High entropy defective fluorite structured rare-earth niobates and tantalates for thermal barrier applications

Rare-earth tantalates and niobates (RE 3 TaO 7 and RE 3 NbO 7 ) have been considered as promising candidate thermal barrier coating (TBC) materials in next generation gas-turbine engines due to their ultra-low thermal conductivity and better thermal stability than yttria-stabilized zirconia (YSZ). However, the low Vickers hardness and toughness are the main shortcomings of RE 3 TaO 7 and RE 3 NbO 7 that limit their applications as TBC materials. To increase the hardness, high entropy (Y 1/3 Yb 1/3 Er 1/3 ) 3 TaO 7 , (Y 1/3 Yb 1/3 Er 1/3 ) 3 NbO 7 , and (Sm 1/6 Eu 1/6 Y 1/6 Yb 1/6 Lu 1/6 Er 1/6 ) 3 (Nb 1/2 Ta 1/2 )O 7 are designed and synthesized in this study. These high entropy ceramics exhibit high Vickers hardness (10.9–12.0 GPa), close thermal expansion coefficients to that of single-principal-component RE 3 TaO 7 and RE 3 NbO 7 (7.9×10 −6 -10.8×10 −6 C −1 at room temperature), good phase stability, and good chemical compatibility with thermally grown Al 2 O 3 , which make them promising for applications as candidate TBC materials.