Enhanced surface segregation in sliding wear tracks

The effect of sliding wear on the rate of surface segregation of sulfur from an OFHC copper sample was investigated. This work 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 was made to slide across the surface with a contact force of 25 g for 9000 cycles. 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 at 390°C is interpreted as due 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