Bridging the Gap Toward Sustainable Tribology: Environmentally Friendly Bonded MoS2-Based Solid Lubricants

MoS₂ remains one of the most extensively studied and widely applied two-dimensional solid lubricants in aerospace and space applications due to its intrinsically low friction coefficient, arising from its layered crystal structure. However, its tribological performance degrades in ambient environments due to its sensitivity to oxygen and humidity. To mitigate this limitation, additives are commonly incorporated to stabilize MoS₂ by interacting with O₂ and H₂O molecules or passivating reactive edge sites. Historically, PbO and Sb₂O₃ have been used to enhance durability; however, their toxicity raises significant environmental and health concerns, motivating the development of sustainable alternatives.

In this study, MoS₂-based coatings containing lead compounds and Sb₂O₃ were considered as the baseline. As a short-term solution for improved environmental friendliness, lead compounds were replaced with metal oxides (i.e., Ag₂O, Bi₂O₃, TiO₂, SnO). Subsequently, lead compounds were replaced with additional Sb₂O₃, aligning with the commercial Everlube 620C formulation. Based on this formulation, and maintaining the same volume concentration of additives, the medium-term solution involved replacing Sb₂O₃ with Bi₂O₃ and TiO₂. The long-term solution focused on replacing Sb₂O₃ with a high-performance polymer (PEEK) and a lamellar ceramic (hBN). Coatings were deposited via a spray-bonding process with an average thickness of ~9 μm and evaluated using ball-on-flat reciprocating tribological testing. Surface and subsurface analyses, including 3D profilometry, SEM, FIB, XRD, and Raman spectroscopy, were conducted to investigate coating integrity, wear mechanisms, and tribofilm evolution.

The results demonstrate that coating performance is strongly governed by additive type and its role in tribofilm formation. Coatings containing Sb₂O₃ combined with metal oxides, particularly Ag₂O, exhibit superior performance (low risk) due to enhanced load support and stable tribofilms. In Sb₂O₃-free systems, MoS₂–Bi₂O₃ coatings show promising performance (medium risk), balancing environmental compatibility and durability. In contrast, coatings incorporating PEEK and hBN exhibit inferior performance (high risk), requiring further optimization.