OXIDATION-KINETICS OF HEXAGONAL BORON-NITRIDE POWDER

The isothermal oxidation of hexagonal boron nitride powders was carried out at 900-1050-degrees-C in dry oxygen and air. The oxidation kinetics were found to obey linear rate law and were described by the surface chemical reaction-controlled shrinking cylindrical model. The apparent activation energies were found to be 298 and 330 kJ mol-1 in dry oxygen and air, respectively. The oxygen partial pressure dependence of the oxidation rate constant at 1000-degrees-C is well represented by a Langmuir-type equation. Microscopic examination of the oxidized sample after removal of the oxide scale with water suggested that the rate of attack by oxygen was determined by an anisotropy due to the crystallographic direction, similar to oxidation in graphite. The volatilization of B2O3 was observed only in dry oxygen and obeyed a linear rate law, and was found not to affect the oxidation reaction.