Key Engineering Materials Vols. 336-338

Abstract: In this paper, 3D Cf/Si-Ti-C-O composites were prepared with a polytitanocarbosilane (PTC) via polymer infiltration and pyrolysis (PIP) process, and accordingly the mechanical, oxidation resisting and thermal shock resisting properties were investigated. The composites with density of 1.93g·cm-3 show rather high flexural strength (485.3MPa) and fracture toughness (19.95MPa⋅m1/2), and typical non-brittle fracture failure mode. After oxidation treatment at 1300°C for 10 minutes in air, the flexural strength of the samples is 306.2MPa, with 63% strength retention. After 5 times of thermal shock tests from room temperature to 1300°C, the flexural strength of the samples is 408.9MPa, about 85% strength retention. SEM observation also testifies non-brittle failure because many fibers are pulled out on the fracture surface. After oxidation treatment, the matrix shows no changes from XRD measurement.

Abstract: 3D C/SiC composites were fabricated by polycarbosilane (PCS) infiltration and pyrolysis process. The influence of pretreatment of carbon fiber, including pyrolytic carbon coating and thermal treatment, on mechanical properties of C/SiC composites was investigated. The results showed that the composites without fiber pretreatment had a flexural strength of 154MPa and a fracture toughness of 4.8 MPa•m1/2, while those with carbon coating or thermal treatment had much higher strength and toughness, that is, more than 400MPa and 15MPa•m1/2 respectively. Weak interfacial bonding and better in-situ strength of carbon fiber were main reasons for mechanical property improvement with pretreated carbon fiber.

Abstract: A method of waved-thermal field chemical vapor infiltration was introduced. And interphases of silicon carbide layer and carbon layer were processed via the route. The preforms with the interfacial coatings were densified by method of forced-flow thermal-gradient chemical vapor infiltration (FCVI) employing hexamethyldisilazane (HMDS) as precursor material of the matrix. The matrix of the composites annealed at 1400°C consists of nano-polycrystalline silicon carbide. The configuration of fracture surface was observed by scanning electronic microscopy (SEM). The interphases behaved successfully as mechanical fuse for the reinforcing fibers.

Abstract: SR-249, a kind of polysiloxane (PSO), was used as the precursor for the first time to fabricate carbon fiber cloth reinforced silicon oxycarbide (2D Cf/Si-O-C) composites. The cure and pyrolysis of the SR-249 as well as the mechanical properties, oxidation and thermal shock behavior of the composites were investigated in the paper. The flexural strength and fracture toughness of the composites reached 217.6 MPa and 12.5 MPa·m1/2, respectively. After soaked at 1300°C under the static air for 10 min, the composites retained 58.5% flexural strength and 64.5% fracture toughness. The thermal shock behavior of the composites was studied by water quenched method. The composites retained 49.3% flexural strength and 47.4% fracture toughness after 10 times of quenching from 1200 to 20°C.

Abstract: Continuous SiC fiber reinforced SiC composites were fabricated by PIP route and their microstructures and mechanical properties were characterized. The density, flexural strength and fracture toughness of the composites were 1.88 g/cm3, 307 MPa and 8.24 MPa⋅m1/2, respectively. The low mechanical properties were ascribed to low density and strong interfacial bonding. The composites exhibited poor oxidation and thermal shock resistance; however, they showed desirable thermal stability.

Abstract: Chopped fiber and a hybrid reinforcement of chopped and continuous fibers were used for fabricating SiC/SiC composites. Under the selected sintering pressure, the composite sintered at lower temperature (1820°C) had lower density. Increasing temperature to 1850°C, the density of the composite reached at a higher level. However, pores still existed and mainly distributed in the areas the fibers accumulated, especially inside of the fiber bundles. Densely sintered matrix still could be found in the composite sintered at 1820°C, 15 MPa. In the areas with the fibers accumulated, matrix was relatively weak so that the cracks were easily propagated leading to the delamination during bending test. When continuous fiber was included into the chopped fiber reinforced composite, a hybrid reinforcing mechanism was obtained. This kind of composite had obviously improved toughness and strength. On the fracture surface, the pulled out fibers that were perpendicular to the fracture surface were increased.

Abstract: 2D Fiber Reinforced carbon-carbon composites (C/C) was successfully bonded to itself with Ti-Ni-Ti sandwich interlayer by transient liquid phase bonding at 1050°C for 20~100 min under axial pressure of 0.1 MPa. The microstructures of joints were investigated and bonding strengths were examined, fracture surfaces were characterized by SEM and XRD. The results show that Ti-Ni eutectic liquid infiltrates into the interconnected capillaries of C/C through the open pores of the C/C boding surface. The presents of the breakpoints in joint alloy and the reaction between Ti and C are beneficial to the improvement of bonding strength. The shear strength of joint is as high as 37.4 MPa.

Abstract: SiC whisker was produced through the reaction between carbon and silicon powders during the sintering of Al2O3-ZrO2-C materials. The thermodynamic conditions and growth mechanism were discussed. The results showed that rod-shaped and dumbbell-shaped SiC whiskers are formed, respectively. The main composition of the beads in dumbbell-shaped whisker is SiOX with a small amount of Na and Ca, but the rods are SiC. In addition, the dumbbell-shaped SiC whisker contributed less than straight rod-shaped SiC whisker to improving the physical properties of Al2O3-ZrO2-C materials.

Abstract: In this paper, the unidirectional SiC fiber-reinforced aluminum phosphates composites, in which the SiC fibers were heat-treated at different temperature, time and in different method, were prepared. The dielectric and mechanical properties of the composites were studied. The influences of heat treatment of SiC fiber on the properties of the SiC fiber-reinforced aluminum phosphates composite were investigated in detail. The flexural strength, relative dielectric constant and dielectric dissipation factor of the composite were measured. And the microstructure of the composite was characterized by SEM (scanning electronic microscope). The results show that heat treatment of SiC fiber has a great influence on mechanical and dielectric properties of the composite. The heat treatment decreases the dielectric constant and dielectric dissipation factor of the composite enormously. But at the same time, the heat treatment of the SiC fiber makes an unfavorably strongly bonded SiC fiber/aluminum phosphates matrix interface, which decreases the strength of the composite extraordinarily. And the composite displays a completely brittle failure behavior without fiber debonding and pulling out, which is detected by SEM.

Abstract: The morphologies and textures of the pyrolytic carbon matrix in 2D-C/C composites after graphitization were investigated by means of polarized light microscope (PLM) and high resolution transmission electron microscope (HRTEM). The microstructure parameters of the pyrolytic carbon matrix before and after graphitization were characterized with X-ray diffraction (XRD) technology. It was found that the interplanar distance of (002) planes (d002) of pyrolytic carbon matrix decreases, and the microcrystalline stack height (LC) increases after graphitization. Graphitization treatment resulted in a coarsening of the surface texture and in the formation of circumferential cracks within the matrix. The lattice fringes of the pyrolytic carbon matrix are continuous and longer in each domain and the (002) peak spot is smaller and more intense after graphitization.