Sintering mechanisms and microstructural development of coprecipitated mullite

Abstract

Coprecipitated mullite precursor powders of the bulk compositions 78 wt% Al₂O₃+22 wt% SiO₂ (high-Al₂O₃ material) and 72 wt% Al₂O₃+28 wt% SiO₂ (low-Al₂O₃ material) have been used as starting materials. The precursor powders were calcined at 600, 950, 1000, 1250, and 1650‡ C, and test sintering runs were performed at 1550, 1600, 1650, 1700, and 1750‡ C. Homogeneous and dense ceramics were obtained from cold isostatically pressed (CIPed) powders sintered in air at 1700‡ C. Therefore, all further sintering experiments were carried out at 1700‡ C. After pressureless sintering, sample specimens were hot isostatically pressed (HIPed) at 1600‡ C and 200 bar argon gas pressure. Sintering densifications of low Al₂O₃ materials ranged between ≈94% and ≈95.5%. There was no clear dependency between densification and calcination temperature of the starting powders. High-Al₂O₃ compositions displayed sintering densities which increased from ≈97% at 600‡ C calcination temperature to ≈99% at 950‡ C calcination temperature. Higher calcination temperatures first caused slight lowering of the sintering density to ≈95.5% (calcination temperature 1250‡ C) but later the density strongly decreased to a value of 85% (calcination temperature 1650‡ C). HIPing of pressureless sintered specimens prepared from powders calcined between 600 and 1100‡ C yielded 100% density. At the given sintering temperature of 1700‡ C, the microstructure of sample specimens was influenced by Al₂O₃/SiO₂ ratios and by calcination temperatures of the starting powders. Homogeneous and dense microstructures consisting of equiaxed mullite plus some minor amount of α-Al₂O₃ were produced from high-Al₂O₃ powders calcined between 600 and 1100‡ C. Low-Al₂O₃ sample specimens sintered from precursor powders calcined between 600 and 1100‡ C were less dense than high-Al₂O₃ materials. Their microstructure consisted of relatively large and elongated mullite crystals which were embedded in a fine-grained matrix of mullite plus a coexisting glass phase. The different microstructural developments of high- and low-Al₂O₃ compositions may be explained by solid-state and liquid-phase sintering, respectively. The microstructure of HIPed samples was very similar to that of pressureless sintered materials, but without any pores occurring at grain boundaries.