first studied using the Quasicontinuum method, which is a concurrent multiscale model where regions of small gradients of deformations are represented as a continuum medium by finite elements, and regions of high gradients of deformation are fully-treated atomistically. Two embeddedatom- method potentials for aluminum were used in order to study the effect of the potential on the nanoindentation behavior. The aim is to better understand the effects of a grain boundary network on the plasticity and the underlying mechanisms from an atomistic perspective. Our results show that a grain boundary network is the primary medium of plasticity at the nanoscale, via shear banding that causes flow serration. We also show that although the dislocation mechanisms are the same, the mechanisms involving grain boundaries are different depending on the interatomic potential. In a second part, abnormal grain growth in aluminum thin films under nanoindentation is studied using both the Quasicontinuum method and parallel molecular dynamics simulations. The effects of the potential, the nature of the indenter and of its size on the grain growth under nanoindentation are investigated. Our results show that the potential used, which can be related to the purity of the material, can reduce grain growth. We also show that the size and material used for the indenter both have significant effects on grain growth. More specifically, grain growth under the indenter is found to occur via atom diffusion if the indenter is of the same material as the thin film. Finally, the sample size effects were studied using parallel molecular dynamics simulations on nickel thin films and nanowires. Single crystals with different sizes are modeled in order to investigate the effects of the free boundaries as well as of the thickness of the samples. It is shown that the yield point and the incipient plasticity mechanisms are similar for all simulations. However, the hardness of the nanowires is found to decrease with the nanowire size during nanoindentation, due to the interaction of prismatic loops and dislocations with the free boundaries. This dissertation has shed light on the plastic deformation mechanisms under nanoscale contact. The results obtained will help the scientific community gain a better understanding of the behavior of nanomaterials, which will lead to the fabrication of more reliable nanodevices.