Normal Load and Counter Body Size Influence the Initiation of Microstructural Discontinuities in Copper during Sliding.

Near the interface of two contacting metallic bodies in relative motion, the microstructure changes. This modified microstructure leads to changes in material properties and thereby influences the tribological behavior of the entire contact. Tribological properties such as the friction coefficient and wear rate are controlled by the microstructure, while the elementary mechanisms for microstructural changes are not sufficiently understood. In this paper, the influence of the normal load and the size of the counter body on the initiation of a tribologically induced microstructure in copper after a single sliding pass is revealed. A systematic variation in the normal load and sphere diameter resulted in maximum Hertzian contact pressures between 530 MPa and 1953 MPa. Scanning electron microscopy, focused ion beam, and transmission electron microscopy were used to probe the subsurface deformation. Irrespective of the normal load and the sphere diameter, a sharp line-like feature consisting of dislocations, the so-called dislocation trace line, was identified in the subsurface area at depths between 100 nm and 400 nm. For normal loads below 6.75 N, dislocation features are formed below this line. For higher normal loads, the microstructure evolution directly underneath the surface is mainly confined to the area between the sample surface and the dislocation trace line, which itself is located at increasing depth. Transmission Kikuchi diffraction and transmission electron microscopy demonstrate that the misorientation is predominantly concentrated at the dislocation trace line. The results disclose a material rotation around axes roughly parallel to the transverse direction. This study demonstrates the generality of the trace line phenomena over a wide range of loads and contact pressures and the complexity of subsurface processes under a sliding contact and provides the basis for modeling the early stages in the microstructure evolution.