![]() The properties of the liquid lubricant, including molecular weight and polarity, are also decisive for lubrication 12, 13. In this case, lubrication acts through shearing of the physisorbed functionalized graphene sheets under the contact. However, the lubrication mechanism was effective also without forming mechanically stable graphene tribofilms within the sliding interfaces 11. Lubrication enhancement by GO/rGO additives was described by the formation of low-shear-strength graphene tribofilm by adsorption to metal surfaces 1, 2, 10. Optimization of oxygen functionality in GO/rGO is an important aspect for preserving the chemical stability of the structure while providing sustainable covalent/noncovalent interactions with the lubricant. The layered lattice structure is unique for reducing inter-sheet shear resistance and has shown low friction coefficients combined with the high strength of the graphene sheets 6, 9. A small amount of GO/rGO is capable of effective lubrication due to high surface to volume ratio, effective inter-sheet shearing and sufficient oxygen functionality making GO/rGO compatible for dispersion in polar liquid lubricants 1, 3, 8. However, toxicity of graphene and related risks for the health cannot be ignored 7. These include low shear strength, high yield strength, chemical compatibility for dispersion in polar lubricants, and high thermal conductivity and an ecofriendly composition 5, 6. Several remarkable properties of graphene-oxide/reduced graphene-oxide (GO/rGO) make it an effective solid additive for liquid lubricants 1, 2, 3, 4, 5. Lubricants reduce friction between contacting surfaces and thus increase the energy efficiency of moving machine components. On the other hand, antiwear properties of hydroxyl-terminated rGO were significantly enhanced compared to epoxy-hydroxyl functionalized rGO due to the integrity of graphene sheet clusters. This can be explained by the strong coupling between graphene sheets through hydroxyl units, causing the interaction of PEG with the rGO to be non- effective for lubrication. However, the friction coefficient was unaffected when hydroxyl-terminated rGO was coupled with PEG. In this condition, PEG chains intercalate between the functionalized graphene sheets, and shear can take place between the PEG and rGO sheets. Epoxy-hydroxyl-terminated rGO dispersed in PEG showed significantly smaller values of the friction coefficient. Two different reduced graphene oxide (rGO) derivatives, terminated by hydroxyl and epoxy-hydroxyl groups, were prepared and blended with two different molecular weights of polyethylene glycol (PEG) for tribological investigation. Functionalized and fully characterized graphene-based lubricant additives are potential 2D materials for energy-efficient tribological applications in machine elements, especially at macroscopic contacts. ![]()
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