On the tribological behavior of adsorbed layers, especially moisture

Abstract

The action of adsorbed molecular films, and in particular moisture, to affect friction and sliding mode was studied in the hoop apparatus. The samples were bundles of 50 μm thick gold-plated copper fibers sliding on gold-plated copper, i.e. under gravity inside a gold-plated vertical copper hoop rotating at uniform speed about its horizontal cylinder axis. The results confirmed previous observations that the incidence of stick-slip increases with decreasing hoop speed and with increasing humidity, in the present study in a nitrogen atmosphere. Also, stick-slip increases with increasing normal force and is much more persistent for bulk samples than for fibers. Additional information had been gained previously through detailed examinations of the momentary contact resistance. The data suggest that, although the thickness of adsorbed moisture films increases with humidity, at contact spots all molecular layers except one on each side drain out, regardless of humidity. Two contributions to the coefficient of friction μ result. 1. (i) The displaced water flows about the spots and thereby adds an increment δμ k which rises with humidity. At slow speeds δμ k is proportional to sliding speed but shear thinning of the fluid causes a decrease in δμ k at higher speeds, 2. (ii) Capillary action of the moisture meniscus about the spots effectively adds to the applied normal force and thereby increases μ by an increment δμ s. Relaxation effects arise because neither moisture drainage nor equilibration of the meniscus is instant. The resultant different dependences of δμ k and δμ s on time, speed and on the spot size appear to explain the observations.