Tribological Investigations of Two-Dimensional Nanostructured Lamellar Materials as Additives to Castor-Oil-Derived Lithium Grease

Abstract The present work deals with the tribological evaluation of castor-oil-derived lithium grease having variable concentrations of pristine and chemically functionalized MoS2 and graphene nanosheets. The MoS2 and graphene oxide nanosheets were synthesized by hydrothermal and Hummer's method, respectively. The tribological performance of castor grease with and without two-dimensional (2D) lamellar nanomaterials was evaluated using a four-ball tribo-tester as per ASTM standards. The graphene-based lamellar nanomaterials in castor grease significantly improved the tribological properties by decreasing friction and wear. The optimized concentration of MoS2-ODT, MoS2, GO-ODA, GO, and rGO nanomaterials in castor grease conserved the frictional energy losses by 19%, 34%, 35%, 54%, and 56%, respectively. Among all samples, the rGO nanosheets in castor grease showed a maximum reduction in friction and wear. The spectroscopic analysis of worn surfaces suggested the establishment of graphene-based tribo-film, which reduced the direct interaction of tribo-interfaces and minimized the friction and wear.