Microstructural Changes in Liquid-Phase-Sintered Silicon Carbide during Creep in an Oxidizing Environment

The knowledge of the microstructural evolution during exposure to high temperatures is important to understanding the mechanisms responsible for the creep resistance of silicon carbide (SiC) ceramics. This includes not only the phase transformation of the SiC grains, but also the phase transformations of the oxynitride grain-boundary phases. For this study, a series of SiC specimens were prepared with varying molar ratios of AlN-Y₂O₃ additives. Increased creep resistance was observed in specimens with an additive system containing a 2:3 molar ratio or 60 mol% Y₂O₃. A continuous oxide layer of Y₂Si₂O₇ formed at the surface during elevated temperature testing in air. No blistering or cracking was observed in this oxide coating. Further increase of the creep resistance was achieved by a post-sintering nitrogen anneal.