Influence of Secondary Phase Chemistry on Grain-Boundary Film Thickness in Silicon-Nitride

The microstructures of liquid-phase sintered Si3N4 materials were
investigated using analytical and high-resolution electron microscopy
(AEM, HREM). Microstructural characterization was performed with
emphasis on interfacial grain-boundary structure and chemistry. All
materials investigated consist of large elongated beta-Si3N4 grains
embedded in a globular fine-grained beta-Si3N4 matrix. Amorphous
secondary phases are observed mainly at three- and four-grain junctions
owing to the liquid-phase sintering process involved during
densification. Upon postsintering heat treatment crystalline secondary
phases form at triple grain regions. However, complete crystallization
cannot be achieved. All grains are covered with a thin amorphous
intergranular film at both homo-phase and heterophase boundaries. In
addition, the film thickness of heterophase boundaries is always
greater than homo-phase boundary films. The presence of amorphous
intergranular films profoundly affects the mechanical properties of
Si3N4-based ceramics at room and elevated temperatures. Therefore, a
control of these films is most desirable. Si3N4 materials with
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different rare-earth and transition-element oxide additions as well as
variations in volume content of sintering aids and secondary impurities
were studied. AEM and HREM studies revealed marked differences in
thickness and chemical composition of the different intergranular films
depending on the material analyzed. The results strongly suggest a
dependence of film thickness on chemical composition. At a given
composition each material showed a characteristic intergranular film
thickness, independent of grain misorientation for arbitrary grain
boundaries. Low angle and special grain boundaries are not covered with
an amorphous intergranular film.