MICROSTRUCTURE AND CHEMICAL-REACTION IN A SI3N4-TIC COMPOSITE

The effects of TiC addition to Si3N4 on microstructure and the chemical aspects of Si3N4-TiC interphase reaction were investigated in samples hot-pressed at 1800-degrees-C in Ar and N2. Composition of a TiC1-xNx solid solution was predicted based on thermodynamic calculation, with titanium carbonitride taken to be an ideal solid solution. The predicted value of x = 0.7 is slightly higher than that derived from the measured lattice parameter and Vegard's law (x = 0.67). Four distinguishable areas were observed in samples hot-pressed in nitrogen atmosphere. They were identified as beta-Si3N4, mixtures of TiC and titanium carbonitride solid solution, SiC with twins, and iron silicide. As the duration of hot-pressing increased, more titanium carbonitride was formed, while less TiC phase remained. Thermodymanic calculations indicate one source of nitrogen gas came from the decomposition of Si3N4. The TiC particles also became more irregular, and reactants were found inside or between TiC as the hot-pressing time was extended. Silicon carbide was not detected in samples which were hot-pressed in argon atmosphere; however, numerous pores were found around TiC.