Oxidation kinetics of refractory carbides at low oxygen partial pressures

The interaction kinetics of sintered refractory carbides such as NbC, TiC, VC and ZrC with atmospheres at low oxygen partial pressure is reported from ambient to 1000 degrees C and from 0.8 to 20 Pa at the total pressure of 1 .6 bar (rel) in Ar. Depending on the experimental conditions, the isotherma l reaction rate was found decreasing or increasing on time. This behaviour ha been linked to the parameter Gamma, defined as the ratio between the wei ght change of the sample and the number of moles of oxygen consumed. A decr easing trend of the reaction rate corresponds to a null value of Gamma. In this case, no growth of oxide occurs but an increase of atomic disorder on the surface of the carbide is expected. A positive trend of the reaction ra te implies Gamma not equal 0 as well as the growth of an oxide layer driven by a solid state mechanism. The oxidation kinetics was never found parabol ic. It is linear at the highest temperatures regardless of the oxygen parti al pressure value and it approaches to be parabolic by decreasing the tempe rature and increasing the oxygen partial pressure. (C) 1999 Elsevier Scienc e B.V. All rights reserved.