The effect of extreme temperature in an oxidising atmosphere on dense tantalum carbide (TaC)

This study describes the microstructure development as dense tantalum carbide (TaC), which is subjected to extreme temperature environments (3,000 °C) in the presence of oxygen. These are conditions that structural materials are expected to experience in hypersonic aero-propulsion applications. The conditions produce molten oxide which may provide a temporary resistance to rapid oxidation and may go some way to repair thermal shock cracks, however, at the same time the liquid is observed to attack the dense ceramic both chemically and mechanically. A reaction mechanism is suggested which involves dissolution of TaC in the oxide melt and a two step oxidation; first the reaction of TaC with oxygen to form Ta(O,C) and TaO _x , resulting in dissolved dissociated carbon, followed by the reaction of dissolved carbon with oxygen to produce gas. This microstructural analysis of one of the candidate ultra-high temperature ceramic materials for hypersonic flight provides new insight into the mechanism of TaC oxidation and the role of the liquid oxide layer in acting not only as a protective layer to further oxidation, as is commonly reported, but also as a dynamic component that promotes erosion of the TaC surface and is a source of further oxygenation of the TaC surface. If the formation of the liquid phase can be better controlled and the reaction of the liquid phase with the matrix be slowed and stabilised, then the formation of a liquid phase at the surface of TaC may provide a key to designing materials that can withstand the rigours of hypersonic flight.