A study of the microstructure and oxidation behavior of alumina/yttria-stabilized zirconia (Al2O3/YSZ) thermal barrier coatings

The microstructure and isothermal oxidation behavior of Al2O3/YSZ and conventional YSZ TBCs were investigated. From the microstructure analysis, it was confirmed that a relatively dense (compared to as-sprayed YSZ) Al2O3/YSZ ceramic layer (~ 4 μm thick) was formed/developed on the surface of YSZ top-coat. Results showed that the formation of Al2O3 was helpful to partially seal the surface cracks in YSZ coating. However, the formation of Al2O3/YSZ composite layer did not change the porosity (~ 12%) of the whole YSZ coating. After 200 h isothermal oxidation, the thickness of TGO in YSZ sample was larger than that of TGO thickness in Al2O3/YSZ sample. This was due to the fact that the relatively dense Al2O3/YSZ ceramic layer reduced the void network in top-coat and decreased the ionic conductivity of YSZ coating. It was also found that the existence of Al2O3 in YSZ is beneficial to stabilize the tetragonal prime phase (t′-phase) during oxidation. During cyclic oxidation at 1000 °C for 240 h, the weight gain per unit area of Al2O3/YSZ TBCs was smaller than that of as-sprayed YSZ TBCs.