| The effects of sulfur impurities and platinum incorporation on the oxidation behavior of aluminide coatings were studied. Low-sulfur aluminide coatings were synthesized by aluminizing a desulfinized single-crystal Ni-based superalloy via low-activity chemical vapor deposition (CVD). (Ni,Pt)Al coatings were prepared by aluminizing the superalloy on which a 7- m m Pt layer was first electroplated. Glow-discharge mass spectroscopy (GDMS) was used to assess the coating sulfur level. With the reduced sulfur level (less than ∼ 0.5 ppmw) in the NiAl coating by minor modifications of the CVD reactor, Al2O3 scale adhesion on the NiAl grain surfaces was significantly improved during cyclic oxidation at 1150°C. However, localized spallation preferentially occurred along the coating grain boundaries. For the (Ni,Pt)Al coatings, relatively high sulfur was found near the coating-substrate interface due to contamination during Pt electroplating process. Despite the higher sulfur level, a very adherent scale formed over both the (Ni,Pt)Al coating grain surfaces and grain boundaries during thermal cycling at 1150°C, and no weight loss was observed after 500 cycles. These results strongly suggested that Pt incorporation can mitigate the detrimental effect of sulfur impurities on scale adhesion of the aluminide coatings. The effects of Pt incorporation on the diffusion behavior of alloying elements in aluminide coatings during isothermal oxidation at 1150°C were studied by electron microprobe analysis. No significant difference in refractory element (Ta, W, Re, and Mo) distributions was observed in the (Ni,Pt)Al and NiAl coatings. Segregation of refractory elements to coating grain boundaries was observed for both NiAl and (Ni,Pt)Al coatings. However, the presence of Pt drastically reduced void growth at the oxide-metal interface. Spallation and voids were observed along the NiAl coating grain boundaries after 100h oxidation at 1150°C. In contrast, a very adherent Al2O3 scale with no visible voids were formed on the (Ni,Pt)Al coating after 200h at 1150°C. Oxidation studies of NiAl and (Ni,Pt)Al coatings on un-desulfinized Ni-based superalloys indicated that Pt additions effectively counteracted the detrimental effects on scale adhesion of high sulfur levels in the substrate superalloys. The effects of coating grain boundary ridges, coating grain size and orientation on scale adhesion were also addressed. |
