The superalloys are a class of materials which exhibit desirable mechanical properties at high temperatures. These alloys generally contain major amounts of nickel, cobalt and/or iron either alone or in combination, as their basis material, and alloying additions of elements such as chromium, aluminum, titanium, and the refractory metals. Superalloys have found numerous applications in gas turbine engines.
In most gas turbine applications, it is important to protect the surface of the engine component from oxidation and corrosion degradation, as such attack may materially shorten the useful life of the component, and cause significant performance add safety problems.
Coatings can be used to protect superalloy engine components from oxidation and corrosion. The well known family of coatings commonly referred to as MCrAlY coatings, where M is selected from the group consisting of iron, nickel, cobalt, and various mixtures thereof, can markedly extend the service life of gas turbine engine blades, vanes, and like components. MCrAlY coatings are termed overlay coatings, denoting the fact that they are deposited onto the superalloy surface as an alloy, and do not interact significantly with the substrate during the deposition process or during service use. As is well known in the art, MCrAlY coatings can be applied by various techniques such as physical vapor deposition, sputtering, or plasma spraying. MCrAlY coatings may also include additions of noble metals, hafnium, or silicon, either alone or in combination. They may also include other rare earth elements in combination with or substitution for yttrium. See, e.g., the following U.S. patents which are incorporated by reference: U.S. Pat. Nos. 3,542,530, 3,918,139, 3,928,026, 3,993,454, 4,034,142, and Re. 32,121.
U.S. Pat. No. Re. 32,121 states that MCrAlY coatings are the most effective coatings for protecting superalloys from oxidation and corrosion attack.
Aluminide coatings are also well known in the art as capable of providing oxidation and corrosion protection to superalloys. See, for example, U.S. Pat. Nos. 3,544,348, 3,961,098, 4,000,507 and 4,132,816, which are incorporated by reference.
During the aluminizing process there is significant interaction between the aluminum and the substrate; the substrate chemistry and deposition temperature exert a major influence on coating chemistry, thickness and properties. A disadvantage of aluminide coatings is that in the thicknesses required for optimum oxidation and corrosion resistance, generally taught by the prior art to be about 0.0035 inches, the coatings are brittle and can crack when subjected to the stresses which gas turbine engine blades and vanes typically experience during service operation. These cracks may propagate into the substrate and limit the structural life of the superalloy component; the tendency to crack also results in poor oxidation and corrosion resistance, as discussed in U.S. Pat. Nos. 3,928,026, 4,246,323, 4,382,976, and Re. 31,339.
Aluminide coatings less than about 0.0035 inches thick may have improved crack resistance, but the oxidation resistance of such thin aluminides is not as good as that of the MCrAlY coatings.
In U.S. Pat. Nos. 3,873,347 and 4,080,486, an attempt is made to combine the advantages of MCrAlY coatings and aluminide coatings. Therein, an MCrAlY coating, preferably 0.003-0.005 inches thick, is aluminized in a pack cementation process, wherein radially aligned defects in the MCrAlY coating are infiltrated with aluminum diffusing inwardly from the pack mixture. More importantly, a high concentration of aluminum results at the outer surface of the MCrAlY coating, which improves the high temperature oxidation resistance of the coating as compared to the untreated MCrAYY. Both patents state that in laboratory tests, the aluminized MCrAlY coating exhibited improved corrosion resistance, although this is somewhat at variance with the conventional wisdom that aluminum enrichment improve oxidation resistance rather than corrosion resistance.
According to U.S. Pat. No. Re. 30,995, in order to prevent cracking and spalling of an aluminized MCrAlY coating from the substrate, the aluminum must not diffuse into the substrate; aluminum may diffuse no closer than 0.0005 inches to the MCrAlY/substrate interface. It is also stated that the aluminum content in the aluminized MCrAlY must be less than ten weight percent, in order to achieve the best combination of coating properties.
In U.S. Pat. No. 3,961,098, an MCr powder is flame sprayed onto a metallic substrate in such a manner that the powder particles are substantially non-molten when they strike the substrate surface. Aluminum is subsequently diffused through the overlay coating, and into the substrate surface. Laboratory tests revealed that the aluminizing step must be conducted so that the final aluminum concentration in the coating is less than 20 weight percent, or else the coating will be brittle, and will have unacceptable corrosion and oxidation resistance.
U.S. Pat. No. 4,246,323 teaches a process for enriching an MCrAlY coating with aluminum. The processing is conducted so that Al diffuses only into the outer surface of the MCrAlY. The outer, Al rich portion of the coating is reported to be resistant to oxidation degradation, and the inner, unaluminized MCrAlY reportedly has good mechanical properties.
In U.S. Pat. No. Re. 31,339 an MCrAlY coated superalloy component is aluminized, and then the coated component is hot isostatically pressed. A substantial increase in coating life is reported, which is attributed to the presence of a large reservoir of an aluminum rich phase in the outer portion of the MCrAlY. As in the patents discussed above, the aluminum diffuses only into the MCrAlY outer surface. U.S. Pat. No. 4,152,223 discloses a process similar to that of U.S. Pat. No. Re. 31,339, in which an MCrAlY coated superalloy is surrounded by a metallic envelope, and then hot isostatically pressed to close any defects in the MCrAlY coating and to diffuse a portion of the envelope into the overlay. If aluminum foil is used as the envelope, the foil may melt during hot isostatic pressing and form intermetallic compounds with the substrate. It is stated that these compounds may enhance the oxidation resistance of the coating. However, such intermetallics may have 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 aforementioned advances in coating technology have markedly improved resistance to oxidation degradation. However, these advances have failed to address what is now viewed as the life limiting property for coated superalloys: resistance to thermal mechanical fatigue cracking.