Effect of different Current Densities on Cu/Al Interface Transfer Film and Current-carrying Friction Properties

Cu-Al contact pairs are commonly used in sliding electrical contact applications and are exposed to various complex working environments during their operation. This study investigated the effects of current density on the formation of transfer film at the electrical contact interface and the current -carrying tribological performance. The contact pairs included Cu pin-Al disc and pin-Cu disc. The pin samples were in vertical contact with the rotating discs at a linear velocity of 17.5 m/s, with an applied load of 28 N for a duration of 2 minutes. These conditions ensured that no welding or overheating occurred. As the current density increased from 0 to 0.5 A/mm², both the friction coefficient and contact resistance of the contact pairs slightly decreased. The oxidation and softening caused by the electric current led to an increase in wear loss. The softening of the materials made the surface asperities more prone to deformation, resulting in increase of the number of the contact points and contact area, thereby reducing the contact resistance. Microscopic analysis revealed that at a current density of 0 A/mm², a small amount of transfer film adhered to the surface of the Cu pins. At 0.5 A/mm², the Cu pin surface exhibited a flaky morphology, and the wear mechanism for the Cu pin-Al disc pair transitioned from adhesive wear to arc erosion. In contrast, at 0 A/mm², the Al pin surface showed a scaly plastic deformation. At 0.5 A/mm², the surface exhibited signs of melting. The damage behavior of the Al pin-Cu disc pair transitioned from plastic deformation to arc erosion. Energy-dispersive X-ray spectroscopy (EDS) analysis demonstrated that the Al: Cu atomic ratio on the copper pin surface increased from 0.073 to 10.022, while the Cu: Al atomic ratio on the aluminum pin surface increased from 0.021 to 0.252. Due to the lower softening temperature of aluminum, the transfer film on the copper pin surface was more pronounced under the same conditions. The coverage of transfer film on the Cu surface increased from 52.3% to 96.6% and the film thickness increased from 4.69 μm to 19.37 μm with the increase of the current density. The appearance of surface melting indicated the formation of an arc during the sliding process. X-ray photoelectron spectroscopy (XPS) analysis identified Al₂O₃ was the main component of the transfer film on the copper pin surface, while trace amounts of CuO and Cu present on the aluminum pin. In summary, the mechanisms of surface damage were attributed to mechanical wear, arc erosion, and oxidative wear. The experimental results in this article were all derived from the continuous sliding process. The effect of oxidation on electrical contact performance was not significant, as oxides could be removed during sliding. For current-carrying frictional pairs used intermittently, the presence of Al₂O₃ in the transfer film could reduce the electrical conductivity of the following sliding. The results of this study may provided support for material selection and failure analysis of high-power sliding electrical contact pairs.