Novel (La0.2Ce0.2Pr0.2Nd0.2Gd0.2)2Hf2O7 high-entropy pyrochlore oxide as a promising TBC material

High-entropy pyrochlore oxides are multi-cation solid solutions that have excellent high-temperature structural stability. They find application in fields like thermal insulation, catalysis, sensors, fuel cells, and optical and electronic devices, due to their compositional flexibility. While much research has focused on pyrochlore zirconates as a thermal barrier coating material, high entropy hafnates remain relatively unexplored although they have been reported to have better thermal characteristics. In this study, a novel high entropy pyrochlore hafnate, (La_0.2Ce_0.2Pr_0.2Nd_0.2Gd_0.2)₂Hf₂O₇, denoted as (LCPNG)HO, was designed, synthesized and characterized as a novel thermal barrier coating material. The high entropy oxide was synthesized through a solution combustion process using urea as fuel. The formation of phase-pure pyrochlore crystal structure was confirmed using characterization methods like XRD, Raman and HR-TEM with SAED. (LCPNG)HO exhibits excellent high temperature phase stability up to 1600 ℃ and has a uniform elemental distribution. The material exhibited excellent thermal stability with negligible mass changes in the temperature range of 25 – 1400 ℃, without any endothermic or exothermic processes being present. (LCPNG)HO exhibited a low thermal conductivity of 1.73 W/m.K and a coefficient of thermal expansion of 11.27 × 10⁻⁶ /K at 1000 ℃. The thermal conductivity value of (LCPNG)HO is lower than that of YSZ, a widely used TBC material. (LCPNG)HO also exhibited a wider UV-Vis absorption than YSZ and 20 % enhancement in IR absorbance, making it a suitable candidate for UV-IR shielding. The novel high entropy oxide displays an average hardness of 12.54 GPa and fracture toughness of 2.51 MPa.m^1/2 on dense sintered samples. Thermal ageing and cycling processes at high temperatures established that the high entropy oxide has excellent phase stability even after 150 h of ageing at 1400 ℃ and 15 cycles of heating and sudden cooling. (LCPNG)HO also exhibited excellent chemical compatibility with the thermally grown oxide, Al₂O₃. The results indicate that (LCPNG)HO has outstanding characteristics, suitable for thermal barrier coating application.