The fiber preform first undergoes the CVI process in which it is coated with a dual layer of boron nitride and silicon carbide. The treated fiber preform is then placed in contact with molten aluminum metal in the presence of air at elevated temperatures. The aluminum oxidizes and forms a matrix rich in aluminum oxide. The coatings protect the fibers from the molten aluminum, and the silicon carbide wets to facilitate infiltration of the aluminum. Residual aluminum, present in the matrix as microscopic interconnected channels, is removed from the CFCC. For cylindrical parts the matrix is grown through the preform by reaction of aluminum metal with air, resulting in an aluminum oxide matrix. The process is capable of manufacturing parts 4 ft in diameter and 7 ft high.

McDermott Technologies Corporation has the only oxide/oxide-based CFCC process. It is fabricating CFCCs from a powder slurry and sol-gel (a liquid converted to solids) impregnation technique. Metal oxide fibers are wound onto a mandrel or woven into cloth or preforms; the residual voids are filled with reactive, fine particles of alumina, mullite, and/or yttrium alumina garnet (YAG) matrices. Densification occurs at 1100°C (2012°F). Fiber coatings include carbon as a fugitive interface. This fugitive interface is achieved by depositing a carbon or Scheelite (CaWO4) coating onto the fiber during composite processing. It is oxidized to create fiber-matrix slip. The "fugitive" approach creates a void along the fiber. This void deflects cracks, leaving continuous fibers to bear the load.

McDermott also makes porous CFCC filter tubes. Any one of three methods can be used. One method involves winding high-strength aluminum oxide ceramic fibers (3M Corporation's Nextel™ 610) onto a mandrel and filling a portion of the space between fibers with aluminum oxide particles utilizing a liquid-to-solid sol-gel multiple infiltration process. The second method involves fabrication of an open network skeleton of high-strength silicon carbide ceramic fibers rigidized by chemical vapor deposition, then filling with a porous oxide matrix by vacuum casting. The third method substitutes a silicon carbide matrix for the aluminum oxide matrix in the first method. In all of these methods the final step is the application of heat and pressure to form an oxide-oxide CFCC. All methods result in a CFCC filter of the appropriate porosity, and meeting the filtration and strength specifications of this application.

COI Ceramics Corporation produces CFCC materials by a polymer infiltration and pyrolysis (PIP) process. This is a versatile way to fabricate large, complex-shaped structures. The process uses low-temperature forming and molding steps typically used in plastic matrix composites. A preform, composed of silicon carbide fibers or fibers derived from a polymer composed of silicon, carbon, oxygen, and nitrogen, is impregnated with a polymer matrix and cured by conventional methods. The composite is pyrolyzed to temperatures beyond 980°C (1800°F) to convert the preceramic matrix polymer to a ceramic. Subsequent impregnation and pyrolysis steps are carried out to achieve the desired density. Both the initial shaping and fabrication of the composite are carried out at low temperature.

Continuous fiber-reinforced ceramic composites fabricated by the PIP process can consist of various fiber, interface coating, and matrix chemistries. Fiber architecture preforms can include filament windings, braids, or two- and three-dimensional weaves. An important aspect of this PIP process is its adaptability to polymer matrix processing equipment. Aside from reducing initial capital investment by using existing equipment, PIP works well with various preforming techniques such as hand lay-up, filament winding, braiding, reaction transfer molding, three-dimensional weaving and conventional cure, such as autoclaving. COI Ceramics makes flat plates, cylinders and complex shaped parts of CFCCs.

COI Ceramics fabricates cylinders by two methods. One method involves winding a silicon carbide fiber onto a cylindrical mandrel followed by liquid infiltration and heat. The second method is similar to the first except than the fiber is, woven into a cloth and then wrapped onto the mandrel.

COIs PIP CFCCs are available in two classes: the Sylramic 100 series—a carbon-coated Nicalon™ fiber in an amorphous SiOC matrix for maximum use temperature <450°C (842°F) in oxidizing environments and up to 1100°C (2012°F) in inert environments—and Sylramic™ 200 and 300 series of proprietary coated Nicalon™ fiber in an amorphous SiNC matrix for use up to 1200-1250°C (2192°-2282°F) in an oxidizing environment.

COI Ceramics also produce a CFCC based on a sol-gel derived alumino-silicate matrix that can be combined with a variety of commercially available fiber reinforcements such as 3M Corporation's Nextel™ fibers. This latter fiber provides the highest temperature resistance and creep resistance. The baseline oxide-oxide system relies on controlled matrix porosity for toughness, eliminating the need for fiber coatings.

Tables 3.10 and 3.11 contain data generated on a COI Ceramic Nextel™ 720 reinforced alumino-silicate CFCC at 982°C (1800°F) and 1093°C (2000°F).

The residual strengths after fatigue are equal or greater than the unexposed composite. The creep rupture tests are just as encouraging, with residual stress after 100 h at 150 MPa (21.6 ksi) equal to 30.3 ksi (Table 3.12).

COI Ceramics also manufactures SiC/SiC CFCC composites with properties as shown in Table 3.13.

TABLE 3.10 COI Ceramics Oxide CFCC Properties with Various 3M Oxide Fabrics
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