Enhanced high‐temperature indentation fracture toughness of zirconia ceramics via HfO ₂ doping

Abstract In this research, 3 mol% Y 2 O 3 ‐stabilized ZrO 2 (3YSZ) ceramics doped with varying HfO 2 content (5–30 mol%; denoted as x H3YSZ) were fabricated via pressureless sintering at temperatures ranging from 1400°C to 1600°C. The phase compositions, microstructures, hardness, indentation fracture toughness (at both room and high temperatures) were systematically investigated for the first time. Results revealed that 3YSZ and HfO 2 formed a tetragonal phase solid solution when the HfO 2 content was ≤10 mol%. However, at HfO 2 concentrations of 15 mol% or higher, the ceramics exhibited a mixture of tetragonal and monoclinic phases (t and m phase). The incorporation of HfO 2 significantly enhanced the indentation fracture toughness at both room and elevated temperatures. The room‐temperature indentation fracture toughness of 10H3YSZ ceramics sintered at 1400°C and 1600°C reached 5.76 ± 0.24 MPa m 1/2 and 6.11 ± 0.09 MPa m 1/2 , respectively, markedly higher than that of undoped 3YSZ ceramics (4.25–4.94 MPa m 1/2 ). At high temperatures, the indentation fracture toughness of 10H3YSZ ceramics sintered at 1600°C was at least 1.4 times of 3YSZ ceramics at 200–800°C. The Vickers hardness of x H3YSZ ceramics decreased with sintering temperature. The Vickers hardness of 10H3YSZ was 12.0 ± 0.19 GPa at room temperature, which decreased to 3.13 ± 0.19 GPa at 800°C. The enhanced indentation fracture toughness was attributed to the reduced threshold for initiating the tetragonal‐to‐monoclinic (t → m) phase transformation in HfO 2 ‐doped ceramics, which increased crack shielding due to a larger volume fraction of transformed grains. Additionally, crack deflection and blunting mechanisms, promoted by grain growth and the reduced high‐temperature hardness at a sintering temperature of 1600°C, further contributed to the improved toughness.