US 3869779 A
The surface of a metallic base system is initially coated with a metallic alloy layer that is ductile and oxidation resistant. An aluminide coating is then applied to the metallic alloy layer. The chemistry of the metallic alloy layer is such that the oxidation resistance of the subsequently aluminized outermost layer is not seriously degraded.
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Description (OCR text may contain errors)
United States Patent 1191 Gedwill et al.
[4 1 Mar. 11, 1975 1 DUPLEX ALUMINIZED COATINGS  Inventors: Michael A. Gedwill, North Olmsted;
Salvatore J. Grisaffe, Rocky River, both of Ohio  Assignee: The United States of America as represented by the Administrator of the National Aeronautics and Space Administration, Washington, DC.
22 Filed: Jan. 24, 1974 21 Appl. No: 436,315
Related US. Application Data  Division of Ser. No. 298,156, Oct. 16, 1972.
 US. Cl 29/194, 29/196.2, 29/196.6, 29/197 [5 l] Int. Cl B32b 15/00  Field of Search 29/194, 196.2, 197, 196.6
 References Cited UNITED STATES PATENTS 3,542,530 11/1970 Talboorn et a1 29/196.6 X
3,620,693 11/1971 Sama 29/197 X 3,649,225 3/1972 Simmons 29/194 3,676,085 7/1972 Evans et a1. 29/194 3,741,791 6/1973 Maxwell et a1. 29/194 X 3,754,903 8/1973 Goward et a1. 29/194 X Primary ExaminerL. Dewayne Rutledge Assistant Examiner-E. L. Weise Attorney, Agent, or Firm-N. T. Musial; G. E. Shook; .1. R. Manning 57 ABSTRACT 9 Claims, No Drawings DUPLEX ALUMINIZED COATINGS ORIGIN OF THE INVENTION The invention described herein was made by employees of the United States Government and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefor.
RELATED APPLICATION This application is a division of copending application, Ser. No. 298,156 which was filed Oct. 16, 1972.
BACKGROUND OF THE INVENTION This invention is concerned with coating metallic base systems. The invention is particularly directed to oxidation resistant alloy overlay coatings and claddings for superalloys and dispersion-strengthened alloys.
Aluminide conversion coatings are currently used to protect superalloy components in aircraft gas turbine engines from oxidation, hot corrosion, thermal fatique, and erosion. The majority of such coatings are applied by diffusion controlled aluminum enrichment of the superalloy surface. In such a process the substrate chemistry and the processing temperature exert a major influence on coating chemistry, thickness, and properties. Thus, it is difficult to tailor an aluminide coating to resist a particular engine environment. As engine temperatures increase to improve performance, aluminide conversion coatings alone offer less potential for providing long time oxidation and thermal fatique resistance.
Nickel and cobalt base superalloys and dispersionstrengthened alloys are used as turbine vanes and blades in aircraft and land-based gas turbine engines. Oxidation, hot corrosion, and thermal fatigue cracking are major factors which limit the useful life of those superalloys by providing a more oxidation and hot corrosion resistant surface in which thermal fatigue cracking is reduced.
The aluminide coatings are in themselves made of a hard, brittle outer-layer and a hard, brittle multiphase sub-layer that can crack under high thermal stresses. Once cracked, the oxidizing and/or hot corrosion environment has direct access to the underlying substrate, and deleterious attacks can occur. Also certain elements in the superalloy substrate enter into these coatings. This generally reduces the .environmental resistance of the coatings and makes them less ductile.
SUMMARY OF THE INVENTION According to the present invention the substrate is initially overlayed with a ductile, oxidation resistant metallic alloy layer. This overlay is achieved by foil cladding or other means, such as physical vapor desposition, ion plating, sputtering, plasma spraying, or slurry sintering. Foil cladding requires more preliminary effort and fixturing, but it supplies a well characterized homogeneous material directly on the superalloy. Thus it provides the protection potential and metallurgical interactions for weak, oxidation resistant alloy coatings on strong, less environmentally resistant superalloys and dispersion-strengthened alloys.
The chemistry of the overlay coating is such that the oxidation resistance of the subsequently aluminized outermost layer is not seriously degraded. The aluminide outer layer can be developed by pack cementation, metallizing, dipping, spraying, physical vapor deposition, ion plating, sputtering, or electrophoresis. Thus, a failsafe system is provided. The aluminide outer layer has a tendency to be less embrittled by substrate elements. It has a lessened tendency to crack because it is supported by a ductile layer, not a brittle, multiphase layer that is conventionally the case. If a crack occurs in the aluminide outer-layer, the ductility of the underlayer restricts its propagation. Widespread oxidation of the underlayer does not occur because the metallic underlayer is oxidation resistant.
OBJECTS OF THE INVENTION It is, therefore, an object of the present invention to provide an improved oxidation resitant coating for superalloys and dispersion-strengthened alloys.
Another object of the invention is to provide an aluminized coating having long time oxidation and thermal fatigue resistance for these materials.
A further object of the invention is to provide an improved aluminized coating for nickel base and cobalt base superalloys, dispersion-strengthened alloys, composites, and directional eutectics.
These and other objects of the invention will be apparent from the specification which follows.
PREFERRED EMBODIMENT OF THE INVENTION According to the present invention a ductile, oxidation resistant metallic alloy is initially applied to the superalloy. An aluminide coating is then applied to the metallic alloy.
In order to illustrate the beneficial technical effects of the invention NiCrAlSi and FeCrAlY foil claddings were applied to typical nickel and cobalt base superalloys of the type used in gas turbine engines. The nominal composition of the first mentioned cladding was 15 to 25% chromium, 3 to 6% aluminum, 0.5 to 1.5% silicon, and the remainder nickel. The preferred composition was 18% chromium, 4% aluminum, 1% silicon, and the remainder nickel.
The other cladding had a nominal composition of 15 to 2% chromium, 3 to 6% aluminum, 0.1 to 1% yttrium, and the remainder iron. The preferred composition was 25% chromium, 4% aluminum, 1% yttrium, and the remainder iron.
These claddings were applied to nickel base superalloys known as lN-l00 and Wl-52. The nominal composition of the lN-IOO alloy was 15% cobalt, 9.5% chromium, 5.3% aluminum, 4.3% titanium, 3.2% molybdenum and the remainder nickel. The nominal composition of the Wl-52 was 21% chromium, 11% tungsten, 2.2% iron, 1.9% columbium, 0.9% silicon and the remainder cobalt. The claddings were also applied to WAZ-20 and NX-188 advanced superalloys and to TD- NiCr dispersionstrengthened alloy. The nominal compositions were, for WAZ-20, 20% tungsten, 6.5% aluminum, 1.5% zirconium, 0.2% carbon and the remainder nickel; for NX-l88, 18% molybdenum, 8% aluminum, 0.04% carbon and the remainder nickel; and for TD-NiCr, 20% chromium, 2% thorium dioxide, and the remainder nickel. It is further contemplated that the substrate can be nickel and cobalt base composites and directional eutectic alloys.
Claddings having a thickness of 0.127 millimeter of both materials were applied to the substrate specimens by hot isostatic gas pressure bonding at a helium pressure of 15,000 to 20,000 psi for two hours at 1090C.
Aluminide coatings were then applied to the claddings by pack cementation at 1900 to 2000F in argon using a powder mixture consisting of 1% sodium or amonium halide, 1% aluminum, and the remainder aluminum oxide. It is also contemplated that the aluminide coating can be applied by a sintered or fused slurry, electrodeposition, physical vapor deposition, ion plating, sputtering, hot dipping, or pyrolysis. The electrodeposition can be of the aqueous, fused salt, or electrophoresis type. The spraying can be either a flame or plasma type.
The system performance was primarily evaluated on the basis of weight change, visual appearance, and metallographic change. Weight change results of furnace tests on NiCrAlSi clad 1N-10O and Wi-52 at 1090C for 20 hour exposure cycles were obtained. These tests showed that the clad-cladding alloy was oxidation resistant in that it gained weight in forming a protective oxide and then little further weight change occurred. While NiCrAlSi clad on lN-lOO showed a slight turnaround primarily due to spalling, it was more protective than on Wi-52. Both bare lN-lOO and bare Wi-52 lost weight rapidly. Exposure at 1040C resulted in more protective behavior for both cladding systems for times up to 400 hours.
Metallographic cross sections of the NiCrAlSi cladding on ln-lOO showed this system was relatively uneffected by 200 hour cyclic furnace oxidation at 1090C. NiCrAlSi clad Wi-52 showed considerable surface oxide penetration and internal oxidation in the cladding after only 120 hours of tests.
The FeCrAlY cladding was evaluated in cyclic furnace oxidation on ln-lOO and Wi-52. The 1090C weight change behavior of the clad Wi-52 was almost identical to that of the cladding alloy itself. The clad In- 100, however, showed more rapid weight gains accompanied by significant spalling. A lower exposure temperature of 1040C resulted in less oxidation attack for the claddings on both substrates.
Metallographic and weight change data obtained after 1090C furnace tests on the commercial aluminide coatings were compared with similar data with the most protective claddings on each substrate. These comparisons indicated that both the attack on the microstructure and weight changes of the coating and Ni- CrAlSi cladding on lN-lOO were very similar after 200 hours (20 hour cycles) at lO90C. Here, both protection systems were approximately the same thickness.
The FeCrAlY cladding on Wl-52 was in much better condition than the completely degraded coating, but it was about twice as thick in the as-clad condition. This ease in controlling thickness is a beneficial technical effect of the overlay or cladding process.
The most promising cladding systems based on furnace testing were the NiCrAlSi clad lN-l and the FeCrAlY clad Wl-52; FeCrAlY clad lN-l00 also appeared to have some potential. These systems were subjected to Mach 1 burner rig testing at both 1040 and 1090C using one hour exposure cycles followed by air blast quenching. Such testing imposed significantly greater thermal stress on the protection system and the surface oxide, especially at the leading edges of the burner rig specimens. The FeCrAlY cladding performed better on both lN-100 and WI-52 than did the NiCrAlSi cladding. The thermal fatigue resistance of these clad systems was markedly superior to that of the aluminide coated systems. In all tests, no cracks were observed in the claddings within the test times. Only the FeCrAlY clad WI-52 performed better in oxidation erosion than the aluminide coating.
Some NiCrAlSi clad lN-lOO burner specimens were aluminized to obtain the benefits of both protective systems. Soft ductile claddings had shown superior resistance to thermal fatigue cracking while harder and more brittle aluminide coatings resisted oxidation better. Aluminizing the NiCrAlSi claddings produced a markedly improved protection system for lN-lOO. The system withstood at least 800 hours of Mach 1 burner rig testing at lOC. Based on the time to show weight change turaround, the aluminized cladding was four to five times as protective as the commercial aluminide coating. Its thermal fatigue resistance was about three times better than the aluminide coating.
The primary cause for improvement in thermal fatigue resistance is believed to be the existence of a rather ductile oxidation resistant layer of aluminum enriched cladding under the external aluminide coating. In conventional aluminide coatings on superalloys, a hard, carbide rich zone is typically found here. Benefits may also be derived from the conversion of the relatively simple NiCrAlSi alloy to the aluminide. This aluminide would be expected to contain little of the strengthening elements found in the lN-lOO.
Several aluminized NiCrAlSi clad WAZ-20, NX-188, and TD-NiCr specimens were tested in cyclic furnace oxidation at 1150C to see how effective the coating would be for higher temperature applications. The oxidation life of the clad was well in excess of 500 and 300 hours, respectively, on WAZ-20 and NX-188, and slightly more than 600 hours on TC-NiCr. This is a substantial improvement over aluminide coatings alone on these substrates which generally failed well within hours in the same tests.
Burner rig tests at 1090C and Mach-1 were conducted on aluminized, electron beam melted and physical vapor deposited NiCrAlSi coatings on lN-l00 and NASA-TRW Vl-A. The nominal composition on the coatings as-deposited is 15% chromium, 4% aluminum, 1% silicon, and the remainder nickel. The nominal composition of NASA-TRW Vl-A superalloy is 7.5% cobalt, 6.0% chromium, 5.8% tungsten, 5.4% aluminum, 9.0% tantalum, 2.0% molybdenum, 1.0% titanium, 9.5% columbium, 0.40% rhenium, 0.5% hafnium, 0.1% zirconium, 0.13% carbon, 0.015% boron, and the remainder nickel. After hours of testing in the very severe environment, the specimens showed no evidence of thermal fatigue crackling and the coating had completely protected the superalloy substrates from oxidation and erosion.
While several preferred embodiments of the invention have been described it is contemplated that various modifications may be made without departing from the spirit of the invention or the scope of the subjoined claims. By way of example, claddings of NiCrAl containing one or more of Si, Y, Mn and Th can be used. Also claddings of FeCrAl containing one or more of Y, Si, Mn and Ta can be used.
What is claimed is:
1. A coated article of manufacture comprising a superalloy substrate selected from the group consisting of nickel-base superalloys and cobalt-base superalloys, dispersion-strengthened alloys, com posites, and directional eutectics,
a ductile, oxidation resistant metallic alloy layer covering said substrate, and
an aluminide coating covering said metallic alloy layer.
2. An article of manufacture is claimed in claim 1 wherein the metallic alloy layer comprises a cladding.
3. An article of manufacture as claimed in claim 2 wherein the metallic alloy layer comprises a foil cladding.
4. An article of manufacture as claimed in claim 3 wherein the foil cladding is a NiCrAl alloy containing one or more elements selected from the group consisting of Si, Y, Mn, and Th.
5. An article of manufacture as claimed in claim 4 wherein the foil cladding is an alloy consisting essentially of from to 25% chromium, 3 to 6% aluminum, 0.5 to 1.5% silicon, and the balance nickel.
6. An article of manufacture as claimed in claim 5 wherein the alloy consists essentially of about 18% chromium, about 4% aluminum, about 1% silicon, and the balance nickel.
7. An article of manufacture as claimed in claim 3 wherein the foil cladding is a FeCrAl alloy containing one or more elements selected from the group consisting of Y, Si, Mn, and Ta.
8. An article of manufacture as claimed in claim 7 wherein the foil cladding is an alloy consisting essentially of from 15 to 25% chromium, 3 to 6% aluminum, 0.1 to 1% yttrium, and the balance iron.
9. An article of manufacture as claimed in claim 8 wherein the alloy consists essentially of about 25% chromium, about 4% aluminum, 1% yttrium, and the balance iron.