Microstructures and phase compositions of Y2O3–ZrO2–HfO2 solid solutions

Y2O3–ZrO2–HfO2 solid-solution powders with different HfO2 contents were prepared via high-pressure hydrothermal synthesis and sintered into ceramics. The phase compositions and microstructures of the solid solutions formed were investigated in detail. The results indicated that the solid solution stabilised by 3 mol% Y2O3 consisted of a monoclinic phase and tetragonal phase with an HfO2 content of ≤25 mol%, but the solid solution changed to a mixture of monoclinic and cubic phases with an HfO2 content of ≥50 mol%. The increase in the HfO2 content weakened the stabilising effect of Y2O3 and made the system tend toward the cubic phase. In addition, compared with the other samples, the solid solution with equal molar ratios of Zr and Hf exhibited the lowest thermal conductivity, and had higher hardness and fracture toughness, which was related to the formation of superstructures.