Yttria Doped Si₂N₂O Ceramics

Abstract

Silicon oxynitride ceramics have been prepared from Si₃N₄ and SiO₂ by pressureless sintering at 1600 and 1775°C and by hot isostatic pressing (HIP) at 1600, 1750 and 1900°C, with Y₂O₃ (0–10 wt%) as a sintering aid. Pressurelessly sintered samples were in general porous, whereas the HIPed samples were dense at all temperatures. The reaction to form Si₂N₂O was very sluggish in the absence of Y₂O₃, and the samples mostly consisted of unreacted starting material, whereas addition of Y₂O₃ and high temperatures greatly favoured the formation of Si₂N₂O. Dense Si₂N₂O materials, prepared by HIP at 1900°C, were hard (HV10 = 1550 kgmm⁻²) and brittle (K_1C = 3.3 MPam^1/2) at room temperature for low Y₂O₃ additions. The hardness decreased slightly, but the fracture toughness remained the same with increasing yttria content.