A review on the synthesis of MXenes and their lubrication performance and mechanisms

MXenes (MXs), as an emerging 2D family of transition metal carbides and nitrides, have been considered as new candidates for solid lubrication/lubricant additives due to their mono-layered graphene-like structure with similar mechanical properties, abundant surface terminations (i.e., -O, -OH, -F), relatively low shear strength and inherent self-lubrication ability. In this review, we introduced MXs' synthesis and their lubrication performance and mechanisms. Typically, pure MXs and MXs-based composites like MXs/polymers, MXs/graphene (MoS2) or MXs/metals (metal oxide) were used as reinforcement materials to form protective coatings with excellent mechanical properties and solid lubrication performance. As liquid lubricant additives, MXs can be used as water-/oil-based lubricant additives, and exhibit improved friction and wear. At the same time, chemically functionalized MXs with better dispersibility and compatibility were introduced as the improved oil-based lubricant additives. Some parameters (such as MXs' structural effects and working conditions) affecting MXs' lubrication performance, and the lubrication mechanisms (such as the formation of tribofilm, the hydrophilicity-/interlayer-dependent lubrication mechanism and adsorption effect) were all discussed. Finally, some future perspectives for MXs' lubrication were proposed at the end of this review.