Suspension Plasma Spray Thermal Barrier Coating Systems - Modelling and Performance Characterization Under Isothermal and Cyclic Oxidation Conditions

The oxidation behavior of a vertically cracked thermal barrier coating (TBC) deposited by suspension plasma spraying (SPS) technique was studied in this research under isothermal and cyclic conditions. A peak temperature 1080ºC and the durations of 400, 800, and 1300 hours/ cycles were utilized. Two different Ni-based substrates, INCONEL 625 and HAYNES 233, were first characterized at 1080ºC for the purpose of material selection. INCONEL 625 was selected because of its superior oxidation performance under this condition. The TBC system consisted of an INCONEL 625 substrate, a NiCoCrAl bond coat, and finally a 7YSZ top coat with vertically cracked structure. During cyclic oxidation test, the TBC samples failed after 800 cycles, however, no evidence of failure was observed under isothermal condition for up to 1300 h. After the oxidation tests, microstructure, phase composition, microstrain, and mechanical properties were examined. Based upon the results obtained, samples under cyclic condition assumed faster sintering rate, which in turn shortened the lifetime of SPS TBCs under cyclic conditions more than that under isothermal conditions. Furthermore, stress distributions within the top coat under cyclic condition were calculated using ABAQUS in order to study the crack initiation and propagation under the influences of different TGO thickness, roughness, and mechanical property degradation of the SPS top coat. Moreover, the results of mechanical properties were utilized to predict the crack density distribution within the top ceramic coat following isothermal exposure, with the application of crack numerical density theory. The outcome of this study provides fundamental understating of the sintering mechanism and coating degradation associated with crack initiation and propagation in vertically cracked SPS TBCs under both isothermal and cyclic conditions.