Effect of current polarity on electrical sliding wear behavior of Cu-WS2-graphite-WS2 nanotube composites in air and vacuum conditions

In this paper Cu-WS₂-graphite-WS₂ nanotube composites were fabricated by powder metallurgy hot pressing method. The effect of current polarity on the wear rates and contact voltage drops of the composites were investigated using a brush-on-slip ring tribometer rubbing against Cu-5 wt.% Ag alloy ring in air and vacuum, respectively. The worn surfaces of the composites were analyzed by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Surface profile curves of the worn tracks were measured using the surface profiler. The results demonstrated that the current polarity has a significant effect on the wear rates and contact voltage drops of the composites in both air and vacuum conditions. Positive brush possesses a higher wear rate compared with the negative brush in the air atmosphere since the electrical field direction activates oxidation at the positive brush surface while inhibits oxidation at the negative brush surface. Except for the regular wear losses, the combined effect of molten metal bridge erosion and arc erosion cause the positive brush to lose extra material and the negative brush to gain extra material, so the positive brush shows a higher wear rate in the vacuum condition. The contact voltage drop of the positive brush is lower than that of the negative brush in the air atmosphere, but contrarily, the positive brush shows a higher contact voltage drop in the vacuum condition.