Tribological Evaluation of Sustainable Spray-Bonded MoS2/hBN Solid Lubricant at Low Temperature

Due to their exceptional tribological properties, specifically under vacuum conditions, MoS₂ solid lubricants have been extensively used in several industries. Since the effectiveness of pure MoS₂ tends to deteriorate in humid and oxygen-containing environments, it is co-deposited by conventional Pb-based compounds to improve its oxidation resistance and tribological performance. However, lead-based compounds are required to be replaced with environmentally-friendly alternatives such as hexagonal boron nitride (hBN) due to their toxicity. Additionally, as extreme temperatures can affect the tribological performance of the coatings, understanding the interfacial phenomenon under realistic service conditions is necessary to mimic the operating conditions of aerospace applications. In this study, MoS₂ coatings with various hBN contents (9.5, 11.5, 13.5, 15.5, and 17.5 wt%) were developed using spray bonding process. The friction behavior was evaluated using a ball-on-flat tribometer at low temperature (i.e., − 50 °C). Subsequently, the coatings were characterized by ex-situ analysis techniques such as scanning electron microscopy (SEM), focused ion beam (FIB), Raman spectroscopy, and atomic force microscopy (AFM). The results demonstrated that all coatings exhibited significantly lower steady-state friction at − 50 °C compared to room temperature. A clear distinction was observed between the mechanisms governing the run-in and steady-state stages. The run-in stage was likely influenced by surface morphology and the intrinsic properties of hBN. Increasing hBN content beyond the optimal level led to a prolonged run-in phase and intensified abrasive wear. Conversely, the steady-state performance seemed to be influenced by the formation of a lubricating interfacial ice layer, facilitating low-friction sliding regardless of composition.

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