Abstract:
A ceramic matrix composite (CMC) material ( 10 ) with increased interlaminar strength is obtained without a corresponding debit in other mechanical properties. This is achieved by infusing a diffusion barrier layer ( 20 ) into an existing porous matrix CMC to coat the exposed first matrix phase ( 19 ) and fibers ( 12 ), and then densifying the matrix with repeated infiltration cycles of a second matrix phase ( 22 ). The diffusion barrier prevents undesirable sintering between the matrix phases and between the second matrix phase and the fibers during subsequent final firing and use of the resulting component ( 30 ) in a high temperature environment.

Description:
This application claims benefit of the 22 Jul. 2005 filing date of U.S. provisional patent application No. 60/702,010. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to the field of materials technology, and more particularly to ceramic matrix composite materials. 
     BACKGROUND OF THE INVENTION 
     The current generation two-dimensional laminate porous oxide ceramic matrix composites (CMC) have relatively low interlaminar strength properties. Three-dimensional CMC materials have higher interlaminar strength; however 3D materials are more expensive and have not yet been fully developed for commercial applications, such as for use in the hot gas path of a gas turbine engine. It is known to improve the interlaminar strength of 2D CMC materials by further densifying the porous matrix in a conventional manner with additional sinterable phase matrix material. Unfortunately, as porosity is decreased in such materials, there is a corresponding reduction in in-plane strength (reduced by more than half in some embodiments) and the material becomes brittle as the interconnection between the matrix and the fibers becomes stronger. 
     It is known in both oxide and non-oxide CMC materials to apply an interface coating material to the fiber prior to matrix formation in order to decrease the fiber-matrix interconnection. The interface material functions to deflect cracks forming in the matrix material away from the fibers, thereby preserving the fiber network strength and the resulting in-plane mechanical properties. Unfortunately, fiber tows that are coated with interface coating materials are more difficult and expensive to weave and the coatings tend to spall off of the fibers during weaving. Furthermore, no viable process has yet been demonstrated for solution coating of filaments in fiber form, since close-packed fibers in cross-over points are difficult to coat. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in the following description in view of the drawings that show: 
         FIG. 1  is a schematic illustration of an improved ceramic matrix composite material at a stage of manufacturing wherein ceramic fibers are surrounded by a first phase of a ceramic matrix material. At this stage of manufacture the material is known in the Prior Art. 
         FIG. 2  is the material of  FIG. 1  after further processing to apply a diffusion barrier layer over the first matrix phase and fibers. 
         FIG. 3  is the material of  FIG. 2  after further matrix densification steps. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present inventors have developed innovative processes and resulting novel ceramic matrix composite materials that exhibit improved interlaminar strength without the usual corresponding degree of reduction of other mechanical properties. These improvements are achieved with a matrix material that includes at least two phases separated by a diffusion barrier. The diffusion barrier is effective to limit sintering between the two phases and optionally between one of the matrix phases and the encased ceramic fibers. In one exemplary embodiment, a known oxide/oxide CMC material sold under the brand name A/N720-1 by COI Ceramic, Inc. of San Diego, California is further densified in a bisque fired state with a second matrix phase infused by conventional matrix densification steps only after the porous matrix has been infused with a monazite diffusion barrier material effective to coat exposed surfaces of the bisque fired matrix and embedded fibers. A/N720-1 material utilizes Nextel® N720 fibers (85% alumina and 15% silica in the form of mullite and alumina polycrystals) disposed in an alumina matrix, and the second matrix phase was also selected to be alumina. The monazite diffusion barrier material used in the exemplary embodiment was a lanthanum phosphate (LaPO 4 ). The resulting densified CMC material exhibited fully-fired short beam shear (SBS) in-plane shear strength that was increased by 21% over the known A/N720-1 material not having the densified matrix, and flatwize tensile strength (FWT) interlaminar strength that was increased by 64% over the known material. These improvements were achieved with a corresponding decrease of only 16% in the in-plane tensile strength when compared to the known material. Similar, although less dramatic, improvements have been obtained in test samples of other oxide/oxide CMC materials. 
     The process for forming the improved CMC material  10  of the exemplary embodiment is illustrated schematically in  FIGS. 1-3 , where the CMC material  10  includes ceramic fibers  12  disposed in a ceramic matrix  14 . The matrix  14  of the exemplary embodiment includes a plurality of non-sinterable oxide shapes  16  that provide a degree of dimensional stability to the material  10 . The non-sinterable oxide shapes  16 , which in the exemplary embodiment are dimensionally stable mullite spheres, are interconnected by a sinterable binder material of alumina particles  18  to define the porous matrix  14 . Together, the non-sinterable mullite particles  16  and binding alumina particles  18  may be considered a first matrix phase  19 . The term matrix phase as used herein is meant to include a single type of particles only, or a variety of particle types, or an infused layer of material only, or both particles and infused material together.  FIG. 1  represents the inventive material  10  at a bisque fired stage of manufacture that is known in the art. 
       FIG. 2  illustrates the material of  FIG. 1  after it has been further processed to infuse a diffusion barrier material  20  into the porous matrix material  14 . The diffusion barrier material  20  may coat both the first matrix phase  19 , as illustrated at  20 ′ and the exposed surfaces of the fiber as illustrated at  20 ″. The diffusion barrier material  20  may be infused into the matrix  14  as a precursor material that is subsequently heated to form the diffusion barrier material by processes known in the art. 
       FIG. 3  illustrates the material of  FIG. 2  after it has been further processed through one or more matrix densification steps to deposit a second phase of matrix material  22  to at least partially fill voids in the matrix  14 . The material is then final fired to achieve the improved mechanical properties cited above. The second phase of alumina matrix material may be introduced as aluminum hydroxychloride and then bisque fired to form alumina through one or more cycles as is known in the art to achieve a desired degree of porosity in the matrix  14 . Some voids  24  will remain in the matrix  14 , and in various trials the exemplary embodiment the improved material  10  exhibited a density in the range of 2.89-2.90 g/cc and an open porosity in the range of 18.75-19.65%. This compares to control samples of prior art A/N720-1 material exhibiting a density in the range of 2.86-2.87 g/cc and an open porosity in the range of 19.92-20.06%. Importantly, the diffusion barrier resides between the two matrix phases  19 ,  22 , thereby preventing them from bonding together during sintering. Keeping the two matrix phases from sintering together allows for increased matrix density without increased sintering activity between the two matrix phases. In a secondary role, the diffusion barrier  20  also resides between the fibers  12  and the second matrix phase  22  and also prevents them from sintering together. 
     The diffusion layer compositions may include compositions that form weak debond layers such as traditionally used as fiber/matrix interface coatings; for example monazites, xenotimes, germinates, tungstates, vanadates, zirconia, hafnates, or other material having compatible chemistries and activation energy levels to function effectively as a diffusion barrier for the matrix material. Not only does the present invention provide higher interlaminar strength without a correspondingly high reduction in strain tolerance, notch insensitivity and strength in other material directions, but it also provides a material with higher thermal conductivity, thereby lowering stresses within the material resulting from thermal transients. A further advantage of the diffusion barrier between matrix phases is the prevention of matrix grain growth and continued densification during service. It is known that continued sintering of the alumina particles during service will result in eventual loss of composite ductility and strength. The diffusion barrier of the present invention coats the exposed particle surfaces, thereby preventing sintering associated with surface diffusion. 
     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. For example, the exemplary embodiment of the invention is described as an oxide/oxide CMC material; however other embodiments may include non-oxide/non-oxide or oxide/non-oxide materials. The invention may further be applied to both 2D and 3D laminates. It is believed that a doubling of interlaminar strength and a 25% increase in through-thickness thermal conductivity may be achievable with minimal loss of in-plane strain-to-failure for 2D laminate embodiments of this invention. Even greater improvements in performance may be achievable for 3D laminate embodiments of the invention. Such improvements are significant in applications requiring a tight radius in a constrained geometry, such as when the material  10  is used in a vane  30  of a gas turbine engine. Numerous variations, changes and substitutions may be made without departing from the invention herein. For example, more than two phases of matrix material may be used with corresponding diffusion barriers being disposed between the respective adjacent phases, as is illustrated schematically at region  32  of  FIG. 3 . The various matrix phases  19 ,  22 ,  34  may be the same material or different materials, and the various diffusion barriers  20 ,  36  may be the same material or different materials. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.