Ultra-high temperature oxidation of a hafnium carbide-based solid solution ceramic composite

• Cr3C2 is a useful transient liquid phase sintering agent for carbide-based ceramics.
• Low atomic mass cations (e.g., Cr or Ti) can rapidly diffuse through oxide scales.
• Minor second phase additions can promote limited liquid formation for pore filling.
• Oxycarbide interlayer may comprise nanocrystalline graphite in amorphous oxide.

Hafnium carbide-based solid solution ceramics with yttria additions were consolidated via hot pressing utilizing chromium carbide as a transient liquid sintering agent. Selected samples were oxidized at temperatures exceeding 2200 °C under high velocity airflow to determine their potential for service in ultra-high temperature environments. Cross-sectional examination revealed a complex multi-layer scale comprising a highly porous exterior region, a dense multi-phase interior region, and an oxygen- and carbon-containing interlayer consisting of nanocrystalline graphite in an amorphous or nanocrystalline oxide matrix. Outward diffusion of mobile cations is speculated to play an important role in establishing the observed scale morphology.