Mullite–gadolinium silicate environmental barrier coatings for melt infiltrated SiC/SiC composites

Slurry based mullite/gadolinium silicate (Gd2SiO5) environmental barrier coatings (EBCs) were developed for melt infiltrated (MI) SiC/SiC composites. The coating chemically adhered well on the substrates. Thermal cycling of uncoated MI-SiC/SiC composites conducted between 1350°C and 90°C (one hour hot and 15min cold) in a 96.5% H2O–3.5% O2 environment caused severe oxidation damage after 100 cycles resulting in the formation of dense silica layer of about 25μm maximum thickness. Mullite/Gd2SiO5 EBCs provided excellent protection to MI-SiC/SiC against moisture damage with significantly less oxidation of the substrate; only about a 2μm thick oxide layer formed even after 400 similar thermal cycles. The hair-line cracks formed at the coating/substrate interface after 400 cycles causing partial coating de-lamination. ► Mullite/Gd2SiO5 EBC chemically adhered well on MI-SiC/SiC composites after sintering. ► Severe oxidation damage occurred for un-coated MI-SiC/SiC after 100 thermal cycles. ► EBC protected MI-SiC/SiC had minimal oxidation damage even after 400 thermal cycles.