The effect of the homogeneous mixing of additives using alumina sol on the sinterability and a/b transformation in silicon nitride ceramics were investigated compared with mechanical mixing. Alumina sol was prepared by peptizing with $HNO_3$ after hydrolyzing ASB with $H_2O$ in molar ratio of 1:100, and the composition of the powder system was $90 Si_3N_4-4Al_2O_3-6Y_2O_3$ by weight percent.
The degree of dispersion was measured by electrophoresis method, and stable dispersion could be obtained at pH=3. After an optimum treatment of this slurry, sintering was conducted. To find an optimal pretreatement condition, 3 kinds of pretreatment methods were used. By comparing them, the best thing was the compaction, and calcination during sintering schedule because it was free from hard agglomerates in compact which were known to prevent sintering. And the sintering behavior of this powder was compared with the one obtained by mechanical mixing method. Both powder compacts were sinterd between 1500℃ and 1800℃ for various holding times under 1 atm $N_2$ pressure.
The sintered densities of powder compacts by homogeneous mixing were higher than those obtained by mechanical mixing method in all sintering conditions. The shorter the sintering time and the lower the sintering temperature, the larger the differnce in sintered densities. And the pore distribution of the former was more uniform than the latter one. X-ray diffraction analysis showed that the degree of a to b transformation in the homogeneously mixed specimen was higher, too.
From these results, it is believed that the homogeneous mixing of sintering additives lowers the temperature of liquid formation, aids the uniform occurrence of liquid phase, promotes the sinterability and phase transformation, and results in the uniform microstructure.
