The role of microstructure on high-temperature oxidation behavior of hafnium carbide

Microstructure development of the products formed upon oxidation of hafnium carbide (HfCx, x = 0.65, 0.81, or 0.94) at 1300 degrees C and 0.8 mbar oxygen pressure was investigated using Raman spectroscopy, X-ray diffraction, electron microscopy, and electron energy-loss spectroscopy. For all specimens a multilayered oxide scale was observed featuring an outermost porous hafnia layer and an interlayer adjacent to the parent carbide containing hafnia interspersed with carbon. The outermost hafnia features coarse pores presumably formed during initial stages of oxidation to allow rapidly evolving gaseous products to escape from the oxidation front. As the oxidation scale thickens, diffusional resistance results in slower oxidation rates and smaller quantities of gaseous products that are removed via networks of increasingly fine pores until the local oxygen partial pressure is sufficiently low to selectively oxidize the parent carbide. Electron microscopy studies suggest that the oxidation sequence at this stage begins with the transformation of parent carbide to an amorphous material having empirical formula HfO2Cx that subsequently phase separates into hafnia and carbon domains. Hafnia polymorphs in the phase-separated region vary from cubic to monoclinic as grains coarsen from ca. 2-20 nm, respectively. Immediately adjacent to the phase-separated region is carbon-free mesoporous hafnia whose pore morphology is inherited from that of prior carbon domains. The average pore size and pore volume fraction observed in mesoporous hafnia are consistent with predictions from kinetic models that ascribe gaseous diffusion through a pore network as the rate determining step in oxidation behavior of hafnium carbide. These observations imply that high-temperature oxidation behavior of hafnium carbide under the employed test conditions is linked to microstructure development via phase separation and coarsening behaviors of an initially formed amorphous HfO2Cx product.