The resulting coating is shown in Fig. 2a. The coating consists also of two
layers whose thicknesses are summarized in
Table III. The diffusion layer is b-NiAl with an
aluminum content below 50% as observed in the
linescan of Fig. 2b. This substoichiometric nickel
aluminide is typical for so-called ‘‘low-activity aluminide
coatings’’ conventionally produced via pack
cementation by outward nickel transport and outward
growth of the coating microstructure due to
high temperatures and low availability of aluminum.
Another important feature of this outward
diffusion process is that slowly diffusing elements
from the substrate cannot diffuse as fast as nickel
and instead form large TCP phases in the interdiffusion
zone below a rather pure b-NiAl layer Therefore, the outer zone of this coating is free of
precipitates compared to the inwardly grown, aluminum-
rich coating formed in the precedent case
(compare with Fig. 1). Finally, the formed diffusion
layer has fewer grain boundaries than the one
formed by the conventional slurry coating.
These microstructural features improve the adhesion
and decrease oxide growth kinetics of the diffusion
layer.