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

Abstract

The influences of sulfur impurities and Pt incorporation on the scale adhesion behavior of aluminide coatings were studied and compared. Low-sulfur NiAl coatings were prepared on a desulfurized, yttrium-free, single-crystal Ni-based superalloy by a modified version of a conventional aluminizing procedure based on chemical vapor deposition. The sulfur level in the resulting NiAl coatings was measured to be less than ϳ0.5 ppmw by glow-discharge mass spectroscopy. Platinum-modified aluminide coatings were synthesized by first electroplating a thin layer of Pt (ϳ7 m) on the superalloy, followed by the same low-sulfur aluminizing procedure. The measured 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 electroplating process. A very adherent ␣-Al 2 O 3 scale formed on the grain surfaces of the low-sulfur NiAl coating during cyclic oxidation testing at 1150 ЊC, but scale spallation eventually occurred over many of the NiAl grain boundaries. In contrast, despite the higher level of sulfur in the (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.

Compositional Effects on Aluminide Oxidation Performance: Objectives for Improved Bond Coats

“…These results Current aluminide bond coats often contain Pt, which is now commonly added in conjunction with a low activity CVD aluminization process which results in a single phase β-(Ni,Pt)Al coating. [9] The role of Pt in oxidation has not been fully explained but it is clear from recent results on both cast aluminides [10] and CVD aluminide coatings [2] that Pt improves alumina scale adhesion. This beneficial effect is shown at 1100°C for cast aluminides, Figure 2.…”

“…[13] The comparison of simple and Pt-modified CVD a l u m i n i d e coatings on René N5 also demonstrated that Pt did not (1) reduce the alumina scale growth rate, (2) alter the diffusion of heavy elements into the coating during deposition or oxidation, or (3) alter the coating Al content before or after oxidation. [2,13].…”

“…Protrusions have been observed previously for both cast Pt-Al alloys [15] and for CVD Pt aluminide coatings. [2] Each protrusion was centered on a single alumina grain and extended into it. This is seen clearly in a STEM annular dark field image, Figure 6.…”

“…Many of the beneficial effects often attributed to Pt, such as reducing heavy element diffusion into the coating or increasing the Al content of the coating, have not been confirmed experimentally. [2,13] One issue that requires further study is the interaction between Pt and S. Understanding this mechanism may provide useful insight into the role of S in high temperature oxidation. [33,34] The incorporation of Hf into aluminide coatings offers significant improvements in performance and could preclude the use of Pt.…”

“…This low-sulfur coating process is described in detail elsewhere. [1,2] Isothermal exposures were performed in dry, flowing O 2 and mass gain was measured using a Cahn Instruments model 1000 microbalance. Cyclic testing was performed in two ways.…”