Wear phenomena and tribofilm formation of copper/copper-graphite sliding electrical contact materials

Copper-graphite composites in reciprocating sliding against copper are investigated with regards to friction, wear and contact resistance. The tribological and electrical evaluation is complemented with surface analysis. It is shown that the presence of graphite in the composite greatly reduces the coefficient of friction and wear. However, the amount of graphite is not critical and only has a minor influence on friction and wear. It is observed that the coefficient of friction is slightly lower, while the wear rate is slightly higher, in pure mechanical tests than in tests with current. The contact resistance is greatly reduced by an increase in the copper content in the composite. Chemical analysis of the tribofilm that forms on the copper surface shows that it consists of graphite as well as Cu2O. It is shown that the mating couple with the highest amount of oxide in the tribofilm also has the lowest contact resistance. Hence, it is concluded that oxides are not necessarily detrimental for the contact resistance as long as there is unoxidized copper available. Novel cross-section techniques and images of the tribofilm contribute to a deeper understanding of how sliding electrical contact surfaces are affected by current and sliding motion.