Microstructure and property evolution of Gd3NbO7-GdNbO4 composite thermal barrier coating

To fulfill the multi-functional requirements of advanced thermal barrier coatings, composite oxides are emerging as promising candidates due to their enhanced thermal and mechanical properties. Although numerous composite materials have been developed, a comprehensive investigation of their microstructure and property evolution is still lacking. In this study, we conducted a comprehensive investigation of the microstructure and property evolution of the Gd₃NbO₇-GdNbO₄ composite thermal barrier coating as a case study. The as-sprayed Gd₃NbO₇-GdNbO₄ coating was verified to possess a single fluorite phase with a composition of Nb_xGd_(1-x)O_(1.5+x), which is a metastable phase. After heat treatment, the monoclinic GdNbO₄ phase precipitated without altering the coating microstructure. During high-temperature thermal exposure, rapid sintering occurred, accompanied by grain coarsening and pore healing, leading to an increase in hardness and toughness. The toughness of the Gd₃NbO₇-GdNbO₄ coating was higher than that of rare earth zirconates due to the presence of multiple toughening mechanisms. Additionally, a low thermal conductivity of 1.33 W/(m·K) was achieved for the Gd₃NbO₇-GdNbO₄ coating due to rapid sintering. Notably, there were no abrupt changes in the CTE curves during the heating process, which is crucial for thermal barrier coatings. The CTE reaches 11.0×10⁻⁶ K⁻¹ at 1300 °C, which is close to YSZ, indicating that the Gd₃NbO₇-GdNbO₄ coating is suitable for advanced thermal barrier coatings.