In situ observation of wear particle formation on lubricated sliding surfaces

The topography of a Cu surface was monitored in situ during a sliding experiment with lubrication. Fe and steel pins were used as counter-faces and poly alpha olefin (PAO-8) was utilized as lubricant. 3-D images of a specific area within the wear track were acquired after every cycle. These experiments were carried out with a state-of-the-art tribometer, which uses a holographic microscope in immersion to observe the Cu surface online. The results show the evolution of the wear track, with a particular focus on the generation of flake-like wear particles. For the first time, these phenomena are correlated with the dynamic behavior of the surface. It is shown that the generation of wear particles results from various processes that occur on the surface, apart from crack propagation. The areas where wear takes place spread within the wear track during the experiment. Additional investigations of the subsurface structure were performed by means of focussed ion beam cross-sections. These revealed a gradient from nanocrystalline to ultrafine-crystalline grain structure beneath the Cu surface, as well as an orientation effect when sliding in one direction, linking the present work with the results published in the literature. Finally, X-ray photoelectron spectroscopy depth profiles were obtained from the counter-faces. These indicated material transfer from the softer to the harder surface, but no deposition from the sliding pins onto the Cu surface.