Effects of Pt incorporation on the isothermal oxidation behavior of chemical vapor deposition aluminide coatings

The effects of Pt incorporation on the isothermal oxidation and diffusion b ehavior of low-sulfur aluminide bond coatings were investigated. Aluminide (NiAl) coatings and Pt-modified aluminide (Ni,Pt)Al coatings were synthesiz ed on a low-sulfur, yttrium-free single-crystal Ni-based superalloy by a hi gh-purity, low-activity chemical vapor deposition (CVD) aluminizing procedu re. The isothermal oxidation kinetics and scale adhesion behavior of CVD Ni Al and (Ni,Pt)Al were compared at 1150 degreesC. Compositional profiles of alloying elements in the NiAl and (Ni,Pt)Al coatings before and after isoth ermal oxidation were determined by electron microprobe analysis. Platinum d id not reduce oxide scale growth kinetics. No significant differences in bu lk refractory metal (W, Ta, Re, and Mo) distributions were observed as a re sult of Pt incorporation. Spallation of the alumina scale and the formation of large voids along the oxide-metal interface were commonly observed over the NiAl coating grain boundaries after 100 hours at 1150 degreesC. In con trast, no spallation of Al2O3 scales occurred on (Ni,Pt)Al coating surfaces or grain boundaries, although the sulfur content in the CVD (Ni,Pt)Al coat ings was higher than that of the CVD NiAl coatings. Most significantly, no voids were observed at the oxide-metal interface on (Ni,Pt)Al coating surfa ces or cross sections after 200 hours at 1150 degreesC. It was concluded th at a major beneficial effect of Pt incorporation on an aluminide coatings o xidation resistance is the elimination of void growth at the oxide-metal in terface, likely by mitigation of detrimental sulfur effects.