e an undesired effect on the fatigue strength of the coated component.
In U.S. Pat. No. 4,382,976, an MCrAlY coated superalloy component is aluminized in a pack process wherein the pressure of the inert carrier gas is cyclicly varied. Aluminum infiltrates radially aligned defects of the overlay, and reacts with the MCrAlY to for various intermetallic, aluminum containing phases. The extent of Al diffusion into the substrate alloy was reported to be significantly less than if the aluminizing were carried out directly on the substrate.
In U.S. Pat. No. 4,101,713, high energy milled MCrAlY powders are applied to superalloy substrates by flame spray techniques. It is stated that the coated component can be aluminized, whereby aluminum would diffuse into the MCrAlY coating, and if desired, into the substrate material. However, according to U.S. Pat. No. Re. 30,995 (issued to the same inventor) diffusion of aluminum into the substrate may cause spalling of the MCrAlY coating from the substrate.
Other U.S. patents which disclose aluminized MCrAlY coatings are U.S. Pat. Nos. 3,874,901 and 4,123,595.
In U.S. Pat. No. 4,005,989, a superalloy component is first aluminized and then an MCrAlY overlay is deposited over the aluminized layer. The two layer coating is heat treated at elevated temperatures, but no information is given as to the results of such heat treatment. The coating was reported to have improved resistance to oxidation degradation compared to the aluminized MCrAlY coatings discussed above.
Other patents which indicate the general state of the art relative to coatings for superalloys include U.S. Pat. Nos. 3,676,085, 3,928,026, 3,979,273, 3,999,956, 4,109,061, 4,123,594, 4,132,816, 4,198,442, 4,248,940, and 4,371,570.
As the operating conditions for superalloy components become more severe, further improvements are required in oxidation and corrosion resistance, and resistance to thermal mechanical fatigue. As a result, engineers are continually seeking improved coating systems for superalloys. The aforementio