Macroscale superlubrication induced by synergistic of black phosphorus and acid molecules at ultrahigh contact pressure

Black phosphorus (BP) has been extensively utilized as a lubricant additive owing to its unique layered structure and extreme pressure anti-wear properties. By introducing black phosphorus (BP) nanosheets into Diethylenetriaminepenta (methylenephosphonic) acid (DTPMPA)/ Ethylene glycol (EG) mixture solution as additives (DTPMPA/EG-BP), the macroscopic superlubrication state on Si₃N₄/sapphire friction pair was attained at a high contact pressure of 1.83 GPa, with the coefficient of friction (COF) of 0.0067. The wear rate of DTPMPA/EG-BP (3.14×10^-9 mm³×N^-1×m^-1) exhibited a 92% reduction when compared to pure EG (3.92×10^-8 mm³×N^-1×m^-1). It was noteworthy that the BP nanosheets adsorbed on the wear surface and meanwhile the molecular layer formed by DTPMPA/EG covered the BP surface, demonstrating that the shear interface shifted from the Si₃N₄/Sapphire interface to the BP nanolayer/molecular layer interface. This interfacial transition avoided direct contact between the friction pairs and provided extremely low shear strength, resulting in ultralow COF. Therefore, the synergistic interaction between the BP nanosheets and the acid solution exerted a predominant influence in achieving superlubrication under extremely high contact pressures on the macroscopic scale. This research proposed a novel strategy to realize liquid superlubrication under high-pressure conditions and by leveraging the synergistic cooperation between 2D materials and acid molecules, it expedited the application of liquid superlubrication in industry.