Publications

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After sliding contact of a hard spherical counterface on a metal surface, the resulting wear scar possesses a complex microstructure consisting of dislocations, dislocation cells, ultrafine or nanocrystalline grains, and material that has undergone dynamic recovery. There remains a controversy as to the mechanical properties of the tribolayer formed in this wear scar. To investigate the properties of this thin layer of damaged material in single crystal nickel, we employed two complementary techniques: pillar compression and nanoindentation. In both techniques, the tests were tailored to characterize the near surface properties associated with the top 500 nm of material, where the wear-induced damage was most extensive. Pillar compression indicated that the worn material was substantially softer than neighboring unworn base metal. However, nanoindentation showed that the wear track was substantially harder than the base metal. These apparently contradictory results are explained on the basis of source limited deformation. The worn pillars are softer than unworn pillars due to a pre-straining effect: undefected pillars are nearly free of dislocations, whereas worn pillars have pre-existing dislocations built in. Nanoindentation in worn material behaves harder than unworn single crystal nickel due to source length reduction from the fine-grained wear structure. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Despite the technological importance of body-centered cubic (BCC) metals, models of their plastic deformation are less common than those of face-centered cubic (FCC) metals, due in part to the complexity of slip in BCC crystals caused by the thermal activation of screw dislocation motion. This paper presents a physically based crystal plasticity model that incorporates atomistic models and experimental measurements of the thermally activated nature of screw dislocation motion. This model, therefore, reproduces the temperature, stress, and strain rate dependence of flow in BCC metals in a simple formulation that will allow for large, grain-scale simulations. Furthermore, the results illustrate the importance of correctly representing mechanistic transitions in materials with high lattice friction. © 2012 Elsevier Ltd. All rights reserved.

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Metallic materials in sliding contact typically undergo dislocation-mediated plasticity, which results in stick-slip frictional behavior associated with high coefficients of friction ({mu} > 0.8). Our recent work on two electroplated nanocrystalline Ni alloys reveal that under combined conditions of low stress and low sliding velocity, these metals have very low friction ({mu} < 0.3). The observed frictional behavior is consistent with the transition from dislocation-mediated plasticity to an alternative mechanism such as grain boundary sliding. Focused ion beam cross-sections viewed in the TEM reveal the formation of a subsurface tribological bilayer at the contact surface, where the parent nanocrystalline material has evolved in structure to accommodate the frictional contact. Grain growth at a critical distance below the contact surface appears to promote a shear-accomodation layer. We will discuss these results in the context of a grain-size dependent transition from conventional microcrystalline wear behavior to this unusual wear behavior in nanocrystalline FCC metals.