Communications of the American Ceramic Society Vol. 90. No. 3 Table L. Properties of KD-I SiC Fiber Chemical composition C/Si atom ratio O(wt%) Diameter(um) Density (g/cm) Filament/yarn ensile strength(MPa) KD-I 1.35 l4-16 1800-220 preforms were heated in flowing nitrogen atmosphere to 1200c also be obtained by nitridation of Al particles. At reaction to pyrolyze the polymer temperatures above 800C, wl of the volatile organic Subsequently, the samples underwent several polymer infil- species of polymer phase have ed, the Al particles are tration and pyrolysis processes using PCS as precursor(with no presumed to react primarily wit eactants, e.g., free C. free filler) for Sic derivation. The polymer-to-ceramic conversion Si, and the pyrolysis-derived Sic to form a new generation of process was also conducted at 1200C in nitrogen atmosphere Al4Si3 according to the following equation: The as-pyrolyzed composites were cut and ground into 2.5 m x 4 mm x 36 mm rectangles for density, porosity, and three- Al+SiC→Al4Si3+Al4C3 point-bend testing. The density and porosity of each sample wa three-point-bend testing was conducted on the INSTRON 5566 Al+Si→Al4Si (Instron Corp, Canton, MA)universal testing machine, with a The density variation versus pyrolysis cycle is shown in Fig.2 modulus was calculated from the data recorded during three- point-bend testing. The phase compositions of derived matrix active filler infiltration seemed to affect the subsequent PIP alone were using X-ray diffractometry(XRD; RAX-10A, Ri- efficiency. This effect can be clearly observed in Fig. 2 where gaku Co., Tokyo, Japan) with CuKa radiation. Instron Corp especially for the first several cycles, different density trends versus PIP cycles are shown for preforms infiltrated with and The composite microstructure was investigated by field emission without the active Al filler. The Al-loading infiltration st scanning electron microscope(FESEM; JSM-6700F, JEOL proved to sensibly increase the bulk density in the first cycle Tokyo, Japan)on the polished cross sections. for the high ceramic yield. However, large volume expat also led to formation of increased amount of sealed pores in the lI Results and discussion surface of the samples, which inhibited the subsequent infiltra tion of polymer. As a result, the density of the composite with A Figure I shows an X-ray diffraction pattern of 20 wt%Al- oading was only a little higher loading- PCs pure matrix pyrolyzed at 1200.C with I h soaking Typical microstructures of the polished cross sections after six The result reveals that new phases of AL], ALC3, and aIN are infiltration and pyrolysis cycles at 1200 C by SEM are shown in the polymer-derived matrix. Evol Fig. 3. In Fig. 3(a), a large amount of matrix formed and tions of Al C3 and aIn crystals are the result of reaction esidual pores were still dispersed in the intra-bundle areas = and decomposition fragments of the pre- the reactive atmosphere during pyrol composites. The intra-bundle matrix formation is significantly chemical reactions can be expressed as dependent on the infiltration process. So the matrix distribution n the bundles is mainly achieved by conversion of the infiltrated [SiR2C-+Al-Si-C+Al-C CHx+H2 (1) PCs, which is often accompanied by a large volume contraction During the following PIP treatment, some of the pores may be Al+N,→AN refilled, but some could not. As a result, small residual pores unavoidably located in the intra-bundle areas. According to the where r denotes carbon-containing functional substituents. observation of polished sections, it seems difficult to achieve a As reported by greil, carburization of Al particles can ully dense matrix by using the present Pip process because of result in a volume expansion of 9% with C in solid state and the difficulty in penetrating the polymer into small pores that 53%with gaseous hydrocarbon Volume expansion of 26% can exist in the converted SiC matrix In Fig 3(b), the SEM image of V-ALC 2.0 -AL Si o-unloading loading △ 1.2 体认 102030405060708090 Diffraction angle(20) Fig. 2. Relationship between density variation and pyrolysis cycle for Fig 1. X-ray diffraction pattern of the pyrolyzed matrix at 1200.C. the two compositespreforms were heated in flowing nitrogen atmosphere to 12001C to pyrolyze the polymer. Subsequently, the samples underwent several polymer infil￾tration and pyrolysis processes using PCS as precursor (with no filler) for SiC derivation. The polymer-to-ceramic conversion process was also conducted at 12001C in nitrogen atmosphere. The as-pyrolyzed composites were cut and ground into 2.5 mm
4 mm
36 mm rectangles for density, porosity, and three￾point-bend testing. The density and porosity of each sample was measured by the Archimedes method. The flexural strength by three-point-bend testing was conducted on the INSTRON 5566 (Instron Corp., Canton, MA) universal testing machine, with a cross-head speed of 0.5 mm/min and a span of 24 mm. Young’s modulus was calculated from the data recorded during three￾point-bend testing. The phase compositions of derived matrix alone were using X-ray diffractometry (XRD; RAX-10A, Ri￾gaku Co., Tokyo, Japan) with CuKa radiation. Instron Corp. The composite microstructure was investigated by field emission scanning electron microscope (FESEM; JSM-6700F, JEOL, Tokyo, Japan) on the polished cross sections. III. Results and Discussion Figure 1 shows an X-ray diffraction pattern of 20 wt% Al￾loading-PCS pure matrix pyrolyzed at 12001C with 1 h soaking. The result reveals that new phases of Al4Si3, Al4C3, and AlN are found to crystallize from the polymer-derived matrix. Evolu￾tions of Al4C3 and AlN crystals are the result of reaction between Al particles and decomposition fragments of the pre￾cursor polymer and the reactive atmosphere during pyrolysis process. The overall chemical reactions can be expressed as10 ½SiR2C þ Al ! Si C þ Al C þ CHx þ H2 (1) Al þ N2 ! AlN (2) where R denotes carbon-containing functional substituents. As reported by Greil,10 carburization of Al particles can result in a volume expansion of 9% with C in solid state and 53% with gaseous hydrocarbon. Volume expansion of 26% can also be obtained by nitridation of Al particles. At reaction temperatures above 8001C, when most of the volatile organic species of polymer phase have evolved, the Al particles are presumed to react primarily with solid reactants, e.g., free C, free Si, and the pyrolysis-derived SiC to form a new generation of Al4Si3 according to the following equation: Al þ SiC ! Al4Si3 þ Al4C3 (3) Al þ Si ! Al4Si3 (4) The density variation versus pyrolysis cycle is shown in Fig. 2 for two typical 2.5D preforms. It is interesting to note that the active filler infiltration seemed to affect the subsequent PIP efficiency. This effect can be clearly observed in Fig. 2 where, especially for the first several cycles, different density trends versus PIP cycles are shown for preforms infiltrated with and without the active Al filler. The Al-loading infiltration step proved to sensibly increase the bulk density in the first cycle for the high ceramic yield. However, large volume expansion also led to formation of increased amount of sealed pores in the surface of the samples, which inhibited the subsequent infiltra￾tion of polymer. As a result, the density of the composite with Al loading was only a little higher. Typical microstructures of the polished cross sections after six infiltration and pyrolysis cycles at 12001C by SEM are shown in Fig. 3. In Fig. 3(a), a large amount of matrix formed and residual pores were still dispersed in the intra-bundle areas which is a commonly observed phenomenon in PIP-derived composites.11 The intra-bundle matrix formation is significantly dependent on the infiltration process. So the matrix distribution in the bundles is mainly achieved by conversion of the infiltrated PCS, which is often accompanied by a large volume contraction. During the following PIP treatment, some of the pores may be refilled, but some could not. As a result, small residual pores unavoidably located in the intra-bundle areas. According to the observation of polished sections, it seems difficult to achieve a fully dense matrix by using the present PIP process because of the difficulty in penetrating the polymer into small pores that exist in the converted SiC matrix. In Fig. 3(b), the SEM image of Table I. Properties of KD-I SiC Fiber Type Chemical composition Diameter (mm) Density (g/cm3 C/Si atom ratio O (wt%) ) Filament/yarn Tensile strength (MPa) Elastic modulus (GPa) KD-I 1.35 10 14–16 2.40 800 1800–2200 150–170 Fig. 1. X-ray diffraction pattern of the pyrolyzed matrix at 12001C. Fig. 2. Relationship between density variation and pyrolysis cycle for the two composites. 970 Communications of the American Ceramic Society Vol. 90, No. 3