Tarnishing resistance of silver–palladium thin films

Abstract

Ag–Pd decorative coatings are supposed to provide better tarnishing resistance as compared to pure Ag, while keeping good optical (brightness) and mechanical properties (ductility). Palladium has a similar optical appearance as silver, and forms a thin protective oxide surface layer. Thus, a small incorporation of Pd in Ag could improve its tarnishing resistance. Thin Ag–Pd films were deposited by magnetron co-sputtering from Pd and Ag targets. The Ar gas pressure, target power and substrate temperature were varied to modify the chemical composition of the films and optimise their properties. The optical properties of the films were evaluated by spectroscopic ellipsometry and colorimetry. Increasing Ar gas pressure and substrate temperature results in a drastic decrease of the specular reflectivity of the films. At constant deposition conditions the reflectivity of the Ag–Pd films decreases with increasing Pd content. The film hardness, evaluated by nanoindentation, increases with Pd content. The tarnishing resistance of the films was evaluated by sulphidation tests. The colour change of the films due to the sulphidation was measured by colorimetry. The nature of chemical bonds of the tarnished products was evidenced by XPS. The results suggest an improvement of tarnishing resistance of the Ag–Pd films with increasing Pd content.