Yttria Doped Si2N2O Ceramics
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 (K1C = 3.3 MPam1/2) at room temperature for low Y2O3 additions. The hardness decreased slightly, but the fracture toughness remained the same with increasing yttria content.
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