Moisture Effects Including Stiction Resulting From Adsorbed Water Films

Stiction resulting from moisture effects at small elastic contact spots has been identified and studied using bundles of fine, gold-plated copper fibers sliding on a gold-plated copper surface. The relevant measurements were made in the hoop apparatus which permits simultaneous monitoring of the momentary coefficient of friction and electrical contact resistance. Previous studies made with the hoop apparatus have shown that under the action of high local pressure, adsorbed moisture is expelled from between the contact spots leaving only one monomolecular layer of adsorbed water on each of the contacting surfaces. Additional details of the observations are varied and permit a refined analysis. Stiction results during periods of very slow motion or rest through local energy reduction at the spots as excess water is slowly drained in the course of molecular ordering of the two absorbed layers. Complex variations of kinetic friction with humidity and sliding speed are explained through the interplay of excess molecules between the contact spot surfaces, meniscus formation, fluid drag about the spots, and shear thinning in that flow.