HOT ISOSTATIC PRESSING OF SINTERED SILICON-NITRIDE

Pressureless-sintered silicon nitride with varying additives was hot isostatic pressed under 150 MPa of nitrogen at 1800-degrees-C. Moderate increases in densities were observed when sintered densities exceeded 90% of theoretical. However, density changes became insignificant as the amount of additives exceeded 12 wt%; moreover, density reduction was occasionally observed. Microstructural analysis, after the silicon nitride was reannealed at 1650-degrees-C under 0.1 MPa of nitrogen, revealed that intergranular glass was supersaturated with nitrogen and "bloated" as a result of nitrogen evolution. This result suggested that the effectiveness of container-free hot isostatic pressing of silicon nitride was severely limited by enhanced solubility of nitrogen gas in the glassy phase under high pressure.