High temperature nicocraly coatings

Abstract

A highly ductile coating for the nickel- and cobalt-base superalloys having long term elevated temperature oxidation-erosion and sulfidation resistance and diffusional stability consists essentially of, by weight, 11-48% Co, 10-40% Cr, 9-15% Al, 0.1-1.0% reactive metal selected from the group consisting of yttrium, scandium, thorium, lanthanum and the other rare earth elements, balance essentially Ni, the nickel content being at least about 15%.

Government Interests

BACKGROUND OF THE INVENTION

The invention described in claims 5 and 6 was made in the course of or under a contract or subcontract thereunder with the Department of the Air Force.

Description

The present invention relates to coatings and coated articles and more particularly to coatings for the nickel- and cobalt-base superalloys having high ductility while retaining desirable stability and elevated temperature oxidation and hot corrosion resistance.

Design trends for advanced gas turbine engines are toward ever increasing turbine inlet temperatures, and the demands on turbine materials have increased to the extent where contemporary aluminide coating systems can be the life limiting component of alloy-coating composites. Coatings are prone to failure by a variety of mechanisms. Aluminide coatings can be, for example, a source of fracture initiation in fatigue. Coating ductility has been found to be an important determinant in fatigue life since at relatively low temperatures aluminide coatings tend to crack in a brittle manner at low strains in the tensile portions of the fatigue cycle. Although various coatings, such as the CoCrAlY type coatings described in the patent to Evans and Elam U.S. Pat. No. 3,676,085, the NiCrAlY type coatings described in the patent to Goward, Boone and Pettit U.S. Pat. No. 3,754,903 and the FeCrAlY type coatings described in the patent to Talboom and Grafwallner U.S. Pat. No. 3,542,530 have in the past provided significant improvements in the lifetimes of the superalloys, further improvements are, of course, desirable. In particular, an improved coating having properties comparable to the conventional coating alloys together with significantly improved ductility would be desirable and useful. Such an improved coating is found in the nickel-cobalt-chromium-aluminum-yttrium system as described herein.

SUMMARY OF THE INVENTION

In brief, the present invention relates to a nickel-cobalt-chromium-aluminum-yttrium coating alloy having greatly improved ductility as well as other properties which together render it eminently suitable for use in gas turbine engine hardware and other rigorous environments. The invention more particularly relates to a high ductility coating alloy which possesses both oxidation-erosion and sulfidation resistance and which consists of a particular combination of nickel, cobalt, chromium, aluminum and a reactive metal selected from the group consisting of yttrium, scandium, thorium, lanthanum and the other rare earth elements. The invention contemplates a coating composition consisting essentially of, by weight, 11-48% cobalt, 10-40% chromium, 9-15% aluminum, 0.01-1.0% of a reactive metal selected from the group consisting of yttrium, scandium, thorium, lanthanum and other rare earth elements, balance essentially nickel, the nickel content being at least about 15%. Advantageously, the coating composition consists essentially of, by weight, about 15-40% cobalt, 12-30% chromium, 10-15% aluminum, 0.01-1.0% yttrium, balance essentially nickel, the nickel content being at least about 15%.

In one preferred embodiment, the coating composition consists essentially of, by weight, about 25-40% cobalt, 14-22% chromium, 13-15% aluminum, 0.01-1.0% yttrium, balance essentially nickel.

In another preferred embodiment, the coating composition consists essentially of, by weight, about 15-35% cobalt, 14-22% chromium, 10-13% aluminum, 0.01-1.0% yttrium, balance essentially nickel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph which dramatically illustrates the ductility behavior of various nickel-cobalt-chromium-aluminum-yttrium coating alloys as compared to representative CoCrAlY and NiCrAlY coating alloys.

FIG. 2 is a graph showing ductility as a function of temperature of some NiCoCrAlY coating alloys as compared to representative CoCrAlY and NiCrAlY coating alloys.

FIG. 3 is a graph illustrating the diffusional stability of various nickel-cobalt-chromium-aluminum-yttrium coating alloys as compared to representative CoCrAlY and NiCrAlY coating alloys.

FIG. 4 is a graph illustrating the oxidation characteristics of various nickel-cobalt-chromium-aluminum-yttrium coating alloys as compared to representative CoCrAlY and NiCrAlY coating alloys.

FIG. 5 is a graph illustrating the sulfidation characteristics of various nickel-cobalt-chromium-aluminum-yttrium coating alloys as compared to representative CoCrAlY and NiCrAlY coating alloys.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the description which follows, reference will be made to various of the conventional or contemporary nickel-base and cobalt-base superalloys. Representative of alloys of this nature are those identified in the industry as follows:

NOMINAL COMPOSITION ALLOY DESIGNATION (Percent by weight) ______________________________________ B-1900 8 Cr, 10 Co, 1 Ti, 6 Al, 6 Mo, .11 C, 4.3 Ta, .15 B, .07 Zr, balance Ni MAR-M302 21.5 Cr, 10 W, 9 Ta, .85 C, .25 Zr, 1 Fe, balance Co TD Cobalt Alloy 20 Ni, 18 Cr, 2 ThO.sub.2, balance Co TD Cobalt Alloy 20 Ni, 30 Cr, 3 ThO.sub.2, balance Co IN 100 10 Cr, 15 Co, 4.5 Ti, 5.5 Al, 3 Mo, .17 C, .75 V, .075 Zr, .015 B, balance Ni MAR-M200 9 Cr, 10 Co, 2 Ti, 5 Al, 12.5 W, .15 C, 1 Nb, .05 Zr, .015 B, balance Ni WI 52 21 Cr, 1.75 Fe, 11 W, 2(Nb + Ta), .45 C, balance Co Udimet 700 15 Cr, 18.5 Co, 3.3 Ti, 4.3 Al, 5 Mo, .07 C, .03 B, balance Ni ______________________________________

It will be appreciated that while the superalloys including those which are directionally solidified, taken as a class, are generally oxidation resistant, it is a necessary and usual practice to coat certain of the components formed therefrom in order to improve their oxidation, sulfidation, erosion and thermal shock resistance and thus extend their operating lives in advanced gas turbine engines.

As noted hereinbefore, the CoCrAlY and NiCrAlY coatings have provided significant improvements in the lifetimes of the superalloys. However, it was found that NiCrAlY coatings, while providing extremely high oxidation resistance and diffusional stability required improvement in sulfidation resistance and that CoCrAlY coatings, while providing extremely high sulfidation resistance required improvement in oxidation resistance and diffusional stability. In an effort to develop a better combination of properties, a variety of overlay coatings was evaluated. It was found that coating alloys of a composition, by weight, of 11-48% cobalt, 10-40% chromium, 9-15% aluminum, 0.01-1.0% reactive metal selected from the group consisting of yttrium, scandium, thorium, lanthanum and the other rare earth elements, balance essentially nickel, the nickel content being at least about 15%, preferably 15-40% cobalt, 12-30% chromium, 10-15% aluminum, 0.01-1.0% yttrium, balance essentially nickel, the nickel content being at least about 15%, and most preferably (1) 25-40% Co, 14-22% Cr, 13-15% Al, 0.01-1.0% Y, balance essentially Ni and (2) 15-35% Co, 14-22% Cr, 10-13% Al, 0.01-1.0% Y, balance essentially Ni dramatically and unexpectedly gave an increase in ductility while providing a satisfactory and adjustable balance of oxidation and hot corrosion resistance as well as acceptably low interdiffusional characteristics. While it had been known that certain of the useful NiCrAlY coatings exhibited a ductility higher than certain of the useful CoCrAlY coatings and it had been surmised therefore that a substitution of some nickel for the cobalt in the CoCrAlY composition might improve ductility, it was surprising and unexpected that the nickel-cobalt-chromium-aluminum-yttrium system as defined above would provide a ductility improvement which was markedly superior to either the NiCrAlY or CoCrAlY.

While not completely understood at the present time, it appears that there is a correlation between coating ductility and the phases present. More specifically, chemistry changes which increase the amount and continuity of the (Ni, Co) solid solution phase, .gamma., tend to increase coating ductility while chemistry changes which increase the amount and continuity of the (Ni, Co) Al, .beta., Ni.sub.3 Al, .gamma.', and Cr, .alpha., tend to decrease ductility. Correlation of coating microstructure with coating chemistry indicates that, in the nickel-cobalt-chromium-aluminum-yttrium system herein described, desirable .gamma. - .beta. microstructures are obtained at a higher aluminum content, the increased stability of the .gamma. - .beta. microstructure caused by cobalt additions to NiCrAlY being the result of a significant reduction of the amount of .gamma.' (Ni.sub.3 Al) and .alpha.(chromium) phases which are precipitated at lower temperatures.

Those skilled in the art will recognize that certain other elements are known to be compatible with the basic chemistry of the present alloys. Accordingly, other elements such as tantalum or hafnium may be advantageously added to the alloy as required in certain applications for modification of the mechanical, diffusional or hot corrosion characteristics of the coating.

In coating the nickel-base and cobalt-base turbine blades and vanes the surfaces to be coated are first thoroughly cleaned free of all dirt, grease and other objectional foreign matter followed by conditioning by abrasive blasting. The coating is achieved by vapor deposition from a suitably heated molten pool of the coating material held in a vacuum chamber at 10.sup.-.sup.4 torr or better. The ingot melted and evaporated by electron beam heating has essentially the same chemistry as that of the desired finished coating.

Parts are preferably preheated to 1750.degree.F .+-. 50.degree. for 5 to 6 minutes before deposition is initiated and this temperature is maintained throughout the coating operation. Deposition time varies somewhat but is controlled to obtain the preferred coating thickness of 0.003-0.005 inch. Subsequent cooling to below 1000.degree.F is accomplished in a nonoxidizing atmosphere. Following the coating step, the parts may be heat treated for 1 hour at 1900.degree.F .+-. 25.degree. in vacuum to more fully bond the coating to the substrate and provide for easier peening.

The coated articles may be dry glass bead peened using 0.007-0.011 inch diameter beads with an intensity equivalent to 19 N. In general, the peening is conducted in accordance with the provisions of the processing specification AMS 2430. The parts may then be heated to 1975.degree.F .+-. 25.degree. in dry argon, dry hydrogen or vacuum; held at heat for 4 hours; and cooled in the protective atmosphere at a rate equivalent to air cooling. Blades and vanes so processed exhibit a coating thickness, excluding the diffused zone of 0.003-0.005 inch.

Of course, it will be recognized that other methods for applying the coatings may be practiced, such as sputtering, ion plating or plasma spraying, without departing from the intent of the present invention.

Referring to FIG. 1, a graph is shown of the unexpected ductility behavior of various nickel-cobalt-chromium-aluminum-yttrium coating alloys as compared to representative CoCrAlY and NiCrAlY coating alloys. The results shown therein were obtained by measuring strain to fracture of coatings deposited on tensile specimens of appropriate superalloys. In particular, Curve A is a plot showing the effects of substituting various amounts of cobalt for nickel in a NiCrAlY alloy having a nominal composition of, by weight, Ni-19Cr-14Al-0.5Y while Curve B is a plot showing the effects of substituting various amounts of cobalt for nickel in a NiCrAlY alloy having a nominal composition of, by weight, Ni-19Cr-12.5Al-0.5Y. As is evident from the drawing, dramatic increases in ductility are obtained and it has been found, in general, that NiCoCrAlY, or CoNiCrAlY as the case may be, coating alloys have compositional ranges consisting essentially of, by weight, 11-48% Co, 10-40% Cr, 9-15% Al, 0.1-1.0% reactive metal selected from the group consisting of yttrium, scandium, thorium, lanthanum and the other rare earth elements, balance essentially nickel (at least about 15%), preferably 15-40% Co, 12-30% Cr, 10-15% Al, 0.1-1.0% Y, balance essentially Ni, the nickel content being at least about 15%, will be effective in this regard. As will be appreciated, with the higher Al content, as shown by Curve A, a generally higher range of cobalt is preferred, a preferred coating consisting essentially of 25-40% Co, 14-22% Cr, 13-15% Al, 0.01-1.0% Y, balance essentially Ni. With lower Al content, as shown by Curve B, a generally lower range of cobalt is preferred, a preferred coating consisting essentially of 15-35% Co, 14-22% Cr, 10-13% Al, 0.01-0.1% Y. In FIG. 2, ductility curves for selected coatings show ductility as a function of temperature and indicate the markedly superior tensile cracking resistance of the NiCoCrAlY coatings.

In one series of thermomechanical fatigue tests, a directionally solidified specimen substrate of MAR-M200 (with hafnium) was coated with Ni-24Co-16Cr-12.5Al-40.3Y and run on a thermomechanical fatigue machine which pushes and pulls the specimen in severe fatigue and temperature cycles which simulate the strain-temperature cycle of a cooled turbine blade. A number of identical substrates were coated with Co-20Cr-12Al-0.5Y and another number with a diffusion aluminide coating. Both the CoCrAlY and the diffusion aluminide coated specimens failed after approximately 1,000 cycles or less on the thermomechanical fatigue machine whereas the NiCoCrAlY coated specimen did not fail until after 1,925 cycles.

Referring to FIGS. 3-5, a comparison of the interdiffusional, oxidation resistance and corrosion resistance properties of various NiCoCrAlY alloy coatings is shown. In the drawings, 3-5 mil coatings of NiCoCrAlY alloy consisting essentially of the indicated amounts of cobalt, 18-21% Cr, 13-14% Al and 0.05-0.8% Y were vapor deposited onto B-1900 substrates as well as onto directionally solidified MAR-M200 (plus Hf) substrates (erosion bars). In FIG. 3, the coated samples were aged 100 hours in air at the indicated temperature. In FIG. 4, coated components were subjected to 2000.degree.F cyclic burner-rig oxidation tests (2000.degree.F, 29 minutes -- forced air cool, 1 minute, JP 5 fuel used) for up to 2,100 hours (2,030 hours hot time). In FIG. 5, coated components were treated under cyclic conditions (1,750.degree.F, 3 minutes -- 2000.degree.F, 2 minutes -- cool, 2 minutes) in a high velocity hot gas stream derived from the combustion of JP 5 jet fuel, with 35 ppm salt/air added. As will be appreciated, the claimed NiCoCrAlY coatings, while giving unexpectedly increased ductility also simultaneously give adjustable and satisfactory degrees of interdiffusion and oxidation and hot corrosion resistance.

For a clearer understanding of the invention and, in addition to the data given in the drawings, other specific examples are set forth below.

EXAMPLES 1-5

Five B-1900 Ni-base alloy erosion bars were coated with a 3-5 mil thick alloy having a composition, consisting essentially of, by weight, Co-20Ni-24Cr-15Al-0.75Y generally in accordance with the procedures outlined above. The coated erosion bars were subjected to 62.5 hours of vane cyclic sulfidation testing (1750.degree.F, 3 minutes -- 2050.degree.F, 2 minutes -- cool, 2 minutes with 35 ppm artificial sea salt: air ingested after combustion and using JP 5 fuel). The coatings exhibited a specific life of from 21.1-24.4 hours/mil and were comparable to Fe-27Cr-13Al-.75Y coatings which exhibited specific lifetimes of 22.2-27.9 hours/mil.

EXAMPLE 6

A 3.6 mil coating of Co-20Ni-24Cr-15Al-0.75Y was vapor deposited onto a MAR-M302 Co-base alloy erosion bar and subjected to a modified vane cyclic sulfidation test (1750.degree.F, 3 minutes -- 2150.degree.F, 2 minutes -- cool, 2 minutes with 35 ppm artificial sea salt: air ingested after combustion using JP 5 fuel) in order to evaluate diffusional stability combined with the very high temperature sulfidation. The coating had a failure time of 162 hours and a specific life of 45 hours/mil.

EXAMPLES 7-10

Two B-1900 Ni-base alloy erosion bars and two MAR-M302 Co-base alloy erosion bars were coated with nominally three mil thick coatings of Co-20Ni-24Cr-15Al-0.75Y as above and were subjected to oxidation-erosion testing at 2000.degree.F until failure. The B-1900 coatings failed at 263.2 and 153.7 hours while the MAR-M302 coatings both failed at 309.2 hours.

EXAMPLES 11-14

Coatings consisting essentially of Co-20Ni-20Cr-12Al-0.5Y, Co-20Ni-16Cr-16Al-0.5Y, Ni-32.5Co-20Cr-12Al-0.5Y and Co-20Cr-12Al-0.5Y were vapor deposited to thicknesses of 4.5-5.5 mil on Co-20Ni-18Cr-2ThO.sub.2 alloy airfoil specimens. All coatings were essentially a two phase mixture of beta CoAl or (CoNi)Al and gamma solid solution. The Co-20Ni-16Cr-16Al-0.5Y coatings were predominantly beta with a small volume percent solid solution gamma phase. The beta phase was continuous and represented an undesirable structure because of its potential low strain-to-crack characteristics. The Co-20Ni-20Cr-12Al-0.5Y and the Co-20Cr-12Al-0.5Y coatings also exhibited a continuous beta type structure but contained substantially more gamma. The Ni-32.5Co-20Cr-12Al-0.5Y had a desired two phase plus gamma structure with the gamma phase being the continuous matrix phase.

These systems were exposed in a static air environment for 100 hours at 2000.degree.F, 2100.degree.F, 2200.degree.F and 2400.degree.F to evaluate stability and elemental interactions. The resultant coating hardness after exposure, showed no detrimental change in hardness or brittle layer formation. The Co-20Ni-16Cr-16Al-0.5Y composition retained its continuous beta structure during exposure and, due to its high crack susceptibility was not tested further. The other coating systems retained or transformed to a two phase mixture of beta in a continuous gamma matrix. The best stability was obtained with the Ni-32.5Co-20Cr-12Al-0.5Y coating.

Additional airfoil shaped specimens of Co-20Ni-18Cr-2ThO.sub.2 were vapor deposition coated with Co-20Cr-12Al-0.5Y, Co-20Ni-20Cr-12Al-0.5Y and Ni-32.5Co-20Cr-12Al-0.5Y to a thickness of 4.5-5.5 mil using the same techniques and subjected to 1800.degree.F, 2000.degree.F, 2200.degree.F and 2400.degree.F isothermal oxidation testing, to 2200.degree.F cyclic oxidation testing (1750.degree.F, 3 minutes -- 2200.degree.F, 2 minutes -- cool, 2 minutes) and to 2200.degree.F cyclic hot corrosion testing (1750.degree.F, 3 minutes - 2200.degree.F, 2 minutes - cool, 2 minutes). In all testing the airfoil samples were rotated at 1,750 rpm in a 400-500 feet/second gas stream of combusted JP 5 fuel. For cyclic hot corrosion testing, the fuel was doped with 0.3% butyl disulfide and synthetic sea salt solution was injected into the combusted flame to yield a 3.5 ppm salt concentration in the burner flame.

The 1800.degree.F and 2000.degree.F isothermal oxidation tests were discontinued at 214 and 222 hours, respectively. All specimens shows no visual signs of degradation. Based on metallographic examination of specimens from the 1800.degree.F tests, coating degradation was least for the Ni-32.5Co-20Cr-12Al-0.5Y. Also in the 2000.degree.F test, the NiCoCrAlY coating exhibited the least degradation. The extent of degradation of the CoNiCrAlY and CoCrAlY coatings was approximately equal.

The 2200.degree.F isothermal oxidation test was discontinued at 305 hours. Again the NiCoCrAlY coating showed the least degradation while the CoCrAlY coating showed the most.

The 2400.degree.F isothermal oxidation test was run to coating failure. Of the three coatings systems evaluated, the NiCoCrAlY composition exhibited the longest life, 226 hours.

The cyclic oxidation and cyclic hot corrosion tests were discontinued at 207 (59 hours hot time) and 204 (58 hours hot time) hours, respectively. Coating failure had not occurred. Essentially no difference was observed in the structure between the three samples in the hot corrosion test. However, in the cyclic oxidation test, the Ni-32.5Co-20Cr-12Al-0.5Y coating exhibited a far greater amount of retained beta than either of the other two.

EXAMPLES 15-16

In a series of especially severe engine tests, first stage turbine blades of the alloys indicated were coated as indicated in Table I and run for 297 hours including 2,000 cycles (acceleration to full takeoff power followed by holding for a period of time, rapid deceleration to idle power and holding for a period of time). Over 100 cycles were with water injection (for thrust augmentation) which imposed the severest possible thermal shock to the coatings.

Table I __________________________________________________________________________ Number Number with Percent with Alloy Coating Tested Cracked Coatings Cracked Coatings __________________________________________________________________________ B-1900 & Hf platinum aluminide 8 8 100 " rhodium aluminide 7 7 100 " high temperature pack aluminide 14 13 93 " low temperature pack aluminide 56 56 100 "Ni-18Cr-14Al-0.5Y 4 4 100 " Ni-12Cr-14Al-0.5Y 2 2 100 " Ni-18Cr-10Al-0.5Y 2 2 100 "Ni-12Cr-12Al-0.5Y 3 3 100 " Ni-18Cr-12Al-0.5Y 3 3 100 Directionally solidified MAR-M200 & Hf Ni-18Cr-12Al-0.5Y 7 5 71 B-1900 & Hf Ni-11Co-22Cr-11Al-0.06Y 5 0 0 " Ni-20Co-16Cr-11.5Al-0.05Y 5 0 0 __________________________________________________________________________

While NiCrAlY had not previously cracked in other engine tests and is therefore considered acceptable for most engine conditions, this test was particularly severe and, as shown, only the NiCoCrAlY coated blades were completely free of coating cracks. In similar tests, CoCrAlY coatings consistently cracked.

It has been clearly established that the inventive alloy coatings are effective not only in providing long term oxidation resistance, corrosion resistance and stability but dramatically improved ductility.

What has been set forth above is intended primarily as exemplary to enable those skilled in the art to practice the invention and it should therefore be understood that, within the scope of the appended claims, the invention may be practiced in other ways than as specifically described.

Claims

What is claimed is:

1. A coating composition for the nickel-base and cobalt-base alloys which consists essentially of, by weight, 11-48% cobalt, 10-40% chromium, 9-15% aluminum, 0.01-1.0% of a reactive metal selected from the group consisting of yttrium, scandium, thorium, lanthanum and other rare earth elements balance essentially nickel, the nickel content being at least about 15%.

2. A coating composition for the nickel-base and cobalt-base alloys which consist essentially of, by weight, 15-40% cobalt, 12-30% chromium, 10-15% aluminum, 0.01-1.0% yttrium, balance essentially nickel, the nickel content being at least about 15%.

3. A coating composition for the nickel-base and cobalt-base alloys which consists essentially of, by weight, 25-40% cobalt, 14-22% chromium, 13-15% aluminum, 0.01-1.0% yttrium, balance essentially nickel.

4. A coating composition for the nickel-base and cobalt-base alloys which consists essentially of, by weight, 15-35% cobalt, 14-22% chromium, 10-13% aluminum, 0.01-1.0% yttrium, balance essentially nickel.

5. A coating composition for the nickel-base and cobalt-base alloys which consists essentially of, by weight, 32.5% cobalt, 20% chromium, 12% aluminum, 0.5% yttrium, balance essentially nickel.

6. A coating composition for the nickel-base and cobalt-base alloys which consists essentially of, by weight, 20% nickel, 20% chromium, 12% aluminum, 0.5% yttrium, balance essentially cobalt.

7. A gas turbine engine component comprising a nickel-base or cobalt-base superalloy coated to a thickness of at least about 0.003 inch with a coating consisting essentially of, by weight, 11-48% cobalt, 10-40% chromium, 9-15% aluminum, 0.01-1.0% of a reactive metal selected from the group consisting of yttrium, scandium, thorium and other rare earth elements, balance essentially nickel, the nickel content being at least about 15%.

8. A gas turbine engine component comprising a nickel-base or cobalt-base superalloy coated to a thickness of at least about 0.003 inch with a coating consisting essentially of, by weight, 15-40% cobalt, 12-30% chromium, 10-15% aluminum, 0.01-1.0% yttrium, balance essentially nickel, the nickel content being at least about 15%.

9. A gas turbine engine component comprising a nickel-base or cobalt-base superalloy coated to a thickness of at least about 0.003 inch with a coating consisting essentially of, by weight 25-40% cobalt, 14-22% chromium, 13-15% aluminum, 0.01-1.0% yttrium, balance essentially nickel.

10. A gas turbine engine component comprising a nickel-base or cobalt-base superalloy coated to a thickness of at least about 0.003 inch with a coating consisting essentially of, by weight, 15-35% cobalt, 14-22% chromium, 10-13% aluminum, 0.01-1.0 yttrium, balance essentially nickel.