Superlubricity achieved with two-dimensional nano-additives to liquid lubricants

The topic of superlubricity is attracting considerable interest around the world while humanity is facing an energy crisis. Since various liquid superlubricity systems can be commonly achieved on the macroscale in ambient conditions, it is considered an effective solution to reduce unnecessary energy and material losses. However, certain practical problems such as low load-bearing pressure, dependence on hydrogen ions, and relatively long running-in processes still limit its widespread application. Two-dimensional (2D) nano-additives with ultrathin longitudinal dimensions can lower the shear resistance between sliding solid surfaces, and thus further optimize the applied conditions. In this review, the latest studies on 2D nano-additives with a combination of various water-based lubricants in the state of superlubricity are reported, typically including black phosphorus (BP), graphene oxide (GO), and layered double hydroxide. During the sliding process, composite lubricants effectively improved the load capacity (up to 600 MPa), reduced wear, and accelerated the running-in period (within 1,000 s) of the liquid superlubricity system. Both macromechanical experiments and microscopic tests are conducted to precisely analyze various interactions at the interfaces of the nano-additives and solid surfaces. These interactions can be described as tribochemical reactions, physical protection, and adsorption enhancement, and improved wear resistance. This review provides better guidance for applying 2D nanomaterials in liquid superlubricity systems.