Microstructures and tribological characteristics of electron-beam co-deposited Ag/Mo thin film coatings

The tribological characteristics of surface coatings on a metal substrate are affected by several factors including the microstructure of the coatings, the interactions between the coating and metal substrate, and the mechanical properties of the substrate. Techniques are available to fabricate different types of metastable film structure to aid in unraveling their influences on tribological behavior. In this study, we investigated the friction and wear properties of thin film coatings consisting of Ag and Mo which were co-deposited by electron-beam evaporation in an ultra high vacuum. Ag and Mo were chosen because the metastable structures, such as extended solid solutions or nano-crystalline microstructures, can be formed by co-deposition, although alloys of the two metals cannot exist in thermodynamic equilibrium. Using a Cameron-Plint high frequency pin-on-plate friction machine, test results show that a co-deposited film having an optimum composition ratio of Ag:Mo = 69:31 (denoted by Ag₆₉Mo₃₁), and having a thickness of 150 nm, has the lowest friction (20%-25% reduction) and improved antiwear properties compared with coatings of other compositions. A bare steel plate after dry sliding of 30 h against a steel sphere shows substantial wear and plastic deformation. In contrast, the same plate with a thin film coating of Ag₆₉Mo₃₁ shows a much smoother surface. The improved tribological characteristics may be attributed to the greater wear-resistant and shearing capabilities of nano-crystalline binary mixtures on iron surfaces.