On the Evolution of Friction-Induced Nanostructures in Single Crystal Nickel

We have investigated the fundamental phenomena governing the friction-induced microstructures in single crystal nickel. Friction measurements were made using a unidirectional linear wear tester against a hard Si₃N₄ ball so that deformation is confined to the softer Ni surface. To minimize the environmental effects on friction, we conducted the experiments in dry nitrogen atmosphere. A high precision rotary stage was designed and built to enable friction measurements to be made in specific crystallographic directions. Measurements were made on (100), (110) and (111) crystal faces. Focused ion beam (FIB) microscopy was used to prepare cross-sections of wear scars suitable for electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM) analyses. The EBSD data revealed the formation of low-angle grain boundaries leading to the development of fine-grained equiaxed recrystallized substructues underneath the wear scars. The extent of subsurface deformation and the steady state friction coefficients were strongly dependent on the crystal orientation. At higher contact stresses, TEM analysis confirmed the formation of fine-grained equiaxed nanocrystalline grain structures. Subsequent friction tests on these nanostructured layers performed at much reduced contact stresses showed significant reductions in the friction coefficients. The role of the friction-induced nanostructures on the mechanisms of metallic friction is discussed.