M.G. Holmquist et al./Composites: Part A 34(2003)163-170 phase, combined with a minor part alumina powder (0.30 the maximum sustainable hoop stress and energy release volume fraction) with fine particle size [23, 24, 31,38]. The rates for a propagating delamination crack. alumina can sufficiently densify at 1200C and help bind the mullite particles together without causing shrinkage Mullite will not densify at 1200C and thus ensure a stable 2. Experimental procedure matrix that will not shrink during the heat treatment, the mall volume fraction of alumina avoids braking the 2.1. materials continuity of the mullite network, which otherwise could lead to matrix densification during service [6, 21, 23). The All specimens prepared in this study were based on matrix is further strengthened by subsequent impregnation Nextel 610 alumina fibre cloth in an eight-harness satin and pyrolysis cycles with an alumina precursor, addin weave Sizing applied to the fabric by the manufacturer was more material to the bonds between the mullite particles removed through thermal treatment in air at 900C for a Pressure filtration has been used to produce this type of period of 2 h; otherwise the fabric was used in the as- composite [6, 19, 21]. In this method, a fibre preform(e.g. 3D received condition. The N610 fibres are composed of pure weave, stacked layers of cloth, etc. ) is mounted on a filter (99%)polycrystalline a-alumina and the tows in the within a die cavity. A powder slurry is introduced in th fabric contain approximately 400 filaments with diameters cavity and a pressure is applied to cause the particles to between 10 and 12 um[26, 27]. Although the N610 fibre stream through the preform and build up a consolidated shows very high room temperature strength, relative to layer within the fibre preform. The method requires N720, a mullite/alumina fibre, it is less stable and creep dispersed slurries with highly repulsive interparticle pair asistant at temperatures above 1000C 29. The potentials. This process works well for producing flat particle size and size distribution of mullite and alumina powders were selected to insure relatively high packing Vibrolntrusion is a new fabrication method for proces densities of the final filter-pressed slurry ing porous matrix composites with the advantage that it can powder was MU-107( Showa Denko KK, Tokyo Japan) be used to form complex shapes [22, 23,38) It requires a with a mean particle size(as determined by the manufac- slurry formulated with a short range repulsive potential turer) of 1 um and a particle size distribution of 0.5 between the particles that will create a weakly attractive 2.5 um. Its chemical composition is reported by the particle network [24, 25]. This interparticle pair potential manufacturer as 75.5%0 Al2O3 and 24% Sio2(by weight) with only trace amounts of TiO2, Fe,O3 and NayO. An o allows a powder compact that has been previously alumina powder (AKP-50, Sumitomo Chemicals, Tokyo, consolidated by pressure filtration, to be fluidised. The Japan)was used with a mean particle size of 0.2 um and a fluidised powder compact, called pre-consolidated slurry, is vibrated into a fibre cloth(or any other fibre preform). The particle size distribution of 0.1-0.3 um. Its chemistry essentially pure a-Al2O3(99.995%). An alumina precursor. shear rate thinning behaviour of the pre-consolidated slurry aluminium Ill sec-butoxide, C12H27O3Al (Gelest Inc. causes vibration to reduce its viscosity, which allows rapid Tullytown, PA, USA) was used to impregnate and thus d efficient intrusion of the fibre tows. Prepregs made in this manner can be frozen and stored. Once thawed they are by volume. strengthen the matrix. The precursor yielded 4% of alumina flexible and can be bent cut and formed much like an epoxy/fibre prepreg. Prepregs can be stacked and formed 2.2. Composite tube manufacturing into complex geometries like T-joints, doubly curve and tubes [38. Drying causes only minimal shrinkage due Aqueous slurries were prepared with a 20% volume to the high volume fraction of solid present in the pre- content of solids, comprising 70 vol% mullite and 30 vol% consolidated slurry alumina. Tetramethylammonium hydroxide (TMA-OH The purpose of this study was to demonstrate the was used to maintain the pH at about 11. Zeta-poter usefulness of the VibroIntrusion process in the fabrication measurements on mullite and alumina powders showed that of tubular structures from prepregs. Several potential pH 1l was sufficient to insure a dispersed slurry through the applications for ceramic matrix composites, including development of electrostatic repulsive interactions between high-temperature power piping and hot gas filters in the particles. two weight percentage(relative to the solids) power generation, and tubular combustors or curved of poly-ethylene oxide urethane silane(PEG-silane, Gelest combustor liners in gas turbine engines, are based on this Inc ) was added to induce a steric dispersing effect due to the simple geometry. A mechanical evaluation of tubular- adsorption of molecules by a reaction with -OH surface shaped specimens is also conveniently performed using a sites on both powders [25]. The slurry was contained in a simple pressure test, giving insights on the failure plastic jar that was placed on a mechanical roller for mechanisms and their relation to the fuid containment approximately 12 h and after mixing, tetramethylam- aptitudes of these materials. The failure behaviour monium nitrate(TMA-N)salt was added to make a 0.25M of pressurised tubes was analysed in terms of both solution. The addition of TMa counterions producephase, combined with a minor part alumina powder (0.30 volume fraction) with fine particle size [23,24,31,38]. The alumina can sufficiently densify at 1200 8C and help bind the mullite particles together without causing shrinkage. Mullite will not densify at 1200 8C and thus ensure a stable matrix that will not shrink during the heat treatment. The small volume fraction of alumina avoids braking the continuity of the mullite network, which otherwise could lead to matrix densification during service [6,21,23]. The matrix is further strengthened by subsequent impregnation and pyrolysis cycles with an alumina precursor, adding more material to the bonds between the mullite particles. Pressure filtration has been used to produce this type of composite [6,19,21]. In this method, a fibre preform (e.g. 3D weave, stacked layers of cloth, etc.) is mounted on a filter within a die cavity. A powder slurry is introduced in the cavity and a pressure is applied to cause the particles to stream through the preform and build up a consolidated layer within the fibre preform. The method requires dispersed slurries with highly repulsive interparticle pair potentials. This process works well for producing flat panels. VibroIntrusion is a new fabrication method for proces￾sing porous matrix composites with the advantage that it can be used to form complex shapes [22,23,38]. It requires a slurry formulated with a short range repulsive potential between the particles that will create a weakly attractive particle network [24,25]. This interparticle pair potential allows a powder compact that has been previously consolidated by pressure filtration, to be fluidised. The fluidised powder compact, called pre-consolidated slurry, is vibrated into a fibre cloth (or any other fibre preform). The shear rate thinning behaviour of the pre-consolidated slurry causes vibration to reduce its viscosity, which allows rapid and efficient intrusion of the fibre tows. Prepregs made in this manner can be frozen and stored. Once thawed they are flexible and can be bent, cut and formed much like an epoxy/fibre prepreg. Prepregs can be stacked and formed into complex geometries like T-joints, doubly curved shapes and tubes [38]. Drying causes only minimal shrinkage due to the high volume fraction of solid present in the pre￾consolidated slurry. The purpose of this study was to demonstrate the usefulness of the VibroIntrusion process in the fabrication of tubular structures from prepregs. Several potential applications for ceramic matrix composites, including high-temperature power piping and hot gas filters in power generation, and tubular combustors or curved combustor liners in gas turbine engines, are based on this simple geometry. A mechanical evaluation of tubular￾shaped specimens is also conveniently performed using a simple pressure test, giving insights on the failure mechanisms and their relation to the fluid containment aptitudes of these materials. The failure behaviour of pressurised tubes was analysed in terms of both the maximum sustainable hoop stress and energy release rates for a propagating delamination crack. 2. Experimental procedure 2.1. Materials All specimens prepared in this study were based on Nextel 610e alumina fibre cloth in an eight-harness satin weave. Sizing applied to the fabric by the manufacturer was removed through thermal treatment in air at 900 8C for a period of 2 h; otherwise the fabric was used in the as￾received condition. The N610 fibres are composed of pure (.99%) polycrystalline a-alumina and the tows in the fabric contain approximately 400 filaments with diameters between 10 and 12 mm [26,27]. Although the N610 fibre shows very high room temperature strength, relative to N720, a mullite/alumina fibre, it is less stable and creep resistant at temperatures above ,1000 8C [27–29]. The particle size and size distribution of mullite and alumina powders were selected to insure relatively high packing densities of the final filter-pressed slurry. The mullite powder was MU-107 (Showa Denko KK, Tokyo Japan) with a mean particle size (as determined by the manufac￾turer) of 1 mm and a particle size distribution of 0.5– 2.5 mm. Its chemical composition is reported by the manufacturer as 75.5% Al2O3 and 24% SiO2 (by weight) with only trace amounts of TiO2, Fe2O3 and Na2O. An a￾alumina powder (AKP-50, Sumitomo Chemicals, Tokyo, Japan) was used with a mean particle size of 0.2 mm and a particle size distribution of 0.1–0.3 mm. Its chemistry is essentially pure a-Al2O3 (99.995%). An alumina precursor, aluminium III sec-butoxide, C12H27O3Al (Gelest Inc., Tullytown, PA, USA) was used to impregnate and thus strengthen the matrix. The precursor yielded 4% of alumina by volume. 2.2. Composite tube manufacturing Aqueous slurries were prepared with a 20% volume content of solids, comprising 70 vol% mullite and 30 vol% alumina. Tetramethylammonium hydroxide (TMA-OH) was used to maintain the pH at about 11. Zeta-potential measurements on mullite and alumina powders showed that pH 11 was sufficient to insure a dispersed slurry through the development of electrostatic repulsive interactions between the particles. Two weight percentage (relative to the solids) of poly-ethylene oxide urethane silane (PEG-silane, Gelest Inc.) was added to induce a steric dispersing effect due to the adsorption of molecules by a reaction with –OH surface sites on both powders [25]. The slurry was contained in a plastic jar that was placed on a mechanical roller for approximately 12 h and after mixing, tetramethylam￾monium nitrate (TMA-N) salt was added to make a 0.25M solution. The addition of TMAþ counterions produce 164 M.G. Holmquist et al. / Composites: Part A 34 (2003) 163–170