SINTERING MECHANISMS AND MICROSTRUCTURAL DEVELOPMENT OF COPRECIPITATED MULLITE

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, 1 000, 1250, and 1650-degrees-C, and test sintering runs were performed at 1 550, 1 600, 1 650, 1 700, and 1750-degrees-C. Homogeneous and dense ceramics were obtained from cold isostatically pressed (Cl Ped) powders sintered in air at 1700-degrees-C. Therefore, all further sintering experiments were carried out at 1700-degrees-C. After pressureless sintering, sample specimens were hot isostatically pressed (HIPed) at 1600-degrees-C and 200 bar argon gas pressure. Sintering densifications of low Al2O3 materials ranged between almost-equal-to 94% and almost-equal-to 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 almost-equal-to 97% at 600-degrees-C calcination temperature to almost-equal-to 99% at 950-degrees-C calcination temperature. Higher calcination temperatures first caused slight lowering of the sintering density to almost-equal-to 95.5% (calcination temperature 1250-degrees-C) but later the density strongly decreased to a value of 85% (calcination temperature 1650-degrees-C). HIPing of pressureless sintered specimens prepared from powders calcined between 600 and 1100-degrees-C yielded 100% density. At the given sintering temperature of 1700-degrees-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 alpha-Al2O3 were produced from high-Al2O3 powders calcined between 600 and 1100-degrees-C. Low-Al2O3 sample specimens sintered from precursor powders calcined between 600 and 1100-degrees-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.