Effects of sulfur impurities and platinum incorporation on the oxidation behavior of aluminide coatings synthesized by chemical vapor deposition

Author

Ying Zhang

Date of Award

8-1998

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Metallurgical Engineering

Major Professor

P. K. Liaw

Committee Members

C. R. Brooks, R. A. Buchanan, D. C. Joy, J. A. Haynes, W. Y. Lee, I. G. Wright

Abstract

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 desulfurized single-crystal Ni-based superalloy via chemical vapor deposition (CVD) process. Similarly, Pt-modified aluminide (Ni,Pt)Al coatings were prepared by aluminizing the superalloy on which a thin layer of platinum (~7 µm) was first electroplated. These low-activity aluminide coatings showed a single P-phase structure in the coating outer layer. The chemical composition and microstructure of the resulting NiAl and (Ni,Pt)Al coatings were characterized, with particular emphasis on sulfur content measurement and refractory element distributions.

Depth profiling by glow-discharge mass spectroscopy (GDMS) was used to assess the coating sulfur level. Sulfur contamination in the initial NiAl coatings resulted in rapid scale spallation. Coating sulfur contents were subsequently reduced by minor modifications of the CVD reactor. With the reduced sulfur level (less than ~0.5 ppmw), AI₂O₃ scale adhesion on the NiAl grain surfaces was significantly improved during cyclic oxidation at 1150°C (up to 200 cycles). However, localized spallation eventually occurred at the coating grain boundaries.

For the (Ni,Pt)Al coatings, relatively high sulfur was found near the coating-substrate interface, which originated from contamination during the Pt electroplating process. However, despite the higher level of sulfur in the (Ni,Pt)Al coatings, a very adherent scale formed over both the coating grain surfaces and grain boundaries during thermal cycling at 1150°C. No weight loss was observed after 500 cycles at 1150°C, and an adherent scale was maintained. These results strongly suggested that Pt incorporation can mitigate the detrimental effect of sulfur impurities on scale adhesion of the aluminide coatings, which has not been previously reported.

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 (such as 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 in the as-deposited and oxidized conditions. However, the presence of Pt appears to drastically reduce void growth along the oxide-metal interface. Spallation and void formation were observed along the NiAl coating grain boundaries after lOOh isothermal oxidation at 1150°C. In contrast, a very adherent AI₂O₃ scale formed on the (Ni,Pt)Al coating surface, and no visible voids were formed after 200h at 1150°C.

Finally, oxidation studies of NiAl and (Ni,Pt)Al coatings on un-desulfurized Ni-Based superalloys indicated that Pt additions effectively counteracted the detrimental effects on scale adhesion of high sulfur levels in the substrate superalloys. Results also suggested that the coating grain size and orientation appear to influence scale adhesion behavior. The effect of coating grain ridges on the scale adhesion was also addressed.

Recommended Citation

Zhang, Ying, "Effects of sulfur impurities and platinum incorporation on the oxidation behavior of aluminide coatings synthesized by chemical vapor deposition. " PhD diss., University of Tennessee, 1998.
https://trace.tennessee.edu/utk_graddiss/9413