A Microstructural Study of Wear Mechanisms in Nanocrystalline Metals

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This project is focused on the wear of nanocrystalline metals and alloys. The goal is to understand the wear mechanisms and model accurately wear rate as a function of grain size in nanocrystalline metals and alloys. To this end, bulk nanocrystalline Al and Al-Si will be produced by equal channel angular extrusion of mechanically milled powders. We will also produce bimodal grain structures consisting of mixtures of nanocrystalline and conventionally-grained material, which we expect will have high hardness yet very good ductility. The microstructures will be characterized using transmission electron microscopy and X-ray diffraction. Such bulk nanocrystalline materials should not suffer from the difficulties of interpreting wear data that occur for coatings and modified surface layers where the grain size and strain often vary throughout the layers. Mechanical properties will be measured as a function of grain size under tension at different loading rates. Wear tests will be performed for different grain sizes in air, dry oxygen and inert gas at room temperature. The deformation mechanisms will be examined using transmission electron microscopy, scanning electron microscopy, profilometry and nanoindentation, and the results will be correlated with the wear test results and mechanical properties. We also aim to construct wear mechanism maps for nanocrystalline materials, with the maps including the conditions when oxidation is of major importance as well as the conditions when deformation mechanisms control the wear behavior. While the work focuses on specific materials, the overall goal is to understand the effects of grain size in the nanocrystalline range on the wear rate and wear mechanisms of nanocrystalline metals and alloys as a whole.