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

Abstract

The oxidation behavior of an ultra-high temperature ceramic (UHTC) based on HfB2 with 20vol% SiC and was studied following two 10min arc-jet test cycles with nominal heat flux of 350Wcm−2, stagnation pressure of 7kPa, and a sustained peak surface temperature of 2360°C. Microstructure characterization revealed a modified, layered structure comprising ∼390μm of porous HfO2 at the surface and an underlying ∼740μm 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.