Effect of surface film on sliding friction and wear of copper-impregnated metallized carbon against a Cu-Cr-Zr alloy

A block-on-ring wear test was performed between a copper-impregnated metallized carbon brush and a Cu-Cr-Zr alloy ring under ambient environment. After 50 km of rubbing at 20 A electrical current and 111 kPa normal pressure at a constant velocity of 25 km/h, a surface film, which was composed of Cu2O, CuO, C and water, formed on the copper alloy ring due to graphite transfer from the brush to the copper alloy and oxidation of the copper. A second series of friction and wear tests was carried out both with and without this initial surface film and at different electrical currents. The friction coefficient and wear mass loss were compared. The results showed that the initial surface film could reduce the friction coefficient in the presence of an electrical current, but in the absence of an electrical current, the film's lubricating performance gradually degraded as the test progressed. Wear mass losses caused by the rubbing of the brushes against the rings having an initial surface film were lower when the electrical currents were 0 A, 10 A and 15 A, respectively, while the situation was reversed when the electrical current was 20 A, i.e., the wear mass loss of the brush specimen rubbing against a ring with an initial surface film was higher than that of a ring without it.

► The composition of surface films are different under the condition of with and without electrical current. ► Oxide layer plays the key role in reducing friction coefficient. ► The lubricating performance of surface film is different under different electrical current.