Sialon Ceramics Sintered with Yttria and Rare Earth Oxides

Abstract

ABSTRACTDense single-phase α-, β- and O′-sialon cermics or mixed sialon ceramics without a glassy grain-boundary phase can be prepared at high temperatures and pressures, and these materials are well suited for high-temperature use, but they are usually brittle. Additional quantities of oxides of group IIIB metals in the periodic table are often added as sintering aids to achieve pressureless sintering and thereby to allow more complicated shapes to be manufactured directly and at lower costs. The most common additive is yttria, but the rare earth oxides are also of interest. All these oxides will promote the growth of elongated β crystals in the microstructure, and the fracture toughness will be improved considerably. Low-cost oxides like Nd2O3, La2O3 or CeO2 may replace Y2O3 without significantly impairing the mechanical properties at room temperature. The expensive rare-earth oxides like Sm2O3, Dy2O3 or Yb2O3 have been found to be as good additives as yttria, or even better, but improvements in mechanical properties are generally small and do not justify the use of these additives in large-scale production. The residual intergranular glassy phase usually found in the microstructure of metal-oxide-doped sialons will deteriorate the properties at very high temperatures, and this type of material is best suited for use at operation temperatures below 900-1000°C.