Effects of MoS2 microencapsulation on the tribological properties of a composite material in a water-lubricated condition

The severe abrasion wear of water-lubricated stern bearings usually occurs in the low-speed, heavy-duty working conditions. Consequently, it is vitally important to improve the anti-friction and tribological properties of composite stern bearings. The compound molybdenum sulfide (MoS2) is a priority additive for enhancing the tribological properties of composites. However, the water capillary effect in the defects in the MoS2 crystal can easily lead to oxidation and failure of the lubrication properties of MoS2. In order to eliminate the chemical oxidation of MoS2 in a humid environment, MoS2 was microcapsulated to improve the chemical stability of MoS2 in a humid environment. In this reported work, urea formaldehyde (UF) microcapsules loaded with MoS2 as the core material were synthesized using in-situ emulsion polymerization. The synthesized microcapsules were added to a high-density polyethylene (HDPE) matrix to prepare composites. The ZCuSn(10)Zn(2) bronze which widely utilized in as the water-lubricated bearing bushing in watercraft was chosen as counterparts material. The tribological properties of the composites modified with microcapsules were investigated using a CBZ-1 tribometer under different running conditions, and the surface morphology of the tested composites was analyzed by using a scanning electron microscope. The test results revealed that the tribological property of the composites filled with the MoS2-containing microcapsules was better than the composite that lacked the MoS2 microcapsules. The composites containing the MoS2 microcapsules exhibited lower sliding friction and a smoother surface morphology, which was the result of the protection provided by the microcapsule walls. The UF walls of microcapsules isolated the steam produced by water and fiction, which delayed the oxidation of the MoS2 crystals. Upon fracturing of the walls of the microcapsules, the encapsulated molybdenum disulfide was released between the friction pairs, which reduced the friction coefficient and adhesion wear. This reported study also provided an experimental basis for structural design and performance enhancement of water-lubricated bearings.