Characteristics of films developed in fretting experiments on tin plated contacts

A series of experiments were conducted to characterize oxide films that develop during fretting degradation. It was found that while the rate of change in resistance depends on contact force, the oxide film characteristics depend strongly on the number of fretting cycles. Moreover, after about 1000 fretting cycles, the oxygen content reaches a saturation level which corresponds to tin volume fractions below the percolation limit for metallic conduction (<0.4). Consequently, it is concluded that conduction is primarily due to the semi-conductor properties of tin oxide (SnO). In addition, sub-micron size particles, composed of tin and tin oxide, were found dispersed over the surface. These particles had tin fractions near the percolation limit and may play a role in the mechanism that causes short-term discontinuities in contact resistance.