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

Abstract

To mitigate the environmental and health hazards associated with antimony trioxide (Sb₂O₃) widely used in aerospace solid lubricants, this study investigates bismuth oxide (Bi₂O₃) as a sustainable alternative additive for bonded molybdenum disulfide (MoS₂)-based coatings. MoS₂-Bi₂O₃ coating was fabricated using a spray bonding process and tribologically evaluated against a commercial MoS₂-Sb₂O₃ 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 Bi₂O₃ possesses a favorable low interaction parameter—chemically suitable for lubricity—the MoS₂-Bi₂O₃ coating exhibited higher friction and wear compared to the Sb₂O₃-containing baseline. This performance gap is likely attributed to the higher intrinsic hardness and shear strength of Bi₂O₃, which compromised tribofilm stability, induced micro-cracking, and promoted loose debris formation, thereby intensifying abrasive wear. Conversely, Sb₂O₃ 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 MoS₂-Sb₂O₃ coating was governed by the development of a stable tribo/transfer film, the MoS₂-Bi₂O₃ formulation was dominated by fracture-driven wear mechanism. Despite the observed reduction in tribological performance relative to the benchmark, the MoS₂-Bi₂O₃ 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.