Effects of platinum additions on the adherence of alumina scales to CVD aluminide bond coatings

It is well known that Al₂O₃ scale adherence to aluminide bond coats is enhanced by Pt alloying additions, however, the actual mechanisms of the Pt effect are not defined. Both NiAl and NiPtAl bond coats were fabricated on two types of single-crystal superalloys (de-sulfurized and nominal S content) by a high-purity chemical vapor deposition (CVD) process. Oxide scale adherence to CVD NiAl during thermal cycle testing to 1150°C was dramatically improved by reducing both superalloy and coating S impurities. Platinum additions to CVD aluminides resulted in significant further improvements in scale adherence, especially on the higher-S superalloy. The most obvious manifestation of the Pt effect was elimination of large voids at the oxide-metal interface. Transmission electron microscopy of α-Al₂O₃ scales after isothermal exposure indicated internal void growth on both NiAl and NiPtAl, but scales on NiPtAl also contained Hf from the superalloy. These results further support previous observations that Pt mitigates the detrimental effect of S impurities by modifying the driving force to stabilize or accelerate void growth, although this may not be the only beneficial effect.