Cyclic and Isothermal Oxidation Resistance of ASPS Thermal Barrier Coating Systems

Isothermal and cyclic oxidation behaviours of two types of thermal barrier coatings (TBCs) were investigated in this study. Both isothermal and cyclic oxidations tests were conducted at a peak temperature of 1080C. The TBC systems tested consisted of nickel-base superalloy substrate (CMSX-4 or IN738LC), platinum aluminide bond coat and 8YSZ top coat with either vertical cracks (VC) or columnar structure. Samples with IN738LC substrate demonstrated longer isothermal oxidation life whereas samples with CMSX-4 substrate showed greater cyclic oxidation life. After the oxidation tests, the samples were analyzed using a scanning electron microscope (SEM) and an X-Ray diffractometer. Cr2O3 and spinels (CS) were detected within some of the TBC samples. The growth of spinels highly depended upon coating’s accessibility to oxygen and test duration. Severe outward elemental diffusion and coefficient of thermal expansion (CTE) mismatch between TGO, bond coat and substrate were responsible for early failure of TBCs.