Molecular Dynamics Simulation of Mechanical Deformation of Ultra-Thin Metal and Ceramic Films
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181 Mat. Res. Soc. Symp. Proc. Vol. 389 © 1995 Materials Research Society
The surface indentation technique has been extended to the sub-micrometer length scale in order to study material deformation within a single grain. In the pioneering work of Gane and Bowden [6], a very sharp tip (diameter - 100 nm) was pressed into the surface of a metal crystal. They observed an interesting phenomenon--no permanent penetration occurred until a critical load was reached. Furthermore, the load at critical yielding corresponded to the theoretical shear strength of the metal. Critical yielding of this type has since been observed by many investigators [7] and, as we shall see, is also observed in the molecular dynamics simulations presented here. Chen and Hendrickson [8] used the microindentation technique to study the dynamics of dislocation creation and motion on the (111) surface of silver crystals. They were able to demonstrate the presence of dislocations on the surface by chemically etching the surface after performing the microindentation experiment. The surface preferentially etches along the edge of a dislocation loop (where the atoms are the furthest from equilibrium) and forms pits where the dislocation loop emerges at the surface. These pits form a hexagonal "rosette" pattern reflecting the symmetry of the (111) surface. More recently, Pharr and Oliver [9] have extended the experiments on the silver (111) surface using a nanoindenter (nanometer resolution along the vertical axis) and have found some rather interesting results. Hardness tends to increase with decreasing depth of indentation and the dislocation rosette patterns disappear entirely at very shallow indentations (< 50 nm), suggesting that very small scale indentation plasticity takes place by non-dislocation mechanisms. In this paper we present direct molecular dynamics evidence of this assertion--dislocations are not an efficient mechanism for accommodating strain due to point indentations at the nanometer length scale. Probably, the most useful tool to date for the study of the mechanical properties of surfaces at the nanometer scale is the atomic force microscope (AFM) [10]. The diameter of an AFM tip can be less than 10 nm. Though several important studies of atomic scale tribological processes using the AFM have appeared [11, 12, 13], several important issues concerning the interpretation of the observed forces remain [14, 15]. In particular, the role of water vapor and other adsorbed films (especially oxide layers) is unknown and few, if any, experiments have been performed in ultra-high vacuum. It is with the aim of understanding and providing a model for the mechanics and mechanisms of tip-to-surface interactions that several molecular dynamics and atomistic studies have appeared. In the pioneering work of Landman and coworkers [ 16, 17], the indentation of a sharp metallic tip into a metal surface was simulated and they observed an interesting effect. Beneath a critical separation, the atoms in the tip and surface "jumped-to-contact," forming a
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