Recession behavior of a silicon nitride with multi-layered environmental barrier coating system

A silicon nitride with multi-layered environmental barrier coating (EBC) system was prepared. The multi-layered coating consisted of a thin Lu2Si2O7 bond coat, Lu2Si2O7/mullite eutectic intermediate layer, and a top Lu2Si2O7 layer. A glassy phase was observed on the top Lu2Si2O7 surface of the as-coated sample. The glassy phase was converted into Lu2Si2O7 and Lu2SiO5 phases via a heat treatment at 1500 °C in Ar. The recession test was performed at 1300 °C for 500 h using high-speed steam jet equipment, which was employed to simulate the gas turbine engine conditions. Post-test examinations showed that the multi-layered EBC system well sustained during the high-temperature steam exposure test. Also, the silicon nitride substrate was protected from the oxidation and corrosion, and thus no material recession was observed. However, some cracks that run through the EBC layer was observed in the sample after the test. The substrate near the cracks oxidized, leading to the formation of a thermally grown oxide phase.