Effect of Sr₂Nb₂O₇ addition on fracture toughness and thermal conductivity of Y₂O₃-stabilized HfO₂ ceramics

Low fracture toughness limits the application of cubic-phase HfO₂ based ceramics as a novel thermal barrier coating material. In this study, Sr₂Nb₂O₇ was introduced into the 24 mol% Y₂O₃-stabilized HfO₂ (YSH24) to improve the fracture toughness. A series of dual-phase composite ceramics with a formula of xSr₂Nb₂O₇/(1-x)YSH24 (x = 0, 0.01, 0.02) were prepared by solid-state sintering. The effect of Sr₂Nb₂O₇ addition on microstructure, fracture toughness, and thermal conductivity of the dual-phase composite ceramics was investigated. The results showed that the perovskite (P) phase with a simple cubic structure of Nb₂O₅-doped SrHfO₃ was generated after Sr₂Nb₂O₇ was introduced. The compositions of x = 0.01 and 0.02 consisted of a mixture of the matrix fluorite (F) phase and a small amount of layer-like P phase. The maximum fracture toughness of 2.47 MPa m^1/2 was achieved for xSr₂Nb₂O₇/(1-x)YSH24 dual-phase composite ceramics when x = 0.01, which is approximately 98% higher than that of YSH24. It was surprising that the thermal conductivity of the x = 0.01 composition was as low as 1.63 W m⁻¹ K⁻¹ at 1200 °C, and the radiative thermal conductivity was reduced by 86% compared to YSH24. The findings should provide insight into the simultaneous improvement of fracture toughness and thermal conductivity of ceramic materials and contribute additional assistance in selecting the materials for advanced thermal barrier coatings.