Y. Xu et al./ Ceramics International 27(2001)565-570 (C3H6) prior to densification. Methytrichosilane(MTS, 3. Results and discussion CH3 SiCl3) was used for deposition of Sic and carried by bubbling hydrogen(H2). Typical conditions used for 3. 1. Flexural loading ne densification of silicon carbide matrix are 1100C. a hydrogen to MTS mol ratio of 10, and a pressure of 2-3 The density of the composites was 2.5 g cm-s after the kPa. Argon (Ar) was employed as a diluent gas to slow three dimensional silicon carbide preform was chemical down the chemical reaction rate of deposition vapor infiltrated for 30 h. Fig. 2a showed the typical failure behavior of 3D Hi-Nicalon SiC/Sic textile com- 2. 2. Mechanical properties measurement posites at room temperature. The mechanical behavior was initially linear elastic. Then, a nonlinear region was Mechanical properties of the composite materials observed, reflecting matrix damage which induced sig were characterized under flexural shear, and impact nificantly compliance, and residual displacement. Finally, ding. Flexural strength was measured with a three- the fiber failed, initiating at the maxim load, causing the point-bending method at temperatures ranging from unstable fracture of the composites. As the temperature room temperature up to 1300oC in vacuum. Shear was increased from room temperature to 1300C, the strength was measured by the short beam bending flexural strength of the 3D Hi-Nicalon SiC/SiC compo- method with a span of 15 mm. Fracture toughness was sites was slightly increased but not decreased. The aver determined with single edged-notched beam method. age values of the flexural strength were 920 MPa at The impact tests were performed with an instrumented room temperature and 1010 MPa at 1300oC in vacuum Charpy equipment for the test. The sample size was The failure behavior of the composites changed with 3.0x20x70 mm, and the impact velocity of 3 m s-I was increase of the temperature. At room temperature the imposed for the test stress drop was very gradual after the maximum stress point. However, the failure behavior became brittle and 2.3. Microstructure observation and surface analysis he composites exhibited steep stress drops after the maximum stress point at high temperatures(Fig. 2b) The density of the samples was determined by the The variation of failure behavior of the composites water displacement method. The microstructure of the was attributed to the interfacial bonding between fiber fracture surface was observed by a scanning electron and matrix. Fig 3 showed the typical microstructure of mIcroscope the Hi-Nicalon SiC/SiC composite materials. The pyr- olysis carbon interfacial layer was very uniform and the thickness was 300 nm. It was this layer that ensured the proper interfacial bonding between fiber and matrix as well as the load transfer from the silicon carbide matrix to the Hi-Nicalon SiC fiber. Moreover, it has been reported that the thermal expansion coefficients of the silicon car bide matrix and the Hi-Nicalon fiber were 22x10-6K-I and 5.3x10-6K-, respectively [19-21]. After the com- posites were cooled down from the infiltration tempera braiding ture to room temperature, a tensile stress was generated Fig. I. Structure of a three dimension preform. cross the interfacial layer. As a result, it was easy for the 1000 4.0 2040.6081.01.2141.6 Displacement, mm Fig. 2. Stress-deflection curve under flexural loading(C3H6) prior to densification. Methytrichosilane (MTS, CH3SiCl3) was used for deposition of SiC and carried by bubbling hydrogen (H2). Typical conditions used for the densification of silicon carbide matrix are 1100C, a hydrogen to MTS mol ratio of 10, and a pressure of 2–3 kPa. Argon(Ar) was employed as a diluent gas to slow down the chemical reaction rate of deposition. 2.2. Mechanical properties measurement Mechanical properties of the composite materials were characterized under flexural shear, and impact loading. Flexural strength was measured with a three￾point-bending method at temperatures ranging from room temperature up to 1300C in vacuum. Shear strength was measured by the short beam bending method with a span of 15 mm. Fracture toughness was determined with single edged-notched beam method. The impact tests were performed with an instrumented Charpy equipment for the test. The sample size was 3.02070 mm, and the impact velocity of 3 m s
1 was imposed for the test. 2.3. Microstructure observation and surface analysis The density of the samples was determined by the water displacement method. The microstructure of the fracture surface was observed by a scanning electron microscope. 3. Results and discussion 3.1. Flexural loading The density of the composites was 2.5 g cm
3 after the three dimensional silicon carbide preform was chemical vapor infiltrated for 30 h. Fig. 2a showed the typical failure behavior of 3D Hi-Nicalon SiC/SiC textile com￾posites at room temperature. The mechanical behavior was initially linear elastic. Then, a non1inear region was observed, reflecting matrix damage which induced sig￾nificantly compliance, and residual displacement. Finally, the fiber failed, initiating at the maxim load, causing the unstable fracture of the composites. As the temperature was increased from room temperature to 1300C, the flexural strength of the 3D Hi-Nicalon SiC/SiC compo￾sites was slightly increased but not decreased. The aver￾age values of the flexural strength were 920 MPa at room temperature and 1010 MPa at l300C in vacuum. The failure behavior of the composites changed with increase of the temperature. At room temperature the stress drop was very gradual after the maximum stress point. However, the failure behavior became brittle and the composites exhibited steep stress drops after the maximum stress point at high temperatures (Fig. 2b). The variation of failure behavior of the composites was attributed to the interfacial bonding between fiber and matrix. Fig. 3 showed the typical microstructure of the Hi-Nicalon SiC/SiC composite materials. The pyr￾olysis carbon interfacial layer was very uniform and the thickness was 300 nm. It was this layer that ensured the proper interfacial bonding between fiber and matrix as well as the load transfer from the silicon carbide matrix to the Hi-Nicalon SiC fiber. Moreover, it has been reported that the thermal expansion coefficients of the silicon car￾bide matrix and the Hi-Nicalon fiber were 2.210
6 K
1 and 5.310
6 K
1 , respectively [19–21]. After the com￾posites were cooled down from the infiltration tempera￾ture to room temperature, a tensile stress was generated Fig. 1. Structure of a three dimension preform. cross the interfacial layer. As a result, it was easy for the Fig. 2. Stress–deflection curve under flexural loading. 566 Y. Xu et al. / Ceramics International 27 (2001) 565–570