Effects of Crystallite Orientation on Environmental Stability and Lubrication Properties of Sputtered MoS2Thin Films

The endurance lives of rf sputtered MoS2 lubricant films are severely degraded by exposure to high humidity environments prior to wear testing. Surface chemical and electron microscopic analyses show that the degraded films are oxidized to form MoO3 and that the oxidation rate is a function of the orientation of the crystallites composing the films before any run-in or burnishing activity. If the crystallites are arranged with their basal planes parallel to the substrate surface, the films have lower reactivity (good stability) and longer endurance lives than film with randomly oriented crystallites. The results of surface analyses and wear tests are presented to support these observations. Auger electron spectroscopy results indicate that oxidation of the randomly oriented films (designated Type I films) occurs to a depth of at least 10 nm into the bulk of the films when they are exposed to adverse environments. Oxidation of films with coplanar orientations (Type II films), however, occurs only within 1 nm of their surfaces. X-ray photoelectron spectroscopy shows that the Type I films have a much larger proportion of oxidized molybdenum [i.e., Mo(IV):Mo(VI) ≅ 0.6] than the Type II films [Mo(IV):Mo(VI) ≥ 1.7] after 1 year of storage in air at relative humidities of 50 percent or less. Type II films have significantly better lubrication properties (endurance lives at least a factor of 2 longer) than Type I films.