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 Ar2. 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 Archi￾medean 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 con￾solidated 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 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 length4 mm width1 mm height. Span and crosshead speed were 25 mm and 0.5 mm/min, respectively. Ulti￾mate 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 depres￾sion. 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 fragmenta￾tion 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 (TG￾DTA) 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