Simulation of Nanometer-Scale Deformation of Metallic and Ceramic Surfaces
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MRS BULLETIN/MAY 1993
Chen and Hendrickson4 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 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 Oliver5 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 shallow indentations (less than 50 nm), suggesting that small-scale indentation plasticity may take place by nondislocation mechanisms. In this article, we present molecular dynamics evidence of this assertion: on the nanometer-length scale, plastic deformation due to point indentation takes place through the creation and motion of point defects (vacancies, interstitials, and surface diffusion). At the nanometer scale, the atomic force microscope (AFM) is used to study atomicscale tribological processes such as the nanometer-scale mechanical properties of thin films,6'7 atomic-scale friction of clean surfaces,8 and surfaces coated with poly-
mer lubricants.910 The diameter of an AFM tip may be as small as 10 nm. With the aim of understanding and providing a model for the mechanics and mechanisms of tipto-surface interactions, several molecular dynamics and atomistic studies have appeared. Landman and co-workers" simulated the indentation of a sharp metallic tip into a metal surface and observed an interesting effect. Beneath a critical separation, the atoms in the tip and surface "jumpedto-contact," forming a neck between the tip and surface. This "avalanche in adhesion" had previously been predicted for planar contacts using the Lennard-Jones force model12 and the equivalent crystal force model.13 Related molecular dynamics simulations of tribological processes include physisorbed fluid films of spherical and flexible molecules confined between solid walls and their properties under shear14 and atomic-scale friction of hydrogen-terminated diamond surfaces.15 In our work we have been using the molecular dynamics simulation method to model the indentation and orthogonal cutting of a clean metal surface1617 and of a clean silicon surface18by a hard diamond-like tool. Orthogonal cutting provides a unique opportunity in which both the length scale and the time scale of experiment and simulation overlap. Results from these simulations suggest that the nature of plastic deformation on the microscopic nanometer scale differs from macroscopic plastic deformation. Nanoscale deformation of metal surfaces due to point indentations occurs through
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