UHV-AES Investigation of Sulfur Surface Segregation in Precious Metal Wear Tracks

Cyclic sliding experiments of a palladium-base (ASTM B540) alloy pin On a gold-base (ASTM B541) alloy plate have been completed in an ultrahigh vacuum (UHV) system that was backfilled to one atmosphere with chromatographic-grade helium. The elemental concentrations in the wear tracks were measured in situ by Auger electron spectroscopy (AES) before and after several thousand cycles were performed. The concentration of sulfur in the wear tracks showed an · appreciable increase (a factor of 2.5-6.4) relative to the off-track regions. The sulfur enhancement was dependent on the number Of sliding cycles. Residual gas analysis of the vacuum-system atmosphere before introduction of the helium and analysis of the chromatographic-grade helium detected no sulfur-containing gases; it is estimated that the partial pressure of sulfur-containing gases was less than - 1 x 10
^-11
torr for all experiments. These results complement an earlier study involving Cyclic sliding of the same alloy couple which also showed that sulfur enhancemerit occurred in the wear tracks. The earlier study was completed in a dry box using controlled environments of helium, carbon dioxide, and oxygen; however, the partial pressure of sulfur-bearing gases could have been orders of magnitude larger than those in the present study. It is concluded that the sulfur originates in the bulk of the alloys rather than in the various environments in which the oscillations were carried out.