36 J Nie et al Materials Science and Engineering A 497(2008) 235-238 Fig. 1. Drawing of as-prepared C/SiC specimens for thermal cycling tests in con- trolled environments (all dimensions in mn and a simulated air atmosphere the resonant frequencies were monitored to determine the reduction in the modulus of the com- osite with increasing thermal cycles. And after thermal cycled for 90 cycles, residual tensile strength was measured at room tempe ature to investigate the effect of the thermal cycling in different environments on the mechanical properties of the needled C/Sic Fig. 2. Schematic drawing cooling water, (5)the measurement for temperature, 6) the induction coil for cyclic 2. Experimental procedures temperature, (7) the grip holder and 8)the pressure and flow of the controlled Z.1. Needled C/Sic composite preparation 2.3. Measurement of resonant frequency to determine the relative In the present work, the needled C/SiC composite was fabri cha ated using a three-step and easily controlled process. At first the preform was fabricated by alternatively stacked non-woven cloth The thermal cycling tests were interrupted to measure the res- layers and short-cut-fiber webs using a needling technique and two onant frequency at the given cycles of 10, 30, 50, uccessive non-woven cloth layers were oriented at an angle of vibration tests were conducted at room temperature. One end of 90, which has been described in detail elsewhere [18]. The car- spec was fixed and the other end was excited to vibrate hori- bon fibers used were PAN-based carbon fibers(T700, 12K, from zontally by a modally tuned hammer(PCB 086C05).The responses Toray, Japan). The fiber contents of the non-woven cloth layers, the were monitored using a non-contact laser displacement sensor(LK- ebs and the needling fibers were 21.0, 7.0 and 2.0 vol % respec- G80H, KEYENCE Corp. Japan). Thus, the vibration frequencies of the ively. Secondly, to protect the carbon fibers from damage in the specimens were gained. For a given sample, dimensions, mass, and CVI process and to weaken the interfacial bonding between the shape factors are supposed to be fixed before and after thermal carbon fibers and the Sic matrix[2, 19 a pyrolytic carbon(Pyc) cycling tests. Then, from Eq (1), the relative changes in modulus lyer was deposited on the surface of carbon fibers as fiber/matrix can be derived and expressed as following terphase prior to the densification of SiC matrix. Finally, Cvi was En tions for CvI process were the same as that described in Refs. Eo 18 where Eo and En are the modulus of the composite specimen unquenched and thermal cycled for n cycles, respectively 2. 2. Thermal cycling tests in two controlled environments 2.4. Residual tensile strength measurement and microstructure The dimensions and shape of the specimens are shown in Fig. 1 nalysis nd three specimens were used for thermal cycling test in each Residual tensile strengths of the specimens after thermal cycled Fig. 2, which has been detailed described in Ref (20 In this work, for 90 cycles in both environments were measured on an INSTRON thermal cycling was carried out from 700 to 1200oC controlled by device(8801. INSTRON Ltd. England)according to ASTM C1275 programmable microprocessor. The temperature was measured standard (211. with a crosshead speed of 0.001 mm/s Strains were an infrared pyrometer through a small window in the wall of the recorded using an exter ter with a gauge length of 25 furnace and the wall was internally cut out to enable the circulat- The density and open porosity of the composites were measured ing cold water to reach all over the surfaces. Thermal cycling was by Archimedes' method at room temperature. The microstructures carried out between two selected temperatures and the period was of the fracture surfaces were observed using a scanning electron 180s: holding for 60 s at 700C, heating to 1200C in 60 s and hold- microscope (SEM, JSM-6700F, JEOL, Japan). difference ATA 500.C). Only the middle parts of specimens (about 3. Results and discussion 40-mm long. 3.5-mm wide and 3.5-mm thick ]were kept in the hot zone and were quenched in two typical environments: The density and open porosity of the as-received needled gas; and(ii) a simulated air which was the mixture of Ar(79 vol%) C/Sic composite were 2.1 g/cm and 15%, respectively. And the and o(21 vol%)gas. The flux of gases was accurately controlled microstructure of the composite is shown in Fig 3. It can be found by a mass flow controller (5850 i series of BROOKS in Japan)and its that the adjacent layers were perfectly interlinked together by the precision could reach 0.1 sccm. needling fiber tow236 J. Nie et al. / Materials Science and Engineering A 497 (2008) 235–238 Fig. 1. Drawing of as-prepared C/SiC specimens for thermal cycling tests in con￾trolled environments (all dimensions in mm). and a simulated air atmosphere. The resonant frequencies were monitored to determine the reduction in the modulus of the com￾posite with increasing thermal cycles. And after thermal cycled for 90 cycles, residual tensile strength was measured at room temper￾ature to investigate the effect of the thermal cycling in different environments on the mechanical properties of the needled C/SiC composites. 2. Experimental procedures 2.1. Needled C/SiC composite preparation In the present work, the needled C/SiC composite was fabri￾cated using a three-step and easily controlled process. At first the preform was fabricated by alternatively stacked non-woven cloth layers and short-cut-fiber webs using a needling technique and two successive non-woven cloth layers were oriented at an angle of 90◦, which has been described in detail elsewhere [18]. The car￾bon fibers used were PAN-based carbon fibers (T700, 12K, from Toray, Japan). The fiber contents of the non-woven cloth layers, the webs and the needling fibers were 21.0, 7.0 and 2.0 vol.%, respec￾tively. Secondly, to protect the carbon fibers from damage in the CVI process and to weaken the interfacial bonding between the carbon fibers and the SiC matrix [2,19], a pyrolytic carbon (PyC) layer was deposited on the surface of carbon fibers as fiber/matrix interphase prior to the densification of SiC matrix. Finally, CVI was employed to deposit PyC interphase and SiC matrix. The condi￾tions for CVI process were the same as that described in Refs. [11,18]. 2.2. Thermal cycling tests in two controlled environments The dimensions and shape of the specimens are shown in Fig. 1, and three specimens were used for thermal cycling test in each controlled environment. The thermal cycling apparatus is shown in Fig. 2, which has been detailed described in Ref. [20]. In this work, thermal cycling was carried out from 700 to 1200 ◦C controlled by a programmable microprocessor. The temperature was measured by an infrared pyrometer through a small window in the wall of the furnace and the wall was internally cut out to enable the circulat￾ing cold water to reach all over the surfaces. Thermal cycling was carried out between two selected temperatures and the period was 180 s: holding for 60 s at 700 ◦C, heating to 1200 ◦C in 60 s and hold￾ing for 60 s, then cooling back to 700 ◦C immediately (temperature difference T ≈ 500 ◦C). Only the middle parts of specimens (about 40-mm long, 3.5-mm wide and 3.5-mm thick) were kept in the hot zone and were quenched in two typical environments: (i) pure Ar gas; and (ii) a simulated air which was the mixture of Ar (79 vol.%) and O2 (21 vol.%) gas. The flux of gases was accurately controlled by a mass flow controller (5850 i series of BROOKS in Japan) and its precision could reach 0.1 sccm. Fig. 2. Schematic drawing of atmosphere chamber with a detailed view of the grip holders, the furnace and the specimen (eight major critical points are indicated)[20]. (1) The load alignment, (2) the configuration of the specimen, (3) the heater, (4) the cooling water, (5) the measurement for temperature, (6) the induction coil for cyclic temperature, (7) the grip holder and (8) the pressure and flow of the controlled atmosphere. 2.3. Measurement of resonant frequency to determine the relative change in modulus The thermal cycling tests were interrupted to measure the res￾onant frequency at the given cycles of 10, 30, 50, 70 and 90. The vibration tests were conducted at room temperature. One end of specimen was fixed and the other end was excited to vibrate hori￾zontally by a modally tuned hammer (PCB 086C05). The responses were monitored using a non-contact laser displacement sensor (LK￾G80H, KEYENCE Corp., Japan). Thus, the vibration frequencies of the specimens were gained. For a given sample, dimensions, mass, and shape factors are supposed to be fixed before and after thermal cycling tests. Then, from Eq. (1), the relative changes in modulus can be derived and expressed as following: En E0 = fn f0 2 (2) where E0 and En are the modulus of the composite specimen unquenched and thermal cycled for n cycles, respectively. 2.4. Residual tensile strength measurement and microstructure analysis Residual tensile strengths of the specimens after thermal cycled for 90 cycles in both environments were measured on an INSTRON device (8801, INSTRON Ltd., England) according to ASTM C1275 standard [21], with a crosshead speed of 0.001 mm/s. Strains were recorded using an extensometer with a gauge length of 25 mm. The density and open porosity of the composites were measured by Archimedes’ method at room temperature. The microstructures of the fracture surfaces were observed using a scanning electron microscope (SEM, JSM-6700F, JEOL, Japan). 3. Results and discussion The density and open porosity of the as-received needled C/SiC composite were 2.1 g/cm3 and 15%, respectively. And the microstructure of the composite is shown in Fig. 3. It can be found that the adjacent layers were perfectly interlinked together by the needling fiber tows.