The Microstructure of Metal Oxide Additive Silicon Nitride Ceramics
This work is concerned with Si3N4 ceramics fabricated with different metal oxide additives, viz. Y2O3, Al2O3, ZrO2 and ZrO2(+3 mol% Y2O3), which have been densified by hot isostatic pressing (HIP). The resulting microstructures, characterized by x-ray diffractometry and analytical electron microscopy, have been related to oxide additive, phase content, formation process, i. e. time and temperature of HIP, and room temperature hardness and indentation fracture toughness.
The different metal oxide additives made it possible to control microstructural features such as grain size, grain shape and the amount of residual intergranular glassy phase. Al2O3 had a crucial influence on the densification behaviour; at a low HIP temperature, 1550 °C, Al2O3 was required in order to achieve full density. The use of ZrO2 and/or Al2O3 made it possible to form ceramics with an extremely small volume fraction of residual glass. A separate addition of Y2O3 increased the volume fraction of glass, and promoted the development of high aspect ratio .beta.-Si3N4 grains. So did also Y2O3 added in the form of stabilized ZrO2 powder, although the volume fraction of glass in these materials was considerably lower. The fracture toughness was primarily determined by the shape of the .beta.- or .beta.'-Si3N4 grains; a higher aspect ratio gave a higher toughness. However, the hardness increased with increasing amount of retained .alpha.-Si3N4. Thus, it has been demonstrated that by control of composition and fabrication process it is possible to "microstructurally design" an Si3N4 ceramic in order to obtain a suitable combination of hardness and fracture toughness.
The microstructures of selected Si3N4 - ZrO2 ceramics were characterized before and after exposure to air at 500, 1100, 1250 and 1400 °C. Strength in four-point bending was also determined for these materials.
The microstructures after oxidation were also characterized by TEM of cross sections containing oxide scale, subscalar region and bulk microstructure. The nature of the oxide scale was dependent upon microstructure and composition of the as-received materials. The ZrO2 structures were characterized in detail by HRTEM before and after heat treatment at 1250 °C. Materials formed under a HIP pressure with Al2O3 additions were not subjected to internal oxidation of the ZrO2 structure during heat treatment.