Effect of superalloy substrate composition on the performance of a thermal barrier coating system

An investigation was carried out to determine the performance of a thermal barrier coating system consisting of (ZrO₂-8% Y₂O ₃)/(Pt) on two single-crystal Ni-base superalloys. Coating/alloy behavior was studied with reference to: (i) initial microstructural features, (ii) oxidation properties, (iii) thermal stability characteristics, and (iv) failure mechanism. All thermal exposure tests were carried out at 1150°C in still air with a 24-h cycling period to room temperature. Failure of the coating system was indicated by macroscopic spallation of the ceramic top coat. Scanning electron microscopy combined with energy dispersive X-ray spectroscopy as well as X-ray diffraction were used to characterize the microstructure. Decohesion between the thermally grown oxide and bond coat was found to be the mode of failure of the coating system for both alloys. This was correlated with the formation of Ti-rich and/or Ti+Ta-rich oxide particles near the oxide-bond coat interface degrading the adherence of the thermally grown oxide. However, the thickening rate of the oxide had very little or no effect on the relative coating performance. It was concluded that the coating performance is critically dependent on alloy substrate composition particularly the concentration of elements, which could have adverse effects on oxidation resistance such as Ti.