Low thermal conductivity, compatible thermal expansion coefficient, and good calcium–magnesium–aluminosilicate (CMAS) corrosion resistance are critical requirements for environmental barrier coatings used on silicon-based ceramics. RE2Si2O7 (RE = rare earth) has been widely recognized as one of the most promising candidates for environmental barrier coatings due to its good water vapor corrosion resistance. However, the relatively high thermal conductivity and poor resistance to CMAS corrosion have limited its practical application. Inspired by the high entropy effect, in this work, a novel rare earth disilicate (Lu1/7Yb1/7Sc1/7Er1/7Y1/7Ho1/7Dy1/7)2Si2O7 ((7RE1/7)2Si2O7) has been designed and synthesized by a solid reaction process. (7RE1/7)2Si2O7 showed a low thermal conductivity of 1.81 W·m−1·K−1 at 1273 K. Furthermore, the thermal expansion coefficient of (7RE1/7)2Si2O7 (4.07×10−6 ℃−1 from room temperature (RT) to 1400 ℃) is close to that of the SiC-based ceramic matrix composites (SiC-CMCs) ((4.5–5.5)×10−6 ℃−1). Additionally, (7RE1/7)2Si2O7 exhibited excellent resistance to CMAS corrosion. When exposed to CMAS at 1300 ℃ for 48 h, the reaction layer thickness was 22 μm. The improved performance of (7RE1/7)2Si2O7 highlights its potential as a promising candidate for thermal/environmental barrier coatings.
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During flight, many silicates (sand, dust, debris, fly ash, etc.) are ingested by an engine. They melt at high operating temperatures on the surface of thermal barrier coatings (TBCs) to form calcium-magnesium-aluminum-silicate (CMAS) amorphous settling. CMAS corrodes TBCs and causes many problems, such as composition segregation, degradation, cracking, and disbanding. As a new generation of TBC candidate materials, rare-earth zirconates (such as Sm2Zr2O7) have good CMAS resistance properties. The reaction products of Sm2Zr2O7 and CMAS and their subsequent changes were studied by the reaction of Sm2Zr2O7 and excess CMAS at 1350 ℃. After 1 h of reaction, Sm2Zr2O7 powders were not completely corroded. The reaction products were Sm-apatite and c-ZrO2 solid solution. After 4 h of reaction, all Sm2Zr2O7 powders were completely corroded. After 24 h of reaction, Sm-apatite disappeared, and the c-ZrO2 solid solution remained.
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A series of Sm2Zr2O7-SiC composites doped with different volume fraction and particle size of SiC were prepared by hot pressing at 1300 ℃. The phase of the composites prepared is P-Sm2Zr2O7 and C-SiC, and no other diffraction peaks exist, which indicates that Sm2Zr2O7 has great chemical compatibility with SiC. The thermal conductivity and phonon thermal conductivity of the Sm2Zr2O7-SiC composites are measured by the laser pulse method. The photon thermal conductivity of the composites is obtained by subtracting the phonon thermal conductivity from the total thermal conductivity. The results show that the photon thermal conductivity of Sm2Zr2O7-SiC composites is lower than that of pure Sm2Zr2O7. The photon thermal conductivity of Sm2Zr2O7-SiC composites decreases first and then increases with the increase of SiC particle size. Sm2Zr2O7-(5 vol%, 10 μm)SiC composite has the lowest photon thermal conductivity.