Analytical Electron Microscopy And High-Resolution Electron Microscopy Studies Of Grain-Boundary Films In Silicon Nitride-Based Ceramics

Abstract

ABSTRACTThe microstructures of post-sintered reaction-bonded Si3N4 materials (SRBSN) were investigated. The materials consist of large elongated β-Si3N4 grains embedded in a finegrained matrix. Amorphous secondary phases exist at triple grain junctions owing to the liquid phase sintering involved during densificacition. Those amorphous phases can be crystallized (nearly completely) by post-sintering heat treatment. Apart from these crystalline grain pockets the grains of all materials are covered with thin (<l-2 nm) amorphous intergranular films on both Si3N4/Si3N4 grain boundaries as well as on secondary-phase/Si3N4 phase boundaries. A control of the intergranular films is most desirable since they limit the high-temperature mechanical properties of Si3N4-based ceramics. Therefore, the required characterization was performed by analytical and high-resolution transmission electron microscopy (AEM/HREM). Si3N4 materials with different rare-earth and transition-element oxide additions were studied. AEM and HREM investigations revealed marked differences in thicknesses and chemical compositions of the different intergranular films depending on the system analyzed indicating a strong dependence of film thickness on chemical composition. However, a given composition of each investigated material showed a characteristic intergranular film thickness, independent of grain misorientation, with the only exception of low-energy grain boundaries. The thickness of the intergranular films was constant within 0.2 nm. In addition, the film thickness of phase boundaries was always greater (by 1–2 nm) compared to grain-boundary films.