Microstructural characterization of the oxide scale on nitride bonded SiC-ceramics

The microstructure of the oxide scale formed at 1510 °C by oxidation of silicon nitride-bonded SiC-ceramics was studied by scanning electron microscopy (SEM). Etching by diluted HF etchants was used to help microstructural observation. This method revealed individual cristobalite crystallites as well as interfaces or interlayers between silica and non-silica phases. The ceramics-scale interface was covered by cristobalite crystallites showing that the devitrification of oxide scale begins here. Crystallites grown on SiC were smaller than those grown on binding phase. A thin HF-soluble interlayer was observed between SiC grains and cristobalite. No interlayer was found between cristobalite and oxynitride-type binding phases. The applied etching procedure gave supplementary information on the cracking, too. Partial etching by diluted HF delineated cracks in the scale. These cracks originate in the cristobalite and extend nearly perpendicularly to the substrate through the whole glassy part of the scale.