New class of high-entropy defect fluorite oxides RE2(Ce0.2Zr0.2Hf0.2Sn0.2Ti0.2)2O7 (RE = Y, Ho, Er, or Yb) as promising thermal barrier coatings

High-entropy ceramics exhibit great application potential as thermal barrier coating (TBC) materials. Herein, a series of novel high-entropy ceramics with RE 2 (Ce 0.2 Zr 0.2 Hf 0.2 Sn 0.2 Ti 0.2 ) 2 O 7 (RE 2 HE 2 O 7 , RE = Y, Ho, Er, or Yb) compositions were fabricated via a solid-state reaction. X-ray diffraction (XRD) and energy dispersive spectrometry (EDS) mapping analyses confirmed that RE 2 HE 2 O 7 formed a single defect fluorite structure with uniform elemental distribution. The thermophysical properties of the RE 2 HE 2 O 7 ceramics were investigated systematically. The results show that RE 2 HE 2 O 7 ceramics have excellent high-temperature phase stability, high thermal expansion coefficients (10.3–11.7 × 10 −6 K -1 , 1200 ℃), and low thermal conductivities (1.10-1.37 W m -1 K -1 , 25 ℃). In addition, RE 2 HE 2 O 7 ceramics have a high Vickers hardness (13.7–15.0 GPa) and relatively low fracture toughness (1.14-1.27 MPa m 0.5 ). The outstanding properties of the RE 2 HE 2 O 7 ceramics indicate that they could be candidates for the next generation of TBC materials.