Medium-entropy (Me,Ti)0.1(Zr,Hf,Ce)0.9O2 (Me = Y and Ta): Promising thermal barrier materials for high-temperature thermal radiation shielding and CMAS blocking

• Two medium-entropy (ME) oxide ceramics, ME (Y,Ti) 0.1 (Zr,Hf,Ce) 0.9 O 2 and ME (Ta,Ti) 0.1 (Zr,Hf,Ce) 0.9 O 2 , were designed and prepared successfully. • Good high-temperature thermal radiation shielding performance and CMAS resistance of the two ME oxides were demonstrated for the first time. • The relationship between high-temperature thermal radiation shielding performance, i.e., infrared absorbance at the waveband of 1 to 5 μm and band gap in ME (Me,Ti) 0.1 (Zr,Hf,Ce) 0.9 O 2 (Me= Y and Ta) was investigated systematically. • The mechanism for the CMAS corrosion resistance was proposed for ME (Me,Ti) 0.1 (Zr,Hf,Ce) 0.9 O 2 (Me= Y and Ta). With continuous enhancement of gas-turbine inlet temperature and rapid increase of radiant heat transfer, thermal barrier coating (TBC) materials with a combination of low thermal conductivity and good high-temperature thermal radiation shielding performance play vital roles in ensuring the durability of metallic blades. However, yttria-stabilized zirconia (YSZ), as the state-of-the-art TBC and current industry standard, is unable to meet such demands since it is almost translucent to high-temperature thermal radiation. Besides, poor corrosion resistance of YSZ to molten calcia-magnesia-alumina-silicates (CMAS) also impedes its application in sand, dust, or volcanic ash laden environments. In order to improve the high-temperature thermal radiation shielding performance and CMAS resistance of YSZ and further reduce its thermal conductivity, two medium-entropy (ME) oxide ceramics, ME (Y, Ti) 0.1 (Zr, Hf, Ce) 0.9 O 2 and ME (Ta, Ti) 0.1 (Zr, Hf, Ce) 0.9 O 2 , were designed and prepared by pressureless sintering of binary powder compacts in this work. ME (Y, Ti) 0.1 (Zr, Hf, Ce) 0.9 O 2 presents cubic structure but a trace amount of secondary phase, while ME (Ta, Ti) 0.1 (Zr, Hf, Ce) 0.9 O 2 displays a combination of tetragonal phase (81.6 wt.%) and cubic phase (18.4 wt.%). Both ME (Y, Ti) 0.1 (Zr, Hf, Ce) 0.9 O 2 and ME (Ta, Ti) 0.1 (Zr, Hf, Ce) 0.9 O 2 possess better high-temperature thermal radiation shielding performance than YSZ. Especially, the high-temperature thermal radiation shielding performance of ME (Ta, Ti) 0.1 (Zr, Hf, Ce) 0.9 O 2 is superior to that of ME (Y, Ti) 0.1 (Zr, Hf, Ce) 0.9 O 2 due to its narrower band gap and correspondingly higher infrared absorbance (above 0.7) at the waveband of 1 to 5 μm. The two ME oxides also display significantly lower thermal conductivity than YSZ and close thermal expansion coefficients (TECs) to YSZ and Ni-based superalloys. In addition, the two ME oxides possess excellent CMAS resistance. After attack by molten CMAS at 1250 °C for 4 h, merely ∼2 μm thick penetration layer has been formed and the structure below the penetration layer is still intact. These results demonstrate that ME ( Me , Ti) 0.1 (Zr, Hf, Ce) 0.9 O 2 ( Me = Y and Ta), especially ME (Ta, Ti) 0.1 (Zr, Hf, Ce) 0.9 O 2 , are promising thermal barrier materials for high-temperature thermal radiation shielding and CMAS blocking.