July 2004 anufacturing SiC-Fiber-Reinforced SiC Matrix Composites 1207 50 um Fig. 2. Polished cross section of a 3-D composite processed with the Sic Fig. 4. Polished cross section of a 2-D composite processed with the SiC seven PIP cycles. The mage in Fig. 4, taken at low slurry infiltration at low magnification showing homogeneity of matrix magnification, indicates mogeneity of the densification densification (SEM). process and the almost absence of macroporosity of a 3-D composite processed SiC powder filtration step( CP-7 PIP specimen. Table D) es, presenting the same level of porosity. Moreover, the presence and grain size of SiC powder in the matrix affected (2) Thermal and Mechanical Properties neither the elastic modulus values of the posites, nor the bending strength, which is mainly influenced by the fiber content and 3-D composites: 2-D composites processed only by PIP had a and architecture. Two typical stress-deflection curves are illus. low thermal diffusivity in the thickness direction(<0.030 cm"/s) and significant strength retention after initial failure the corresponding thermal diffusivity values were doubled. The 3-D composites thermal diffusivity densified only by PIP was (3) High-Temperature Treatment significantly higher than 2-D composites because of the fraction of The specimens characterized by the highest thermal diffusivity fibers in the thickness direction. In this case. the SiC powder values were subjected to a further high temperatur infiltration step induced a further increase of thermal diffusivity up treatment(700C, 10 min) with the aim to induce polymer to-0.075cm2/s derived SiC matrix crystallization, and to investigate the effects of Concerning mechanical properties, MOR values of the polymer this treatment on structural, thermal, and mechanical properties. filtrated samples were improved from -400-480 MPa in both Polymer-derived SiC was subjected to the same treatment to 2-D and 3-D composites increasing PIP cycles from 7 to 14 follow the structural evolution of the material. X-ray diffraction because of the porosity decrease. Composites obtained by slurry attern evidenced a significant peak sharpening indicating growth infiltration and only 7 PIP cycles were characterized by a MOR of crystallite size up to 20-30 nm in agreement with the results 500 MPa)slightly higher than in samples processed with 14 PIP obtained by Berbon et al. In addition, the high-temperature treatment of the SiC from polymer pyrolysis increased the densit measured by He-picnometry, from 2.51 to 2.71 g/em and induced a further 5% mass loss. Composite density decreased following the high-temperature treatment with a concurrent porosity and pore subjected to two more PIP cycles with ing. Some specimens were before characterization. The main properties measured after the high-temperature treatment and after the two additional PIP cycles ollected in Table Ill The most significant effect of the high-temperature treatment was the increase in thermal diffusivity which almost doubled (and more than doubled for 3-D composites containing SiC fine powder). If high-thermal conductivity is of prime importance for material designers, polymer-derived SiC matrix crystallization seems unavoidable for PIP-processed composites Two additional PIP cycles seemed to be effective in regaining e initial density with a concurrent slight further increase in hermal conductivity. On the contrary, mechanical properties reouce d possibly indicating that Tyranno SA fibers or the interphase or the fiber-carbon interface are not completely um fected by the thermal treatment at 1700"C Figure 6 shows a polished cross section of a 3-D specimen processed with the SiC powder infiltration step, treated at high temperature and subjected to two additional PIP cycles: it evi Fig 3. Polished cross section of a 2-D composite processed only by PIP. dences dense matrix material in which the SiC powder is bound by