Key Engineering Materials Vols. 512-515

Abstract: The core-shell structure precursor powder was synthesized through coat-mix process, and a small amount of Al₂O₃, SiO₂ and Y₂O₃ composite additives were added. Porous silicon carbide ceramics were produced after molding, carbonization and sintering, and the crystalline phase, weight loss, porosity, bending strength, and thermal expansion coefficient were measured. The effect of aging pH value of coat-mix process on the characterization of porous SiC ceramics had been analyzed. The results indicate that the crosslink degree and curing rate of the phenolic resin is adjusted by changing the aging pH value, and the coating effect in the coat-mix process is improved. Thereby, the flexural strength, thermal expansion coefficient, porosity and pore distribution of the porous SiC ceramics could be adjusted according to the application requirements. When the pH value ranges from 3 to 5 or higher than 7, the phenolic resin shows a lower crosslink degree and char yield, while the ceramics show a higher porosity and a lower thermal expansion coefficient; and the flexural strength decreases as the aging environmental pH value increases.

Abstract: SiC-Si₃N₄ powders and modified SiC-based ceramic matrix composites (CMCs) were fabricated using polycarbosilane (PCS), divinylbenzene (DVB) and Si₃N₄ filler. Si₃N₄ was introduced into CMCs fabricated through polymer infiltration and pyrolysis (PIP) to lower down the carbon content by in-situ carbothermal reaction, which derived from pyrolyzed PCS-DVB. The oxidation resistance and three point bending strength of modified C/SiC composites were effectively enhanced. The phase composition, microstructure of SiC-Si₃N₄ powders and modified C/SiC composites were investigated by XRD, SEM and TEM.

Abstract: The dispersion of SiC nanopowders in ethanol solution was studied by sedimentation test, particle size measurement and TEM analysis. The dispersion behavior of SiC nanopowders in ethanol solution was strongly dependent on the pH values, types and amounts of dispersant. PEI was found to be effective for the dispersion of SiC nanopowders in ethanol solution. With the addition of PEI, the isoelectric points of SiC nanopowders in ethanol solution were at pH 9.5, and shift to pH 12.3. The stability of SiC suspension increased with the dispersant content increasing until reached 2.5 wt% PEI. The suitable pH value for the dispersion of SiC nanopowders should lower than 10.

Abstract: Amorphous Si–B–C–N ceramics synthesized by polymer thermolysis possess excellent chemical, thermal and mechanical stability at high temperature. Many attentions have been devoted to study the crystallization and stability of Si–B–C–N ceramics. The SiC nanocrystallite reinforced amorphous Si–B–C–N ceramics have been fabricated by pyrolysis of single-source precursors at high temperature. The heat-treatment at high temperature commonly impaired the thermal stability of the composites. Crack and pore limit the development of the composites. New processing routes that overcome those problems are desirable. Here we reported the thermolysis of binary-source precursors mixed by polycarbosilane and boron-modified polyvinylsilazanen to fabricate Si–B–C–N composites with relative densities up to 96%. Green bodies were obtained by compaction of precursors using warm pressing at different temperature. The obtained composites were characterized by Scanning Electron Microscope and X-ray Diffraction.

Abstract: Chemical vapor deposition (CVD) is an effective method of preparing silicon carbide whiskers or films and chemical vapor infiltration (CVI) can be successfully used as the preparation of SiC composites. In this paper, silicon carbides whiskers were firstly deposited on substrates of RB-SiC by CVD process and then silicon carbide composites were prepared by chemical vapor infiltration in the SiC whiskers in an upright chemical vapor deposition furnace of Φ150mm×450mm with methyltrichloride silicane (MTS) as precursor gas, H2 as carrier gas and Ar as dilute gas. The morphologies of the SiC whiskers grown on RB-SiC substrate and SiC composites infiltrated in SiC whiskers were determined by scanning electron microscope (SEM), and the crystalline phase of the final deposits were confirmed with X-ray diffractometry (XRD) As a result, the curly defects of whiskers decrease with the addition of dilute gas. And by chemical vapor infiltration in SiC whiskers the, SiC composites were successfully prepared. Finally the deposits were determined as β-SiC.

Abstract: With increasing of the speed of the train, the requirement of the performance of the braking friction materials is more and more higher. It is urgent to find new braking materials to satisfy the rigorous using environment. Cf/SiC has the broad application prospects when it is used as a new kind of braking material. On this article, in order to shorten preparation cycle and reduced production costs, short carbon fiber is substituted continuous carbon fiber felt. Cf/SiC brake materials were prepared by the way of molding compression-pressureless sintering with resin binder. The effect of short carbon fiber dispersion on bending properties of brake materials was researched. The experimental results show that the bending strength of Cf/SiC brake materials, which manufactured by dispersing short carbon fibers, is higher than the composites manufactured by using fiber bundle. When the distribution of carbon fiber in composites is in single fiber state, the interfacial strength between carbon fibers and matrix were increased. At the experimental condition, compared to the sample with fiber bundle, the bending strength of the specimen with dispersed fiber is increased 40.79%. The toughening mechanism of carbon fiber debonding and fiber pull were generated in the process of bending fracture. The fracture model of composites is pseudo-ductile type.

Abstract: PAN and Pitch short carbon fiber-reinforced SiC matrix composites were fabricated by using a melting infiltration technique. The microstructure of the composites was characterized by scanning electron microscopy. The flexural strength of the composites was measured at room and high temperatures. The thermal conducitivity of the composites were evaluated. Effects of fibers on mechanical properties and thermal conductivity were assessed. The experimental results showed that the silicification of fibers during siliconizing was prevented due to the presence of barrier layer on the surface of fiber. Also, flexural strength and thermal conductivity depended on fibers used and content of fibers.

Abstract: The conversion of the liquid polycabosilane (LPCS) into silicon carbide was investigated by IR, XRD, which indicated the feasibility of the transition from LPCS to SiC ceramics above 900°C. The FTIR spectra and XRD Pattern of the C_f/SiC composites show that the matrix deposited at 1200°C has silicon carbide structure with the crystallite size of β-SiC phase of about 41 nm, while the SiC phase is amorphous at 900°C. The carbon fiber reinforced silicon carbide composites (C_f/SiC) were hereby prepared at 900°C and 1200°C, through chemical liquid-vapor deposition (CLVD) process using LPCS as precursor. Flexural strength of 224 MPa for C_f/SiC specimen with density of 1.81g·cm^-3 was obtained after being prepared at 1200°C for 30 minutes. The load-deflection curve has shown that the fracture behavior of the C_f/SiC composites is a typical non-brittleness. The results indicate that the CLVD process has a great advantage and prospect to prepare C_f/SiC composites in future.

Abstract: A new double-layer lattice structure based on C/SiC composite material is described and being investigated as a means to increase the service temperature of thermal protection structure. The design incorporates a C/SiC double-layer sandwich comprising two pyramidal truss cores. The outer layer of the sandwich structure is the thermal protection layer, which can make the heat redistribute. The internal layer is the insulation layer, which can decrease the temperature of the hot components and increase their reliability. The temperature field of C/SiC lattice thermal protection structures with different geometrical parameters was calculated by the finite element software ANSYS. It is found that the thermal behavior of the double-layer lattice thermal structure is affected by the truss geometry, such as truss length and inclination angle. The thermal protection capacity of C/SiC lattice structure is analyzed and compared with the equivalent solid structure. The results indicated that C/SiC lattice thermal protection structure has lower density and better thermal protection property than the traditional thermal protection structures.

Abstract: SiC-TiB2 composite was prepared at 2298 K by using pressureless sintering. The microstructures and mechanical properties of the SiC-TiB2 composite were investigated. The SiC-TiB2 composite with 5 mass % B4C-C showed the optimized comprehensive properties. The volume density, hardness, flexural strength and fracture toughness of that were 3.43 g/cm3, 18.5 GPa, 353 MPa and 4.67 MPa•m1/2, respectively. The excessive B4C-C content might decrease the mechanical properties of the composite.