The effect of the crystallographic orientation of Ag thin films on their tribological performance

Abstract

The correlation between the lubricity of Ag films with nanometric thickness and their crystallographical orientation on an Si substrate surface has been studied in ultrahigh vacuum (UHV) environment. The Ag films with 5 and 57 nm in thicknesses were prepared on both 7×7 and √3-Ag surfaces using water-cooled Knudsen-Cell in the UHV condition at a pressure of less than 4×10−8 Pa. The growth process of Ag film is epitaxial when the films deposited on the Si (111) 7×7 clean surface (7×7 surface), whereas the Ag films become polycrystalline on √3-Ag surface at temperatures of less than 303 K. The friction coefficient of polycrystalline films was 3 to 4 times higher than that of epitaxial films. These results indicate that the crystallographic orientation of soft metallic solid lubricant layers strongly effected to their tribological performance. Reflection high-energy electron diffraction (RHEED) observations on the worn surfaces of polycrystalline films with a thickness of 5 nm showed the same patterns of Ag (111) surface, which means that crystallographic orientation of polycrystalline film was changed by mechanical rubbing motion of diamond pin. The effect of the crystallographic orientation of Ag film on its tribological performance is discussed.