A kinetic and thermoanalytical study on oxidation of powder and single-crystal samples of niobium carbide

The oxidation of two kinds of NbC, powder and single-crystal samples, was carried out isothermally at temperatures of 390 to 610°C at an oxygen partial pressure of 8 kPa using an electromicrobalance. The kinetics results suggested that the oxidation of both powder and crystal samples proceeds essentially by a phase-boundary-controlled reaction. It was found that cracking of grains or particles in the powder samples, which occurs in the later stage, accelerates oxidation below 485°C, and extremely rapid reaction proceeds above that temperature. Simultaneous TG-DTA-MS measurement showed that the reaction above 485°C is due to a large heat evolution resulting from rapid oxidation of bare NbC surfaces produced by cracking of grains or particles, which was partly caused by immediate release of CO₂ accumulated in them. Scanning electron microscopy of the crystals showed that columnar, porous grains with the major axis normal to the surface formed, which were responsible for the phase-boundary-controlled reaction of the oxidation of both powder and crystal samples.