Journal of the American Ceramic Society-Holmquist and lange disintegrated matrix in the form of powder. Thus, the ability of the mullite grains and have higher creep resistance and high rous matrix to isolate the fibers from matrix cracks will allow temperature stability, but lower room temperature strength, com the fibers to fail in a manner similar to what is seen for di pared with N610 fibers, which are high purity (99%o) polycry bundles.The high failure strain of the fiber bundle will also talline a-alumina become the failure strain of the composite. The mullite powder used was MU-107 (Showa Denko KK. Two methods have been developed at University of California Tokyo, Japan), which has a mean particle size of-I um, a particl at Santa Barbara(UCSB) to produce porous matrix composites. size distribution of 0.5-2.5 um, and a BET surface area of 7 The first uses pressure filtration to pack particles around fibers 2/g. It has a chemistry of 75.5% AL,O, and 24% SiO,(by within a preform. The powder surrounding the fibers is then weight) with only trace amounts of TiO2. Fe=O3, and Na2O strengthened by the cyclic infiltration and pyrolysis of a precur sor.21..25.2 For this method, the slurry is formulated so that the pan)was selected as the alumina powder and has a mean particle particles are repulsive to themselves and also to the fibers. Mullite size of -0.2 Hm, a more narrow particle size distribution (0. 1-0.3 has been chosen as the matrix material because of its lack of um), and a BET surface area of 10.6 m /g(manufacturers data densification at temperatures below -1300oC. Levi et al. Its chemistry is essentially pure a-Al, O,(99.995%). An AL,O reported that alumina powder with a much smaller particle size precursor, aluminum( Ill) sec-butoxide, C12H27O, Al( Gelest, Inc (-200 nm)could be added to the mullite powder to aid in Tullytown, PA), was used to strengthen the matrix. The precursor strengthening the powder matrix. At processing temperatures is 95% pure and has a yield of 4% of Al]O, by volume. Before around 1200 C(chosen to avoid degradation of the fibers)the infiltration the precursor was diluted with 25%(by volume) of alumina sinters to form bridges between the larger mullite particles sec-butyl alcohol(Sigma-Aldrich, Milwaukee, wI) and between the mullite particles and the fibers. In this case, the 1200C, is sufficient (20.70)to prevent shrinkage of the mixed ratio was selected to achieve high packing densities of the powder powder matrix. body and low shrinkage in the following sintering step. Lower The second method was first introduced by Haslam et al alumina content would give fewer sintering necks between mullite Instead of packing the particles around the fibers via pressure rticles, whereas mixtures with higher alumina content will filtration, the powder is first consolidated to a very high volume densify relatively fast above 1200C. Tetraethylammonium hy raction and then infiltrated into the fiber preform via vibration- droxide (TEA-OH)was used to maintain the pH above I assisted flow. As detailed elsewhere. the initial slurry must be allowing electrostatic repulsive interactions to develop between formulated so that the particles are weakly attracted to one another. the oxide particles, A 2 wt% amount(relative to the solids) of The formulation of the the development of a short-range repulsive particle potentialthat was added to induce a steric dispersing effect. The PEG-silane pressure filtration, thus allowing the network to retain its interpar--M-OH (M- metal atom)surface sites, y reacting with the prevents the particles from being pushed into contact during molecules chem-absorbed to the particles ticle potential in the consolidated state agglomerates was promoted by ultrasonic agitation for 5 min. The In the present investigation we used a powder slurry with a slurry was placed on a mechanical roller for 12 h, and then special interparticle pair potential (weakly attractive produced by a tetramethylammonium nitrate(TMA-N) salt was added (25M)to short-range repulsive)that allows a powder compact, which has form weakly attractive pair potentials between the particles. As described elsewhere, TMA counter ions aid in producing a The preconsolidated slurry with a relative density of 0.54 was weakly attractive particle network. The network can be packed on a fiber cloth, and since it shows shear rate thinning, to a high density via pressure filtration, and the so-formed ibration reduces the viscosity and allows rapid and effi consolidated body can subsequently be fluidized again via vibra- cient intrusion into the fiber tows. The technique to make c tion. The slurry was consolidated by pressure filtration at 4 MPa to posites, called VibroIntrusion, has been described elsewhere. 2 form disk-shaped bodies that were stored in sealed plastic bags The vibration of the preconsolidated slurry was conducted between The consolidation pressure of 4 MPa was lower than the critical could be frozen and stored. Once thawed they were flexible and which would obviate fluidization after consolidation, a>o contact astic sheets to avoid ev uld be bent, cut, and formed much like an epoxy/fiber prepreg Prepregs could be stacked and formed into cox?) Drying only x geometries the consolidated bodies was determined to 54+ 1%0 like T-joints, doubly curved shapes. and tubes The consolidated powder compact was fluidized by subjecting it causes minimal shrinkage since a high dry content could be to mechanical vibration Fiber cloths (cut to -60 X 60 mm")were reached in the matrix slurry. Subsequent precursor impregnation/ put in separate plastic bags, and an excess of the preconsolidated pyrolysis cycles were done to strengthen the matrix. slurry was dispensed to both sides of each fiber cloth. Assisted by This new method to manufacture CCCs produced material vibration, the slurry was manually rolled across the surface of th with similar matrixes as a previous route fiber cloth with a piece of aluminum rod until the cloth was fully between this new method and the older method to produce porous infiltrated. Since the preconsolidated slurry exhibits shear-rate thinning, the vibration reduces the viscosity difference is that the mullite/alumina volume ratio, 70/30 used intrusion of the particles into the fiber cloth a and allows rapid matrix CFCCs will be made throughout the text. The main here, is lower than that(80/20) used earlier. The purpose was to Prepregs were frozen to aid removal from the plastic bag, or show that the process produces composite material with similar they could be stored for later use. To produce the composite, properties but with the added advantage of allowing complex frozen prepregs were removed from the plastic bags and stacked pes to be made on top of each other. The pile of prepregs was packed in a plastic bag, evacuated, and placed between two flat steel plates using two spacer bars to fix the thickness, The number of prepregs was IL. Experimental Procedure chosen to give the desired fiber volume fraction of the composite (in this case 13 layers of prepregs and a nominal thickness of 3. 18 (I) Materials and Composite Processing mm). After thawing, the assembled layers were put on a vibrating Reinforcement fibers used in this work were Nextel 610 M and table and pressed lightly to cause the preconsolidated slurry to Nextel 720M(N610 and N720, 3M Corp. St. Paul MN)woven flow and complete the infiltration. To remove trapped air as much into eig harness satin fabrics. The tows in the fabric contain as possible(which later on could give rise to large-scale porosity ) -400 filaments with diameters between 10 and 12 um. N720 the vacuum was kept on a level of -10 torr during the vibration fibers are composed of a mixture of sub-micrometer alumina and step(-5 min). It should be noted that the so-formed green ceramic