J Zhong et al. Journal of Materials Processing Technology 190(2007)358-362 Table I The main characters of HMDS and physical parameters of carbon fiber preform Molecular structural formula (CH3 )3SiNHSI(CH3)3 61×41×11 Purity (%o) 990 Colorless and transparent liquid Bulk density (g/cm. 0.596 Preform Tolal Boiling point°C) Non-toxicity (g) specimen 6 Fig. 1. Schematic diagram for three-point-bending test(all dimensions were in mm). as innermost layer, followed by a fine quartz layer with a thickness of 0. 1 mm, a disordered fiber zone(zone B) in Fig. 4. The morphology of arse quartz sand layer with a thickness of 2-3 mm at outermost, was set into magnified zone B was showed in Figs. 5 and 6 From Fig. 6, it the pyrolysis batch and the pyrolysis was carried out at 950C with a pressure of was observed that the thickness of pyrocarbon layer was about 0.1 MPa, N2 used as protective atmosphere. Then a pyrocarbon layer was formed on the surface of carbon fiber of the preform. The preform with pyrocarbon layer 1.5 um as immersed into the container filled with HMDS, and pyrolysis was perf The zone C whose fibers were normal to fracture surface ubsequently. The pyrolysis processing was the same as that of pitch except that appeared the features of brittle failure. Smooth fracture and lit- pyrolysis temperature was 850C. The sample was densified by repeating four tle fiber pull-out were observed(Fig. 4). The morphology of cycles of infiltration and pyrolysis of HMDS. The overall procedure could be magnified zone C was showed in Fig. 5. It could be seen that Length direction of bend specimens was parallel to one of continuous carbon the thickness of pyrocarbon interface was about 0.5 um, and the xx 3.5 mm. Schematic diagram boundary between pyrocarbon layer and matrix was not clear, for three-point-bending test was illustrated in Fig. 1. The span for bending test which might suggest the strong interfacial bonding was 60mm. ASTM C1341-97 recommended that span-to-depth ratio was 16: 1 Fig. 7 was the magnification of zone A in Fig 4. There were However, when the span-to-depth ratio was larger than 8: 1, the test results would a lot of micro-cracks being approximately vertical to fiber and a be satisfactory [7] few cracks being parallel to fiber. These cracks partially formed 0.5 mm/min on a vacuum super-high-temperature testing machine at room tem. in the manufacturing process, and partially derived from damage perature and 1300C Flexural strength o was calculated from process during bending test. At the manufacturing temperature there were no cracks generating in the composite. However, when the composite was cooled to room temperature, owing to mismatch of the thermal expansion between fiber and matrix, where P is the maximum load before failure of the sample. L the length of span, b the width of the sample, h is the thickness of sample. The mor- phology of the sample was observed in the SEM. The chemical composition and the phase structure of matrix were examined with the EDS and XRD, 3. Results and discussion 3.1. The structure of the sample Figs. 2 and 3 showed that the macrostructure of as-prepare material was dense and homogeneous with some pores whose sizes were less than 5 um between fibers and less than 100 um between the carbon fiber bundles, respectively Because the plates used as the preform were woven and punctured with two-dimensional orthogonal continuous car- bon fiber and short carbon fiber. the laminated structure was observed on the fracture, each layer appeared different frac- ture characteristic (Fig. 4). There was fiber pull-out on theJ. Zhong et al. / Journal of Materials Processing Technology 190 (2007) 358–362 359 Table 1 The main characters of HMDS and physical parameters of carbon fiber preform HMDS Molecular structural formula (CH3)3SiNHSi(CH3)3 Preform Physicaldimension (mm) 361 × 41 × 11 Purity (%) ≥99.0 Characterization Colorless and transparent liquid Bulk density (g/cm3) 0.596 Flash point (◦C) 27 Tolal mass (g) 97 Boiling point (◦C) 126 Toxicity Non-toxicity Fig. 1. Schematic diagram for three-point-bending test (all dimensions were in mm). as innermost layer, followed by a fine quartz layer with a thickness of 0.1 mm, a coarse quartz sand layer with a thickness of 2–3 mm at outermost, was set into the pyrolysis batch and the pyrolysis was carried out at 950 ◦C with a pressure of 0.1 MPa, N2 used as protective atmosphere. Then a pyrocarbon layer was formed on the surface of carbon fiber of the preform. The preform with pyrocarbon layer was immersed into the container filled with HMDS, and pyrolysis was performed subsequently. The pyrolysis processing was the same as that of pitch except that pyrolysis temperature was 850 ◦C. The sample was densified by repeating four cycles of infiltration and pyrolysis of HMDS. The overall procedure could be finished within several days. Length direction of bend specimens was parallel to one of continuous carbon bundles. The specimen size was 70 mm × 5mm × 3.5 mm. Schematic diagram for three-point-bending test was illustrated in Fig. 1. The span for bending test was 60 mm. ASTM C1341-97 recommended that span-to-depth ratio was 16:1. However, when the span-to-depth ratio was larger than 8:1, the test results would be satisfactory [7]. Three-point-bending strength was measured with loading velocity of 0.5 mm/min on a vacuum super-high-temperature testing machine at room tem￾perature and 1300 ◦C. Flexural strength σ was calculated from σ = 3PL 2bh2 where P is the maximum load before failure of the sample, L the length of span, b the width of the sample, h is the thickness of sample. The mor￾phology of the sample was observed in the SEM. The chemical composition and the phase structure of matrix were examined with the EDS and XRD, respectively. 3. Results and discussion 3.1. The structure of the samples Figs. 2 and 3 showed that the macrostructure of as-prepared material was dense and homogeneous with some pores whose sizes were less than 5m between fibers and less than 100m between the carbon fiber bundles, respectively. Because the plates used as the preform were woven and punctured with two-dimensional orthogonal continuous car￾bon fiber and short carbon fiber, the laminated structure was observed on the fracture, each layer appeared different frac￾ture characteristic (Fig. 4). There was fiber pull-out on the disordered fiber zone (zone B) in Fig. 4. The morphology of magnified zone B was showed in Figs. 5 and 6. From Fig. 6, it was observed that the thickness of pyrocarbon layer was about 1.5m. The zone C whose fibers were normal to fracture surface appeared the features of brittle failure. Smooth fracture and lit￾tle fiber pull-out were observed (Fig. 4). The morphology of magnified zone C was showed in Fig. 5. It could be seen that the thickness of pyrocarbon interface was about 0.5m, and the boundary between pyrocarbon layer and matrix was not clear, which might suggest the strong interfacial bonding. Fig. 7 was the magnification of zone A in Fig. 4. There were a lot of micro-cracks being approximately vertical to fiber and a few cracks being parallel to fiber. These cracks partially formed in the manufacturing process, and partially derived from damage process during bending test. At the manufacturing temperature there were no cracks generating in the composite. However, when the composite was cooled to room temperature, owing to mismatch of the thermal expansion between fiber and matrix, Fig. 2. Cross section of the C/C/SiC composite.