Metallic surfaces that are subjected to tribological loading often suffer from accelerated oxidation, which may significantly influence the resulting friction and wear behavior. This in turn can be a critical issue for many tribological applications. However, the underlying mechanisms of tribooxidation remain poorly understood. In order to engineer materials with better friction and wear properties, it is important to gain a fundamental understanding of the oxidation processes at play.
To this end, a model system with a ball-on-flat contact consisting of copper single crystals paired with sapphire sphere counterbodies was chosen. Experiments have been carried out using a set up for reciprocating linear sliding in a controlled environment with 50% RH and at room temperature. Over the course of the experiments, the number of sliding cycles was varied in order to monitor the sequence of stages in the microstructural evolution and to identify diffusion pathways. As previous research has shown, oxidation under tribological loading is governed by diffusion along defects such as dislocations [1]. In order to further investigate this, two sets of experiments were conducted with different orientations of the sliding direction respective to the crystallographic direction. By sliding along hard- and easy-glide directions, the aim was to enable different degrees of dislocation mobility along the sliding direction and to compare the resulting oxide formation.
Investigation and characterization of the oxides as well as of the general microstructure was performed using scanning electron and scanning transmission electron microscopy (SEM and STEM) as well as atom probe tomography (APT). Results showed that the experiments with different sliding directions lead to very different appearances of the resulting oxides; first insights will be presented.
In the long term, understanding the underlying and fundamental mechanisms of tribo-oxidation will enable the targeted development of friction- and wear-optimized surfaces.

References
[1] Julia S. Rau, S. Balachandran, R. Schneider, P. Gumbsch, B. Gault, C. Greiner, “High diffusivity pathways govern massively enhanced oxidation during tribological sliding”, Acta Mater., Vol. 221,
2021.