Effects of alumina sources on the microstructure and properties of nitrided Al2O3-C refractories

Fused white alumina, tabular alumina with a plate-like morphology, reactive alumina powder with a high specific surface area and industrial alumina composed of 40 to 76% γ-Al2O3 and 60 to 24% α-Al2O3 were selected as alumina sources and grouped in this work. The effects of alumina sources on the microstructure of nitrided Al2O3-C refractories were investigated. A large number of β-Sialon phases and a small number of SiC phases were formed when the alumina source varied, and α-Si3N4 phase was only formed when using tabular alumina. The β-Sialon phase was deemed to be the major ceramic bonding phase, which generated the morphologies of column and tabular column. Columned β-Sialon crystals with conical tips were formed by the direct nitriding of liquid silicon following a VLS (vapor-liquid-solid) growth mechanism, the transformation of the VLS growth mechanism into a VS (vapor-solid) growth mechanism was observed when using industrial alumina with smooth tips on the β-Sialon crystals. β-Sialon crystals with a morphology of tabular column were formed through nitriding of SiO (g) and Si (g) following the VS growth mechanism. SiC whiskers were formed by the reacting of CO (g) and SiO (g) following a CVD (chemical-vapor-deposition) growth mechanism. The physical, mechanical and thermal properties of these groups after the nitriding process were also investigated and compared. When using reactive alumina powder and fused white alumina as the alumina sources, the optimal cold crushing strength (CCS) and cold modulus of rupture (CMOR) were generated due to the dense reticular structure, and also the optimal hot modulus of rupture (HMOR) was achieved due to the formation of large size of O’-Sialon tabular whiskers in the test atmosphere. Improved thermal shock resistance and oxidation resistance were also observed.