Tribological Investigation of Environmentally Friendly Bonded MoS2-Based Solid Lubricants for Aerospace Applications - CME Seminar Series Fall 2025

MoS2-based solid lubricants are widely used in space/aerospace applications for their low friction and wear resistance, especially under vacuum condition. However, their performance degrades in humid or oxygen-rich environments, prompting the use of lead-based additives to enhance stability. While such lead-containing formulations deliver reliable lubrication, the recognized toxicity of Pb and its adverse environmental and health impacts have driven a transition toward greener alternatives.

To improve environmental sustainability, this study reformulates conventional coatings by eliminating Pb₃(PO4)2 and incorporating greener alternatives. Conventional formulations for MoS2-based solid lubricants containing hazardous additives (i.e., Sb2O3 and Pb₃(PO₄)2) were used as baselines.

To address environmental concerns with a short-term approach, Pb₃(PO4)2 was replaced with eco-friendly additives such as SnO, Ag2O, Bi2O3, AgNO3, and TiO2.

Furthermore, the commercial Everlube 620C formulation was developed by substituting Pb₃(PO4)2 with additional Sb2O3. Based on this formulation, as a medium-term strategy, Sb2O3 was further substituted with greener oxides, including Bi2O3 and TiO2. For long-term solutions, hexagonal boron nitride (hBN) and polyetheretherketone (PEEK) were also explored as environmentally friendly alternatives for Sb2O3.

The tribological performance of the developed coatings was evaluated using a reciprocating ball-on-flat tribometer at different test conditions. Subsequently, the unworn and worn surfaces of the coatings were characterized by ex-situ analysis techniques.

The results showed that the short-term formulations provided lower risk and comparable performance to conventional lead-containing coatings, while significantly improving sustainability.

Overall, this study aims to develop next-generation, environmentally-friendly MoS2-based solid lubricants by optimizing both the type and concentrations of additives to enhance tribological performance under various application-relevant conditions.