MATERIALS CHARACTERIZATION 59(2008)975-978 977 testing machine, with a cross-head speed of 0.5 mm/min and a span of 24 mm. For each test, five samples were tested. The composite microstructure was investigated by scanningelectron microscopy(SEM) of the polished cross sections. 3. Results And discussion 2 shows the cross-sectional micrographs of the fabricated Cd/SiC composite. As shown in Fig. 2(a), some isolated small -19 pores could be observed in the intra-bundle areas even after several infiltration-pyrolysis cycles. This is a commonly 0.0 06 observed phenomenon in the PIP-derived samples. These dispersed residual pores were ascribed to the shrinkage of the Displacement (mm) infiltrated PCS on pyrolysis and the difficulty for achieving Fig 3-Stress/displacement curves forthe fabricated composites the first cycle slurry infiltration. Generally, during PIP process with single-layer and multilayered PyC interphases the size and number of residual pores left in the inter-and cycles proceeded and thus hindering any further polymer Pycinterphases exhibited a relatively higher strength of 211 MPa, infiltration. When the residual pores were small enough, the about 12% higher than that of composites with a single pyc viscous PCS solution could not be effectively infiltrated into interphase by calculation. This phenomenon demonstrates that the consolidated body. It is advisable to stop the process at this the multilayered Pyc interphase was more resistant to fracture point In Fig. 2(b), SEM image of higher magnification shows behavior Typical stress-displacement curves derived from the and Sic interphase, as well as the derived matrix. As shown in bending test for the two composites are shown in Fig 3. It is g 2(b), the thickness of the deposited Pyc and sic interphase observed that the composites with multilayered Pyc inter were about 200 nm. No obvious circular cracks around the phase displays a higher strength but a slightly lower modulus fiber surface were observed, as commonly observed in the of elasticity which can be observed from the slope of the linear uncoated C/sic composite [7]. These cracks result from the stage in the stress-displacement curves For the two compo interphase helps to modify the thermal mismatch between value. This characteristic might be ascribed to the relatively the fiber and matrix. At the same time, the Al particles reacted loose matrix and weak interphase material, providing the with the carbon-containing volatile fragments from polymer lower elastic modulus. Even though the weak interphase decomposition and the reactive atmosphere, which led to material is beneficial for fiber pullout and crack bridging, it is formation of new phases such as AL,C3 and AIN. As reported by simultaneously detrimental for load transfer from the matrix Greil [8], a specific volume expansion of 53% and 26% can be to the fibers through such a weak interphase [9 For the achieved for conversion of active Al with gaseous species multilayered composites, a still weaker interfacial bonding ALC3 and AIN, respectively. Thus, the expansion of the was achieved, so a slightly lower slope was observed for the infiltrated Al particles during pyrolysis process will compel initial stage of linear deformation, demonstrating relatively sate for the volume shrinkage caused by polymer-to-ceramic lower elastic modulus. The matrix cracks might be deflected conversion and chances for micro-crack formation can be within the multilayered PyC interphase, and the shear stress extensively reduced. The derived inter-bundle matrix is in the crack tip can be reduced, which hampers the abrupt failure of the fiber bundles near the initial matrix cracks, at Table 1 lists the physical and mechanical properties of the two the same time, the lengthened cracks resulting from crack composites. Bulk densities of the two composites with multilay deflection have larger capability of deformation- energy ered and single Pyc interphases were 1.72 and 1.71 g/cm, sorptionin the fracture process. Therefore, the composite with respectively, indicating that PyC microstructure has no effect on multilayered Pyc interphase has a slightly higher bending the infiltration process and final density Bending test results at strength than that with a single one. Considerable failure room temperature suggest that the composites with multilayered displacement was achieved for both composites Table 1-Effect of interphase microstructure on the Conclusions physical and mechanical properties Interphase Density Porosity Bending Failure 2D C/SiC composites, with two kinds of Pyc interphases were 2)(%) strength displacement abricated by slurry infiltration and PIP process. The high magnification micrograph of the fiber boundary confirmed that a homogeneous deposition of PyC and Sic interphases 1.71±0.02 188±11 was achieved by the ICVI process. The microstructure of the 1.72±0.03 211±13 Pyc interphase has noticeable effect on the flexural strengthtesting machine, with a cross-head speed of 0.5 mm/min and a span of 24 mm. For each test, five samples were tested. The compositemicrostructure was investigated by scanning electron microscopy (SEM) of the polished cross sections. 3. Results And Discussion Fig. 2 shows the cross-sectional micrographs of the fabricated Cf/SiC composite. As shown in Fig. 2(a), some isolated small pores could be observed in the intra-bundle areas even after several infiltration-pyrolysis cycles. This is a commonly observed phenomenon in the PIP-derived samples. These dispersed residual pores were ascribed to the shrinkage of the infiltrated PCS on pyrolysis and the difficulty for achieving effective polymer infiltration after the matrix was formed in the first cycle slurry infiltration. Generally, during PIP process, the size and number of residual pores left in the inter-and intra-bundle areas would gradually decrease when the PIP cycles proceeded and thus hindering any further polymer infiltration. When the residual pores were small enough, the viscous PCS solution could not be effectively infiltrated into the consolidated body. It is advisable to stop the process at this point. In Fig. 2(b), SEM image of higher magnification shows the microstructure of the fiber boundary, the deposited PyC and SiC interphase, as well as the derived matrix. As shown in Fig. 2(b), the thickness of the deposited PyC and SiC interphase were about 200 nm. No obvious circular cracks around the fiber surface were observed, as commonly observed in the uncoated Cf/SiC composite [7]. These cracks result from the thermal expansion mismatch of the fiber and matrix. The PyC interphase helps to modify the thermal mismatch between the fiber and matrix. At the same time, the Al particles reacted with the carbon-containing volatile fragments from polymer decomposition and the reactive atmosphere, which led to formation of new phases such as Al4C3 and AlN. As reported by Greil [8], a specific volume expansion of 53% and 26% can be achieved for conversion of active Al with gaseous species to Al4C3 and AlN, respectively. Thus, the expansion of the infiltrated Al particles during pyrolysis process will compen￾sate for the volume shrinkage caused by polymer-to-ceramic conversion and chances for micro-crack formation can be extensively reduced. The derived inter-bundle matrix is shown in Fig. 2(c). No obvious cracks were observed. Table 1 lists the physical andmechanical properties of the two composites. Bulk densities of the two composites with multilay￾ered and single PyC interphases were 1.72 and 1.71 g/cm3 , respectively, indicating that PyC microstructure has no effect on the infiltration process and final density. Bending test results at room temperature suggest that the composites withmultilayered PyC interphases exhibited a relatively higher strength of 211 MPa, about 12% higher than that of composites with a single PyC interphase by calculation. This phenomenon demonstrates that the multilayered PyC interphase was more resistant to fracture behavior. Typical stress-displacement curves derived from the bending test for the two composites are shown in Fig. 3. It is observed that the composites with multilayered PyC inter￾phase displays a higher strength but a slightly lower modulus of elasticity which can be observed from the slope of the linear stage in the stress-displacement curves. For the two compo￾sites, bending strength decreased gradually after the peak value. This characteristic might be ascribed to the relatively loose matrix and weak interphase material, providing the lower elastic modulus. Even though the weak interphase material is beneficial for fiber pullout and crack bridging, it is simultaneously detrimental for load transfer from the matrix to the fibers through such a weak interphase [9]. For the multilayered composites, a still weaker interfacial bonding was achieved, so a slightly lower slope was observed for the initial stage of linear deformation, demonstrating relatively lower elastic modulus. The matrix cracks might be deflected within the multilayered PyC interphase, and the shear stress in the crack tip can be reduced, which hampers the abrupt failure of the fiber bundles near the initial matrix cracks. At the same time, the lengthened cracks resulting from crack deflection have larger capability of deformation-energy ab￾sorption in the fracture process. Therefore, the composite with multilayered PyC interphase has a slightly higher bending strength than that with a single one. Considerable failure displacement was achieved for both composites. 4. Conclusions 2D Cf/SiC composites, with two kinds of PyC interphases were fabricated by slurry infiltration and PIP process. The high magnification micrograph of the fiber boundary confirmed that a homogeneous deposition of PyC and SiC interphases was achieved by the ICVI process. The microstructure of the PyC interphase has noticeable effect on the flexural strength Fig. 3 – Stress/displacement curves for the fabricated composites with single-layer and multilayered PyC interphases. Table 1 – Effect of interphase microstructure on the physical and mechanical properties Interphase Density (g/cm3 ) Porosity (%) Bending strength (MPa) Failure displacement (mm) Single 1.71 ± 0.02 27 188 ± 11 0.28 Multiple 1.72 ± 0.03 27 211 ± 13 0.33 MATERIALS CHARACTERIZATION 59 (2008) 975 – 978 977