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. How￾ever, 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 dis￾perse pores finely [15,16]. In addition, filler particles could replenish pores if those are active to produce car￾bide [17–19]. Contrary to this, it might decline impreg￾nation 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 perfor￾mance polymer-derived SiC/SiC composite was per￾formed. 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, impregna￾tion 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 conven￾tional polycarbosilane [20]. Owing to continuous ther￾mosetting 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 con￾sequence, the effects of the process parameters on den￾sity 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 character￾ized by flexural test. 2. Experimental procedure The polymer used as the matrix precursor was poly￾vinylsilane, 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 differ￾ential 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). Mor￾phological analysis was performed for isolated inter￾mediates of PVS in room temperature. Densities of the intermediates were measured by picnometry using dis￾tilled 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 quan￾tity of evolved gas was monitored as a function of tem￾perature 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 Car￾bon Co., Ltd. (Japan), was employed as the reinforce￾ment. SiC particles of mean particle size of 0.27 um were utilized as the filler material. It was commercially man￾ufactured as ultra fine grade of BetarundumTM by IBI￾DEN 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