US 3846159 A
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Nav. 5, 1974 N. s. BoRNsrElN ETAL EUTECTI ALLOY COATING Filed Aug, 18 1972 2 Sheets-Sheet 1 NOV- 5 1974 N. s. BoRNs'rx-:IN ErAL 3.345.159
EUTECTIC ALLOY COATING 2 Sheets-Sheet 2 Filed Aug. 18, 1972 f-Uaiasafes Patent one@ 3,846,159 Patented Nov. 5, 1974 3,846,159 EUTECTIC ALLOY COATING Norman S. Bornstein, West Hartford, and Edwin H. Kraft, Hebron, Conn., assignors to United Aircraft Corporation East Hartford, Conn.
Filed Aug. 18, 1972, Ser. No. 281,868 Int. Cl. C23c 9/00, 17/00 U.S. Cl. 117-71 M 16 Claims ABSTRACT F THE DISCLOSURE BACKGROUND OF THE INVENTION 'Ihe invention herein described was made in the course of or under a contract or subscontract thereunder, with the Department of the Navy.
The capabilities of the current nickel and cobalt base superalloys are being severely taxed in the advanced gas turbine engines since they are exposed to high stress levels at temperatures in excess of 85 percent of their melting points. Even though the performance and endurance of these alloys have been improved by coating techniques utilizing the aluminide coatings as well as those of CoCrAlY, NiCrAlY or FeCrAlY, by design techniques such as air cooling and by manufacturing techniques such as unidirectional solidification, such measures offer only interim solutions to the basic problem.
It is known that a number of alloys, principally the eutectcs, may be directionally solidified from a melt to form an ordered microstructure wherein one phase solidifies in Whisker or lamellar form in a matrix of a second phase. A number of eutectics of this nature and the techniques to produce in situ whisker or lamel'lae-strengthened alloys are described, for example, in U.S. 3,124,452 to W. Kraft, U.S. 3,528,808 and 3,552,953 to Lemkey and Thompson, U.S. 3,554,817 to Thompson and U.S. 3,564,- 940 to Thompson and Lemkey, all sharing a common assignee with the present invention.
While a number of these directionally solidified eutectics may be particularly attractive for gas turbine engine use since their melting points and strengths are high, the need for protective coatings in oxidizing and sulfidizing environments at the temperatures associated with gas turbine engine operation has remained. Because of the unique nature of the directionally solidified eutectcs, however, previously developed coating techniques utilized for protection of nickel and cobalt base superalloys are generally ineffective for use on directionally solidified eutectic alloys. The eutectic microstructure generally consists of relatively hard reinforcing plates or rods of one phase dispersed in a matrix of the softer phase. In the application of conventional coatings thereto, it has been found that this aligned two-phase structure is prone to being carried into the surface coating. The resultant lamellar or rod interfaces in the coating can act as crack initiation sites and cause cracks to propagate into the substrate and thereby degrade its properties. Further, while deformation of mating surfaces in diffusion bonding is considered necessary so that asperites do not cause voids at the bond line, the rod or lamellar reinforcement in eutectic composites are typically nondeformable and, in some cases,
brittle so that surface deformation is not considered pos sible. Thus diffusion bonding of the eutectic is believed to result in fracture reinforcement which may later act as crack nuclei` The region of chemical composition of a eutectic alloy is restricted by nature to those wherein two or more phases in thermodynamic equilibrium are solidified simultaneously as contrasted to a solid solution alloy whose composition can be chosen anywhere within the solubility limits. Eutectics are further distinguished from alloys based on solid solutions in that their microstructure cannot be reformed once disturbed as can occur during surface treatments. It will be appreciated that this fact also distinguishes the eutectic alloys from the mechanical dispersion hardened alloys such as TD-NiCr. At any rate, it has been found, as indicated, that the generation of, for example, an aluminized coating directly on a directionally solidified eutectic alloy produces unwanted phases and phase morphologies which do not occur on alloys which are based on solid solution. In the case of the eutectic, the result of such direct coating is the retention of an aligned biphase coating structure wherein preferential cracking and/or oxidation occurs along the phase boundaries causing premature failure of the coating.
In the present invention, a process is described wherein protective coatings which would otherwise form the unwanted phases, may be successfully applied to directionally solidified eutectic substrates to yield excellent oxidation erosion resistance. In a preferred embodiment, an aluminide coating based 0n the system Ni-Cr-Al is formed on a Ni3Cb reinformed nickel rich directionally solidified eutectic alloy which not only eliminates the aligned biphase structure in the coating but also forms an intermediate layer between the directionally solidified eutectic alloy substrate and coating which is more ductile than the coating. The ductile layer, in effect, reduces the propensity to form cracks and acts to blunt any cracks that are formed to prevent them from extending into the substrate.
While aluminide coatings based on the Ni-Cr-Al system are not, per se, unknown, their efficiency for usage with directionally solidified eutectic alloys has not been apparent. The prior patents U.S. 3,290,126 to Monson and U.S. 3,338,733 to Rowady both describe Ni-Cr-Al coatings achieved in a technique which includes a chromizing step. These patents, however, teach a chromizing step wherein a mixed or prealloyed powder, containing both Ni and Cr, are present. As indicated above, the inventors herein have found that such a technique will not work on the eutectic alloys. Investigation has revealed that unless the elements of the coating are in thermodynamic stability with the phases of the eutectic substrate, deleterious phases Iwill be formed. It has been found, for example, that pure chromium cannot be allowed to contact the surface of the NiaCb reinforced nickel rich eutectic alloy because the addition of chromium thereto modifies the eutectic structure and alters the mechanical properties of the alloy. It has also been found that too high a concentration of Al will create the same problem. It was further discovered that if carbon is present, it can result in the formation of a new low melting phase which compromises the alloy.
SUMMARY OF THE INVENTION `nickel base directionally solidified eutectic type alloys are herein defined as those alloys which are undirectionally solidified according to the eutectic type reaction wherein two or more phases freeze simultaneously from a multicomponent liquid to form a nickel rich matrix and a reinforcing phase of generally parallel rods or lamellae such as, for example, NiaCb or the metals of Group VIB of the Periodic Table (i.e., Cr, Mo and W).
The invention contemplates a method, as well as the article produced thereby, for depositing an oxidation erosion resistant coating which, in itself, is not compatible with the directionally solidified eutectic type alloy substrate onto that substrate. By compatible is meant the ability of the coating to be in thermodynamic equilibrium with the substrate, i.e., to remain in contact with the phases which comprise the substrate without forming additional or deleterious phases. Examples of such oxidation erosion resistant coatings are the aluminides, CoCrAl, FeCrAl, NiCrAlor CoCrAl, FeCrAl, NiCrAl with rare earth additions (e.g., yttrium), as disclosed for example, in U.S. 3,649,225 to Simmons. More particularly, the invention contemplates the provision of a compatible intermediate layer between the directionally solidified eu tectic type substrate and incompatible coating, such as a layer of a transition metal such as cobalt, a metal of the nickel family or a nickel cobalt alloy preferably Ni, Co or Ni-Co alloy.
In one embodiment, the inventive process is directed toward depositing an oxidation erosion resistant aluminide coating on a Ni3Cb reinforced nickel rich directionally solidified eutectic comprising, in sequence, the steps of depositing an intermediate coating layer of nickel, cobalt or Ni-Co alloy on the substrate, diffusing the nickel, cobalt or Ni-Co alloy into the eutectic alloy substrate either as a separate step or as part of the subsequent chromizing step, chromizing the surface of the intermediate layer, diffusing the chromium into the intermediate layer either as a separate step or as part of the subsequent aluminizing step to establish a concentration gradient across the nickel, cobalt or Ni-Co alloy layer with the initial chromium content at the intermediate layer eutectic interface not exceeding the maximum solubility of chromium in the eutectic phases, aluminizing the surface of the chromium modified intermediate layer including a diffusion heat treatment at a temperature sufiicient to effect an alloying reaction to form NiAl (or CoAl) containing chromium with excess chromium, if any, precipitated therein.
The coating system of the present invention is particularly satisfactory for the NiSCb reinforced nickel rich directionally solidified eutectics, especially those selected from the group consisting of Ni3Al-Ni3Cb, Ni-Ni3Cb, (Ni, Cr)-Ni3Cb and Ni-Ni3Al-Ni3Cb. In the process a thin layer of nickel, cobalt or nickel-cobalt alloy preferably 0.5-3.0 mils thick, is deposited on the eutectic by any suitable method, such as pack cementation, electrophoretic, dlame or plasma spraying, vapor deposition or electroplating, although the latter two techniques are preferred. The coated part is then heat treated for 1-16 hours at 13002000 F. to diffuse the nickel, cobalt or Ni-Co alloy into the eutectic alloy substrate, although, as will be appreciated, this diffusion heat treatment may be eliminated as a sepaarte step if it is an integral part of the following step, such as is the case with a following pack cementation step.
In the second step, the coated substrate is chromized; that is, exposed to a source of chromium so that the composition of the coating is changed from that of pure nickel, cobalt or nickel-cobalt alloy, to an alloy containing Cr. Initially, the chromium is deposited by one of the same processes employed for the deposition of the intermediate coating metal as previously described, with the preferred technique being that of pack cementation. The chromium coated part is then heat treated, either separately or as part of the subsequent step, for 1 16 hours at 13002000 F. to diffuse the chromium into 75 the intermediate coating metal layer with the Cr content being graded so that it is greatest at the outer surface and is virtually nil at the intermediate coating metal-eutectic interface. It is important that the concentration gradient be established across the intermediate metal-coating so that the initial Cr content at the intermediate metaleutectic interface does not exceed the maximum solubility of Cr in the eutectic phases.
In the third step, the chromium modified intermediate metal coated substrate is aluminized; that is, exposed to a source of aluminum so that Al is diffused into the Cr modified intermediate metal coating. Aluminum is deposited by a suitable technique such as vapor deposition, flame or plasma spray, electrophoresis, electroplating, slurry coating or pack cementation, with the latter being preferred. Subsequently, the aluminum coated part is heated for 4-16 hours at 16002000 F. to diffuse the Al into the Cr modified intermediate metal coating to form the intermetallic NiAl or CoAl which is preferably saturated, or nearly saturated, with Cr or any excess Cr being precipitated in the NiAl or CoAl matrix.
The coating formed by the inventive process is free of any aligned biphase structure and forms a buffer zone between the coating and the substrate.
BRIEF DESCRIPTION OF THE DRAWINGS An understanding of the invention will become more apparent to those skilled in the art by reference to the following detailed description when viewed in light of the accompanying drawings, wherein:
FIG. 1 is a photomicrograph showing, in longitudinal section, the microstructure of an aluminized directionally solidified Ni3Al-Ni3Cb eutectic (200X before reduction);
FIG. 2 is a photomicrograph showing, in transverse section, the microstructure of the aluminized eutectic of FIG.
l (200X before reduction);
FIG. 3 is a photomicrograph showing, in transverse section an undesirable Ni-Cr-Al-Cb phase at the interface of a directionally solidified NisAl-NiaCb eutectic and a Cr-NiAl coating (500x before reduction); and
FIG. 4 is a photomicrograph showing, in transverse section, the microstructure of a directionally solidified NiaAl-NiaCb eutectic having an intermediate Ni layer provided between the eutectic aluminide and the coating (500 before reduction).
DESCRIPTION OF THE PREFERRED EMBODIMENTS As previously described, the inventive process relates to a method for bonding an oxidation erosion resistant metallic coating, such as an aluminide, CoCrAl, FeCrAl, NiCrAl or CoCrAl, FeCrAl, NiCrAl modified by a rare earth metal addition such as yttrium coating to a directionally solidified eutectic type alloy by first providing on the eutectic substrate a relatively ductile intermediate layer of chemically compatible metal. Use of the intermediate layer between the coating and the substrate not only prevents formation of phase boundaries in the coating which are associated with the interphase boundaries of the eutectic but also substantially reduces the likelihood of crack propagation from the coating into the substrate. It further provides a zone for the required deformation in the case of diffusion bonding.
While not considered essential, it s preferred, prior to depositing the intermediate layer, to heavily etch the substrate to selectively remove one phase of the alloy to a depth on the order of the interphase spacing. This is particularly beneficial if intermediate layer deposition is achieved by electroplating. In this way, the composite microstructure is used to enhance the mechanical bond of the applied layer; the unetched phase protruding into the applied layer to act as a key. Suitable etchants will of course vary with the composition of the eutectic. 'For the NiaCb reinforced nickel rich eutectics, HNOa-HF-HZO solutions will give satisfactory results.
The protective intermediate layer is deposited onto the substrate by any suitable technique, such as by vapor deposition, pack cementation, electrophoretic, ame or plasma spraying or by the electrochemical processes, the latter being preferred. The intermediate layer must be compatible with the eutectic substrate in the sense that it must be able to be in thermodynamic equilibrium therewith and thus remain in contact with the phases which comprise the substrate without forming any deleterious phases. Intermediate layers considered suitable are such metals as Ni, Co, Ni-Co alloys, Fe and the precious metals. While the thickness of the layer may vary, a layer approximately 0.5-3.0 mils thick is considered satisfactory.
After the intermediate layer is deposited, it may be subjected to a separate diffusion heat treatment step in order to diffuse the metal of the layer into the substrate prior to the application of the oxidation erosion resistant coating. If, however, the following step (the first step in the application of the oxidation erosion resistant coating) incorporates such an effect, as in the case of a pack cementation step in the application of an aluminide coating system or even in the case of a vacuum vapor coating process for applying a FeCrAl, CoCrAl or NiCrAl type coating as taught, for example, in U.S. 3,639,151 to Krutenat, the separate diffusion heat treatment may be omitted. By virtue of the intermediate layer, the coating is formed free of any aligned Lbiphase structure and a buffer zone is formed between the coating and the directionally solidified eutectic alloy substrate. Thus it will be apparent that those protective coatings which contain elements which, if allowed to directly contact the substrate, would otherwise form deleterious phases are prevented from doing so. Thus, for example, to provide an aluminide coating based on the Ni-Cr-Al system on a eutectic which will form a deleterious phase upon contact with Cr or with an excess of Al, as in the case of the detailed example below, it is necessary to provide the compatible layer, in this case nickel, prior to chromizing or aluminizing. The same need for a buffer layer is true for the application of a NiCrAlY, CoCrAlY or FeCrAlY coating.
In one investigation, a directionally solidified NisAl-NiaCb lamellar eutectic was coated with a 0.5-3.0 mil thick layer of nickel, following mechanical cleaning, by an electrochemical process comprising plating in a Watts bath at 25 amps/ft.2 at 100 F. for approximately one hour. While the technique of electroplating is preferred, it is not critical and any suitable method, e.g., vapor deposition, pack cementation, electrophoretic or ame or plasma spraying, is considered satisfactory.
The electroplated nickel coating was heat treated for four hours at 1975 F., which is preferred, although treatment for 1-16 hours at 13002000 F. is considered satisfactory. Next, the nickel plated part was chromized by a pack cementation technique. Although there are many suitable packs, one comprising, by weight, 25-75% A1203, 25-75% Cr and 0.1-l% NH4C1 is preferred. In the present case, the part was packed in a granular mixture of, by weight 50% A1203, 49.5% Cr and 0.5% NH4Cl, then heat treated in an argon environment for four hours at 2000 F. The chromizing may also be carried/out by any 0f the suitable techniques mentioned above in connection with the nickel plating, followed by a heat treatment in a noncontaminating atmosphere for 1-16 hours at 13002000 F. The key here is to diffuse the chromium into the applied metal layer with the gradient being such that its Cr content is substantially nil at the nickel eutectic interface.
The part was finally aluminized by a pack cementation step wherein it is preferred that the pack mixture comprise, by weight, approximately 75-95% A1203, 5-25% Al and 0.1-1% NHiCl. In this example, the part was heat treated for two hours at 1400 F. in a granular pack containing, by weight, 89.5% A1203, 10.0% Al and 0.5 NHgCl. Subsequently, the part was removed from the pack and heated for eight hours at 1975 F. in argon. The
aluminum may also be deposited satisfactorily by vapor deposition, flame or plasma spraying, electrophoresis, electroplating or slurry coating and the diffusion heat treatment may last from 4-16 hours at 1600-2000 F.
The resulting coating was free of any aligned biphase structure. The coating, which comprised a ductile intermediate layer with the oxidation erosion resistant layer superimposed thereon was subjected to electron microbeam probe analysis. The intermediate layer consisted essentially of gamma nickel as its principal component with minor amounts of Al, Cb and Cr in solid solution. The concentration of Al was relatively uniform across the coating at 3.6 li- 0.5 by weight. The Cr was present as a concentration gradient of substantially zero at the nickeleutectic interface to approximately 7% at the interface with the outer layer while the Cb was present at approximately 18% at the eutectic interface and approximately 1% at the outer layer interface. The outer layer consisted essentially of nickel aluminide (NiAl) saturated with chromium in solid solution and with the excess chromium precipitated out and distributed therethrough as alpha chromium. The composition, by weight, of the outer coating was: 18.2% Al, 7.2% Cr, trace amount of Cb, balance Ni.
The coated eutectic was subjected to oxidation erosion tests at 2100 F. and did not fail after 250 hours. The coating exhibited oxidation erosion resistance generally as good as conventional aluminide coatings on Ni base superalloys. The coated part was also subjected to diffusional stability tests and, after isothermal exposure showed substantially no increase in outer and intermediate coating layer thickness. Overal diffusional stability was deemed excellent and at least as good or better than most conventional aluminide coatings on Ni base superalloys. The coated member was additionally subjected to a severe mechanical treatment test whereby cracks were produced in the outer coating layer by abrasive wheel cutting of the part. The cracks would not, however, propagate across the intermediate buffer layer. In this regard, it should be noted that the buffer layer also exhibited good oxidation resistance so that even if such cracking did occur during engine operation, oxidation would not rapidly ensue.
In another investigation, a directionally solidified NiaAl-NigCb lamellar eutectic was coated with a two mil thick layer of cobalt and then processed in a manner identical to the detailed example described above. The resulting coating was free of any aligned biphase structure and comprised a ductile intermediate layer consisting essentially of cobalt with minor amounts of Al, Cb and Cr in solid solution. The outer layer consisted essentially of CoAl saturated with chromium in solid solution with excess chromium being precipitated out and distributed throughout the CoAl. The coated eutectic was subjected to diffusional stability tests and, after isothermal exposure showed substantialy no increase in outer and intermediate coating layer thickness. Overall diffusional stability was deemed excellent.
What has been set forth above is intended primarily as exemplary to enable those skilled in the art in the practice of 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.
What we claim is: 1. A method for depositing an oxidation erosion resistant coating on a nickel base directionally solidified eutectic type alloy comprising, in sequence, the steps of: depositing an intermediate coating layer of nickel, co-
balt or nickel-cobalt alloy on said substrate;
diffusion heat treating the intermediate metal layer either separately or as part of the subsequent chromizing step to diffuse the intermediate metal into the eutectic alloy substrate;
chromizing the surface of the intermediate coating layer;
diffusion heat treating either separately or as part of the subsequent aluminizing step to establish a concentration gradient across the intermediate layer with the initial chromium content at the intermediate metal-eutectic interface not exceeding the maximum solubility of chromium in the eutectic phases; aluminizing the surface of the chromium modied intermediate metal layer including a diffusion heat treatment at a temperature and for a time sufficient to effect an alloying reaction to form on an outer portion of said intermediate layer an outer layer of NiAl or CoAl substantially saturated with chromium and having any excess chromium precipitated therein.
2. The method of claim 1 wherein said nickel base directionally solidified eutectic type alloy comprises a Ni3Cb reinforced nickel rich directionally solidified eutectic.
3. The method of claim 1 wherein said eutectic alloy substrate is etched prior to deposition of said intermediate metal.
4. The method of claim 3 wherein said intermediate metal layer is deposited to a thickness of 0.5-3.0 mil.
5. The method of claim 4 wherein the intermediate layer is nickel and the diffusion heat treatment of the nickel is conducted in a noncontaminating atmosphere at 13002000 F. for 1-16 hours.
6. The method of claim 5 wherein the diffusion heat treatment of the chromized nickel layer is conducted in a noncontaminating atmosphere at 13002000 F. for l- 16 hours.
7- The method of claim 6 wherein the diffusion heat treatment of the aluminized layer is conducted in a noncontaminating atmosphere at 16002000 F. for 4-16 hours.
8. A method for imparting oxidation erosion resistance at temperatures above 2000 F. to a directionally solidified eutectic selected from the group consisting of Ni3A1-Ni3Cb, Ni-NiaAl-NiaCb and (Ni, Cr)-Ni3Cb comprising, in sequence, the steps of:
depositing a layer of nickel 0.5-3.0 mil thick thereon;
diffusion heat treating in a noncontaminating atmosphere for approximately 1-16 hours at approximately l300-2000 F. either separately or as part of the following chromizing step: chromizing the surface of the nickel layer by embedding the nickel coated eutectic in a pack mixture of approximately, by weight, 25-75% A1203, 25-75% Cr and O.l-1% NH4Cl and heat treating in a noncontaminating atmosphere for a time and at a temperature sufficient to establish a concentration gradient across the nickel layer with the chromium content at the nickel eutectic interface not exceeding the maximum solubility of chromium in the eutectic phases and the chromium content at the outer surface of the nickel being no greater than approximately 7%, by weight;
aluminizing the surface of the chromized nickel layer by embedding the coated substrate in a pack mixture of approximately, by weight, 75-95% A1203, 5-25% A1 and 0.l-l% NI-LlCl for approximately two hours termediate layer including minor amounts of Al and Cr O present in solid solution, the Cr being present in a concentration gradient across said intermediate layer with the initial Cr content at the intermediate metal-eutectic interface not exceeding the maximum solubility of Cr in the eutectic phases, and an oxidation erosion resistant outer coating layer bonded to said intermediate metal layer selected from the group consisting of the aluminides, CoCrAl, FeCrAl, NiCrAl and CoCrAl, FeCrAl or NiCrAl with a rare earth element addition.
10. The article of claim 9 wherein said substrate is a directionally solidified eutectic selected from the group consisting of Ni3Al-Ni3Cb, Ni-NigAl-NiCb, Ni-Ni3Cb and (Ni, Cr)-Ni3Cb.
11. The article of claim 9 wherein said intermediate metal layer is selected from the group consisting of Ni, Co and Ni-Co alloys.
12. The article of claim 11 wherein said substrate is a directionally solidied NisCb reinforced nickel rich eutectic alloy.
13. The article of claim 12 -wherein said intermediate layer consists essentially of gamma nickel having minor amounts of aluminum, columbium and chromium present in solid solution.
14. The invention of claim 13 wherein said nickel layer contains, in solid solution, approximately, by weight, 3.6% Al, a concentration gradient extending from the nickeleutectic interface to the outer nickel layer surface of approximately 07% Cr and l8-1% Cb.
1S. The invention of claim 14 wherein said nickel layer is 0.5-3.0 mils thick.
16. The invention of claim 15 wherein the outer coating consists essentially of approximately, by weight, 7.2% Cr, 18.2% Al, balance nickel, with trace amounts of Cb.
References Cited UNITED STATES PATENTS 3,676,085 7/1972 Evans et al 29-197 3,649,225 3/ 1972 Simmons 29-196.6 3,647,517 3/1972 Milidantri et al. 117-107.2 P 3,640,815 2/1972 Schwartz et al. 117-107.2 P 3,564,940 2/1971 Thompson et al. 75-170 3,554,817 1/1971 Thompson et al. 148-32 3,338,733 8/1967 Rowady 117-50 3,290,126 12/1966 Monson 117-107.2 P
CHARLES E. VAN HORN, Primary Examiner M. BALL, Assistant Examiner U.S. C1. X.R.