Mitigation of Micropitting Wear with Ti-doped Molybdenum Disulfide Coatings

Micropitting and scuffing wear of rolling element bearings and gears in gas turbine engines are common occurrences and cannot usually be mitigated by lubricant additives or base oils. Therefore, it is of interest to explore other strategies, such as the use of tribological coatings. Temperatures and speeds of critical components in gas turbine engines limit the types of coating that can tribologically function effectively in the demanding environments. Dichalcogenide-based coatings, such as MoS₂ can perform well at elevated temperatures and speeds, but their tribological performance becomes significantly degraded with moisture and humidity. Doping MoS₂ with transition metals such as Ti greatly decreases the negative effects of moisture and humidity on the friction and wear of the coating. In this work, the ability of Ti-MoS₂ coatings to affect the onset of micropitting on AISI M50 steel rollers was evaluated as a function of slide to roll ratio and water content in a high performance aircraft-type gas turbine lubricant. Experiments were performed in a micropitting rig that utilizes three rings radially loaded against a roller. Since the rotational speeds of the rings and roller can be controlled independently, experiments can be conducted with a defined slide to roll ratio. In tests performed with a slide to roll ratio of +10%, Ti-MoS₂ coatings delayed the onset of micropitting and scuffing wear by at least an order of magnitude. The proposed mechanism by which the coatings were able to deliver these desirable tribological benefits is through tribochemical interactions occurring between the constituents of the coating and the lubricant that created much thicker and more durable antiwear films on the rings and rollers than were created from the lubricant alone.