Effect of ball milling on hexagonal boron nitride (hBN) and development of Al-hBN nanocomposites by powder metallurgy route

This study reports on the exfoliation of bulk hexagonal boron nitride (hBN) by high-energy ball milling and the development of Al-hBN (alumninum-hexagonal boron nitride) nanocomposites by the powder metallurgy (PM) route via the incorporation of the exfoliated hBN in the Al matrix as a nanoreinforcement. The effect of ball milling on the morphology, crystallite size, lattice strain, and thermal stability of hBN powder have also been reported in this paper. Commercially available bulk hBN was ball milled for up to 30 hours in a high-energy planetary ball mill in order to exfoliate the hBN. Although no new phases were formed during milling, which was confirmed by the XRD (x-ray powder diffraction) spectra, ball milling resulted in the attachment of functional groups like hydroxyl (OH) and amino (NH₂) groups on the surface of the hBN, which was confirmed by FTIR (Fourier Transform Infrared Spectroscopy) analysis. HRTEM (high resolution transmission electron microscopy) analysis confirmed the synthesis of hBN having few atomic layers of hBN stacked together after 20 hours of milling. After 20 hours of milling, the hBN particle size was reduced from ~1 μm to ~400 nm, while the crystallite size of the 20-hour-milled hBN powder was found to be ~18 nm. Milling resulted in a flake-like structure in the hBN. Although milling involved both exfoliation as well as reagglomeration of the hBN particles, a significant decrease in the diameter of the hBN particles and their thickness was observed after a long period of milling. The average thickness of the 20-hour-milled hBN flakes was found to be ~32.61 nm. HRTEM analysis showed that the hexagonal structure of the milled hBN powder was maintained. Al-based nanocomposites reinforced with 1%, 2%, 3%, and 5% by weight hBN were fabricated by PM route. The Al-hBN powder mixtures were cold-compacted and sintered at 550°C for 2 hours in argon (Ar) atmosphere. The maximum relative density of ~94.11% was observed in the case of Al-3 wt.% hBN nanocomposite. Al-3 wt.% hBN nanocomposite also showed a significant improvement in hardness and wear resistance compared to the pure Al sample that was developed in a similar fashion. The maximum compressive strength of ~999 MPa was observed in the case of Al-3 wt.% hBN nanocomposite and was approximately twice that of the pure Al sample developed in a similar fashion.