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The hardness and strength of metal tribofilms: An apparent contradiction between nanoindentation and pillar compression

Acta Materialia

Battaile, Corbett C.; Boyce, Brad L.; Weinberger, Christopher R.; Prasad, Somuri V.; Michael, Joseph R.; Clark, Blythe C.

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.

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TYPE Conference YEAR 2012
ScopusOSTI

Sliding friction in electrodeposited nanocrystalline Ni Alloys : transitional behavior associated with grain size, sliding speed, and contact stress

Padilla, Henry A.; Prasad, Somuri V.; Battaile, Corbett C.; Kotula, Paul G.

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.

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TYPE Conference YEAR 2010
OSTI

The role of crystallography and nanostructures on metallic friction

Prasad, Somuri V.; Michael, Joseph R.; Battaile, Corbett C.; Kotula, Paul G.

In ductile metals, sliding contact is often accompanied by severe plastic deformation localized to a small volume of material adjacent to the wear surface. During the initial run-in period, hardness, grain structure and crystallographic texture of the surfaces that come into sliding contact undergo significant changes, culminating in the evolution of subsurface layers with their own characteristic features. Here, a brief overview of our ongoing research on the fundamental phenomena governing the friction-induced recrystallization in single crystal metals, and how these recrystallized structures with nanometer-size grains would in turn influence metallic friction will be presented. We have employed a novel combination of experimental tools (FIB, EBSD and TEM) and an analysis of the critical resolved shear stress (RSS) on the twelve slip systems of the FCC lattice to understand the evolution of these friction-induced structures in single crystal nickel. The later part of the talk deals with the mechanisms of friction in nanocrystalline Ni films. Analyses of friction-induced subsurfaces seem to confirm that the formation of stable ultrafine nanocrystalline layers with 2-10 nm grains changes the deformation mechanism from the traditional dislocation mediated one to that is predominantly controlled by grain boundaries, resulting in significant reductions in the coefficient friction.

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TYPE Conference YEAR 2010
OSTI
Results 51–75 of 122
Results 51–75 of 122