Effects of substrate material and TBC structure on the cyclic oxidation resistance of TBC systems

The effects of different substrates (CMSX-4 and IN738LC) on cyclic oxidation behavior of two thermal barrier coatings (TBCs) were investigated in this study. Cyclic oxidation test was conducted at a peak temperature of 1080 °C with 30 min hold time. The TBC systems tested consisted of two nickel-base superalloy substrates (CMSX-4 and IN738LC), a platinum aluminide bond coat and two types of 8YSZ top coat (vertical cracks (VC) and columnar structure). Samples with CMSX-4 substrate showed greater cyclic oxidation lifetimes than that with IN738LC, and VC YSZ on CMSX-4 offered longer cyclic life than those with columnar structure. After the oxidation test, the samples were analyzed using a scanning electron microscope (SEM) and an X-Ray diffractometer. The failure location of all TBC systems examined was found to be in the vicinity of TGO, consistent with past research of similar coating systems. Alumina and spinels (CS) were detected on all spalled surfaces although the amount of each phase differs from sample to sample. Ta-rich oxide was found on spalled coating surfaces with CMSX-4 substrate while islands of YSZ on the exposed TGO surface of samples with IN738LC substrate. It is believed that alloy composition, TGO rumpling and stresses induced by TGO growth and coefficient of thermal expansion (CTE) mismatch between different layers all contributed to the reduced cycles to failure of TBC systems with IN738LC substrate.