Film-thickness effect of Ag lubricant layer in the nano-region

Abstract

The tribological properties of Ag films with a wide range of thicknesses on an Si(1 1 1)-7×7 surface have been studied using a pin-on-plate type tribometer under ultrahigh-vacuum (UHV) conditions. Reflection high-energy electron diffraction (RHEED) patterns show that the Ag film deposited on an Si(1 1 1)-7×7 surface at room temperature (RT) forms a two-dimensional fiber crystal structure and gradually grows into a single-crystal Ag film as the film thickness increases. The coefficient of friction of 170 nm as-deposited Ag film was 0.06–0.08, and that coefficient decreased to 0.01–0.03 when the thickness decreased to 8.4–1.5 nm. In the thickness decreased from 1.5 to 0 nm, the coefficient of friction increase as the thickness decreased. The clean Si(1 1 1) surface showed a coefficient of friction of 0.8. The worn track widths on both the 1.5 nm Ag film and 170 nm Ag films were comparable. Auger electron spectroscopy (AES) measurements showed that the Ag MNN intensity ratio of the worn track to non-slide surfaces was 0.9–0.6 after frictional experiments. By annealing the 0.4 and 1.5 nm films at 500 °C, the morphology of the film changed significantly; one atomic layer of Ag formed a 3 × 3 Ag surface ( 3 Ag), and the residual Ag formed fine crystals. The population of the crystals increased as the film thickness increased. Virtually all of the 0.4 nm annealed Ag film area was 3 Ag surface, and the film showed a coefficient of friction of 0.1. The 3 Ag was covered by fine Ag crystals with the 1.5 nm annealed film, which showed a coefficient of friction less than 0.01. The high-lubricity mechanism of the Ag film in the nanometer-thickness region is discussed in terms of its mechanical properties such as the shear strength of Ag depending on the film thickness.