Preparation of Black Phosphorus Nanosheets and Their Lubricating Extreme Pressure Properties in Castor Oil

Black phosphorus (BP) nanosheets were synthesized through high-energy ball milling and liquid-phase exfoliation. The impact of BP on the tribological characteristics of castor oil (CO) was evaluated by using a four-ball test. The lubrication and extreme pressure performance of oil-based lubricants containing various mass fractions of BP nanosheets were examined in steel/steel frictional pairs, with the results compared to pure CO. Experimental results demonstrate that BPNS significantly fortifies the load-carrying capacity and antiwear resistance of the base oil, achieving a 43% reduction in friction and an 83% decrease in wear volume, alongside a 54% increase in extreme pressure performance. Molecular dynamics simulations revealed that when BP was introduced into the system, the friction force decreased by 44.4%. These enhancements are primarily attributed to the formation of the tribo-chemical layer consisting of iron oxides (Fe2O3, Fe3O4) and POx/P–O–C and the physical adsorption film on the worn surface. The interlayer shear properties of BP nanosheets also contribute significantly to reducing friction and mitigating wear.