FIELD OF THE INVENTION
This invention relates to airfoils in high-temperature environments, and particularly to thermal barrier coatings on vanes and blades in the turbine section of a gas turbine engine.
BACKGROUND OF THE INVENTION
Airfoils in high-temperature environments, such as vanes and blades in the hottest rows of a gas turbine, require thermal protection and cooling. Thermal barrier coatings (TBCs) are used to reduce heat flux into the airfoil and allow hotter surface temperatures on the airfoil. Currently, TBCs can only be applied as thin layers, since thermal gradients cause differential expansion within the coating and between the coating and substrate, which weakens the coating and its adhesion to the substrate. However, a thin TBC means that a substantial amount of air or steam cooling of the component is needed to maintain temperature limits of the substrate.
One technology to increase TBC thickness while maintaining its integrity and adhesion is called a back-filled honeycomb. This is a metallic honeycomb attached to a metal substrate surface and filled with a ceramic thermal barrier material. Examples of this technology are found in U.S. Pat. Nos. 6,846,574; 6,641,907; 6,235,370; and 6,013,592. A prefabricated honeycomb structure can be welded to a substrate. Alternately, a honeycomb may be fabricated by depositing a metal-ceramic material in a mask on the substrate and heating it to produce cohesion and a solid-state diffusion bond with the substrate. Back-filled honeycomb technology provides a metal-to-ceramic friendly bond, and allows thicker thermal barrier coatings.
A prefabricated honeycomb structure is useful for relatively flat surfaces, but cannot be conveniently bonded to a curved surface, such as an airfoil surface. The honeycomb masking/deposition method as in U.S. Pat. No. 6,846,574 can be used on curved surfaces, but is difficult to apply on highly curved or sharp surfaces, such as the leading and trailing edges of an airfoil.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is explained in the following description in view of the drawings that show:
FIG. 1 is a transverse section of a turbine airfoil core with ceramic-filled honeycombs on the pressure and suction surfaces, and a leading-edge CMC boot according to aspects of the invention.
FIG. 2. is a detail surface view of the ceramic insulation-filled honeycomb.
FIG. 3. is a detail view of a pin retaining the boot on the airfoil core.
FIG. 4. is a view as in FIG. 3 showing a variation in an outer coating on the CMC boot.
FIG. 5. is a detail view of the trailing edge of FIG. 1.
FIG. 6. is a view as in FIG. 1, showing an alternate pin embodiment.
FIG. 7. is a detail view of another leading edge CMC boot embodiment.
FIG. 8. is a detail view of another leading edge CMC boot embodiment.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows a transverse section of an airfoil 20, such as a gas turbine blade or vane, with a pressure side 22, a suction side 24, a leading edge 26, a trailing edge 28, a metal or ceramic/metal load-bearing core 30, main cooling channels 32, a trailing edge cooling channel 34, and a trailing edge coolant flow 36. The airfoil 20 may have a substantially consistent sectional geometry along a length of the airfoil, or it may vary. For example the airfoil may taper from a first end to a second end. Turbine vanes commonly span between radially inner and outer platforms that form respective inner an outer shroud rings in the turbine. These aspects are known in the art in various forms.
A metallic or metallic/ceramic honeycomb 40A, 42A may be formed and bonded to the pressure and/or suction sides of the core 30 by a method as taught in U.S. Pat. No. 6,846,574 B2 of the present assignee, which is incorporated herein by reference. In summary, this involves depositing a masking material on a substrate, which in the present invention is a surface 31 of the airfoil core 30; then selectively removing portions of the mask, for example by photolithography or laser etching, to form a honeycomb void pattern in the mask material; then depositing a metal or a graded metal/ceramic particulate wall material into the honeycomb void pattern, for example by electro deposition; then heating the wall material to produce cohesion within the wall material and a solid-state diffusion bond between the wall material and the substrate. This forms a honeycomb wall structure bonded to the substrate. The remaining mask material is then removed to form a void pattern of cells within the honeycomb walls. An insulating ceramic particulate material 50 and a bonding agent are then deposited into the honeycomb cells, for example by electro-deposition, and is heated to produce cohesion within the insulating material and bonding to the honeycomb walls and substrate. The insulating material 50 may include hollow ceramic spheres 52 as also taught in U.S. Pat. No. 6,846,574 or other voids. Such voids provide insulation and abradability, which allows surface wear from particle impacts without deep spalling. Alternately, a metal or ceramic/metal honeycomb may be bonded to the substrate by brazing or welding.
The honeycomb walls can be formed at any angle to the substrate, depending on the direction of the etching process, for example the direction of a laser beam. Thus, the honeycomb walls 40A can be substantially parallel to each other as shown on the pressure side 22 of FIG. 1, or the walls 42A can be normal to the substrate surface as shown on the suction side 24 of FIG. 1. The honeycomb walls 40A, 42A can be applied to follow the curvatures of the pressure and suction sides of the airfoil in either a normal or a parallel honeycomb wall orientation, or combinations of these orientations and others as desired. The walls may be formed in any polygonal pattern that provides a generally uniform honeycomb wall thickness throughout the pattern, including, but not limited to, hexagonal, square, rectangular, and triangular cells. FIG. 2 shows a surface view of a hexagonal honeycomb wall structure 40A, 42A filled with ceramic insulation 50 containing hollow ceramic spheres 52.
On the highly curved leading edge of the airfoil, a honeycomb structure becomes less desirable, because the cell walls would be highly divergent or highly oblique to the substrate in some areas. Thus, according to the present invention, the leading edge 26 may be covered with a boot 60A of ceramic matrix composite (CMC) material formed of ceramic fibers in a ceramic matrix. The fibers may be random, oriented, or woven into a fabric as known in the art. The boot may have a C or U-shaped cross section as shown. First and second ends 61, 62 of the section define first and second edges of the boot.
FIG. 1 shows a CMC boot 60A attached to an airfoil core 30 with pins 63A having enlarged heads 68. FIG. 3 shows a detail of the pin attachment mechanism of this embodiment. The pins 63A may be formed of a metal or metal/ceramic material bonded to the core 30 by solid-state diffusion bonding 66. This can be done by forming and curing the CMC boot 60A using fabrication methods known in the art, then forming respective pin-shaped holes 64 for the pins 63A in the CMC boot 60A by any machining method, such as milling or laser etching. The boot 60A may then be clamped against the leading edge of the core 30. The holes 64 now serve as molds for the pins, and may be filled with metal particles, or with a gradient mixture of metal and ceramic particles, with mostly or all metal particles at the bottom, and mostly or all ceramic particles in the head 68. The pin material may then be heated to a temperature of internal cohesion and solid-state diffusion bonding 66 with the substrate 30. The pin materials and heating may be the same as for the honeycomb walls 40A, 42A. This pin fabrication method forms a perfectly tight yet stress-free pin that is integrally bonded with the substrate. The remainder of the boot 60A between the edges 61, 62 may be unbonded to the substrate to allow limited slippage of the boot relative to the substrate during differential expansion. This aspect may be further developed to provide a cooling channel as later described for FIG. 7.
FIG. 3 shows that the ceramic insulation 50 of the honeycomb 42A may extend to cover the CMC boot 60A. The honeycomb 42A may have a substantially consistent height over most of the surface 31. However, one or more shorter rows of cells 44 may be provided adjacent the leading and/or trailing edges 26, 28 to anchor the insulation 50 beside the boot 60A and/or to anchor the insulation over the trailing edge.
FIG. 4 shows a detail as in FIG. 3 using a different ceramic insulation 51 covering the boot 60A than the insulation 50 on the honeycomb 42A. The insulation 51 may be optimized differently than the insulation 50 in the honeycomb to provide advantages such as increased adhesion to CMC and/or impact resistance. For example the insulation 51 may be a ceramic without voids but with an anisotropic crystal lattice structure for low thermal conductivity, such as taught in U.S. application Ser. No. 12/101,460 filed 11 Apr. 2008 and assigned to the present assignee, which is incorporated herein by reference.
FIG. 5 shows a detail of a trailing edge 28 with shoulders 33 formed in the core member 30. Each shoulder defines a transition between the respective pressure and/or suction side 22, 24 and the trailing edge 28 portion of the core. Each shoulder 33 defines a first thickness of the ceramic insulation 50 over the core member trailing edge 28 that is less than a second thickness of the ceramic insulation over the pressure and suction sides 22, 24 of the core 30.
FIG. 6 shows an embodiment 20 with both a leading edge CMC boot 60A and a trailing edge CMC boot 59, in which each boot is attached to the core 30 with metal pins or rivets 63B. These pins or rivets may have two heads as shown, formed by a riveting tool, or they may have only a distal head 68 in the boot and a cylindrical shaft pressed into a cylindrical bore in the core.
FIG. 7 shows a CMC boot 60B with retention flanges 65 inserted into respective retention slots 69 in an airfoil core 30. The slots 69 can be formed by machining, casting, or extrusion of the core. This boot can be formed and cured, then elastically spread and clipped onto the core from ahead of the leading edge. Alternately the boot may be slid onto the core from one end of the airfoil. For example if a vane airfoil has a removable inner or outer platform, the boot 60B can be slid onto the core 30 from an end of the airfoil. Optionally, a gap 35 may be provided between the core and the boot. This may serve as a cooling channel, and also allows differential expansion of the boot 60B relative to the core 30. A central portion 71 of the boot may be thicker than the boot edges 61, 62, for impact resistance. This variation in thickness may be achieved by increasing layers of ceramic fabric or fibers toward the center 71, or by a ceramic inclusion in the middle of the boot (not shown). A TBC 50 may be applied over the boot. This TBC may be either continuous over the boot and the honeycomb, or it may be discontinuous at the boot/honeycomb interface. This boot is replaceable by cutting away the TBC along the edges 61, 62 of the boot, and tapping/sliding the boot off an end of the airfoil or by cutting through the middle of the boot and removing it from the slots 69 in halves. FIG. 7 also illustrates an embodiment of a honeycomb 40B, 42B that extends only part-way to the surface of the TBC 50.
FIG. 8 shows a CMC boot 60C with hooked retainer flanges 65 in retention slots 69 in an airfoil core 30. A hook 67 on each flange positively prevents slippage of the flange out the retention slot 69 normal to the surface of the core member. To fabricate this embodiment, the slots 69 can be formed by extruding the core or by including a fugitive material in casting the core. Then the boot can be slid onto the core from one end of the airfoil. Alternately, the boot 60C can be formed and fully cured, then inserted into a mold. Then material for the core can be poured into the mold, imbedding the flange 65 and hook 67 in the core. With the latter method, the boot cannot be removed and replaced, but the TBC 51 on the boot can be replaced by etching or machining away the old TBC and applying a new one.
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. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.
Patent Application
20100074726
