Aluminide coatings are the most widely used to protect nickel-and cobalt-based super-alloys for gas turbine applications. They are generally formed by pack cementation: the components are immersed in a powder mixture in a semi-sealed retort. The retort is then placed inside a furnace and heated at high temperature(between 750C and 1150℃) under a protective atmosphere. In the case of an aluminizing process, aluminium is transferred from the pack to the substrate surface where it reacts and an aluminidelayer forms by diffusion with the substrate. Due to this mode of formation this technique is a particular case of "reactive chemical vapour deposition(CVD)". Pack cementation is a simple process to operate, easily controlled and inexpensive to run.These advantages justify its wide use for protecting superalloys, but due to its mode of formation, it suffers from several limitations: the range of accessible coating compositions and structures remains rather narrow(only one element in practice can be transferred in the gas phase from the pack) and in some applications it would be desirable to enrich the substrate surface not only with aluminium, but also with chromium, yttrium, etc. Moreover, the composition and structure of the substrate alloy can seriously affect the performances of the coating.
Other diffusion coatings have been developed: silicon carbide on carbon-based materials and silicides on niobium alloys. In the latter case the addition elements are brought to the surface as a slurry.
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