Influence of TiO2 Additive on the Tribological Performance of Bonded MoS2 Solid Lubricants

To elucidate the role of environmentally friendly oxide additives in a molybdenum disulfide (MoS2)-based solid lubricant, this study investigates the tribological behavior of a MoS2–TiO2 coating deposited via a spray-bonding process and compares it with a commercial Sb2O3-containing formulation (Everlube 620C). Interfacial characteristics and wear-related mechanisms were systematically analyzed using scanning electron microscopy (SEM), focused ion beam (FIB), Raman spectroscopy, and X-ray diffraction (XRD). The MoS2–TiO2 coating exhibited a higher steady-state coefficient of friction (0.35–0.45) and wear compared to the baseline. Its wear behavior was governed by fracture-induced three-body abrasion, driven by the hard and brittle nature of TiO2, which promotes stress concentration at particle–matrix interfaces, crack initiation, particle pull-out, and debris generation. These processes suppress the formation of a desirable MoS2-rich tribo/transfer film, leading to deformation-dominated friction. Overall, the findings indicate that the intrinsic mechanical properties and interfacial behavior of TiO2 limit its effectiveness as an additive in MoS2-based coatings, highlighting the importance of additive selection and compatibility in achieving optimal tribological performance. Notably, this study was performed at an additive volume fraction equivalent to that of Sb2O3 in Everlube 620C, serving as a foundation and indicating that further optimization of TiO2 particle size and concentration is required to achieve comparable performance.