Fractography studies were accomplished on two reaction-sintered Si3N4 materials (Norton NC-350 Si3N4 and KBI (Kawecki-Berylco) Si3N4) which had been tested in four-point bending at room temperature, 1200, 1350, and 1500°C in air. At room temperature NC-350 exhibited uniform planar features representing a transgranular fracture mode, uniformly distributed small pores having a size range of 1–2 μm or less, and a few isolated large pores of ∼10 μm. In contrast, the KBI Si3N4 at room temperature exhibited nonuniform, diffuse, and ill-defined fracture features, small pores having a size range of 2–5 μm, and many large pores of ∼50 μm in size. At high temperatures, the fracture appearance of NC-350 Si3N4 was ill defined and diffuse, indicating a change in fracture mode. The fracture originating flaws in NC-350 were primarily the large pores or pore agglomerates. At all temperatures, in the KBI material, it was not possible to observe the mirror and, therefore, the site of fracture initiation. For NC-350, KIC values were 1.2–1.6 MN/m3/2 at room temperature and 2.1–2.4 MN/m3/2 at high temperatures.

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