Cyclic and Isothermal Oxidation Resistance of ASPS Thermal Barrier Coating Systems

Abstract

To systematically study the oxidation performances of thermal barrier coatings (TBCs) deposited by axial suspension plasma spraying (ASPS) method, isothermal and cyclic oxidation tests were conducted at a peak temperature of 1080C. The TBC systems are based on two different nickel-base superalloy substrates (CMSX-4 or IN738LC), platinum aluminide bond coat and yttria stabilized zirconia (8YSZ) top coat with either vertical cracks (VC) or columnar structure. After the oxidation tests, the samples were analyzed using a scanning electron microscope (SEM) and an X-Ray diffractometer. Samples with IN738LC substrate demonstrated longer isothermal oxidation lives whereas samples with CMSX-4 substrate showed greater cyclic oxidation lives. Outward elemental diffusion (of W and Ta) in TBC system containing CMSX-4 contributed to reduced isothermal oxidation life while the mismatch of coefficients of thermal expansion (CTE) between TGO, bond coat and substrate was believed to be responsible for early failure of TBC system with IN738LC substrate. TBC systems with columnar YSZ overall had longer isothermal oxidation life while vertically cracked YSZ seemed to result in longer cyclic life. Cr2O3 and spinels (CS), in addition to thermally grown aluminum oxide (TGO), were detected on tested TBC samples. The growth of spinels highly depended upon a coating's accessibility to oxygen and test duration and was found to contribute the spallation of the coating layer.iv