Effect of TiO2 and AlN additions on the sintering behavior of the Si3N4-Y2O3-Al2O3 system

Si3N4 - Y2O3 - Al2O3 system with different amounts of TiO2 and AlN additions was fired at the temperatures from 1300 to 1900degreesC for 1h. TEM and EDS analysis showed that spherical TiN particles were located in grain boundary and grain. The sample adding both TiO2 and AlN fired at 1700degreesC showed the bulk density above 3.25gcm(-3). In-situ measurement of shrinkage during sintering was carried out. As a result, shrinkage of green body of the Si3N4 - Y2O3 - Al2O3 - TiO2 - AlN system started at lower temperature compared with that of the Si3N4 - Y2O3 - Al2O3 system. Although three peaks of the shrinkage rate in the sample without TiO2 and AlN system were observed, the third peak disappeared in the case of TiO2 and AlN addition. Furthermore, it was confirmed that the maximum temperature in the shrinkage rate dropped about 70degreesC in the sample with TiO2 and AlN addition. XRD analysis showed that alpha to beta phase transformation in the sample with TiO2 addition is delayed and that with TiO2 and AlN addition was advanced in comparison to that without TiO2 and AlN addition. According to thermodynamics, added TiO2 should be reacted with Si3N4 in the case of only TiO2 addition and with AlN in the case of simultaneous addition of TiO2 and AlN to form SiO2 and Al2O3, respectively. It was probable that the increase in the amount of SiO2 result in the increase in liquid phase with high viscosity, so that the densification was advanced at high temperature in spite of delay of alpha to beta phase transformation. On the other hand, in the case of formation of A1203, much liquid phase with relatively low viscosity seemed to advance both the densification and the phase transformation. Consequently, simultaneous addition of TiO2 and AlN was found to work well for densification of Si3N4 at low temperature.