Abstract:
A stitching geometry and method for selective interlaminar reinforcement of a CMC wall ( 20 A). The CMC wall is formed of ceramic fiber layers ( 22 ) individually infused with a ceramic matrix, stacked, and at least partially cured. A row of holes is formed in the wall, and a ceramic fiber thread ( 25 ) is infused with a wet ceramic matrix and passed through the holes to form stitches ( 28, 30, 31 ). The stitches are then cured, causing them to shrink more than any remaining wall shrinkage, thus tensioning the stitches and compressing the wall laminae together. The stitches may have through-wall portions ( 30, 31 ) that are angled differently in different wall areas as a function of interlaminar shear over interlaminar tension, optimizing wall reinforcement locally depending on magnitude and direction of shear. Alternate rows of stitches ( 54, 56 ) may have offsets in a stitch direction ( 34 ) and/or different through-wall angles (A 1 , A 2 ).

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to ceramic matrix composite (CMC) walls formed of laminated sheets of ceramic fibers, and particularly to interlaminar reinforcement methods and mechanisms for such walls. 
       BACKGROUND OF THE INVENTION 
       [0002]    Ceramic matrix composites (CMC) are used for components in high-temperature areas of gas turbines. CMC walls may be formed by laminating multiple layers of ceramic fabric or fibers in a ceramic matrix. However, interlaminar stresses work to damage the interlayer bonds and separate the layers. Alternately, three-dimensionally woven CMC walls offer improved through-thickness properties over 2D laminated walls. 3D walls are reinforced at the preform stage, during which a ceramic fiber structure is woven to nearly final shape by weaving, braiding, or knitting. However, this technique is not feasible for oxide-based CMCs without considerable investment, because infiltrating a particulate loaded ceramic slurry into a thick, tightly packed, brittle fiber preform is extremely difficult. It can result in incomplete and heterogeneous matrix infiltration, even when using state-of-the-art processing, and even for relatively thin 3D preforms such as 3-4 mm. The problem is worse with more realistic component wall thicknesses such as 5 mm or more. U.S. Pat. Nos. 4,568,594, 4,888,311, 4,921,822, 5,077,243, 5,294,387, 5,306,554, and 5,460,637 teach oxide matrix CMCs in which the largest matrix particles are preferred to be greater than 1 micron in diameter. This size is taught as preferable both for sintering shrinkage control and high temperature stability. However, these large particle sizes are especially difficult to infiltrate into a thick, densely packed fibrous preform, because the preform acts as a filter. All of the above patents also teach bimodal particle size distributions including a smaller size range much less than 1 micron. The smaller particles infiltrate nicely, but they are segregated from the larger particles via the filtration effect. 
         [0003]    In addition, 3D preforms have the following constraints: 
         [0004]    Limited to simple shapes such as extruded, flat, or cylindrical, depending on production method 
         [0005]    Not conducive to localized optimization of reinforcement geometry 
         [0006]    High development costs for complex shapes, due to custom loom setups, etc. 
         [0007]    3D preforms often cannot be compacted for maximum fiber volume without losing the through-thickness reinforcement benefit 
         [0008]    Long development time &amp; expense precludes iterative design approaches 
         [0009]    Expertise resides in a limited number of specialty shops, none of which offer all available options. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The invention is explained in the following description in view of the drawings that show: 
           [0011]      FIG. 1  is a sectional view of a CMC wall formed of layers with interlaminar reinforcement stitching in a first geometry. 
           [0012]      FIG. 2  is a sectional view of a CMC wall formed of layers with interlaminar reinforcement stitching in a second geometry. 
           [0013]      FIG. 3  is a sectional view of a CMC wall formed of layers with interlaminar reinforcement stitching and surface pads. 
           [0014]      FIG. 4  is a sectional view of a CMC wall formed of layers with interlaminar reinforcement stitching with surface loops anchoring a thermal barrier layer. 
           [0015]      FIG. 5  is a sectional view of a CMC airfoil with reinforcement stitching angles varying by local stress type. 
           [0016]      FIG. 6  is a perspective transparent view of an edge portion of a gas turbine shroud ring segment with two overlapping rows of stitching angled to resist interlaminar shear. 
           [0017]      FIG. 7  is a front transparent view of  FIG. 6 . 
           [0018]      FIG. 8  is a top transparent view of  FIG. 6 . One row of stitches is dashed to distinguish it visually from the other row. 
           [0019]      FIG. 9  is a left side transparent view of  FIG. 6 . One row of stitches is dashed to distinguish it visually from the other row. 
           [0020]      FIG. 10  is a front transparent view as in  FIG. 7  with optional additional pairs of stitching rows around an edge of the ring segment. 
           [0021]      FIG. 11  is a perspective transparent view of an edge portion of a gas turbine shroud ring segment with two overlapping rows of doubly angled stitching to resist interlaminar shear at any angle. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    The invention provides selective through-thickness reinforcement applied to a CMC laminated wall after the laminae are stacked and partially or fully cured. Holes may be formed in the wall after at least partial curing, and a ceramic fiber thread is passed through the holes to form a row of stitches. The thread is infiltrated with a wet ceramic matrix before or after stitching, and then heated/cured after stitching. The ceramic fibers and matrix of the thread shrink during firing, thus creating a tensile preload in the stitches. Selection of a firing temperature for the CMC wall, and selection of the type of thread fiber determines the amount of tensile preload created. 
         [0023]    For example, bisque firing of the CMC wall can partially shrink the wall in order to leave a remaining shrinkage approximately equal to the full shrinkage of a thermal barrier coating (TBC) during a final curing. The present stitching may be done after the bisque firing and before applying the TBC. If wet stitching is done on a bisque-fired CMC laminate, shrinkage of the thread fibers and thread matrix can be greater than that of the bisque-fired laminate during final curing, thus imparting tension on the stitching. This provides a corresponding through-thickness compression on the laminate. Holes in the CMC wall for the stitches may be formed using means such as laser or water jet cutting, or the holes may be formed by using either fugitive or removable pins. For example, holes may be formed in the wet lay-up stage by insertion of pins or rods through the wet preform (e.g., through apertures in the tooling). The pins may be removed following drying and partial cure of the laminate and prior to bisque firing. Holes with diameters less than 1 mm in diameter have been shown to have little effect on in-plane properties in CMC walls, which are relatively notch-insensitive. Larger diameter holes are also possible, but the resultant in-plane strength debits would have to be accounted for. 
         [0024]    A weaving thread  25  may comprise a single tow or multiple tows of fibers. For example, a single tow or bundle of fibers may have a cross section of about 0.5 mm in a circular cross section. In another example it may be 1 mm wide by 0.25 mm thick. A thread  25  may be on the order of the ply thickness or larger. Multiple tows may be used to form larger threads. These example dimensions are not intended to be limiting. 
         [0025]      FIG. 1  shows a reinforced CMC wall  20 A formed of laminated ceramic fiber sheets  22 . The wall  20 A has first and second exterior surfaces  24 ,  26 . A ceramic fiber thread  25  has surface-spanning portions  28 , oblique through-wall portions  30 , and a stitch direction and length  34 . Herein “oblique” means neither parallel nor perpendicular to a CMC wall surface plane, or to a tangent plane in the case of a curved wall surface. “More oblique” means more divergent from perpendicular to the CMC wall surface plane or tangent plane. Angle A 1  illustrates an angle of divergence of a through-wall portion  31  of a stitch from a line perpendicular to the CMC wall surface plane or tangent plane along the row of stitches. Oblique stitching as in  FIG. 1  resists interlaminar shear in the stitch direction  34 . 
         [0026]      FIG. 2  shows a reinforced CMC wall  20 B formed of laminated ceramic fiber sheets  22 . The wall  20 B has first and second exterior surfaces  24 ,  26 . A ceramic fiber thread  25  has surface-spanning portions  28 , and perpendicular through-wall portions  30 . Perpendicular stitching as in  FIG. 2  resists interlaminar tension or separation. 
         [0027]      FIG. 3  shows a reinforced CMC wall  20 C formed of laminated ceramic fiber sheets  22 . The wall  20 C has first and second exterior surfaces  24 ,  26 . A ceramic fiber thread  25  has surface-spanning portions  28 , and oblique through-wall portions  30 . Pads or bars  32  under the surface-spanning portions  28  distribute stress between the thread  28  and the surface  22 ,  24 . The pads  32  may also provide gripping surfaces for any applied thermal barrier coating (not shown) on the first surface  24  and/or any ceramic backing or core (not shown) on the second surface  26 . The pads may also serve to increase the local bending radius of the stitching fibers to minimize stress concentrations due to bending. The pads  32  may be ceramic or CMC. 
         [0028]      FIG. 4  shows a reinforced CMC wall  20 D formed of laminated ceramic fiber sheets  22 . The wall  20 D has first and second surfaces  24 ,  26 . A ceramic fiber thread  25  has surface-spanning portions  27 ,  28 , and oblique through-wall portions  30 . The surface-spanning portions  27  on the first wall surface  24  are formed as loops during stitching. Then a wet ceramic thermal barrier coating  42  is applied and flows through and under the loops and between the thread  25  and the fiber sheets  22 . During final curing, the loops  27  anchor the thermal barrier coating. Shrinkage of the thread creates a preloaded connection between the wall and the TBC. Pads or bars  32  are shown under the surface-spanning portions  28  of the thread  25  on the second wall surface  26 . This distributes stress between the thread  28  and the wall surface  26 . One may appreciate that the pads or bars may also be used on surface  24  in conjunction with coating  42 . 
         [0029]      FIG. 5  shows a hollow CMC airfoil  40  with a wall formed of laminated ceramic fiber sheets  22  with a thermal barrier coating  42 . The airfoil  40  has a leading edge  43 , a trailing edge  44 , a pressure side  45 , and a suction side  46 . In this example, it may be assumed that the leading and trailing edges experience mostly interlaminar tension, while the pressure and suction sides experience mostly or largely interlaminar shear. The through-wall portions  30  of the stitches  25  vary from substantially normal to the lamina  22  at the leading and trailing edges to oblique at the pressure and suction sides of the airfoil. This provides local optimization of reinforcement. 
         [0030]    With a varying stitch geometry such as in  FIG. 5 , each through-wall portion  30  of a stitch may have an angle that departs from a line normal to the CMC wall as a function of the ratio of interlaminar shear over interlaminar tension, up to a maximum angle A 1  such as 60 degrees, for example. Thus, a stitch that resists mainly interlaminar tension has approximately perpendicular through-wall portions, and a stitch geometry that resists largely interlaminar shear has oblique through-wall portions. 
         [0031]    In  FIG. 5  the stitches  28 ,  30  may follow a path around the leading edge  43  generally transverse to a span of the airfoil  40 . The trailing edge  44  shows a similar geometry for a first row of stitches  25 . In addition, a second row of the stitches  33 , shown as a dashed line, follows a path approximately parallel to that of the first row of stitches  25 . Each stitch of the second row  33  is offset from each respective stitch of the first row  25  by approximately 50% of each respective stitch length. In other words, the peaks of the second row are adjacent to the valleys of the first row. Such alternating rows of stitches may be placed all along the span of the airfoil. Alternatively, the lines of stitches may be oriented along the span. This is particularly useful if the leading or trailing edges require perpendicular through-wall stitches. 
         [0032]      FIG. 6  shows an edge portion of a CMC shroud ring segment  50 , with first  54  and second  56  rows of stitches, each stitch having a surface-spanning portion  28  and a through-wall portion  30 . A thermal barrier layer  42  is shown on the ring segment  50  as known in the art. The first row of stitches  54  resides substantially in a first stitching plane  60  ( FIG. 7 ) that is oblique to the wall structure  52 . The second row of stitches  56  resides in a second stitching plane  62  that is substantially a mirror image of the first stitching plane across a mirror plane  64  normal to the wall structure and parallel to the first row. Each stitch of the first row  54  is offset from each respective stitch of the second row  56  in the stitch direction by approximately 25% of each respective stitch length. This allows the two rows of stitches to overlap without intersecting threads. Angle A 2  ( FIG. 7 ) illustrates an angle of a stitching plane  62  relative to the mirror plane  64 . 
         [0033]      FIGS. 8 and 9  show top and left side views of  FIG. 6 . The second row of stitches  56  is dashed for visual clarity.  FIG. 10  shows a view as in  FIG. 7  with additional pairs of stitching rows. The stitch geometry of  FIGS. 6-10  especially resists interlaminar shear perpendicular to an edge of the ring segment. 
         [0034]      FIG. 11  shows a stitch geometry similar to that of  FIG. 6 , except that each stitch has wall-crossing portions  31  that are oblique to wall spanning portions  28  within the stitch plane  60 ,  62  in addition to having oblique stitching planes  60 ,  62 . Each stitch of the first row  55  is offset from each respective stitch of the second row  57  by approximately 25% of each respective stitch length, allowing the two rows of stitches to overlap without intersecting threads. This stitch geometry resists interlaminar shear in all directions. 
         [0035]    A CMC wall formed of laminated ceramic fiber sheets made of a mullite-based fiber, such as the alumina-rich mullite fibers sold under the trademark Nextel™ 720 may be stitched with fibers having a higher alumina content fiber, for example Nextel™ 610 or Nextel™ 650 fibers. While the higher alumina content Nextel™ 610 and Nextel™ 650 fibers have lower strength at temperature and experience more rapid grain growth and strength degradation than Nextel™ 720 fibers, even in their degraded form they offer much greater strength than the through-thickness matrix-dominated CMC laminate. While such higher alumina fibers may degrade somewhat on the surface of the wall, which is the hottest portion of the structure, the subsurface properties will be affected to a much lesser degree. Interlaminar shear and tensile stressed tend to peak at mid-thickness (e.g. interlaminar tension is zero at the free surfaces) where the stitching fibers are maintained at a more moderate temperature. Thus, the somewhat lower temperature capability fibers with higher alumina content can be used effectively. Advantageously, the higher alumina Nextel™ 610 or Nextel™ 650 fibers experience greater shrinkage at typical CMC firing temperatures, thereby resulting in greater prestress potential. Such fibers also have a higher coefficient of thermal expansion, resulting in greater prestress at ambient temperature or any temperature less than the sintering temperature. These fibers also have higher thermal conductivity, resulting in improved laminate through-thickness conductivity and lower temperature gradients and thermal stresses. They are also more amenable to tight radius stitching. 
         [0036]    The formula below can be applied to angle A 1  as in  FIG. 1  and/or to angle A 2  as in  FIG. 7  or to both A 1  and A 2  independently for different shear directions as in  FIG. 11 . 
         [0000]        A =( S/T )* C    
         [0037]    where: A is Angle A 1  and/or A 2 , limited to a given maximum
       S is Interlaminar shear in the direction of the angle A 1  or A 2     T is Interlaminar tension (perpendicular tension between lamina)   C is a Constant, such as 10, or a variable or function       
 
         [0041]    Benefits of the invention include: 
         [0042]    Selective reinforcement in areas of high stress only 
         [0043]    Stitch geometry can be optimized for each local wall area differently. 
         [0044]    Avoids high development cost of 3D fiber preforms and tooling. 
         [0045]    Allows matrix infiltration of each thin lamina individually, avoiding problematic and expensive thick-wall infiltration for 3D preforms. 
         [0046]    Preloads CMC walls with through-thickness compression. 
         [0047]    Not limited by looms and textile machinery. 
         [0048]    Experimentation can be performed within a testing laboratory with rapid feedback for iterative design. 
         [0049]    Stitch fibers on CMC surfaces add grip for thermal barrier coatings and ceramic backings or cores. 
         [0050]    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.