Friction and wear of mechanical systems greatly contribute to global energy expenditure. In times of rapidly progressing global climate crisis, the need for tailoring materials towards low friction and small wear rates is readily apparent. In order to achieve these goals, a precise understanding of fundamental deformation mechanisms occurring in metal sliding interfaces is of great importance. During tribological loading of metal surfaces, plastic strain is commonly accommodated by dislocation-mediated plastic deformation, including crystal rotation and texture formation, which in turn influence friction and wear. Especially the early stages of lattice rotations induced by sliding are not yet sufficiently understood.

In the present work, the early stages of tribologically induced crystal rotation kinematics are therefore analyzed in great detail by performing electron backscatter diffraction (EBSD) directly on the bulk surface of a high-purity copper bicrystal, after a single sliding pass with a sapphire sphere. Analysis reveals three distinct types of crystal lattice rotation. First, crystal rotations around the transverse direction (TD) are shown to be at the heart of crystal lattice rotation kinematics in both grains, irrespective of load and sliding direction (SD). Reverting the sliding direction primarily inverts the sign of the crystal rotations observed, but does not impact the fundamental nature of the rotation process. Second, a lower proportion of data points rotates further around TD, exhibiting superimposed rotations around ±SD. Finally, combined lattice rotation around TD and deformation twinning are observed under specific conditions. These insights are physically rationalized regarding wear track topography, slip traces and anisotropic friction.

In conjunction, these insights set the stage for more applied research, e.g. into crystallographic textures especially resistant or susceptible to combined lattice rotation and shear. The novel approach of using EBSD directly on surfaces mildly deformed by tribological loading offers superior statistics and is less prone to artifacts than commonly used techniques such as Transmission Kikuchi Diffraction, requiring destructive specimen preparation. It has the potential to be applied to a wide variety of materials and systems.