Tribology of single crystal nickel: Interplay of crystallography, microstructural evolution, and friction

This paper describes the evolution of friction-induced microstructures underneath the wear surfaces of nickel single crystals, and their role in influencing the steady state coefficient of friction. Using a custom built tribometer, friction measurements were made on {001} and {011} crystal faces along several crystallographic directions in unidirectional linear mode with a Si3N4 ball. Cross-sections of wear scars were prepared by focused ion beam (FIB) microscopy, and electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM) were used to analyze the evolution of crystallographic texture and recrystallization. The formation of a thin (10-15 nm) ultra-nanocrystalline layer in the {011}<211> orientation resulted in significant reductions in the coefficient of friction for that configuration, from the initial maximum of 0.64 to a steady state value of 0.32, after approximately 500 cycles of linear, unidirectional sliding. Crystal plasticity theory was used to describe the evolution of dislocation content in the worn material, which in turn provides an estimate of the characteristic grain size as a function of the imposed strain. An analysis of grain boundary sliding in ultra-fine-grained material suggests a mechanism for friction reduction. Model predictions of the contribution of grain boundary sliding (relative to plastic deformation) to friction provide a phenomenological description of the lubrication due to ultra-nanocrystalline surface microstructures.