Effect of additive concentration and temperature on the tribological performance of eco-friendly bonded MoS2-PEEK solid lubricants for aerospace applications

Spray-bonded molybdenum disulfide (MoS 2 ) coatings are widely used in space and aerospace components for their low friction and wear resistance. Conventional formulations often rely on toxic additives/dopants such as Pb-based compounds and Sb 2 O 3 to improve performance, raising environmental and health concerns. This study investigates polyetheretherketone (PEEK) as an eco-friendly alternative additive and evaluates how it affects the tribological behavior of bonded MoS 2 coatings at both room temperature (25 °C) and low-temperature (–50 °C). Coatings with PEEK concentrations (1.4, 1.9, 2.4, 2.9, and 3.4 wt.%) were prepared and systematically tested under ball-on-flat reciprocating sliding. The coatings were then characterized through nanoindentation, confocal microscopy, SEM/EDS, FIB-SEM, and Raman spectroscopy. Results showed that PEEK had a significant effect on wear behavior, while the steady-state friction coefficient remained statistically unchanged. Increasing PEEK content increased H/E and H 3 /E 2 ratios, enhancing matrix ductility and crack resistance. Wear resistance improved up to 2.9 wt.% PEEK, where the coating exhibited the shallowest wear track and lowest wear rate. This optimum composition balanced matrix compliance and load-bearing capacity, promoting stable tribo/transfer film. Conversely, excessive PEEK content (3.4 wt.%) disrupted MoS 2 percolation pathways and uniform load distribution, resulting in reduced wear resistance. Although MoS 2 –PEEK coatings showed inferior tribological performance than commercial MoS 2 –Sb 2 O 3 coating (Everlube 620C), they offer a sustainable route, with additive optimization potentially closing the performance gap. At –50 °C, all coatings showed negligible wear and low steady-state friction coefficient (0.04–0.06), likely attributed to the formation of interfacial ice films that masked compositional differences.