Nickel aluminides coatings

Fig. 10. Schematic diagram showing the as-deposited structure of a low activity (left) and a high activity (right) diffusion aluminide coating on a nickel-base superalloy. The high activity coating would receive a heat treatment to convert the Ni2Al3 to NiAl.

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

Since many years the intermetallic phases based on aluminides have been an important topic for research and development, because of their high melting points, low densities and excelent corrosion resistance at high temperatures. Especially, nickel aluminides have been of great interest as coating materials for several high temperature applications. 

Figure 10.6 Optical micrograph showing the cross-section of an as-processed modified aluminide coating on a nickel-base superaUoy. This particular coating was fabricated under conditions which produced a second phase, PtAl2 (white contrast), in a matrix of S-NiAl.

Figure 10.16 Optical micrograph showing the cross-section of a Pt-modified aluminide coating on a nickel-base single-crystal superalloy after oxidation at 1200 °C for 20 h. The original grain structure of the /3-phase is evident and y has begun to nucleate at /6 grain boundaries as a consequence of A1 depletion.

Rare earths incorporated into aluminide coatings. The enhanced oxidation resistance and scale adhesion imparted to AI2O3 forming alloys has stimulated a quest for novel approaches for incorporating rare-earth elements into nickel-aluminide (p-NiAl) coatings on superalloy turbine-blade materials. 

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. 

Voudouris N, Christoglou C, Angelopoulos G N, Formation of aluminide coatings on nickel by a fluidized bed CVD process . Surf Coat TechnoL, 2001 141 275-282... 

Calorised Coatings The nickel- and cobalt-base superalloys of gas turbine blades, which operate at high temperatures, have been protected by coatings produced by cementation. Without such protection, the presence of sulphur and vanadium from the fuel and chloride from flying over the sea promotes conditions that remove the protective oxides from these superalloys. Pack cementation with powdered aluminium produces nickel or cobalt aluminides on the surfaces of the blade aerofoils. The need for overlay coatings containing yttrium have been necessary in recent times to deal with more aggressive hot corrosion conditions. 

Aluminides based on the intermetallic phases Ni3Al and Fe3Al are considered both as structural materials and as coatings for high temperature applications [1-6]. Their excellent corrosion resistance is due to their forming a dense, protective alumina scale. Alumina, especially ot-Al203, shows low rate constants even at temperatures above 1000°C [7]. Unlike chromia, which is formed on conventional stainless steels and nickel base alloys, alumina does not evaporate above 1000°C [8] and it is even stable in oxygen deficient atmospheres. 

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