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

The effect of humidity on gold-plated contacts has been studied in the hoop apparatus using a solid slider and fiber bundles. The specimens slide smoothly (for significantly positive d mu /dv), execute harmonic oscillations (for dv/dt near zero), or stick-slip (for significantly negative d/dv). 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 approximately 0.5-nm-thick adsorption film at the contact spots, independent of coefficient of friction, sliding mode, or ambient atmosphere including vacuum. Minute traces of contaminants can inhibit the humidity effect. The observations are explained using a model of adsorbed films in which all but one monomolecular layer on each side are squeezed out from between load-bearing areas, and that the excess molecules flow about the contact spots during sliding. 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.>