Superlubricity of Black Phosphorus as Lubricant Additive.

Superlubricity is defined as a sliding regime in which friction, or the resistance to sliding of two relatively moving surfaces, almost vanishes. From a practical point of view, the development and use of new materials that can enable superlubricity (coefficient of friction, COF < 0.01) in moving mechanical systems will have huge positive impact on energy-saving and emission reduction. In this work, the use of a new two-dimensional material, black phosphorus (BP) as a high-performance water-based lubricant additive that can significantly reduce friction and achieve superlubricity has been explored. A lowest COF value of 0.0006 ever measured by an application-orientated ball-on-plate tribometer has been found. Robust superlubricity in the aqueous solution with ultrafine BP nanosheets modified by NaOH (BP-OH) has been observed for a wide range of additive concentrations, contact pressures, and sliding velocities owing to the very low shear resistance of the water layer retained by BP-OH nanosheets. This finding has the potential of opening up a new approach to dramatically reduce or even eliminate friction by using BP nanomaterials as lubricant additives.