S Tang et al /Materials Science and Engineering A 465(2007)290-294 Table 1 Weight and mole compositions of C and Si in the SiC matrix at the center and the edge of the cross-section of the C/SiC composites according to EPMA Point Edge C(wt %) Si(wt %) SiC(wt %) C(mol %) Si(mol%) C(wt %) Si(wt %) SiC(wt %) C(mol %) Si(mol %) 9839 52.33 0.11 47.57 52.24 51.87 69. 47.82 52.18 Average 753 48.14 51.86 920 Silicon-Ko Carbon.Ka Chlorine and nitrogen Fig 2 SEM micrograph of SiC growth morphology surrounding the carbon Fig. 3. Load and fracture toughness vs displacement curves for the C/Sic fiber and EDS analysis. to the EPMa are exhibited in Table 1. The deposit is reaching the maximum value for the stress, and then the stress stoichiometric or has a very slight silicon excess (Si to C gradually decreases with the displacement increasing. This indi- about 1.08-1.09)and the compositions are similar at the cates a damage-tolerant fracture behavior, which is also verified and at the edge by micrographs of the fracture surface(Fig 4). The fracture Fig 2 presents the PyC interphase and the Sic matrix sur- surface is jagged. There is obvious subsequent breaking of the rounding the carbon fiber and the eds result. It can be clearly whole fiber bundles because the Sic matrix is firstly deposited seen that a distinct PyCinterphase with a thickness about 1.3 um on the surface of the fibers and then on the surface of the fiber is present on the surface of the fiber and a large amount of bundles. These fiber bundles also have jagged surfaces with the Si accumulates in the PyC, gradually increasing from the fiber fiber pull-out and are not broken in one plane. In some areas, periphery to the PyC periphery. A radical magnetic attraction- many long pull-out fibers are observed, which once more points electric deposition mechanism has been proposed to explain the out the damage tolerance of the developed material. It should be rapid growth rate of the SiC matrix [14. The Si diffusion into emphasized that the majority of the pull-out fibers are smooth the PyC interphase should be attributed to the strong electro- and their diameters are about 7-8 um, proving the crack mainly magnetic affinity of the carbon fibers to the radicals. It should occurring at the interface between the fiber and the PyC. Fur be noted that the interface between the PyC and the fiber is ther, it can be obviously seen that in Fig. 4(c)that many holes smooth and reflect well the fiber shape, possibly revealing the and the Pyc interphases leave behind after the fibers are pulled weak interfacial bonding, whereas the interface between the Pyc out. It is well known that carbon fibers and SiC matrix exhibit and the SiC matrix is tortuous and uneven, probably indicating a siginificantly different CTEs. The CTEs for the fibers are a the strong bonding radial CTE (afr)of 7.0 x 10-6oC-I and a longitudinal CTE The mechanical test results are summarized in Table 2. The (an) of-(0. 1-1. 1)x10-6oC-I, and that of the Sic matrix is curves of load and fracture toughness versus displacement of 4. x 10-C(am)[16, 17]. Since afr >am, the fibers have a the C/SiC composites in the SENB tests are shown in Fig. 3. tendency to radically contract within the SiC matrix upon cool- The composites present a slight deviation from linearity before ing with de-cohesion between the fibers and the PyC interphase Density, porosity and mechanical properties of the C/SiC composites prepared by the HCVi process sity(g/cm) Porosity (%6) Flexural strength(MPa ulus(GPa) Compressive streng gth(MPa)Fracture toughness(MPama) 2.32±004 9.8±0.3 163±8 26.5±2.2 304±55(D276±92(1) 6.5±1.2292 S. Tang et al. / Materials Science and Engineering A 465 (2007) 290–294 Table 1 Weight and mole compositions of C and Si in the SiC matrix at the center and the edge of the cross-section of the C/SiC composites according to EPMA Point Center Edge C (wt.%) Si (wt.%) SiC (wt.%) C (mol.%) Si (mol.%) C (wt.%) Si (wt.%) SiC (wt.%) C (mol.%) Si (mol.%) 1 27.59 70.80 98.39 47.67 52.33 27.19 70.11 97.30 47.57 52.43 2 27.58 70.54 98.12 47.76 52.24 27.65 69.68 97.33 48.13 51.87 3 27.41 69.92 97.33 47.82 52.18 28.40 69.88 98.28 48.72 51.28 Average 27.53 70.42 97.95 47.75 52.25 28.75 69.89 97.64 48.14 51.86 Fig. 2. SEM micrograph of SiC growth morphology surrounding the carbon fiber and EDS analysis. to the EPMA are exhibited in Table 1. The deposit is near￾stoichiometric or has a very slight silicon excess (Si to C ratio, about 1.08–1.09) and the compositions are similar at the center and at the edge. Fig. 2 presents the PyC interphase and the SiC matrix sur￾rounding the carbon fiber and the EDS result. It can be clearly seen that a distinct PyC interphase with a thickness about 1.3 m is present on the surface of the fiber and a large amount of Si accumulates in the PyC, gradually increasing from the fiber periphery to the PyC periphery. A radical magnetic attraction￾electric deposition mechanism has been proposed to explain the rapid growth rate of the SiC matrix [14]. The Si diffusion into the PyC interphase should be attributed to the strong electro￾magnetic affinity of the carbon fibers to the radicals. It should be noted that the interface between the PyC and the fiber is smooth and reflect well the fiber shape, possibly revealing the weak interfacial bonding, whereas the interface between the PyC and the SiC matrix is tortuous and uneven, probably indicating the strong bonding. The mechanical test results are summarized in Table 2. The curves of load and fracture toughness versus displacement of the C/SiC composites in the SENB tests are shown in Fig. 3. The composites present a slight deviation from linearity before Fig. 3. Load and fracture toughness vs. displacement curves for the C/SiC composites. reaching the maximum value for the stress, and then the stress gradually decreases with the displacement increasing. This indi￾cates a damage-tolerant fracture behavior, which is also verified by micrographs of the fracture surface (Fig. 4). The fracture surface is jagged. There is obvious subsequent breaking of the whole fiber bundles because the SiC matrix is firstly deposited on the surface of the fibers and then on the surface of the fiber bundles. These fiber bundles also have jagged surfaces with the fiber pull-out and are not broken in one plane. In some areas, many long pull-out fibers are observed, which once more points out the damage tolerance of the developed material. It should be emphasized that the majority of the pull-out fibers are smooth and their diameters are about 7–8 m, proving the crack mainly occurring at the interface between the fiber and the PyC. Fur￾ther, it can be obviously seen that in Fig. 4(c) that many holes and the PyC interphases leave behind after the fibers are pulled out. It is well known that carbon fibers and SiC matrix exhibit a siginificantly different CTEs. The CTEs for the fibers are a radial CTE (αfr) of 7.0 × 10−6 ◦C−1 and a longitudinal CTE (αfl) of −(0.1–1.1) × 10−6 ◦C−1, and that of the SiC matrix is 4.8 × 10−6 ◦C−1(αm) [16,17]. Since αfr > αm, the fibers have a tendency to radically contract within the SiC matrix upon cool￾ing with de-cohesion between the fibers and the PyC interphase Table 2 Density, porosity and mechanical properties of the C/SiC composites prepared by the HCVI process Density (g/cm3) Porosity (%) Flexural strength (MPa) Modulus (GPa) Compressive strength (MPa) Fracture toughness (MPa m1/2) 2.32 ± 0.04 9.8 ± 0.3 163 ± 8 26.5 ± 2.2 304 ± 55(||) 276 ± 92(⊥) 6.5 ± 1.2