How transfer film formation in bronze/silver-graphite sliding contact drives its electrical performance

Recent interest in improving lifespan of electrical slip rings for application in wind turbines has renewed the need for deeper understanding of sliding electrical contact operation. In contact slip rings using graphite-based brush technology, the formation of transfer film, or “patina”, on the surface of the metal ring is a well-known phenomenon [1], [2]. In this scope, the study of its formation in a bronze/silver-graphite contact has been conducted. The test bench is based on a real industrial slip ring specific for wind turbine application. The evolution of the transfer film was investigated by means of different surface characterization methods. EDX analyses permitted to establish a direct relation between transfer chemical composition, transfer thickness (h) and electrical resistance of the contact (Rc), highlighting the role of the transfer film in slip ring electrical performance. In particular, the presence of contaminating Cu 2 O oxides is emerging as a key factor in controlling both h and Rc. Optical microscopy and profilometry, allowed to propose a scenario of build-up following by destruction of the transfer film, explaining at the same time the origin of these Cu 2 O oxides. Focused Ion Beam (FIB) nanomachining, followed by Transmission Electron Microscopy (TEM) enrich this scenario and correlates the surface observations, revealing a strong evolution of the composition and morphology of the transfer pads from very carbonaceous, poorly graphitized transfers to a much more metallic and graphitized composition. In this last state, a lamellar structure is observed, with successive lamellae of silver separated by thin graphite layers. Raman spectroscopy also permits to study the crystalline structure of the graphite transferred on the ring surface and shows a progressive re-organization of the crystallites. While this reorganisation should be beneficial in a tribological perspective, it is important to note that this mechanism is entirely independent of both the growth-destruction of the transfer film and the evolution of Rc.