Electrical sliding friction and wear properties of Cu-MoS2-graphite-WS2 nanotubes composites in air and vacuum conditions

Cu-MoS2-graphite-WS2 nanotubes composites were fabricated by the P/M hot pressing method. The tribological properties and contact voltage drops of the composites were investigated using a block-on-ring friction and wear tester in air and vacuum respectively, rubbing against Cu-5 wt% Ag alloy ring with different electric current. Tribo-films formed on the worn surfaces were characterized by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Results illustrated that the friction coefficients and wear rates in air were lower than those in vacuum due to the synergistic effect of lubrication from the mixture of MoS2 and graphite in air and the limited lubrication properties of graphite in vacuum. The friction coefficients and wear rates increased gradually with the increasing current density because of the additional Joule heat resulting from the release of current. The contact voltage drops were low in the initial stage, and then increased to a stable value owing to the gradual formation of tribo-films on the sliding surface which changed the nature asperities from metal-metal to metal-film-metal type on the contact interfaces of composites and rings.