S. Wu et al /Composites: Part A 37(2006)1396-140 1397 using a four-step three-dimensional (4-step 3D) braiding 3. Results method, and was supplied by the Nanjing Institute of chemical vapor infiltration(LPCVI) process was em- degradatior change and residual flexural strength Glass Fiber, People's Republic of China. Low pressure 3. 1. Weight ployed to deposit pyrolytic carbon interphase and the sil- icon carbide matrix. The volume fraction of fibers was Fig. I shows weight change of the SiC/SiC composite about 40% and the braiding angle was about 200. The specimens with temperature after corrosion in environ interfacial layer of pyrocarbon(PyC) was deposited for ments containing Na2SO4 vapor, oxygen and water vapor I h at 870C and 5 kPa with C3H6. The deposited Pyc for different time Below 1200C, the weight gain increased interphase layer is about 0.2 um. Methyltrichlorosilane slowly as the temperature or experimental time increases (MTS, CH3 SiCl3) was used for the deposition of the From 1200 to 1300C, the weight gain strongly increased SiC matrix. MTS vapor was carried by bubbling hydro- with the temperature increasing. Additionally, the weight gen. The conditions for deposition of Sic matrix were as gain strongly increased with the experimental time increas- follows: the deposition temperature 1100C, pressure ing for the primordial 5 h, then decreased with the experi 5 kPa, time 20 h, the molar ratio of H, to methyltrichlo- mental time increasing. Above 1300C, the weight gain rosilane(MTS)10. Argon was employed as the dilute gas kept the same value for the primordial 2 h, and then it in to slow down the chemical reaction rate of deposition creased as the experimental time increases while decreased [10]. Specimens with dimension of 2.4 4.2 X 30.0 mm with the experimental time increases after 5 h. As experi were machined from the as-received composite and pol- mental time exceeded 2 h, the weight gain decreased as ished. The Sic coating was prepared on the substrates the temperature increased for 5 h to seal the open ends of the fibers after cutting ig. 2 shows the effect of temperature on the residual from the prepared composite flexural strength of SiC/SiC composite after corrosion in environments containing Na2SO4 vapor, oxygen and water 2. Corrosion tests vapor for 10 h. Below 1200C, the residual flexural strength nearly kept the same value as that of as-received The corrosion tests were conducted in simulated specimens(1193 MPa). Above 1200C, the residual flex- combustion environments containing Na SO4 vapor, oxy- ural strength strongly decreased with an increasing and water vapor at temperatures to 1500C for 10 h in a special device [6]. The Na2SO vapor was obtained by volatilization of Na 2SO4 in a corundum crucible at 900C after Na2SO4 powder was ntered at 900C for I h. The Na2SO4 vapor was carried into the reaction region by the mixed gas containing argon, oxygen and water vapor. The concen- tration of Na2SO4 was about 100 ppm, which was calcu lated by the measured weight loss of crucible with melted salt inside. The partial pressure of oxygen and water vapor were about 8000 and 14000 Pa, respectively. Three pecimens were used for each experimental condition 100011001200130014001500 The mass of the specimens were recorded after the specimens were corroded for 0, 2, 5, and 10 h at the Temperature(C) given temperature (1000, 1100, 1200, 1300, 1400, Fig. 1. Weight change of the SiC/SiC composite with temperature after 1500C),respectively. They were measured using an corrosion for different time electronic balance (METTLER TOLEDO AG204 sensitivity =0.I mg) 1200 2.3. Measurements of the composite specimens The flexural strength of the specimens before and after 10 h corrosion was measured by a three-point bending method. which was carried out on an Instron 1195 machine at room temperature. The span dimension was 20 mm and the loading rate was 0.5 mm/min. The fracture sections and the surfaces of the corrosive specimens were observed on a 100011001200130014001500 SEM(model JEOL JXA-840, SEM). The corrosion prod- ucts were analyzed by EDS (INCA 300)and XRD(Rigaku Fig. 2. Flexural strength change of the Sic/SiC composite with temper- DMAX-2400) ature after corrosion for 10 husing a four-step three-dimensional (4-step 3D) braiding method, and was supplied by the Nanjing Institute of Glass Fiber, Peoples Republic of China. Low pressure chemical vapor infiltration (LPCVI) process was em￾ployed to deposit pyrolytic carbon interphase and the sil￾icon carbide matrix. The volume fraction of fibers was about 40% and the braiding angle was about 20. The interfacial layer of pyrocarbon (PyC) was deposited for 1 h at 870 C and 5 kPa with C3H6. The deposited PyC interphase layer is about 0.2 lm. Methyltrichlorosilane (MTS, CH3SiCl3) was used for the deposition of the SiC matrix. MTS vapor was carried by bubbling hydro￾gen. The conditions for deposition of SiC matrix were as follows: the deposition temperature 1100 C, pressure 5 kPa, time 20 h, the molar ratio of H2 to methyltrichlo￾rosilane (MTS) 10. Argon was employed as the dilute gas to slow down the chemical reaction rate of deposition [10]. Specimens with dimension of 2.4 · 4.2 · 30.0 mm were machined from the as-received composite and pol￾ished. The SiC coating was prepared on the substrates for 5 h to seal the open ends of the fibers after cutting from the prepared composite. 2.2. Corrosion tests The corrosion tests were conducted in simulated combustion environments containing Na2SO4 vapor, oxy￾gen, and water vapor at temperatures ranging from 1000 to 1500 C for 10 h in a special device [6]. The Na2SO4 vapor was obtained by volatilization of Na2SO4 in a corundum crucible at 900 C after Na2SO4 powder was sintered at 900 C for 1 h. The Na2SO4 vapor was carried into the reaction region by the mixed gas containing argon, oxygen and water vapor. The concen￾tration of Na2SO4 was about 100 ppm, which was calcu￾lated by the measured weight loss of crucible with melted salt inside. The partial pressure of oxygen and water vapor were about 8000 and 14000 Pa, respectively. Three specimens were used for each experimental condition. The mass of the specimens were recorded after the specimens were corroded for 0, 2, 5, and 10 h at the given temperature (1000, 1100, 1200, 1300, 1400, 1500 C), respectively. They were measured using an electronic balance (METTLER TOLEDO AG204, sensitivity = 0.1 mg). 2.3. Measurements of the composite specimens The flexural strength of the specimens before and after 10 h corrosion was measured by a three-point bending method, which was carried out on an Instron 1195 machine at room temperature. The span dimension was 20 mm and the loading rate was 0.5 mm/min. The fracture sections and the surfaces of the corrosive specimens were observed on a SEM (model JEOL JXA-840, SEM). The corrosion prod￾ucts were analyzed by EDS (INCA 300) and XRD (Rigaku D/MAX-2400). 3. Results 3.1. Weight change and residual flexural strength degradation Fig. 1 shows weight change of the SiC/SiC composite specimens with temperature after corrosion in environ￾ments containing Na2SO4 vapor, oxygen and water vapor for different time. Below 1200 C, the weight gain increased slowly as the temperature or experimental time increases. From 1200 to 1300 C, the weight gain strongly increased with the temperature increasing. Additionally, the weight gain strongly increased with the experimental time increas￾ing for the primordial 5 h, then decreased with the experi￾mental time increasing. Above 1300 C, the weight gain kept the same value for the primordial 2 h, and then it in￾creased as the experimental time increases while decreased with the experimental time increases after 5 h. As experi￾mental time exceeded 2 h, the weight gain decreased as the temperature increased. Fig. 2 shows the effect of temperature on the residual flexural strength of SiC/SiC composite after corrosion in environments containing Na2SO4 vapor, oxygen and water vapor for 10 h. Below 1200 C, the residual flexural strength nearly kept the same value as that of as-received specimens (1193 MPa). Above 1200 C, the residual flex￾ural strength strongly decreased with an increasing temperature. –0.4 –0.2 0 0.2 0.4 0.6 0.8 1 1000 1100 1200 1300 1400 1500 Temperature(°C) Weight change(%) 2h 5h 10h Fig. 1. Weight change of the SiC/SiC composite with temperature after corrosion for different time. 500 600 700 800 900 1000 1100 1200 Residual flexural strength(MPa) 1000 1100 1200 1300 1400 1500 Temperature(°C) Fig. 2. Flexural strength change of the SiC/SiC composite with temper￾ature after corrosion for 10 h. S. Wu et al. / Composites: Part A 37 (2006) 1396–1401 1397