Platinum-aluminide coatings

Diffusion Zone f".g.ll. Schematic diagram of a platinum aluminide coating on a nickel-base superalloy. 

Pack cementation is the most widely used process for making diffusion aluminide coatings. Diffusion coatings are primarily aluminide coatings composed of aluminum and the base metal. A nickel-based superalloy forms a nickel-aluminide, which is a chemical compound with the formula NiAl. A cobalt-based superalloy forms a cobalt-aluminide, which is a chemical compoimd with the formula CoAl. It is common to incorporate platinum into the coating to improve the corrosion and oxidation resistance. This is called a platinum-aluminide coating. Diffusion chrome coatings are also available. 

For a platinum-aluminide coating, a thin (typically 8-p,m) layer of platinum is first deposited onto the substrate, usually by a plating process. The second step involves aluminizing for several hours using the conventional packed cementation process to form the platinum-aluminide coating. 

Turbine blades of jet engines are coated with a protective layer of platinum aluminide to impart high temperature corrosion resistance. Platinum is electroplated onto the blade using P-salt or Q-salt electroplating solutions (28,29). The platinum is then diffusion-treated with aluminum vapor to form platinum aluminide. Standards for the inspection and maintenance of turbine blades have become more stringent. Blades are therefore being recoated several times during their lifetime. 

G. Fisher, W.Y. Chan, P.K. Datta, J.S. BumeU-Gray, Noble metal aluminide coatings for gas turbines, Platinum Met. Rev. 43 (1999) 59-61. 

Platinum improves scale adhesion at 1000-1200 C in cast NiAl alloys (e.g. Pint et al., 1998a see also Fig. 6-11) and is used in commercial Pt-Aluminide coatings (Leh-nert and Meinhardt 1972 Smith and Boone, 1990 Warnes and Punola, 1997). Interestingly, at 950 °C a Pt layer approximately 50 nm thick deposited on single-crystal NiAl did not improve scale adhesion and, in fact, inhibited the 0 to a phase transformation (Roux et al, 1993). 

J. A. Haynes, Y. Zhang, W. Y. Lee, B. A. Pint, I. G. Wright, and K. M., Cooley, Effects of platinum additions and sulfur impurities on the microstructure and scale adhesion behavior of single-phase CVD aluminide bond coatings. In Elevated Temperature Coatings, eds. J. M. Hampikian and N. B. Dahotre, Warrendale, PA, TMS, 1999, p. 51. 

Both aqueous and fused-salt electrol3rtes have been used for plating the platinum group elements. Platinum has been used as a diffusion-barrier layer in aluminiding nickel-base alloys and MCrAlY coatings. Platinum... 

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