In situ Examination of Segregation and Wear Processes of Precious Metal Electrical Contact Alloys

The electrical contact resistance and friction coefficient of gold-based (ASTM B541) and palladium-based (ASTM B540) precious metal electrical contact materials have been measured simultaneously with au in situ pin-on-plate device in a scanning Auger analytical system. Intermittent surface analysis of the wear track was performed as wear of the contact surface progressed without repositioning of the pin in the wear track. The wear process was one of adhesive wear. Initially, a prow formed on the pin, and subsequently, debris was transferred back and forth between the pin and plate. The identity and partial pressures of gases in contact with the wear surface were controlled precisely, thereby avoiding exposure to unwanted gas phase contaminants. The friction coefficient varied from 2.2 to 2.6 for high vacuum ambients containing oxygen (10-6 torr) or nitrogen (10-7 torr) but was 1.6 for an atmospheric ambient (700 torr) of 0.05 oxygen in nitrogen. In comparison, the friction coefficient was <0.5 in laboratory air. The electrical contact resistance was < m for all test conditions but varied inversely with the friction coefficient. Segregation of sulfur to the wear track was observed in situ in the 700-torr ambient after as few as ten wear cycles. The friction coefficients for these tests were larger than for previous experiments; the increase was attributed to surface depletion of copper.