Comparison of the oxidation behaviour of two dense hot isostatically pressed tantalum carbide (TaC and Ta2C) materials

Isothermal oxidation of dense HIPed tantalum carbide materials TaC and Ta2C, has been performed in flowing oxygen between 750 and 850 degrees C. The behaviour of the two carbides: i.e. TaC (NaCl type structure) and Ta2C (hexagonal type), is characterized by the growth of a non-protective oxide scale which, on square section samples, forms a maltese cross. X-Ray diffraction analysis has only shown the formation of tantalum hemipentoxide beta Ta2O5. The oxidation of TaC proceeds by an interfacial reaction process. For Ta2C, the mechanism could be more complex due to the presence of an intermediate oxycarbide layer TaCxOy which has been detected at the Ta2C-Ta2O5 interface. Indeed in this case, it is not possible to exclude a diffusion limiting process thought this oxynitride sublayer of constant thickness, with time. (C) 1997 Elsevier Science Limited.