Diffusion behaviour and corrosion rate of rare earth monosilicate-based EBCs under CMAS exposure

Abstract

The thermochemical corrosion behaviour of different rare earth (RE)-based (Yb, Lu and Er) environmental barrier coatings (EBCs) between 1 and 48 h in low calcium–magnesium–aluminosilicate (CMAS) load environments at 1300 °C was studied. The EBCs were coated on reaction-bonded SiC–B4C substrates using the dip-coating method. Each coating is composed of a dense RE2SiO5-based bottom coat and a porous RE2O3 topcoat. The microstructures and compositions of the corroded samples were examined using scanning electron microscopy and energy dispersive spectroscopy. CMAS infiltrated the topcoat through pores and cracks, settled in these defects, and dissolved in the neighbouring coating areas to precipitate as a Ca–Mg–Si–RE garnet. The diffusion coefficients of Ca and Si in the CMAS–EBC corroded specimens were calculated. Yb and Lu silicates have the lowest diffusion coefficient values, whereas Er silicate has the highest values. The formed garnet filled the pores and inhibited further diffusion of corrosive species in the coat. The corrosion rates were calculated based on the physical properties of the coat and found to be lower than those of coatings produced using atmospheric plasma spraying. The diffusion coefficient and corrosion rate were successfully used to evaluate various corrosion-resistant coatings. The results show that single-layer dip-coated Yb silicate is a cost-effective CMAS-resistant RE-based EBC.