Enhanced sulfur segregation in plastically deformed OFHC Cu and the effect of surface segregated sulfur on electric contact resistance

Plastic deformation of metals has been known to enhance the diffusion of impurities as a result of the increased diffusivity along dislocation cores. Surface segregation of impurities tends to modify the surface properties of metals. In the present work we have examined the effect of sulfur surface segregation on the contact resistance for OFHC Cu. Two copper specimens were prepared by different surface treatments-one was abraded with SiC paper and another was polished with a metal polishing paste. As expected, the kinetics of thermal sulfur segregation for the two specimens were different due to the differing amounts of cold work done on their surfaces. Contact resistances were measured in situ in UHV at dry circuit levels and at different loads (minimum 1 g) as a function of the sulfur content on the specimen surfaces. The surface concentration of sulfur increased from zero by annealing the specimens at increasing temperatures. As the surface concentration of sulfur increased to approximately 10 a/o, the contact resistances decreased for all loads. However, above a critical sulfur concentration of about 10 a/o, the contact resistance increased sharply. Further increases in the sulfur concentration did not cause an appreciable change in the contact resistance and a saturation was observed. No polarity effects were found for the two specimens. Air exposure of the specimens caused the sulfur to disappear from the surfaces, being replaced by oxygen and carbon. Correspondingly, the contact resistance increased approximately linearly with time of exposure