Oxidation behavior and interface evolution of tri-layer Si/Yb2SiO5/LaMgAl11O19 thermal and environmental barrier coatings under isothermal heat treatment at 1300 °C

A tri-layer Si/Yb2SiO5/LaMgAl11O19 thermal and environmental barrier coatings (TEBCs) was prepared for protecting SiCf/SiC substrate by using atmospheric plasma spraying technology (APS). While the introduction of the thermal barrier coatings (TBCs) improves the temperature capability of the environmental barrier coatings (EBCs), it generates vertical crack in the as-deposited TEBCs due to the mismatch of coefficient of thermal expansion (CTE) between Yb2SiO5 EBCs and LaMgAl11O19 TBCs. Isothermal oxidation tests of coated samples were performed at 1300 °C in air for 50, 100, 200, and 300 h. The oxidation behavior and interface evolution of TEBCs were investigated. After 50 h of isothermal oxidation, the crack healing was observed in the coating due to the crystalline of amorphous phase and grain growth. However, the width of vertical crack increased with the oxidation times due to the increasing of residual stresses, which provide rapid diffusion channel for oxygen into surface of Si, resulting in the formation of SiO2 layer. Eventually, the mechanical spallation of coating was observed in the surface of bond coat due to the thermal mismatch and the growth of SiO2 layer. After isothermal oxidation, the Yb3Al5O12 reaction layer was formed in the interface between Yb2SiO5 and LaMgAl11O19 layers due to the inter-diffusion of atoms. Furthermore, the growth of thermally grown oxide (TGO) and Yb2Si2O7 layer at the interface between Si and Yb2SiO5 layers followed approximately the parabolic law.