Abstract:
A ceramic matrix composite (CMC) airfoil assembled from a pressure side wall ( 42 ) and a suction side wall ( 52 ) joined by interlocking joints ( 18, 19 ) at the leading and trailing edges ( 22, 24 ) of the airfoil to produce a tapered thin trailing edge. The trailing edge ( 24 ) is thinner than a combined thicknesses of the airfoil walls ( 42, 52 ). One or both of the interlocking joints ( 18, 19 ) may be formed to allow only a single direction of assembly, as exemplified by a dovetail joint. Each joint ( 18, 19 ) includes keys ( 44 F,  54 F,  56 F,  46 F) on one side and respective keyways ( 44 K,  54 K,  56 K,  46 K) on the other side. Each keyway may have a ramp ( 45 ) that eliminates indents in the airfoil outer surface that would otherwise result from the joint.

Description:
FIELD OF THE INVENTION 
     This invention relates to ceramic matrix composite (CMC) airfoil structures, and particularly to gas turbine vanes. 
     BACKGROUND OF THE INVENTION 
     Ceramic matrix composites (CMC) are used for components in high temperature environments, such in gas turbine engines. Oxide based CMC is typically formed by combining ceramic fibers with a ceramic matrix, and heating the combined material to a sintering temperature. The fibers add tensile strength in the directions of the fibers. The resulting material has a higher operating temperature range than metal, and can be optimized for strength by fiber orientations and layering. 
     A curved CMC wall has a minimum radius of curvature if any of the ceramic fibers wrap around the curve, due to limited flexibility of the fibers. This presents problems in CMC airfoil fabrication, because an airfoil preferably has a thin trailing edge.  FIG. 1  shows an airfoil with a leading edge  22 , a trailing edge  24 , a pressure side  26 , a suction side  28 , a ceramic matrix material  24 , and multiple ceramic fiber layers  36 . The minimum radius of curvature of the CMC fiber is used for the outermost layer at the trailing edge to form a thin trailing edge  24 . This minimum radius also applies to the inner fiber layers, which creates separations and voids  38  between the layers. 
       FIG. 2  shows a known airfoil design in which the CMC trailing edge  24  has an outer radius that is concentric with the inner layers. Only the innermost layer is limited by the minimum bending radius. This provides uniform layer spacing around the trailing edge  24  without voids. However, an additional trailing edge member  32 , made of metal for example, must be added to provide a thin final trailing edge  33 . 
     Vane airfoils in gas turbines are often hollow and internally pressurized with a coolant gas such as air or steam. This pressure causes a stress concentration at the inner curve  25  of the trailing edge. For laminated constructions, this internal pressure results in an interlaminar tensile stress concentrated at the inner radius of the trailing edge, which tends to be a design-limiting feature. Gas pressures inside and outside of the airfoil vary with engine cycles, causing cyclic stress at the trailing edge. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in the following description in view of the drawings that show: 
         FIG. 1  illustrates a prior art problem that occurs in attempting to form a CMC airfoil with a thin trailing edge by ceramic fiber layer wrapping. 
         FIG. 2  shows a prior solution to the problem of  FIG. 1  by adding a metal trailing edge. 
         FIG. 3  shows a finger joint as known in cabinetry. 
         FIG. 4  shows an assembly direction of the finger joint of  FIG. 3 . 
         FIG. 5  shows a dovetail joint as known in cabinetry. 
         FIG. 6  shows the only assembly direction of the dovetail joint of  FIG. 5 . 
         FIG. 7  is an exploded perspective view of a two-part CMC airfoil per the invention. 
         FIG. 8  is perspective rear view of an airfoil assembled per  FIG. 7 . 
         FIG. 9  is a perspective pressure side view of an airfoil assembled per  FIG. 7 . 
         FIG. 10  is a sectional top view taken on a plane through a suction side key of  FIG. 8 . 
         FIG. 11  is a sectional top view taken on a plane through a pressure side key of  FIG. 8 . 
         FIG. 12  is perspective rear view of an airfoil with both pressure and suction side trailing edge ramps, eliminating the filler of  FIG. 8 . 
         FIG. 13  is a sectional top view taken on a plane through a pressure side key of  FIG. 12 . 
         FIG. 14  is perspective rear view of another embodiment of the invention. 
         FIG. 15  is a sectional top view taken on a plane through the keys and keyways of  FIG. 14 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The inventors have recognized that a thin CMC trailing edge can be achieved by forming an airfoil in two parts, a pressure side wall part and a suction side wall part, tapering the trailing edges of the two parts and bonding them together. However, the resulting trailing edge joint might not be sufficiently resistant to separation and delamination from the stresses previously described, so they conceived the following interlocking airfoil assembly. 
       FIGS. 3 and 4  show a finger joint known in cabinet making. This joint interlocks and prevents relative vertical movement of the two joined parts. It also provides an increased bonding surface with very strong separation resistance.  FIGS. 5 and 6  show a dovetail joint known in cabinet-making. This joint can only be assembled along a single direction shown by the arrow, so it prevents separation in all directions except the reverse of the assembly direction. However, application of these types of woodworking joints to an airfoil is not straightforward because, unlike in woodworking applications, the pressure and suction side walls meet at a narrow angle at the trailing edge, they must merge into a thin edge, and the joint must form an aerodynamically smooth outer surface. 
       FIGS. 7-11  show an airfoil  40  according to aspects of the invention. A pressure side wall part  42  has leading edge keys  44 F separated by leading edge keyways  44 K, and trailing edge keys  46 F separated by trailing edge keyways  46 K. A suction side wall part  52  has leading edge keys  54 F separated by leading edge keyways  54 K, and trailing edge keys  56 F separated by trailing edge keyways  56 K. The keys of each part  42  and  52  interlock in respective keyways of the opposite part along the leading and trailing edges  22 ,  24  of the airfoil to form leading and trailing edge joints  18 ,  19 . 
     The keys have side surfaces  43 ,  55 ,  47 ,  57  that are angled according to the desired joint type and assembly direction. For example, in  FIG. 7  the key side surfaces  47  and  57  are angled to provide a dovetail interlock. In  FIG. 7  the dot-dash lines connect representative points on the two parts  42 ,  52  that meet in the assembly shown in  FIG. 8 . These dot-dash lines indicate one possible assembly direction. However they do not represent a required assembly direction, which is determined by the angles of the side surfaces of the keys. A possible assembly direction D is shown in  FIG. 10 . In this example, it is approximately tangent to a camber line  21  at the leading edge  22 . 
     Two mechanisms for smoothly merging the pressure and suction side trailing edges are shown in the example embodiment of  FIGS. 7-11 . The pressure side wall part  42  has feather-edge ramps  45  between the trailing edge keys  46 F. Each ramp  45  receives a respective suction side trailing edge key  56 F along the ramp such that the final thickness T of the trailing edge equals the thickness of the pressure side wall part. The ramp prevents an indent in the pressure side outer surface that would occur without the ramp. 
     To illustrate an alternative to ramps, the suction side trailing edge keys  56 F are separated by keyways  56 K without ramps. This results in local indents in the outer surface of the suction side that may be filled with ceramic  60  as shown in  FIGS. 8 and 11 . This filler material may be a ceramic insulation material as known in gas turbine shroud coatings. 
       FIG. 10  is a top sectional view through the suction side keys  54 F,  56 F, showing a ramp  45 .  FIG. 11  is a top sectional view through the pressure side keys  44 F,  46 F, showing a filler  60 . The distal ends of the pressure and suction side keys  46 F,  56 F may each have a generally cylindrical radius R of about ½ T, with these radii being coaxial along the trailing edge of the airfoil. 
     A camber line  21  is a mean curve between the pressure and suction surfaces  26 ,  28 . Assembly direction may be tangent to the camber line at the leading edge  22  as indicated in  FIG. 10  or tangent to the camber line  21  at the trailing edge  24 , or along the ramps  45 , among other directions, depending on the angles of the key sides  43 ,  55 ,  47 ,  57 . The embodiment of  FIGS. 7-11  can be assembled along a direction D tangent to the camber line  21  at the leading edge  22 . This assembly direction happens to be approximately perpendicular to the camber line at the trailing edge  24  in the illustrated airfoil geometry. However, this is not a requirement. The side surfaces  43 ,  55 ,  47 ,  57  of the keys must be angled to allow at least one assembly direction, and they may be angled to allow only one assembly direction by using a dovetail geometry. 
       FIG. 12  is perspective rear view of an airfoil with both pressure side trailing edge ramps  45  and suction side trailing edge ramps  64 . These ramps eliminate the need for the filler  60  shown in  FIGS. 8 and 11 .  FIG. 13  is a sectional top view taken on a plane through a pressure side key  46 F of  FIG. 12 . 
       FIGS. 14-15  illustrate an embodiment in which the leading edge keyways  82  are holes in an inwardly extending flap  80  of the suction side part  52 . The trailing edge keyways  84  are holes in the trailing edge of the suction side part  52 . The leading edge keys  72  of the pressure side part  42  are inserted into the leading edge keyways  82 . The trailing edge keys  76  of the pressure side part  42  are inserted into the trailing edge keyways  84 . The leading and trailing edge keyways and keys may be all aligned in substantially the same direction D, allowing assembly in that direction. Surface continuity flaps  74  may be provided between the leading edge keys  72  on the pressure side part  42 . Likewise, surface continuity flaps  78  may be provided between the trailing edge keys  76  on the pressure side part  42 . Ceramic filler may be applied between the continuity flaps to provide an aerodynamically smooth surface over the joints. Alternately, the continuity flaps may not be provided, and ceramic or CMC filler alone may fill and smooth the indents caused by the joints. 
     The keys, keyways, and ramps herein may be formed by machining the CMC parts  42 ,  52  after firing each part. Any indents in the outer surface of the airfoil after assembly can be aerodynamically smoothed by filling with a ceramic filler, such as an insulating ceramic, and/or by applying a thin ceramic fabric or fiber ply overwrap. The airfoil assembly can be fabricated as follows: 
     1. Lay-up the basic CMC parts  42  and  52   
     2. Bisque-fire or a fully fire the parts 
     3. Machine the keys, ramps, tabs per the embodiment 
     4. Apply ceramic adhesive to the keys, and join the two parts 
     5. Fill and/or overwrap any indents in the airfoil outer surface 
     6. Optionally overwrap the joints or the whole airfoil 
     7. Co-cure the assembly 
     The above description refers to typical oxide-based CMC available commercially (e.g., from COI Ceramics Inc.). Analogies to non-oxide CMCs are evident. For example, parts may be machined at an intermediate stage of matrix densification or pyrolysis, assembled and co-processed through subsequent steps to final density. 
     Fabrication is simplified from both a tooling and a lay-up perspective because each part is effectively a curved 2D lay-up, rather than a tube. 3D geometric constraints are minimized in each part, resulting in better microstructure properties in the final material. Drying and sintering shrinkages tend to pull apart laminates with constrained shapes, but each airfoil part  42 ,  52  is less constrained than a layered tube with bends. 
     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.