Abstract
The effects of superalloy composition and Pt content on the high temperature oxidation behavior of -’ NiPtAl diffusion coatings were investigated over the temperature range 1050 – 1150oC. Simple NiPtAl diffusion coatings with 7 or 12 m electroplated Pt thickness were evaluated in 1h cycles in dry O2 for up to 2,500 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 4%Ti alloy 1483 were protective at 1050oC, but exhibited severe scale spallation at 1100oC, with extensive formation of Ti- and Ni-rich oxides at the gas interface. Coatings with 7µm Pt on X4 were extremely protective at 1100oC, but failed rapidly at 1150oC, which also was associated with the formation of Ti-rich oxides. Increasing the coating Pt content on X4 improved the 1150oC oxidation behavior. Coatings on Ti-free N5 showed the best performance at 1150oC, especially with 12µm Pt. Although -’ 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.
