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)1341-1350K. 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