Smart overlay coatings — concept and practice

Smart overlay coatings are a functionally gradient coating system designed to provide high temperature corrosion protection over a wide range of operating conditions. The SMARTCOAT design consists of a MCrAlY base, enriched first in chromium, then aluminium to provide a chemically graded structure. At elevated temperatures, above 900°C (1650°F), the coating oxidises to form a protective alumina scale. However, at lower temperatures this alumina scale does not reform rapidly enough to confer protection under type II hot corrosion conditions. The coating is therefore designed with an intermediate chromium-rich interlayer, which permits the rapid formation of chromia healing areas of type II corrosion damage. Laboratory and burner rig tests have been carried out on a series of developmental smart overlay coatings. These have shown that the development of an intermediate chromium-rich phase provides protection under low temperature hot corrosion conditions, while the aluminium-rich surface layer provides resistance to high temperature oxidation and type I hot corrosion. Thus, the single application of SMARTCOAT permits operation over a broad range of industrial and marine turbine conditions.