Hot Isostatic Pressing of Sintered Silicon Nitride

Abstract

Silicon nitride ceramics were fabricated by pressureless sintering and then hot isostatic pressed under 150 MPa of nitrogen at 1800°C. Density changes by hot isostatic pressing (HIPing) varied significantly depending on sintered densities before HIPing and on the compositions of sintering additives and their amounts. Moderate increases in densities were observed when sintered densities exceeded 90% of the theoretical. However, fully dense components could not be obtained; even occasional density reduction was observed when the amount of additives exceeded 12 wt %. Further reduction in the final density were observed as HIPed silicon nitride was reannealed at 1650°C under 0.1 MPa of nitrogen. It is thus concluded that sinter-HIPing of silicon nitride is severely limited by the increase in solubility of nitrogen in glassy phases under high pressure. HIPing under similar conditions using mixed gas of nitrogen and argon produced fully dense components.