Fundamental measurements of the friction of clean and oxygen-covered VC(100) with ultrahigh vacuum atomic force microscopy: evidence for electronic contributions to interfacial friction

Abstract

Atomic force microscopy (AFM) has been employed in an ultrahigh vacuum (UHV) environment to measure the frictional properties of single-crystal vanadium carbide (VC) as a function of reaction with oxygen. The VC(100) surface, prepared by sputtering and annealing in vacuum, was found to be free of oxygen and to exhibit good crystallographic order with atomically flat terraces ∼100–200 Å in width. The coefficient of friction between a silicon nitride probe tip and the clean VC surface was measured as 0.52±0.04. Chemical modification of this surface was accomplished by exposure to molecular oxygen which produced a saturation coverage of chemisorbed atomic oxygen. The coefficient of friction between the same silicon nitride tip and the “oxidized” VC surface in the wearless regime was measured as 0.32±0.05, representing a ∼40% reduction in friction. This reduction in friction is associated with a reduction in the density of metal d electrons nearest the Fermi level which occurs upon oxygen adsorption. These changes provide evidence for the contribution of an electronic mechanism of energy dissipation at the sliding interface of a solid–solid point contact.