Covalently attached graphene-ionic liquid hybrid nanomaterials: synthesis, characterization and tribological application

Hybrid nanomaterials offer task-specific functional properties besides the individual properties of their constituent materials/elements. Herein, graphene-ionic liquid (Gr-IL) hybrid nanomaterials are synthesized to integrate the friction-reducing properties of both ionic liquids and graphene nanosheets. Importantly, the remarkable mechanical strength of graphene improves the anti-wear performance, whereas covalently grafted ionic liquids facilitate the dispersion of the Gr-IL in the polyethylene glycol (PEG 200) synthetic lube base oil. Graphene, prepared by a controlled chemical approach, is used for the covalent grafting of an imidazolium ring. Three variable Gr-IL hybrid nanomaterials, containing bis(salicylato) borate (BScB), oleate (OL), and hexafluorophosphate (PF6) anions, are synthesized to control their surface properties. Detailed chemical and microstructural features of the Gr-IL hybrid nanomaterials are studied using FTIR, XPS, Raman, XRD, FESEM, and HRTEM analyses. The Gr-IL hybrid nanomaterials as lubricant additives for PEG 200 significantly reduced the friction and the wear. Further, the Gr-IL hybrid nanomaterials offer remarkably improved anti-wear properties (55-78%) compared to that of the corresponding ionic liquid (7-39%) blends in PEG 200 and this was attributed to the high mechanical strength of graphene, which protects the contact interfaces against material loss. The elemental and micro-Raman results of the worn surfaces suggested the formation of a tribo-chemical thin film composed of Gr-IL and its tribo-chemical products with steel interfaces. The BScB anion constituted Gr-BScB showed the maximum reduction in friction, whereas the OL analogue exhibited the smallest wear. This study reveals the potential of task-specific Gr-IL hybrid nanomaterials as novel additives for diversified tribological applications.