Size-dependent plastic flow behavior is manifested in nanoindentation, microbending, and pillar-compression experiments and plays a key role in the contact mechanics and friction of rough surfaces. Recent experiments using a hard flat plate to compress single-crystal Au nano-pyramids and others using a Berkovich indenter to indent flat thin films show size scaling into the 100-nm range where existing mechanistic models are not expected to apply. To bridge the gap between single-dislocation nucleation at the 1-nm scale and dislocation-ensemble plasticity at the 1-m scale, we use large-scale molecular dynamics (MD) simulations to predict the magnitude and scaling of hardness H versus contact size ഞ c in nano-pyramids. Two major results emerge: a regime of near-power-law size scaling H Ϸ ഞ c ؊ exists, with MD Ϸ 0.32 compared with expt Ϸ 0.75, and unprecedented quantitative and qualitative agreement between MD and experiments is achieved, with H MD Ϸ 4 GPa at ഞc ؍ 36 nm and Hexpt Ϸ 2.5 GPa at ഞ c ؍ 100 nm. An analytic model, incorporating the energy costs of forming the geometrically necessary dislocation structures that accommodate the deformation, is developed and captures the unique magnitude and size scaling of the hardness at larger MD sizes and up to experimental scales while rationalizing the transition in scaling between MD and experimental scales. The model suggests that dislocation-dislocation interactions dominate at larger scales, whereas the behavior at the smallest MD scales is controlled by nucleation over energy barriers. These results provide a basic framework for understanding and predicting size-dependent plasticity in nanoscale asperities under contact conditions in realistic engineered surfaces.atomistics ͉ dislocation interactions ͉ plasticity ͉ size-effects ͉ surfaces I t is now well established that plastic flow behavior in metallic materials is size dependent, with flow stress (1, 2) or hardness (3) increasing with decreasing volume of material under load. Experimental studies of nanoindentation (4-6) and wire and thin-film bending (7-10) and tension (11-17) clearly elucidate the size effects at micrometer scales. The rational for size-dependent plasticity has been attributed to the need for ''geometrically necessary'' defect structures, typically dislocations, which accommodate the rapid strain gradients that must exist in many small-scale deformation conditions and the influence of these dislocations in hardening the material. Phenomenological strain-gradient-plasticity theories (18-21) and discrete dislocation models (22) both qualitatively exhibit the observed size effects under varying conditions. However, the conceptual bases of these models typically rely on interactions between preexisting, statistically generated, and geometrically necessary dislocations that are not thought to be applicable at nanometer scales where relatively few dislocations exist. Atomistic studies using molecular dynamics (MD) at the nanometer scale can exhibit size effects (23-26), but they are usually ass...
Surface roughness plays an important role in the delamination wear caused by rough surface contact. A recent dislocation model analysis predicts that nano-scale contacts of surface steps induce nucleation of dislocations leading to pro-load and anti-load dislocation segregation near the contact surface. Suh dislocation segregation generates a sub-layer of tensile residual stress in a much thicker layer of compressive residual stress near the surface. The tensile sub-layer thickness is expected to be about 50 to 100 times the step height. In order to verify the predictions of the model analysis, contact experiments are carried out on polycrystalline aluminum surface to determine the existence of the tensile sub-layer. The variation of the residual stress along the thickness direction is measured using a newly developed high sensitivity curvature-measurement interferometer. The residual stress distribution measured with sub-nanometer spatial resolution indicates that contact loading leads to formation of a highly stressed sub-layer of tensile residual stress within a much thicker layer of compressive residual stress. Implications of tensile residual stress for delamination wear are discussed.
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