Turbine blade or turbine vane made of a ceramic foam joined to a metallic nonfoam, and preparation thereof

[0002] This invention relates to a turbine blade or turbine vane formed of a metallic nonfoam region and a ceramic foam region.

[0003] The property requirements of a turbine blade or turbine vane vary greatly according to location within the article. For example, the attachment (dovetail) must be strong and fatigue resistant at intermediate temperatures, the root region of the airfoil must be strong, fatigue resistant, and resistant to environmental damage at higher temperatures, and the tip region of the airfoil must retain a form factor and have excellent resistance to environmental damage at the highest temperatures. Different parts of a single region may require different properties, as for example the pressure side and the suction side of the airfoil. There is a large incentive to raise the combustion gas temperature of the engine. However, there is also a large incentive to decrease the weight of the turbine blades as much as possible, because a reduction in turbine blade weight leads to reductions in disk weight, shaft weight, bearing weight, and support weight that in turn increase the weight efficiency of the engine.

[0004] In most cases, the different property requirements are met with a single material of construction that may not be optimal for any one location but instead achieves a good balance of properties for all of the locations. The currently preferred material of construction for most turbine blade and turbine vane applications is nickel-base superalloys, which may be coated to protect against environmental damage at the highest temperatures.

[0005] Composite materials have been developed for use at room temperature and mildly elevated temperatures. Such composite materials include the familiar fiber-reinforced organic matrix composites such as graphite fiber-epoxy composites. Structures made of such materials may have their properties tailored according to the location within the article, by changing the direction of the fibers, the volume fraction of the fibers, the type of fibers, and the like.

[0006] There have been attempts to apply these principles of composite construction to high-temperature applications such as turbine blades and vanes. Research studies have been underway for many years to apply composite-construction principles to high-temperature components such as turbine blades. These efforts have focused on superalloys that are reinforced by particles, fibers, or whiskers of ceramic materials. Although there have been some advancements, these efforts have not been successful in the sense that there are no such composite articles in regular service today. Gas turbine blades are typically made of nickel-base superalloys that may be made hollow to reduce weight and to allow cooling air to be conveyed through the interior of the blades. The use of a composite construction would offer the promise of reducing weight while maintaining performance, but no operable approach has been proposed as yet.

[0007] There is, accordingly, a need for an improved approach to turbine blades and vanes that must operate at elevated temperatures, must have property requirements that vary substantially at different locations of the article, and must be as light in weight as possible. The present invention fulfills this need, and further provides related advantages.

[0008] The present invention provides a composite construction that is applicable to turbine blades and turbine vanes which operate at high temperatures. The structure utilizes a combination of metallic regions and ceramic foam regions to tailor the properties as required for excellent mechanical properties and for low weight. The approach of the invention allows the designer of the article to determine the required properties for the various locations, and then the article is manufactured with different materials optimized for each location.

[0009] An article of manufacture comprises an article selected from the group consisting of a turbine blade and a turbine vane. The article further comprises a metallic nonfoam region, and a ceramic foam region joined to the metallic nonfoam region. The ceramic foam region comprises an open-cell solid foam made of ceramic cell walls having intracellular volume therebetween. The ceramic cell walls are preferably alumina. The intracellular volume may be empty porosity or an operable intracellular metal such as an intracellular nickel-base superalloy. The ceramic foam region may even be varied within itself, to have a first ceramic foam subregion having an intracellular volume that is empty porosity, and a second ceramic foam subregion having an intracellular volume comprising the intracellular metal. The metallic nonfoam region may be any operable metal, such as a primary nickel-base superalloy. The nonfoam region and the ceramic foam region are joined by any operable approach, such as a weld joint, a diffusional joint, or a casting joint.

[0010] In one approach, a method is provided for preparing an article selected from the group consisting of a turbine blade and a turbine vane. The method comprises the steps of preparing an airfoil region by the steps of providing a piece of a sacrificial ceramic having the shape of the airfoil region, and contacting the piece of the sacrificial ceramic with a reactive metal which reacts with the sacrificial ceramic to form an oxidized ceramic of the reactive metal and a reduced form of the ceramic. The resulting structure comprises a ceramic foam of the oxidized ceramic compound of the reactive metal with ceramic cell walls and an intracellular volume between the ceramic cell walls, the intracellular volume comprising a reaction-product metal. The reaction-product metal may be removed to create empty porosity, replaced with a replacement metal, or left unchanged. The method further includes joining the airfoil region to an attachment region by any operable approach.

[0011] The present approach provides a great deal of flexibility in precisely tailoring the structure and properties of a turbine blade or turbine vane. These structures have in common an airfoil that is joined to an attachment structure. The ceramic foam material used in the airfoil is lighter in weight than a comparable superalloy, and the weight may be reduced even further by removing the reaction-product metal from the intracellular volume where mechanical property requirements are minimal and the material functions largely to define a form. Where the mechanical property requirements are higher, the reaction-product metal may be replaced with the intracellular nickel-base superalloy to produce a ceramic foam whose intracellular volume is filled with the superalloy.

[0012] The joining of the ceramic foam regions and the nonfoam regions is accomplished by any operable approach. In one technique, the regions are each fabricated separately and then joined by welding such as electrical resistance welding, solid-state diffusional joining, liquid-phase joining that may be possible in some cases, or brazing with a brazing metal. In another technique, the ceramic foam region is fabricated, and the metallic nonfoam region is cast around it.

[0013] The result is a turbine blade or vane that has the metallic nonfoam region where required for strength and ductility, typically in the attachment, and the ceramic foam region that has a high-temperature shape-retention capability but is not as strong and ductile as the metallic nonfoam region. The metallic nonfoam material is typically used to form the attachment, and the ceramic foam material is typically used to form some or all of the airfoil. Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.

[0014] FIG. 1 is a schematic perspective view of a gas turbine blade;

[0015] FIG. 2 is a schematic view of an article made of a ceramic foam joined to a metallic nonfoam;

[0016] FIG. 3 is a schematic view of a second embodiment of an article made of a ceramic foam joined to a metallic nonfoam;

[0017] FIG. 4 is a schematic view of a third embodiment of an article made of a ceramic foam joined to a metallic nonfoam;

[0018] FIG. 5 is a schematic enlarged detail of the article of FIG. 2, showing the microstructure in region 5—5;

[0019] FIG. 6 is a block flow diagram of an approach for fabricating the article; and

[0020] FIG. 7 is a schematic microstructure of the ceramic foam precursor material resulting from the immersion step.

[0021] This invention relates to the structure and preparation of a turbine blade or a turbine vane used in a turbine engine such as a gas turbine engine. FIG. 1 depicts such an article in the form of a solid or hollow gas turbine blade 10. A solid or hollow turbine vane is similar in relevant aspects. The turbine blade 10 includes an airfoil 12 against which a flow of hot combustion gas is directed during service of the gas turbine engine. The turbine blade 10 is mounted to a turbine disk (not shown) by an attachment such as a dovetail attachment 14 which extends downwardly from the airfoil 12 and engages a slot on the turbine disk. A platform 16 extends longitudinally outwardly from the area where the airfoil 12 is joined to the dovetail 14. In the hollow form of the gas turbine blade 10 or turbine vane, a number of cooling channels extend through the interior and the wall of the airfoil 12, ending in openings 18 in the surface of the airfoil 12. A flow of cooling air is directed through the cooling channels, to reduce the temperature of the airfoil 12.

[0022] The present invention deals with a turbine blade or turbine vane having a composite structure with a metallic nonfoam region and a nonmetallic ceramic foam region. The composite structure may be used in a number of different approaches related to turbine blades and turbine vanes. The following discussion is presented first in a general form applicable to a wide variety of these different approaches, and then applied to some specific turbine blade and turbine vane applications.

[0023] FIGS. 2-4 depict in general form an article 20 comprising a metallic nonfoam region 22 and a ceramic foam region 24 joined to the metallic nonfoam region 22. In the embodiment of FIG. 2, the metallic nonfoam region 22 forms one side of the article 20, and the ceramic foam region 24 forms the other side. In the embodiment of FIG. 3, the ceramic foam region 24 is embedded within the metallic nonfoam region 22, so that the metallic nonfoam region 22 surrounds the ceramic foam region 24. In the embodiment of FIG. 4, the metallic nonfoam region 22 is embedded within the ceramic foam region 24, so that the ceramic foam region 24 surrounds the metallic nonfoam region 22. All of these embodiments are within the scope of the invention. The ceramic foam region 24 comprises an open-cell solid ceramic foam made of ceramic cell walls having intracellular volume therebetween, as will be discussed in greater detail subsequently.

[0024] Referring again to FIG. 1, the principles of FIGS. 2-4 may be applied to the gas turbine blade 10 or vane. For example, the airfoil 12 may be made of the metallic nonfoam material, except that a tip end 17 may be made of the ceramic foam. In another example, the airfoil 12 may be made of the ceramic foam, except that a leading edge 19 may be made of the metallic nonfoam material. In another example, the dovetail attachment 14 may be made of the metallic nonfoam material, and the entire airfoil 12 may be made of the ceramic foam. In another example, the platform 16 may be made of ceramic foam, and the airfoil 12 and dovetail attachment 14 may be made of the metallic nonfoam material. Inserts may be used at other locations, such as at the trailing edge or on the concave side of the airfoil. Combinations of these various approaches may also be used, such as a metallic nonfoam dovetail attachment 14, a ceramic foam platform 16, and a metallic airfoil 12 except for a ceramic foam tip end 17 and leading edge 19. The present approach is not concerned with the specific form and locations of the composite elements, but instead provides turbine blade and turbine vane designers with the structure and approach for making such composite turbine blades and vanes.

[0025] FIG. 5 illustrates the microstructure of the article 20 shown in FIG. 2, but it is equally applicable to the embodiments of FIGS. 3-4. The article 20 includes the metallic nonfoam region 22. The metallic nonfoam region 22 is a metal of any operable type that is not a foam, but is preferably a nickel-base superalloy. (The metal of the metallic nonfoam region 22 is termed the “primary” metal, to distinguish it from the “intracellular” metal that may be present in the intracellular volume of the ceramic foam.) A nickel-base alloy has more nickel than any other element. The nickel-base alloy may additionally be a nickel-base superalloy, meaning that it is of a composition which is strengthened by the precipitation of gamma-prime phase. A typical nickel-base alloy has a composition, in weight percent, of from about 1 to about 25 percent cobalt, from about 1 to about 25 percent chromium, from about 0 to about 8 percent aluminum, from 0 to about 10 percent molybdenum, from about 0 to about 12 percent tungsten, from about 0 to about 12 percent tantalum, from 0 to about 5 percent titanium, from 0 to about 7 percent rhenium, from 0 to about 6 percent ruthenium, from 0 to about 4 percent niobium, from 0 to about 0.2 percent carbon, from 0 to about 0.15 percent boron, from 0 to about 0.05 percent yttrium, from 0 to about 1.6 percent hafnium, balance nickel and incidental impurities. Specific alloys are known in the art. The metallic nonfoam region 22 may be reinforced with particles, fibers, whiskers, wires, or other reinforcement, as long as that reinforcement is not a foam.

[0026] The article 20 further includes the ceramic foam region 24, joined to the metallic nonfoam region 22. The ceramic foam region 24 may be uniform throughout, or it may include a first ceramic foam subregion 30 and a second ceramic foam subregion 32. Additional ceramic foam subregions may be present as well, if desired. In all cases, the ceramic foam region 24 is formed of an open-cell solid ceramic foam 40 comprising ceramic cell walls 42 and an intracellular volume 44 therebetween. The cell walls 42 and the intracellular volume 44 are each interpenetrating, continuous regions. The ceramic cell walls 42 are any operable ceramic, but are preferably an alumina-based material. Alumina is synonymous herein with aluminum oxide. The ceramic cell walls 42 preferably constitute at least about 60 percent by volume, and most preferably from about 60 to about 80 percent by volume, of the ceramic foam 40. The intracellular volume 44 preferably occupies the remainder of the volume of the ceramic foam material 40. Although it cannot be readily discerned from FIG. 5, the ceramic cell walls 42 are internally continuous, and the intracellular volume 44 is internally continuous. All portions of either phase 42 and 44 are respectively continuous, so that there is a continuous path from the external surfaces to any location within either phase.

[0027] The intracellular volume 44 may be filled with an intracellular metal or may be empty porosity. Where the intracellular volume 44 is a metal, it is preferably a nickel-base superalloy (within the class discussed above), but other types of metals may be used as the intracellular metal. Where the ceramic foam region 24 is uniform throughout, the entire intracellular volume 44 is either the intracellular metal or the empty porosity.

[0028] FIG. 5 illustrates the case where the ceramic foam region 24 is divided into subregions, in this case the first ceramic foam subregion 30 and the second ceramic foam subregion 32. These subregions 30 and 32 differ in the nature of the intracellular volume 44. In the illustrated embodiment, the intracellular volume 44 of the first ceramic foam subregion 30 is filled with intracellular metal, and the intracellular volume 44 of the second ceramic foam subregion 32 is empty porosity. This arrangement is selected because the intracellular volume 44 of the first ceramic foam subregion 30 aids in bonding of the ceramic foam region 24 to the metallic nonfoam region 22.

[0029] FIG. 6 illustrates a preferred method for forming the article 20. The ceramic foam region 24 is provided as a freestanding element, numeral 60. The ceramic foam region 24 has a shape and size as required for the intended application. The ceramic foam is preferably prepared by first fabricating a sacrificial ceramic form. The sacrificial ceramic form is prepared by an operable approach, and a preferred approach is illustrated in FIG. 6. In this preferred approach, a slip of a sacrificial ceramic material is prepared and cast into a mold that has the same shape, but slightly larger dimensions, than the required dimensions of the ceramic foam, numeral 62. The preferred sacrificial ceramic material is silica (silicon dioxide) particles. Additions of modifiers may be made to the ceramic slip. For example, additions that modify the firing behavior of the ceramic, such as calcia (calcium oxide) in the case of silica sacrificial ceramic, may be made. Additions that modify the porosity of the final reacted ceramic material, such as mullite, may be made. Additions that modify the properties of the final reacted ceramic material, such as boron nitride or sol gel alumina to increase the wear resistance of the final reacted ceramic material, may be made. Additions that modify the chemical composition of the final reacted ceramic, such as boron, may be made.

[0030] The slip casting of silica particles is well known in other applications, and the same procedures are used here. Typically, a slurry of silica particles and acrylic binder in water is prepared and poured into the mold. The mold and its contents are dried, numeral 64, to remove the carrier liquid. The dried slip cast material is thereafter heated to an elevated temperature to fire and fuse the ceramic, numeral 66. In the case of silica, a typical firing temperature is about 2000° F. and a firing time is about 4 hours. The original slip casting is made slightly oversize to account for the shrinkage during drying and firing. The required oversize is known in the art because slip casting is so widely employed for other applications, but is typically about 1-15 percent.

[0031] The steps 62, 64, and 66 together provide the sacrificial ceramic having the shape of the ceramic foam region.

[0032] The sacrificial ceramic form is thereafter contacted to a molten reactive metal, most preferably an aluminum-base metal. The contacting is preferably accomplished by immersing the sacrificial ceramic form into the molten reactive metal, numeral 68. The preferred approach is disclosed in U.S. Nos. Pat. 5,214,011 and 5,728,638, whose disclosures are incorporated by reference. The metal may be a pure metal, or it may be an alloy containing the reactive metal. Most preferably, the reactive metal, when in alloy form, contains more of the reactive element than any other element. The reactive metal may optionally be mixed with nonreactive metals such as a large fraction of nickel and other elements of the nickel-base alloy of interest for some applications, as disclosed in the '638 patent.

[0033] While the sacrificial ceramic form is immersed in the reactive metal, the ceramic of the sacrificial ceramic form is chemically reduced and the reactive metal is chemically oxidized. (Reduction and oxidation are broadly interpreted in the sense of electron transfer.) The reactive metal becomes an oxide or oxidized form, aluminum oxide in the preferred case. As a result of a mechanism involving volume changes and internal fracturing and discussed in the '011 patent, the foam or sponge structure is formed throughout the sacrificial ceramic as it transforms from the sacrificial form-composition to the final composition. The intracellular volume that results is filled with the reaction-product metal resulting from the reaction process. The result of the immersion step 68 is a reacted ceramic foam blade preform.

[0034] The ceramic foam preform material 90 is shown in FIG. 7. This material is the precursor for the structures shown in FIGS. 2-5 and is similar in many ways. The open-cell solid foam material 90 comprises two interpenetrating, continuous regions 92 and 94. The region 92 is the ceramic that constitutes the cell walls, and the region 94 is a reaction-product metal. In the preferred case where the sacrificial ceramic is silica and the reactive metal is an aluminum-base metal, the region 92 is alumina (plus any modifiers that were added to the original sacrificial ceramic and remain). The intracellular region 94 is an aluminum-base metal that is also a reaction product but will, in general, have a different composition than that of the initial reactive metal. The region 92 is internally continuous within itself, and the intracellular region 94 is internally continuous within itself. All portions of either region are continuous, so that there is a continuous path from the external surfaces to any location within either phase. A consequence of this structure is that either the region 92 or the region 94, or both regions 92 and 94, may be modified to improve the bondability of the structure, numeral 70. The step 70, if performed, occurs after the immersion step 68 and before subsequent steps.

[0035] Two modification techniques are of particular interest in step 70. In one, all or part of the reaction-product metal in the intracellular region 94 is replaced with another metal to improve the bondability and performance of the ceramic foam, numeral 74. Generally, the metal in the intracellular region y be replaced with a metal that is more suitable for particular applications. In the case of most interest, it is desired that the intracellular volume 44 be filled with a nickel-base superalloy for its elevated-temperature properties. The use of the nickel-base superalloy also enhances the bonding of the ceramic foam to the metallic nonfoam region 22. In the preferred case, where the intracellular region 94 is filled with an aluminum-base material after the immersion step, this intracellular reaction-product metal is replaced with an intracellular nickel-base superalloy to form the intracellular volume 44. 68 is immersed into a bath of the replacement liquid metal, such as nickel-base or copper-base alloys. The preform is maintained in the replacement liquid metal for a period of time, which depends upon the thickness of the composite material. This immersion allows diffusion to take place such that the aluminum is replaced by the liquid replacement metal from the bath. As an example, the aluminum/aluminum oxide composite material may be immersed in a nickel-base alloy for 8 hours at 1600° C. to effect the substantially complete replacement of the aluminum phase by the nickel-base alloy.

[0036] In a second modifying approach, numeral 72, the reaction-product metal may be removed from the intracellular region 94. The approach to removing the reaction-product metal in the intracellular region 94 will vary according to the composition of the metal. In the preferred case, all or part of the aluminum-base reaction-product metal may be chemically removed by dissolution in an appropriate chemical. For example, aluminum-base metals may be removed by reaction with HCI or NaOH. An electrical field may be applied so that the metal is removed anodically.

[0037] An advantage of the present invention is that the size, shape, and/or dimensions of the ceramic foam regions 22, as well as their precursor structures, may be adjusted as necessary at any of several steps in the process. For example, the fired material of step 66, which is silica in the preferred embodiment, may be reshape or resized by glass shaping techniques or machining. After the immersion step 68, or after the steps 70, 72 or 74, the ceramic foam region may be coarse machined and/or fine machined to adjust its size and dimensions, or to add detail features.

[0038] The modification techniques 72 and 74 may be used to produce different structures in different parts of the ceramic foam region 24, resulting in a structure having the subregions 30 and 32 of FIG. 5. The selective replacement or removal may be readily accomplished using conventional masking techniques. The selection of which areas are to experience metal removal or replacement depends upon the specific application. However, it is usually desirable that the portion of the ceramic foam region 24, here the first ceramic foam subregion 30, that is adjacent to the metallic nonfoam region 22 have the intracellular volume 44 filled with the intracellular metal that is somewhat similar in composition to the metal of the nonfoam region 22. This similarity of composition aids in achieving an acceptable bond at a joint 46 between the ceramic foam region 24 and the metallic nonfoam region 22.

[0039] A coating 50 may be applied to all or a portion of an external surface 52 of the ceramic foam region 24, FIG. 5. The coating may serve to seal porosity in the subregion 32 or to protect the intracellular metal in the subregion 30. The coating 50 may be a ceramic, such as a ceramic paste that is applied and fired. For example, an alumina paste may be applied to seal the porosity of the subregion 32. The coating 50 may instead be a protective layer such as a diffusion aluminide or overlay aluminide coating, with an optional overlying thermal barrier coating. Such coatings are known in the art for other purposes. For example, a thermal barrier coating system may be applied overlying the subregion 30 (and the nonfoam region 22).

[0040] This completes the preparation of the ceramic foam region 24 as a freestanding precursor component.

[0041] The metallic nonfoam region 22 is provided, numeral 76. The metallic nonfoam region 22 is fabricated by any operable technique. The fabrication of the metallic nonfoam region 22 is known in the art, and does not form a part of the present invention, except as discussed next. In the case of a turbine blade or turbine vane, the metallic nonfoam region 22 is typically cast from a nickel-base superalloy and solidified. The solidified may be directional and with or without a seed, constriction, or other feature to form single crystals. The solidification may be non-directional as well.

[0042] The ceramic foam region 24 is joined to the metallic nonfoam region 22 at the joint 46, numeral 78. The joining may be accomplished by any operable technique that achieves a joint 46 between the ceramic foam region 24 and the metallic nonfoam region 22. A metallurgical bond is preferred as the joint. The bond may be produced by electrical resistance welding, in which an electrical current is applied through the ceramic foam region 24 and the metallic nonfoam region 22 to produce heating, melting, and interdiffusion at the interface 30. The bond may instead be produced by pressing the ceramic foam region 24 and the metallic nonfoam region 22 together and heating the assembly in a furnace to cause the metal of the ceramic foam region 24 and the metal of the metallic nonfoam region 22 to interdiffuse, either in the solid state or the liquid state. For this approach, the ceramic foam region 24 would necessarily constitute the first ceramic foam subregion 30 with an intracellular metal. In a third approach, a brazing metal with a melting temperature lower than the metals of the ceramic foam region 24 and the metallic nonfoam region 22 may be placed into the interface between the elements to be joined, and melted and thereafter cooled, whereupon the ceramic foam 24 and metallic nonfoam region 22 are bonded together.

[0043] In each of the first three joining approaches, the ceramic foam region 24 and the metallic nonfoam region 22 are first prepared as freestanding elements and then joined together. A fourth joining approach 78 differs in that the ceramic foam region 24 is prepared as a freestanding element, but the metallic nonfoam region 22 is furnished as a liquid metal and then cast around the metallic nonfoam region 22. The ceramic foam region 24 is positioned within a casting mold, and then the liquid metal is provided and cast into the mold and solidified. Solidification may be directional to produce an oriented polycrystal or single crystal (if a seed, constriction, or other growth source is used), or nondirectional to produce a generally equiaxed structure.

[0044] The coating 50 as discussed earlier may be applied at this stage of the processing as well.

[0045] Thus, in a preferred application and referencing FIGS. 1 and 5, the metallic nonfoam region 22 comprises the attachment 14 of the blade 10 (or of a vane). The metallic nonfoam region 22 is preferably a primary nickel-base superalloy. The ceramic foam region 24 comprises the airfoil 12. Near a root 100 of the airfoil 12, the airfoil has the structure of the first ceramic foam subregion 30, wherein the intracellular volume 44 is filled with an intracellular nickel-base superalloy. The intracellular. nickel-base superalloy is preferably, but not necessarily, of the same composition as the primary nickel-base superalloy. The first ceramic foam subregion 30 is joined to the metallic nonfoam region 22 at the joint 46. Near a tip 102 of the airfoil 12, the airfoil has the structure of the second ceramic foam subregion 32, with the intracellular volume empty porosity. The regions 22 and 30 may optionally be coated with protective coatings 50 such as a thermal-barrier coating system.

[0046] Although particular embodiments of the invention have been described in detail for purposes of illustration, various modifications and enhancements may be made without departing from the spirit and scope of the invention. Accordingly, the invention is not to be limited except as by the appended claims.