Wear of Silver-Graphite Brushes Against Various Ring Materials at High-Current Densities

A study has been carried out to evaluate noble metals for potential use as ring materials for high-speed high-current applications. A pin-on-disk friction apparatus was constructed so as to allow sliding tests to be carried out at high speeds and high-current densities in controlled environments. The pin material was a silver-graphite brush composite while the flat material was one of the elemental metals nickel, copper, silver, gold, platinum, palladium, rhodium, ruthenium, rhenium, or iridium. The pressure was 0.2 N/mm2, the current density was 0.8 A/mm2, and the sliding speed was 4 m/s. The friction, wear, and electrical resistance were determined in wet CO2 and dry air. In general, and surprisingly, materials which give high friction give low wear although, in fact, both the friction and the electrical resistance values covered a narrow range. The wear results are not in agreement with earlier data obtained during tests of silver pins sliding against the same metals at much lower loads, speeds, and electrical current densities, in which case the low wear metals were those with low metallurgical compatibility against silver, or else metals with a hexagonal crystal structure. In these new tests, the wear behavior at high speeds and high-current densities with silver-graphite brushes obeys essentially the same compatibility relationships, but referred to the graphite. The best performance in terms of low wear was provided by the metals rhodium, palladium, and platinum.