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.

Graphene oxide (GO) nanosheets and onion-like carbon (OLC) nanoparticles were synthesized from natural graphite powder and candle soot, respectively, and characterized by transmission electron microscopy and Raman spectroscopy. The lubricating performances of GO and OLC as lubricant additives in water were comparatively evaluated using a ball-on-disc tribometer. The effects of sand blasting of a steel disc on its morphology and tribological property were evaluated. The results show that the two nanomaterials, GO and OLC, when used as lubricant additives in water effectively reduce the friction and wear of the sliding discs, which is independent of the disc surface treatment. On applying heavy loads, it is observed that GO exhibits superior friction-reducing and anti-wear abilities compared to those of OLC—a trace amount of GO can achieve a lubricating ability equivalent to that of an abundant amount of OLC. Furthermore, it is observed that sand blasting cannot improve the wear resistance of the treated steel disc, even though the hardness of the disc increased after the treatment. The possible anti-wear and friction-reducing mechanisms of the GO and OLC as lubricant additives in water are discussed based on results for the wear surfaces obtained by scanning electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy