  In the present work, it is shown that the surface preparation method used on
two Ni-based superalloys prior to aluminizing chemical vapor deposition (CVD)
is one of the most important factors determining the oxidation resistance of
aluminized Ni-based superalloys. It was found that grit blasting the substrate
surface negatively affects the oxidation resistance of the aluminized coatings.
For grit blasted and aluminized IN 625, a thicker outer NiAl coating was formed
compared to that of IN 738. In contrast, no effect on NiAl coating thickness
was found for grit blasted and aluminized IN 738. However, a thicker
interdiffusion zone (IDZ) was observed. It was shown that the systems with
grit-blasted surfaces reveal worse oxidation resistance during thermal shock
tests, namely, a higher mass loss was observed for both grit blasted and
aluminized alloys, as compared to ground and aluminized alloys. A possible
reason for this effect of remaining alumina particles originating from surface
grit blasting on the diffusion processes and stress distribution at the
coating/substrate is proposed.
