3198 S. Wu et al. / Materials Letters 60(2006)3197-3201 2. Experimental procedure Interphase layer 2.1. Preparation of 3D Hi-Nicalon/PyC/SiC composite specimens Hi-NicalonTM silicon carbide fiber from Japan Nippon Matrix micro crack Carbon was employed. The fiber preform was prepared using four-step three dimensional (4-step 3D) braiding method, and was supplied by the Nanjing Institute of Glass Fiber, Peoples epublic of China. Low pressure chemical vapor infiltration (LPCVD) process was employed to deposit pyrolytic carbon (PyC) interphase and the silicon carbide matrix. The volume Fig. 2. Fine matrix micro cracks in the 3D Hi-Nicalon/PyC/SiC composite. fraction of fibers was about 40% and the braiding angle was about 20. The interfacial layer of PyC was deposited for I h at 870C and 5 kPa with C3H6. The deposited PyC interphase mal Analysis System(thermobalance with a sensitivity of layer is about 0.2 um Methyltrichlorosilane (MTS, CH3SiCl3) +0.0001 mg) for 900 min. The simulated air was a mixture of was used for the deposition of the SiC matrix. MTS vapor was oxygen and argon with a volume ratio of 22 to 78. The gas flow carried by bubbling hydrogen. The conditions for deposition of speed was about 3. 5 cms. Weight changes related to oxidation SiC matrix were as follows: the deposition temperature was time of specimens was recorded with TG mode. The oxidation 1100C, pressure was 5 kPa, time was 120 h, the molar ratio of test procedure was as follows: Firstly, the specimen was hold for H2 to methyltrchlorosilane (MTS)was 10. Argon was employed 15 min at 25C in argon. Secondly, heat up the specimen in as the dilute gas to slow down the chemical reaction rate of argon with controlled heating rate. Finally, introduce the deposition. The dimension of as-received SiC/SiC sample was simulated air at the desired temperature and start oxidation 2.5x42x 30.0 mm. The Sic coating was prepared on the process specimens for 20 h to seal the open ends of the fibers after cutting from the prepare 2.3. Measurements of the composite specimens 2.2. Oxidation tests The flexural strength of the specimens before and after oxidation was measured by a three-point bending method, The oxidation of the Hi-Nicalon/PyC/SiC in a simulated which was carried out on an Instron 1195 machine at room air was conducted in METTLER TOLEDO STAR Ther- temperature. The span dimension was 20 mm and the loading rate was 0.5 mm/min. The fracture sections and the surfaces of the specimens were observed on a scanning electron microscope (SEM,S4700). 3. Results and discussion 3.1. Microstructure of the 3D Hi-Nicalon/Py C/SiC composite Fig. I showed the surface and interior section morphologies of the 3D Hi-Nicalon/PyC/SiC composite Cracks were neither observed in the outer CVD SiC coating, nor in the SiC matrix. However, finely micro cracks could be observed in the matrix for the Hi-Nicalon/PyC/ Unsealed pore Fig. 1. Surface and interior section morphologies of the 3D Hi-Nicalon/PyC Fig 3. Unsealed pores in CVD SiC coating.2. Experimental procedure 2.1. Preparation of 3D Hi–Nicalon/PyC/SiC composite specimens Hi–Nicalon™ silicon carbide fiber from Japan Nippon Carbon was employed. The fiber preform was prepared using four–step three dimensional (4–step 3D) braiding method, and was supplied by the Nanjing Institute of Glass Fiber, People’s Republic of China. Low pressure chemical vapor infiltration (LPCVI) process was employed to deposit pyrolytic carbon (PyC) interphase and the silicon carbide matrix. The volume fraction of fibers was about 40% and the braiding angle was about 20°. The interfacial layer of PyC was deposited for 1 h at 870 °C and 5 kPa with C3H6. The deposited PyC interphase layer is about 0.2 μm. Methyltrichlorosilane (MTS, CH3SiCl3) was used for the deposition of the SiC matrix. MTS vapor was carried by bubbling hydrogen. The conditions for deposition of SiC matrix were as follows: the deposition temperature was 1100 °C, pressure was 5 kPa, time was 120 h, the molar ratio of H2 to methyltrchlorosilane (MTS) was 10. Argon was employed as the dilute gas to slow down the chemical reaction rate of deposition. The dimension of as–received SiC/SiC sample was 2.5 × 4.2 × 30.0 mm. The SiC coating was prepared on the specimens for 20 h to seal the open ends of the fibers after cutting from the prepared composite. 2.2. Oxidation tests The oxidation of the Hi-Nicalon/PyC/SiC in a simulated air was conducted in METTLER TOLEDO STARe Ther￾mal Analysis System (thermobalance with a sensitivity of ± 0.0001 mg) for 900 min. The simulated air was a mixture of oxygen and argon with a volume ratio of 22 to 78. The gas flow speed was about 3.5 cm·s–1 . Weight changes related to oxidation time of specimens was recorded with TG mode. The oxidation test procedure was as follows: Firstly, the specimen was hold for 15 min at 25 °C in argon. Secondly, heat up the specimen in argon with controlled heating rate. Finally, introduce the simulated air at the desired temperature and start oxidation process. 2.3. Measurements of the composite specimens The flexural strength of the specimens before and after oxidation 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 specimens were observed on a scanning electron microscope (SEM, S4700). 3. Results and discussion 3.1. Microstructure of the 3D Hi–Nicalon/PyC/SiC composite Fig. 1 showed the surface and interior section morphologies of the 3D Hi–Nicalon/PyC/SiC composite. Cracks were neither observed in the outer CVD SiC coating, nor in the SiC matrix. However, finely micro cracks could be observed in the matrix for the Hi–Nicalon/PyC/ Fig. 1. Surface and interior section morphologies of the 3D Hi–Nicalon/PyC/ SiC composite (a) Surface; (b) interior section. Fig. 2. Fine matrix micro cracks in the 3D Hi–Nicalon/PyC/SiC composite. Fig. 3. Unsealed pores in CVD SiC coating. 3198 S. Wu et al. / Materials Letters 60 (2006) 3197–3201