EBC development for hot-pressed Y2O3/Al2O3 doped silicon nitride ceramics

Multilayer EBCs of alcohol and sol-based slurries containing (a) 45% SiO2–34% Y2O3–%Al2O3, (b) mullite/Gd2SiO5 (88/12 wt.%) with and without B2O3 addition, (c) mullite/rare-earth silicates (94/6 wt.%, (Gd2SiO5, Lu2SiO5, Er2SiO5, and HfSiO4)), and (d) only mullite, were applied on hot pressed Y2O3/Al2O3 doped Si3N4 substrates. Of the four major EBC systems studied, only mullite/Gd2SiO5 (88/12 wt.%) EBCs sintered in air at 1400 °C for 3 h showed good bonding with the substrate. In all other EBC systems studied, the Y2O3/Al2O3 additives present in Si3N4 reacted with silica and rare-earth silicates to form low melting point constituents during sintering causing bubble formation and/or de-bonding at the substrate/coating interface. Thermal cyclic resistance of silicon nitride coupons coated with mullite/Gd2SiO5 (88/12 wt.%) EBC was investigated in a moisture containing environment (90% H2O–10% O2) for 100 cycles from 1350 °C to room temperature (1 h hot and 15 min cold). Moisture exposure resulted in accumulation of large pores in the coating, mostly aligned along the substrate/coating interface.