Patent Application
20100086680
The present disclosure generally relates to techniques for coating an internal surface of an article.
Nickel- and/or cobalt-based superalloys effective for use in high temperature environments are commonly used by the aerospace and power industries. While such alloys are inherently strong, they are generally not particularly resistant to corrosion by high temperature gases. Corrosion and oxidation-resistant coatings are therefore often applied to the superalloys to improve corrosion and/or oxidation resistance.
Internal coatings to protect those superalloy parts can be applied by several different methods. For example, chemical vapor deposition (CVD) methods using a reactor to generate a reactive Al-bearing species, or CVD methods that use above-the-pack technology, have been used. Both methods are generally effective, but require a sizable investment in equipment and the like. Moreover, in the case of above-the-pack processes, there is a significant amount of waste generated, including corrosive gases that require scrubbers to capture active species and/or convert them to innocuous products.
In general, the disclosure is directed to a mixture and techniques for coating an internal cavity of an article. In some embodiments, the article may include a turbine vane or turbine blade, and may be formed of a superalloy substrate comprising Ni or Co. The article may include one or more cavities, which may or may not be interconnected. The coating techniques may generally include introducing a mixture into a cavity of the article and heating the article and cavity. The mixture may be in dry powder form, or may be a slurry or suspension in a liquid carrier, and may include an aluminum source and an organo halocarbon activator. When heated, the organo halocarbon activator may decompose and release a halogen species, which reacts with aluminum in the aluminum source to form an aluminum halide. The aluminum halide may then diffuse to a surface of the cavity, where the aluminum reacts with and is deposited on the surface.
In one aspect, the present disclosure is directed to a method including introducing a mixture comprising an aluminum source and an organo halocarbon activator into an internal cavity of an article, and heating the article and the mixture to a temperature sufficient to form an aluminum halide, which deposits on a surface of the internal cavity to form a coated article.
In another aspect, the present disclosure is directed to a method including depositing on an external surface of an article a first layer comprising Pt, Si, and a reactive element selected from the group consisting of Hf, Y, La, Ce, Zr, and combinations thereof, introducing a mixture comprising aluminum and an organo halocarbon activator into an internal cavity of the article, and depositing a second layer comprising Al on the first layer to form an alloy including a γ-Ni+γ′-Ni3Al phase constitution, where the second layer is deposited with the organo halocarbon activator.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.
In one aspect, the present disclosure is generally directed to techniques for depositing a coating on a surface of a cavity of an article. The coating may be an aluminide coating and may be deposited with an organo halocarbon activator. In some embodiments, the coating on the surface of the cavity of the article may be applied substantially concurrently with application of a coating to an external surface of the article.
The coating may be applied to any article, and may be particularly well suited for application on superalloy articles including Ni and Co. An article including any conventional superalloy may be used, including, for example, those available from Martin-Marietta Corp., Bethesda, Md., under the trade designation MAR-M247; those available from Cannon-Muskegon Corp., Muskegon, Mich., under the trade designation CMSX-4, CMSX-10; and the like. Typical superalloy constituents such as, for example, Cr, Co, Mo, Ta, Ti, W, Re, and combinations thereof, may optionally be added to or present in the alloy composition. In some embodiments, the article may be component of a high temperature mechanical system, such as, for example, a turbine blade or turbine vane that is used in a gas turbine engine.
In the embodiment illustrated in
Turbine blade 100, and more specifically airfoil 102, may also include a plurality of trailing edge exit slots 116. As is best seen in
Airfoil 102 further includes a plurality of film cooling holes 118. First film cooling hole 118a is in fluid communication with first cavity 114a, while second film cooling hole 118b and third film cooling hole 118c are in fluid communication with second cavity 114b and third cavity 114c, respectively. The plurality of film cooling holes 118 may be located proximate tip 120 of blade 100, as shown in
As described briefly above, turbine blade 100 may be a component of a high temperature mechanical system, such as a gas turbine engine. When used in a high temperature mechanical system, blade 100 may experience high temperatures and may be prone to corrosion or oxidation. Thus, turbine blade 100 may include a protective coating that reduces or substantially eliminates the effects of oxidation or corrosion on blade 100. For example, leading edge 106, trailing edge 108, pressure sidewall 110 and suction sidewall 112 may be coated with a thermal barrier coating (TBC), which is a porous ceramic coating that provides thermal insulation for exterior surfaces of blade 100. The TBC may be attached to blade 100 by a bond coat, which in some embodiments may be a coating including a γ-Ni+γ′-Ni3Al phase constitution. In the present application, a γ-Ni+γ′-Ni3Al coating refers to a coating that includes a Pt-group metal, Ni and Al in relative concentration such that a γ-Ni+γ′-Ni3Al phase constitution results in some portion of the coating. In some embodiments, the concentration of Al is limited with respect to the concentration of Ni and the Pt-group metal such that substantially no β-NiAl phase structure, preferably no β-NiAl phase structure, is present in the alloy, and the γ-Ni+γ′-Ni3Al phase constitution predominates. Exemplary alloys and coatings with a γ-Ni+γ′-Ni3Al phase constitution are described in, for example, U.S. Pat. No. 7,273,662, which is incorporated by reference herein in its entirety.
While external coatings including a γ-Ni+γ′-Ni3Al phase constitution are generally preferred, the methods described herein may be used to make external coatings including up to about 33 at. % Al, which results in a coating having a substantially β-NiAl phase structure. Thus, while this disclosure is directed in part to producing external coatings having a γ-Ni+γ′-Ni3Al phase constitution, the invention is not limited to such coatings.
Cavities 114 may also require protection from oxidation and/or corrosion. In many embodiments, the surfaces of cavities 114 may not receive a TBC, and may instead receive a protective aluminide coating, which oxidizes to form an aluminum oxide coating that resists further oxidation or corrosion. As described in further detail hereinafter, the aluminide coating may be applied to the surface of cavities 114 by introducing a mixture including an aluminum source, an organo halocarbon activator and, optionally, filler into the cavities 114 and heating blade 100 and the mixture. In some embodiments, the mixture may be in dry powder form, while in other embodiments, a liquid carrier may carry the mixture in a suspension or slurry.
The technique further includes introducing a mixture into the cavities 114 of the article (204). The mixture may include an aluminum source, an organo halocarbon activator and, optionally, a filler. In some embodiments, the mixture may be in powder form, while in other embodiments, the mixture may be carried in a liquid carrier.
The aluminum source may include elemental Al, or may include alloy powders such as, for example, (by weight) 55Al:45Cr, 42Al:40Cr:18Mn, 10Al:30Cr, other aluminum-chromium alloys, organo-aluminum compounds, and the like.
The organo halocarbon activator may act as an activator for the aluminum during a subsequent heat treatment step. The organo halocarbon may include, for example, fluoropolymers, choropolymers, and the like.
The organo halocarbons are preferably compatible with the liquid carrier in the suspension and can preferably be produced in granular form. One preferred fluoropolymer is polytetrafluoroethylene (PTFE), which may be obtained from DuPont Fluoroproducts, Wilmington, Del., under the trade designations DuPont Teflon PTFE 60 and DuPont Zonyl MP-1200, which is a micronized PTFE powder.
The mixture may also include filler. The filler may be a non-reactive oxide powder, such as aluminum oxide, zirconium oxide, titanium oxide, hafnium oxide, and the like. The size of the filler particles may vary, and may affect the ease with which the mixture infiltrates the cavity of the article. The size of the filler particles may also influence the ease with which any residual mixture is removed from cavities 114 after completion of the coating technique. For example, relatively larger filler particles may facilitate removal of the residual mixture from cavities 114, while relatively smaller filler particles may make removal of the residual mixtures from cavities 114 more difficult. On the other hand, relatively smaller filler particles may improve the ease with which the mixture infiltrates cavities 114 of blade 100 compared to relatively larger filler particles, such as, for example, when the cavities 114 include portions having a small diameter, serpentine path, or other restrictions. For example, an average size of the filler particles may range from about 1 micron (14,000 grit) to about 160 microns (90 grit). Other suitable filler particles sizes may include about 135 microns (120 grit), about 115 microns (150 grit), and about 90 microns (220 grit).
When the mixture is carried in a liquid carrier, such as in a suspension or slurry, the liquid carrier may vary widely so long as it is compatible with the solids therein. In some embodiments, an organic solvent or mixture of organic solvents may be used. In addition, the liquid carrier may be essentially anhydrous, i.e., may be essentially free of water. Preferred liquid carriers also have at least one of a high specific gravity and a high dielectric constant. The liquid carrier may dissolve an electrophoretically active agent, which is described below, up to a concentration of about 5 g/L. Additionally, it may be preferred that the liquid carrier does not react with an activating agent, such as those described in further detail below.
Example liquid carriers may include, for example, nitroalkanes, alcohols containing about 1 to about 6 backbone carbons, glycols, and the like. In some embodiments, the liquid carrier may include a mixture of isopropanol and nitromethane in a ratio from about 55 wt. % isopropanol and 45 wt. % nitromethane to about 65 wt. % isopropanol and about 35 wt. % nitromethane. One exemplary liquid carrier may include about 60 wt. % isopropanol and about 40 wt. % nitromethane.
When the mixture is carried in a liquid carrier, the suspension or slurry may also include an electrophoretically active agent such as zein, gliadin, synthetic methyl cellulose, and the like. The electrophoretically active agent, in conjunction with an activating agent, may form a chelated complex, which has a positive charge, with the aluminum source in the suspension. When present in the suspension or slurry, the electrophoretically active agent may be present at a concentration of about 1 g/L to about 30 g/L. In some embodiments, the electrophoretically active agent may be present at a concentration of about 1 g/L to about 3 g/L, and the electrophoretically active agent may be present in the mixture at a concentration of about 2.2 g/L.
The suspension or slurry may further include an activating agent. In some embodiments, the activating agent may include a transition metal nitrate having a divalent (+2) charge that can form a chelated complex with the electrophoretically active agent. In some embodiments, preferred activating agents include nickel nitrate hexahydrate, cobalt nitrate hexahydrate, and the like. When present in the suspension or slurry, the activating agent may be present at a concentration of about 0.08 g/L to about 0.20 g/L, and in some embodiments, may be present at a concentration of about 0.14 g/L.
When the mixture is carried in a liquid carrier, the suspension or slurry may include any total solids content that provides a slurry or suspension of a desired consistency (e.g., a consistency that allows the slurry or suspension to be injected into the cavity). For example, the mixture may include a total solids content of about 15 g solids per L liquid carrier to about 2000 g solids per L liquid carrier. In some embodiments, about 100 g solids may be mixed with 0.050 L of a zein solution to form a slurry.
Typically, the powder mixture, or the solids in the suspension or slurry, may include about 0.5 wt. % to about 25 wt. % of the Al source, about 1 wt. % to about 10 wt. % of the organo halocarbon activator, and the balance filler. In some embodiments, the powder mixture, or the solids in the slurry or suspension, may include about 1 wt. % to about 6 wt. % of the Al source, about 1 wt. % to about 10 wt. % of the organo halocarbon activator, and the balance filler. By adjusting the composition of the powder mixture or solids in the suspension or slurry, the thickness of the resulting coating on the surface of cavities 114 may be controlled. For example, higher aluminum source content may result in a thicker coating on the surface of cavities 114, while lower aluminum source content may result in a thinner coating on the surface of cavities 114.
Once the mixture is introduced into the cavities 114 of turbine blade 100, blade 100 and the mixture are heated (206). While not wishing to be bound by any theory, presently available evidence indicates that the heating may cause the organo halocarbon activator to decompose and release a halogen species. The halogen species may react with aluminum in the aluminum source to form an aluminum halide, thus activating the aluminum. The aluminum halide may the diffuse to the surface of cavities 114, and the aluminum may react with and be bound to the surface. The reaction of the aluminum with the surface of cavities 114 liberates the halogen, which may then react with and activate another aluminum atom in the aluminum source. Thus, the heat treatment step can bind the aluminum to the surface of cavities 114. Further, heating blade 100 and the mixture may allow aluminum to diffuse into the surface of cavities 114.
At least some of the aluminum halides formed by the reaction of the halogen species with aluminum in the aluminum source may be present in a vapor phase. This may facilitate more uniform coating of the surface of cavities 114. For example, cavities 114 may include portions which have a small diameter, are serpentine or otherwise restricted, and which do not come in intimate contact with the mixture, in powder, slurry, or suspension form. In conventional processes, this may lead to reduced coating thicknesses in these portions, or even no coating in these portions. However, in techniques according to the present disclosure, the formation of vapor phase aluminum halides allows aluminum to be deposited on surfaces of cavities 114 that are not contacted with the powder, slurry or suspension mixture. Accordingly, coatings formed by the techniques of the current disclosure may be deposited on portions of the surface of cavities 114 on which conventional techniques do not deposit a coating. Additionally, coatings formed by the techniques of the current disclosure may have a more uniform thickness than coatings formed by conventional techniques.
By selecting the heating time and heating temperature of turbine blade 100 and the mixture, the total amount of Al bound to the surface of cavities 114 can be controlled. In some embodiments, turbine blade 100 and the mixture may be heated to about 1400° F. to about 1975° F. for about 1 hour to about 4 hours activate the organo halocarbon activator and deposit the aluminum on the surface of cavities 114. In some embodiments, the heat treatment may be carried out in an oven under an inert atmosphere, such as a partial pressure (e.g., about −5 inches Hg) of argon.
In some embodiments, the aluminum coating is further heat treated to diffuse the aluminum into the surface of cavities 114, as will be described in further detail below with respect to
Masking of at least some of trailing edge exits slots 116 and/or film cooling holes 118 may also prevent the aluminum halide vapor formed by the reaction of the aluminum in the aluminum source and the organo halocarbon activator from escaping cavities 114 when mixture 404 and blade 100 are heated (e.g., step 106 of
In some embodiments, masking material 402 may include a thermal or UV curable resin. For example, Testors cement (available from The Testors Corporation, Rockford, Ill.), Dymax 706-H (available from Dymax Corporation, Torrington, Conn.) or a RTV (Room Temperature Vulcanizing) rubber may be applied to at least some of trailing edge exit slots 116 and/or film cooling holes 118 and cured using thermal or UV energy.
In some embodiments, it may be desired that the masked areas also receive some form of aluminum or aluminide coating. In embodiments such as these, masking material 402 may be mixed with an aluminum source. The aluminum source may include, for example, elemental Al, or may include alloy powders such as, for example, (by weight) 55Al:45Cr, 42Al:40Cr:18Mn, 10Al:30Cr, other aluminum-chromium alloys, organo-aluminum compounds, and the like. The aluminum source may be mixed into a slurry including masking material 402, applied to at least some of trailing edge exit slots 116 and/or film cooling holes 118, and cured. When heated, the aluminum from the aluminum source may diffuse to the surface of the masked areas and be deposited on the surface, forming a coating. Further details regarding mixing masking material 402 with an aluminum source may be found in U.S. Pat. No. 6,586,052, the contents of which are incorporated herein by reference in their entirety.
Masking material 402 may be applied to at least some of trailing edge exit slots 116 and/or film cooling holes 118, and the surfaces of turbine blade 100 adjacent trailing edge exit slots 116 and/or film cooling holes 118 may be cleaned to remove any excess masking material 402. Masking material 402 may then be cured using heat or UV radiation, and the surfaces of turbine blade 100 adjacent the cured masking material 402 may be cleaned with a light grit blasting following the cure. The grit blasting may be performed using aluminum oxide or another abrasive.
Once at least some of trailing edge exit slots 116 and/or film cooling holes 118 have been masked, a mixture 404 is injected into the cavities 114 (304), as illustrated in
When mixture 404 is carried in a liquid carrier as a suspension or slurry, cavities 114 may be only partially filled with the suspension or slurry and the blade 100 may be rotated or agitated to coat the surfaces of cavities 114. The excess suspension or slurry may then be allowed to run out metering plate orifices 122. This may reduce the amount of mixture 404 required to form the coating on the surfaces of cavities 114. In other embodiments, cavities 114 may be substantially filled with mixture 404 when mixture 404 is carried in a liquid carrier.
Once mixture 404 has been introduced into cavities 114, metering plate orifices 122 are masked with masking material 402 (306). Similar to masking at least some of trailing edge exit slots 116 and/or film cooling holes 118, masking metering plate orifices 122 may prevent the aluminum halide vapor formed by the reaction of the aluminum in the aluminum source and the organo halocarbon activator from escaping cavities 114 when mixture 404 and blade 100 are heated (e.g., step 106 of
In some embodiments, masking material 402 may include a thermal or UV curable resin. For example, Testors cement (available from The Testors Corporation, Rockford, Ill.) or Dymax 706-H (available from Dymax Corporation, Torrington, Conn.) may be used to mask metering plate orifices 122 and be cured using thermal energy, UV energy, or a combination of thermal energy and UV energy. In some embodiments, the same masking material 402 is used to mask trailing edge exit slots 116, film cooling holes 118 and metering plate orifices 122, while in other embodiments, a different masking material 402 is used to mask metering plate orifices 122 than is used to mask trailing edge exit slots 116 and film cooling holes 118.
Once metering plate orifices 122 are masked, turbine blade 100 and mixture 404 are heated, as described above with reference to step 206 of
The solids in suspension 606 may include a Pt-group metal such as, for example, Pt, Pd, Ir, Rh, Ru, and combinations thereof. The solids in suspension 606 may further include at least one reactive element such as, for example, Hf, Y, La, Ce, Zr and combinations thereof. In some embodiments, the Pt-group metal may include Pt and the reactive element may include Hf. The addition of such reactive elements tends to stabilize the γ′ phase in the final external coating composition. Therefore, if sufficient reactive metal is incorporated into the external coating composition, the resulting phase constitution may be predominately γ′, or solely γ′. In addition, the solids in the suspension 606 may also include Si.
In some embodiments, the solids may be prepared by forming an alloy including the Pt-group metal, reactive element, and Si. The alloy may then be ground into a powder and deposited in the suspension 606. The alloying and subsequent grinding may result in an alloy powder having a substantially homogeneous composition of Pt-group metal, reactive element, and Si.
Grinding may result in an alloyed powder that includes particles with a size distribution. That is, not all particles in the alloyed powder are necessarily the same size (e.g., diameter). The alloyed powder may include a relatively wide range of particle sizes, depending on the parameters of the electrophoretic deposition process. In certain embodiments, the size of a majority of particles in the alloyed powder may range from about 1 μm to about 20 μm, and more preferably from about 5 μm to about 10 μm. Grinding may also introduce small amounts of impurities into the alloyed powder, which may include, for example, Al and O, primarily in the form of Al2O3.
The alloyed powder is preferably substantially free of impurities, but may include up to about 5 wt. % impurities. The alloyed powder also preferably includes less than about 3 wt. % O2. If oxygen impurities combine with the reactive element in the alloy, such as Hf, the Hf may not be fully incorporated into the substrate during diffusion heat treatment. Special precautions such as using a dry box with high purity argon during the preparation of the alloy mixture, pressing the alloy mixture into a compact, or performing the alloying in extremely air-tight furnace that eliminates traces of oxygen during the alloying process have been shown to be beneficial in producing alloy powders containing Pt, Si, and Hf. Additionally, a stepped heat treatment used during alloying is useful in producing a homogenous alloy.
In one embodiment, to provide a Pt-group-metal modified γ-γ′ alloy in first layer 616, the solids in suspension 606 may include about 84 wt. % to about 93 wt. % Pt-group metal, about 3 wt. % to about 12 wt. % Si, and about 2 wt. % to about 6 wt. % of the at least one reactive element, based on the total weight of solids in suspension 606.
The total solids content in suspension 606 may be about 15 grams solids per liter liquid carrier to about 40 grams solids per liter liquid carrier, preferably about 30 grams solids per liter liquid carrier.
The liquid carrier in suspension 606 may vary widely so long as it has compatibility with the solids therein, and a variety of organic solvents may be used. The liquid carrier may be essentially anhydrous, i.e., may be essentially free of water. Preferred liquid carriers also have at least one of a high specific gravity and a high dielectric constant. Preferred liquid carriers also dissolve an electrophoretically active agent up to a concentration of about 5 g/L. Additionally, preferred liquid carriers do not react with an activating agent such as those described in further detail below.
In one embodiment useful for making γ-Ni+γ′-Ni3Al alloys, the liquid carrier may include about 55 wt. % to about 65 wt. % isopropanol and about 35 wt. % nitromethane to about 45 wt. % nitromethane, more preferably about 60 wt. % isopropanol and about 40 wt. % nitromethane.
Suspension 606 may further include an electrophoretically active agent such as zein, gliadin, synthetic methyl cellulose, and the like. The electrophoretically active agent, in conjunction with an activating agent, may form a chelated complex with the metals in suspension 606 that has a positive charge. The electrophoretically active agent may be present in suspension 606 at about 1 g/L to about 3 g/L, more preferably about 2.2 g/L.
Suspension 606 may further include an activating agent. Suitable activating agents include transition metal nitrates having divalent (+2) charges that can form a chelated complex with the electrophoretically active agent. In some embodiments, the activating agents may include nickel nitrate hexahydrate, cobalt nitrate hexahydrate, and the like. The activating agent may be present in the suspension at a concentration from about 0.08 g/L to about 0.20 g/L, and a concentration of about 0.14 g/L is suitable.
The chelated complex formed by the electrophoretically active agent and the activating agent may surround the metal particles in suspension 606 and impart a slight positive charge that causes the encased particle to migrate to and deposit on the negatively charged turbine blade 602 (i.e., the cathode).
In some embodiments, electrophoretic deposition of the Pt-group metal, reactive element, and Si may be carried out for up to about 6 minutes at a current density of about 1 to about 2 mA/cm2 between the anode and cathode (turbine blade 602). Suitable voltages may range from about 35 V to about 250 V, and may be chosen to provide the desired current density. The necessary voltage may be influenced by the composition of electrophoretic bath 608 and the distance between the anode and cathode (turbine blade 602). The suspension may be maintained by mechanical agitation including, for example, an impeller 610, as describe above. Further details regarding electrophoretic deposition processes are provided in U.S. Pats. No. 5,057,196 and 5,958,204 both to Creech et. al, which are incorporated herein by reference in their entirety. The weight of first layer 616 deposited on the exterior surface of turbine blade 602 may range from about 5 mg/cm2 to about 30 mg/cm2, preferably about 10 mg/cm2 to about 20 mg/cm2.
After completion of the electrophoretic deposition of the Pt-group metal, Si and at least one reactive element, turbine blade 602 including first layer 616 is removed from electrophoretic bath 608. Turbine blade 602 coated with first layer 616 may then be subjected to a heat treatment step (not shown). The heat treatment enables the Pt-group metal, Si, and at least one reactive element present in layer 616 to diffuse into the substrate and the substrate elements to diffuse into the first layer 616. The heat treatment step may be performed at temperatures ranging from about 1800° F. to about 2200° F. for about 15 minutes up to about 1 hour. Preferably, the diffusion heat treatment step occurs in an inert atmosphere, such as under a vacuum.
Following the heat treatment step, first layer 616 may include less than about 30 at. % of the Pt-group metal, less than about 2 at. % of the reactive element, and less than about 28 at. % Si. In some embodiments, first layer 616 may include about 10 at. % to about 30 at. % of the Pt-group metal, about 0.3 at. % to about 2 at. % of the reactive element, and about 2 at. % to about 28 at. % Si. In some embodiments, first layer 616 may also include less than about 20 at. % Al, less than about 8 at. % Cr, and less than about 8 at. % Co as a result of diffusion from turbine blade 602, as well as Ni and other superalloy substituents. In some embodiments, first layer 616 may include about 10 at. % to about 20 at. % Al, about 2 at. % to about 8 at. % Cr, and about 4 at. % to about 8 at. % Co. All of the above values are nominal, and may vary up to about ±1-2 at. %.
As an example, in one embodiment, the diffusion heat treatment may result in a coating including about 13 at. % Al, about 25 at. % Pt, about 5 at. % Si, about 1.5 at. % Hf, about 4 at. % Cr, about 6 at. % Co, incidental amounts of other superalloy substrate elements, and the balance Ni. In the same example, the phase constitution of the resulting coating was about 85% γ′-Ni3Al and about 15% γ-Ni.
A second layer 816 of the external coating may then be deposited on first layer 616 (504), as illustrated in
Coating thickness and composition may be controlled by selecting appropriate pack cementation times and temperatures, and by controlling the composition of the pack. In some embodiments, the pack may include about 0.5 wt. % to about 1 wt. % Al, about 1 wt. % to about 5 wt. % organo halocarbon powder, and the balance 90 grit aluminum oxide.
Turbine blade 602 including first layer 616 and the pack are then heated to a temperature between about 1400° F. to about 1975° F. for about 1 hour to about 4 hours, which causes the coating materials in the pack to deposit on to first layer 616.
Additionally, one or more reactive elements may be included in the pack mixture in amounts up to about 5 wt. %, if desired. The inclusion of one or more reactive element in second layer 816 may increase the concentration of the one or more reactive element in the final external coating.
Alternatively, the deposition of second layer 816 may be accomplished using a second electrophoretic deposition system 800, as illustrated in
The electrophoretic suspension 806 may include solids, a liquid carrier, and any necessary additives, such as activators and the like.
Suitable liquid carriers include nitroalkanes, alcohols containing about 1 to about 6 backbone carbons, glycols, and the like, and mixtures of isopropanol and nitromethane are preferred. In some embodiments, the liquid carrier comprises about 55 wt. % to about 65 wt. % isopropanol and about 35 wt. % to about 45 wt. % nitromethane, and may include about 60 wt. % isopropanol and about 40 wt. % nitromethane.
Electrophoretic suspension 806 may further include an activating agent and an electrophoretically active agent, including those described above. Suitable electrophoretically active agents include zein, and electrophoretic suspension 806 may include about 1 g/L to about 3 g/L, preferably about 2.2 g/L. Suitable activating agents include cobalt nitrate hexahydrate, and electrophoretic suspension 806 may include an activating agent concentration of about 0.08 g/L to about 0.20 g/L. In some embodiments, electrophoretic suspension 806 may include an activating agent concentration of about 0.14 g/L.
The electrophoretically active agent and the activating agent may form a chelated complex which may surround the metal particles in the electrophoretic suspension 806 and impart a slight positive charge that causes the encased particle to migrate to and deposit on the negatively charged cathode (turbine blade 602/first layer 616).
The solids present in electrophoretic suspension 806 may include an Al source, an organo halocarbon activator, and filler. In some embodiments, electrophoretic suspension 806 may include a total solids content (including the coating composition) of about 15 g solids per L liquid carrier to about 40 g solids per L liquid carrier, preferably about 30 g solids per L liquid carrier.
Based on presently available evidence, the organo halocarbon can act as an activator for the Al. The use of the organo halocarbon activator is particularly preferred for the electrophoretic deposition process, as conventional halide salt activators are not compatible with this process, but may also be used for pack cementation.
Suitable organo halocarbons include, for example, fluoropolymers, such as polytetrafluoroethylene (PTFE), chloropolymers, such as polyvinylchloride (PVC), and the like.
Suitable organo halocarbons can preferably be produced in granular form and are compatible with the liquid carrier in the electrophoretic suspension 806. One particularly preferred fluoropolymer is polytetrafluoroethylene (PTFE), which may be obtained from DuPont Fluoroproducts, Wilmington, Del., under the trade designation DuPont Teflon PTFE 60, and the trade designation Dupont Zonyl MP-1200.
During the electrophoretic process, the organo halocarbon and Al are substantially concurrently deposited on first layer 616, forming second layer 816, as shown in
The Al source may include elemental Al, or may include alloy powders such as, for example (by weight) 55Al:45Cr, 42Al:40Cr:18Mn, 70Al:30Cr, other aluminum-chromium alloys, organo-aluminum compounds, and the like.
The filler may be a non-reactive oxide powder, such as aluminum oxide, zirconium oxide, titanium oxide, or hafnium oxide, and the like. The solids in electrophoretic suspension 806 may include about 0.5 wt. % to about 25 wt. % of the Al source, about 1 wt. % to about 10 wt. % of the organo halocarbon activator, and the balance filler.
Electrophoretic deposition of the Al may be carried out for up to about 6 minutes at a current density of about 1 mA/cm2 to about 2 mA/cm2 between the anode and cathode (article 402/first layer 414). Suitable voltages may range from about 35 V to about 250 V, and may be chosen to provide the desired current density. The necessary voltage may be influenced by the composition of electrophoretic bath 808 and the distance between the anode and cathode (turbine blade 602/first layer 616). The suspension may maintained by mechanical agitation including, for example, an impeller 810, as described above. Further details regarding electrophoretic deposition processes are provided in U.S. Pat. Nos. 5,057,196 and 5,958,204 both to Creech et al., which are incorporated herein by reference in their entirety. Adjusting either the current density or deposition time, or both, may allow control over the green coat weight. The green coat weight is the weight of second layer 816 applied to first layer 616 per unit area of first layer 616, and green coat weight affects the total amount of Al available to diffuse into the coating during subsequent heat treatment steps. Preferred green coat weights range from about 12 mg/cm2 to about 65 mg/cm2, preferably about 15 mg/cm2 to about 30 mg/cm2.
After application of the second layer 816, a mixture (e.g., mixture 404) may be introduced into cavities 614 of turbine blade 602 (204) and turbine blade 602 and mixture 404 may be heated to activate mixture 404 (206), as described above in further detail with reference to
The heating of turbine blade 602 and mixture 404 may substantially simultaneously activate the aluminum present in second layer 816. For example, the heat treatment may cause the organo halocarbon activator to decompose and release halogens, which react with the aluminum in the second layer 816 to form an aluminum halide. The aluminum halide may diffuse to the surface of first layer 616 and the aluminum may react with and be bound to the surface of first layer 616. Further, the heat treatment step (206) may facilitate interdiffusion between turbine blade 602, first layer 616 and second layer 816.
As described above, heating of turbine blade 602 and mixture 404 may occur at a temperature of about 1400° F. to about 1975° F. for about 1 hour to about 4 hours. In some embodiments, the heat treatment may be carrier out in an oven under an inert atmosphere, such as a partial pressure (e.g., about −5 inches Hg) of argon. The heat treatment may also result in thermal decomposition and removal of any masking material 402.
After heating the turbine blade 602 and mixture 404, the exterior surface of turbine blade 602 may optionally be cleaned to remove any excess green coat that is not chemically bound to the exterior surface of turbine blade 602. Cleaning may be accomplished by ultrasonic cleaning in water or a solvent such as methanol, or simply mechanically removing the residual unreacted bisque with a soft bristle brush, or the like. For example, the exterior surface of turbine blade 602 may be ultrasonically cleaned twice in methanol for about 5 minutes.
The turbine blade 602 and mixture 404 may then undergo a final heat treatment (506). The final heat treatment facilitates further diffusion of the coating elements and produces a more uniform coating. The final heat treatment step is preferably carried out at about 1975° F. to about 2080° F. for about 1 to about 4 hours, although any convenient temperature may be used.
Following the final heat treatment, cavities 614 may optionally be cleaned to remove any excess mixture 404. For example, in some embodiments a high pressure water blast may be used to remove any remaining residue.
While the technique illustrated in
A turbine blade having a part number 23072440 was electrophoretically coated with a PtSiHf alloy including about 91.6 wt. % Pt, about 3.5 wt. % Si and about 4.8 wt. % Hf and diffused at 2175° F. for 15 minutes in a vacuum. The heat treatment in vacuum may also result in vacuum cleaned surfaces of the internal cavities of the blade. After a light 220 grit blast with aluminum oxide at about 20 to 25 psi, a deposit of approximately 6 wt. % aluminum, 3.5 wt. % polytetrafluoroethylene and the balance 1 micron aluminum oxide filler (XP-101) was electrophoretically applied to the external surfaces according to a method described in U.S. patent application Ser. No. 11/963,249. After the electrophoretic deposition, a composition including approximately 6 wt. % aluminum, 3.5 wt. % polytetrafluoroethylene and the balance 1 micron aluminum oxide filler was mixed with zein in a 60 wt. % isopropanol and 40 wt. % nitromethane vehicle in the approximate ratio of 100 grams of solids to 50 milliliters of zein solution. The composition was injected into the internal cavity of the turbine blade until the slurry composition exited the trailing edge opening and film cooling holes at the tip of the blade. The two holes on the bottom of the blade dovetail were filled with Dymax 706-H (available from DYMAX Corporation, Torrington, Conn.) and cured for approximately 5 minutes under an ultraviolet light to harden the plugs.
The composition was activated by a heat treatment at 1500° F. for about 1 hour under a −5″ Hg partial pressure of argon gas. Upon completion of the heat treatment, undiffused bisque left on the external surfaces of the blade was removed by vigorous brushing with a soft bristle brush. Following this, the blade was placed in a beaker containing methanol and ultrasonically cleaned for about 5 minutes. The ultrasonic cleaning was repeated in fresh methanol for about 5 minutes. The coating was further heat treated at about 2080° F. for about 1 hour in a vacuum at approximately 10−4 Torr to form the external PtAl coating and the internal aluminide coating.
After the post-coat heat treatment, the blade was heat tinted to demonstrate complete coating coverage on the external surfaces. The heat-tinted blade showed the dovetail serrations were free of coating per the design of the Dymax 706-H masking plugs in the bottom two holes of the blade stalk. A high pressure water blast was used to remove any remaining residue from the internal cavity of the blade. The blade was sectioned at approximately 15%, 50% and 85% of the length of the blade to document the coating thicknesses on the internal surfaces of the blade. Table 1 shows the coating thicknesses on the external and internal surfaces produced by this method, and
A turbine blade having part number 23072440 was subjected to “pickling” for 30 minutes in a 50% nitric acid and 50% water solution at approximately 150° F. to clean the internal surfaces of the blade in preparation for the internal coating. The blade was coated with the same PtSiHf alloy as described in Example 1 and diffused at 2175° F. for 15 minutes in a vacuum. After a light grit blast, the blade was masked by filling the trailing edge and film cooling holes near the blade tip with Testors cement (available from The Testor Corporation, Rockford, Ill.). The excess Testors cement was wiped off prior to curing and lightly grit blasted with 220 grit aluminum oxide after curing to remove any excess. In this example, the surfaces just inside the trailing edge and the inside surfaces of the film cooling holes thus have thin or no coating.
With the masking in place, a dry pack mix consisting of approximately 1 wt. % aluminum, 1 wt. % fine polytetrafluoroethylene powder and the balance 90 grit aluminum oxide was poured into the blade cavity and vibrated until the dry pack mix was near the top of the two holes in the bottom of the blade dovetail. As in Example 1, Dymax 706-H was injected into the two holes to seal off the internal cavity and prevent gaseous aluminum-containing species from depositing aluminide coating on the machined dovetail surface of the blade during heat treatment. As before, the Dymax 706-H was cured with ultraviolet radiation.
Following this, the XP-101 coating was electrophoretically deposited onto the external surfaces of the blade. Initial diffusion of the Al-containing external and internal coatings was performed at a temperature of about 1500° F. for about 1 hour under a −5″ Hg partial pressure of argon. After brushing off the residual undiffused bisque from the external surfaces and ultrasonically cleaning the exterior surfaces in methanol twice, as in Example 1, the blade was subjected to the same post-coat heat treatment of about 2080° F. for about 1 hour in a vacuum oven at approximately 10−4 Torr.
Various embodiments of the invention have been described. These and other embodiments are within the scope of the following claims.
