Appeal No. 94-4428
Application 08/024,034
We reverse.
The subject matter on appeal generally relates to the
formation of protective coatings on nickel- and cobalt-based
superalloy articles of the type used as gas turbine parts. It
is known in the prior art to deposit oxidation-resistant
coatings on the surfaces of such articles and typically, such
coatings are nickel or cobalt aluminide having surfaces which
oxidize to form an aluminum oxide (alumina) scale which is
tough, adherent and resistant to attack by oxidation
(Specification, page 1, lines 27-33). A problem with alumina
scales formed by the prior art process is that they tend to
spall from the surface of the part, largely as a result of the
difference between the coefficient of thermal expansion of the
underlying superalloy and the alumina scale (Specification,
page 2, lines 1-8). A result of such spallation is further
oxidation of the surface aluminide layer to alumina, which can
ultimately deplete the aluminum available on the surface
(Specification, page 2, lines 8-16).
Appellant’s invention is said to be based on the
discovery of a method to produce a spallation-resistant
alumina layer on an aluminided superalloy article. This is
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