Enhanced surface segregation in sliding wear tracks

The effect of sliding wear on the rate of surface segregation of sulfur from an oxygen-free, high-conductivity (OFHC) copper sample was investigated. The study was carried out in an ultra-high-vacuum system having a residual gas pressure of 5×10^-11 torr. Wear tracks were formed on OFHC Cu that had previously been annealed and argon-ion sputter cleaned. A bent pin (palladium-based alloy, ASTM B540) was made to slide across the surface with a contact force of 25 g for 9000 cycles. Auger electron spectroscopy was used to characterize the composition of the surface on and off the wear track. No surface segregation was observed to occur as a result of forming the wear track. Subsequent in situ isothermal annealing between 310°C and 470°C produced S enhancement on the specimen surface. It was observed that the rate of S segregation on the wear track was much faster than off track up to approximately 390°C, where this rate decreased significantly. At higher temperatures, the rate of S segregation on-track approximately equaled the off-track rate. The drop in the rate of S segregation on the track of 390°C is attributed to annealing-out of the short-circuit diffusion paths associated with the defects formed when the wear track was made. The subsequent increase in S concentration at higher temperatures is due to the normal surface segregation phenomenon which occurs in annealed samples