Correlation of processing technique and microstructure of platinum aluminide bond coats with performance of thermal barrier coatings deposited on nickel base superalloy

We show that the performance of thermal barrier coating systems is critically dependent upon the processing technique and microstructure of platinum aluminides utilised as bond coats. It is demonstrated by thermal exposure tests at 1150 degrees C in air with 24 h cycling period to room temperature that the average useful life of a coating system employing zirconia-7 wt-% yttria as top coat and alloy MAR M002DS as substrate is increased from 192 to 480 h by replacing a three-layer bond coat aluminised by conventional pack cementation with a two-layer bond coat aluminised by chemical vapour deposition. Before each aluminising process, the superalloy has been electroplated with a platinum layer about 7 mu m in thickness. Microstructural characterisation using scanning electron microscopy combined with energy dispersive X-ray spectroscopy, electron-probe microanalysis, transmission electron microscopy and X-ray diffraction indicates that the superior performance provided by the two-layer bond coat is related to its higher thermal stability enhancing the adhesion of the thermally grown oxide. However, both coating systems are found to fail by the same mechanism involving loss of adhesion between the thermally grown oxide and bond coat.