High temperature oxidation regime transitions in hafnium carbide

Abstract

AbstractUnderstanding the oxidation behavior of hafnium carbide is crucial to its application in extreme environments. In this work, the transition in high‐temperature oxidation kinetics regimes in hafnium carbide is explained based on phase equilibria considerations supported by observed changes in oxide scale microstructure evolution associated with different transformation pathways. Below, a composition‐dependent critical temperature and oxygen pressure, hafnium carbide first transforms to an amorphous material with nominal composition HfO2C followed by phase separation into carbon and hafnia domains. Subsequently, gaseous transport through a nanometric pore network formed by oxidative removal of phase‐separated carbon becomes rate‐limiting. Above this critical point, the oxidation sequence involves a direct transformation from hafnium carbide to hafnia and gaseous products, leading to dissimilar scale morphologies responsible for the reported transition from gaseous to solid‐state diffusion‐limited oxidation regimes at ultra‐high temperatures.