Morphological effect of epitaxial Ag films with thickness of the order of nanometres on the tribological properties of a single-crystal silicon surface in an ultra-high-vacuum environment

Abstract

The tribological properties of thin Ag films deposited on an Si (111) 7 × 7 surface have been studied in sliding contact with a diamond as a function of the film thickness ranging from 0.4 to 170nm in an ultra-high-vacuum environment. Observations by scanning tunnelling microscopy (STM) and atomic force microscopy on the morphology of the epitaxial Ag film showed three kinds of structure as the thickness increased: an island structure (at 1.5 nm or less), a network structure (between 1.5 and 20 nm) and a continuous rough film (thicker than 20 nm). The coefficient of friction reached at 0.007 for an Ag film thickness of 5 nm with the network structure, and it increased as the film thickness increased more than 5 nm. The coefficient of friction was 0.06 for a thickness of 170 nm; therefore, the coefficient of friction correlates with the morphology of the film. Observations by means of STM and reflection high-energy electron diffraction showed that the fine Ag (111) sheets with the network structure were sheared and rotated by sliding of the diamond surface. The high lubricity of the thin Ag film was attributed to a perfect film without shearing in spite of the nanometric thickness and much lower shear strength of the film than that of the Si substructure, and the lubricity was influenced by the morphology of the film.