Macroscale superlubrication induced by synergistic effects 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 BP nanosheets into a mixture of diethylenetriaminepenta (methylenephosphonic) acid (DTPMPA) and ethylene glycol (EG) as additives (DTPMPA/EG–BP), the macroscopic superlubrication of the Si₃N₄/sapphire friction pair was attained at a high contact pressure of 1.83 GPa, with a coefficient of friction (COF) of 0.0067. The wear rate of DTPMPA/EG–BP (3.14×10⁻⁹ mm³·N⁻¹·m⁻¹) was 92% lower than that of pure EG (3.92×10⁻⁸ mm³·N⁻¹·m⁻¹). Notably, the BP nanosheets adsorbed on the worn surface, and 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 prevented direct contact between the friction pairs and provided extremely low shear strength, resulting in an ultralow COF. Therefore, the synergistic interaction between the BP nanosheets and the acid solution exerted a predominant influence on achieving superlubrication under extremely high contact pressures at the macroscopic scale. This research proposed a novel strategy to realize liquid superlubrication under high-pressure conditions, and by leveraging the synergistic cooperation between two-dimensional (2D) materials and acid molecules, it expedited the application of liquid superlubrication in industry.