FAILURE-MECHANISM OF A THERMAL BARRIER COATING SYSTEM ON A NICKEL-BASE SUPERALLOY

An investigation was carried out to determine the failure mechanism of a thermal barrier coating system on an Ni-base superalloy. The coatin g system consisted of an outer layer of yttria-stabilized zirconia (to p coat), and an inner layer of Pt-aluminide (bond coat). Specimens wer e exposed at 1010 and 1150 degrees C with a 24-h cycling period to roo m temperature. Scanning electron microscopy combined with energy dispe rsive X-ray spectroscopy as well as X-ray diffraction were used in mic rostructural characterization. Spallation of the oxide scale developed by the bond coat was found to be the mode of failure. Experimental re sults indicated that the breakdown of oxide was affected by internal o xidation of Hf diffusing from the alloy substrate into the bond coat s urface 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 it s oxidation rate and permitting outward diffusional transport of eleme nts from the sutstrate. (C) 1998 Chapman & Hall.