Tribological behavior of an Ag sub-nanometer-thick film on an Si (1 1 1) 7 × 7 surface at elevated temperatures

The tribological characteristics of a 0.3 nm thick silver film on an Si (1 1 1) 7×7 surface in sliding contact with diamond have been studied as a function of the substrate temperature in an ultrahigh vacuum (UHV) environment. The morphological change of the Ag film at elevated substrate temperatures was observed in situ reflection high-energy electron diffraction (RHEED). The morphology of the Ag film changed abruptly at a temperature of approximately 470 K (Tc), the Ag film shows a fiber-texture structure at temperatures lower than 470 K, whereas a 3×3-R30° superstructure appears at temperatures between 470 and 700 K. The average coefficient of friction of a 0.3 nm thick Ag film is 0.02 at room temperature (RT), and increases up to 0.6 as the temperature increases to 700 K. The coefficient of friction abruptly changes at Tc, and the change indicates that the friction property of the film strongly depends on the morphology of the film. The amount of Ag on a worn surface, determined by in situ Auger electron spectroscopy (AES), decreased with an increase of substrate temperature. The lubricity of an Ag film in the sub-nano region is discussed in terms of the morphological change of the film.