Four kinds of sintered Si3N4 were HIP'ed without container. The effects of HIP condition on density, weight change, microstructure and flexural strength were studied. The results showed that nitrogen was more preferable than argon as the pressure medium of HIP, and crystal grains of Si3N4 were considerably grown by HIP treatment at high temperatures around 1900°C. Moreover, the flexural strength and Weibull modulus of sintered Si3N4 which contained 6% Y2O3, 2% Al2O3 and 3% MgO as sintering additives were improved from 537 to 805MPa and from 5.2 to 8.0, by HIP treatment, respectively.
