REACTION SINTERING AND PROPERTIES OF SILICON OXYNITRIDE DENSIFIED BY HOT ISOSTATIC PRESSING

Silicon oxynitride ceramics were reaction sintered and fully densified by hot isostatic pressing in the temperature range 1700-degrees-C to 1950-degrees-C from an equimolar mixture of silicon nitride and silica powders without additives. Conversion to Si2N2O increases steeply from a level around 5% of the crystalline phases at 1700-degrees-C to greater-than-or-equal-to 80% at 1800-degrees-C, and increases a few percent further at higher temperatures. Alpha-Si3N4 is the major residual crystalline phase below 1900-degrees-C. The hardness level for materials containing greater-than-or-equal-to 85% Si2N2O is approximately 19 GPa, comparable with the hardness of Si3N4 hot isostatically pressed with 2.5 wt% Y2O3, while the fracture toughness level is around 3.1 MPa.m1/2, being approximately 0.8 MPa.m1/2 lower. The three-point bending strength increased with HIP temperature from approximately 300 to 500 MPa.