From ultra-low friction to superlubricity state of black phosphorus: Enabled by the critical oxidation and load

Based on the density functional theory (DFT), we investigate the friction properties of inevitable oxidized black phosphorus (o-BP). o-BP with the weaker interlayer adhesion exhibits their great potential as a solid lubricant. At the zero load, the friction property of o-BP is adjusted by its oxidation degree. Expressly, ultra-low friction of P4O2 (50% oxidation, O : P = 2 : 4 = 50%) is obtained, which is attributed to the upper O atoms with lower sliding resistance in the O channel formed by lower layer O atoms. More attractive, we observe superlubricity behavior of o-BP at the critical load/distance due to the flattening potential energy surface (PES). The flattening PES is controlled by the electrostatic role for the high-load (P4O3, O : P = 3 : 4 = 75%), and by the electrostatic and dispersion roles for the low-load (P4O2). Distinctly, the transform from ultra-low friction to superlubricity state of black phosphorus (BP) can be achieved by critical oxidation and load, which shows an important significance in engineering application. In addition, negative friction behavior of o-BP is a general phenomenon (Z > Z_min, Z_min is the interlayer distances between the outermost P atoms of minimum load.), while its surface-surface model is different from the fold mechanism of the tip-surface model (Z₀ < Z < Z_min, Z₀ is the interlayer distances between the outermost P atoms of equilibrium state.). Thus, this phenomenon cannot be captured due to the jump effect with instability of the atomic force microscopy (AFM) (Z > Z_min). In summary, o-BP improves the friction performance and reduces the application limitation, comparing to graphene (Gr), MoS₂, and their oxides.