Thermal conductivity, phase stability, and oxidation resistance of Y_3Al_5O_(12) (YAG)/Y_2O_3-ZrO_2 (YSZ) thermal-barrier coatings

This paper describes a new multilayer TBC that incorporates a 10 μm-thick oxygen barrier layer of yttrium–aluminum garnet (YAG) into a typical YSZ TBC system. The thermal conductivity of as-sprayed YAG/YSZ coatings was reduced due to excessive porosity and amorphous areas in the YAG layer. After long-term heat treatments, the conductivity of the multilayer was unaffected by the presence of YAG. Sintering and recrystallization of the amorphous YAG regions occurred during high-temperature heat treatments. While YAG itself possesses excellent phase stability, its presence also improved the phase stability of zirconia near the YAG/YSZ interface, inhibiting the outward diffusion of yttrium from high-yttria t-ZrO_2 The YAG layer reduced the NiCoCrAlY bond-coat oxidation rate by a factor of three during isothermalfurnace tests conducted at 1200° C.