Wear experiments on bare, single-crystal Ni films indicate that a thin (approximately 250 nm) layer of nanocrystalline Ni can form at the wear interface, and that this layer serves to lubricate the contact. Furthermore, the phenomenon is qualitatively sensitive to the crystallographic orientation of the frictional loading. For example, when a 1 N normal load and 3.75 mm/s tangential speed are applied to a 1/8" diameter Si₃N₄ ball in contact with electropolished single-crystal Ni, the measured friction coefficient is usually in the range 0.6 to 0.8. However, when the Ni surface is of the {110} type and the sliding direction is <211>, the friction coefficient gradually decreases from 0.6 to 0.5 during the first 400 cycles, at which point a sustainable nanocrystalline film forms and the friction coefficient abruptly drops to 0.3, where it remains indefinitely. Modeling of the wear behavior, based on crystal plasticity, microstructure formation, and grain bounda ry sliding, suggests that this self-lubrication phenomenon is due to the capacity of ultra-fine-grained microstructures to support grain rotation. Friction experiments on bulk nanocrystalline Ni deposits confirm this hypothesis by demonstrating low friction coefficients (around 0.3) and virtually no wear-in under low loads and sliding speeds, and higher friction (around 0.6) under high loads and speeds. This presentation will provide an overview of the experiments and modeling of nanocrystalline film formation on single-crystal Ni, detail the results from friction experiments on bulk nanocrystalline Ni, and discuss model validation of the phenomenon's strain rate sensitivity.

Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.94AL85000.