Effect of phase composition and microstructure on fracture toughness and thermal conductivity of gadolinia stabilized hafnia ceramics

Abstract

Due to excellent thermophysical properties, cubic (C) phase HfO₂ is a promising candidate for thermal barrier coatings. Improving its fracture toughness is still a critical challenge before its engineering application. In this study, we proposed a phase-engineering strategy by incorporating the monoclinic (M) phase into the cubic (C) matrix to improve fracture toughness. A series of GdO_1.5-doped HfO₂ ceramics, with general chemical formula Gd_xHf_1-xO_2-x/2 (0≤x ≤ 0.20), were prepared by solid-state sintering at 1600 °C. Dual-phase M + C composite ceramics were obtained when 0 < x < 0.2. Notably, a percolation-like cluster structure was generated when M phase content reached 35 vol%. In which the M phase formed percolating interconnected network inside the C phase matrix. This unique architecture enabled a remarkable 90 % enhancement in fracture toughness, reaching a maximum value of 2.46 MPa m^1/2 for Gd_0.14Hf_0.86O_1.93. Additionally, Gd_0.14Hf_0.86O_1.93 also maintained a relatively low thermal conductivity (2.0 W m⁻¹ K⁻¹ at 1200 °C), and the percolation-like clusters block radiative transport at high temperatures, resulting in thermal conductivities of the composite ceramics remaining constant. These findings demonstrate a synergistic approach to enhance mechanical durability and thermal insulation performance in advanced TBC materials, offering valuable insights for next-generation high-temperature applications.