Porous matrix composites

The range of potential components that could be fabricated from porous matrix ceramic composites may be limited because of the poor mechanical properties of the porous matrix. Porous matrices generally yield composites with low compressive and interlaminar properties, making it difficult to design structures capable of carrying out-of-plane loads. Furthermore, it is difficult to attach porous matrix composites to neighboring structures. Three-dimensional fiber arehiteetures rather than two-dimensional laminate structures will probably be required for the components made from a porous matrix eeramie composite. 

One should note that the terms porous-matrix composite and dense-matrix composite are often used to differentiate between families of composite behavior and are used loosely with regard to the actual density of the material. Current technology composites of the porous-matrix type are not fully sintered and have approximately 35-50% matrix porosity/25-40% total porosity (see Table 2 for examples and references). (The composite consists of a volume fraction of fibers,/. The remainder of the volume, (1-/) is considered to be matrix. The term matrix porosity (p ) refers to the pore or void fraction of the matrix, so that, as the fibers are assumed to be fully dense, the composite or total porosity is r= For example if/= 0.4 a 30% matrix porosity would correspond to 18% total... 

The mechanical behavior of porous-matrix composites has been modeled, but the compromises between matrix properties and toughness are not yet fully quantified. Initial estimates indicated that a matrix porosity of 30% is needed for crack deflection within a porous matrix [4, 43]. Subsequent efforts have been aimed at better quantifying the transition between porous and dense CMC failure behavior [44] however, further investigation is essential, especially considering the temperature dependent nature of the porous microstructure. 

FIGURE 3. Low magnification views showing dramatic difference between a) a fibrous failure normally seen with porous matrix composites and b) a brittle failure [30]. Reprinted with permission of The American Ceramic Society, www.ceramics.org.copyright 2003. All rights reserved. 

FIGURE 4. Typical microstructure of porous matrix composite (Nextel 720/mullite-alumina, UCSB material)... 

While there are abundant reasons for academic consideration of porous-matrix oxide-oxide composites, much of the practical interest, and opportunity to pursue them, is owed to two factors the economical availability of numerous types of fibers, and the absence of the necessity to coat fibers. Both of these factors significantly affect the ease and cost of experimentation, and, more importantly, the ultimate cost of the materials. An additional significant advantage is that in many cases fabrication can be very similar to that employed for polymeric composites.The majority of this section is therefore focused on porous matrix composites, since these materials are the most mature. Research on composites containing interface coatings is also addressed in this section (4.3). 

Another relatively mature porous-matrix composite, WHIPOX (i ound Highly orous Oxide Ceramic Composite) has been developed this material is the subject of a preceding chapter in this Handbook and will therefore not be addressed in this text. These materials utilize either Nextel 720 or 610 fibers filament wound into a highly porous mullite or alumina... 

Infiltration (67) provides a unique means of fabricating ceramic composites. A ceramic compact is partially sintered to produce a porous body that is subsequently infiltrated with a low viscosity ceramic precursor solution. Advanced ceramic matrix composites such as alumina dispersed in zirconia [1314-23-4] Zr02, can be fabricated using this technique. Complete infiltration produces a homogeneous composite partial infiltration produces a surface modified ceramic composite. 

Directed Oxidation of a Molten Metal. Directed oxidation of a molten metal or the Lanxide process (45,68,91) involves the reaction of a molten metal with a gaseous oxidant, eg, A1 with O2 in air, to form a porous three-dimensional oxide that grows outward from the metal/ceramic surface. The process proceeds via capillary action as the molten metal wicks into open pore channels in the oxide scale growth. Reinforced ceramic matrix composites can be formed by positioning inert filler materials, eg, fibers, whiskers, and/or particulates, in the path of the oxide scale growth. The resultant composite is comprised of both interconnected metal and ceramic. Typically 5—30 vol % metal remains after processing. The composite product maintains many of the desirable properties of a ceramic however, the presence of the metal serves to increase the fracture toughness of the composite. 

All VGCF was graphitized prior to composite consolidation. Composites were molded in steel molds lined with fiberglass reinforced, non-porous Teflon release sheets. The finished composite panels were trimmed of resin flash and weighed to determine the fiber fraction. Thermal conductivity and thermal expansion measurements of the various polymer matrix composites are given in Table 6. Table 7 gives results from mechanical property measurements. 

Bone is a porous tissue composite material containing a fluid phase, a calcified bone mineral, hydroxyapatite (HA), and organic components (mainly, collagen type). The variety of cellular and noncellular components consist of approximately 69% organic and 22% inorganic material and 9% water. The principal constiments of bone tissue are calcium (Ca ), phosphate (PO ), and hydroxyl (OH ) ions and calcium carbonate. There are smaller quantities of sodium, magnesium, and fluoride. The major compound, HA, has the formula Caio(P04)g(OH)2 in its unit cell. The porosity of bone includes membrane-lined capillary blood vessels, which function to transport nutrients and ions in bone, canaliculi, and the lacunae occupied in vivo by bone cells (osteoblasts), and the micropores present in the matrix. 

In the early stages of bone formation, the osteons dominate the bone structure to make an overall structure of fiber-matrix composite. While the primary bone has a dense structure, the secondary bone structure is this composite. As a result, the cortical bone structure becomes very complex. It is microscopically porous, has a lamellar structure, and is also a fiber-matrix composite. Size and packing of osteons and canals, and their orientation, determine the mechanical properties of these bones. 

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