Influences of Superalloy Composition and Pt Content on the Oxidation Behavior of Gamma-Gamma Prime NiPtAl Bond Coatings

The effects of superalloy composition and Pt content on the high-temperature oxidation behavior of gamma-gamma' NiPtAl diffusion coatings were investigated over the temperature range of 1050-1150 A degrees C. Simple NiPtAl diffusion coatings with 7 or 12 A mu m electroplated Pt thickness were evaluated in 1-h cycles in dry O-2 for up to 2500 cycles on four superalloys: directionally solidified (DS) alloy 142, 1st generation single-crystal (SX) alloy 1483, and 2nd generation SX alloys X4 and N5. Coatings on high-Hf alloy 142 experienced severe internal oxidation of Hf at all temperatures. Coatings on similar to 5 at.% Ti alloy 1483 were protective at 1050 A degrees C, but exhibited severe scale spallation at 1100 A degrees C, with extensive formation of Ti- and Ni-rich oxides at the gas interface. Coatings with 7-A mu m Pt on X4 were extremely protective at 1100 A degrees C, but failed rapidly at 1150 A degrees C, which also was associated with the formation of Ti-rich oxides. Increasing the coating Pt content on X4 improved the 1150 A degrees C oxidation behavior. Coatings on Ti-free N5 showed the best performance at 1150 A degrees C, especially with 12-A mu m Pt. Although gamma-gamma' coatings can exhibit outstanding cyclic oxidation resistance with minimal Al depletion, they appear to be sensitive to substrate composition, as well as eventual Pt depletion due to interdiffusion.