Use of the ultrahigh vacuum methods of surface science allows the preparation and study of highly characterized solid–solid interfaces for tribological investigations. One of the limiting factors in the progress of our understanding of tribology has been the inability to generate solid–solid interfaces with well-defined, reproducible, atomic scale structures and compositions. The preparation of single crystalline surfaces of metals and the characterization of adsorbed molecules on these

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... has been one of the major accomplishments of the field of ultrahigh vacuum surface science. Combining surface science instrumentation with an ultrahigh vacuum tribometer has made it possible to bring to the study of tribology the same quality of experimental measurement as is available for many other surface phenomena. One phenomenon that has been explored in great detail is the effect of the coverage of adsorbed species on friction between metal surfaces. Other measurements have included the study of friction anisotropy; the effect on the friction between two single crystal surfaces of their relative crystallographic orientations across the sliding interface. Finally, recent results have correlated the friction between adsorbate covered surfaces with the desorption energy of the adsorbed molecules. These results and measurements anticipated for the future offer the opportunity to address truly fundamental questions in tribology and complement many of the other new tools recently developed for the study of tribological phenomena.