Orientation Dependence of High-Angle Grain Boundary Formation during Sliding Wear in Copper Single Crystals

Sliding wear tests were conducted on copper single crystals having (110) and (111) surface, and polycrystalline copper. Evolution of high-angle grain boundaries during the sliding wear was investigated by the electron backscatter diffraction (EBSD) technique. The high-angle grain boundaries, which were formed in the vicinity of the worn surface, could be classified into two kinds from their morphology: one is parallel to the worn surface (Type A high-angle boundary) and the other is a grain boundary surrounding an equiaxed fine grain (Type B high-angle boundary). At Type A high-angle boundaries, a rotational axis between adjoining grains was almost parallel to z-axis which is defined as the direction perpendicular to both wear direction and worn surface normal. Grain boundary character distribution of Type A boundaries was sensitive to crystallographic orientation of the z-axis. When the z-axis was < 110 >, orientation relationship of Sigma 33a had high frequency. On the other hand, high Sigma 31a frequency was obtained at the sliding wear occurring under < 111 > z-axis condition. It is concluded that the evolution of Type A boundaries was caused by lattice rotation induced by sliding wear. For Type B high-angle boundaries, fractions of low-Sigma coincident site lattice (CSL) boundaries were high, and the frequency distribution of CSL boundaries was almost independent of wear direction and worn surface orientation. Unlike Type A boundaries, rotational axes at Type B boundaries showed no preferred orientation. These crystallographic features suggest that recrystallization is the most plausible origin for Type B boundary evolution. Consequently, the high-angle boundaries were produced probably by two different processes during sliding wear.