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 isothermal reaction rate was found decreasing or increasing on time. This behaviour ha been linked to the parameter Gamma, defined as the ratio between the weight change of the sample and the number of moles of oxygen consumed. A decreasing 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 rate 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 parabolic. It is linear at the highest temperatures regardless of the oxygen partial pressure value and it approaches to be parabolic by decreasing the temperature and increasing the oxygen partial pressure. (C) 1999 Elsevier Science B.V. All rights reserved.