Tribology and sliding electrical contact resistance of e-beam hard Au: Effects of annealing

Nanocomposite Au-ZnO thin films in the dilute oxide (<5.0 vol%) regime were synthesized by electron beam (e-beam) evaporation, as alternatives to electroplated Au hardened with Ni. Tribological measurements of e-beam hard Au were made while passing current through sliding contacts; electrical contact resistance (ECR) and friction data were simultaneously acquired during the test. The friction, wear and ECR behaviour were studied for the as-deposited film condition, and after annealing at 250 °C and 350 °C in air. The study revealed that the 250 °C annealed Au-2 vol% ZnO film exhibited the lowest, stable friction coefficient s (µ~0.25) and ECR (~35 mΩ) during sliding. Furthermore, the wear rate of this 250 °C annealed ZnO hardened Au nanocomposite film was an order of magnitude lower at 1.5×10−5 mm3/N m than for a typical Ni hardened, electroplated Au film at 1.3×10−4 mm3/N m. Cross-sectional transmission electron microscopy studies inside the wear surfaces revealed that the extremely stable, low friction coefficients and wear rate of annealed Au-2 vol% ZnO film was due to partial coverage of the wear surface with a ZnO tribofilm that reduced the adhesive contact contribution to wear with minimal impact on ECR. The potential implications of this study in the search for an environmentally friendly alternative to widely used electroplated hard Au are discussed.