Corrosion behaviour of rare-earth monosilicates in CMAS exposure

The thermochemical reactions between calcium-magnesium-aluminosilicate (CMAS) and three different rare earth (RE)-based (Yb, Lu and Er) environmental barrier coatings (EBCs) were studied at 1300 degrees C for 1-48 h. The EBCs were applied on a reaction bonded SiC-B4C substrate using dip coating technique. The coating's layout consists of a dense RE2SiO5 bottom coat and a protective RE2O3 topcoat. The microstructure and phase composition of the CMAS-EBC specimens were examined using scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The results showed that CMAS dissolved in RE2O3 to form protective Ca-RE garnet phase in the case of Yb and Lu-based coatings and unprotective apatite in the case of Er-based coating. CMAS melts penetrated the RE2O3 topcoat after 6 h. Yb and Lu-silicate EBCs offer higher resistance to CMAS attack compared to Er-silicate coat. The use of RE2O3 topcoat proves beneficial strategy to limit CMAS corrosion to the RE2SiO5 coating.

Automated summary

The study found that CMAS dissolved in Yb and Lu-based coatings to form protective Ca-RE garnet phase, while in Er-based coating, unprotective apatite was formed. Yb and Lu-silicate EBCs offer higher resistance to CMAS attack, and the use of RE2O3 topcoat proves to be a beneficial strategy to limit CMAS corrosion.