Mullite–YSZ multilayered environmental barrier coatings tested in cycling conditions under water vapor atmosphere

Plasma sprayed mullite/YSZ graded coatings on SiC substrates were examined as environmental barrier coatings (EBCs). Several coating systems with various layer arrangements were subjected to thermal aging at 1300 °C in cycling (50 and 100 cycles, i.e. 100 and 200 h) conditions under rich water vapor atmosphere. The level of porosity and the crack population were quantified using cross section SEM images of these coatings. The thickness of the thermally grown silica (TGS) layer developed was estimated and the presence of cristobalite in its structure was evidenced by micro-Raman spectroscopy. Hardness and elastic modulus were also determined by the use of instrumented indentation for each layer in the different coatings. The evolution of all these parameters confirmed that EBC architectures formed by compliant porous layers resulted more effective in reducing cracks and the effects of aging than EBCs with dense graded compositions.

► Thermal cycling aging produced crack density reduction by coalescence of cracks.
► Thermal cycling aging did not produce porosity changes.
► Silica scale is formed by amorphous phase and cristobalite.
► Void formation is detected close to the thermally grown silica scale.
► The best coating is formed by a three-layered Si-bi-modal mullite and nanostructured ZrO2 architecture.