Abstract:
A toughened composite material, having a first phase defining a matrix and a plurality of typically second phase particles dispersed in the first phase matrix. Each respective particle is characterized by a predetermined geometric architecture, such as a spiral shape. The presence of the geometrically distinct dispersed second phase operates to deflect and attenuate crack propagation.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to co-pending U.S. Provisional Patent Application Ser. No. 60/972,493 filed on Sep. 14, 2007. 
    
    
     GRANT STATEMENT 
     Research leading to this invention was federally supported by grant No. FA9550-06-1-0125 from the U.S. Air Force Office of Scientific Research. The U.S. government has certain rights in this invention. 
    
    
     TECHNICAL FIELD 
     The novel technology relates generally to the materials science, and, more particularly, to a method for producing a toughened ceramic material through the dispersal of a second phase therethrough characterized by a generally spiral architecture. 
     BACKGROUND 
     Ceramic materials are typically strong in compression but are generally weak in tension or under torsional forces. Typically, ceramic materials fail in tension and/or under torsion via a crack propagation mechanism. Ceramic materials may be toughened by adding a second phase, such as carbon fibers, to form a composite material. However, the addition of such a second phase may complicate the formation process, adding expense. Further, the operating range of both phases may be very different; for example, carbon fibers may oxidize under high temperature refractory conditions and thus may not be an optimal toughening choice for refractory materials. Thus, there remains a need for a means to toughen refractory ceramic materials. The present novel technology addresses this need. 
     SUMMARY 
     The present novel technology relates generally to the toughening of refractory ceramic materials, such as zirconium diboride, and, more particularly, to a method and apparatus for preparing and forming two or more dissimilar materials into an architecture consisting of a first phase characterized by interpenetrating spirals dispersed in a second matrix phase. One object of the present novel technology is to provide an improved composite material system. Related objects and advantages of the present novel technology will be apparent from the following description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically illustrates a process for making a composite material according to a first embodiment of the present novel technology. 
         FIG. 2  is a schematic view of two sheets of materials made of respective first and second phases rolled together according to  FIG. 1 . 
         FIG. 3  is a schematic view of the extrusion step of  FIG. 1 . 
         FIG. 4  is an enlarged photographic view of the rolled spiral sheets of  FIG. 1 . 
         FIG. 5  tabularly represents several compositions of the present novel technology. 
         FIG. 6  schematically illustrates a process for making a composite material having spiral silicon carbide members embedded in a zirconium diboride matrix according to a second embodiment of the present novel technology. 
         FIG. 7A  is a photomicrograph illustrating a first composite composition of the present novel technology fired to 1900 degrees. 
         FIG. 7B  is a photomicrograph illustrating a first composite composition of the present novel technology fired to 2000 degrees. 
         FIG. 7C  is a photomicrograph illustrating a third composite composition of the present novel technology. 
         FIG. 7D  is an enlarged photomicrographic view of the embodiment of  FIG. 7C . 
         FIG. 8A  is a first photomicrograph illustrating crack propagation in the composition of  FIG. 7A . 
         FIG. 8B  is a second photomicrograph illustrating crack propagation in the composition of  FIG. 7A . 
         FIG. 8C  is a third photomicrograph illustrating crack propagation in the composition of  FIG. 7A . 
         FIG. 8D  is a fourth photomicrograph illustrating crack propagation in the composition of  FIG. 7A . 
         FIG. 9  is a first graph illustrating the toughness of a composite material having a dispersed spiral phase. 
         FIG. 10  is a second graph illustrating the toughness of a composite material having a dispersed cylindrical phase. 
         FIG. 11A  is a photomicrograph illustrating a composite material having a dispersed second phase characterized by a spiral architecture. 
         FIG. 11B  is a photomicrograph illustrating a composite material having a dispersed second phase characterized by a first alternate geometric architecture. 
         FIG. 11C  is a photomicrograph illustrating a composite material having a dispersed second phase characterized by a second alternate geometric architecture. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     For the purposes of promoting an understanding of the principles of the novel technology, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the novel technology is thereby intended, such alterations and further modifications in the illustrated device, and such further applications of the principles of the novel technology as illustrated therein being contemplated as would normally occur to one skilled in the art to which the novel technology relates. 
     As generally illustrated in  FIGS. 1-11C , the present novel technology relates to a composite material  10  wherein two or more dissimilar materials or phases  15 ,  20  are combined into an architecture consisting of interpenetrating first phase members  15 , each characterized by a predetermined, discrete geometry, such as spirals, dispersed in a second phase matrix  20 . This architecture enables the production of high fracture toughness composite materials  10  combining advantageous properties of two or more dissimilar phases  15 ,  20 . In addition to increased fracture toughness, the composite material  10  typically enjoys the enhancement of one or more material properties, tailored through the choice of the individual phases  15 ,  20  so combined. For example, both thermal shock resistance and oxidation resistance of zirconium diboride  20  are generally improved by the addition of SiC particles  15  having a spiral geometry to the ZrB 2  matrix  20 . Physical properties of the bulk matrix material  10  such as creep resistance, dielectric behavior, thermal conductivity, electrical conductivity, dielectric constant and the like may also be tailored through the material choice, geometry, and orientation of the added particles of the first phase material  15  having a predetermined geometry or geometries. 
     The properties of the end composite  10  may be influenced by such factors as the physical properties of their constituent phases  15 ,  20 , the relative concentrations of the constituent phases  15 ,  20 , the orientation of the dispersed phase(s)  15  in the matrix  20 , and the like. For example, if the dispersed phase members  15  are properly and substantially uniformly oriented, some of the physical properties of the resultant composite material  10  may be made highly anisotropic; alternately, if the dispersed phase members  15  are randomly oriented, the physical properties of the resultant composite material  10  may still be altered while remaining isotropic. The present novel technology achieves these results in fewer steps than previous coextrusion techniques, allowing for such benefits as increased processing efficiency, reduced production costs, accelerated production of components and the like. 
     In one embodiment, the dispersed first phase material  15  may be prepared from powder polymer blends incorporating ceramic or like precursor materials to be formed into predetermined geometric shapes, such as spirals, spheres, cones, cylinders, ellipsoids, cubes, tetrahedrons, parallelepipeds, pyramids, and the like (see  FIGS. 11A-C ). Such geometric architectures are achieved by mixing between about  40  and about  60  volume percent of a first desired powder material  25  with a thermoplastic polymer  30  suitable for extrusion (see  FIG. 1 ); the about 40 to about 60 volume percent range is typical, although gratifying results may be achieved with compositions outside this range. While in this example the precursor materials  25 ,  30  are powders, each respective precursor  25 ,  30  may alternately be introduced in liquid form, granular form, or any convenient form. The ceramic and polymer precursor powders  25 ,  30  are typically mixed to yield a substantially homogeneous admixture or blend  35  with the ceramic phase  25  dispersed in a polymer matrix  30 . Once a substantially homogenous blend  35  has been formed, the dispersed-phase precursor material  35  is typically formed  37  into a sheet  40  of the desired thickness  42 . This process is then repeated for a second desired powder material  45  to yield sheets  55  of a second composition  60  (a second desired powder material  45  dispersed in a second thermoplastic resin matrix  50 ) and characterized by a second desired thickness  62 . Typically the second desired composition  60  and the second desired thickness  62  will be different from the respective first desired admixture composition  35  and the first desired thickness  42 ; however, one or both may be the same. These sheets  40 ,  55  are then layered one on top of the other (typically with alternating compositions) and rolled up  63 , such as from one edge, until a rolled member  65  of the desired diameter is achieved (see  FIG. 2 ). The relative thickness  42 ,  62  of these sheets, one to another, defines the final geometry  70  of the resultant spiral  80 , as well as the number of turns the spiral  80  will consist of for a given diameter. This rolled member  65  is then consolidated  67 , typically in a cylindrical die, to form a solid billet or feedrod  85 . The feedrod  85  is then extruded  90 , typically in one step, in order to obtain a component  95  of the desired diameter (see  FIGS. 3 and 4 ). The extruded filament  95  can then be incorporated into the final product  10 , be it as short chopped lengths  97 , as continuous lengths of filament  98 , or some combination thereof  99 . The added first phase particles  97  may be made of any convenient size. Typically, the particles are between about 25 μm and about 2 cm in diameter, but may be made larger or smaller if desired. 
       FIG. 5  illustrates in tabular form a few possible matrix compositions. The listings in  FIG. 5  are not exhaustive, but are instead intended to represent a few example compositions. It should be noted that the matrix  20  and dispersed phases  15  may be of the same material, with only the geometry of the dispersed spiral phases  15  being different. 
     An example of the procedure for producing a typical composite material  10  is detailed below. In this example, illustrated as  FIG. 6 , the material  10  is a composite of ZrB 2  and SiC, with SiC spirals  15  dispersed in a ZrB 2  matrix  20 . First, about  54  volume percent ZrB 2  powder  25  was blended with a thermoplastic polymer  30  and a small amount of plasticizer  32  (less than 10 volume percent) using a heated high shear mixer until a first homogeneous blend  35  was formed. This process was repeated using (57 volume percent) SiC powder  45  blended with a thermoplastic resin  50  to yield a second homogeneous blend  60 . The first and second respective powder polymer blends  35 ,  60  were then each pressed  37  into respective sheets  40 ,  55 , each with a thickness of about 20 mils, using a heated hydraulic press and shims to control the final thickness. Strips  100 ,  105  about 3 inches wide by about 8 inches long were then cut from each respective sheet  40 ,  55 . The SiC strip  105  was placed atop of the ZrB 2  strip  100  and heated  107  to ˜130° C. on a heated platen. After the material became pliable, the strips  100 ,  105  were rolled up  63  from one end to yield a rolled member or rod  65  characterized by the spiral architecture. The rod  65  was then placed in a die of about 0.86 inches in diameter, heated to 130° C., and consolidated  67  into a feedrod  85  using a hydraulic press. Using an extruder, the feedrod  85  was then passed through a heated spinneret  90  reducing the diameter to about 300 microns to yield a filament  98  while maintaining the original geometry of the spiral feedrod  85 . The filament  98  was chopped into 1 mm lengths  97  which were then mixed with additional ZrB 2  powder  20  in order to form a mixture  110  that contained about 30 volume percent SiC. This mixture  110  was hot-pressed in order to form the final ZrB 2 -matrix billet  10  containing 30 volume percent SiC spirals. The amount of second phase material  15  added could vary widely from about 5 to about 95 volume percent. The choice of how much of the first phase material  15  is desired to be added to the second phase matrix  20  to produce a desired and advantageous result would depend on the physical property of properties being manipulated. For mechanical properties, a range of between about 20 and about 40 volume percent would typically be selected. For the manipulation of electrical or thermal properties, a range of between about 5 and about 25 volume percent would typically be appropriate. 
       FIGS. 7A and 7B  are photomicrographs illustrating the dispersed first phase spirals  15  in the second phase matrix  20 .  FIGS. 8A-8D  are photomicrographs graphically illustrating the crack propagation deflection and attenuation properties of the composite materials  10 . As can be seen, crack propagation is blunted by first phase spiral particles  15 , with the crack either stopped or redirected.  FIGS. 9 and 10  graphically illustrate the increase in toughness of the composite material  10  over a test material. Typically, the dispersed first phase  15  is characterized by a spiral architecture, although other geometries (cylinders and the like) may likewise prove advantageous. Likewise, the first and/or second phase  15 ,  20  may be ceramic, but may also be metallic, polymeric, vitreous, amorphous or the like. 
     Crack defection can occur for multiple reasons. Often times in ceramics propagating cracks may be deflected or attenuated by running into to a difference in elastic modulus between two phases; likewise, deflection may occur at the interface between two phases when the interface is weaker than either phase. The tensile stresses generated at the interface between two phases of dissimilar thermal expansions can also draw a crack along the interface as opposed to allowing it to propagate across the interface. Differences in fracture toughness between two phases can also lead to crack deflection as a crack tries to propagate from the low toughness phase into the high toughness phase. 
     While the novel technology has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiment has been shown and described and that all changes and modifications that come within the spirit of the novel technology are desired to be protected.