The microstructure and boundary phases of in-situ reinforced silicon nitride

The microstructure of an in-situ reinforced silicon nitride, gas pressure sintered with La2O3, Y2O3, and SrO additives and then heat treated, is examined with X-ray diffraction, SEM, and high-resolution TEM. Two crystalline rare-earth apatite phases, La5Si3O12N and Y5Si3O12N, are identified at the grain pockets and at the two-grain boundaries. The thickness of the crystalline phases at the two-grain boundaries is approximately 1.7 nm, in compliance with the suggested equilibrium intergranular spacing. A glassy phase is also present at the grain pockets and at the two-grain boundaries due to incomplete crystallization of the boundary phases. The thickness of the amorphous phase at the two-grain boundaries varies from 0.7 to 3.0 nm, suggesting that compositional inhomogeneities exist in these areas. Based on the microstructural observations, the structures of the crystalline boundary phases, the equilibrium intergranular film thickness, and the mechanisms causing incomplete recrystallization of the glassy phase in the in-situ reinforced silicon nitride are discussed.