Toward greener alternative: Comparative tribological study of Sb2O3 and Bi2O3 additives in bonded MoS2-based solid lubricants for aerospace applications

To mitigate the environmental and health hazards associated with antimony trioxide (Sb 2 O 3 ) widely used in aerospace solid lubricants, this study investigates bismuth oxide (Bi 2 O 3 ) as a sustainable alternative additive for bonded molybdenum disulfide (MoS 2 )-based coatings. MoS 2 -Bi 2 O 3 coating was fabricated using a spray bonding process and tribologically evaluated against a commercial MoS 2 -Sb 2 O 3 benchmark (Everlube 620C) under reciprocating sliding conditions. Interfacial phenomena and wear mechanisms were characterized utilizing SEM, FIB, Raman spectroscopy, and XRD. Results indicated that while Bi 2 O 3 possesses a favorable low interaction parameter—chemically suitable for lubricity—the MoS 2 -Bi 2 O 3 coating exhibited higher friction and wear compared to the Sb 2 O 3 -containing baseline. This performance gap is likely attributed to the higher intrinsic hardness and shear strength of Bi 2 O 3 , which compromised tribofilm stability, induced micro-cracking, and promoted loose debris formation, thereby intensifying abrasive wear. Conversely, Sb 2 O 3 demonstrated a predominantly shear-driven wear mechanism, likely due to its superior mechanical compatibility with the matrix, facilitating more desirable tribo/transfer film formation and basal plane alignment parallel to the sliding direction. Thus, while the tribological behavior of the MoS 2 -Sb 2 O 3 coating was governed by the development of a stable tribo/transfer film, the MoS 2 -Bi 2 O 3 formulation was dominated by fracture-driven wear mechanism. Despite the observed reduction in tribological performance relative to the benchmark, the MoS 2 -Bi 2 O 3 system yielded performance comparable to—or better than—other eco-friendly alternatives reported in the literature, establishing it as a viable candidate for non-toxic solid lubrication subject to further compositional optimization. F0B7 MoS 2 –Bi 2 O 3 solid lubricant coating was developed via a spray-bonding process and systematically evaluated under reciprocating sliding conditions, with comparison to the commercial MoS 2 –Sb 2 O 3 coating (Everlube 620C). F0B7 Although the Bi 2 O 3 -containing formulation underperformed relative to the Sb 2 O 3 benchmark, it exhibited tribological behavior comparable to—or better than—other environmentally friendly MoS 2 -based systems reported in the literature. F0B7 Sb 2 O 3 promoted superior MoS 2 (002) basal-plane alignment and supported the formation of a stable tribofilm and transfer film due to its better mechanical compatibility with the matrix. F0B7 The higher hardness and shear resistance of Bi 2 O 3 contributed to microcracking and generated more loose debris, resulting in reduced tribofilm stability and a less compact transfer film during sliding. F0B7 Wear behavior was dominated by a shift in wear mechanism when transitioning from Sb 2 O 3 to Bi 2 O 3 additives, indicating that mechanical compatibility and desirability of tribo/transfer film—not hardness alone—governed overall wear performance.