Progress of Functionalized Graphene Nanomaterials and Their Applications as Water-Based Lubricating Additives

The phenomenon of friction and wear is widespread in industrial production and daily life, resulting in a large amount of energy consumption and waste of resources. Therefore, it is particularly important to reduce friction and wear and improve the lubrication performance of mechanical equipment. The key to improve the lubrication performance of water-based lubricant and thus extend the service life of mechanical equipment is high-performance lubricants. Traditional oil-based lubricants have good lubricating effects and are not volatile. However, they usually show poor cooling performance during use and are likely to cause environmental pollution. In recent years, green and environmentally friendly water-based lubricant has gradually attracted the attention of scientists. Due to its low viscosity and volatile characteristics, neat water has poor lubrication effect as a lubricant. Therefore, it is urgent to develop efficient water-based lubricating additives. Graphene has excellent mechanical properties, thermal conductivity, anti-friction and anti-wear properties, which is a potential candidate as a high-performance additive for water-based lubricant. Therefore, this review summarizes the different functionalizations of graphene-based nanomaterials and their applications as water-based lubricating additives. This article mainly includes three parts: the various functionalization approaches of graphene-based nanomaterials and their applications in water lubrication; the corresponding mechanisms of friction-reduction and anti-wear of graphene-based nanomaterials; and the outlook of current problems of graphene-based nanoadditives in water lubrication. 　Firstly, four types of graphene-based nanohybrids with different functionalizations and their research progress as water-based lubricating additives were introduced. (1) Inorganic nanoparticles-functionalized graphene nanoadditives. Inorganic nanoparticles, e.g. metal nanoparticles, metal oxide or sulfide nanoparticles, and nanocarbons were usually used as nanoadditives to improve the friction and wear properties of oil or water-based lubricating fluids. In order to enhance the tribological properties of graphene, graphene-inorganic nanoparticles nanohybrids were prepared by combining zero-dimensional nanoparticles and two-dimensional graphene, which can effectively improve the friction and wear performance of graphene in water. Typically, the graphene-inorganic nanoparticles nanohybrids containing lanthanum trifluoride (LaF₃), nanodiamond, aluminium oxide (Al₂O₃), silicon dioxide (SiO₂) were described. The results demonstrated that the synergistic effect between the two components would improve the friction-reduction and antiwear properties of water. (2) Organic molecules-functionalized graphene nanoadditives. The modification of graphene nanomaterials with organic molecules, including ionic liquids (ILs), surfactants, ethylene glycol could improve the dispersion stability of graphene and then lubrication performance. ILs have been long established as versatile lubricants. In this part, ILs assisted exfoliation and following modification of graphene nanomaterials were summarized. In addition, the super lubrication behaviors of graphene oxide in water modified with ILs or ethylene glycol were also discussed. (3) Polymer-modified graphene nanoadditives. This was one of the most potential graphene-based nanoaddtives for water lubrication. Generally, graphene-polymer nanohybrids can be prepared by either non-covalent functionalization or covalent functionalization. Covalent modification was an important way to modify graphene nanomaterials by covalently grafting polymer chains to graphene surfaces. In contrast, non-covalent modification was realized by adsorption of polymer chains on graphene surfaces via van der Waals force, π-π interaction, hydrogen bonding or electrostatic interaction. The pros and cons of both covalent modification and non-covalent modification were briefly discussed. Then some examples on polymer-modified graphene used as water lubricating additives were presented, which demostrated excellent friction reduction and antiwear properties. (4) Graphene-polymer bulk composite materials. Functionalized graphene was a typical two-dimensional nanofiller to prepare graphene-polymer bulk composites. Because of high elastic modulus and tensile strength of graphene, the mechanical properties of graphene-polymer composites were significantly enhanced. Furthermore, the incorporation of graphene in the composites would also reduce the coefficient of friction and improve the load-bearing capacity of composites due to low interlayer shear strength when using pure water as lubricating fluid. Some engineering polymers including ultrahigh molecular weight polyethylene (UHMWPE), polyimide (PI) and epoxy resin (EP) were presented as polymer matrixes to prepare graphene-polymer composites in this part. 　Secondly, the possible mechanism of graphene-based nanomaterials on friction-reduction and antiwear in water was briefly discussed. At present, research showed that graphene as a two-dimensional nanoadditive played the main role in following four aspects: (1) formation of a friction protective film. During the friction process, the layered graphene was easy to enter the contact area and can bear a certain load to reduce friction and wear. Under contact pressure and shear conditions, the functionalized graphene with high surface energy was prone to adsorbing on friction interfaces and forming a physical protective film. The adsorbed carbonaceous protective film can bear normal load and avoid the direct contact between the friction pairs, thereby achieving effective anti-friction and anti-wear effect. Futhermore, the mechanical force and heat generated by the friction process promoted the tribochemical reaction between the lubricating additive and the friction interface. The formation of a chemical transfer film on the friction interface could provide extremely low shear strength. (2) sliding or rolling effect. The extremely low interlayer shear force and nanoscale size of graphene can be regarded as particle rolling under shear condition, showing similar functions with spherical nanoparticles additives. (3) self-repairing effect. The continuous deposition of graphene nanoparticles can fill up the defects of the worn surface, repair the damaged transfer film, and realize self-repairing effect. (4) smoothing and polishing effect. The graphene nanomaterial can also cut the edge of the contact surface during the sliding process, thereby producing a micro-polishing effect, and further reducing the coefficient of friction and improving the anti-wear performance. In most cases, the anti-friction and anti-wear properties of graphene-based nanoaddtives was a combination of two or more mechanisms, resulting in a synergistic lubrication mechanism. 　Finally, the current problems of graphene as water-based lubricating additives and the urgent research topics in the future were proposed. Numerous research on functionalizations of graphene with inorganic nanoparticles, surfactants, ionic liquids, polymers had been conducted. The reported graphene-based nanohybrids exhibited enormous potentials on water-based lubrication. However, the applications of graphene-based nanoadditives for water lubrication and related lubrication mechanisms were still in their infancy. Researchers still need to conduct in-depth studies in the following aspects: (1) improve the agglomeration of graphene in water or other liquids caused by the strong π-π stacking interaction and van der Waals force between graphene nanosheets; (2) the anti-friction and anti-wear mechanism of graphene-based nanomaterials is still not fully understood. Thus, more in-depth and systematic study should be conducted; (3) the systematic evaluation of biocompatibility of graphene-based nanomaterials and the potential applications in bio-lubrication and super lubrication; (4) macroscale preparation of graphene nanomaterials with low cost and environmental compatibility.