S Wu et al. Materials Science and Engineering A 435-436(2006)412-417 posite fabricated by four-step three-dimensional (4-step 3D) long, 3 mm wide and 3 mm thick) were kept in the hot zone ater-containing environment have not been found and oxidizing atmospheres. Deionized water was used to engen- The present investigation presents the tension-tension fatigue der water vapor. The partial pressure of water vapor, oxy damage behaviors of a 3D PyC-interphase SiC/SiC composite gen and argon was 15, 8 and 78 kPa, respectively. The flux (reinforced by Hi-Nicalon SiC fibers)with a CVD SiC coating of gases was accurately controlled by a mass flow controller in a H2O-O2-Ar environments at 1300C. Much of analysis (5850 i series from BROOKS, in Japan) with a precision of and discussion will then focus on the fatigue characteristics of 0.1 SCCM. The tests were run under a maximum applied stress the composite of 120 MPa, at a frequency of I Hz and with a stress ratio of R=0.5(R=Omin/o max), with a sinusoidal wave form. In order to 2. Materials and experimental procedure gain knowledge of the tension-tension fatigue life of the SiC/SiC in H2O-02-Ar environment, the tests were carried out until the 2.1. Sample preparation specimen fully failed. Microstructures of the samples before and after tests were SiC fiber(Hi-Nicalon, Nippon Carbon Co., Japan) pre- examined using a scanning electron microscope(SEM, $4700 form was prepared using four-step three-dimensional (4-step 3D)braiding method. Low pressure chemical vapor infiltra-3.Results tion(LPCvd) process was employed to deposit pyrolytic carbon (PyC) as an interphase and silicon carbide as a matrix. The vol- 3. 1. Monotonic tensile behavior of the SiC/SiC composite ume fraction of SiC fiber was about 40% and the braiding angle was about 200. PyC layer was deposited on the fiber by decom- The average ultimate tensile strength(UTS)of the SiC/SiC positions of C3 Hs at 870C for I h at reduced pressure of 5 kPa composites at room temperature measured from the monotonic in a CVI reactor, arriving to a thickness of 0. 2 um Methyl- tension test was 456+40MPa trichlorosilane(MTS, CH3 SiCl3) was used for the deposition of Fig. 2 shows the typical load-extension curve of the SiC/SiC SiC, carried by bubbling hydrogen in gas phase and argon measured under monotonic tension the dilute gas to slow down the chemical reaction rate during load-extension curve indicated initially a linear elastic behay deposition SiC matrix was prepared at 1100"C for 120h(30h ior up to the proportional limit of 1000N(corresponding to per times) at reduced pressure of 5 kPa, and the molar ratio of H2 about 130MPa), which was about 29% of the ultimate ten- to MTS was 10. The as-received SiC/SiC was cut into dog-bone sile strength. Then the curve showed a continuously decreasing emples(showed in Fig. 1)along the fibers longitudinal direc- slope and associated non-linear displacements. After that, the n. Density and open porosity of the samples were 2.7gcm-3 curve showed a linear portion till the fracture of the composite and 7.3%, respectively, measured by Archimedes method. A It should be noted that the load-extension curve for all spec CVD SiC coating was deposited on the samples for 20h to seal imens showed a slight steep drop in the final portion at the the open ends of the fiber load range from 2500 to 2800N (326-365 MPa), then the curve quickly restored. This monotonic tensile curve of the 4-step 3D 2.2. Test details SiC/SiC in this study is different from those observed in 2D Sic/SiC and those in 3D SiC/SiC with an orthogonal architecture The monotonic tensile tests were performed at room temper- [11,18) ature to determine the tensile properties of the material, namely, It has been supposed that the linear proportion in the ultimate tensile stress and to compare the damage character- load-extension curve represents the portion before any appre- istics. The load increased at a constant rate of 0.001 mms-up ciable amount of matrix crack, developed in the composite [ll] to fracture of the specimens. The displacement, load and strain The non-linear in curve or the transition was due to the sub were monitored Tension-tension fatigue tests of the SiC/SiC composite were conducted in a H20-02-Ar environment at 1300C. Tension-tension fatigue tests were conducted with integrated system including a resistance heating furnace and a servo- hydraulic machine(Model INSTRoN 8801 from INSTRON Ltd, England). Only the gauge parts of the specimens(40 mm 4-R8.5 Fig 1. A schematic showing of the dog-bone shape SiC/SiC specimen(the unit Fig. 2. Typical load-extension curves of the 3D SiC/SiC composite in monotonicS. Wu et al. / Materials Science and Engineering A 435–436 (2006) 412–417 413 composite fabricated by four-step three-dimensional (4-step 3D) in water-containing environment have not been found. The present investigation presents the tension–tension fatigue damage behaviors of a 3D PyC-interphase SiC/SiC composite (reinforced by Hi-Nicalon SiC fibers) with a CVD SiC coating in a H2O–O2–Ar environments at 1300 ◦C. Much of analysis and discussion will then focus on the fatigue characteristics of the composite. 2. Materials and experimental procedure 2.1. Sample preparation SiC fiber (Hi-Nicalon, Nippon Carbon Co., Japan) pre￾form was prepared using four-step three-dimensional (4-step 3D) braiding method. Low pressure chemical vapor infiltra￾tion (LPCVI) process was employed to deposit pyrolytic carbon (PyC) as an interphase and silicon carbide as a matrix. The vol￾ume fraction of SiC fiber was about 40% and the braiding angle was about 20◦. PyC layer was deposited on the fiber by decom￾positions of C3H6 at 870 ◦C for 1 h at reduced pressure of 5 kPa in a CVI reactor, arriving to a thickness of 0.2
m. Methyl￾trichlorosilane (MTS, CH3SiCl3) was used for the deposition of SiC, carried by bubbling hydrogen in gas phase and argon as the dilute gas to slow down the chemical reaction rate during deposition. SiC matrix was prepared at 1100 ◦C for 120 h (30 h per times) at reduced pressure of 5 kPa, and the molar ratio of H2 to MTS was 10. The as-received SiC/SiC was cut into dog-bone samples (showed in Fig. 1) along the fibers longitudinal direc￾tion. Density and open porosity of the samples were 2.7 g cm−3 and 7.3%, respectively, measured by Archimedes method. A CVD SiC coating was deposited on the samples for 20 h to seal the open ends of the fibers. 2.2. Test details The monotonic tensile tests were performed at room temper￾ature to determine the tensile properties of the material, namely, the ultimate tensile stress and to compare the damage character￾istics. The load increased at a constant rate of 0.001 mm s−1 up to fracture of the specimens. The displacement, load and strain were monitored. Tension–tension fatigue tests of the SiC/SiC composite were conducted in a H2O–O2–Ar environment at 1300 ◦C. Tension–tension fatigue tests were conducted with integrated system including a resistance heating furnace and a servo￾hydraulic machine (Model INSTRON 8801 from INSTRON Ltd., England). Only the gauge parts of the specimens (40 mm Fig. 1. A schematic showing of the dog-bone shape SiC/SiC specimen (the unit was mm). long, 3 mm wide and 3 mm thick) were kept in the hot zone and oxidizing atmospheres. Deionized water was used to engen￾der water vapor. The partial pressure of water vapor, oxy￾gen and argon was 15, 8 and 78 kPa, respectively. The flux of gases was accurately controlled by a mass flow controller (5850 i series from BROOKS, in Japan) with a precision of 0.1 SCCM. The tests were run under a maximum applied stress of 120 MPa, at a frequency of 1 Hz and with a stress ratio of R = 0.5 (R = σmin/σmax), with a sinusoidal wave form. In order to gain knowledge of the tension–tension fatigue life of the SiC/SiC in H2O–O2–Ar environment, the tests were carried out until the specimen fully failed. Microstructures of the samples before and after tests were examined using a scanning electron microscope (SEM, S4700). 3. Results 3.1. Monotonic tensile behavior of the SiC/SiC composite The average ultimate tensile strength (UTS) of the SiC/SiC composites at room temperature measured from the monotonic tension test was 456 ± 40 MPa. Fig. 2 shows the typical load–extension curve of the SiC/SiC measured under monotonic tension at room temperature. The load–extension curve indicated initially a linear elastic behav￾ior up to the proportional limit of 1000 N (corresponding to about 130 MPa), which was about 29% of the ultimate ten￾sile strength. Then the curve showed a continuously decreasing slope and associated non-linear displacements. After that, the curve showed a linear portion till the fracture of the composite. It should be noted that the load–extension curve for all spec￾imens showed a slight steep drop in the final portion at the load range from 2500 to 2800 N (326–365 MPa), then the curve quickly restored. This monotonic tensile curve of the 4-step 3D SiC/SiC in this study is different from those observed in 2D SiC/SiC and those in 3D SiC/SiC with an orthogonal architecture [11,18]. It has been supposed that the linear proportion in load–extension curve represents the portion before any appre￾ciable amount of matrix crack, developed in the composite [11]. The non-linear in curve or the transition was due to the sub￾Fig. 2. Typical load–extension curves of the 3D SiC/SiC composite in monotonic tension at room temperature