An investigation was carried out to determine the failure mechanism of a thermal barrier coating system on an Ni-base superalloy. The coating system consisted of an outer layer of yttria-stabilized zirconia (top coat), and an inner layer of Pt-aluminide (bond coat). Specimens were exposed at 1010 and 1150 °C with a 24-h cycling period to room temperature. Scanning electron microscopy combined with energy dispersive X-ray spectroscopy as well as X-ray diffraction were used in microstructural characterization. Spallation of the oxide scale developed by the bond coat was found to be the mode of failure. Experimental results indicated that the breakdown of oxide was affected by internal oxidation of Hf diffusing from the alloy substrate into the bond coat surface developing localized high levels of stress concentration at the oxide–bond coat interface. It was concluded that the cause of failure was degradation of thermal stability of the bond coat accelerating its oxidation rate and permitting outward diffusional transport of elements from the substrate.

It is the objective of this paper to compare the thermal stability characteristics and isothermal oxidation behavior of an aluminide coating and a Pt-aluminide coating of the same Al content on a Ni-base superalloy. Addition of Pt to an aluminide coating was found to improve its thermal stability as well as its capability for selective oxidation of Al resulting in a purer scale of slower growth rate. This was correlated with the greater diffusional stability of the Pt-aluminide coating restricting the transport of substrate elements into the outer coating layers.