S. Wu et al. Materials Letters 60(2006)3197-3201 3199 1.8 Cvd SiC 200400600800100012001400 Fig. 4. Relations of CTE to temperatures for the 3D Hi-Nicalon/PyC/SIC composite and CVD Sic SiC composite under higher magnification as shown in Fig. 2. Between the fiber and matrix a Pyc interphase layer could be observed and gaps were scarcely observed either between the fiber and the interphase or between the matrix and the interphase. Moreover, some pores existed the Cvd sic coating as shown in Fig 3. As for fiber reinforced ceramic matrix composites, cracks were resulted from the mismatch of thermal expansion coefficient(CTE between the composite and coating, and between the fibers and matrix, respectively. The CtE of Hi-Nicalon SiC fiber and SiC matrix w about3.1~3.5×10-6Cand4.6×10-°C, respectively[2,10,13]Fig. 4 showed the relations of CTE to temperatures of the 3D Hi-Nicalon/ PyC/Sic composite and CVD SiC from 25 C to 1400oC. The relations Fig. 6. Morphology of nonuniform consumption of PyC phase in Hi-Nicalon/ of CTE to temperature of Hi-Nicalon/Py C/SiC composite have a PyC/SiC composite at inat:(a)600°C;(b) similar tendency to those of CVD SiC and the difference between them was very little. Thus, a little tensile stress was produced in the Sic coating or matrix during cooling process and the thermal cracks were scarcely produced in them. Some matrix pores formed between the fiber bundles were open on the surface of the composite after machined. Thus, open pores could be observed in the SiC coating 3.2. Oxidation behavior of the 3D Hi-Nicalon/Py C/SiC composite Fig. 5 showed the TGA results of the Hi-Nicalon/PyC/SiC composite after oxidation in the simulated air for 900 min. Below 55065075085095010501150125013501450 1100C, oxidation of the composite led to a weight loss. Above 1100C, oxidation resulted in a weight gain in a parabolic-linear 600 Time(min) 5006007008009001000I1001200130014001500 Fig. 5.(a)Relations of weight changes to temperature of the 3D Hi-Nicalon/ PyC/SiC composite after oxidized for 900 min(b) Typical TG curves during the Fig. 7. Relations of residual strength to tures of the Hi-Nicalon/PyC/SiC first 400 min composite after oxidized for 900 min.SiC composite under higher magnification as shown in Fig. 2. Between the fiber and matrix a PyC interphase layer could be observed and gaps were scarcely observed either between the fiber and the interphase or between the matrix and the interphase. Moreover, some pores existed in the CVD SiC coating as shown in Fig. 3. As for fiber reinforced ceramic matrix composites, cracks were resulted from the mismatch of thermal expansion coefficient (CTE) between the composite and coating, and between the fibers and matrix, respectively. The CTE of Hi–Nicalon SiC fiber and SiC matrix was about 3.1∼3.5 × 10–6 /°C and 4.6 × 10–6 /°C, respectively [2,10,13]. Fig. 4 showed the relations of CTE to temperatures of the 3D Hi–Nicalon/ PyC/SiC composite and CVD SiC from 25 °C to 1400 °C. The relations of CTE to temperature of Hi–Nicalon/PyC/SiC composite have a similar tendency to those of CVD SiC and the difference between them was very little. Thus, a little tensile stress was produced in the SiC coating or matrix during cooling process and the thermal cracks were scarcely produced in them. Some matrix pores formed between the fiber bundles were open on the surface of the composite after machined. Thus, open pores could be observed in the SiC coating. 3.2. Oxidation behavior of the 3D Hi–Nicalon/PyC/SiC composite Fig. 5 showed the TGA results of the Hi–Nicalon/PyC/SiC composite after oxidation in the simulated air for 900 min. Below 1100 °C, oxidation of the composite led to a weight loss. Above 1100 °C, oxidation resulted in a weight gain in a parabolic–linear manner. Fig. 4. Relations of CTE to temperatures for the 3D Hi–Nicalon/PyC/SiC composite and CVD SiC. Fig. 5. (a) Relations of weight changes to temperature of the 3D Hi–Nicalon/ PyC/SiC composite after oxidized for 900 min (b) Typical TG curves during the first 400 min. Fig. 6. Morphology of nonuniform consumption of PyC phase in Hi–Nicalon/ PyC/SiC composite after oxidized for 900 min at : (a) 600 °C; (b) 900 °C. Fig. 7. Relations of residual strength to temperatures of the Hi–Nicalon/PyC/SiC composite after oxidized for 900 min. S. Wu et al. / Materials Letters 60 (2006) 3197–3201 3199