Abstract:
A laminated ceramic matrix composite structure is strengthened with one or more layers of a metal reinforcement. The metal reinforcement is selected to provide optimal strength and thermal compatibility with the ceramic matrix composite. The metal reinforcement includes an outer oxidized layer that bonds to the ceramic matrix composite. It may also include a barrier layer on the surface of the metal that helps prevent further oxidation. The structure is formed using standard composite prepreg layup techniques.

Description:
TECHNICAL FIELD 
     This disclosure generally relates to laminated composite structures, especially those using a ceramic matrix, and deals more particularly with a method for making a hybrid metal-reinforced ceramic matrix composite structure, as well as a composite structure produced thereby. 
     BACKGROUND 
     Ceramic matrix composite (CMC) structures may be used in aerospace and other applications because of their ability to withstand high operating temperatures. For example, CMC structures may be used where parts are subjected to high temperature exhaust gases in aircraft applications. Generally, laminated CMC composite structures may have relatively low impact resistance, particularly where the impact is localized as a result of sudden point loads. This low impact resistance stems in part from the fact that these CMC laminates may be formed from fibers held in a ceramic matrix, which may have less than optimal ability to absorb or dampen the energy resulting from localized impacts. 
     One solution to the problem mentioned above consists of adding additional layers of CMC laminate materials in order to strengthen the structure, however this solution may be undesirable in some applications because of the additional weight it adds to the aircraft component. 
     Hybrid laminate materials are known in which composite layers comprising continuous fibers in a resin matrix are interspersed with layers containing metal. For example, TiGr laminates have been developed comprising interspersed layers of graphite composite and titanium. Similarly, laminates having glass composite layers interspersed with aluminum layers are also known. However, none of these prior material systems is readily adaptable for use in strengthening CMC structures. 
     Accordingly, there is a need for a hybrid metal-ceramic matrix composite structure in which the CMC laminates are reinforced to resist localized impact loads, but yet avoid materials that add substantial weight to the structure. There is also a need for a method of making the hybrid structures mentioned above that is both repeatable and well suited for production environments. 
     SUMMARY 
     The disclosed embodiments provide a method of making a hybrid metal-ceramic matrix composite structure exhibiting greater resistance to localized impact loading and improved ductility. Additional benefits may also include, but are not limited to, enhanced lightning strike capability and higher thermal conductivity. 
     According to one disclosed method, a hybrid metal-ceramic matrix composite structure is fabricated by: forming a reinforcing layer containing a metal reinforcement; forming an oxide on the surface of the metal reinforcement; forming a layup including placing the reinforcing layer between layers of continuous ceramic fibers pre-impregnated with a ceramic matrix; and, curing the layup to bond the layers of ceramic fibers to the reinforcing layer. Forming the reinforcing layer may include rolling a mesh pattern into a sheet of metal. The layer of oxide may be formed by applying a metal coating on the surface of the metal reinforcement, and oxidizing the metal coating. The method may further include sintering the ceramic matrix by heating the cured layup in a furnace for a pre-selected period of time. 
     According to another disclosed method embodiment, a composite structure is fabricated by: providing multiple plies of continuous ceramic fibers pre-impregnated with a ceramic matrix; forming at least one reinforcing ply containing a continuous metal reinforcement having a coefficient of thermal expansion (CTE) generally matching the CTE of the ceramic fibers; forming a layup by placing the reinforcement ply between multiple plies of ceramic fibers; and, bonding the reinforcing ply to the ceramic matrix by curing the layup at elevated temperature. The reinforcing ply may be formed by weaving metal and ceramic fibers together to form a metal-ceramic mesh. The reinforcing ply may also be formed by: providing a sheet of nickel-cobalt ferrous alloy having the thermal expansion characteristics of borosilicate glass; applying a nickel coating on the alloy sheet, and heating the alloy sheet to a temperature sufficient to oxidize the nickel coating. 
     According to another disclosed embodiment, a laminated composite structure is provided, comprising: multiple layers of ceramic fibers held in a ceramic matrix; and, at least one reinforcing layer including a metal bonded to the ceramic matrix and having a coefficient of thermal expansion (CTE) generally matching the CTE of the ceramic fibers. The metal in the reinforcing layer may comprise a mesh, a perforated metal foil, a woven braid, foil strips, or wires. The surface of the metal includes an oxide barrier coating which may be a metal, a glass or a layered impermeable oxide. 
     The disclosed embodiments satisfy a need for ceramic matrix composite structures that are reinforced to resist mechanical impact loads as well as stress from thermal cycling, while remaining light-weight. 
     Other features, benefits and advantages of the disclosed embodiments will become apparent from the following description of embodiments, when viewed in accordance with the attached drawings and appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE ILLUSTRATIONS 
         FIG. 1  is a perspective view of an airplane having high temperature, jet engine components that may be fabricated according to the disclosed embodiments. 
         FIG. 2  is a perspective view of a section of a hybrid metal-ceramic composite structure according to one of the disclosed embodiments. 
         FIG. 3  is a sectional view taken along the line  3 - 3  in  FIG. 2 . 
         FIG. 4  is a plan view of a rolled metal screen mesh used as reinforcement in the composite structure shown in  FIG. 3 . 
         FIG. 5  is a plan view showing a perforated metal foil mesh comprising an alternate form of the reinforcement. 
         FIG. 6  is a plan view showing a woven metal braid comprising another alternate form of the reinforcement. 
         FIG. 7  is a plan view of interwoven metal and ceramic fiber comprising another alternate form of the reinforcement. 
         FIG. 8  is a perspective view showing two orthogonally arranged sets of foil strips comprising another alternate form of the reinforcement. 
         FIG. 9  is a sectional view of a CMC structure reinforced with continuous, interleafed sheets of metal comprising another alternate form of the reinforcement. 
         FIG. 10  is a sectional view of a CMC structure with orthogonally arranged metal wires comprising another alternate form of the reinforcement. 
         FIG. 11  is a simplified flow diagram of one method embodiment. 
         FIG. 12  is a more detailed diagrammatic flow diagram showing another method embodiment. 
         FIG. 13  is an enlarged, plan view of a portion of the wire mesh produced by the method shown in  FIG. 12 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring first to  FIGS. 1-4 , a hybrid metal-ceramic matrix composite (CMC) structure  20  may be used in parts subjected to high temperatures, such as, without limitation, the exhaust nozzle  22  and exhaust plug  24  on jet engines  26  of an aircraft  28 . As used herein, the term “ceramic matrix composite” refers to a composite created from continuous fibers bound in a ceramic matrix. The fibers can be in tape or cloth form and may include, but are not limited to, fibers formed from silicon carbide, alumina, aluminosilicate, aluminoborosilicate, carbon, silicon nitride, silicon boride, silicon boronitride, and similar materials. The ceramic matrix may include, but is not limited to, matrices formed from aluminosilicate, alumina, silicon carbide, silicon nitride, carbon, and similar materials. 
     The hybrid metal-CMC structure  20  broadly includes one or more reinforcing layers  30  interleafed between multiple layers  32  comprising continuous ceramic fibers held in a ceramic matrix. In the illustrated example, the hybrid metal CMC structure  20  comprises, from top to bottom as viewed in  FIG. 3 , two layers  32  of ceramic matrix composite, a single reinforcing layer  30 , eight layers  32  of ceramic matrix composite, one reinforcing layer  30 , and two layers  32  of ceramic matrix composite. A variety of other sandwich constructions are possible depending on the application. The hybrid metal-CMC structure  20  may contain as few as one reinforcing layer  30  or a plurality of such layers  30  interleafed at various positions between the layers  32  of ceramic matrix composite. 
     In the embodiment illustrated in  FIGS. 2-4 , the reinforcing layers  30  each include a metal screen mesh  40  having openings  42  that may be penetrated by the ceramic matrix during fabrication of the hybrid metal-CMC structure  20 , resulting in fusion of layers  30 ,  32 . The mesh  40  includes interconnected metal elements  34  having an outer oxide layer  36  that is bonded to the surrounding ceramic matrix. 
     The reinforcing layer  30  includes metal that may be in any of various continuous forms. For example, as shown in  FIG. 5 , the reinforcing layer  30  may comprise a metal foil sheet  44  containing perforations  46 . Alternatively, as shown in  FIG. 6  the reinforcing layer  30  may comprise a woven metal braid  48 . As shown in  FIG. 7 , it may be possible to form the reinforcing layer  30  from interwoven metal fibers  50  and ceramic fibers  52 . 
       FIG. 8  illustrates another form of the reinforcing layer  30  in which the metal reinforcement is formed by parallel strips  54  of metal foil. The parallel strips  54  of metal foil in multiple layers  30   a ,  30   b  may be arranged at differing angles for example, orthogonally, where more than one reinforcing layer  30  is used to strengthen the hybrid structure  20 . 
       FIG. 9  illustrates the possibility of using continuous, flat metal foil sheets  56  sandwiched between layers  32  of ceramic matrix composite. Still another embodiment is shown in  FIG. 10  in which the reinforcing layers  30  are formed by parallel metal wires  58  that may be orthogonally arranged in multiple layers  30 . 
     The metal  34  used in the reinforcing layer  30  may have a coefficient of thermal expansion (CTE) that generally matches, and may be as close as possible to, the CTE of the ceramic matrix composite. Where the ceramic matrix composite comprises alumina fibers in an aluminosilcate matrix, a metal  34  may be selected that is relatively soft and has a relatively low CTE in order to form a satisfactory bond with the CMC. For example, iron and nickel-based metal alloys such as KOVAR® and Alloy 42 may be good candidates for use with alumina fiber based CMCs. KOVAR® is a nickel-cobalt ferrous alloy having thermal expansion characteristics similar to borosilicate glass which are approximately 5×10 −6 /K between 30° C. and 200° C., to approximately 10×10−6/K at 800° C. KOVAR® typically comprises 29% nickel, 17% cobalt, 0.2% silicon, 0.3% manganese, and 53.5% iron (by weight). The term KOVAR® is sometimes used as a general term for FeNi alloys exhibiting the thermal expansion properties mentioned above. 
     Other “superalloys” in which the base alloying element is usually nickel, cobalt, or nickel-iron, may also be suitable. Superalloys exhibit good mechanical strength and creep resistance at high temperatures, good surface stability, and corrosion and oxidation resistance. It may be possible, however to employ metals  34  that are harder and have higher CTEs, depending upon the materials used as the ceramic matrix  38  (shown in  FIG. 13 ). 
     The exact geometry of the reinforcing layer  30  will vary depending upon the application, and consideration may be given to a variety of parameters in selecting feature size and geometry of the alloy metal  34  included in reinforcing layer  30 , including, but not limited to: gauge or thickness; open area per square inch; distribution per square inch; and, patterns and angles. 
     Attention is now directed to  FIG. 11  which illustrates, in simplified form, the steps of one method embodiment for making the hybrid metal-CMC structure  20 . Beginning at  60 , the metal reinforcement  34  is fabricated using any of various processes such as roll forming a metal foil, weaving, braiding, or extrusion, to name only a few. Next, at step  62 , it may be necessary to prepare the surface of the metal reinforcement  34 , as will be discussed in more detail below. For example, it may be necessary to apply a barrier coating (not shown) to the metal reinforcement  34  in order to protect the underlying metal alloy from excessive oxidation or other chemical changes during the subsequent processing steps or after the hybrid metal-CMC structure  20  is placed into use. 
     At step  64 , an oxide coating  36  ( FIG. 3 ) is formed over the surface of the metal reinforcement  34 , or over the barrier coating where applicable. As will be discussed later, the oxide coating  36  applied at  64  is intended to enhance the bond created between the metal reinforcement  34  and the ceramic matrix  38 . The exact type of metal oxide will depend upon the type of ceramic oxide used in the ceramic matrix  38 . 
     Next, at step  66 , a layup is formed comprising multiple CMC layers  32  between which one or more of the reinforcing layers  30  have been interleafed. At  68 , the layup formed at  66  is compacted and cured using conventional techniques and equipment, such as heated presses, vacuum bagging and autoclaving. Finally, as shown at step  70 , the cured layup is subjected to post cure processing that may include, but without limitation, sintering in which the cured layup is heated in a furnace in order to fuse the ceramic matrix  38 . 
     Attention is now directed to  FIGS. 12 and 13  which diagrammatically illustrate additional details of a method for fabricating the hybrid metal-CMC structure  20  using selected materials. Beginning at  72 , a selected metal alloy foil  74 , such as 0.005 inch thick KOVAR® is slit and stretched using a roll tool  76  to form a metal mesh  40  having, for example, 100 openings per square inch. After being slit and stretched, the mesh  40  may be somewhat uneven in cross section, as shown at  78 . Consequently, the mesh  40  is rolled and flattened at  80  so that the openings in the mesh  40  are even, as shown at  82 . The mesh  40  is then coated at  84  with a suitable metal such as nickel. As shown at  86 , the nickel coating  88  surrounds the KOVAR® mesh  40 . At step  90 , the nickel coated KOVAR® mesh  40  is heat treated, for example at 1500° F. for three hours in order to oxidize the surface of the nickel coating  88  and thereby produce an outer layer  92  of nickel oxide covering the nickel coating  88 . It should be noted here that while a nickel coating  88  has been illustrated in connection with the disclosed embodiment, other suitable barrier coatings are possible, including glass type coatings and complex layered, impermeable oxides. 
     At step  94 , woven or knitted sheets  95  of ceramic fibers are immersed in a ceramic slurry at  96  to form prepreg ceramic fiber sheets  98 . At  99 , a layup  98  is formed by stacking the prepreg sheets  98  with one or more interleafed reinforcing layers  30  containing the metal reinforcement  34 . In one embodiment, a suitable layup  98  may comprises two plies of N610 CMC prepreg sheets  98 , followed by one sheet of the mesh  40 , eight plies of the prepreg sheets  98 , one ply of the mesh  40 , followed by two plies of the prepreg sheets  98 . 
     Next, at  102 , the layup  100  is placed between caul plates  104  and is sealed in a vacuum bag (not shown). The vacuum bagged layup  100  is then placed on a platen press (not shown) or is placed in an autoclave  108 , as shown at step  106 . The layup  100  is cured at appropriate temperatures and pressures for a pre-selected period of time. For example, the layup  100  described above may be subjected to a low temperature cure profile that may range from 150 to 450° F. and pressures up to 100 psi. 
     At step  110 , the part  100  may be subjected to post-cure processing, such as sintering within a furnace (not shown). For example, the part  100  may be subjected to an elevated temperature, pressureless post cure profile in a furnace that may range from 500° F. to 2200° F. 
       FIG. 13  illustrates more clearly a section of the KOVAR® mesh  40  having a nickel coating  88  covered by a layer of nickel oxide  92  that forms an interfacial bond with the surrounding, ceramic matrix  38 . The strength of the bond between the nickel oxide  92  and the ceramic matrix  38  is tailored to optimize the properties of the hybrid composite. 
     The process described in connection with  FIG. 12  utilizes the nickel coating  88  as a barrier between the nickel oxide coating  92  which may be required to bond the KOVAR®  40  to the ceramic matrix  38 . As previously mentioned, the underlying nickel coating  88  prevents excessive oxidation of the KOVAR® mesh  40  which may occur either during the fabrication stages of the hybrid metal-CMC structure  20 , or as a result of sustained elevated temperatures when the structure  20  is placed in service. However, depending upon the metal alloy that is chosen for the metal reinforcement layer  30 , it may not be necessary to employ a barrier coating, such as the nickel coating  88 , but rather it may be possible to directly oxidize the outer surface of the base metal from which the reinforcement layer  30  is fabricated. Also, other techniques may be used to control the possible continued oxidation of the underlying base metal  40 , where a barrier layer  88  is not used. 
     Although the oxide coating  92  is produced by oxidizing the underlying barrier coating  88  in the embodiments illustrated above, alternately, it may be possible to apply and bond an oxide coating to the underlying barrier layer  88 , comprising an oxide other than that of the base metal forming the barrier layer  88 . 
     Although the embodiments of this disclosure have been described with respect to certain exemplary embodiments, it is to be understood that the specific embodiments are for purposes of illustration and not limitation, as other variations will occur to those of skill in the art.