Abstract:
A method of manufacturing a gas turbine engine component ( 10 ) and the component so formed. The method includes: stacking a plurality of CMC layers ( 16 ) along a metal core ( 30 ) to form a stack of disconnected CMC layers, wherein adjacent edge faces ( 46 ) of the layers define a surface ( 44 ); additively depositing ceramic material ( 14 ) to only selected portions of the surface ( 44 ) to bond together at least some of the layers at their respective edge faces; and selecting locations for the depositing of the ceramic material to achieve a predetermined mechanical characteristic of the resulting component.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to gas turbine components formed by joining a stack of CMC layers, and more specifically, to a method of joining such CMC layers with a ceramic deposit additively deposited onto the stack. 
       BACKGROUND OF THE INVENTION 
       [0002]    Economics and environmental demands are driving the efficiency of combined cycle power plants with gas turbine engine topping cycles increasingly higher. In order to achieve this efficiency, the gas turbine cycle needs to operate at turbine inlet temperatures as high as 1600 to 1800 degrees Centigrade. At these temperatures, material operating limits are being reached and/or cooling flow requirements increase so much that the benefit of the higher inlet temperature is offset. One technique that has been used to address this challenge is to use ceramic matrix composite (CMC) materials for hot gas path surfaces such as a turbine vanes or blades, etc. An example of a suitable material class is oxide-oxide composites. Monolithic construction of such materials is problematic, and the use of CMC layers that are stacked to complete the component has been proposed. U.S. Pat. No. 7,247,003 to Burke at al. discloses such a structure. However, one challenge with such construction is that the CMC layers are not bonded, but instead they are bolted onto a metal backing. Very high heat loads on both the pressure and the suction side can lead to structural damage of the metal backing in this configuration. In addition to structural requirements, CMC stacks can pose an issue for overlay coating adherence, where the overlay coating may be, for example, a ceramic thermal barrier coating (TBC). Accordingly, there remains room in the art for improvement. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0003]    The invention is explained in the following description in view of the drawings that show: 
           [0004]      FIG. 1  is a perspective illustration of an exemplary embodiment of a gas turbine component formed of a CMC stack and a ceramic deposit thereon. 
           [0005]      FIG. 2  is a sectional view of the ceramic deposit of  FIG. 1  along line  2 - 2  illustrated after an overlay coating has been added. 
           [0006]      FIG. 3  is a perspective illustration of an alternate exemplary embodiment of a gas turbine component. 
           [0007]      FIG. 4  is a schematic illustration of an interface between adjacent CMC layers. 
           [0008]      FIG. 5  schematically illustrates a method of forming the ceramic deposit of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0009]    The present inventors have devised an innovative CMC laminate structure that provides for improved structural integrity, improved sealing between layers, and improved adherence of any applied overlayer. The proposed structure includes a ceramic deposit additively formed on the CMC stack. The ceramic deposit may be applied such that it bonds at least two adjacent CMC layers to each other. It may also be deposited such that it forms a raised structure that will increase adherence of an overlayer. The ceramic deposit may be the only way the CMC layers are bonded together, and the ceramic deposit may form a gas tight seal so combustion gases do not pass between the CMC layers. Alternately, the CMC layers may also be bonded together and sealed using conventional means, such as with adhesive, such that an interface between adjacent CMC layers may be bonded and sealed using a combination of one or more ceramic deposits and adhesive. The inventors have also devised a method for applying the ceramic deposit using a laser beam to heat and melt ceramic powder to form the ceramic deposit via an additive manufacturing process. 
         [0010]    It is known to melt an edge of a single CMC layer, as disclosed in U.S. Publication number 2007/0075455 to Marini et al. However, Marini discloses merely sealing a free edge of a single layer in order to improve wear resistance or hardness, and this results in a smooth coating/deposit. The method disclosed herein bonds plural CMC layers together along their adjoining edges with a ceramic deposit that may be rougher and therefore more suited for overlayer adherence than the smooth coating of Mariana. As used herein each CMC layer is a discrete structure prior to any bonding operation. That is to say that while each CMC layer may include resin material as part of its composition, abutting CMC layers are not bonded together by the matrix material that may be present within any individual CMC layer. Accordingly, while the CMC layer itself may be a laminate in that it may include fiber layers bonded together by a resin material, each CMC layer is considered a single, discrete CMC layer herein. 
         [0011]      FIG. 1  is an illustration of an exemplary embodiment of a gas turbine component  10  formed of a CMC stack  12  and a ceramic deposit  14  thereon. The CMC stack  12  includes a plurality of CMC layers  16 , such as an oxide-oxide composite. In this exemplary embodiment, each CMC layer  16  is in the form of a layer  18  of an airfoil portion  20  of the component  10 , where the component  10  may be a gas turbine engine blade or vane. Also included is a metal core  30 . In this exemplary embodiment the metal core  30  is partially hollow, with cavities that may function as cooling channels. In this configuration the CMC layers  16  of the CMC stack  12  protect the metal core  30  from combustion gases while the metal core  30  provides strength for the component  10 . However, the disclosure is not meant to be limited to such a specific structure and the teaching may be applied more broadly as would be understood by those of ordinary skill in the art. 
         [0012]    The ceramic deposit  14  is in the shape of a bead that bonds adjacent CMC layers  32  together, similar to an edge weld bead. The adjacent CMC layers  32  define an interface  34  there between (e.g. an area defined by faying surfaces) having a perimeter  36 . A ceramic deposit  14  may extend along part of the perimeter  36  or it may extend along the entire perimeter  36 . Various embodiments of the CMC stack  12  may include ceramic deposits  14  that extend along part of the perimeter  36 , ceramic deposits  14  that extend along the entire perimeter  36 , or a combination of the two. The selection of full or part extension of the ceramic deposit  14  and/or the use of adhesive between adjacent CMC layers  32  may be based on a desired/predetermined mechanical characteristic of the component  10  when complete. For example, partial edge bonding with a ceramic deposit  14  allows for some flexibility within the structure, whereas adhesive alone or adhesive and edge bonding may provide a stronger/less flexible structure. Any combination of edge bonding, adhesive and/or bolting may be used to achieve a desired mechanical characteristic in the component  10 . 
         [0013]    Moreover, the porosity of the ceramic deposit  14  may be controlled by controlling the deposition process to be from approximately forty percent to ninety percent to achieve a desired mechanical characteristic including, for example, permeability and rigidity. When formed as a non-permeable (gas-tight) ceramic deposit, and when formed between adjacent CMC layers  32 , the ceramic deposit  14  seals the adjacent CMC layers  32  such that combustion gases cannot pass there between to reach the metal core  30 . The porosity of the ceramic deposit  14  also controls the modulus of elasticity (rigidity) of the ceramic deposit  14 . The strain tolerance of the ceramic deposit  14  is associated with the modulus of elasticity. Therefore, controlling the porosity can control the rigidity of the ceramic deposit as well as the strain tolerance. Accordingly, if a compliant bond (securement) between the adjacent CMC layers is desired, the ceramic deposit  14  may be made more porous. Alternately, if a rigid bond is preferred, the ceramic deposit  14  may be made less porous. The mechanical characteristics may be controlled such that they are uniform throughout the ceramic deposit  14 , or so that they vary locally from one ceramic deposit  14  to another, or within a given ceramic deposit  14  as desired. 
         [0014]      FIG. 2  is a sectional view of the ceramic deposit  14  of  FIG. 1  along line  2 - 2 , to which an overlayer  38  has been added. The ceramic deposit  14  forms a bead that joins corners  40  of the adjacent CMC layers  32 , thereby forming a seal  42  there between that prevents combustion gases from passing through the interface  34 . The ceramic deposit  14  is raised with respect to a surface  44  of the component  10  formed by edge faces  46  of respective CMC layers  16 . Accordingly, in an exemplary embodiment, the ceramic deposit does not cover the entire edge face  46 . If a ceramic deposit  14  is formed on both corners of one edge face  46 , there may still be a remainder  48  of the edge face  46 , and hence of the surface  44 , that is not covered with the ceramic deposit  14 . The elevated nature of the ceramic deposit  14  relative to surface  44  provides a greater surface area that increases adherence for the overlayer  38 . The ceramic deposit  14  may also be shaped to include features that may better engage the overlayer  38 , such as grooves, overhangs, etc. These, in turn, improve design life and spallation resistance of the overlayer  38 . 
         [0015]      FIG. 3  is an illustration of an alternate exemplary embodiment where the ceramic deposit  14 ′ forms a pattern on the surface  44  of the component. The ceramic deposit  14 ′ is bonded to respective edge faces  46  of at least two adjacent CMC layers  32 , and because it spans the respective interface  34 , the ceramic deposit  14 ′ secures the adjacent CMC layers  32  to each other. As above, the mechanical characteristics can be controlled as desired within the pattern to produce predetermined mechanical characteristics. For example, toward a trailing edge  50 , the ceramic deposit  14 ′ may be deposited to be denser, and hence more rigid, for structural integrity. Toward a leading edge  52 , the ceramic deposit  14 ′ may be more porous and flexible, thereby increasing its ability to absorb impacts, thereby reducing foreign object damage (FOD). In another example, the ceramic deposit  14 ′ may be formed to be gas-tight, yet porous enough to permit minor deformation of the CMC stack  12  proximate the metal core  30 , which provides the ultimate structural stability where present. 
         [0016]    While a crisscross pattern is shown, any pattern may be used as will be understood by those of ordinary skill in the art. For example, beads of the pattern may be spaced closer together where greater overlayer adherence is sought. Likewise, a height, width, aspect ratio (e.g. 3:1 to 5:1 in terms of height/thickness to width), cross sectional shape, and surface roughness of the ceramic deposit  14 ,  14 ′ may also be controlled locally to achieve the balance of structural integrity, flexibility, and overlayer adherence sought. 
         [0017]      FIG. 4  is a schematic illustration of adjacent CMC layers  32  and the interface  34  between the adjacent CMC layers  32 . The interface  34  is defined by an area in between the adjacent CMC layers  32 , akin to a faying area. Openings  60  in the CMC layers  16  receive the metal core  30  (not shown) and the interface  34  stands between combustion gases outside the CMC stack  12  and the openings  60 . Therefore, the interface  34  may be sealed to prevent intrusion of the combustion gases between the CMC layers  16  so that the combustion gas does not reach the openings  60  and the metal core  30  therein. The seal may be achieved by forming the ceramic deposit  14  around the entire perimeter  36  of the interface  34 . Alternately, the seal may be achieved by combining one or more ceramic deposits  14  with adhesive  62  in a manner that provides a continuous seal around the perimeter  36 . 
         [0018]    The adhesive  62  may permit little relative movement between the adjacent CMC layers  32  where applied. The ceramic deposit  14  secures the edges  40  of the adjacent CMC layers  32 , but does not extend into the interface  34 , and therefore may permit more relative movement between the adjacent CMC layers  32 . Accordingly, the interface  34  can be tailored to control relative movement locally within each interface  34  depending on design requirements. 
         [0019]      FIG. 5  schematically illustrates an exemplary embodiment of a method of forming the ceramic deposit  14 ,  14 ′, and in particular the ceramic deposit  14  of  FIG. 1 . In this exemplary embodiment, the ceramic deposit is formed by traversing an energy beam  70  emitted from an energy beam source  72 , such as a laser, to melt ceramic material. The molten ceramic material then cools to form the ceramic deposit  14 . The energy beam source  72  may be a green laser system with  512  nanometer wavelength and may generate a laser beam with a spot size of approximately fifty micrometers. 
         [0020]    The process may be autogenous such that the ceramic that is melted is ceramic from the CMC layers  16 . Alternately, or in addition, ceramic powder  74  may be used as filler and preplaced on the surface  44  where the ceramic deposit  14  is to be formed. The ceramic powder  74  may include particles from one (1) micron and above. Alternately, or in addition, the ceramic powder  74  may be fed to a process location  76  via a ceramic powder stream  78  delivered from a ceramic powder source  80  via a delivery tube  82 . Other embodiments may use a paste, tape or ribbon to provide the ceramic filler material for the ceramic deposit. The ceramic to be melted, whether part of the CMC layers  16  or a separate filler material, may be semi or non-transparent to the selected energy beam  70  in order to capture the heat energy. Filler material may be provided with or without a binder material. 
         [0021]    The process for forming the ceramic deposit  14  may be iterative. In such an exemplary embodiment, the ceramic deposit  14  may be built up in layers, where each layer is produced by melting ceramic in the manner disclosed above. Each layer may be from ten (10) microns thick to two (2) millimeters thick. The component  10  may be positioned in a bed of ceramic powder (not shown), a respective layer formed, the component lowered, and the next layer formed on the previously formed layer. Such a process would allow for one dimensional (1D) prints (ceramic deposit  14 ), two dimensional (2D) prints (ceramic deposit  14 ′), and three dimensional (3D) ceramic deposits, meaning that in the sectional view of  FIG. 2 , a cross-sectional shape of the ceramic deposit  14  could engineered as desired to better adhere the overlayer  38  to the CMC stack  12 , such as with an overhang or undercut. 
         [0022]    The innovative component and method proposed herein enables the manufacture of gas turbine components having improved structural integrity and overlayer adherence. These improvements can be tailored locally between adjacent CMC layers as well as locally in regions of the component spanning plural CMC layers, thereby increasing design flexibility. Accordingly, this represents a significant improvement in the art. 
         [0023]    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.