Elemental Surface Composition of Slip Ring Copper as a Function of Temperature

The surfaces of four copper samples--oxygen-free high-conductivity (OFHC) (99.98 percent) Cu, 99.99 percent Cu, 99.9999 percent Cu, and a dilute Ag/Cu alloy (0.086 weight percent (wt %) Ag in OFHC Cu)--were examined in an ultrahigh vacuum system by Auger electron spectroscopy as a function of temperature between 25 and 600°C. It was found that the elemental surface composition of each sample varied drastically with temperature. For example, carbon was a surface constituent in amounts ranging from 0 to about 100 at %, depending upon surface treatment. An interesting finding was the rapid surface segregation of sulfur upon heating in all samples, leading to surface concentrations ranging up to 32 at % in some cases. These results suggest that the higher temperatures occurring at the current-carrying surface asperities will alter the composition of the interface. Hence, the electrical resistance of the interface and its friction and wear properties may be grossly affected.