Synthesis and cyclic oxidation behavior of a (Ni, Pt) Al coating on a desulfurized Ni-base superalloy

The influences of sulfur impurities and Pt incorporation on the scale adhes ion behavior of aluminide coatings were studied and compared. Low-sulfur Ni Al coatings were prepared on a desulfurized, yttrium-free, single-crystal N i-based superalloy by a modified version of a conventional aluminizing proc edure based on chemical vapor deposition. The sulfur level in the resulting NiAl coatings was measured to be less than similar to 0.5 ppmw by glow-dis charge mass spectroscopy. Platinum-modified aluminide coatings were synthes ized by first electroplating a thin layer of Pt (similar to 7 mu m) on the superalloy, followed by the same low-sulfur aluminizing procedure. The meas ured sulfur content in the (Ni, Pt)Al coating was substantially higher than that of the low-sulfur NiAl coating due to contamination during the Pt ele ctroplating process. A very adherent alpha-Al2O3 scale formed on the grain surfaces of the low-sulfur NiAl coating during cyclic oxidation testing at 1150 degrees C, but scale spallation eventually occurred over many of the N iAl grain boundaries. In contrast, despite the higher level of sulfur in th e (Ni Pt)Al coating, a very adherent scale was formed over both the coating grain surfaces and grain boundaries during thermal cycling. These results suggest that Pt additions can mitigate the detrimental influence of sulfur on scale adhesion.