CMC anchor for attaching a ceramic thermal barrier to metal

A ceramic matrix composite (CMC) anchor (20, 100) joining a metal substrate (40) and a ceramic thermal barrier (38). The CMC anchor extends into and interlocks with the ceramic barrier, and extends into and interlocks with the metal substrate. The CMC anchor may be a honeycomb (20) or other extending-into-and-interlocking geometry. A CMC honeycomb may be formed with first (22) and second (24) arrays of cells (26) with open distal ends (28) on respective opposite sides of a sheet (30). The cells may have walls (32) with transverse passages (36). A metal (40) may be deposited into the cells and passages on one side of the sheet, forming a metal substrate locked into the honeycomb. A ceramic insulation material (38) may be deposited into the cells and passages on the opposite side of the sheet, forming a layer of ceramic insulation locked into the honeycomb.

FIELD OF THE INVENTION

The invention relates generally to mechanisms for attaching ceramic coatings to metal structures, and more particularly to attaching a ceramic thermal barrier coating to a metallic component by means of a mutually interlocked ceramic matrix composite (CMC) honeycomb.

BACKGROUND OF THE INVENTION

Metal structures in high temperature environments such as in gas turbines may be coated with a protective ceramic insulating layer called a thermal barrier coating (TBC). Various processes and thermal barrier compositions have been used, but usually have been limited to layers less than 2 mm thick due to thermal expansion differences between the coating and the metal. This limits the amount of protection provided by these coatings, and leads to high thermal gradients in the coating, which can cause spalling. Differential thermal expansion can crack the coating and weaken the bond with the protected substrate material.

Other approaches to adhering ceramic coatings to metal substrates include the use of metal foams or feltmetals. U.S. Pat. No. 5,605,046 (Liang) and others use fibrous metallic layers brazed to metal substrates and used as a compliant layer for ceramic TBCs. Improvements, such as U.S. Pat. No. 6,499,943 (Beeck, et al) focus on improving the temperature capability of the compliant metallic interlayer.

The problem with all these solutions is the temperature limitation of the metallic interlayers. For porous or thin-walled metal structures, oxidation resistance is severely compromised by high surface area and rapid depletion of protective oxide forming elements. Thus, the compliant member becomes the temperature limiting feature of such designs. For applications where high heat flux and/or temperatures necessitate the use of ceramic thermal barrier coatings, improvements over these state-of-the-art solutions is desired.

Thus, there has been a long-standing need for thicker coatings with improved bonding and durability on metal structures for high temperature environments.

The present invention provides a high temperature, oxidation-resistant compliant layer between a structural metal substrate and an insulating ceramic coating. The compliant interlayer comprises a fiber-reinforced ceramic composite structure which is integrally tied to both metallic and ceramic coating members and is arranged in such a manner as to provide compliance for differential thermal expansion.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1illustrates a CMC honeycomb20with first22and second24arrays of cells26with open distal ends28on respective first and second sides of a CMC sheet30. The CMC may have carbon or ceramic fibers31that may be continuous between the CMC sheet30and the cells26, thereby providing a degree of tensile strength between the first22and second24array of cells. Such a continuous CMC honeycomb can be made using a 3-dimensional fabric weave followed by a ceramic matrix infusion as known in the art, or it can be achieved by mixing random fibers in a ceramic matrix, and molding the honeycomb. This structure is generally similar to the comb structure of honeybees, except the cells need not be hexagonal. They can be any tubular shape, including square as shown. The cells may have walls32with passages36that join or communicate between adjacent cells on the same side of the sheet.

Alternate methods of fabricating a CMC honeycomb include:

1. Stacking corrugated CMC sheets offset laterally to form channels, and bonding or curing the sheets together

2. Stacking and bonding a plurality of CMC braided tubes

3. Stacking and bonding a plurality of CMC 3D woven sheets of tubes.

FIG. 2shows two opposed interior cells26of the CMC honeycomb ofFIG. 1separated by a CMC sheet30. Here “interior cell” means a cell that is surrounded by other cells. InFIGS. 1 and 2each interior cell is connected by four passages36to four adjacent cells.FIG. 3shows a configuration in which a cell of the second array has two levels of passages36. The passages may be formed differently for the cells on opposed sides of the sheet30or they may be the same on both sides of the sheet. The depth of the cells from the sheet to the respective open distal end of the cells may be the same or different on opposed sides of the sheet.FIG. 4shows a configuration in which a cell is connected by two passages36to two adjacent cells. These configurations and/or others may be used for interior and peripheral cells.

FIG. 5shows a ceramic insulation material38injected or poured into the cells and passages of the first array22using a mold46, thus forming a continuous web of ceramic insulation that is locked into the CMC honeycomb20. The first array22is shown on top inFIG. 5, in case a gravity pour is used for the ceramic38. However, this vertical orientation is not necessary if injection molding is used. The mold46is shown schematically. While the CMC sheet30is illustrated as being planar, one skilled in the art will appreciate that other embodiments may include infusing the cells in a curved orientation, such as may be necessary when the end product being formed has a curved shape, such as for an airfoil, ring segment or combustor of a gas turbine engine, for example.

Once the ceramic material of the CMC honeycomb and ceramic composite structure ofFIG. 5has dried or cured sufficiently, it is ready to receive metal material as described with regard toFIG. 6.FIG. 6shows a metal structure40formed by pouring or injecting a molten metal40into the cells and passages of the second array24(now on top) using a mold48, thus forming a continuous web of metal that is locked into the honeycomb20. The metal40and/or ceramic material38may overflow the cells26during the respective pouring operation as shown, forming a continuous metal wall44and/or a continuous ceramic insulation layer42covering respective sides of the honeycomb20. The result is an insulated metal structure101as inFIG. 7, with an integrated CMC honeycomb20that locks the ceramic insulation38and the metal40together. The honeycomb structure includes ceramic fibers extending from the metal side of the structure to the ceramic side of the structure for providing an interconnection there between that is effective to enhance a bond between the metal layer40and the ceramic layer38.

Surfaces39,41with specific shapes may be formed on the ceramic insulation38and/or on the metal structure40in the molds46,48. For example, the ceramic insulation surface39may be formed as a surface of rotation, including a cylindrical surface. For example, shapes can be formed for gas turbine components such as ring seal segments, vane segment shrouds, transitions, and combustors. The ceramic38may be poured first. Then the ceramic38and CMC honeycomb20may be fired. Then the metal40may be poured. One or both surfaces39,41may be later machined to final specifications.

As shown inFIG. 7, the metal40may shrink more than the CMC during cooling of the metal pouring, which may separate the metal from some or all of the CMC surfaces. This process leaves clearances52that will later accommodate differential thermal expansion during use of the structure101in a variably high temperature environment. In prior art, differential thermal expansion and shrinkage during processing and usage can weaken the bond between the TBC and metal, and/or can crack the TBC. The present interlocking CMC honeycomb overcomes this problem without needing to match expansion characteristics of the TBC and metal, and without needing to manage and tolerate TBC cracking.

FIG. 8illustrates a method70for making the structure101by forming72the CMC honeycomb20, then partially or fully curing it74, then placing76the honeycomb20into a mold46, then pouring78ceramic insulation38into the cells26of the first array22, then drying and at least partially curing80the ceramic38in the honeycomb, then placing82the honeycomb into a second mold48, then pouring84metal into the cells26of the second array22. The ceramic insulation material may include hollow ceramic spheres in a ceramic matrix as known in the art.

While oxide CMC's can survive most metal casting processes, the high temperatures required for Ni-based superalloys may degrade the CMC properties—particularly its strain tolerance. Even in this degraded state, the CMC still offers much better bond reinforcement and compliance than a straight metal-to-ceramic bond. However, methods that deposit metal at lower bulk temperatures may be used to minimize this effect on the CMC. Such methods may include:

1. Selective laser sintering (laser locally densifies thin layers of metal powder deposit—may not heat underlying material in bulk)

2. Physical vapor deposition

3. Active metal brazing (allows joining at a lower temperature than the metal melting temp. Can diffuse active species to increase thermal capacity of joint. Requires matching joint design with tolerance requirements.)

4. Powder metal approaches using sintering temperatures that are below melting point

5. The above can be used in combination with each other and/or casting. The term “deposit” may be used generically to describe any process for applying or forming the metal, ceramic, or other layers to form the structure described herein.

FIG. 9illustrates an intermediate fabrication step that may be used in certain embodiments described below. A leachable, non-wetting ceramic core53may be deposited in a layer at the desired metal/ceramic coating interface prior to metal pouring and other steps as described below.

FIG. 10illustrates an insulated metal structure102with a void54between an inner surface45of the metal substrate40and an inner surface35of the ceramic thermal barrier38. This void54can be used for cooling or added compliance. It can be formed by the following steps:

1. Fill the CMC honeycomb20with a leachable, non-wetting ceramic core53in a layer at the desired metal/ceramic coating interface as inFIG. 9.

2. Cast or deposit metal40against one side of the core53.

3. Cast or deposit ceramic38against the opposite side of the core53.

4. Leach away the core53, leaving a void54.

FIG. 11illustrates an embodiment103with no separation sheet or void between the metal40and ceramic38.

1. Fill the CMC honeycomb20with a leachable, non-wetting ceramic core53in a layer at the desired metal/ceramic coating interface as inFIG. 9.

2. Cast or deposit metal40against one side of the core53.

3. Leach away the core53.

4. Cast or deposit ceramic38against the metal.

FIG. 12illustrates an embodiment104with a compliant ceramic buffer layer such as a fibrous ceramic felt or blanket55between the metal40and ceramic38.

1. Fill the CMC honeycomb20with a leachable, non-wetting ceramic core53in a layer at the desired metal/ceramic coating interface as inFIG. 9.

2. Cast or deposit metal40against one side of the core53.

3. Leach away the core53.

4. Deposit ceramic fibers55against the metal inner surface.

5. Cast or deposit ceramic38against the ceramic fibers55.

In another fabrication method, ceramic38can be deposited into the CMC anchor, and then cured, creating the ceramic layer38with an inner surface or interface plane. For embodiment103the metal40may then be deposited against the ceramic inner surface, allowing some infusion of the metal into the ceramic layer porosity. If infusion is not wanted, a non-wetting layer can be applied to the ceramic layer prior to applying the metal. For embodiments102and104, a leachable, non-wetting ceramic core material53(embodiment102) or ceramic fibers55(embodiment104) can be deposited in a layer on the ceramic38inner surface as shown inFIG. 9prior to metal deposition.

Other CMC wall geometries besides honeycombs may be used. The term “anchor” may be used to describe any CMC wall structure that joins a metal substrate40and a ceramic barrier layer38as described and claimed herein. For example,FIGS. 13 and 14illustrate a fabrication technique for a CMC anchor100in which a flat pattern with cutouts is formed in a CMC sheet57to create tabs56, which are folded up and down in a desired pattern (e.g., alternating). This creates alternating tabs56connected to a face sheet57. The tabs may then be embedded in a metal substrate and a ceramic coating on opposite sides of the face sheet57. The tabs56may have transverse passages for interlocking with the ceramic38and metal40(not shown in these two figures).

FIG. 15illustrates an embodiment105in which CMC anchor walls32protrude through the metal substrate40, and are attached to the backside of the metal. This allows a lower-temperature joining process, such as brazing58or even mechanical attachment in a manner that creates an interlock between the metal40and passages36in the CMC anchor.

While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. For example, the honeycomb structure may be a fully infused CMC material, or it may be only partially infused with matrix material so that it is somewhat porous, thereby allowing the infusion of the respective ceramic38or metal40layers to further strengthen those layers. Alternatively, the sheet30may be infused to a different degree than the walls32of the honeycomb, such as to allow more interconnection between the cells on respective sides of the sheet but still maintaining a CMC boundary between the ceramic38and metal40layers. In another embodiment it may be desired to omit the sheet30and to form the ceramic38to only partially fill the honeycombs, then to complete the fill of the honeycombs with the metal40. This embodiment maintains the integrity of continuous ceramic fibers extending across the boundary between the metal and its protective ceramic insulating layer, thereby improving the bonding there between.