《复合材料 Composites》课程教学资源（学习资料）第五章 陶瓷基复合材料_C-SiC-18

MATERIALS SIENCE& EGIEERIL ELSEVIER Materials Science and Engineering A 390(2005)154-158 Effects of pyrolysis processes on the microstructures and mechanical properties of Cf/sic composites using polycarbosilane Ke Jian, Zhao-Hui Chen, Qing-Song Ma, Wen-Wei Zheng Key Laboratory of Advanced Ceramic Fibers and Composites, National University of Defense Technology, Changsha 410073, PR China Received 12 March 2004; received in revised form 27 July 2004; accepted 27 July 2004 Abstract Three-dimensional braided carbon fiber reinforced silicon carbide composites(3D-B Cr/SiC)were prepared through eight cycles of vacuum infiltration of polycarbosilane(PCs)and subsequent pyrolysis under an inert atmosphere. The influences of heating rate and pyrolysis temperature on the microstructure and mechanical properties of Cr/Sic were discussed. It was found that the heating rate had great effect on the mechanical properties of Cr/SiC composites. With the increase of heating rate, the density of CySiC composites increased and the interfacial bonding was weakened. As a result, the flexural strength of Cy/sic was enhanced from 145 to 480 MPa when the heating rate was increased from 0.5 to 15C/min. The results showed that the flexural strength of the C/sic composites fabricated at a heating rate of 15C/min could be increased from 480 to 557 MPa if the pyrolysis temperature of the sixth cycle was elevated from 1200 to 1600C, which was also attributed to the desirable interfacial structure and increased density. When tested at 1300C in vacuum, the Casic showed higher flexural strength(680 MPa)than that(557 MPa)at room temperature 2004 Elsevier B. V. All rights reserve ywords: Cr/SiC composites; Pyrolysis temperatures; Heating rates; Interface; Density; Flexural strength 1. Introduction as low processing temperature, controllable ceramic com- positions, and near-net-shape technologies. Many different SiC ceramics have wide application in various industrial preceramic polymers have been invented since their origi- fields because of their excellent high strength and modulus, nal development by Yajima et al. [4]. Polycarbosilane(PCS), low density, excellent oxidation resistance and good thermal polysilazane(PSz), and polysiloxane(PSO), which produce stability, etc. [ 1]. However, they are generally notch-sensitive SiC, Si-C-N, and Si-O-C ceramics after pyrolysis, are often and exhibit low toughness; hence, they are unreliable as struc- used as preceramic precursors for the PlP routes tural materials. Continuous carbon fiber reinforcement has The mechanical properties of Ct/SiC composites are de- been demonstrated to be the most effective way for improv- termined by their microstructures. Interfacial structure and ing the toughness of SiC ceramics, and CsIc composites are density are the two most important factors [5,6]. Weak in- considered as desirable high-temperature structural materials terfacial bonding and high density can result in high me- 2,3] chanical properties. The microstructures of C/SiC compos There are several methods to fabricate Cr/SiC composites, ites are governed by fabrication process. The manufacturing such as chemical vapor infiltration(CVI), slurry infiltration of Cf/SiC composites via PIP includes three stages. In the combined with hot-press and polymer infiltration pyrolysis stage 1, the fiber preform is infiltrated with precursor solu- (PIP),etc. Of these methods, the PlP route is being actively tion. In the stage 2, the precursor is cured into a solid. In developed because it offers many potential advantages such the stage 3, the cured solid is pyrolyzed in an inert or re- active atmosphere. Several infiltration cure pyrolysis cycles Corresponding author. Tel. +867314576441 were required to densify composites because the evolution of x:+867314573165 gaseous products and volume shrinkage occur during pyroly E-mailaddressjianke_nudt(@sina.com(k.Jian). sis, resulting in micro-cracks and pores in matrix Of the three 0921-5093/S-see front matter 2004 Elsevier B v. All rights reserved doi:10.1016/msea2004.07.064

Materials Science and Engineering A 390 (2005) 154–158 Effects of pyrolysis processes on the microstructures and mechanical properties of Cf/SiC composites using polycarbosilane Ke Jian∗, Zhao-Hui Chen, Qing-Song Ma, Wen-Wei Zheng Key Laboratory of Advanced Ceramic Fibers and Composites, National University of Defense Technology, Changsha 410073, PR China Received 12 March 2004; received in revised form 27 July 2004; accepted 27 July 2004 Abstract Three-dimensional braided carbon fiber reinforced silicon carbide composites (3D-B Cf/SiC) were prepared through eight cycles of vacuum infiltration of polycarbosilane (PCS) and subsequent pyrolysis under an inert atmosphere. The influences of heating rate and pyrolysis temperature on the microstructure and mechanical properties of Cf/SiC were discussed. It was found that the heating rate had great effect on the mechanical properties of Cf/SiC composites. With the increase of heating rate, the density of Cf/SiC composites increased and the interfacial bonding was weakened. As a result, the flexural strength of Cf/SiC was enhanced from 145 to 480 MPa when the heating rate was increased from 0.5 to 15 ◦C/min. The results showed that the flexural strength of the Cf/SiC composites fabricated at a heating rate of 15 ◦C/min could be increased from 480 to 557 MPa if the pyrolysis temperature of the sixth cycle was elevated from 1200 to 1600 ◦C, which was also attributed to the desirable interfacial structure and increased density. When tested at 1300 ◦C in vacuum, the Cf/SiC showed higher flexural strength (680 MPa) than that (557 MPa) at room temperature. © 2004 Elsevier B.V. All rights reserved. Keywords: Cf/SiC composites; Pyrolysis temperatures; Heating rates; Interface; Density; Flexural strength 1. Introduction SiC ceramics have wide application in various industrial fields because of their excellent high strength and modulus, low density, excellent oxidation resistance and good thermal stability, etc. [1]. However, they are generally notch-sensitive and exhibit low toughness; hence, they are unreliable as struc￾tural materials. Continuous carbon fiber reinforcement has been demonstrated to be the most effective way for improv￾ing the toughness of SiC ceramics, and Cf/SiC composites are considered as desirable high-temperature structural materials [2,3]. There are several methods to fabricate Cf/SiC composites, such as chemical vapor infiltration (CVI), slurry infiltration combined with hot-press and polymer infiltration pyrolysis (PIP), etc. Of these methods, the PIP route is being actively developed because it offers many potential advantages such ∗ Corresponding author. Tel.: +86 731 4576441; fax: +86 731 4573165. E-mail address: jianke nudt@sina.com (K. Jian). as low processing temperature, controllable ceramic com￾positions, and near-net-shape technologies. Many different preceramic polymers have been invented since their origi￾nal development by Yajima et al. [4].Polycarbosilane (PCS), polysilazane (PSZ), and polysiloxane (PSO), which produce SiC, Si–C–N, and Si–O–C ceramics after pyrolysis, are often used as preceramic precursors for the PIP routes. The mechanical properties of Cf/SiC composites are de￾termined by their microstructures. Interfacial structure and density are the two most important factors [5,6]. Weak in￾terfacial bonding and high density can result in high me￾chanical properties. The microstructures of Cf/SiC compos￾ites are governed by fabrication process. The manufacturing of Cf/SiC composites via PIP includes three stages. In the stage 1, the fiber preform is infiltrated with precursor solu￾tion. In the stage 2, the precursor is cured into a solid. In the stage 3, the cured solid is pyrolyzed in an inert or re￾active atmosphere. Several infiltration cure pyrolysis cycles were required to densify composites because the evolution of gaseous products and volume shrinkage occur during pyroly￾sis, resulting in micro-cracks and pores in matrix. Of the three 0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2004.07.064

K Jian et al/ Materials Science and Engineering A 390(2005)154-158 stages, the pyrolysis is the most important stage because the 23. Evaluation of the mechanical properties and the interfacial structure and the content and morphology of pores microstructures of the composites are mainly determined in this stage. Consequently, the pyrol ysis process plays a key role in determining the mechanical The bulk densities of the composites were cor properties of PIP-derived Cr/SiC composites the weight-to-volume ratio. Three-point bending tests were In this paper, three-dimensional braided carbon fiber rein- used to evaluate flexural strength of the composites with the forced silicon carbide composites(3D-B Cr/SiC)were fab- span/height ratio of 15 and a crosshead speed of 0.5 mm/min ricated via PIP route, and the effects of heating rate and py The flexural strengths of composites at room temperature rolysis temperature on the microstructures and mechanical were tested at Central South University, China, using a uni properties of 3D-B Cr/SiC composites were investigated versal testing machine(Instron-1342) The flexural strengths of composites at 1300C in vacuum were tested at North- western Polytechnical University, China, using a flexural and 2. Experimental procedure tensile testing machine(YKM-2200) For mechanical prop- erties test, three specimens were measured for each compos- ite and the size of these samples was approximately 3 mm x 4 mm x 70mm. Fracture surfaces of the specimens af- Three-dimensional braided carbon fiber(3D-B C) was ter three-point bending tests were examined on the scanning Ised as the reinforcement for this study. It is two-step braided electron microscopy(SEM) in Nanjing Fiberglass Research and Design Institute. Fiber volume fraction in x, y, and directions of 3D-B Cr preforms were 8: 1: 1. The fibers are Jitan fibers (Jilin Carbon Corp 3. Results and discussion China), and the tensile strength and elastic modulus of the 3. 1. The influence of heating rate on the microstructures carbon fiber were 3000 MPa and 210 GPa, respectively and mechanical properties of composites Polycarbosilane(PCs), the precursor of SiC matrix, with nolecular weight 1300 and soften point -210C,was The mechanical properties of Al200-0.5, A1200-5 synthesized in our laboratory. Divinylbenzene(DVB)was Al200-15. and A1600-15 are shown in Table 2. It was clear used as solvent and cross-linking reagent for PCs In previ- that the flexural strength increased remarkably when the heat ous work the PCS/dvB solution with a mass ratio of 1: 0.4 was demonstrated to be proper to prepare C/SiC composites A& rate was enhanced from 0.5 to 15C/min. A1200-05, lich was fabricated at a heating rate of05C/min, exhibited the lowest flexural strength, only 145 MPa. When the heating rate was enhanced to 15C/min, the flexural strength reached 2. 2. Preparation of 3D-B CSic 480 MPa, about three times higher than that of Al200-05 The density of the four samples as a function of pyroly The preparation of 3D-B Cf/SiC included three stages. sis cycles is plotted in Fig. 1. It could be observed that the In the stage 1, 3D-B Cr preforms were infiltrated with density was enhanced sharply with increasing heating rate PCS/DVB solution in vacuum. In the stage 2, the preforms According to Ref [8], the shape and size of pores in matrix filled with precursor were cured at 150C for 6h. In the of ceramics could be affected greatly by the heating rates stage 3, the cured preforms were pyrolyzed in an inert atmo- When a fast pyrolysis rate is employed, only large pores sur- sphere. In order to densify the composites, the other seven vive in the material, which were propitious for the infiltration infilitration-cure-pyrolysis cycles were repeated. Four sam- in the following cycles. On the contrary, when the heating ples, which were denoted as A1200-0.5, Al200-5, A1200-15, rate was slow, small and closed pores existed in the materi and A1600-15 were fabricated with different pyrolysis pro- als. It was very difficult to impregnate the PCS/DVB solution cesses,and the detailed pyrolysis parameters are shown in into these close pores in the following cycles that resulted in Table 1 lower density of the Ce/SiC composites. So, the density of the 1st-5th and 7th and 8th cycles A1200-0.5 1200

K. Jian et al. / Materials Science and Engineering A 390 (2005) 154–158 155 stages, the pyrolysis is the most important stage because the interfacial structure and the content and morphology of pores are mainly determined in this stage. Consequently, the pyrol￾ysis process plays a key role in determining the mechanical properties of PIP-derived Cf/SiC composites. In this paper, three-dimensional braided carbon fiber rein￾forced silicon carbide composites (3D-B Cf/SiC) were fab￾ricated via PIP route, and the effects of heating rate and py￾rolysis temperature on the microstructures and mechanical properties of 3D-B Cf/SiC composites were investigated. 2. Experimental procedure 2.1. Raw materials Three-dimensional braided carbon fiber (3D-B Cf) was used as the reinforcement for this study. It is two-step braided in Nanjing Fiberglass Research and Design Institute. Fiber volume fraction in x, y, and z directions of 3D-B Cf preforms were 8:1:1. The fibers are Jitan fibers (Jilin Carbon Corp., China), and the tensile strength and elastic modulus of the carbon fiber were 3000 MPa and 210 GPa, respectively. Polycarbosilane (PCS), the precursor of SiC matrix, with molecular weight ∼1300 and soften point ∼210 ◦C, was synthesized in our laboratory. Divinylbenzene (DVB) was used as solvent and cross-linking reagent for PCS. In previ￾ous work, the PCS/DVB solution with a mass ratio of 1:0.4 was demonstrated to be proper to prepare Cf/SiC composites [7]. 2.2. Preparation of 3D-B Cf/SiC The preparation of 3D-B Cf/SiC included three stages. In the stage 1, 3D-B Cf preforms were infiltrated with PCS/DVB solution in vacuum. In the stage 2, the preforms filled with precursor were cured at 150 ◦C for 6 h. In the stage 3, the cured preforms were pyrolyzed in an inert atmo￾sphere. In order to densify the composites, the other seven infilitration-cure-pyrolysis cycles were repeated. Four sam￾ples, which were denoted as A1200-0.5, A1200-5, A1200-15, and A1600-15 were fabricated with different pyrolysis pro￾cesses, and the detailed pyrolysis parameters are shown in Table 1. Table 1 Pyrolysis parameters of each cycle of 3D-B Cf/SiC composites Samples Pyrolysis parameters of each cycle 1st–5th and 7th and 8th cycles 6th cycle Heating rate (◦C/min) Temperature (◦C) Heating rate (◦C/min) Temperature (◦C) A1200-0.5 0.5 1200 0.5 1200 A1200-5 5 1200 5 1200 A1200-15 15 1200 15 1200 A1600-15 15 1200 15 1600 2.3. Evaluation of the mechanical properties and the microstructures of the composites The bulk densities of the composites were computed from the weight-to-volume ratio. Three-point bending tests were used to evaluate flexural strength of the composites with the span/height ratio of 15 and a crosshead speed of 0.5 mm/min. The flexural strengths of composites at room temperature were tested at Central South University, China, using a uni￾versal testing machine (Instron-1342). The flexural strengths of composites at 1300 ◦C in vacuum were tested at North￾western Polytechnical University, China, using a flexural and tensile testing machine (YKM-2200). For mechanical prop￾erties test, three specimens were measured for each compos￾ite and the size of these samples was approximately 3 mm × 4 mm × 70 mm. Fracture surfaces of the specimens af￾ter three-point bending tests were examined on the scanning electron microscopy (SEM). 3. Results and discussion 3.1. The influence of heating rate on the microstructures and mechanical properties of composites The mechanical properties of A1200-0.5, A1200-5, A1200-15, and A1600-15 are shown in Table 2. It was clear that the flexural strength increased remarkably when the heat￾ing rate was enhanced from 0.5 to 15 ◦C/min. A1200-0.5, which was fabricated at a heating rate of 0.5 ◦C/min, exhibited the lowest flexural strength, only 145 MPa. When the heating rate was enhanced to 15 ◦C/min, the flexural strength reached 480 MPa, about three times higher than that of A1200-0.5. The density of the four samples as a function of pyroly￾sis cycles is plotted in Fig. 1. It could be observed that the density was enhanced sharply with increasing heating rate. According to Ref. [8], the shape and size of pores in matrix of ceramics could be affected greatly by the heating rates. When a fast pyrolysis rate is employed, only large pores sur￾vive in the material, which were propitious for the infiltration in the following cycles. On the contrary, when the heating rate was slow, small and closed pores existed in the materi￾als. It was very difficult to impregnate the PCS/DVB solution into these close pores in the following cycles that resulted in lower density of the Cf/SiC composites. So, the density of the

K Jian et al. Materials Science and Engineering A 390(2005)154-158 0.25 The properties of 3D-B Cr/SiC composites fabricated by different pyrolysis mples Density (g cm) Porosity (% Flexural strength(MPa) A1200-15 A1200-0.51.772 1448 A1200-5 329 A1200-151.969 479.5 556.7 A1200-0.5 02g f samples fabricated on different pyrol- Al200-0.5 delayed fracture behavior of A1200-15 and brittle fracture AL200-5 behavior of al200-05 that was accordant with the results ▲A1200-15 derived from SEM micrographs A1600-15 It was the different pyrolysis processes that resulted in different interfacial structures. It had been known that the atoms in matrix could diffuse into carbon fibers driven by high temperature during pyrolysis to create strong fiber-matrix bonding and a red in fiber strength because the Fig. 1. Bulk density depending on the number of pyrolysis cycle. rates of Al200-05 were very low, there was quite a long t for the atoms in matrix to diffuse into carbon fibers, resulting composites increased sharply with increasing heating rates. in strong interfacial bonding and a great reduction in fiber The increase of density meant the decrease of the porosity strength[9]. with the increase of the heating rates, the atoms that resulted in improvement of the mechanical properties of in matrix had much less time to diffuse into carbon fibers, the composites resulting in weak fiber-matrix bonding and a little reduction The fracture surfaces of A1200-05 and A1200-15 are fiber strength. given in Fig. 2. In Fig. 2, Al200-15 illustrated an extensive Based on the results with the increase of heating rates the fiber de-bonding and pull-out, whereas Al200-05 exhibited density of composites increased and the interfacial bonding little fiber pull-out and the length of the pull-out fibers was weakened. As a result, the flexural strength of Cr/Sic was ery short. It was well known that interfacial bonding strength enhanced from 145 to 480 MPa when the heating rate was could be evaluated by the morphology of fracture surfaces. increased from 0.5 to 15C/min An extensive fiber pull-out of fibers indicated a relatively weak fiber/matrix interfacial bonding, while little fiber pull- 3.2. The infiuences of pyrolysis temperature on the out and short pull-out length indicated a strong fiber/matrix microstructure and the mechanical properties of interfacial bonding. Therefore, the SEMmicrographs showed composites that the fiber/matrix interfacial bonding of A1200-15 was more desirable than those in a.5 and a l200-5. The The influence of pyrolysis temperature on the microstruc pull-out mechanism was further supported by a typical load- ture and mechanical properties of composites was also inves- tigated in this paper. As shown in Table 2, the flexural strength Fig. 2. SEM micrographs of samples fabricated on different heating rates. (a)A1200-05 and (b)A1200-15

156 K. Jian et al. / Materials Science and Engineering A 390 (2005) 154–158 Table 2 The properties of 3D-B Cf/SiC composites fabricated by different pyrolysis processes Samples Density (g cm−3) Porosity (%) Flexural strength (MPa) A1200-0.5 1.772 18.4 144.8 A1200-5 1.886 14.0 329.7 A1200-15 1.969 10.8 479.5 A1600-15 2.001 9.60 556.7 Fig. 1. Bulk density depending on the number of pyrolysis cycle. composites increased sharply with increasing heating rates. The increase of density meant the decrease of the porosity that resulted in improvement of the mechanical properties of the composites. The fracture surfaces of A1200-0.5 and A1200-15 are given in Fig. 2. In Fig. 2, A1200-15 illustrated an extensive fiber de-bonding and pull-out, whereas A1200-0.5 exhibited little fiber pull-out and the length of the pull-out fibers was very short. It was well known that interfacial bonding strength could be evaluated by the morphology of fracture surfaces. An extensive fiber pull-out of fibers indicated a relatively weak fiber/matrix interfacial bonding, while little fiber pull￾out and short pull-out length indicated a strong fiber/matrix interfacial bonding. Therefore, the SEM micrographs showed that the fiber/matrix interfacial bonding of A1200-15 was more desirable than those in A1200-0.5 and A1200-5. The pull-out mechanism was further supported by a typical load￾displacement curve that was shown in Fig. 3. It showed typical Fig. 3. Load-displacement curves of samples fabricated on different pyrol￾ysis conditions. delayed fracture behavior of A1200-15 and brittle fracture behavior of A1200-0.5 that was accordant with the results derived from SEM micrographs. It was the different pyrolysis processes that resulted in different interfacial structures. It had been known that the atoms in matrix could diffuse into carbon fibers driven by high temperature during pyrolysis to create strong fiber–matrix bonding and a reduction in fiber strength. Because the heating rates of A1200-0.5 were very low, there was quite a long time for the atoms in matrix to diffuse into carbon fibers, resulting in strong interfacial bonding and a great reduction in fiber strength [9]. With the increase of the heating rates, the atoms in matrix had much less time to diffuse into carbon fibers, resulting in weak fiber–matrix bonding and a little reduction in fiber strength. Based on the results, with the increase of heating rates, the density of composites increased and the interfacial bonding weakened. As a result, the flexural strength of Cf/SiC was enhanced from 145 to 480 MPa when the heating rate was increased from 0.5 to 15 ◦C/min. 3.2. The influences of pyrolysis temperature on the microstructure and the mechanical properties of composites The influence of pyrolysis temperature on the microstruc￾ture and mechanical properties of composites was also inves￾tigated in this paper. As shown in Table 2, the flexural strength Fig. 2. SEM micrographs of samples fabricated on different heating rates. (a) A1200-0.5 and (b) A1200-15

K Jian et al. Materials Science and Engineering A 390(2005)154-158 Fig 4. SEM micrograph of sample A1600-15 tested at room temperature. Fig. 5. SEM micrograph of sample A1600-15 tested at 1300C in vacuum of Al200-15 was 480 MPa, while the flexural strength of energy was needed when the composites were ruptured. As result, the flexural strength increased. The mechanical prop- Al600-15 was enhanced to 557 MPa. The essential effect erties of Cf/SiC composites at higher temperatures would be of pyrolysis temperature could be explained from two as- tested in further investigation pects. First, it was reported [9] that the interphase between the Sic matrix and carbon fibers resulting from the diffu- Sion of Si atoms into carbon fibers consisted of Si. C. and o 4. Conclusions According to the reference, the interphase will decompose above 1400C[10]. As the results of decomposition of inter- 3D-B Cr/SiC composites were prepared with four differ- phase, fiber-matrix bonding may be weaker which could be ent pyrolysis processes, through eight cycles of infiltration of proved from SEM photos. Fig. 4 showed the fracture surface PCS/DVB and subsequent pyrolysis in an inert atmosphere of Al600-15 To compare with Fig 2a, It is observed that Their mechanical properties and microstructures were eval although the pull-out fiber length of A1600-15 was no longer uated. The follow uated. The following conclusions can be drawn from the than that of A1200-15, the number of pull-out fibers increased present investigations sharply, indicating that fiber-matrix bonding become weaker Secondly, when the pyrolysis temperature was elevated (a)With the elevation of heating rates, the density and flex from 1200 to 1600C, the cured PCS/DVB would be py- ral strength of Ct/SiC composites increased. A1200-05 lyzed more completely and many closed pores in the com- reached the density of 1.772 gcm-3and flexural strengt posites opened, so that the composites may be filled with 145 MPa, while A1200-15 reached 1.951 gcm-' and more PCS/DVB solution in the following cycles. As a re- 480 MPa. The higher mechanical properties were at- sult, the density increased from 1.951 to 2.011 gcm-3.The tributed to the desirable interfacial structure and the in- elevation of density meant the decrease of porosity that was creased density beneficial to the mechanical properties of composites (b)It was found that the pyrolysis temperature could af- Because of the reasons mentioned above. A 1600-15 ex- fect the mechanical properties of Cf/SiC. Pyrolysis at hibited higher flexural strength than that of A 1200-1 1600C in the sixth cycle could also elevate the density and weaken the interfacial bonding. As a result, the flex 3.3. The mechanical properties of Cysic at high ural strength of Cr/SiC composites was increased from 480 to 557 MPa when the pyrolysis temperature elevated temperature from 1200 to 1600C in the sixth cycle Because Cr/SiC composites have been developed for high- (c)Tested at 1300C in vacuum, the flexural strength and temperature applications such as the components of turbine modulus of A1600-15 reached 680 MPa and 109 GPa engines and the re-entry thermal protection system of space respectively craft, it is necessary to know mechanical properties of CsIc samples at high temperature. The flexural strength of A1600 References 5 was tested at 1300 C in vacuum The flexural strength and flexural modulus of A1600-15 at 1300C in vacuum were 680 MPa and 109 GPa, while those of A1600-15 at room [2]AG.Evans, J Am. Ceram Soc. 73(2)(1990)187-206 temperature were 557 MPa and 135 GPa, respectively. The [3]T Mah, M.G. Mendiratta, Am. Ceram Soc. Bull. 66(2)(1987) decrease in the flexural modulus at high temperature wa mainly attributed to the softening of the SiC matrix. Fig [4]Y. Hasegawa, M. Iimura, S. Yajima, J. Mater. Sci. 15(1980) shows the fracture surface of A 1600-15 that had been tested at 720-727 5]K Suzuki, J. Ceram, Jpn. Soc. 106(3)(1998)364-368 high temperature. As comparison to Fig 4, the length of pull-(6jBZ. Jang, L R. Hwang, J.W. Fergus, Compos. Sci. Technol. 56(12) out carbon fibers was much longer. It meant that much more (1996)1341-1350

K. Jian et al. / Materials Science and Engineering A 390 (2005) 154–158 157 Fig. 4. SEM micrograph of sample A1600-15 tested at room temperature. of A1200-15 was 480 MPa, while the flexural strength of A1600-15 was enhanced to 557 MPa. The essential effect of pyrolysis temperature could be explained from two as￾pects. First, it was reported [9] that the interphase between the SiC matrix and carbon fibers resulting from the diffu￾sion of Si atoms into carbon fibers consisted of Si, C, and O. According to the reference, the interphase will decompose above 1400 ◦C [10]. As the results of decomposition of inter￾phase, fiber–matrix bonding may be weaker which could be proved from SEM photos. Fig. 4 showed the fracture surface of A1600-15. To compare with Fig. 2a, it is observed that although the pull-out fiber length of A1600-15 was no longer than that of A1200-15, the number of pull-out fibers increased sharply, indicating that fiber–matrix bonding become weaker. Secondly, when the pyrolysis temperature was elevated from 1200 to 1600 ◦C, the cured PCS/DVB would be py￾rolyzed more completely and many closed pores in the com￾posites opened, so that the composites may be filled with more PCS/DVB solution in the following cycles. As a re￾sult, the density increased from 1.951 to 2.011 g cm−3. The elevation of density meant the decrease of porosity that was beneficial to the mechanical properties of composites. Because of the reasons mentioned above, A1600-15 ex￾hibited higher flexural strength than that of A1200-15. 3.3. The mechanical properties of Cf/SiC at high temperature Because Cf/SiC composites have been developed for high￾temperature applications such as the components of turbine engines and the re-entry thermal protection system of space￾craft, it is necessary to know mechanical properties of Cf/SiC samples at high temperature. The flexural strength of A1600- 15 was tested at 1300 ◦C in vacuum. The flexural strength and flexural modulus of A1600-15 at 1300 ◦C in vacuum were 680 MPa and 109 GPa, while those of A1600-15 at room temperature were 557 MPa and 135 GPa, respectively. The decrease in the flexural modulus at high temperature was mainly attributed to the softening of the SiC matrix. Fig. 5 shows the fracture surface of A1600-15 that had been tested at high temperature. As comparison to Fig. 4, the length of pull￾out carbon fibers was much longer. It meant that much more Fig. 5. SEM micrograph of sample A1600-15 tested at 1300 ◦C in vacuum. energy was needed when the composites were ruptured. As a result, the flexural strength increased. The mechanical prop￾erties of Cf/SiC composites at higher temperatures would be tested in further investigation. 4. Conclusions 3D-B Cf/SiC composites were prepared with four differ￾ent pyrolysis processes, through eight cycles of infiltration of PCS/DVB and subsequent pyrolysis in an inert atmosphere. Their mechanical properties and microstructures were eval￾uated. The following conclusions can be drawn from the present investigations: (a) With the elevation of heating rates, the density and flexu￾ral strength of Cf/SiC composites increased. A1200-0.5 reached the density of 1.772 g cm−3 and flexural strength 145 MPa, while A1200-15 reached 1.951 g cm−3 and 480 MPa. The higher mechanical properties were at￾tributed to the desirable interfacial structure and the in￾creased density. (b) It was found that the pyrolysis temperature could af￾fect the mechanical properties of Cf/SiC. Pyrolysis at 1600 ◦C in the sixth cycle could also elevate the density and weaken the interfacial bonding. As a result, the flex￾ural strength of Cf/SiC composites was increased from 480 to 557 MPa when the pyrolysis temperature elevated from 1200 to 1600 ◦C in the sixth cycle. (c) Tested at 1300 ◦C in vacuum, the flexural strength and modulus of A1600-15 reached 680 MPa and 109 GPa, respectively. References [1] A.J. Klein, Adv. Mater. 64 (5) (1986) 130. [2] A.G. Evans, J. Am. Ceram. Soc. 73 (2) (1990) 187–206. [3] T. Mah, M.G. Mendiratta, Am. Ceram. Soc. Bull. 66 (2) (1987) 304–308. [4] Y. Hasegawa, M. Iimura, S. Yajima, J. Mater. Sci. 15 (1980) 720–727. [5] K. Suzuki, J. Ceram, Jpn. Soc. 106 (3) (1998) 364–368. [6] B.Z. Jang, L.R. Hwang, J.W. Fergus, Compos. Sci. Technol. 56 (12) (1996) 1341–1350

K Jian et al. Materials Science and Engineering A 390(2005)154-158 7 Su Bo, Wu Xiaojing, Liu Fengrong, et al., in: Proceedings of the 8th 9]QS Ma, Z.H. Chen, w.w. Zheng, H.F. Hu, Mater. Sci Eng. A352 National Conference of Composites Meterials, 1994 (in Chinese). (1-2)(2003)212-216 [8]H. Schmidt, D. Koch, G. Grathwohl, P. Colombo, J. Am. Ceram [10Q.S. Ma, Z.H. Chen, w.w. Zheng, H F. Hu, J. Natl. Univ. Defen. So.84(10)(2001)2252-2255 Tech. 23(5)(2001)40-44(in Chinese

158 K. Jian et al. / Materials Science and Engineering A 390 (2005) 154–158 [7] Su Bo, Wu Xiaojing, Liu Fengrong, et al., in: Proceedings of the 8th National Conference of Composites Meterials, 1994 (in Chinese). [8] H. Schmidt, D. Koch, G. Grathwohl, P. Colombo, J. Am. Ceram. Soc. 84 (10) (2001) 2252–2255. [9] Q.S. Ma, Z.H. Chen, W.W. Zheng, H.F. Hu, Mater. Sci. Eng. A352 (1–2) (2003) 212–216. [10] Q.S. Ma, Z.H. Chen, W.W. Zheng, H.F. Hu, J. Natl. Univ. Defen. Tech. 23 (5) (2001) 40–44 (in Chinese)