EFFECTS OF RARE-EARTH-OXIDE ADDITION ON THE SINTERING OF MULLITE

Effects of various kinds of rare earth oxides (Y2O3, Er2 O3, Sm2O3, La2O3 and CeO2) addition on the sintering of mullite and the precipitation behavior of secondary phases in the mullite sintered bodies were investigated. The sintering of the mullite compacts, to which a small amount of these rare earth oxides was added, was performed at 1500-degrees - 1600-degrees-C. The morphologies of the rare earth silicates precipitated and remained on the grain boundaries of the sintered bodies could be classified into three categories, i.e., crystalline, glassy and decomposed substances, depending upon the kinds of the rare earth oxides added. The microstructures of the mullite sintered bodies changed as the amount of the additives varied. The three point bending strength of the sintered mullite increased with the addition of 0.5 wt% of Y2O3 or CeO2. On the other hand, excessive amount of these additives would result in the decrease of the three point bending strength. It was found that the maximum value of three point bending strength was evidently related to the melting point of the secondary phases existed in the sintered bodies of mullite.