High-Temperature Oxidation of NbC and TaC at Low Oxygen Pressures

The oxidation of hot-pressed NbC and TaC was studied between 2000 and 2500 K at 10/sup -5/ to 10/sup -3/ mbar oxygen pressure by gravimetric methods. By CO-formation a metallic surface layer is formed which simultaneously reacts with oxygen forming volatile metal oxides. A steady-state layer thickness is reached if the rates of layer formation and metal evaporation are equal. Far away from the steady-state conditions the weight loss and the metal layer thickness deviate essentially from the values predicted by a reaction model. This suggests that for Va metal carbides the model should be modified by considering the early stage of the oxidation.