Effect of oxidation on room temperature strength of ZrB 2 - and HfB 2 - based ultra-high temperature ceramics

Abstract

The effect of oxidation on room temperature (RT) flexure strength degradation in SiC-reinforced ultra-high temperature ceramics (UHTCs) and La₂O₃-doped UHTCs has been characterised in the temperature range 1400–1600°C for oxidation times of up to 32 h. Flaw healing was identified for oxide scale thicknesses < 50 μm, although second phase agglomerates limit the flaw healing effect to scale thicknesses of 20 μm. A linear degradation of RT strength with oxide scale thickness was observed for oxide scale thickness >50 μm. Two oxide scale configurations have been proposed to minimise RT strength degradation. The most promising is the scale with a porous layer containing non-interconnected porosity (85–90% dense) of either MeLa₂O₇ or MeO _x C _y (Me = Zr or Hf).