Ultrahigh vacuum studies of the boundary lubrication of metals

Abstract

A novel ultrahigh vacuum (UHV) tribometer was used to study the tribological behavior of two Cu(111) surfaces modified by the adsorption of various atomic (sulfur, chlorine, iodine) and molecular (ethanol, trifluoroethanol, butanol, and heptafluorobutanol) species as a function of coverage. The friction behavior of clean Cu(111) surfaces was characterized by stick-slip sliding behavior and high static friction coefficients.

Of the atomic species studied, only contaminant films formed by exposure to the atmosphere were effective as lubricants, in the sense that the sliding behavior changed from "stick-slip" to "slip" and the friction coefficient was significantly lower than that of the clean surfaces. As the contaminant film thickness was reduced by sputtering, a transition to higher friction coefficients and stick-slip behavior occurred at film thicknesses $\sim$5-8 A. The problem of the poorly defined thickness and composition of the contaminant films was addressed by making friction measurements on surfaces that were cleaned and then modified by the adsorption of sulfur, chlorine and iodine. Compared to the clean surfaces, the friction coefficient was reduced only slightly (sulfur, chlorine) or not at all (iodine), even at saturation. From these results, it can be concluded that the minimum thickness is greater than $\sim$2 A, the effective ionic radius of sulfur, chlorine or iodine atoms. The difference in the lubricating ability of the three species was small and of uncertain significance.

For the alcohols, friction measurements were made at coverages ranging from submonolayer to several multilayers. These alcohols showed similar friction behavior as a function of coverage. At coverages below one monolayer the surfaces were not well lubricated--the sliding behavior was stick-slip and the friction coefficients were high, as for the surfaces modified with sulfur, chlorine and iodine. At coverages greater than one monolayer the friction coefficient was significantly reduced and decreased monotonically with increasing coverage. The transition from "stick-slip" to "slip" sliding behavior was reached at coverages of 2-7 ML. The friction coefficient continued to decrease with increasing coverage, reaching a limiting value only at coverages of several multilayers.