Sintering and Properties of Si₃N₄ with and without Additives by HIP Treatment

Abstract

Hot isostatic pressing of Si₃N₄ powders with and without additives was performed using a glass container, and also various kinds of pressure-less sintered Si₃N₄ were HIP'ed without a container. The effects of HIP treatment on density, microstructure, flexural strength, microhardness and fracture toughness of Si₃N₄ ceramics were studied. By HIPing of Si₃N₄ ceramics using a glass container, it was difficult to reach the theoretical density. The microhardness of HIP'ed Si₃N₄ without additives was low, and the fracture toughness of HIP'ed Si₃N₄ with and without additives was about 5MN/m^3/2. Thermal conductivity of HIP'ed Si₃N₄ without additives was 22∼25W/m·K, and it decreased with increasing the amount of additives. The density, flexural strength and microhardness of pressure-less sintered Si₃N₄ which contained Al₂O₃ and Y₂O₃ as oxide additives were remarkably improved by HIP treatment using nitrogen as a pressure transmitting gas.
It is very important to select the sintering conditions for fabricating the pre-sintered body of Si₃N₄ in order to improve the mechanical properties of Si₃N₄ ceramics by HIPing treatment.