Transformation Of Boron Nitride From Cubic To Hexagonal Under 1-Atm Helium

The transformation of boron nitride from the cubic to hexagonal phase has been investigated between 1560 and 1660 degrees C. High-purity cBN powders of three different particle sizes (< 0.5 mu m, 2-4 mu m and 35-45 mu m) were heat treated at ambient pressure in a dry flowing helium atmosphere over this temperature range for up to 8 h. Microscopy reveals surface growth of hBN on the cBN particles, although the morphology and texture of the hBN phase is different between the largest particles (35-45 mu m) and the smaller particle sizes (< 0.5 mu m and 2-4 mu m). X-ray diffraction was used to measure the weight fraction of each phase in the powder samples. Transformation rates as measured by weight fraction of the hexagonal phase over time correspond to a zero-order transformation of cBN to hBN, which becomes inhibited as the particle size decreases. Variations in surface growth morphology between the particle sizes suggest growth inhibition due to a reduction in nucleation site availability. Activation energies as determined from Arrhenius plots are 357 +/- 29 kJ.mol(-1), 738 +/- 168 kJ.mol(-1) and 1490 +/- 65 kJ.mol(-1) for the < 0.5 mu m, 2-4 mu m and 35-45 mu m, respectively. Increasing surface area of the powders by decreasing the particle size corresponds with both decreased activation energy and collision frequency factor.