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Preparation of continuous fiber ceramic composites using a combination of steric-stabilization and depletion-flocculation phenomena
Composites: Part A 30 (1999) 231–237
Preparation of continuous fiber ceramic composites using a combination of steric-stabilization and depletion-flocculation phenomena Judith A. Koltay 1, Donald L. Feke* Department of Chemical Engineering, Case Western Reserve University, Cleveland, OH 44106-7217, USA Received 1 June 1998; accepted 9 July 1998
Abstract A combination of depletion-flocculation and steric-stabilization phenomena has been used in the preparation of highly fluid, concentrated suspensions. Both of these characteristics are beneficial for the fabrication of fiber-reinforced composite materials formed by infiltration of single- and multiple-ply fiber weaves by suspension. To illustrate this processing approach, suspensions of submicron silicon particles in cyclohexane were prepared via the depletion-steric method. These suspensions were used to infiltrate weaves of five different types of ceramic fibers. Scanning electron micrographs of the fired composites indicate the high density and uniformity of the infiltration achieved using this approach. q 1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Fibres; colloidal processing
1. Introduction There have been many recent advances in the development of continuous ceramic fibers with excellent mechanical properties and good environmental stability characteristics. The combination of the widespread availability of such ceramic fibers and the persistent need for superior materials for structural and mechanical applications has motivated the development of advanced composite processing methods. Often, such advances can stem from improvements in understanding the fundamental properties that govern processing behavior. There are a variety of techniques practiced for the fabrication of fiber-reinforced ceramic composites. Often, the continuous ceramic fibers are woven into two-dimensional cloths, which can then be stacked, shaped and laminated together to form complicated three-dimensional objects. Subsequently, the material that becomes the matrix phase is introduced to the interstitial space between the fibers, and the whole assembly (known as the ‘green composite’) is then sintered or reaction-bonded together. Often, the introduction of the matrix precursor to the fiber weaves poses a great deal of processing difficulty owing to the small interfiber distances through which this material must pass. * Corresponding author. Tel.: 1 1-216-368-2750; fax: 1 1-216-3683016. 1 Present address: Hewlett Packard, Sonoma, CA, USA.
Several methods for the introduction of the matrix precursor have been attempted. These include chemical vapor deposition (CVD) approaches in which a gas-phase reaction precipitates solids directly within the fiber weaves. Alternatively, it is possible to infiltrate the fiber weaves with an inorganiccontaining polymer liquid, which is subsequently pyrolyzed thereby leaving behind the inorganic fraction. In these cases, however, the nature of the matrix phase is limited to those materials that can be derived from CVD or pyrolyis. Another common method is the infiltration of the assembly of the woven fibers by a slurry containing small ceramic particles that become the continuous matrix phase upon sintering. Since these particles can be derived from any source or synthesis route, this approach allows greater flexibility in the choice of the matrix material for the composite. In principle, this method relies on capillary action to draw slurry into the fiber weave or an externally applied pressure field to drive the slurry in. Subsequently, as the solvent is driven off the particles are deposited in the interstitial regions within the weave. However, unfavorable interactions that occur between the ceramic particles and/or between the particles and the fibers can prevent a uniform impregnation of the particles throughout the material. Often the fiber weave itself acts as a filter, and the particles from the slurry are deposited only on the outside of the weave. The success of the slurry infiltration method depends on the nature and strength of the particle–particle and particle– fiber interaction forces. Colloidal processing strategies have
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been gaining in popularity in wet ceramics processing since the aim of such strategies is to gain control over these types of interactions. There are two slurry characteristics that are desirable for the slurry-infiltration approach. The first is a high degree of stability in the slurry since agglomerates of particles would have difficulty penetrating into the small inter-fiber gaps. A slurry with a low zero-shear viscosity (with no apparent yield stress) signals such a highly stable state. This rheological characteristic also enables deep and uniform impregnation of the fiber weaves. The second desirable characteristic is a high solid loading in the slurry. In order to form a dense matrix phase, the packing density of particles within the green composite must be high and thus infiltrating with a highly concentrated slurry benefits the ultimate packing density in the green composite. The primary goal of this study is to illustrate how the colloidal phenomena of depletion flocculation could be used in combination with steric stabilization to prepare a slurry with characteristics favorable to the infiltration of fiber weaves. The method is general, and can apply to a wide variety of slurry systems. To illustrate this approach, ceramic composites consisting of a silicon nitride matrix (derived from a green composite containing silicon particles) and five different types of fiber weaves were prepared. Depletion flocculation involves the use of polymeric additives that dissolve in the processing medium but do not adsorb onto the solids. The action of these additives is to drive the particles towards flocculation. Steric stabilization involves the use of additives that are grafted to the surfaces of the particles to prevent the flocculation from actually occurring. We use the two colloidal effects to influence the state of the slurry on two length scales. The depletion effect occurs over the longer range (initially over the length scale of the particles and subsequently over the length scale of the vessel containing the slurry). The depletion effect is used to generate small volumes of high solid loading and also a very low (or zero) solid loading over large portions of the suspension. The steric effect occurs over a shorter range (corresponding to the thickness of the grafted layer) to prevent particles from colliding and sticking to each other. The depletion-flocculation effect has been known for more than a decade [1–4]. Additives used for depletion flocculation are chosen to be highly soluble in the processing fluid but should not extensively adsorb onto the surface of the suspended solids. As the particles experience random Brownian motion, they will occasionally pass closer to one another than the size of the dissolved additive molecules. Fig. 1a shows a schematic representation of this effect. In this case, the additive is effectively depleted from the region between the particles, temporarily creating an osmotic pressure imbalance between the bulk solution (which contains the polymer additive) and the microreservoir of additivefree processing fluid located between the particles. This osmotic pressure differential will tend to drain fluid from between the particles, and this effect drives particles
together. Eventually, the groups of particles formed by these depletion forces may be large enough to settle at observable rates, and the suspension will gravity-separate into two phases: an upper phase devoid of solids, and a lower phase containing all of the solids within the dispersion. The strength of the depletion effect depends on the concentration of the depletion additive in solution. At too low a concentration, Brownian motion could separate the particles before the fluid draining would occur, and the dispersion would remain stable. Thus, there is a minimum concentration of the depletion additive necessary to initiate the phase separation process. In contrast, steric additives are selected to be grafted or strongly adsorb onto the surfaces of the particles. Upon close approach of two particles, these adsorbed layers can interact, thereby imparting a short-range force to stabilize the particles against flocculation. Fig. 1b illustrates the steric effect. The steric additive serves the secondary role of blocking the particle surface from adsorption by the depletion additive. The strength of the steric stabilization depends on the characteristics of the additive (such as its molecular weight or chain length) and the degree to which it interacts with the processing fluid. For low molecular weight steric additives, it is possible that the depletion effect can overwhelm the steric-stabilizing effect at high concentrations of the depletion additive. Thus, there may exist another critical concentration of the depletion additive; processing above this minimum value can induce flocculation even in the presence of the steric stabilizer. Processing with the depletion additive in the concentration range between the phase-separation and the induced-flocculation concentration values results in highly stable slurries with relatively high solid concentration.
2. Experimental procedure To illustrate this approach, reaction-bonded silicon nitride continuous-fiber composites were prepared. To produce these composites, weaves of continuous fiber were infiltrated with silicon powder and were subsequently nitrided to form the silicon nitride matrix phase. The silicon powder, initially 1–10 mm in size (Union Carbide, lot #50054) was attrition milled in cyclohexane for 72 h. A particle size analysis obtained using dynamic light scattering (Nicomp model 370, Santa Barbara, CA) showed that the silicon powder was milled to a size range of 0.136– 0.23 mm. A thin steric barrier of approximately 7 wt% relative to the solids was provided to the silicon particles by reacting 1-octadecanol (Aldrich Chemical Company) to surface silanol groups. Details of this pseudo-esterification reaction procedure are outlined in earlier work [5]. Experiments involving the depletion effect and using the resulting slurries to coat AVCO SCS-6 (monofilament
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Fig. 1. Schematics of (a) the depletion-flocculation effect and (b) the steric-stabilization effect.
silicon carbide) have been described in earlier work [5, 6]. These results were used to determine the optimum concentration of the depletion additive for the work reported here. Monodisperse polystyrene (Pressure Chemical Company) with molecular weight 5780 was used as the depletion additive. Slips were prepared by ultrasonically dispersing (model W-225 Ultrasonic Processor, Heat Systems, Farmingdale, NY) 5 wt% of the modified powder in cyclohexane in a separating funnel. Solutions of polystyrene were pipetted to the silicon dispersion to give an overall polystyrene concentration of 0.78 mg/cm 3. The dispersion was thoroughly mixed and tightly sealed to prevent evaporation of the cyclohexane and was allowed to stand in a water bath at 408C. Within an hour, two phases separated by a sharp interface could be distinguished. The clear upper phase was devoid of solids while the turbid lower phase was rich in silicon. Up to 24 h was allowed to ensure a complete phase separation. The lower phase, which contained approximately 40 wt% solids, was collected and used for the fiber weave infiltration. A total of five different commercially available types of fibers and/or weave styles was used in infiltration experiments to test the robustness of the technique and the quality of the infiltration.
Two sets of experiments used ceramic grade Nicalon fibers. Nicalon is a multifilament silicon carbide fiber manufactured through a polymer pyrolysis process by Nippon Carbon, Japan, and is available in North America from Dow Corning Corporation, Midland, MI. The fiber is homogeneously comprised of ultrafine beta-SiC crystals with excess carbon and 9–11% oxygen as SiO2 (58% silicon, 31% carbon, 11% oxygen). Some variety in the crosssectional size and shape of the fibers (round to oval) is typical. There are 250–500 filaments/tow and the fiber diameter is approximately 12–15 mm. For the infiltration trials, these were woven into both a plain square weave and a satin weave style. The as-received Nicalon fibers were coated with sizing (polyvinylacetate) which was burnt off at 5508C prior to infiltration. Single pieces of woven cloth (1.2 × 12.7 cm) were dipped with tweezers between two and ten times into the lower phase slurries prepared as described above. The number of dips required depended on the density of the fiber weave style. Similarly the amount of time required for the cyclohexane solvent to evaporate at room temperature between dips depended on the weave style and filament density. Between each dip most fiber weaves required approximately 30 s for the cyclohexane to evaporate. Satin
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Fig. 2. Electron micrographs of the cross-section of a 1-ply Nicalon square weave composite.
weaves required several minutes to dry due to the greater density of the weave style. A third set of experiments was performed with Stackpole Panex carbon WCG fiber. This is a non-continuous pitch based fiber. Small lengths of fiber are spun into a thread which is woven into different weave styles. As a result, the shape of the fiber is circular with a highly jagged, irregular surface. The fourth set of experiments was performed with X96371 HPZ ceramic fiber (Dow Corning Corporation, Midland, MI). HPZ is an inorganic silicon nitride type fiber manufactured by a pyrolysis process using hydridopolysilizane polymer. The ceramic fiber is amorphous with a typical elemental composition of 57% silicon, 28% nitrogen, 10% carbon and 4% oxygen. At the time of this study, HPZ fiber was a developmental product which was not commercially available. There were 200–800 filaments/ tow with a filament diameter of 10–12 mm. The weave thickness was approximately 0.028–0.030 cm. The product is supplied with surface treatments and sizing which were
removed prior to slurry infiltration in a similar manner to that for the Nicalon fibers. The fifth set of experiments was performed with Thornel carbon fiber T-650/35 (Amoco Performance Products, Ridgefield, CT). Thornel is a continuous length, high strength, high modulus fiber made from a polyacrylonitrile precursor. The fiber surface had been treated to increase the interlaminar shear strength in a resin matrix composite. The fiber surface consists of oxidized carbon functionality with minor levels of silicon, oxygen, nitrogen and sulfur contaminants. There are 3000 filaments/strand and the filament diameter is 6.8 mm. The thickness of the weave was 0.036–0.041 cm. The cross-sectional Thornel carbon fiber shape is mostly ‘kidney-bean’ shape and the surface of Thornel carbon fibers consists of cranulated grooves resulting from the process of spinning the polyacrylonitrile precursor. The density of the filaments/tow is approximately 4–15 times the density of the fibers in the HPZ satin weave and 6–12 times the density of the Nicalon weaves. After infiltration with the silicon slurries, the woven cloth pieces were dipped in a binder–plasticizer mix for handlability. The mix consisted of 6.7 wt% silicon powder ultrasonically dispersed in a mixture of 76.5 wt% tetrahydrofuran, 4.2 wt% polybutylmethacrylate (Aldrich Chemical Company) and 2.6 wt% dibutylphthalate (Aldrich Chemical Company). The 1–10 mm silicon powder was attrition milled in naptha (Cuyahoga Chemical Corporation) for 24 h to 0.17–0.26 mm size range. Polybutylmethacrylate was added as a binder, and dibutylphthalate acted as a plasticizer. The infiltrated Nicalon weaves were made into 1-ply and 4-ply composites. All other weaves were made into 1-ply composites. The 1.2 × 12.7 cm woven cloth pieces were wrapped between two 1.2 × 12.7 cm strips of 0.05 cm thick silicon–Teflon tape. The silicon–Teflon (Dupont Company, Wilmington, DE) tape was a 90g/10g mix blended in 227 cm 3 of naphtha, filtered to form a paste and rolled into tape. The infiltrated cloth placed between two strips of silicon–Teflon tape was cold pressed (Carver Laboratory Press, Menomonee Falls, WI) at 4450 N. A 0.01 cm thick layer of Si3N4 –Teflon tape was placed on top and underneath the composite, which was then wrapped in graphite tape (Union Carbide, Cleveland, OH). The Si3N4 –Teflon tape was used to aid in grafoil removal after hot pressing. The grafoil tape was used to protect the graphite die and composite. The composite was then hot pressed (Pathways Thermal Tech, City of Industry, CA) in a vacuum to produce handleable green compacts. Hot pressing consisted of a multistep procedure. Under a vacuum of 10 22 Pa the temperature was ramped up at a rate of 108C/min from 20 to 1758C. The temperature was then raised at a slower rate of 28C/min to 5008C during the binder burnoff stage while 0.87 MPa pressure was applied to the sample. After binder burnoff the temperature was raised from 500 to 8008C at 108C/min, and then held at 8008C for 15 min. Subsequently, the pressure applied to the sample
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3. Results and discussion
Fig. 3. Electron micrographs of the cross-section of a 1-ply Nicalon satin weave composite.
was released. Nitrogen was used to aid in cooling of the hot press and sample. The grafoil was removed from the composite before nitridation. To assemble the 4-ply composites, four strips of infiltrated cloth were stacked up with a piece of silicon–Teflon tape on the top and bottom of the stack. The 4-ply composites were consolidated in the same manner as the 1-ply composites described above. The consolidated green composites were transferred to a horizontal nitridation furnace consisting of a recrystalized Al2O3 reaction tube with stainless steel end caps. The heat was provided by external silicon carbide heating elements. The composites were located in the central hot zone of the furnace where the temperature variation could be held to ^ 28C. Commercial purity nitriding gas was further purified prior to introduction into the furnace by passing through an oxygen trap. The O2 and H2O contents were reduced to less than 1 ppm. The composites were nitrided at 1250 C for 4 h with a nitrogen flow of 23.6 cm 3/s at atmospheric pressure.
Figs 2–6 show a series of SEM micrographs of crosssections of composites prepared as described above. Each figure shows two different magnifications of the same crosssection. The micrographs presented here were chosen as representative of the overall quality as seen throughout the composite. Fig. 2 shows a cross-section of a composite derived from a 1-ply Nicalon plain square weave. The larger circular zones are the cut ends of the Nicalon fibers, and the material located between the fibers is the silicon nitride formed from the silicon particles. Fig. 3 shows the cross-section of a composite formed using a 1-ply Nicalon satin weave. The quality of the matrix residing between the fibers indicated that the infiltration of slurry particles was good across the entire cross-section in both cases. The square weave was fairly uniform in size and was measured to be approximately 0.013 cm thick. Due to the different weave design, the dimension of the satin weave composite was variable and measured 0.07–0.09 cm thick. Fig. 4 shows micrographs of a composite formed from infiltrating a square weave of the Stackpose Panex WGC carbon fibers. The dark regions are the fibers, and again the material located between the fibers was derived from the infiltrated silicon particles. The original fiber weave was very fine and lightweight with a thickness of 0.013– 0.018 cm. It was difficult to infiltrate this fine cloth via the dipping procedure without creasing and crumpling the weave as it was lowered into the slurry. As a result, this weave was only dipped three times in the silicon slurry. From the micrographs, however, it can be seen that despite these difficulties and the surface irregularities of the fiber, a very high quality and uniform infiltration was achieved. The micrographs in Fig. 5 show the cross-section of a 1ply satin weave of HPZ fibers. This composite was produced as a result of dipping five times in the silicon slurry. The elongated oval fiber geometry contrasts with the highly spherical Nicalon fibers. The distribution of HPZ fibers in the weave design does not appear as ordered as in the Nicalon weaves and varies greatly in density throughout the weave. The high quality of the matrix shows that the infiltrated HPZ satin weave was highly loaded with matrix powder throughout. Results derived from the Thornel fiber are illustrated in Fig. 6. Despite such a high filament density, the fiber surface roughness, the kidney-bean shape of the fiber and its irregular fiber orientation, the micrographs show good infiltration was achieved. The whole of the matrix region has very low voidage throughout. The whole series of micrographs indicate high quality infiltration results. All weave styles and fiber types attempted in infiltration experiments were highly loaded with matrix powder and show low voidage. The results of infiltrating with the suspensions prepared via the depletionsteric method were much improved in comparison to
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Fig. 4. Electron micrographs of the cross-section of a Stackpole Panex WCG fiber composite.
infiltration with unmodified slurries. For example, Nicalon square weaves were dipped into slurries of bare untreated silicon powder in cyclohexane solvent. This silicon powder (expected to exhibit an oxidized silanol and siloxane surface) was milled for 72 h prior to use. The silicon powder was ultrasonically pulsed in the cyclohexane solvent to break up any agglomerates that may be present prior to dipping the cloth strips into the slurry. Electron microscopy of the cross-section of the Nicalon square composites showed minimal infiltration with matrix powder. The particles were deposited primarily as a surface layer, with little to no infiltration towards the center of the cloth. The voidage was high throughout. The poor infiltration quality obtained in this case results from the rapid flocculation of the untreated silicon powder. In the absence of any repulsive stabilization mechanism (steric layer), and in the poor cyclohexane dispersive medium for silicon, after ultrasonication the onset of flocculation of the primary silicon particles is rapid. The larger, irregular shape and size of the
Fig. 5. Electron micrographs of the cross-section of a 1-ply HPZ satin weave composite.
agglomerates cannot physically infiltrate the small interfiber spaces. As a result, the agglomerates tend to deposit on the outer surface of the cloth, thereby providing a physical barrier and blocking possible weave infiltration routes during later immersion into the slurry. Fiber weave infiltration was also performed using a silicon–acetone slurry. Here, 25 wt% of silicon was ultrasonically dispersed in acetone to break up any agglomerates. The silicon was 1–10 mm Union Carbide powder attrition milled in naphtha for 24 h to 0.1–2.6 mm size. The silicon was not treated and no additives were used in the acetone slurry. Processing followed the same dipping, composite consolidation and nitridation procedure as outlined for the studies reported above. To ensure that no agglomerates formed between weave dips, the silicon–acetone slurry was ultrasonically pulsed at intervals. Composites prepared via acetone slurries were also examined under the scanning electron microscope. The infiltration quality of these slurries was a significant improvement over the cyclohexane slurries. There was, however, a noticeable level of voidage.
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phenomena of depletion flocculation with steric stabilization were found to exhibit properties desirable for the infiltration of fiber weaves with slurry. These properties included a highly stable (unagglomerated) state and a relatively high concentration of solids. The approach appears to be robust, and should be applicable to a wide variety of slurry systems. For the specific silicon slurry used to illustrate the approach, the colloidal parameters that were determined from experiments using large single fibers were found to apply for the infiltration of small-diameter ceramic-fiber weaves. The infiltrated weaves were cold pressed, hot pressed and nitrided to enable investigation of the infiltration quality via SEM examination of cross-sectional samples of the formed composite. SEM micrographs revealed that a uniform infiltration of matrix powder had been achieved regardless of fiber type, size and shape, surface characteristics, or weave design.
Acknowledgements The authors are grateful to the NASA Lewis Research Center for support of this work under Grant NAG3-1141 and Co-operative agreement NCC 3-298. The authors also wish to thank Raymond Babuder of the NASA Lewis Research Center for performing the infiltration experiments involving untreated silicon powder.
References Fig. 6. Electron micrographs of the cross-section of a Thornel carbon fiber composite.
Although acetone is a good solvent for silicon dispersion [7] and the silicon particles remained well dispersed after ultrasonic pulsing, a small degree of flocculation qualitatively could be observed to be occurring. Hence continued ultrasonic treatment at intervals was necessary. Due to the unavoidable generation of some flocculation, infiltration was significantly improved but not complete. Additionally, as a result of the heat generated by the continued ultrasonic pulsing to prevent agglomerate formation, rapid evaporation of the acetone solvent was a problem. 4. Conclusions Slurries prepared using a combination of the colloidal
[1] Clark J, Vincent B. Nonaqueous silica dispersions stabilized by terminally anchored polystyrene: the effect of added polymer. J Colloid Interface Sci 1981;82:208–216. [2] de Hek H, Vrij A. Phase separation in non-aqueous dispersions containing polymer molecules and colloidal spheres. J Colloid Interface Sci 1979;70:592–594. [3] de Hek H, Vrij A. Interactions in mixtures of colloidal silica spheres and polystyrene molecules in cyclohexane. J Colloid Interface Sci 1981;84:409–422. [4] Pathmamanoharan CH, de Hek H, Vrij A. Phase separation in mixtures of organophilic spherical silica particles and polymer molecules in good solvents. Colloid and Polymer Science 1981;259:769–771. [5] Wernet JH. Composite and microcomposite fabrication via depletion stabilization routes. Ph.D. dissertation, Case Western Reserve University, 1994. [6] Koltay JA, Feke DL. Depletion flocculation strategy for the fabrication of ceramic powder coatings on ceramic powders. Journal of the American Ceramic Society, in press. [7] Bleier A. The role of van der Waals forces in determining wetting and dispersion properties of silicon powder. J Phys Chem 1983;87:3493– 3500.
Preparation of continuous fiber ceramic composites using a combination of steric-stabilization and depletion-flocculation phenomena Judith A. Koltay 1, Donald L. Feke* Department of Chemical Engineering, Case Western Reserve University, Cleveland, OH 44106-7217, USA Received 1 June 1998; accepted 9 July 1998
Abstract A combination of depletion-flocculation and steric-stabilization phenomena has been used in the preparation of highly fluid, concentrated suspensions. Both of these characteristics are beneficial for the fabrication of fiber-reinforced composite materials formed by infiltration of single- and multiple-ply fiber weaves by suspension. To illustrate this processing approach, suspensions of submicron silicon particles in cyclohexane were prepared via the depletion-steric method. These suspensions were used to infiltrate weaves of five different types of ceramic fibers. Scanning electron micrographs of the fired composites indicate the high density and uniformity of the infiltration achieved using this approach. q 1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Fibres; colloidal processing
1. Introduction There have been many recent advances in the development of continuous ceramic fibers with excellent mechanical properties and good environmental stability characteristics. The combination of the widespread availability of such ceramic fibers and the persistent need for superior materials for structural and mechanical applications has motivated the development of advanced composite processing methods. Often, such advances can stem from improvements in understanding the fundamental properties that govern processing behavior. There are a variety of techniques practiced for the fabrication of fiber-reinforced ceramic composites. Often, the continuous ceramic fibers are woven into two-dimensional cloths, which can then be stacked, shaped and laminated together to form complicated three-dimensional objects. Subsequently, the material that becomes the matrix phase is introduced to the interstitial space between the fibers, and the whole assembly (known as the ‘green composite’) is then sintered or reaction-bonded together. Often, the introduction of the matrix precursor to the fiber weaves poses a great deal of processing difficulty owing to the small interfiber distances through which this material must pass. * Corresponding author. Tel.: 1 1-216-368-2750; fax: 1 1-216-3683016. 1 Present address: Hewlett Packard, Sonoma, CA, USA.
Several methods for the introduction of the matrix precursor have been attempted. These include chemical vapor deposition (CVD) approaches in which a gas-phase reaction precipitates solids directly within the fiber weaves. Alternatively, it is possible to infiltrate the fiber weaves with an inorganiccontaining polymer liquid, which is subsequently pyrolyzed thereby leaving behind the inorganic fraction. In these cases, however, the nature of the matrix phase is limited to those materials that can be derived from CVD or pyrolyis. Another common method is the infiltration of the assembly of the woven fibers by a slurry containing small ceramic particles that become the continuous matrix phase upon sintering. Since these particles can be derived from any source or synthesis route, this approach allows greater flexibility in the choice of the matrix material for the composite. In principle, this method relies on capillary action to draw slurry into the fiber weave or an externally applied pressure field to drive the slurry in. Subsequently, as the solvent is driven off the particles are deposited in the interstitial regions within the weave. However, unfavorable interactions that occur between the ceramic particles and/or between the particles and the fibers can prevent a uniform impregnation of the particles throughout the material. Often the fiber weave itself acts as a filter, and the particles from the slurry are deposited only on the outside of the weave. The success of the slurry infiltration method depends on the nature and strength of the particle–particle and particle– fiber interaction forces. Colloidal processing strategies have
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been gaining in popularity in wet ceramics processing since the aim of such strategies is to gain control over these types of interactions. There are two slurry characteristics that are desirable for the slurry-infiltration approach. The first is a high degree of stability in the slurry since agglomerates of particles would have difficulty penetrating into the small inter-fiber gaps. A slurry with a low zero-shear viscosity (with no apparent yield stress) signals such a highly stable state. This rheological characteristic also enables deep and uniform impregnation of the fiber weaves. The second desirable characteristic is a high solid loading in the slurry. In order to form a dense matrix phase, the packing density of particles within the green composite must be high and thus infiltrating with a highly concentrated slurry benefits the ultimate packing density in the green composite. The primary goal of this study is to illustrate how the colloidal phenomena of depletion flocculation could be used in combination with steric stabilization to prepare a slurry with characteristics favorable to the infiltration of fiber weaves. The method is general, and can apply to a wide variety of slurry systems. To illustrate this approach, ceramic composites consisting of a silicon nitride matrix (derived from a green composite containing silicon particles) and five different types of fiber weaves were prepared. Depletion flocculation involves the use of polymeric additives that dissolve in the processing medium but do not adsorb onto the solids. The action of these additives is to drive the particles towards flocculation. Steric stabilization involves the use of additives that are grafted to the surfaces of the particles to prevent the flocculation from actually occurring. We use the two colloidal effects to influence the state of the slurry on two length scales. The depletion effect occurs over the longer range (initially over the length scale of the particles and subsequently over the length scale of the vessel containing the slurry). The depletion effect is used to generate small volumes of high solid loading and also a very low (or zero) solid loading over large portions of the suspension. The steric effect occurs over a shorter range (corresponding to the thickness of the grafted layer) to prevent particles from colliding and sticking to each other. The depletion-flocculation effect has been known for more than a decade [1–4]. Additives used for depletion flocculation are chosen to be highly soluble in the processing fluid but should not extensively adsorb onto the surface of the suspended solids. As the particles experience random Brownian motion, they will occasionally pass closer to one another than the size of the dissolved additive molecules. Fig. 1a shows a schematic representation of this effect. In this case, the additive is effectively depleted from the region between the particles, temporarily creating an osmotic pressure imbalance between the bulk solution (which contains the polymer additive) and the microreservoir of additivefree processing fluid located between the particles. This osmotic pressure differential will tend to drain fluid from between the particles, and this effect drives particles
together. Eventually, the groups of particles formed by these depletion forces may be large enough to settle at observable rates, and the suspension will gravity-separate into two phases: an upper phase devoid of solids, and a lower phase containing all of the solids within the dispersion. The strength of the depletion effect depends on the concentration of the depletion additive in solution. At too low a concentration, Brownian motion could separate the particles before the fluid draining would occur, and the dispersion would remain stable. Thus, there is a minimum concentration of the depletion additive necessary to initiate the phase separation process. In contrast, steric additives are selected to be grafted or strongly adsorb onto the surfaces of the particles. Upon close approach of two particles, these adsorbed layers can interact, thereby imparting a short-range force to stabilize the particles against flocculation. Fig. 1b illustrates the steric effect. The steric additive serves the secondary role of blocking the particle surface from adsorption by the depletion additive. The strength of the steric stabilization depends on the characteristics of the additive (such as its molecular weight or chain length) and the degree to which it interacts with the processing fluid. For low molecular weight steric additives, it is possible that the depletion effect can overwhelm the steric-stabilizing effect at high concentrations of the depletion additive. Thus, there may exist another critical concentration of the depletion additive; processing above this minimum value can induce flocculation even in the presence of the steric stabilizer. Processing with the depletion additive in the concentration range between the phase-separation and the induced-flocculation concentration values results in highly stable slurries with relatively high solid concentration.
2. Experimental procedure To illustrate this approach, reaction-bonded silicon nitride continuous-fiber composites were prepared. To produce these composites, weaves of continuous fiber were infiltrated with silicon powder and were subsequently nitrided to form the silicon nitride matrix phase. The silicon powder, initially 1–10 mm in size (Union Carbide, lot #50054) was attrition milled in cyclohexane for 72 h. A particle size analysis obtained using dynamic light scattering (Nicomp model 370, Santa Barbara, CA) showed that the silicon powder was milled to a size range of 0.136– 0.23 mm. A thin steric barrier of approximately 7 wt% relative to the solids was provided to the silicon particles by reacting 1-octadecanol (Aldrich Chemical Company) to surface silanol groups. Details of this pseudo-esterification reaction procedure are outlined in earlier work [5]. Experiments involving the depletion effect and using the resulting slurries to coat AVCO SCS-6 (monofilament
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Fig. 1. Schematics of (a) the depletion-flocculation effect and (b) the steric-stabilization effect.
silicon carbide) have been described in earlier work [5, 6]. These results were used to determine the optimum concentration of the depletion additive for the work reported here. Monodisperse polystyrene (Pressure Chemical Company) with molecular weight 5780 was used as the depletion additive. Slips were prepared by ultrasonically dispersing (model W-225 Ultrasonic Processor, Heat Systems, Farmingdale, NY) 5 wt% of the modified powder in cyclohexane in a separating funnel. Solutions of polystyrene were pipetted to the silicon dispersion to give an overall polystyrene concentration of 0.78 mg/cm 3. The dispersion was thoroughly mixed and tightly sealed to prevent evaporation of the cyclohexane and was allowed to stand in a water bath at 408C. Within an hour, two phases separated by a sharp interface could be distinguished. The clear upper phase was devoid of solids while the turbid lower phase was rich in silicon. Up to 24 h was allowed to ensure a complete phase separation. The lower phase, which contained approximately 40 wt% solids, was collected and used for the fiber weave infiltration. A total of five different commercially available types of fibers and/or weave styles was used in infiltration experiments to test the robustness of the technique and the quality of the infiltration.
Two sets of experiments used ceramic grade Nicalon fibers. Nicalon is a multifilament silicon carbide fiber manufactured through a polymer pyrolysis process by Nippon Carbon, Japan, and is available in North America from Dow Corning Corporation, Midland, MI. The fiber is homogeneously comprised of ultrafine beta-SiC crystals with excess carbon and 9–11% oxygen as SiO2 (58% silicon, 31% carbon, 11% oxygen). Some variety in the crosssectional size and shape of the fibers (round to oval) is typical. There are 250–500 filaments/tow and the fiber diameter is approximately 12–15 mm. For the infiltration trials, these were woven into both a plain square weave and a satin weave style. The as-received Nicalon fibers were coated with sizing (polyvinylacetate) which was burnt off at 5508C prior to infiltration. Single pieces of woven cloth (1.2 × 12.7 cm) were dipped with tweezers between two and ten times into the lower phase slurries prepared as described above. The number of dips required depended on the density of the fiber weave style. Similarly the amount of time required for the cyclohexane solvent to evaporate at room temperature between dips depended on the weave style and filament density. Between each dip most fiber weaves required approximately 30 s for the cyclohexane to evaporate. Satin
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Fig. 2. Electron micrographs of the cross-section of a 1-ply Nicalon square weave composite.
weaves required several minutes to dry due to the greater density of the weave style. A third set of experiments was performed with Stackpole Panex carbon WCG fiber. This is a non-continuous pitch based fiber. Small lengths of fiber are spun into a thread which is woven into different weave styles. As a result, the shape of the fiber is circular with a highly jagged, irregular surface. The fourth set of experiments was performed with X96371 HPZ ceramic fiber (Dow Corning Corporation, Midland, MI). HPZ is an inorganic silicon nitride type fiber manufactured by a pyrolysis process using hydridopolysilizane polymer. The ceramic fiber is amorphous with a typical elemental composition of 57% silicon, 28% nitrogen, 10% carbon and 4% oxygen. At the time of this study, HPZ fiber was a developmental product which was not commercially available. There were 200–800 filaments/ tow with a filament diameter of 10–12 mm. The weave thickness was approximately 0.028–0.030 cm. The product is supplied with surface treatments and sizing which were
removed prior to slurry infiltration in a similar manner to that for the Nicalon fibers. The fifth set of experiments was performed with Thornel carbon fiber T-650/35 (Amoco Performance Products, Ridgefield, CT). Thornel is a continuous length, high strength, high modulus fiber made from a polyacrylonitrile precursor. The fiber surface had been treated to increase the interlaminar shear strength in a resin matrix composite. The fiber surface consists of oxidized carbon functionality with minor levels of silicon, oxygen, nitrogen and sulfur contaminants. There are 3000 filaments/strand and the filament diameter is 6.8 mm. The thickness of the weave was 0.036–0.041 cm. The cross-sectional Thornel carbon fiber shape is mostly ‘kidney-bean’ shape and the surface of Thornel carbon fibers consists of cranulated grooves resulting from the process of spinning the polyacrylonitrile precursor. The density of the filaments/tow is approximately 4–15 times the density of the fibers in the HPZ satin weave and 6–12 times the density of the Nicalon weaves. After infiltration with the silicon slurries, the woven cloth pieces were dipped in a binder–plasticizer mix for handlability. The mix consisted of 6.7 wt% silicon powder ultrasonically dispersed in a mixture of 76.5 wt% tetrahydrofuran, 4.2 wt% polybutylmethacrylate (Aldrich Chemical Company) and 2.6 wt% dibutylphthalate (Aldrich Chemical Company). The 1–10 mm silicon powder was attrition milled in naptha (Cuyahoga Chemical Corporation) for 24 h to 0.17–0.26 mm size range. Polybutylmethacrylate was added as a binder, and dibutylphthalate acted as a plasticizer. The infiltrated Nicalon weaves were made into 1-ply and 4-ply composites. All other weaves were made into 1-ply composites. The 1.2 × 12.7 cm woven cloth pieces were wrapped between two 1.2 × 12.7 cm strips of 0.05 cm thick silicon–Teflon tape. The silicon–Teflon (Dupont Company, Wilmington, DE) tape was a 90g/10g mix blended in 227 cm 3 of naphtha, filtered to form a paste and rolled into tape. The infiltrated cloth placed between two strips of silicon–Teflon tape was cold pressed (Carver Laboratory Press, Menomonee Falls, WI) at 4450 N. A 0.01 cm thick layer of Si3N4 –Teflon tape was placed on top and underneath the composite, which was then wrapped in graphite tape (Union Carbide, Cleveland, OH). The Si3N4 –Teflon tape was used to aid in grafoil removal after hot pressing. The grafoil tape was used to protect the graphite die and composite. The composite was then hot pressed (Pathways Thermal Tech, City of Industry, CA) in a vacuum to produce handleable green compacts. Hot pressing consisted of a multistep procedure. Under a vacuum of 10 22 Pa the temperature was ramped up at a rate of 108C/min from 20 to 1758C. The temperature was then raised at a slower rate of 28C/min to 5008C during the binder burnoff stage while 0.87 MPa pressure was applied to the sample. After binder burnoff the temperature was raised from 500 to 8008C at 108C/min, and then held at 8008C for 15 min. Subsequently, the pressure applied to the sample
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Fig. 3. Electron micrographs of the cross-section of a 1-ply Nicalon satin weave composite.
was released. Nitrogen was used to aid in cooling of the hot press and sample. The grafoil was removed from the composite before nitridation. To assemble the 4-ply composites, four strips of infiltrated cloth were stacked up with a piece of silicon–Teflon tape on the top and bottom of the stack. The 4-ply composites were consolidated in the same manner as the 1-ply composites described above. The consolidated green composites were transferred to a horizontal nitridation furnace consisting of a recrystalized Al2O3 reaction tube with stainless steel end caps. The heat was provided by external silicon carbide heating elements. The composites were located in the central hot zone of the furnace where the temperature variation could be held to ^ 28C. Commercial purity nitriding gas was further purified prior to introduction into the furnace by passing through an oxygen trap. The O2 and H2O contents were reduced to less than 1 ppm. The composites were nitrided at 1250 C for 4 h with a nitrogen flow of 23.6 cm 3/s at atmospheric pressure.
Figs 2–6 show a series of SEM micrographs of crosssections of composites prepared as described above. Each figure shows two different magnifications of the same crosssection. The micrographs presented here were chosen as representative of the overall quality as seen throughout the composite. Fig. 2 shows a cross-section of a composite derived from a 1-ply Nicalon plain square weave. The larger circular zones are the cut ends of the Nicalon fibers, and the material located between the fibers is the silicon nitride formed from the silicon particles. Fig. 3 shows the cross-section of a composite formed using a 1-ply Nicalon satin weave. The quality of the matrix residing between the fibers indicated that the infiltration of slurry particles was good across the entire cross-section in both cases. The square weave was fairly uniform in size and was measured to be approximately 0.013 cm thick. Due to the different weave design, the dimension of the satin weave composite was variable and measured 0.07–0.09 cm thick. Fig. 4 shows micrographs of a composite formed from infiltrating a square weave of the Stackpose Panex WGC carbon fibers. The dark regions are the fibers, and again the material located between the fibers was derived from the infiltrated silicon particles. The original fiber weave was very fine and lightweight with a thickness of 0.013– 0.018 cm. It was difficult to infiltrate this fine cloth via the dipping procedure without creasing and crumpling the weave as it was lowered into the slurry. As a result, this weave was only dipped three times in the silicon slurry. From the micrographs, however, it can be seen that despite these difficulties and the surface irregularities of the fiber, a very high quality and uniform infiltration was achieved. The micrographs in Fig. 5 show the cross-section of a 1ply satin weave of HPZ fibers. This composite was produced as a result of dipping five times in the silicon slurry. The elongated oval fiber geometry contrasts with the highly spherical Nicalon fibers. The distribution of HPZ fibers in the weave design does not appear as ordered as in the Nicalon weaves and varies greatly in density throughout the weave. The high quality of the matrix shows that the infiltrated HPZ satin weave was highly loaded with matrix powder throughout. Results derived from the Thornel fiber are illustrated in Fig. 6. Despite such a high filament density, the fiber surface roughness, the kidney-bean shape of the fiber and its irregular fiber orientation, the micrographs show good infiltration was achieved. The whole of the matrix region has very low voidage throughout. The whole series of micrographs indicate high quality infiltration results. All weave styles and fiber types attempted in infiltration experiments were highly loaded with matrix powder and show low voidage. The results of infiltrating with the suspensions prepared via the depletionsteric method were much improved in comparison to
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Fig. 4. Electron micrographs of the cross-section of a Stackpole Panex WCG fiber composite.
infiltration with unmodified slurries. For example, Nicalon square weaves were dipped into slurries of bare untreated silicon powder in cyclohexane solvent. This silicon powder (expected to exhibit an oxidized silanol and siloxane surface) was milled for 72 h prior to use. The silicon powder was ultrasonically pulsed in the cyclohexane solvent to break up any agglomerates that may be present prior to dipping the cloth strips into the slurry. Electron microscopy of the cross-section of the Nicalon square composites showed minimal infiltration with matrix powder. The particles were deposited primarily as a surface layer, with little to no infiltration towards the center of the cloth. The voidage was high throughout. The poor infiltration quality obtained in this case results from the rapid flocculation of the untreated silicon powder. In the absence of any repulsive stabilization mechanism (steric layer), and in the poor cyclohexane dispersive medium for silicon, after ultrasonication the onset of flocculation of the primary silicon particles is rapid. The larger, irregular shape and size of the
Fig. 5. Electron micrographs of the cross-section of a 1-ply HPZ satin weave composite.
agglomerates cannot physically infiltrate the small interfiber spaces. As a result, the agglomerates tend to deposit on the outer surface of the cloth, thereby providing a physical barrier and blocking possible weave infiltration routes during later immersion into the slurry. Fiber weave infiltration was also performed using a silicon–acetone slurry. Here, 25 wt% of silicon was ultrasonically dispersed in acetone to break up any agglomerates. The silicon was 1–10 mm Union Carbide powder attrition milled in naphtha for 24 h to 0.1–2.6 mm size. The silicon was not treated and no additives were used in the acetone slurry. Processing followed the same dipping, composite consolidation and nitridation procedure as outlined for the studies reported above. To ensure that no agglomerates formed between weave dips, the silicon–acetone slurry was ultrasonically pulsed at intervals. Composites prepared via acetone slurries were also examined under the scanning electron microscope. The infiltration quality of these slurries was a significant improvement over the cyclohexane slurries. There was, however, a noticeable level of voidage.
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phenomena of depletion flocculation with steric stabilization were found to exhibit properties desirable for the infiltration of fiber weaves with slurry. These properties included a highly stable (unagglomerated) state and a relatively high concentration of solids. The approach appears to be robust, and should be applicable to a wide variety of slurry systems. For the specific silicon slurry used to illustrate the approach, the colloidal parameters that were determined from experiments using large single fibers were found to apply for the infiltration of small-diameter ceramic-fiber weaves. The infiltrated weaves were cold pressed, hot pressed and nitrided to enable investigation of the infiltration quality via SEM examination of cross-sectional samples of the formed composite. SEM micrographs revealed that a uniform infiltration of matrix powder had been achieved regardless of fiber type, size and shape, surface characteristics, or weave design.
Acknowledgements The authors are grateful to the NASA Lewis Research Center for support of this work under Grant NAG3-1141 and Co-operative agreement NCC 3-298. The authors also wish to thank Raymond Babuder of the NASA Lewis Research Center for performing the infiltration experiments involving untreated silicon powder.
References Fig. 6. Electron micrographs of the cross-section of a Thornel carbon fiber composite.
Although acetone is a good solvent for silicon dispersion [7] and the silicon particles remained well dispersed after ultrasonic pulsing, a small degree of flocculation qualitatively could be observed to be occurring. Hence continued ultrasonic treatment at intervals was necessary. Due to the unavoidable generation of some flocculation, infiltration was significantly improved but not complete. Additionally, as a result of the heat generated by the continued ultrasonic pulsing to prevent agglomerate formation, rapid evaporation of the acetone solvent was a problem. 4. Conclusions Slurries prepared using a combination of the colloidal
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