Papers by Author: Li Tong Zhang

Abstract: Three types of three-dimensional stitched carbon fiber reinforced silicon carbide composites (3DS C/SiCs) with 4, 9 and 16 Z-yarn/cm² were fabricated by chemical vapor infiltration, respectively. Both iosipescu specimens without enough representative volume elements (RVEs) and ±45° tension specimens containing enough RVEs were tested to attain the in-plane shear properties of 3DS C/SiCs. The results showed that these two methods produced similar results. This demonstrated that the dimensions of iosipescu specimens were not necessary to cover enough RVEs for 3DS ceramic matrix composite in-plane shear testing. With respect to the shear modulus, both two methods would be regarded as the effective methods. However, in terms of the in-plane shear strength, iosipescu method would be a more reliable method.

Abstract: Si₃N₄/SiBCN composite ceramics were prepared by infiltrating and pyrolyzing liquid polyborosilazane in porous Si₃N₄ ceramics. To increase their wave-absorbing ability, SiC nanoparticles and SiC film obtained by chemical vapor infiltration were separately introduced into the composite ceramics. The surface morphology, element and phase composition of ceramics were analyzed by means of scanning electron microscopy, energy dispersive spectrometer and X-ray diffraction. Dielectric and electromagnetic wave absorbing property researches show that the permittivity and dielectric loss of the ceramics were effectively improved and the electromagnetic reflection coefficient was visibly decreased when SiC was loaded. It is indicated that SiC is an effective dielectric lossy absorbent, and the Si₃N₄/SiBCN composite ceramics containing SiC possess the great potential in the application of wave-absorbing material.

Abstract: Carbon fiber reinforced silicon carbide matrix composites (C/SiC) are promising candidate materials for high-temperature structural applications. However, in oxidizing environments the two main constituents, that is, carbon fiber and pyrolytic carbon interphase which bears and transfers loads respectively are susceptible to deplete rapidly for oxidation. In this paper, the oxidation behavior of carbon fiber and pyrolytic carbon were investigated by simulating environmental experiments and scanning electron microscopy. The reactivity discrepancy in the carbonaceous constituents and in the different zone of carbon fiber was discerned. After oxidation, the morphology of carbon phase broken before oxidation were compared with that of those broken after oxidation. Based on the microstructural model, the contrast results of morphology were well interpreted from the reactive preference and selectivity.

Abstract: Materials characterization is a crucial issue in the development and application of new materials. Materials characterization aims to mine and acquire characteristic information and their evolution in the materials. It mainly includes three important topics which are microstructural characterization, properties characterization, and environmental degradation. In this paper, characterization techniques about these topics were discussed for C/SiC composites and a characterization system was preliminarily established. All these characterization research and their results further the better understanding of the relationship between microstructure and properties and of the failure mechanisms in the C/SiC composites.

Abstract: Three-dimensional (3D) carbon fiber reinforced silicon carbide matrix composites (C/SiC) were prepared by a low-pressure chemical vapor infiltration method. The thermal shock behaviors of the composites in different environments were researched using an advanced acoustic emission (AE) system. Damage initiation and propagation were easily detected and evaluated by AE. The thermal shock damage to C/SiC composites mainly occurred at the process of cooling and was limited at argon but unlimited at wet oxygen atmosphere. Also correlations have been established between the different damage mechanisms and the characteristics of acoustic emission signals obtained during thermal shock tests. In this way, the paper contributes to the development of the acoustic emission technique for monitoring of damage development in ceramic-matrix composites.

Abstract: Internal friction of 2D C/SiC composites fabricated by chemical vapor infiltration (CVI) method was measured by dynamical mechanical analysis (DMA) at different frequencies from room temperature (RT) to 400°C in air atmosphere. Internal friction of 2D C/SiC composites increased gradually with increasing temperature and then decreased after damping peak appeared in the temperature range of 250°C to 300°C. Damping capacity and peak value decreased gradually with increasing frequency, accompanied with a shift of damping peak towards lower temperatures. Moreover, the effect of interphase thickness on damping behavior of 2D C/SiC composites was investigated. The results showed that damping peak of the composites increased gradually and the temperature of the peak shifted to the lower temperature with increasing PyC interphase thickness, when the interphase thickness is in the range of 90~296nm. The influence of interphase thickness on interfacial bonding strength, sliding resistance and the microstructure of SiC matrix was discussed, which was considered to be responsible for the results.

Abstract: Nano-yttria powder can be synthesized by yttrium citrate-urea precursor, combusted at 600°C in air. The CVD SiC coated on graphite (CVD SiC/Graphite) infiltrated by the yttrium citrate-urea precursor, combusted at of 600°C, and then sintered at 1450°C, the thin yttria film can be achieved. The SEM morphology and EDS result of the thin yttria film show a mass of needle-shaped pining into the CVD SiC layer, which improves the combination of CVD SiC layer and wash yttria coating. Therefore, it is an effective transition layer between CVD SiC coating and wash yttria layer.

Abstract: The conventional ultimate performance test by applying a component in its true application (i.e., in an engine) is often very expensive and impractical when dealing with developmental materials. Simpler, less expensive, and more practical test methods must be utilized. The present work aims toward the applications of an innovative methodology for testing environmental performance of advanced Ceramic Matrix Composites (CMCs) in the presence of combined mechanical, thermal, and environmental applied conditions. To obtain a comprehensive understanding of how a composite might perform in certain application environments, a newly developed environmental performance testing system, which is able to provide the fundamental damage information of the composites in simulating service environments including variables such as temperature, mechanical and thermal stresses, flowing oxidizing gases and high gas pressure, is proposed. The system comprises of two subsystems: (1) equivalent experimental simulating subsystem, and (2) wind tunnel experimental simulating subsystem. The evolution mechanisms of the composites properties and microstructures can be achieved by the former, and then be validated and modified by the latter. Various loading (e.g. fatigue, creep), various atmospheres (e.g. argon, oxygen, water vapor, wet oxygen and molten salt vapor) and various temperature conditions (e.g. constant or cyclic temperatures) can be conducted on the system. Some typical experimental results are presented in this paper. Large quantities of tests have demonstrated the extraordinary stability and reliability of the system.