MAVERAL ELSEVIER Materials Science and Engineering A195(1995)145-150 Processing of damage-tolerant, oxidation-resistant ceramic matrix composites by a precursor infiltration and pyrolysis method FF. Lange W.C. Tu, A.G. Evans Materials Department, College of Engineering, University of California at Santa Barbara, Santa barbara, CA 93106 USA Abstract methonage-tolerant, continuous fiber ceramic matrix composites een produced by an inexpensive method. according to this particles are heat treated to form a porous framework with out shrinkage, which is then strengthened with an inorganic synthesized from a precursor. High particle packing densities can be achieved within the fiber preform provided that the particle-to-fiber diameter ratio is small. Filling the interstices with a increased the composite density and also limits the size of the crack-like voids within the matrix. In this review we descr mechanical properties of partially dense materials produced from powders to show that a porous matrix can be strong. We strate that the packing density of particles around fibers is highest when the particle-to-fiber diameter ratio is small. The kinetics and mechanical behavior of composite systems is summarized to demonstrate the requirements of damage-tolerant properties. An all oxide ceramic matrix composite produced by this method is discussed Keywords: Infiltration; Ceramics; Composites; Pyrolysis 1. Introduction made strong(above 200 MPa)by a cyclic precursor d. In additio Liquid precursor infiltration and pyrolysis can be the porous matrix itself can induce cracking mecha used for processing ceramics and their composites nisms that provide damage-tolerant behavior. this [1-7 The precursor is a liquid, comprising metal discovery implies that "weak "interfaces are not neces- organic molecules dissolved in an appropriate solvent. sarily a requirement of damage-tolerant, high strain to The excess solvent is removed by evaporation and the failure ceramic matrix composites reinforced with solid precursor molecules are decomposed( pyrolyzed) strong, ceramic fibers. For two different composite sys- to the desired inorganic with a heat treatment. a tems explored in this study, the fibers are well bonded powder compact can be infiltrated with a liquid pre- to the matrix and do not appear to be degraded by the cursor and pyrolyzed to synthesize an inorganic phase processing. In one of these systems, since both the within the porous ceramic[1-4]. A variety of unique matrix(mullite)and fibers(alumina)are oxides, high microstructures(graded, multiphase, partially porous temperature degradation will not occur in oxidizing to fully dense etc. )having unique thermomechanical environments properties can be fabricated. In addition, the pyrolyzed he processing method reviewed here involves precursor can be used both to increase the relative three steps: (i) the packing of powder around fibers by density and to strengthen the powder compact without pressure filtration, (i)a heat treatment to strengthen shrinkage l5」 he powder matrix without shrinkage and (iii)the The lack of powder shrinkage during strengthening additional strengthening of the n advantage for ceramic composites. Conventional trated solution precursor that pyrolyzes to a desired strengthening by densification is constrained by the inorganic material. The inorganic can have a different fibers and leads to the formation of crack-like voids 8. composition from either the fibers or the matrix Moreover, a powder matrix surrounding fibers can be Because the precursor does not completely fill the 221-5093/95/$95001995-Elsevier Science S.A. All rights reserve SSD0921-5093(94)06513-6
MATERIALS SCIENCE & ENGINEERING A ELSEVIER Materials Science and Engineering A195 (1995) 145-150 Processing of damage-tolerant, oxidation-resistant ceramic matrix composites by a precursor infiltration and pyrolysis method F.F. Lange, W.C. Tu, A.G. Evans Materials Department, College of Engineering, University of California at Santa Barbara, Santa Barbara, CA 93106, USA Abstract Damage-tolerant, continuous fiber ceramic matrix composites have been produced by an inexpensive method. According to this method, the space between the fibers is filled with a powder. The powder particles are heat treated to form a porous framework without shrinkage, which is then strengthened with an inorganic synthesized from a precursor. High particle packing densities can be achieved within the fiber preform provided that the particle-to-fiber diameter ratio is small. Filling the interstices with a powder increased the composite density and also limits the size of the crack-like voids within the matrix. In this review we describe the mechanical properties of partially dense materials produced from powders to show that a porous matrix can be strong. We demonstrate that the packing density of particles around fibers is highest when the particle-to-fiber diameter ratio is small. The kinetics and mechanical behavior of composite systems is summarized to demonstrate the requirements of damage-tolerant properties. An alloxide ceramic matrix composite produced by this method is discussed. Keywords: Infiltration; Ceramics; Composites; Pyrolysis I. Introduction Liquid precursor infiltration and pyrolysis can be used for processing ceramics and their composites [1-7]. The precursor is a liquid, comprising metal organic molecules dissolved in an appropriate solvent. The excess solvent is removed by evaporation and the solid precursor molecules are decomposed (pyrolyzed) to the desired inorganic with a heat treatment. A powder compact can be infiltrated with a liquid precursor and pyrolyzed to synthesize an inorganic phase within the porous ceramic [1-4]. A variety of unique microstructures (graded, multiphase, partially porous to fully dense etc.) having unique thermomechanical properties can be fabricated. In addition, the pyrolyzed precursor can be used both to increase the relative density and to strengthen the powder compact without shrinkage [5]. The lack of powder shrinkage during strengthening is an advantage for ceramic composites. Conventional strengthening by densffication is constrained by the fibers and leads to the formation of crack-like voids [8]. Moreover, a powder matrix surrounding fibers can be made strong (above 200 MPa) by a cyclic precursor infiltration-pyrolysis processing method. In addition, the porous matrix itself can induce cracking mechanisms that provide damage-tolerant behavior. This discovery implies that "weak" interfaces are not necessarily a requirement of damage-tolerant, high strain to failure ceramic matrix composites reinforced with strong, ceramic fibers. For two different composite systems explored in this study, the fibers are well bonded to the matrix and do not appear to be degraded by the processing. In one of these systems, since both the matrix (mullite) and fibers (alumina) are oxides, high temperature degradation will not occur in oxidizing environments. The processing method reviewed here involves three steps: (i) the packing of powder around fibers by pressure filtration, (ii) a heat treatment to strengthen the powder matrix without shrinkage and (iii) the additional strengthening of the matrix with an infiltrated solution precursor that pyrolyzes to a desired, inorganic material. The inorganic can have a different composition from either the fibers or the matrix. Because the precursor does not completely fill the 0921-5093/95/$9.50 © 1995 - Elsevier Science S.A. All rights reserved SSD10921-5093(94)06513-6
146 F.F. Lange et al./ Materials Science and Engineering A195(1995)145-150 volume between the particles after pyrolysis, the matrix powder is packed from the slurry state by pressure phase contains a residual void phase even after multi- filtration, with interparticle forces controlled by surface ple infiltration-pyrolysis chemical methods through pH control, by polyelectro- We begin this review by describing the mechanical lytes etc. The highest packing density is achieved when properties of partially dense materials produced from the particles are repulsive. Whether long range or short powders to show that a porous matrix can be strong. range, the repulsive fo rce ac cts as a lubricant to allow Secondly, it will be shown that the packing density of particles to rearrange and pack to their highest density particles around fibers is highest when the particle-to- [26, 27. Also, above a critical pressure, their packing fiber diameter ratio is small. Thirdly, the kinetics and density is not affected by the applied pressure [26, 27 mechanics of the infiltration-pyrolysis process will be In contrast, the packing density achieved in bodies described. Lastly, the damage-tolerant mechanical produced from flocced slurries is much lower and pres- behavior of two composite systems will be demon- sure sensitive because strongly attractive interparticle forces produce a cohesive, connective network before they are packed[26, 27. In effect, the coefficient friction between particles is high when they are 2. Mechanical properties of partially dense strongly attractive and particle rearrangement during materials formed from powders packing is inhibited. Dry powders behave much like the flocced slurry. Repulsive particles are required to Porous materials can have high specific mechanical achieve a high packing density properties [9. Such solids, when formed from The particle-to-fiber diameter ratio R greatly affects powders, transmit force through structural units. These the packing density of particles around fibers [25]. One units have morphological characteristics which differ reason for the lower packing density is the"wall effect from those for cellular materials [10-24] and, unlike shown in Fig. 1(a). When particles are packed against a cellular materials, the mechanical properties are linear wall, such as a fiber surface, extra spaces exist that functions of porosity [24 when normalized by the would have been partially filled with particles if the corresponding values for the fully dense material. The surface did not exist. Zok et al. 28 have shown that the normalized Young's modulus E and the critical stress extra void volume introduced by the "wall effect intensity factor K are given by [24] increases with the ratio r. when r>0.1 a limited number of particles can fill the interstices between the E - and K (1) fibers as shown in Fig. 1(b). The geometrical restriction powder and inhibits the application of powder where po is the relative density of the initial powder methods in forming composite systems unless sub- compact. Similar relations are also obtained when micron particles of the desired powder can be increased density is achieved by cyclic precursor infil- obtained tration and pyrolysis [6]. Hence, both E and Kc can be optimized by starting with a powder compact having he highest possible particle packing density. More importantly, the strength of the partially dense body is controlled by the size of the crack-like flaws Fiber that pre-exist within the initial powder compact [24] Thus, partially dense powder compacts(and matrices in fiber-reinforced composites) could be strong, residual porosity, if the particles were packed Fiber disrupt particle packing Missing to a high relative density and the flaw size within the at surface Wall Effect Volume powder was minimized 3. Packing of particles and reinforcements Increasing particle fiber diame ter Ratio Particle morphology, interparticle forces and the Fig. 1. (a)The packing of particles at the fiber surface particle size distribution are the major factors control- a greater amount of porosity because of the"wall"eff ling the density to which mass can be consolidated the form of powders [25]. The effect of interparticle eter ratio is above 0.1, fewer particles can be packed within the forces on packing density is best illustrated when fiber interstices
146 E E Lange et al. / Materials Science and Engineering A195 (1995) 145-150 volume between the particles after pyrolysis, the matrix phase contains a residual void phase even after multiple infiltration-pyrolysis. We begin this review by describing the mechanical properties of partially dense materials produced from powders to show that a porous matrix can be strong. Secondly, it will be shown that the packing density of particles around fibers is highest when the particle-tofiber diameter ratio is small. Thirdly, the kinetics and mechanics of the infiltration-pyrolysis process will be described. Lastly, the damage-tolerant mechanical behavior of two composite systems will be demonstrated. 2. Mechanical properties of partially dense materials formed from powders Porous materials can have high specific mechanical properties [9]. Such solids, when formed from powders, transmit force through structural units. These units have morphological characteristics which differ from those for cellular materials [10-24] and, unlike cellular materials, the mechanical properties are linear functions of porosity [24] when normalized by the corresponding values for the fully dense material. The normalized Young's modulus /~ and the critical stress intensity factor/(c are given by [24] E =P-Po and I?2c =p-p~° (1) 1-P0 1-P0 where P0 is the relative density of the initial powder compact. Similar relations are also obtained when increased density is achieved by cyclic precursor infiltration and pyrolysis [6]. Hence, both/~ and/(c can be optimized by starting with a powder compact having the highest possible particle packing density. More importantly, the strength of the partially dense body is controlled by the size of the crack-like flaws that pre-exist within the initial powder compact [24]. Thus, partially dense powder compacts (and matrices in fiber-reinforced composites) could be strong, despite residual porosity, if the particles were packed to a high relative density and the flaw size within the powder was minimized. 3. Packing of particles and reinforcements Particle morphology, interparticle forces and the particle size distribution are the major factors controlling the density to which mass can be consolidated in the form of powders [25]. The effect of interparticle forces on packing density is best illustrated when powder is packed from the slurry state by pressure filtration, with interparticle forces controlled by surface chemical methods through pH control, by polyelectrolytes etc. The highest packing density is achieved when the particles are repulsive. Whether long range or short range, the repulsive force acts as a lubricant to allow particles to rearrange and pack to their highest density [26,27]. Also, above a critical pressure, their packing density is not affected by the applied pressure [26,27]. In contrast, the packing density achieved in bodies produced from flocced slurries is much lower and pressure sensitive because strongly attractive interparticle forces produce a cohesive, connective network before they are packed [26,27]. In effect, the coefficient of friction between particles is high when they are strongly attractive and particle rearrangement during packing is inhibited. Dry powders behave much like the flocced slurry. Repulsive particles are required to achieve a high packing density. The particle-to-fiber diameter ratio R greatly affects the packing density of particles around fibers [25]. One reason for the lower packing density is the "wall effect", shown in Fig. l(a). When particles are packed against a wall, such as a fiber surface, extra spaces exist that would have been partially filled with particles if the surface did not exist. Zok et al. [28] have shown that the extra void volume introduced by the "wall effect" increases with the ratio R. When R >0.1, a limited number of particles can fill the interstices between the fibers as shown in Fig. l(b). The geometrical restriction of particle packing limits the packing density of the powder and inhibits the application of powder methods in forming composite systems unless submicron particles of the desired powder can be obtained. Fiber disrupts particle packing Missing at surface = Wall Effect Particle Volume Increasing particle/fiber diameter Ratio ) Fig. I. (a) The packing of particles at the fiber surface introduces a greater amount of porosity because of the "wall" effect where portions of particles (dark portions of particles) would have existed if fiber was missing. (b) When the particle-to-fiber diameter ratio is above 0.1, fewer particles can be packed within the fiber interstices
F.F. Lange et al materials Science and Engineering A195(1995)145-150 The problem of packing powders within three- pressurized, entrapped gas to the surface. Such gas dimensional preforms was solved by Jamet et al. [29] diffusion is controlled by its solubility at higher pres using pressure filtration. Our adaptation is shown in sures Fig. 2. In this process the consolidated layer builds up The flow of liquid into a porous medium by differ- within the preform, fixed to a filter. Powders can be ential pressure AP is described by Darcy's law[33, 34] packed within the preform provided that three condi- tions are satisfied (25)J. First, the particles must be small h-2KAP /2 nough to flow through the preform channels and smaller yet(R <0.05)to achieve high packing densities for the reasons described above. Second, the particles where h is the distance of liquid intruded within a n the slurry must be repulsive( flocced slurries clog the period t, K is the permeability of the porous body and hannels). Third, repulsive surface forces must exist n is the viscosity of liquid. Gas diffusion obeys Fick's between the preform material and the particles High pressures are desirable because of the para- h=(2DBP /ri/2 bolic kinetics of pressure filtration and the low per- meability of highly packed, submicron powders. where De is the diffusion coefficient of the gas within During filtration, both the reinforcement material and the liquid, B is Henry's constant and Pi is the pressure the surrounding powder are compressed. Both relieve of the entrapped gas. Although both phenomena are their stored strain when the pressure is removed. Since concurrent, the flow of liquid due to differential pres each has different strain recoveries, stresses arise, sure initially dominates. Once the gas within the which can damage these bodies, as shown in Fig. 2(b). compact is sufficiently compressed, gas diffusion Bodies formed from dispersed slurries still flow after becomes dominant onsolidation and can dissipate stresses [26]. The When the intruded precursor is converted to an recovery is time dependent[26]. Thus, the rheology of inorganic during heat treatment, the void space is the consolidated body must be understood and con- partially filled with pyrolyzed precursor without trolled to avoid processing damage induced by the shrinkage of the powder. The kinetics of subsequent reinforcements [301 cycles depends on the permeability of the pyrolyzed precursor, which in turn depends on microstructural development during the heat treatment subsequent to pyrolysis Surface cracks can form within the powder 4. Infiltration physics and kinetics compact during either precursor drying or pyrolys They can be avoided by strengthening the powder The infiltration of a dry, porous medium containing compact by forming small necks between touching gas occurs by two different mechanisms [31, 32]. First, particles by evaporation-condensation [5]. Moreover, capillary plus applied pressures cause a wetting liquid precursor molecules concentrate near the surface as to flow into a granular medium; flow will diminish and the solvent is removed by drying [5]. This can be pre- then stop when the pressure of the entrapped gas vented by gelling the precursor prior to drying. For causes the differential pressure to approach zero. example, a Zr acetate is gelled by soaking the infiltrated Second, gas can diffuse through the liquid from the bodies in aqueous NH OH [5] Iloeetd slur Stift Bod Rcisforernt Prior 嬗 ceas●latl Powdr 88 a) reinforcement preforms by pressure filtration. When the pressure is removed, the re or less strain than the powder compact. (b) Differential strain can induce damage in the
F.F. Lange et al. / Materials Science and Engineering A195 (1995) 145-150 147 The problem of packing powders within threedimensional preforms was solved by Jamet et al. [29] using pressure filtration. Our adaptation is shown in Fig. 2. In this process the consolidated layer builds up within the preform, fixed to a filter. Powders can be packed within the preform provided that three conditions are satisfied [25]. First, the particles must be small enough to flow through the preform channels and smaller yet (R < 0.05) to achieve high packing densities for the reasons described above. Second, the particles in the slurry must be repulsive (flocced slurries clog the channels). Third, repulsive surface forces must exist between the preform material and the particles. High pressures are desirable because of the parabolic kinetics of pressure filtration and the low permeability of highly packed, submicron powders. During filtration, both the reinforcement material and the surrounding powder are compressed. Both relieve their stored strain when the pressure is removed. Since each has different strain recoveries, stresses arise, which can damage these bodies, as shown in Fig. 2(b). Bodies formed from dispersed slurries still flow after consolidation and can dissipate stresses [26]. The recovery is time dependent [26]. Thus, the rheology of the consolidated body must be understood and controlled to avoid processing damage induced by the reinforcements [30]. 4. Infiltration physics and kinetics The infiltration of a dry, porous medium containing gas occurs by two different mechanisms [31,32]. First, capillary plus applied pressures cause a wetting liquid to flow into a granular medium; flow will diminish and then stop when the pressure of the entrapped gas causes the differential pressure to approach zero. Second, gas can diffuse through the liquid from the pressurized, entrapped gas to the surface. Such gas diffusion is controlled by its solubility at higher pressures. The flow of liquid into a porous medium by differential pressure AP is described by Darcy's law [33,34]: = -- t 1/2 (2) where h is the distance of liquid intruded within a period t, K is the permeability of the porous body and r/is the viscosity of liquid. Gas diffusion obeys Fick's law [35]: h =(2OgflPi)l/ztff2 (3) where Dg is the diffusion coefficient of the gas within the liquid, fl is Henry's constant and Pi is the pressure of the entrapped gas. Although both phenomena are concurrent, the flow of liquid due to differential pressure initially dominates. Once the gas within the compact is sufficiently compressed, gas diffusion becomes dominant. When the intruded precursor is converted to an inorganic during heat treatment, the void space is partially filled with pyrolyzed precursor without shrinkage of the powder. The kinetics of subsequent cycles depends on the permeability of the pyrolyzed precursor, which in turn depends on microstructural development during the heat treatment subsequent to pyrolysis. Surface cracks can form within the powder compact during either precursor drying or pyrolysis. They can be avoided by strengthening the powder compact by forming small necks between touching particles by evaporation-condensation [5]. Moreover, precursor molecules concentrate near the surface as the solvent is removed by drying [5]. This can be prevented by gelling the precursor prior to drying. For example, a Zr acetate is gelled by soaking the infiltrated bodies in aqueous NH4OH [5]. [ Slurry __ . St ¢i~ o rot ~.~:~t l~tor'~. Co~ ol.i~td l~ow~v StJt! 3 o<l.y~ Di.+~,trtl Slur~/' W Bo~y ~1o w'~ Ilo I) ~m.lti~ (a) (b) Fig. 2. (a) Powder can be packed within reinforcement preforms by pressure filtration. When the pressure is removed, the reinforcement material can recover either more or less strain than the powder compact. (b) Differential strain can induce damage in the powder compact unless stresses are dissipated by body flow
FF Lange et al. Materials Science and Engineering A195(1995)145-150 The size distribution of the crack-like voids pro- fiber preform and packed by filtration under the capil duced within a pyrolyzed precursor is proportional to lary pressure provided by the plaster of Paris. After the size distribution of the voids within the initial complete powder consolidation, the bodies were powder compact [5. Thus partly filling of the void partially dried in the mold, removed and then fully phase within a fiber preform decreases the size of the dried at 60C. The excess powder layer, on top of the crack-like voids during precursor pyrolysis to the size fiber-powder composite, was removed to prevent of the particle interstices, instead of the much larger matrix cracking on further processing interstices between fibers The Si3n4 matrix composite compacts were subse- quently heat treated at 1250C for 10 h under flowing n2 to strengthen partially the Si3N4 matrix without 5.“ Ceramic wood shrinkage by forming necks between the touching Si3N4 particles, by evaporation-condensation. To When two intrinsically brittle materials are com- strengthen this powder matrix fully, infiltration, pyroly- bined, damage tolerance can be achieved whenever sis and heat treatment procedures were implemented cracks can be induced to deflect along planes parallel using polysilazane [5]. This precursor pyrolyzes to an to the loading direction[36]. Most damage-tolerant amorphous"Si, N4". Up to three cycles were typically ceramic matrix composites(CMCs) have implemented employed with heat treatment at 1200'C for 4 h after this requirement by using a thin interphase between the each pyrolysis. A similar procedure was used to pro- fiber and the matrix. The interphases used in most duce a mullite matrix; a mixed alkoxide was used to commercial products consist of either C or BN. These strengthen the mullite powder matrix after gelation, interphases oxidize and cause embrittlement 37, 38]. drying and heat treatment permits the creation of low cost, damage-tolerance and fractured sections o big by was used on polished Here, a new concept is developed and exploited that nning electron CMCs, inherently resistant to oxidation embrittlement, features. Fig 3 illustrates a typical polished sect 9 nlight the microstr without the requirement for a matrix-fiber interphase. the Al2O3-Si3 N4 composite showing that the cube-like The ensuing composites have preformance character- Si, Ni4 particles are bonded together with amor istics similar to those demonstrated by various natural phous"Si3N4"and the Al,O3 fibers are bonded to the materials, particularly wood 39 matrix with the same amorphous"Si3N4". The dimen c The materials described here use high strength sion of the pores seen in Fig 3 are limited in size to the ramic fibers in a porous ceramic matrix. The space between particles materi als are selected to satisfy the two basic criteria Flexural testing demonstrates the fracture mode and needed to achieve damage-tolerant behavior, accord- damage tolerance of these materials. Fig 4 shows the ing to the scheme elaborated elsewhere [7].(i)The load-displacement behavior of both the Al,O-Si3N fibers have a larger thermal expansion coefficient than the matrix. In consequence, the fiber bundles are esidual tension, whereas the matrix regions are in residual compression. (ii)The matrix consists of a fine- scale, porous framework having a relatively low resis tance k to crack extension but good tensile strength The latter criterion can be satisfied by using the CMC processing method described above, i.e. packing powders around fibers by the pressure filtration of particles within a dispersed slurry(Section 3),and strengthening the powder matrix by the cyclic infiltra- tion of a ceramic precursor (Section 4). The behavior is exemplified by Al,O3 fibers in porous matrices of either Si, N, or mullite [7 In order to create these composites, dispersed aqueous Si3n4 and mullite slurries with a particle size up to I um were first prepared Slip casting was used to pack the particles around fiber bundles. The as lum received AL2O, fibers were cut into 35 mm lengths. Fig. 3. Polished section of an AlO-SigN, composite showing Fiber bundles were dip coated into the slurry and that the cubic-like Si N particles are bonded together with stacked in a Teflon mold in contact with plaster of amorphous, precursor derived "Si, N, "and the AL,O, fibers are Paris. The slurry was poured into the mold to cover the bonded to the matrix with the same amorphous"Si, A
148 F.F. Lange et al. / Materials Science and Engineering A195 (1995) 145-150 The size distribution of the crack-like voids produced within a pyrolyzed precursor is proportional to the size distribution of the voids within the initial powder compact [5]. Thus partly filling of the void phase within a fiber preform decreases the size of the crack-like voids during precursor pyrolysis to the size of the particle interstices, instead of the much larger interstices between fibers. 5. "Ceramic wood" When two intrinsically brittle materials are combined, damage tolerance can be achieved whenever cracks can be induced to deflect along planes parallel to the loading direction [36]. Most damage-tolerant ceramic matrix composites (CMCs) have implemented this requirement by using a thin interphase between the fiber and the matrix. The interphases used in most commercial products consist of either C or BN. These interphases oxidize and cause embrittlement [37,38]. Here, a new concept is developed and exploited that permits the creation of low cost, damage-tolerance CMCs, inherently resistant to oxidation embrittlement, without the requirement for a matrix-fiber interphase. The ensuing composites have preformance characteristics similar to those demonstrated by various natural materials, particularly wood [39]. The materials described here use high strength ceramic fibers in a porous ceramic matrix. The materials are selected to satisfy the two basic criteria needed to achieve damage-tolerant behavior, according to the scheme elaborated elsewhere [7]. (i) The fibers have a larger thermal expansion coefficient than the matrix. In consequence, the fiber bundles are in residual tension, whereas the matrix regions are in residual compression. (ii) The matrix consists of a finescale, porous framework having a relatively low resistance K c to crack extension but good tensile strength. The latter criterion can be satisfied by using the CMC processing method described above, i.e. packing powders around fibers by the pressure filtration of particles within a dispersed slurry (Section 3), and strengthening the powder matrix by the cyclic infiltration of a ceramic precursor (Section 4). The behavior is exemplified by AI203 fibers in porous matrices of either Si3N 4 or mullite [7]. In order to create these composites, dispersed aqueous Si3N 4 and mullite slurries with a particle size up to 1 ktm were first prepared. Slip casting was used to pack the particles around fiber bundles. The asreceived AIeO 3 fibers were cut into 35 mm lengths. Fiber bundles were dip coated into the slurry and stacked in a Teflon mold in contact with plaster of Paris. The slurry was poured into the mold to cover the fiber preform and packed by filtration under the capillary pressure provided by the plaster of Paris. After complete powder consolidation, the bodies were partially dried in the mold, removed and then fully dried at 60 °C. The excess powder layer, on top of the fiber-powder composite, was removed to prevent matrix cracking on further processing. The Si3N 4 matrix composite compacts were subsequently heat treated at 1250 °C for 10 h under flowing N2 to strengthen partially the Si3N 4 matrix without shrinkage by forming necks between the touching Si3N 4 particles, by evaporation-condensation. To strengthen this powder matrix fully, infiltration, pyrolysis and heat treatment procedures were implemented using polysilazane [5]. This precursor pyrolyzes to an amorphous "Si3N4". Up to three cycles were typically employed with heat treatment at 1200 °C for 4 h after each pyrolysis. A similar procedure was used to produce a mullite matrix; a mixed alkoxide was used to strengthen the mullite powder matrix after gelation, drying and heat treatment. Scanning electron microscopy was used on polished and fractured sections to highlight the microstructural features. Fig. 3 illustrates a typical polished section of the A1203-Si3N 4 composite showing that the cube-like Si3Ni 4 particles are bonded together with amorphous "Si3N4" and the AI203 fibers are bonded to the matrix with the same amorphous "Si3N4". The dimension of the pores seen in Fig. 3 are limited in size to the space between particles. Flexural testing demonstrates the fracture mode and damage tolerance of these materials. Fig. 4 shows the load-displacement behavior of both the AI203-Si3N 4 Fig. 3. Polished section of an AI203-Si3N n composite showing that the cubic-like Si3N 4 particles are bonded together with amorphous, precursor derived "Si3N4" and the A1203 fibers are bonded to the matrix with the same amorphous "Si3N4
F.F. Lange et al Materials Science and Engineering A195(1995)145-150 5 400500600 2cyde.1250°c 100um b Fig 4. Load-displacement behavior of (a Al,O3-Si3N and(b) Al2O3-mullite composites. 10 (Fig. 4(a))and the Al2O3-mullite(Fig. 4(b))com posites. Extensive inelastic deformation is evident. The failure.(b) Typical fracture surface of Al, O,-SiN, composite step-wise load drops, beyond the peak stress, are showing fiber bundles with attached matrix characteristic of the behavior of laminar composites with crack-deflecting interfaces [40 Fig. 5(a)illustrates the"wood"-like fracture path of the Al,O3-Si3N4 composite. Fig. 5(b) illustrates a crack-like voids within the matrix and thus optimizes typical fracture surface, showing that fiber bundles are its strength. The matrix itself can act as a crack bonded together with the powder matrix, and that deflecting phase such that an all-oxide CMc can be crack deflection occurs within the matrix fabricated 6. Concluding remarks Acknowledgments Damage-tolerant continuous fiber CMCs can be This research was sponsored by the Defense luced by a powder route that packs particles within Advance Research Projects Agency through the a fiber preform by pressure filtration and then strength- University Research Initiative of UCSB under ONR ening the powder matrix by a cyclic precursor infiltra- Contract N00014-92-J-1808. Portions of this review tion method. High particle packing densities can be that concerned interparticle tentials that control achieved within the fiber preform provided that the particle packing density and the rheology of the slurry particle-to-fiber diameter ratio is small. Filling the and consolidated body were supported by the Office of interstices with particles first limits the size of the Naval Research under No0014-90-J-1441
F.F. Lange et al. / Materials Science and Engineering A195 (1995) 145-150 149 500 400 3OO 100 0 0 a I I I I 100 200 300 400 500 Displac~a~at (Ima) 300, b 2 yd,.IZ 0*C ] 20O !f/ a. • 150 a i 0 o ~oo 200 aoo 40o 50o soo Displacement (~m) Fig. 4. Load-displacement behavior of (a)AI203-Si3N 4 and (b) Al:O3-mullite composites. (Fig. 4(a)) and the A1203-mullite (Fig. 4(b)) composites. Extensive inelastic deformation is evident. The step-wise load drops, beyond the peak stress, are characteristic of the behavior of laminar composites with crack-deflecting interfaces [40]. Fig. 5(a) illustrates the "wood"-like fracture path of the AIzO3-Si3N 4 composite. Fig. 5(b) illustrates a typical fracture surface, showing that fiber bundles are bonded together with the powder matrix, and that crack deflection occurs within the matrix. Fig. 5. (a) Fracture path of A1203-Si3N 4 composite in flexural failure. (b) Typical fracture surface of AI203-Si3N 4 composite showing fiber bundles with attached matrix. crack-like voids within the matrix and thus optimizes its strength. The matrix itself can act as a crackdeflecting phase such that an all-oxide CMC can be fabricated. 6. Concluding remarks Damage-tolerant continuous fiber CMCs can be produced by a powder route that packs particles within a fiber preform by pressure filtration and then strengthening the powder matrix by a cyclic precursor infiltration method. High particle packing densities can be achieved within the fiber preform provided that the particle-to-fiber diameter ratio is small. Filling the interstices with particles first limits the size of the Acknowledgments This research was sponsored by the Defense Advance Research Projects Agency through the University Research Initiative of UCSB under ONR Contract N00014-92-J-1808. Portions of this review that concerned interparticle potentials that control particle packing density and the rheology of the slurry and consolidated body were supported by the Office of Naval Research under N00014-90-J-1441
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