Two-dimensional hexagonal boron nitride by cryo-milling: microstructure and oxidation behavior at elevated temperature

Boron nitride (BN) nanosheets in bulk quantity and reproducible quality were generated by a top-down approach from coarse (micron) grade hexagonal boron nitride (m-hBN) via cryo-milling under optimized process conditions followed by sonication. The as-produced boron nitride nanosheets showed a lower average thickness of about 15–25 nm when compared to previously reported low-yield techniques like sonication and specific solvent-induced liquid exfoliation and chemical vapor deposition (CVD). Interestingly, some fraction of the hexagonal boron nitride (hBN) was transformed to rhombohedral BN (rBN) under cryo-milling conditions. Such transition assisted the generation of the layered 2D nanosheet structure due to higher interlamellar spacing of the basal planes in rBN. Thermo-gravimetric and differential scanning calorimetric (TG/DSC) studies showed thermal stability of hBN/rBN nanosheets up to about 800 °C when compared to about 1000 °C in m-hBN, with the formation of BNOx and B₂O₃ phases. Multiple DSC peaks for oxidation were observed corresponding to different particle size ranges in the powder. Thus, cryo-milling could be an effective route for the bulk production of 2D-BN nanosheets for various applications like high-temperature lubricants.