In modern applications involving very high temperatures or severe hot corrosion problems, aluminide coatings provide relatively limited protection. They are nevertheless still widely used in less demanding applications.

Consitution

Diffusion aluminide coatings are based on the intermetallic compound β-NiAl (see phase diagram). Although different processes exist to form them, pack cementation is the most widely used as it is inexpensive and well adapted to coating of small parts.

The Al-Ni phase diagram.

Pack cementation falls in the category of chemical vapour deposition. In this process, the components to be coated are immersed in a powder mixture containing Al₂O₃ and aluminium particles. About 1-2 wt% of ammonium halide activators are added to this `pack'. This is then heated to temperatures around 800-1000 ^oC in argon or H₂ atmosphere. At these temperatures, aluminium halides form which diffuse through the pack and react on the substrate to deposit Al metal.

The activity of Al maintained at the surface of the substrate defines two categories of deposition methods: low and high activity, also referred to as outward and inward diffusion respectively.

In cements with low aluminium contents (low activity/outward), the formation of the coating occurs mainly by Ni diffusion, and results in the direct formation of a nickel rich NiAl layer. The process requires high temperature (1000-1100 ^oC). In service, the interdiffusion with the substrate is very limited and the gradient of Al in β is low.

In cements with high aluminium contents (high activity/inward), the coating forms mainly by inward diffusion of aluminium and results in formation of Ni₂Al₃ and possibly β-NiAl. Aluminizing temperatures can be lower (700-950 ^oC). There can be a high Al concentration gradient in the coating, and also significant interdiffusion with the substrate during service. For these reasons, a diffusion heat-treatment is generally given at 1050-1100 ^oC to obtain a fully β layer.

Microstructures of two types of aluminides coating on superalloy, left: high activity/inward diffusion, right: low activity/outward diffusion, from M. Eskner, PhD thesis, Royal Insitute of Technology, Stockholm, available here.

The structure and composition of the coating depends on the substrate, implying that coatings must be tailored for a given alloy. Aluminide coatings lack ductility below 750 ^oC. One of the major problems faced by aluminide coatings is thermomechanical fatigue, as cyclic strains induced by temperature gradients in the blades can lead to thermal fatigue cracks.

Influence of the substrate

The substrate composition strongly influences the final microstructure of the system, in a manner which also depends on the process.

In low activity/outward diffusion coatings, the alloying elements present in the substrate will also tend to diffuse into the coating layer, to an extend limited by their solubility. In high activity/inward diffusion coatings, they enter in solution the compound layer in formation, or as precipitates potentially forming during the treatment.

Of the elements present in the alloy, titanium is thought to be particularly detrimental to the oxidation resistance of such coatings, as it leads to the formation of TiO₂ crystals which break the alumina layer.

An typical microstructure of low activity aluminide coating is illustrated below. The external zone is typically Al rich β-NiAl, while the internal one is Ni rich.

Schematic illustration of aluminide coating obtained by low activity pack cementation. After R. Pichoir, in High Temperature Alloys for Gas Turbines, D. Coutsouradis et al., eds. (Appl. Sci. Pub., London, 1978), p. 191.

Further reading

1. R. Pichoir, Aluminide coatings on nickel or cobalt-base superalloys, in High Temperature Alloys for Gas Turbines, D. Coutsouradis et al., eds., Appl. Sci. Pub., London, 1978, p191.
2. Metallic and Ceramic Coatings, Production, High Temperature Properties and Applications, M.G. Mocking et al., Longman Scientific and Technical, Marlow
3. Cocking J. L. et al., Surf. Sci. Techn., 36:1988, 37-47, 'Comparative durability of six coating systems on first-stage turbine blades in the engines of a long-range maritime patrol aircraft'.
4. Sivakumar R. et al., Surf. Sci. Techn., 37:1989, 139-160, 'High temperature coatings for gas turbine blades: a review'.
5. DeMasi-Martin J. T. et al., Surf. Coat. Techn., 68/69:1994, 1-9, 'Protective coatings in the gas turbine engine'.
6. Zhang Y., 'Aluminide coatings for power-generation applications', Technical Report, ORNL/Sub/01-47035/01, available here.