The Surface Segregation of Sulfur During Oscillatory Pin-On- Plate Wear Studies of Precious Metal Electrical Contact Alloys

An oscillatory pin-on-plate experimental configuration was used to investigate friction and electrical contact resistance phenomena of a gold-base (ASTM B541) and a palladium-base (ASTM B540} alloy couple. Measurements were made in controlled environments of dry helium, dry carbon dioxide, and dry air at atmospheric pressure. No lubrication was employed. Friction coefficients were initially high but decreased to a steady value of approximately 0.2 as the number of oscillatory cycles increased for dry carbon dioxide and dry air. The friction results for dry helium were much more erratic. Friction coefficients are much less sensitive to the nature of the contact interface than is electrical contact resistance. The electrical contact resistance always had initial values of 1-10 mµ, but it tended to increase by up to three orders of magnitude in a nonreproducible manner for all three gases. Sharp fluctuations in electrical contact resistance were common. Electrical polarity was reversed, but due to the large variability in contact resistance no effect was identified. The surface compositions and morphologies of wear tracks were characterized using scanning Auger microscopy combined with Ar-ion beam sputtering. The analyses revealed that the wear tracks formed in all three atmospheres contained an approximately 6-rim thick surface film consisting of S, Ag, Cu, C, O, and Pd; the Pd was transferred from the pin as small debris paMicles. The S in this thin surface film may be present as Ag and/or Cu sulfides. The observed increase in the contact resistance and decrease in the coefficient of kinetic friction are attributed to the growth of thin sulfide layers on the wear track surfaces.