Thermochemical Degradation of Rare Earth-Based Environmental Barrier Coatings

Silicon carbide ceramic matrix composites (SiC-CMCs) and refractory metal alloys (RMAs) are candidate material replacements for nickel-base superalloy gas turbine components since they can achieve higher working temperatures and improve engine efficiency. However, both material classes are susceptible to environmental degradation, requiring both thermal and environmental barrier coating systems.

Rare earth silicates, RE2Si2O7 and RE2SiO5, are candidate EBC systems for SiC-CMCs. The steam and siliceous deposit resistance of these materials are well studied. However, Na2SO4 -induced hot corrosion resistance is relatively unknown. Examination of RE2Si2O7 and RE (RE = Y, Yb) coating systems exposed to 2.5 mg/cm2 of Na2SO4 at temperatures from 825°C to 1316°C in a 0.1% SO2-O2 (g) environment was conducted. Characterization from scanning electron microscopy (SEM), X-ray diffraction (XRD), and inductively coupled plasma-optical emission spectrometry (ICP-OES) discovered water-soluble RE and Na-RE sulfate products formed in the corrosion layer causing significant damage to the sample surface for the 825 °C exposures. However, at higher temperatures, the reactivity is seen for Na2O from Na2SO4 and SiO2 in the RE-silicate phases reactive forming various Na-Silicate products.

High entropy rare earth oxides (HERO) are candidates for thermal and environmental T/EBC coatings on Nb-based RMAs. Rare earth oxides (RE2O3) are valued for their stability in high-temperature turbine environments. By utilizing the high entropy approach, altering the characteristics and properties of the coating can be achieved. An example of an examined thermochemical property in this work is for HERO compositions (Y0.33Yb0.33Er0.33)2O3 and (Y0.2Yb0.2Er0.2Ho0.2Nd0.2)2O3 demonstrating calcium-magnesium aluminosilicates (CMAS) infiltration resistance. CMAS exposures conducted at 1500°C in air showed differences in reaction product characteristics because of the RE cations available in each HERO composition. Further demonstrating the ability to tailor EBC properties with high entropy mixing of RE2O3 compositions.