MICROSTRUCTURAL CHARACTERIZATIONS OF PLASMA-SPRAYED ZrO2/Al2O3 THERMAL BARRIER COATINGS AS RELATED TO OXIDATION BEHAVIOR

Thermally grown oxide scales can provide high-temperature oxidation protection if they are slow growing, sound and adherent to the substrate. In thermal barrier coatings (TBCs), failure occurs near or at the interface between the metallic bond coat and its thermally grown alumina layer. Oxide scales consisting mainly of alumina were grown on duplex (ZrO2/bondcoat) and functionally-graded (ZrO2/Al2O3/bondcoat) systems through static heat treatment at 1050°C. Electron microscopy was used to analyze the microstructures of oxide layers grown on various TBC systems. Different thickness of grown oxide scales resulting in the systems will be discussed in the light of the effectiveness of alumina interlayer acting as an additional oxidation barrier. Spallation observed at higher temperature for functionally-graded (FG) system is believed to be due to phase transformation of γ-alumina to α-alumina, and hence inducing additional residual stress causing a premature failure. The effect of incorporating alumina in the FG system, in terms of minimizing the thermal residual stress at the interfaces, was evaluated from the standpoint of Finite Element Modeling.