Silicon oxynitride ceramics were reaction sintered and fully densified by hot isostatic pressing in the temperature range 1700°C to 1950°C from an equimolar mixture of silicon nitride and silica powders without additives. Conversion to SizNLO increases steeply from a level around 5% of the crystalline phases at 1700°C to 280% at 1800°C, and increases a few percent further at higher temperatures. a-Si3N4 is the major residual crystalline phase below 1900°C. The hardness level for materials containing 285% Si2Nz0 is approximately 19 GPa, comparable with the hardness of SiJN4 hot isostatically pressed with 2.5 wt% Y z 0 3 , while the fracture toughness level is around 3.1 MPa m"', being approximately 0.8 MPa mi" lower. The three-point bending strength increased with HIP temperature from approximately 300 to 500 MPa. [Key words: silicon oxynitride, sintering, hot isostatic pressing, X-ray diffraction, mechanical properties.]
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