Synthesis and thermal conductivity of ytterbium silicate doped  with Sm and Gd for environmental barrier coatings application

Abstract

SiC/SiC ceramic matrix composite parts have begun to be used in the hot section of gas turbine engines. It is essential to prevent atmospheric corrosion in these parts. Therefore, the development of coating materials with superior properties is of particular scientific interest. The thermal conductivity of coating ceramics is also essential for protecting parts in adverse high-temperature service conditions. In this study, ytterbium silicate-based (YbSi) ceramics used as the top layer in environmental barrier coating were produced by traditional powder metallurgy methods. In addition, they were doped with the rare earth elements Sm and Gd to improve some of their properties. The ceramic samples were structurally characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The thermal conductivity of rare earth elements–silicate samples was measured by the laser flash method. The characterization of samples allowed determining two phases in the rare earth elements–silicate pellets: mono- and disilicate. As a result of Gd doping, the percentage of the disilicate phase increased in the structure. Doping with Sm reduced the amount of this phase. The thermal diffusivity and heat capacity of YbSi ceramic were decreased by doping with Sm and Gd rare earth elements. Consequently, thermal conductivity was significantly reduced by doping Gd and Sm to YbSi ceramics. The thermal conductivity of undoped YbSi ceramics was calculated as 1.98 W(m × K)^–1. However, the thermal conductivity values of YbSi ceramics doped with Gd and Sm amounted to 1.38 and 1.01 W(m × K)^–1, respectively. As a result of Sm doping, the thermal conductivity of YbSi ceramic was reduced by 50%. Thus, RE-doped YbSi ceramics can be a promising candidate for environmental barrier coating applications.