Abstract
The combined chemical and structural analysis capabilities of the analytical electron microscope (AEM) make it an important tool for understanding the nature of protective coatings on nickel-base superalloys. One of the important coating systems for protecting turbine blades from oxidation and hot corrosion is a MCrAlY alloy applied by electron beam physical vapor deposition (where M = Ni, Co, or Fe). In the as deposited condition this coating consists of a fairly open, generally columnar structure. A post-coating heat treatment on the order of 2 to 4 hours at approximately 1100°C is normally given to the coating. The major coating constituents are a β[(Ni,Co)Al] intermetallic phase and a γ[Co,Cr,Ni] solid solution phase. This heat treatment results in the formation of a ∼ 10 μm interdiffusion zone of phase transformation products at the coating/substrate interface and a thin protective layer (< 1 μm) of Al2O3 at the coating surface (Fig. l). While the coating contains just a few tenths of a percent yttrium, this yttrium plays an important role in improving the adherence of the protective Al2O3 layer. It has previously been proposed that the yttrium causes oxide pegs to develop at the Al2O3/coating alloy interface and that these pegs improve oxide adherence by mechanically keying the Al2O3 to the coating alloy.
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