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

Kleebe, H. J.; Hoffmann, M. J.; Ruhle, M.

Abstract: 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 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.

Zugehörige Institution(en) am KIT Institut für Keramik im Maschinenbau (IKM)
Publikationstyp Zeitschriftenaufsatz
Jahr 1992
Sprache Englisch
Identifikator ISSN: 0044-3093
KITopen ID: 1000007796
Erschienen in Zeitschrift für Metallkunde
Band 83
Heft 8
Seiten 610-617
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