Tailoring the microstructure of hot-pressed SiC by heat treatment

In the present work, monolithic NITE-SiC produced with the addition of Al₂O₃, Y₂O₃ and SiO₂ were annealed at different temperatures to observe microstructure evolution. The weight loss of specimens annealed under 1400°C is less than 3%. However, specimens annealed at 1500°C or higher show pockets of erosion and heavy weight loss. The second phase in as-fabricated NITE-SiC, an oxide glassy phase, is gradually crystallized through heat treatment at 1300°C or higher. These oxide phases are identified as yttrium-aluminum garnet (YAG) and Y ₂Si₂O₇ by X-ray diffractometry. During heat treatment the softened oxide glassy phase diffuses from the center to surface and reacts with SiC matrix. It can be concluded that the secondary oxide phase is reduced gradually through movement of the liquid phase toward surface and successive volatilization.