Regulating the Nb₂C nanosheets with different degrees of oxidation in water lubricated sliding toward an excellent tribological performance

Novel two-dimensional (2D) Nb₂C nanosheets were successfully prepared through a simple lultrasonic and magnetic stirring treatment from the original accordion-like powder. To further study their water-lubrication properties and deal with common oxidation problems, Nb₂C nanosheets with different oxidation degrees were prepared and achieved long-term stability in deionized water. Scanning electron microscope (SEM), transmission electron microscope (TEM), scanning probe microscope (SPM), X-ray powder diffraction (XRD), Raman, and X-ray photoelectron spectrometer (XPS) experiments were utilized to characterize the structure, morphology, and dispersion of Nb₂C nanosheets with different degrees of oxidation. The tribological behaviors of Nb₂C with different degrees of oxidation as additives for water lubrication were characterized using a UMT-3 friction testing machine. The wear scars formed on the 316 steel surface were measured using three-dimensional (3D) laser scanning confocal microscopy. The tribological results showed that a moderately oxidized Nb₂C nanosheet, which owned the composition of Nb₂C/Nb₂O₅/C, displayed excellent tribological performance, with the friction coefficient (COF) decreasing by 90.3% and a decrease in the wear rate by 73.1% compared with pure water. Combining the TEM and Raman spectra, it was shown that Nb₂O₅ nanoparticles filled in the worn zone, and the layered Nb₂C and C were adsorbed into the surface of the friction pair to form a protective lubricating film. This combined action resulted in an excellent lubricating performance.