Thermochemical compatibility of ytterbia–(hafnia/silica) multilayers for environmental barrier coatings

Environmental barrier coating (EBC) systems consisting of multiple layers tailored to address individual protection needs may offer improved performance relative to conventional architectures. If the requirements of thermochemical and thermomechanical compatibility are met, the deposition of a segmented thermal barrier coating on a dense rare earth silicate EBC could provide additional thermal protection and resistance to attack by molten deposits. The thermochemical compatibility between silicates in the YbO1.5–SiO2 system and phases in the YbO1.5–HfO2 system was investigated by equilibrating powder compacts of selected ternary compositions; diffusion couples were used to simulate interactions at the layer interfaces in the proposed architectures. The deduced 1500 °C ternary isothermal section reveals that the ordered δ-Yb4Hf3O12 and H3–Yb6HfO11 phases are only compatible with ytterbium monosilicate (Yb2SiO5) EBC. Implementation of these hafnates in contact with ytterbium disilicate (Yb2Si2O7) leads to interfacial reactions that facilitate layer debonding. The results provide criteria to guide the design of future thermal/environmental barrier coating architectures.