S Ochiai et al./ Composites: Part A 35(2004)33-40 amorphous fiber into B-SiC, involving the generation of both the vacuum and air exposures Tensile test was carried out at Sio and Co gases even in Ar atmosphere [6]. This suggests room temperature at a crosshead speed of 8.3 X 10 m/s t the decomposition in vacuum will be enhanced due to for a gage length 50 mm. The fracture surface of the asier generation of the gases in comparison with that in A composite was observed with scanning electron microscope According to Chollon et al. [7]. the decomposition takes (SEM (Jeol, JSM-541OLS)) place at the fiber surface, resulting in a silicon depleted layer and Sic crystal growth, and these features are thought to include the formation of surface flaws of large size and thus a 3. Results and discussion decrease in fiber strel If the surface defects are the strength-determining factor 3. 1. Residual strength of vacuum-exposed composite for the fiber exposed in vacuum at high temperatures fracture mechanical approach can be applied for description Fig. I shows the weight change due to the exposure in of the fiber strength. It is expected that the fracture vacuum and air For 973 K-exposure, the weight change was mechanical model, proposed in the preceding paper [1] to minor in both environments. With increasing exposure describe the residual strength of the air-exposed composite, temperature, the weight tends to increase in the case of can be employed with a slight modification for description exposure. On contrary, the weight tends to decrease in of the residual strength of the vacuum-exposed composite In this work. first the residual strength and fracture mode of he composite exposed in vacuum at high temperatures will be studied experimentally to reveal the differences and 8 similarities of the degradation behavior between the air and vacuum exposures. Then a model will be presented to describe the variation of the strength of the vacuum-exposed 5 function of by modifying the former model [1] and will be applied to the experimental results 2. Experimental procedure Exposure time, t(s) The same SiC/SiC mini-composite specimens used for the air exposure test [1] were used for the present vacuum exposure test. As the fiber, Si-Zr-C-O fiber(ZMI fiber with a composition of Sizr<o0r C14400.32) modified by a pecial treatment was used. The modifie ed fiber had the 1473K following microstructural features [10]: (a) the top surface is a carbon layer with a thickness of a few nanometers and (b) the carbon concentration is graded in the circumferential o Vacuum region within 20-30 nm from the top surface. For the matrix, the same precursor as the ZMI fiber with dispersed ZrSiO4 particles was used. The specimens were fabricated Exposure time, t(s) by the polymer impregnation and pyrolysis(PIP)method at Jbe Companies [1, 11]. The fiber volume fraction and cross sectional area of thus fabricated composite specimens were 0.45 and 0.17 mm- on an average, respectively 1673K 1573and1673Kfor3.6×102-36×105 s in a vacuum O Vacuum chamber, which was evacuated continuously by a vacuum 5 pump to the residual pressure of around 2.7x 10 Pa. All specimens were heated at a constant rate of 0.67 K/s up to the prescribed temperature, and after the exposure for the prescribed time, they were cooled down to room tempera- ture at the same rate of 0.67 K/s as well as those for air Exposure time, t(s) The weight change due to the exposure was measured to Fig. 1. Weight change of the composite due to the exposure in vacuum and monitor the difference in degradation process between in air at 973, 1473 and 1673 K.amorphous fiber into b-SiC, involving the generation of both SiO and CO gases even in Ar atmosphere [6]. This suggests that the decomposition in vacuum will be enhanced due to easier generation of the gases in comparison with that in Ar. According to Chollon et al. [7], the decomposition takes place at the fiber surface, resulting in a silicon depleted layer and SiC crystal growth, and these features are thought to include the formation of surface flaws of large size and thus a decrease in fiber strength. If the surface defects are the strength-determining factor for the fiber exposed in vacuum at high temperatures, fracture mechanical approach can be applied for description of the fiber strength. It is expected that the fracture mechanical model, proposed in the preceding paper [1] to describe the residual strength of the air-exposed composite, can be employed with a slight modification for description of the residual strength of the vacuum-exposed composite. In this work, first the residual strength and fracture mode of the composite exposed in vacuum at high temperatures will be studied experimentally to reveal the differences and similarities of the degradation behavior between the air and vacuum exposures. Then a model will be presented to describe the variation of the strength of the vacuum-exposed composite as a function of exposure temperature and time by modifying the former model [1] and will be applied to the experimental results. 2. Experimental procedure The same SiC/SiC mini-composite specimens used for the air exposure test [1] were used for the present vacuum exposure test. As the fiber, Si–Zr–C–O fiber (ZMI fiber with a composition of SiZr,0.01C1.44O0.32) modified by a special treatment was used. The modified fiber had the following microstructural features [10]; (a) the top surface is a carbon layer with a thickness of a few nanometers and (b) the carbon concentration is graded in the circumferential region within 20–30 nm from the top surface. For the matrix, the same precursor as the ZMI fiber with dispersed ZrSiO4 particles was used. The specimens were fabricated by the polymer impregnation and pyrolysis (PIP) method at Ube Companies [1,11]. The fiber volume fraction and cross￾sectional area of thus fabricated composite specimens were 0.45 and 0.17 mm2 on an average, respectively. The specimens were exposed at 973, 1123, 1273, 1473, 1573 and 1673 K for 3.6 £ 102 –3.6 £ 105 s in a vacuum chamber, which was evacuated continuously by a vacuum pump to the residual pressure of around 2.7 £ 1023 Pa. All specimens were heated at a constant rate of 0.67 K/s up to the prescribed temperature, and after the exposure for the prescribed time, they were cooled down to room tempera￾ture at the same rate of 0.67 K/s as well as those for air exposure. The weight change due to the exposure was measured to monitor the difference in degradation process between the vacuum and air exposures. Tensile test was carried out at room temperature at a crosshead speed of 8.3 £ 1026 m/s for a gage length 50 mm. The fracture surface of the composite was observed with scanning electron microscope (SEM (Jeol, JSM-5410LS)). 3. Results and discussion 3.1. Residual strength of vacuum-exposed composite Fig. 1 shows the weight change due to the exposure in vacuum and air. For 973 K-exposure, the weight change was minor in both environments. With increasing exposure temperature, the weight tends to increase in the case of air exposure. On contrary, the weight tends to decrease in Fig. 1. Weight change of the composite due to the exposure in vacuum and in air at 973, 1473 and 1673 K. 34 S. Ochiai et al. / Composites: Part A 35 (2004) 33–40