1734 Journal of the American Ceramic Sociery-Holmquist and lange Vol. 86. No. 10 disintegrated matrix in the form of powder. Thus, the ability mullite grains and have higher creep resistance and high porous matrix to isolate the fibers from matrix cracks wi temperature stability, but lower room temperature strength, com- the fibers to fail in a manner similar to what is seen pared with N610 fibers, which are high purity (99%) polycrys- bundles. The high failure strain of the fiber bundle wi talline a-alumil 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 particle at Santa Barbara(UCSB) to produce porous matrix composites. size distribution of 0.5-2.5 um, and a Bet surface area of 7.5 The first uses pressure filtration to pack particles around fibers m-g. It has a chemistry of 75.5% Al,O3 and 24% SiOz(by within a preform. The powder surrounding the fibers is ther weight) with only trace amounts of TiO,, Fe2O3, and Na, strengthened by the cyclic infiltration and pyrolysis of a precur- (manufacturers' data). AKP-50(Sumitomo Chemicals, Tokyo sor.2,24, 25,28For this method, the slurry is formulated so that the Japan)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 um, a more narrow particle size distribution(0.1-0 densification at temperatures below -1300C25 Levi et al28 um), and a BET surface area of 10.6 m2/g(manufacturers'data) has been chosen as the matrix material because of its lack Its chemistry is essentially pure a-Al2O3(99.995%). An Al, O3 reported that alumina powder with a much smaller particle size precursor, aluminum(Ill) sec-butoxide, C12H27O3Al(Gelest, Inc (-200 nm) could be added to the mullite powder to aid in Tullytown, PA), was used to strengthen the matrix. The recurs engthening the powder matrix. At processing temperatures ound 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, Wn) and between the mullite particles and the fibers. In this case, the persed, aqueous slurries containing 20 vol% solids(mullite/ volume fraction of mullite powder, which does not shrink at alumina proportions, 70/30)were prepared The mullite to alumina 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 step2 428Lower The second method was first introduced by Haslam et al.- alumina content would give fewer sintering necks between mullite stead of packing the particles around the fibers via pressure particles, whereas mixtures with higher alumina content will filtration, the powder is first consol to a very high volume densify relatively fast above 1200oC. Tetraethylammonium hy- fraction and then infiltrated into the fiber preform via vibration- droxide (TEA-OH) was used to maintain the pH above 11 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 weakly attractive particle network requires poly(ethylene oxide) urethane silane(PEG-silane, Gelest, Inc the development of a short-range repulsive particle potential-that was added to induce a steric dispersing effect. The PEG-silane events the particles from being pushed into contact during molecules chem-absorbed to the pa acting wit oressure filtration, thus allowing the network to retain its interpar--M-OH (M= metal atom)surface sites. 37,38 Dispersion of soft ticle potential in the consolidated state gglomerates was promoted by ultrasonic agitation for 5 min. The In the present investigation we used a powder slurry with a rry was placed on a mechanical roller for 12 h, and then special interparticle pair potential weakly attractive produced by a tetramethy lammonium nitrate(TMA-N)salt was added (0. 25M)to hort-range repulsive")that allows a powder compact, which has orm weakly attractive pair potentials between the particles. A been previously consolidated by pressure filtration, to be fluidized. 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 placed on a fiber cloth, and since it shows shear rate thinning, Itration. and the so-for added vibration 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 com- tion. The slurry was consolidated by pressure filtration at 4 MPa te posites, called Vibrolntrusion, has been described elsewhere.",- form disk-shaped bodies that were stored in sealed plastic bags The vibration of the preconsolidated slurry was conducted betweer The consolidation pressure of 4 MPa was lower than the critical plastic sheets to avoid evaporation. Prepregs made in this manner pressure where a large number of particles are pushed into contact, could be frozen and stored. Once thawed they were flexible and which would obviate fluidization after consolidation. 32,37,38Using could be bent, cut, and formed much like an epoxy/fiber prepreg. the weight difference method, the volume fraction of solids within Prepregs could be stacked and formed into complex geometries the consolidated bodies was determined to 54 1% like T-joints, doubly curved shapes, and tubes. 33,34 Drying only 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 mm2)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 CFCCs produced material vibration, the slurry was manually rolled across the surface of the with similar matrixes as a previous route, 24,25,128-30 comparisons 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 and allows rapid difference is that the mullite/alumina volume ratio, 70/30 used intrusion of the particles into the fiber cloth.22,23 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 shapes to be made. on top of each other. The pile of prepregs was packed in a plastic ed between two flat steel plates usin 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 enforcement fibers used in this work were Nextel 610 table and pressed lightly to cause the preconsolidated slurry to Nextel 720(N610 and N720, 3M Corp, St Paul, MN) flow and complete the infiltration. To remove trapped air as much nto eight-harmess satin fabrics. The tows in the fabric as possible(which later on could give rise to large-scale porosity 400 filaments with diameters between 10 and 12 um the vacuum was kept on a level of -10 torr during the vibration bers are composed of a mixture of sub-micrometer alumina and step(-5 min). It should be noted that the so-formed green ceramicdisintegrated matrix in the form of powder. Thus, the ability of the porous matrix to isolate the fibers from matrix cracks will allow the fibers to fail in a manner similar to what is seen for dry bundles. The high failure strain of the fiber bundle will also become the failure strain of the composite. Two methods have been developed at University of California at Santa Barbara (UCSB) to produce porous matrix composites. The first uses pressure filtration to pack particles around fibers within a preform. The powder surrounding the fibers is then strengthened by the cyclic infiltration and pyrolysis of a precur￾sor.21,24,25,28 For this method, the slurry is formulated so that the particles are repulsive to themselves and also to the fibers. Mullite has been chosen as the matrix material because of its lack of densification at temperatures below
1300°C.25 Levi et al.28 reported that alumina powder with a much smaller particle size (
200 nm) could be added to the mullite powder to aid in strengthening the powder matrix. At processing temperatures around 1200°C (chosen to avoid degradation of the fibers) the alumina sinters to form bridges between the larger mullite particles and between the mullite particles and the fibers. In this case, the volume fraction of mullite powder, which does not shrink at 1200°C, is sufficient (
0.70) to prevent shrinkage of the mixed powder matrix. The second method was first introduced by Haslam et al.23 Instead of packing the particles around the fibers via pressure filtration, the powder is first consolidated to a very high volume fraction and then infiltrated into the fiber preform via vibration￾assisted flow. As detailed elsewhere, the initial slurry must be formulated so that the particles are weakly attracted to one another. The formulation of the weakly attractive particle network requires the development of a short-range repulsive particle potential32 that prevents the particles from being pushed into contact during pressure filtration, thus allowing the network to retain its interpar￾ticle potential in the consolidated state. In the present investigation we used a powder slurry with a special interparticle pair potential (weakly attractive produced by a short-range repulsive22) that allows a powder compact, which has been previously consolidated by pressure filtration, to be fluidized. The preconsolidated slurry with a relative density of 0.54 was placed on a fiber cloth, and since it shows shear rate thinning, added vibration reduces the viscosity and allows rapid and effi￾cient intrusion into the fiber tows. The technique to make com￾posites, called VibroIntrusion, has been described elsewhere.22,23 The vibration of the preconsolidated slurry was conducted between plastic sheets to avoid evaporation. Prepregs made in this manner could be frozen and stored. Once thawed they were flexible and could be bent, cut, and formed much like an epoxy/fiber prepreg. Prepregs could be stacked and formed into complex geometries like T-joints, doubly curved shapes, and tubes.33,34 Drying only causes minimal shrinkage since a high dry content could be reached in the matrix slurry. Subsequent precursor impregnation/ pyrolysis cycles were done to strengthen the matrix. This new method to manufacture CFCCs produced material with similar matrixes as a previous route;24,25,28–30 comparisons between this new method and the older method to produce porous matrix CFCCs will be made throughout the text. The main difference is that the mullite/alumina volume ratio, 70/30 used here, is lower than that (80/20) used earlier. The purpose was to show that the process produces composite material with similar properties but with the added advantage of allowing complex shapes to be made. II. Experimental Procedure (1) Materials and Composite Processing Reinforcement fibers used in this work were Nextel 610TM and Nextel 720TM (N610 and N720, 3M Corp., St. Paul, MN) woven into eight-harness satin fabrics. The tows in the fabric contain
400 filaments with diameters between 10 and 12 m. N720 fibers are composed of a mixture of sub-micrometer alumina and mullite grains and have higher creep resistance and high￾temperature stability, but lower room temperature strength, com￾pared with N610 fibers, which are high purity (99%) polycrys￾talline -alumina.35,36 The mullite powder used was MU-107 (Showa Denko KK, Tokyo, Japan), which has a mean particle size of
1 m, a particle size distribution of 0.5–2.5 m, and a BET surface area of 7.5 m2 /g. It has a chemistry of 75.5% Al2O3 and 24% SiO2 (by weight) with only trace amounts of TiO2, Fe2O3, and Na2O (manufacturers’ data). AKP-50 (Sumitomo Chemicals, Tokyo, Japan) was selected as the alumina powder and has a mean particle size of
0.2 m, a more narrow particle size distribution (0.1–0.3 m), and a BET surface area of 10.6 m2 /g (manufacturers’ data). Its chemistry is essentially pure -Al2O3 (99.995%). An Al2O3 precursor, aluminum(III) sec-butoxide, C12H27O3Al (Gelest, Inc., Tullytown, PA), was used to strengthen the matrix. The precursor is 95% pure and has a yield of 4% of Al2O3 by volume. Before infiltration the precursor was diluted with 25% (by volume) of sec-butyl alcohol (Sigma-Aldrich, Milwaukee, WI). Dispersed, aqueous slurries containing 20 vol% solids (mullite/ alumina proportions, 70/30) were prepared. The mullite to alumina ratio was selected to achieve high packing densities of the powder body and low shrinkage in the following sintering step.24,28 Lower alumina content would give fewer sintering necks between mullite particles, whereas mixtures with higher alumina content will densify relatively fast above 1200°C. Tetraethylammonium hy￾droxide (TEA-OH) was used to maintain the pH above 11, allowing electrostatic repulsive interactions to develop between the oxide particles. A 2 wt% amount (relative to the solids) of poly(ethylene oxide) urethane silane (PEG-silane, Gelest, Inc.) was added to induce a steric dispersing effect. The PEG-silane molecules chem-absorbed to the particles by reacting with the –M–OH (M  metal atom) surface sites.37,38 Dispersion of soft agglomerates was promoted by ultrasonic agitation for 5 min. The slurry was placed on a mechanical roller for 12 h, and then tetramethylammonium nitrate (TMA-N) salt was added (0.25M) to form weakly attractive pair potentials between the particles. As described elsewhere, TMA counter ions aid in producing a weakly attractive particle network.37,38 The network can be packed to a high density via pressure filtration, and the so-formed consolidated body can subsequently be fluidized again via vibra￾tion. The slurry was consolidated by pressure filtration at 4 MPa to form disk-shaped bodies that were stored in sealed plastic bags. The consolidation pressure of 4 MPa was lower than the critical pressure where a large number of particles are pushed into contact, which would obviate fluidization after consolidation.32,37,38 Using the weight difference method, the volume fraction of solids within the consolidated bodies was determined to 54  1%. The consolidated powder compact was fluidized by subjecting it to mechanical vibration. Fiber cloths (cut to
60  60 mm2 ) were put in separate plastic bags, and an excess of the preconsolidated slurry was dispensed to both sides of each fiber cloth. Assisted by vibration, the slurry was manually rolled across the surface of the fiber cloth with a piece of aluminum rod until the cloth was fully infiltrated. Since the preconsolidated slurry exhibits shear-rate thinning, the vibration reduces the viscosity and allows rapid intrusion of the particles into the fiber cloth.22,23 Prepregs were frozen to aid removal from the plastic bag, or they could be stored for later use. To produce the composite, frozen prepregs were removed from the plastic bags and stacked 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 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 mm). After thawing, the assembled layers were put on a vibrating table and pressed lightly to cause the preconsolidated slurry to flow and complete the infiltration. To remove trapped air as much as possible (which later on could give rise to large-scale porosity), the vacuum was kept on a level of
10 torr during the vibration step (
5 min). It should be noted that the so-formed green ceramic 1734 Journal of the American Ceramic Society—Holmquist and Lange Vol. 86, No. 10