Low-temperature phase stability of ceramics in the ZrO2—Y2O3—CeO2 system produced after heat treatment of the starting powders at 850 °C

Abstract

The low-temperature phase stability of materials in the ZrO2—Y2O3—CeO2 system with compositions, % (mol.): 97ZrO2—3Y2O3; 95ZrO2—3Y2O3—2CeO2; 92,5ZrO2—2,5Y2O3—5CeO2; 90ZrO2—2Y2O3—8CeO2; 88ZrO2—12CeO2 was studied. Treatment of materials in hydrothermal conditions under an increased holding time (14 hours) was used. The starting powders were produced by the hydrothermal synthesis in an alkaline medium and heat-treated at 850 °C. The samples were sintered at 1350 oC. The materials properties were investigated by the X-ray phase analysis and electron microscopy. The aging stability of ceramics was determined by the degree of phase transformation T-ZrO2 → M-ZrO2 under experimental conditions. Porous microstructures were formed in the samples, which differ in the size distribution of both grains and pores. A characteristic feature is the presence of various amounts of fine-grained fragments with a regular microstructure and the formation of both rounded and elongated grains.The phase transformation T-ZrO2 → M-ZrO2 leads to an increase of the samples porosity. This, in turn, contributes to the intensification of the ceramics aging. After 14 h the phase transformation T-ZrO2 → M-ZrO2 was found in four samples. In the sample 97ZrO2—3Y2O3, 46% of M-ZrO2 was formed; in the 95ZrO2—3Y2O3—2CeO2 sample, 48% of M-ZrO2 was formed; in the 92,5ZrO2—2,5Y2O3—5CeO2 sample, 39% of M-ZrO2 was formed. In the 90ZrO2—2Y2O3–8CeO2 sample ≈1% of M-ZrO2 appeared, and in the 88ZrO2—12CeO2 sample M-ZrO2 was not identified. Formation features of the solid solution during the doping of zirconia with yttrium oxide and cerium oxide, the amount of cerium oxide in a ZrO2-based solid solution, the phase transformation F-ZrO2 → T-ZrO2 during the sintering and the formation of a homogeneous microstructure contribute to increasing the low-temperature phase stability of samples both 90ZrO2—2Y2O3—8CeO2 and 88ZrO2—12CeO2 composition. During the microstructural design of ceramics in the ZrO2—Y2O3—CeO2 system with increased low-temperature phase stability, it is necessary to establish such a ratio of Y2O3 and CeO2 in the solid solution based on ZrO2 that would provide the necessary strength behavior according to the ceramics use. Keywords: ZrO2—Y2O3—CeO2 system, ZrO2-based solid solution, M-ZrO2 phase, aging, low-temperature phase stability.