Molecular layering effects on friction at Ni(100)/Ni(100) interfaces

The combined use of an ultrahigh vacuum tribometer and a variety of surface science techniques has enabled us to explore the tribological properties o f interfaces between Ni(100) surfaces and to observe phenomena attributable to molecular layering. Friction measurements have been made between a pair of clean Ni(100) surfaces, modified by the presence of adsorbed atomic sul fur with and without adsorbed ethanol. Friction measurements made with etha nol coverages ranging from 0 to 10 monolayers (ML) on each Ni(100) surface reveal that the friction coefficient is discontinuous in coverage and can b e correlated to the coverage dependence of the ethanol desorption energy. D uring shearing, sliding never commences between clean Ni(100) surfaces or s ulfided Ni(100) surfaces without adsorbed ethanol. In the submonolayer cove rage regime of either atomic sulfur or adsorbed ethanol, the behavior is ch aracterized by a high friction coefficient (mu (s) > 5.5) accompanied by hi gh adhesive forces (mu (ad) = 1.5 +/- 0.7) An abrupt decrease in both the f riction coefficient and adhesion coefficient occurs at a coverage of 1 ML o f ethanol on each surface. The friction coefficient drops to mu (s), = 3.1 +/- 1, while the adhesion coefficient is lowered to mu (ad) approximate to 0.25. At coverages between 1.0 and 2.5 ML of ethanol on each Ni(100) surfac e, the static friction coefficient decreases in a stepwise manner that is c orrelated with discontinuities in the ethanol desorption energy. This stepw ise decrease in both the friction coefficient and the desorption energy may be due to molecular layering of the ethanol.