Selective active oxidation in hafnium boride-silicon carbide composites above 2000 °C

The oxidation behavior of an ultra-high temperature ceramic (UHTC) based on HfB₂ with 20vol% SiC and was studied following two 10 min arc-jet test cycles with nominal heat flux of 350 W cm⁻², stagnation pressure of 7 kPa, and a sustained peak surface temperature of 2360 °C. Microstructure characterization revealed a modified, layered structure comprising ∼390 μm of porous HfO₂ at the surface and an underlying ∼740 μm porous region containing un-oxidized HfB₂ over the bulk UHTC, unaffected below the oxidation front. The SiC presumably undergoes active oxidation, as commonly reported for temperatures above ∼1600 ± 100 °C. However, unlike typical of exposures below ∼2000 °C no molten silicate phase was present at the surface to mediate the exchange of oxidant and gaseous reaction products. Additionally, a HfC impurity phase oxidizes concurrently with SiC rather than HfB₂. A thermodynamic analysis is provided to explain the observed behavior and the differences with lower temperature scenarios in the literature.