K. Shimoda et aL/Composites Science and Technology 69(2009)1623-1628 volume fraction. Therefore infiltration and densification of the m 410 MPa. the pls was 360 MPa and the elastic modulus was trix in the intra-fiber-bundles becomes a challenge for this process. 360 GPa Dong et al. enhanced the intra- fiber-bundle matrix densification by In Fig. 4, the stress-strain curves of typical Sic/SiC composites pregnation during PIP pretreatment using a polymer containing during monotonic tensile tests are presented. High ductility type slurry(polycarbosilane(PCs)mixed with a filler, which was com- in Fig. 1b was obtained at lower fabrication temperature posed of Sic nano-powder and sintering additives)[13, 14 Even (1800C). In addition, the fracture behavior composites were cat- or SiC/SiC composites using Sic submicron-powder, the improve- egorized in two types from the difference on the densification in ment of the intra-fiber-bundle matrix densification by PCS impreg- the intra-fiber-bundle regions, as shown in Fig. 3a and C. the was reported by Yoshida et al [25]. In this present case dense intra-fiber-bundle composites with low-V exhibited very ut PIP treatment, a highly-densified composite with high-V large ductile fracture region after the PLS with the relatively high btained at 1900C under 20 MPa, where near-full densifica- UTS(380 MPa). The porous intra-fiber-bundle composites with temperature increases, liquid phase formation with lower viscosity PLS with the relative low UTS(320 MPa). Typical fractur er the tion could be achieved in the intra-fiber-bundles. As fabrication high-V displayed relatively large ductile fracture region a more effectively promotes grain-boundary sliding, so that fiber faces of those composites with low- and high-Vf are shown in arrangement is continuously enhanced under pressure. That Fig 5a and b, respectively. High strength type in Fig. la was fab- mote the sufficient infiltration and densification of Sic ricated at higher fabrication temperature(1900C)with high-VE powder in the intra-fiber-bundles. As a consequence, densifi This composites displayed the very high mechanical values, such of the composites with high-Vf is contributed mainly to as the UTS (410 MPa), the PLS(360 MPa)and the elastic mod matrix infiltration and densification of Sic nano-powder in the in- ulus (360 GPa). The failure of the composites occurred soon afte tra-fiber-bundles involving fiber rearrangement pressure via liquid the Pls and far from the matrix crack saturation. This behavior phase of sintering additives. However, high external pressure on after the Pls inhibited load transfer from matrix to fibers and re- the surfaces of the fibers with PyC interface might cause potential stricted the longer fiber pull-out, as shown in Fig. 5C. However, damages to the fibers and PyC interface with the increase of fabri- this implies that the enhanced matrix densification of the cation temperature involving pressure omposites with high-V through sufficient infiltration and densi- fication of SiC nano-power via liquid phase is beneficial at least in 3.3. Mechanical properties and fracture behaviors erms of first fracture properties. Cracks were deflected along Pyc matrix interface in spite of fabrication temperature and Some average physical and mechanical properties of the com- ume fraction as shown in Fig 6a. In addition, on posites under two fiber volume fractions incorporating different temperature fabricated composites, some cracks fabrication temperatures are listed in Table 1. The PLS, the UTs PyC/fiber interface were observed as shown in lower and the elastic modulus in monotonic tensile tests were deter- temperature, less-consolidated matrix before 3rd stage might mined following the general guidelines of ASTM C-1275. The Pls form the relatively weak interface bonding between matrix and was the stress corresponding to a 0.0005% offset strain For com- PyC interface required for high ductility type, which exhibits a parison, the three-point bending strength and the elastic modulus wide ductile domain after the Pls with longer fiber pull-out. with in bending were determined by the load-displacement curves longer fiber pull-out, the fibers bridging the matrix cracks can ab according to ASTM C-1341. As shown in Table 1, the bulk densi sorb much more energy to avoid non-catastrophic fracture behav and open porosity had the strong dependence on fiber volume frac- ior due to the friction between matrix and fibers when fibers pull tion of the composites at lower fabrication temperature. Also, the out from matrix, resulting in higher fracture toughness. On the value for three-point bending strength and elastic modulus in other hand, at higher fabrication temperature, well-consolidated bending showed the similar characteristics. When fabrication tem- matrix might form strong interface bonding between matrix and perature increased, the bulk density increased and open porosity Py c interface with efficient load transfer from matrix to fibers re- decreased for the composites with high-V. At 1900C, the open quired for high strength type. The very high PLS with the higl porosity reached the lowest value less than 1%. This could be attri- elastic modulus could be mainly attribution from the strength bution to the intra-fiber-bundle densification, as shown in Fig 3. ened interaction bonds between Py c interface and matrix, as Meanwhile, the values for three-point bending strength and elastic shown in Fig. 6b. Through this study, interface bonding between modulus in bending gradually increased, and the highest value Pyc interface and matrix could be concluded as a significant key vere obtained at 1900C. The bending strength under this fabrica- for tailoring of fracture behaviors. In CFCCs, increasing of volume ion temperature was over 850 MPa Date analysis indicated a sim- fraction of fibers with high tensile strength and modulus as rein- ilar variation trend for both monotonic tensile and three-point forcement should enhance mechanical values of composites in bending test, with increasing fabrication temperature from 1800 strength In NITE process, however, fiber volume fraction strongly to 1900C. Both the Uts and the pls were the highest at affected the porosity and microstructure of the composites, in par 900C. Under this fabrication temperature, the UTs was ticular in the intra-fiber-bundles, and therefore increasing of fiber volume fraction prevented improving the mechanical values (1800C). This study revealed that Effects of fiber volume fraction incorporating fabrication temperature on the average of fabrication temperature was effective way to enhance infiltra- physical and mechanical properties of the composites. tion and densification of Sic nano-powder in the intra-fiber-bun Fabrication temperature(C) 18501900 e interrace bonding. For the composites fabrication by hot-pressing, individual fiber deformation is detect Fiber volume fraction(volg) 33abeinafewcases,indicatedfberdeformatonincudingpote m3) tial fiber creep. For the present study, such a creep formation Ultimate beind ing strength (MPa) 517 375 41 690 860 does not seem to have the significant affected mechanical perfor Elastic modulus in bending(GPa) of the composites. The fibers were well protected by ngth(MPa) 380 50 358 induced-PyC coating and well-consolidation with sufficient-infil- rtional limit stress(MPa tration of SiC nano-powder in the intra- fiber-bundle and could Elastic modulus(GPa) Strain at fracture(%) 02860.1760.1350.1450.127 contribute excellent mechanical performances with non-cata- strophic fracture behavior.volume fraction. Therefore, infiltration and densification of the ma￾trix in the intra-fiber-bundles becomes a challenge for this process. Dong et al. enhanced the intra-fiber-bundle matrix densification by impregnation during PIP pretreatment using a polymer containing slurry (polycarbosilane (PCS) mixed with a filler, which was com￾posed of SiC nano-powder and sintering additives) [13,14]. Even for SiC/SiC composites using SiC submicron-powder, the improve￾ment of the intra-fiber-bundle matrix densification by PCS impreg￾nation was reported by Yoshida et al [25]. In this present case without PIP treatment, a highly-densified composite with high-Vf was obtained at 1900 C under 20 MPa, where near-full densifica￾tion could be achieved in the intra-fiber-bundles. As fabrication temperature increases, liquid phase formation with lower viscosity more effectively promotes grain-boundary sliding, so that fiber rearrangement is continuously enhanced under pressure. That could promote the sufficient infiltration and densification of SiC nano-powder in the intra-fiber-bundles. As a consequence, densifi- cation of the composites with high-Vf is contributed mainly to matrix infiltration and densification of SiC nano-powder in the in￾tra-fiber-bundles involving fiber rearrangement pressure via liquid phase of sintering additives. However, high external pressure on the surfaces of the fibers with PyC interface might cause potential damages to the fibers and PyC interface with the increase of fabri￾cation temperature involving pressure. 3.3. Mechanical properties and fracture behaviors Some average physical and mechanical properties of the com￾posites under two fiber volume fractions incorporating different fabrication temperatures are listed in Table 1. The PLS, the UTS and the elastic modulus in monotonic tensile tests were deter￾mined following the general guidelines of ASTM C-1275. The PLS was the stress corresponding to a 0.0005% offset strain. For com￾parison, the three-point bending strength and the elastic modulus in bending were determined by the load-displacement curves according to ASTM C-1341. As shown in Table 1, the bulk density and open porosity had the strong dependence on fiber volume frac￾tion of the composites at lower fabrication temperature. Also, the value for three-point bending strength and elastic modulus in bending showed the similar characteristics. When fabrication tem￾perature increased, the bulk density increased and open porosity decreased for the composites with high-Vf. At 1900 C, the open porosity reached the lowest value less than 1%. This could be attri￾bution to the intra-fiber-bundle densification, as shown in Fig 3. Meanwhile, the values for three-point bending strength and elastic modulus in bending gradually increased, and the highest value were obtained at 1900 C. The bending strength under this fabrica￾tion temperature was over 850 MPa. Date analysis indicated a sim￾ilar variation trend for both monotonic tensile and three-point bending test, with increasing fabrication temperature from 1800 to 1900 C. Both the UTS and the PLS were the highest at 1900 C. Under this fabrication temperature, the UTS was 410 MPa, the PLS was 360 MPa and the elastic modulus was 360 GPa. In Fig. 4, the stress–strain curves of typical SiC/SiC composites during monotonic tensile tests are presented. High ductility type in Fig. 1b was obtained at lower fabrication temperature (1800 C). In addition, the fracture behavior composites were cat￾egorized in two types from the difference on the densification in the intra-fiber-bundle regions, as shown in Fig. 3a and c. The dense intra-fiber-bundle composites with low-Vf exhibited very large ductile fracture region after the PLS with the relatively high UTS (380 MPa). The porous intra-fiber-bundle composites with high-Vf displayed relatively large ductile fracture region after the PLS with the relative low UTS (320 MPa). Typical fracture sur￾faces of those composites with low- and high-Vf are shown in Fig. 5a and b, respectively. High strength type in Fig. 1a was fab￾ricated at higher fabrication temperature (1900 C) with high-Vf. This composites displayed the very high mechanical values, such as the UTS (410 MPa), the PLS (360 MPa) and the elastic mod￾ulus (360 GPa). The failure of the composites occurred soon after the PLS and far from the matrix crack saturation. This behavior after the PLS inhibited load transfer from matrix to fibers and re￾stricted the longer fiber pull-out, as shown in Fig. 5c. However, this implies that the enhanced matrix densification of the composites with high-Vf through sufficient infiltration and densi- fication of SiC nano-power via liquid phase is beneficial at least in terms of first fracture properties. Cracks were deflected along PyC/ matrix interface in spite of fabrication temperature and fiber vol￾ume fraction as shown in Fig. 6a. In addition, only in the higher temperature fabricated composites, some cracks deflected along PyC/fiber interface were observed as shown in Fig. 6b. At lower temperature, less-consolidated matrix before 3rd stage might form the relatively weak interface bonding between matrix and PyC interface required for high ductility type, which exhibits a wide ductile domain after the PLS with longer fiber pull-out. With longer fiber pull-out, the fibers bridging the matrix cracks can ab￾sorb much more energy to avoid non-catastrophic fracture behav￾ior due to the friction between matrix and fibers when fibers pull￾out from matrix, resulting in higher fracture toughness. On the other hand, at higher fabrication temperature, well-consolidated matrix might form strong interface bonding between matrix and PyC interface with efficient load transfer from matrix to fibers re￾quired for high strength type. The very high PLS with the high elastic modulus could be mainly attribution from the strength￾ened interaction bonds between PyC interface and matrix, as shown in Fig. 6b. Through this study, interface bonding between PyC interface and matrix could be concluded as a significant key for tailoring of fracture behaviors. In CFCCs, increasing of volume fraction of fibers with high tensile strength and modulus as rein￾forcement should enhance mechanical values of composites in strength. In NITE process, however, fiber volume fraction strongly affected the porosity and microstructure of the composites, in par￾ticular in the intra-fiber-bundles, and therefore increasing of fiber volume fraction prevented improving the mechanical values at lower temperature (1800 C). This study revealed that increasing of fabrication temperature was effective way to enhance infiltra￾tion and densification of SiC nano-powder in the intra-fiber-bun￾dles as well as the interface bonding. For the composites fabrication by hot-pressing, individual fiber deformation is detect￾able in a few cases, indicated fiber deformation including poten￾tial fiber creep. For the present study, such a creep formation does not seem to have the significant affected mechanical perfor￾mances of the composites. The fibers were well protected by induced-PyC coating and well-consolidation with sufficient-infil￾tration of SiC nano-powder in the intra-fiber-bundle, and could contribute excellent mechanical performances with non-cata￾strophic fracture behavior. Table 1 Effects of fiber volume fraction incorporating fabrication temperature on the average physical and mechanical properties of the composites. Fabrication temperature (C) 1800 1850 1900 Fiber volume fraction (vol%) 31 52 55 31 53 Bulk density (g/cm3 ) 2.94 2.81 2.96 3.06 3.11 Open porosity (%) 2.1 6.2 4.4 1.0 0.6 Ultimate bending strength (MPa) 517 375 711 690 860 Elastic modulus in bending (GPa) 160 122 174 184 277 Ultimate tensile strength (MPa) 380 322 356 350 358 Proportional limit stress (MPa) 216 177 209 259 408 Elastic modulus (GPa) 289 277 345 338 354 Strain at fracture (%) 0.286 0.176 0.135 0.145 0.127 1626 K. Shimoda et al. / Composites Science and Technology 69 (2009) 1623–1628