Low thermal conductivity in high-entropy rare-earth pyrosilicate solid-solutions for thermal environmental barrier coatings

Dense thermal environmental barrier coatings (TEBCs) with low thermal conductivities are needed for the protection and insulation of SiC-based ceramic matrix composites (CMCs) in future gas-turbine engines. Here we demonstrate experimentally that the high-temperature thermal conductivity of dense beta-Yb2Si2O7 pyrosilicate ceramic can be reduced progressively in beta-(Y0.1Yb0.9)(2)Si2O7 and beta-(Y0.5Yb0.5)(2)Si2O7 solid-solutions for TEBC application. We also demonstrate that by going to a dense five-component equi-atomic single-phase beta-(Y0.2Yb0.2Sc0.2Gd0.2Lu0.2)(2)Si2O7 pyrosilicate high-entropy ceramic (HEC), the thermal conductivity can be reduced further to 1.52 W.m.K-1 (at 10 00 degrees C). This is attributed to enhanced phonon scattering due to the increased mass-disorder with the incorporation of the additional RE3+ cations in the HEC. Although the high-entropy aspect and bonding-disorder do not appear to contribute significantly to the phase stability or the low thermal conductivity in such HECs, they are strong candidates for future TEBCs for CMCs. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.