A KINETIC-STUDY ON OXIDATION OF NIOBIUM CARBIDE

The isothermal oxidation of NbC powders was carried out at temperature s of 420-600-degrees-C and oxygen pressures of 4, 8 and 16 kPa, using an electro-microbalance. The oxidation process was divided into three steps I to III. In step I, oxidation occurs linearly in initial period (a degree of oxidation (alpha) < 10-20%). In step II (10% < alpha < 3 0%), oxidation proceeds by a diffusion-controlled process below 490-de grees-C, whereas oxidation was extremely accelerated by large heat evo lution due to the crystallization of Nb2O5 formed above that temperatu re. A phase boundary-controlled process operates in step III (alpha > 30%).