| Summary: | HfO<sub>2</sub> has become a promising thermal barrier coating material due to its similarity in structure and chemical properties with ZrO<sub>2</sub> and its higher phase structure transition temperature. However, the fracture toughness of HfO<sub>2</sub> is not ideal, greatly limiting its application. In this report, we find a special sandwich structure of ceramics, comprising a cubic (C) phase /monoclinic (M) phase/cubic (C) phase. The microstructural evolution and mechanical properties of these ceramics were investigated under annealing at 1500 °C. The results indicate that, with an increase in annealing duration, there was a gradual augmentation in the proportion of the monoclinic (M) phase and the fracture toughness increased from 2.18 MPa·m<sup>0.5</sup> to 2.83 MPa·m<sup>0.5</sup> after 48 h of annealing, which is higher than many potential TBC materials. The residual compressive stress present in the M phases during the progression of crack propagation served to facilitate the bridging and deflection of cracks. As such, this process led to the alleviation of stress concentration at the crack tip, ultimately enhancing the toughening effect.
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