We have investigated crystallization behavior of cordierite-based oxynitride glass made by melting procedure of the powder mixture of cordierite and silicon nitride. The results clearly indicated that two polymorphs exist, a stable α form that has significant low coefficient of thermal expansion and a metastable μ form. TTT diagram of the crystallization of α form shows a typical C-shape curve that has a nose around 1420 K, above 1373 K, the α form can be directly obtained by isothermal crystallization of the oxynitride glass and the μ form inverts to the α form at temperatures below 1273 K. The rate of the crystallization of cordierite-based oxynitride glass is found to be smaller in comparison with that of the oxide system, which can be explained with the rate-controlling step of the formation of the α-cordierite grain and the diffusivity in the oxynitride glass. We also have fabricated α-cordierite ceramics that have the nitrogen content of 1.55mass% (2.2at%) which is revealed to be much smaller than that of the batch composition (5mass% (7.0at%) ) , which is due mainly to the decomposition reaction of nitrogen in the oxynitride melts and glasses during heat-treatments. However, the fabricated nitrogen-containing α-cordierite ceramics had the coefficient of thermal expansion of 10-7 K-1, which is comparable with the oxide α-cordierite ceramics. Moreover, the bending strength and the Young's modulus of the α-cordierite ceramics were found to be significantly improved with adding nitrogen, which would be attributed to the incorporation of nitrogen into the microstructure of α-cordierite.
