Enhanced tribo-chemical properties of oxygen functionalized mechanically exfoliated hexagonal boron nitride nanolubricant additives

Two dimensional (2D) materials with layered lattice structure as nanofluid additives are useful to improve the tribological properties of metallic sliding interfaces. To enhance the tribological efficiency, the bulk crystalline hexagonal boron nitride (h-BN) powder sample was mechanically exfoliated by ball milling, and further processed through ultrasonication for de-aggregation. High resolution X-ray diffraction (HR XRD) and high resolution transmission electron microscopy (HR TEM) results clearly indicate the exfoliation of bulk h-BN into thinner two-dimensional (2D) crystalline sheets without creating noticeable structural defects. The topography of exfoliated nanosheets is well confirmed by atomic force microscopy (AFM). Oxygen functionalization into the h-BN nanosheets after the mechanical exfoliation was investigated by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). Oxygen functionalization of h-BN improved the cohesive compatibility between h-BN and commercial 10W30 lubricant oil for stable dispersion. Friction coefficient and wear of sliding metallic interfaces were reduced significantly in the presence of few layered exfoliated h-BN nanofluid as compared to neat lubricated oil. Micro- XPS and energy-disperse X-ray spectroscopy (EDX) analysis demonstrated the presence of adsorbed h-BN tribofilm in the metallic wear track. Thin 2D sheets of h-BN nanofluid was effective as an additive for low shear resistance under the tribo stressed condition which is the main reason for significant reduction in friction coefficient. Moreover, the enhanced wear resistance of exfoliated h-BN additives was explained by low shear resistance and high compressive/tensile strength of planer sheets which restricted the mechanical damage and protected the metallic interfaces against deformation and wear.