Friction anisotropy at Ni(100)/Ni(100) interfaces

The combined use of an ultrahigh vacuum tribometer and a number of surface science techniques has enabled us to explore the tribological properties of interfaces between single-crystal metal surfaces and to address the fundam ental issue of frictional anisotropy. Friction measurements have been made between a pair of Ni(100) surfaces which were prepared to be truly clean or modified by the presence of adsorbed atomic sulfur with and without adsorb ed ethanol. Measurements made with systematic variation of the relative cry stallographic orientations of the two Ni(100) surfaces have revealed that t he friction coefficient is anisotropic with respect to lattice orientation. When aligned (theta similar to 0 degrees) and sliding along the [110] dire ction, the friction coefficient between the two clean Ni(100) surfaces was mu (s) = 816 +/- 2.5. The minimum static friction coefficient occurred when the two clean Ni(100) surfaces were misoriented by theta similar to 45 deg rees (mu (s) = 2.5 +/- 1). This orientational anisotropy is consistently ob served even in the presence of adsorbed atomic sulfur and up to 4 monolayer s of adsorbed ethanol, although these modified surfaces no longer have the same surface lattice periodicity as the Ni(100) substrate. The effect of la ttice orientation is damped out at the point that the surfaces are separate d by >20 monolayers of adsorbed ethanol. The friction anisotropy observed b etween Ni(100) surfaces suggests that surface lattice commensurability is n ot the only cause of friction anisotropy in this system.