Basic research on friction has been deeply impacted by the fundamental work of Bowden and Tabor starting in the first half of the twentieth century. To this day, many tribological researchers adhere to the paradigm that the friction force in a tribological contact is comprised of two components: A “shearing” contribution originating from the shearing of junctions near the contact interface and a “ploughing” contribution necessary to displace bulk material.
Given the above, it follows readily that the friction force is governed by material properties both near the interface and in the bulk of the two contacting bodies. In the case of anisotropic crystalline materials such as metals, the observed friction thus depends on the crystal orientation of the material in contact and the direction of sliding with respect to the interface. Previous studies of such anisotropic friction behavior have shown that both friction components play a significant role in its emergence. The precise magnitudes of each contribution in a specific system, however, remain ambiguous. They are strongly dependent on the experimental conditions as well as the evolution of respectively governing material properties of the sliding contact.
Here, model experiments with sapphire spheres repeatedly sliding unidirectionally on high-purity copper single crystals were conducted to further elucidate the origin and evolution of friction anisotropy. The influence of pronouncing either the shearing or ploughing component of friction was studied by varying the experimental conditions accordingly. Scanning and transmission electron microscopy were used in conjunction with Electron Backscatter Diffraction and Transmission Kikuchi Diffraction to link microstructural changes during the course of silding with the observed friction behavior. Once friction anisotropy in single crystals is more clearly understood, manifold applications arise also with respect to polycrystalline materials, for example by applying beneficial crystallographic textures to reduce friction coefficients in real-world applications.