Introduction

As discussed in previous sections, aluminide coatings strongly interact with the substrates and must therefore be tailored for each different alloy.

Overlay coatings as opposed to diffusion coatings, provide more independence from the substrate alloy, but also more flexibility in design as compositions can be modified depending on the degradation mechanisms expected to prevail.
Typical MCrAlY bond coats (M=Fe,Co or Ni) contain at least 4 elements, which means that coating methods such as pack cementation are considerably more difficult to use, as the activity of each element in the pack would have to be controlled carefully so as to obtain a coating of required composition.

The presence of a significant amount of Cr give these coatings excellent corrosion resistance combined with good oxidation resistance.

Alternative methods are therefore preferred, such as air plasma spray (APS), low pressure plasma spray (LPPS), or electron beam physical vapour deposition (EBPVD). These are detailed in other sections. Deposition is followed by a high-temperature heat-treatment in vacuum to allow interdiffusion and therefore improve adhesion (for example, Richard 1996) .

Microstructure

MCrAlY coatings typically exhibit a two-phase microstructure β+ γ. The presence of γ increases the ductility of the coating thereby improving thermal fatigue resistance.
As for β-NiAl coatings, high temperature exposure results in depletion of the Al both to the TGO (thermally grown oxide) and to the substrate by interdiffusion. As the amount of Al decreases, the β phase tends to dissolve. For this reason, it is often described as an aluminium reservoir, and coating life often measure in terms of depletion of β.

Schematic illustration of MCrAlY microstructure.	Al diffusion to the oxide layer and the substrate result in depletion of beta from both sides.

Composition and role of additions

The M of MCrAlY stands for either Ni or Co, or a combination of both (when applied to steels, it can also be Fe), depending on the type of superalloy. Co-based appear to have superior resistance to corrosion.

Cr provides hot-corrosion resistance, but the amount that can be added is limited by the effect it is expected to have on the substrate, and the formation of Cr-rich phases in the coating.

Al content is typically around 10-12 wt%. Since oxidation life is essentially controlled by the availability of Al, it would be tempting to increases the alulminium content. However, this results in significant reduction of ductility (for example, Sivakumar, 1989).

MCrAlY also typically contain 1 wt% yttrium (Y), which enhances adherence of the oxide layer. It was initially thought that Yttrium helped the formation of oxide pegs which helped anchor the oxide layer to the coating.
However, it has been shown that there is little if any correlation (Smeggil, 1987), and it is now believed that the main role of Y is to combine with sulfur and prevent its segregation to the oxide layer, which is otherwise detrimental to its adhesion.
Additions of hafnium (Hf) play a similar role

The effect of other additions has also been investigated (Nicoll, 1982). It was found that silicon (Si) significantly improved cyclic oxydation resistance, however it also decreases the melting point of the coating. 5 wt% are enough to lower the melting temperature to about 1140 ^oC. There is also evidence that it affects phase stability. For cyclic oxidation at 1000 ^oC, 2.5 wt% was found to be the optimum content. Further additions were detrimental.

Additions of rhenium (Re) have been shown to improve isothermal or cyclic oxidation resistance, and thermal cycle fatigue (Czech et al., 1994).

Additions of tantalum (Ta) can also increase the oxidation resistance.

Bibliography

1. Nicoll A. R. et al., Thin Solid Films, 95:1992, 21-34, 'The effect of alloying additions on M-Cr-Al-Y systems - An experimental study'.
2. Smeggil J. L., Mater. Sci. Eng., 87:1987, 261-265, 'Some comments on the role of yttrium in protective oxide scale adherence'.
3. Sivakumar R. et al., Surf. Coat. Techn., 37:1989, 139-160, 'High temperature coatings for gas turbine blades: a review'.
4. Czech et al., Surf. Coat. Techn., 68:1994, 17-21, 'Improvement of MCrAlY coatings by additions of rhenium'.
5. Richard C. S. et al., Surf. Coat. Techn., 82:1996, 99-109, 'The influences of heat-treatments and interdiffusion on the adhesion of plasma-sprayed NiCrAlY coatings'.