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

Abstract

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)0.9O2 and ME (Ta, Ti)0.1(Zr, Hf, Ce)0.9O2, were designed and prepared by pressureless sintering of binary powder compacts in this work. ME (Y, Ti)0.1(Zr, Hf, Ce)0.9O2 presents cubic structure but a trace amount of secondary phase, while ME (Ta, Ti)0.1(Zr, Hf, Ce)0.9O2 displays a combination of tetragonal phase (81.6 wt.%) and cubic phase (18.4 wt.%). Both ME (Y, Ti)0.1(Zr, Hf, Ce)0.9O2 and ME (Ta, Ti)0.1(Zr, Hf, Ce)0.9O2 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)0.9O2 is superior to that of ME (Y, Ti)0.1(Zr, Hf, Ce)0.9O2 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)0.9O2 (Me = Y and Ta), especially ME (Ta, Ti)0.1(Zr, Hf, Ce)0.9O2, are promising thermal barrier materials for high-temperature thermal radiation shielding and CMAS blocking.