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 degrees C. Microstructure characterization revealed a modified, layered structure comprising similar to 390 mu m of porous HfO2 at the surface and an underlying similar to 740 mu m 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 similar to 1600 +/- 100 degrees C. However, unlike typical of exposures below similar to 2000 degrees 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. (C) 2016 Elsevier Ltd. All rights reserved.