Slurry based multilayer environmental barrier coatings for silicon carbide and silicon nitride ceramics — I. Processing

Multilayer mullite/gadolinium silicate (Gd2SiO5) environmental barrier coatings (EBCs) were deposited on α-SiC (Hexaloy) and Si3N4 (SN282) substrates through cost-effective slurry based dip-coat processing. Coatings applied by two approaches, alcohol and sol-based slurries, were examined and optimized in terms of their recipes and air sintering temperatures. A significant increase in densification rates was found for the sol-based EBCs applied on both SiC and SN282 substrates due to the fine mullite particles formed during reaction sintering of well-mixed silica and alumina sols. Mechanical alloying of the starting powder mixtures instead of their simple rotary-blending was found to be beneficial in terms of enhanced coat sintering kinetics. Dense thick coatings that were well-bonded to the substrate were obtained.