Functionalization of graphene-based materials: Effective approach for enhancement of tribological performance as lubricant additives

The performance of lubricants greatly depends on the additives in their formulation. Considerable attention has been paid to graphene for the formulation of lubricant as anti-wear or anti-friction additives. In the lubricant additive industry, the introduction of graphene-based materials will enable improvement in the lubricant's fundamental characteristics by increasing its strength and load bearing capability. It is then no wonder that the race to commercialize graphene has been on for the past few years since the discovery of the material in 2004. Pristine graphene has a high electrical conductivity; thus, it is desirable for applications such as semiconductors, battery, and sensors. Meanwhile, functionalized graphene is desirable for other applications, including lubricant additives, to achieve a stable dispersion matrix or medium. The graphene structure can be tuned via chemicals or physical functionalization methods to meet the requirement needed by specific applications. In this review, the limitation of graphene as lubricant additives is discussed. Subsequently, methods on the functionalization of graphene are reviewed. Finally, the recent progress on functionalized graphene as lubricant additives are discussed in detail.