Eutectic Studies in the System Si3N4-Si2N2O-Mg2SiO4

Melting experiments have established three important eutectics in this system: (1) the Si3N4-Mg2SiO4 binary eutectic composition, 0.07Si3N4+0.93Mg2SiO4 at 1560°C, (2) the Si2N2O-Mg2SiO4 binary eutectic composition, 0.17Si2N2O+ 0.83Mg2SiO4 at 1525°C, and (3) the Si3N4-Si2N2O-Mg2SiO4 ternary eutectic composition, 0.04Si3N4+0.14Si2N2O+0.82Mg2SiO4 at 1515°C. Systematic replacement of MgO with CaO in the ternary eutectic reduced its melting temperature to 1325°C for a CaO/MgO molar ratio of 0.67. The results of this study are discussed in relation to fabrication, microstructure, and high-temperature strengths.