Adsorption films, humidity, stick-slip and resistance of sliding contacts

The effect of humidity oil gold-plated contacts is studied in a hoop apparatus using a solid slider and fiber bundles. The specimens slide smoothly, execute harmonic oscillations, or stick-slip. High humidity and low speed favor stick-slip, while low humidity and high speed favor smooth sliding. The tendency for stick-slip is greater for a solid slider than for fiber bundles. The electrical resistance is indicative of an about 0.5-nm-thick adsorption film at the contact spots, independent of the coefficient of friction, sliding mode, or ambient atmosphere including vacuum. Minute traces of contaminants can inhibit the humidity effect. All of the observed effects can be explained qualitatively by means of a simple contact spot model. Electrical noise is due to minor variations of film thickness at the load-bearing areas. It is larger for a solid slider than for fiber bundles, and larger for stick-slip than other sliding modes