COMPOSITES SCIENCE AND TECHNOLOGY ELSEⅤIER Composites Science and Technology 62(2002)2179-2188 www.elsevier.com/locate/compscitech Consolidation of polymer-derived Sic matrix composites processing and microstructure Masaki Kotania,*. I, Takahiro Inoue, Akira Kohyama, Kiyohito Okamura Yutai Katoh Graduate School of Energy Science, Kyoto Univ Yoshidahonmachi, Sakyo-ku, Kyoto 606-8501, Japan bNational Institute of Advanced Industrial Science and Technology, 1-2-1 Namiki, Tsukuba, Ibaraki, 305-8942 Jape Institute of Advanced Energy, Kyoto University, Gokasho, Uji, Kyoto, 611-0011 Japan dOsaka prefecture University, 1-1 Gakuen-cho, Sakai, Osaka, 599-8531 Japan Received 19 October 2001: received in revised form 26 July 2002: accepted 27 July 2002 Abstract SiC fiber reinforced Sic matrix composite(SiC/SiC composite) has been developed by polymer impregnation and pyrolysis (PIP) method, which consists of impregnation, curing, consolidation, and re-impregnation and pyrolysis. As a prospective approach to fabricate a high performance composite, consolidation conditions, such as curing temperature to make a green body, pressure and heating rate during consolidation, were systematically controlled for effective consolidation. Because of its advantage in controlling hysical characteristic, polyvinylsilane(pvs) that is liquid thermosetting organo-silicic compound was utilized as the matrix pre- cursor. Based on the pyrolytic behavior of PVs, effects of the process conditions on microstructure of the consolidated bodies were accurately characterized. To relate those microstructure with mechanical property, flexural tests were performed for the composites fter multiple PIP processing. Consequently, process conditions to make a high performance composite could be appeared. Struc- C 2002 Elsevier Science Ltd. All rights reserved. er tural conditions to be optimized for further improvement in mechanical and environmental properties were also discussed Keywords: A Ceramic-matrix composites(CMCs): A. Preceramic polymer; B Curing: B Porosity: B Mechanical properties 1. Introduct shape and geometry, microstructural control, and cost Since silicon carbide possesses such superior proper Main challenge of this process has been made to ties as strength at elevated temperature, oxidation resis- reduce pores and cracks which were formed due to gas tance and microstructural stability under irradiation, evolution and volumetric shrinkage of a preceramic there have been many efforts on r& d of Sic/Sic polymer during pyrolysis [8-11]. Gas evolution causes composite for use in aerospace vehicles and fusion the inhomogenization of matrix. Volumetric shrinkage power reactor [1-5]. Among potential fabrication pro- directly gives rise to the formation of pores. After a cesses of ceramics matrix composites (CMCs), PIP polymer is hardened, both events lead to crack initia- method is one of most promising methods because of its tion. At that time, fiber distribution is determined advantages in the viewpoints of impregnation efficiency Although repetition of PIP processing is much useful to among fibers, large-scale fabrication with complicated fill such defects with polymer-pyrolyzed product microstructure produced in first PIP processing would be influential on final microstructure. Therefore, effec- Corresponding author at present address. Tel: +81-298-68-2336: tive consolidation to yield minimum amount of crack initiation and appropriate fiber distribution are very and Development, National Space Development Agency e Present address: Thermal Engineering Group, Office important technical issue a high per mance SiC/Sic composite Tsukuba Space Center 2-1-1 Sengen, Tsukuba, Ibaraki, n order to control the distribution of fibers and pores, utilization of high volumetric yield polymer, fille 0266-3538/02/S. see front matter C 2002 Elsevier Science Ltd. All rights reserved. PII:S0266-3538(02)00151-3
Consolidation of polymer-derived SiC matrix composites: processing and microstructure Masaki Kotania,*,1, Takahiro Inoueb, Akira Kohyamac , Kiyohito Okamurad, Yutai Katohc a Graduate School of Energy Science, Kyoto University, Yoshidahonmachi, Sakyo-ku, Kyoto 606-8501, Japan bNational Institute of Advanced Industrial Science and Technology, 1-2-1 Namiki, Tsukuba, Ibaraki, 305-8942 Japan c Institute of Advanced Energy, Kyoto University, Gokasho, Uji, Kyoto, 611-0011 Japan dOsaka Prefecture University, 1-1 Gakuen-cho, Sakai, Osaka, 599-8531 Japan Received 19 October 2001; received in revised form 26 July 2002; accepted 27 July 2002 Abstract SiC fiber reinforced SiC matrix composite (SiC/SiC composite) has been developed by polymer impregnation and pyrolysis (PIP) method, which consists of impregnation, curing, consolidation, and re-impregnation and pyrolysis. As a prospective approach to fabricate a high performance composite, consolidation conditions, such as curing temperature to make a green body, pressure and heating rate during consolidation, were systematically controlled for effective consolidation. Because of its advantage in controlling physical characteristic, polyvinylsilane (PVS) that is liquid thermosetting organo-silicic compound was utilized as the matrix precursor. Based on the pyrolytic behavior of PVS, effects of the process conditions on microstructure of the consolidated bodies were accurately characterized. To relate those microstructure with mechanical property, flexural tests were performed for the composites after multiple PIP processing. Consequently, process conditions to make a high performance composite could be appeared. Structural conditions to be optimized for further improvement in mechanical and environmental properties were also discussed. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Ceramic-matrix composites (CMCs); A. Preceramic polymer; B. Curing; B. Porosity; B. Mechanical properties 1. Introduction Since silicon carbide possesses such superior properties as strength at elevated temperature, oxidation resistance and microstructural stability under irradiation, there have been many efforts on R & D of SiC/SiC composite for use in aerospace vehicles and fusion power reactor [1–5]. Among potential fabrication processes of ceramics matrix composites (CMCs), PIP method is one of most promising methods because of its advantages in the viewpoints of impregnation efficiency among fibers, large-scale fabrication with complicated shape and geometry, microstructural control, and cost [6,7]. Main challenge of this process has been made to reduce pores and cracks which were formed due to gas evolution and volumetric shrinkage of a preceramic polymer during pyrolysis [8–11]. Gas evolution causes the inhomogenization of matrix. Volumetric shrinkage directly gives rise to the formation of pores. After a polymer is hardened, both events lead to crack initiation. At that time, fiber distribution is determined. Although repetition of PIP processing is much useful to fill such defects with polymer-pyrolyzed product, microstructure produced in first PIP processing would be influential on final microstructure. Therefore, effective consolidation to yield minimum amount of crack initiation and appropriate fiber distribution are very important technical issue for making a high performance SiC/SiC composite. In order to control the distribution of fibers and pores, utilization of high volumetric yield polymer, filler 0266-3538/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0266-3538(02)00151-3 Composites Science and Technology 62 (2002) 2179–2188 www.elsevier.com/locate/compscitech * Corresponding author at present address. Tel.: +81-298-68-2336; fax: +81-298-68-2968. E-mail address: kotani.masaki@nasda.go.jp (M. Kotani). 1 Present address: Thermal Engineering Group, Office of Research and Development, National Space Development Agency of Japan, Tsukuba Space Center 2-1-1 Sengen, Tsukuba, Ibaraki, 305-8505 Japan
M. Kotani et al. /Composites Science and Technology 62(2002)2179-2188 addition to the polymer, and pressurization during -SiHy-CHy-CHy-and-C(SiH3)H-CHy-, with the ratio consolidation is expected to be much beneficial. How- of 1: 1 [21]. It was synthesized by radical polymerization ever, even with an advanced polymer of high ceramic of vinylsilane(CH2=CH-SiH3)in an autoclave Density yield of more than 80 wt % large volumetric shrinkage and molecular weight of the polymer were 0.91 Mg m-3, unavoidably occurs due to big change of density from and 957(number average )/2780(weight average)(Mw/ polymer to ceramic [12-14]. Filler loading into polymer Mn=2.9), respectively. It is transparent liquid with the can improve apparent volume shrinkage, and also dis viscosity of about 70 cP at room temperature. Pyrolysis perse pores finely [15, 16]. In addition, filler particles chemistry during polymer-to-ceramic conversion has could replenish pores if those are active to produce car- been studied so far [22] bide [17-19]. Contrary to this, it might decline impreg- To take advantage of PVSs superior characteristics, nation efficiency and weaken a pyrolyzed product. its pyrolytic behaviors such as thermosetting, shrinkage Pressurization is expected to fix a fibrous preform andand gas evolution were precisely inspected prior to eliminate pores outside, as far as the polymer retains composite fabrication. Thermogravimetric and differ fluidity. Even after a polymer is hardened, it might be ential thermal analysis (TG-DTA)was performed under beneficial for reducing crack initiation formed due to the heating rate of 300 K/h and the flowing rate of Ar of internal gas pressure. As another option to reduce pores 0. 2 1 /min As the reference material, high purity alumina and cracks in a consolidated body, curing prior to powder was utilized. DTA curve was determined by pressurization is prospective, because the volumetric deducting a blank from the original analytical data fraction of a final pyrolyzed product out of a polymeric Differently from this experiment, TGa were performed precursor is increased at various heating rates in flowing Ar(1 I/ min). Mor- In this work, process development for high perfor- phological analysis was performed for isolated inter mance polymer-derived SiC/Sic composite was per- mediates of PVS in room temperature. Densities of the formed. As the matrix precursor, PVS, which was intermediates were measured by picnometry using dis liquid polycarbosilane with a lot of functional Si-H tilled water. For these experiments, the samples were bonds, was applied because of its advantages of suffi- prepared by heating the polymer in same conditions as cient stability at ambient temperature, low viscosity, curing(400 K). According to those data, volumetric and continuous thermosetting behavior. Rheological residues at various temperatures were estimated with the properties such as viscosity and wettability are much following equation important characteristic in filler dispersion, impregna tion into small area among fibers. PVS and its slurry (a1473/or) UT= with Sic particles appeared to be impregnated very well into a continuous SiC fiber preform without dilution Recent work also demonstrated its superior rheological property in fiber production, namely finer SiC fiber was where v is volumetric residue(%), o mass residue (%) successfully synthesized by blending PVS with conven- and p density(Mg m-3). Subscripts of the characters tional polycarbosilane [20]. Owing to continuous ther- correspond to pyrolyzing temperature(K). Total quan mosetting behavior during pyrolysis, its physical tity of evolved gas was monitored as a function of tem- characteristics could be accurately controlled by heat perature at intervals of 100 K. The sample was heated in treatment. To make a composite of high density and a closed silica tube of already known volume at 300 K/h uniform fiber distribution, main efforts was paid for in vacuum. The quantities were estimated from the optimizing consolidation conditions; such as curing change of gas pressure temperature to prepare a green body, pressure and In the fabrication of composite, Hi-Nicalon"M,which heating rate to make a consolidated body. In con- is continuous SiC-C) fiber produced by Nippon Car sequence, the effects of the process parameters on den- bon Co, Ltd (Japan), was employed as the reinforce- sity and microstructure were clearly revealed. And those ment SiC particles of mean particle size of 0. 27 um were were discussed on the basis of pyrolytic behavior of the utilized as the filler material. It was commercially man polymer. The relationship between microstructure and factured as ultra fine grade of Betarundum by IB mechanical properties of the composites was character- DEN Co, Ltd ( Japan). All samples were unidirectional ized by flexural test composites. Those were fabricated in the following procedures composed of four steps 2. Experimental procedure 1. To prepare a unidirectional fibrous preform, the fiber tow was uniformly wound in size of 40x20 The polymer used as the matrix precursor was poly- mm. Then it was heated up to 873 K in vacuum vinylsilane, which is developed by Mitsui chemical, inc. for removing sizing agent that quite a little (Japan). It was composed of two kinds of unit structure applied by the manufacturer
addition to the polymer, and pressurization during consolidation is expected to be much beneficial. However, even with an advanced polymer of high ceramic yield of more than 80 wt.%, large volumetric shrinkage unavoidably occurs due to big change of density from polymer to ceramic [12–14]. Filler loading into polymer can improve apparent volume shrinkage, and also disperse pores finely [15,16]. In addition, filler particles could replenish pores if those are active to produce carbide [17–19]. Contrary to this, it might decline impregnation efficiency and weaken a pyrolyzed product. Pressurization is expected to fix a fibrous preform and eliminate pores outside, as far as the polymer retains fluidity. Even after a polymer is hardened, it might be beneficial for reducing crack initiation formed due to internal gas pressure. As another option to reduce pores and cracks in a consolidated body, curing prior to pressurization is prospective, because the volumetric fraction of a final pyrolyzed product out of a polymeric precursor is increased. In this work, process development for high performance polymer-derived SiC/SiC composite was performed. As the matrix precursor, PVS, which was a liquid polycarbosilane with a lot of functional Si–H bonds, was applied because of its advantages of suffi- cient stability at ambient temperature, low viscosity, and continuous thermosetting behavior. Rheological properties such as viscosity and wettability are much important characteristic in filler dispersion, impregnation into small area among fibers. PVS and its slurry with SiC particles appeared to be impregnated very well into a continuous SiC fiber preform without dilution. Recent work also demonstrated its superior rheological property in fiber production, namely finer SiC fiber was successfully synthesized by blending PVS with conventional polycarbosilane [20]. Owing to continuous thermosetting behavior during pyrolysis, its physical characteristics could be accurately controlled by heat treatment. To make a composite of high density and uniform fiber distribution, main efforts was paid for optimizing consolidation conditions; such as curing temperature to prepare a green body, pressure and heating rate to make a consolidated body. In consequence, the effects of the process parameters on density and microstructure were clearly revealed. And those were discussed on the basis of pyrolytic behavior of the polymer. The relationship between microstructure and mechanical properties of the composites was characterized by flexural test. 2. Experimental procedure The polymer used as the matrix precursor was polyvinylsilane, which is developed by Mitsui chemical, inc. (Japan). It was composed of two kinds of unit structures, –SiH2–CH2–CH2– and –C(SiH3)H–CH2–, with the ratio of 1:1 [21]. It was synthesized by radical polymerization of vinylsilane (CH2¼CH–SiH3) in an autoclave. Density and molecular weight of the polymer were 0.91 Mg m3 , and 957 (number average)/2780 (weight average) (Mw/ Mn=2.9), respectively. It is transparent liquid with the viscosity of about 70 cP at room temperature. Pyrolysis chemistry during polymer-to-ceramic conversion has been studied so far [22]. To take advantage of PVS’s superior characteristics, its pyrolytic behaviors such as thermosetting, shrinkage and gas evolution were precisely inspected prior to composite fabrication. Thermogravimetric and differential thermal analysis (TG-DTA) was performed under the heating rate of 300 K/h and the flowing rate of Ar of 0.2 l/min. As the reference material, high purity alumina powder was utilized. DTA curve was determined by deducting a blank from the original analytical data. Differently from this experiment, TGA were performed at various heating rates in flowing Ar (1 l/min). Morphological analysis was performed for isolated intermediates of PVS in room temperature. Densities of the intermediates were measured by picnometry using distilled water. For these experiments, the samples were prepared by heating the polymer in same conditions as curing (400 K). According to those data, volumetric residues at various temperatures were estimated with the following equation. T ¼ ð Þ !1473=!T 1473 T ð1Þ where is volumetric residue (%), ! mass residue (%), and density (Mg m3 ). Subscripts of the characters correspond to pyrolyzing temperature (K). Total quantity of evolved gas was monitored as a function of temperature at intervals of 100 K. The sample was heated in a closed silica tube of already known volume at 300 K/h in vacuum. The quantities were estimated from the change of gas pressure. In the fabrication of composite, Hi-NicalonTM, which is continuous SiC–(C) fiber produced by Nippon Carbon Co., Ltd. (Japan), was employed as the reinforcement. SiC particles of mean particle size of 0.27 um were utilized as the filler material. It was commercially manufactured as ultra fine grade of BetarundumTM by IBIDEN Co., Ltd. (Japan). All samples were unidirectional composites. Those were fabricated in the following procedures composed of four steps. 1. To prepare a unidirectional fibrous preform, the fiber tow was uniformly wound in size of 4020 mm. Then it was heated up to 873 K in vacuum for removing sizing agent that quite a little applied by the manufacturer, 2180 M. Kotani et al. / Composites Science and Technology 62 (2002) 2179–2188
M. Kotani et al. /Composites Science and Technology 62(2002)2179-2188 2181 make a cured sheet, the preform was dippe 3. Results and discussion into PVS's slurry at the filler content of 25 wt% in an ambient environment, and then heated up 3. 1. Pyrolytic behavior of the polymer to curing temperature at 300 K/h in Ar, To make a consolidated body, the cured sheets Fig. I shows TG-DTA curves of PVs at 300 K/h in were stacked, and then heated up to 1473 K at Ar. Mass degradation continuously occurred from 380 300 K/h in Ar under unidirectional pressure, to 800 K. It was accelerated along with temperature and 4. To make a composite, the consolidated body was became highest between 650 and 700 K. Most of the subjected to six times of re-impregnation and mass change had finished below 700 K. Mass yield of a pyrolysis(subsequent PIP processing) of PVs pyrolyzed product up to 1473 K was 32.6%. As for without pressurization DTA curve, a downward tendency was seen from 400 K. It should be related with the mass degradation due to In all heat treatments, target temperature was kept fo the emission of polymer components. Between 500 and 10 min. Although the composites were fabricated at 600 K, there could be identified a continuous depres almost same amount of fiber, those thickness differed in sion. Based on previous reports [23, 24], it might be the range between 1. 5 and 2.5 mm, depending on the related with cross-linking reaction. Big endothermic consolidation conditions. Thus, fiber volume fraction of peak at 700 K implied a drastic change of the molecular the samples was dependent on consolidation condition. structure to form Si-C backbone, where the fragmenta- Process optimizations were performed by evaluating the tion of polymer structure simultaneously occurred [25] consolidation bodies. Sample I Ds were set to reflect the Appearances of isolated samples of PVs heated up to process conditions (curing temperature/K, pressure various temperatures were exhibited in Table 1. On the applied during consolidation/MPa). whole, the polymer showed continuous thermosetting Densimetry for the samples was performed by Archi- from transparent liquid to porous brownish glassy solid nedean method after every PIP processing. Relative between 600 and 700 K. The polymer pyrolyzed up to density(dg) was defined as ceramic volume fraction in a 583 K was not so much different from original one other bulk, calculated with the following equation than a slight increase of viscosity. Heated up to 603 K gelation was recognized. Referred to the TGa curve, dR a-sor)100%) (2) mass degradation had already reached more than 20 here. Then, the polymer continuously thermoset with further mass degradation and gradual coloration. Below where as is weight of a specimen measured in water, oc 673 K, the pyrolyzed products were free of pore. But, and op weights of a specimen with without water in all pyrolyzed above 693 K, the polymer became brownish open pores measured in atmosphere, and pH, o)r density and quite insoluble in solvents, and frothed vigorously of water at the temperature of T. dR could be estimated These features suggested that the polymer evolved much only for the samples after consolidation, because closed gas and subsequently lost almost its plasticity at this pore might be formed in subsequent PIP processing. temperature Apparent density (dA) was defined as average density of Fig. 2 shows densities and volumetric residues of PVs all constituents, expressed in the following equation as a function of temperature. Although it is not easy to da= P(H2O)7(Mg m-) It depends on the ratio of fiber and matrix in a con- solidated body. According to densities of the fiber(2.74 Mg- m-3)and the matrix theoretically derived from the slurry(2.92 Mgm-3), the density was proportional to matrix content in all constituents, microstructural characterizations were performed after consolidation nd subsequent PIP processing, using optical micro- scope(OM)and scanning electron microscope(SEM) Three-point flexural test was performed at room tem- perature. Dimensions of test specimens were 30 mm length x 4 mm width x I mm height Span and crosshead 20 40060080010001200 speed were 25 mm and 0.5 mm/min, respectively. UIti mate flexural strength (u) and work-of-fracture Temperature /K W..F)were obtained from the peak load and the area Fig. 1. Thermogravimetric and differential thermal analysis(TG of load-crosshead displacement chart. DTA)curves for PVS at a heating rate of 300 K/h in Ar
2. To make a cured sheet, the preform was dipped into PVS’s slurry at the filler content of 25 wt.% in an ambient environment, and then heated up to curing temperature at 300 K/h in Ar, 3. To make a consolidated body, the cured sheets were stacked, and then heated up to 1473 K at 300 K/h in Ar under unidirectional pressure, 4. To make a composite, the consolidated body was subjected to six times of re-impregnation and pyrolysis (subsequent PIP processing) of PVS without pressurization. In all heat treatments, target temperature was kept for 10 min. Although the composites were fabricated at almost same amount of fiber, those thickness differed in the range between 1.5 and 2.5 mm, depending on the consolidation conditions. Thus, fiber volume fraction of the samples was dependent on consolidation condition. Process optimizations were performed by evaluating the consolidation bodies. Sample IDs were set to reflect the process conditions (curing temperature/K, pressure applied during consolidation/MPa). Densimetry for the samples was performed by Archimedean method after every PIP processing. Relative density (dR) was defined as ceramic volume fraction in a bulk, calculated with the following equation. dR ¼ 1 !D !C !S ðH2OÞT 100ð%Þ ð2Þ where !S is weight of a specimen measured in water, !C and !Dweights of a specimen with/without water in all open pores measured in atmosphere, and ðH2OÞT density of water at the temperature of T. dR could be estimated only for the samples after consolidation, because closed pore might be formed in subsequent PIP processing. Apparent density (dA) was defined as average density of all constituents, expressed in the following equation. dA ¼ !D !D !S ðH2OÞT Mg m3 ð3Þ It depends on the ratio of fiber and matrix in a consolidated body. According to densities of the fiber (2.74 Mg.m3 ) and the matrix theoretically derived from the slurry (2.92 Mg.m3 ), the density was proportional to matrix content in all constituents. Microstructural characterizations were performed after consolidation and subsequent PIP processing, using optical microscope (OM) and scanning electron microscope (SEM). Three-point flexural test was performed at room temperature. Dimensions of test specimens were 30 mm length4 mm width1 mm height. Span and crosshead speed were 25 mm and 0.5 mm/min, respectively. Ultimate flexural strength (u) and work-of-fracture (W.O.F) were obtained from the peak load and the area of load-crosshead displacement chart. 3. Results and discussion 3.1. Pyrolytic behavior of the polymer Fig. 1 shows TG–DTA curves of PVS at 300 K/h in Ar. Mass degradation continuously occurred from 380 to 800 K. It was accelerated along with temperature and became highest between 650 and 700 K. Most of the mass change had finished below 700 K. Mass yield of a pyrolyzed product up to 1473 K was 32.6%. As for DTA curve, a downward tendency was seen from 400 K. It should be related with the mass degradation due to the emission of polymer components. Between 500 and 600 K, there could be identified a continuous depression. Based on previous reports [23,24], it might be related with cross-linking reaction. Big endothermic peak at 700 K implied a drastic change of the molecular structure to form Si–C backbone, where the fragmentation of polymer structure simultaneously occurred [25]. Appearances of isolated samples of PVS heated up to various temperatures were exhibited in Table 1. On the whole, the polymer showed continuous thermosetting from transparent liquid to porous brownish glassy solid between 600 and 700 K. The polymer pyrolyzed up to 583 K was not so much different from original one other than a slight increase of viscosity. Heated up to 603 K, gelation was recognized. Referred to the TGA curve, mass degradation had already reached more than 20% here. Then, the polymer continuously thermoset with further mass degradation and gradual coloration. Below 673 K, the pyrolyzed products were free of pore. But, pyrolyzed above 693 K, the polymer became brownish and quite insoluble in solvents, and frothed vigorously. These features suggested that the polymer evolved much gas and subsequently lost almost its plasticity at this temperature. Fig. 2 shows densities and volumetric residues of PVS as a function of temperature. Although it is not easy to Fig. 1. Thermogravimetric and differential thermal analysis (TGDTA) curves for PVS at a heating rate of 300 K/h in Ar. M. Kotani et al. / Composites Science and Technology 62 (2002) 2179–2188 2181
M. Kotani et al. /Composites Science and Technology 62(2002 )2179-2188 Table I 8.0 Appearances of isolated pyrolyzed products of PVs up to various Temp.(K) 6.0 Transparent liquid(100 cP) Transparent viscous liquid 4.0 anslucent rubbery solid, non porous 3.0 Transparent glassy solid, non porous Yellowish glassy solid, non porous 0 Brownish glassy solid, non porous 1.0 0.0 measured accurate density for a light polymer-pyrolyze intermediate by picnometry, it could be apparently Temperature /K shown that the densities were increased almost linearly Fig 3. Gas evolution behavior of PVS as a function of temperature with temperature. Thus, the volumetric residue showed similar behavior to the mass change. where drastic TGA curves for PVS at various heating rates are volumetric shrinkage occurred between 600 and 700 K shown in Fig. 4. Clear effect of heating rate was seen in and most of volume change had finished below 700 K. ceramic yield, where it was improved from 32%(600 K/ Volumetric yield of the polymer after pyrolysis up to h)to 37%(10 K /h). It might be owing to the increase of 1473 K was estimated to be 11% time for cross linking at around 700 K [25]. Another As an unavoidable negative influence on matrix mor- noticeable feature appeared in the mass degradation phology, gas evolution behavior during pyrolysis was behavior of early stage of pyrolysis(400-600 K). It monitored as a function of temperature in Fig 3. Each increased as the heating rate was slowed down. There point represents total amount of gases evolved from 100 fore, it was considered that the ceramics yield of PVs K below to representative temperature As gas evolution vas not so influenced by the amount of loss of low was negligible below 600 K, mass degradation detected molecular fraction, but the degree of cross linking below 600 K was proved to be due to the evaporation of Thus, sufficient time for cross linking was quite impor low molecular oligomers [26]. PVS heated up to more tant for efficient consolidation. than 400 K, those fractions might volatilize from a cru According to these results, it was approved that main cible and deposited on the cool part of the heating decomposition of PVs occurred between 600 and 700 K apparatus. Gas evolution was drastically increased from with great amount of gas evolution and mass degrada 600 to 700 K. This event would be related with the tion. As inorganization was highly proceeded, the poly- fragmentation of polymer structure. It was also sug- mer would turn poor of fluidity above 700 K Since fiber gested by a big endothermic peak detected in Dta distribution and matrix homogeneity in a composite curve. Then, gas evolution gradually declined along cannot be improved after the polymer has lost fluidity with temperature fiber alignment, stacking of prepreg sheets and shaping 1000 1500 Temperature /K Temperature /K Fig. 2. Densities and volumetric residues of Pvs as a function of temperature. Fig 4. TGA curves for PvS at various heating rates in Ar
measured accurate density for a light polymer-pyrolyzed intermediate by picnometry, it could be apparently shown that the densities were increased almost linearly with temperature. Thus, the volumetric residue showed similar behavior to the mass change, where drastic volumetric shrinkage occurred between 600 and 700 K and most of volume change had finished below 700 K. Volumetric yield of the polymer after pyrolysis up to 1473 K was estimated to be 11%. As an unavoidable negative influence on matrix morphology, gas evolution behavior during pyrolysis was monitored as a function of temperature in Fig. 3. Each point represents total amount of gases evolved from 100 K below to representative temperature. As gas evolution was negligible below 600 K, mass degradation detected below 600 K was proved to be due to the evaporation of low molecular oligomers [26]. PVS heated up to more than 400 K, those fractions might volatilize from a crucible and deposited on the cool part of the heating apparatus. Gas evolution was drastically increased from 600 to 700 K. This event would be related with the fragmentation of polymer structure. It was also suggested by a big endothermic peak detected in DTA curve. Then, gas evolution gradually declined along with temperature. TGA curves for PVS at various heating rates are shown in Fig. 4. Clear effect of heating rate was seen in ceramic yield, where it was improved from 32% (600 K/ h) to 37% (10 K/h). It might be owing to the increase of time for cross linking at around 700 K [25]. Another noticeable feature appeared in the mass degradation behavior of early stage of pyrolysis (400–600 K). It increased as the heating rate was slowed down. Therefore, it was considered that the ceramics yield of PVS was not so influenced by the amount of loss of low molecular fraction, but the degree of cross linking. Thus, sufficient time for cross linking was quite important for efficient consolidation. According to these results, it was approved that main decomposition of PVS occurred between 600 and 700 K with great amount of gas evolution and mass degradation. As inorganization was highly proceeded, the polymer would turn poor of fluidity above 700 K. Since fiber distribution and matrix homogeneity in a composite cannot be improved after the polymer has lost fluidity, fiber alignment, stacking of prepreg sheets and shaping Table 1 Appearances of isolated pyrolyzed products of PVS up to various temperatures Temp. (K) Appearance r.t. Transparent liquid (100 cP) 583 Transparent viscous liquid 603 Transparent gel 623 Translucent rubbery solid, non porous 653 Transparent glassy solid, non porous 673 Yellowish glassy solid, non porous 693 Brownish glassy solid, non porous Fig. 2. Densities and volumetric residues of PVS as a function of temperature. Fig. 4. TGA curves for PVS at various heating rates in Ar. Fig. 3. Gas evolution behavior of PVS as a function of temperature. 2182 M. Kotani et al. / Composites Science and Technology 62 (2002) 2179–2188
M. Kotani et al. / Composites Science and Technology 62(2002)2179-2188 2183 should be completed below 700 K. Curing processing Fig. 6(b), the break of a product into fibers was caused prior to consolidation might be effective for densifica- by squeeze of slurry from fiber preform. Successive tion of a body because effective volumetric yield of a consolidation was accomplished under the pressure of 1 precursor was improved [see Formula(1)]. If the poly- 5 and 10 MPa. Similar profiles were shown as a function mer was cured up to 600 and 700 K, it could be of curing temperature under these pressures;i.e.con- improved by almost twice. Also, further densification tinuous change and particular high density could be might be possible by crushing pores with appropriate presented. It was considered that the green bodies were pressure, because the polymer continuously underwent efficiently consolidated owing to well-balanced relation viscous liquid and plastic solid in this range of tem- ship between the physical characteristics of a precursor perature. At that time, a precursor impregnated into and pressure in these conditions. As pressure was fibrous body had to be soft enough for plastic defor- increased, curing temperature at which the particular mation under external forces. In addition, heating rate high relative density appeared rose, and those values during the first pyrolysis should be slow so as not only was increased. It nsidered that this increase of to control crack initiation and dilatation but to improve relative density was owing to the improvement of effec ceramic yield. These requirements have to be fully taken tive yield of a precursor. In this optimization, relative into consideration for process development density could attain to 70% in the condition of (623, 10) Fig. 7 shows apparent densities of the consolidated 3. 2. Optimization of consolidation conditions bodies under various conditions. Apparent density defined in this work provides useful information about 3.2.1. Curing temperature and pressure microstructure, because it depends on the ratio of the As thermosetting of PVS from viscous liquid to solid occurred in very short range between 583 and 663 K curing temperature was precisely controlled with the interval of 20 K. Fig. 5 shows relative densities of the consolidated bodies under various conditions between curing temperature and pressure. Clear effects of the process conditions on relative density could be identi- fied. Fig. 6(a) and(b) exhibits representative appear ances of the samples consolidated in the conditions of g (583, 0) and(603, 20) respectively, showing extreme. 551 cases related with pressure. As a noticeable feature of Fig 5, the results of 0 MPa are notably inferior to those of other pressure. Many large pores among inhomo- geneously distributed fibers and bundles could be seen in Fig. 6(a). This figure indicates that pressure was very important for appropriate fiber distribution and densi- fication, though the fabrication process of a composite without pressurization is very attractive to develop near-net shape production of complicated-shaped com- K ponents. On the contrary, excessive pressure also gave a Fig. 5. Relative densities of as-consolidated bodies under various negative influence for microstructure. As shown in conditions between curing temperature and pressure (b) Fig. 6. Optical micrographs of as-consolidated bodies under the conditions of (a)(583, 0)and(b)(623, 15)
should be completed below 700 K. Curing processing prior to consolidation might be effective for densification of a body because effective volumetric yield of a precursor was improved [see Formula (1)]. If the polymer was cured up to 600 and 700 K, it could be improved by almost twice. Also, further densification might be possible by crushing pores with appropriate pressure, because the polymer continuously underwent viscous liquid and plastic solid in this range of temperature. At that time, a precursor impregnated into a fibrous body had to be soft enough for plastic deformation under external forces. In addition, heating rate during the first pyrolysis should be slow so as not only to control crack initiation and dilatation but to improve ceramic yield. These requirements have to be fully taken into consideration for process development. 3.2. Optimization of consolidation conditions 3.2.1. Curing temperature and pressure As thermosetting of PVS from viscous liquid to solid occurred in very short range between 583 and 663 K, curing temperature was precisely controlled with the interval of 20 K. Fig. 5 shows relative densities of the consolidated bodies under various conditions between curing temperature and pressure. Clear effects of the process conditions on relative density could be identi- fied. Fig. 6(a) and (b) exhibits representative appearances of the samples consolidated in the conditions of (583, 0) and (603, 20) respectively, showing extreme cases related with pressure. As a noticeable feature of Fig. 5, the results of 0 MPa are notably inferior to those of other pressure. Many large pores among inhomogeneously distributed fibers and bundles could be seen in Fig. 6 (a). This figure indicates that pressure was very important for appropriate fiber distribution and densi- fication, though the fabrication process of a composite without pressurization is very attractive to develop near-net shape production of complicated-shaped components. On the contrary, excessive pressure also gave a negative influence for microstructure. As shown in Fig. 6(b), the break of a product into fibers was caused by squeeze of slurry from fiber preform. Successive consolidation was accomplished under the pressure of 1, 5 and 10 MPa. Similar profiles were shown as a function of curing temperature under these pressures; i. e. continuous change and particular high density could be presented. It was considered that the green bodies were efficiently consolidated owing to well-balanced relationship between the physical characteristics of a precursor and pressure in these conditions. As pressure was increased, curing temperature at which the particular high relative density appeared rose, and those values was increased. It was considered that this increase of relative density was owing to the improvement of effective yield of a precursor. In this optimization, relative density could attain to 70% in the condition of (623,10). Fig. 7 shows apparent densities of the consolidated bodies under various conditions. Apparent density defined in this work provides useful information about microstructure, because it depends on the ratio of the Fig. 5. Relative densities of as-consolidated bodies under various conditions between curing temperature and pressure. Fig. 6. Optical micrographs of as-consolidated bodies under the conditions of (a) (583, 0) and (b) (623, 15). M. Kotani et al. / Composites Science and Technology 62 (2002) 2179–2188 2183