Ceramics in the Si3N4-ZrO2-CeO2 system: Phase composition changes and properties
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Silicon nitride-based compositions containing zirconia and ceria additives were prepared by hot pressing in argon atmosphere. The compositions studied can be divided into two groups: (a) with a constant total amount of ZrO2 + CeO2 additive (32 mol.%) within which the CeO2 content was varied from 0 to 6.2 mol.%, and (b) with various contents (10 to 42 mol.%) of ZrO2 + CeO2 additive, the ratio of ZrO2 to CeO2 staying constant at a level of 7.35 : 1. Phase compositions were determined by X-ray powder diffraction (XRD), while energy-dispersive X-ray microanalysis (EDX) was used for compositional analysis of the ZrO2 grains. The results showed that after sintering the Ce4+ ions were reduced to Ce3+. At low initial ceria concentrations, Si2N2O was formed beside beta -Si3N4 and ZrO2 (the majority of which was monoclinic). At higher ceria concentrations, however, Si2N2O disappeared by reaction with ceria and silica to form Ce-N apatite. Nitrogen-containing zirconia phases were not found in the... studied compositions, up to 1750 degreesC.