Papers by Author: Shao Ming Dong

Abstract: C_f/SiC composites were fabricated through in situ growth of carbon nanotubes (CNTs) on three-dimensional needle-punched carbon fabric via chemical vapor deposition and polymer impregnation and pyrolysis process. The mechanical and thermal properties of the composites were investigated. The flexural strength and fracture toughness were decreased due to the fiber volume fraction loss and much shorter pull-out length of fibers which was caused by the higher interfacial bonding strength between fiber and matrix after the growth of CNTs. Brittle fracture character of CNTs was observed due to the strong interfacial bonding strength between CNTs and matrix. The parallel thermal conductivity and perpendicular thermal conductivity were improved to 14.5% and 8.0% respectively.

Abstract: Because of its combined characteristics of metals and ceramics, such as low density, high Young’s modulus, thermal and chemical resistance with low hardness, high electrical and thermal conductivity, it was expected that the introduction of Ti3SiC2 to fiber reinforced ceramic matrix can make the composite own some unique properties. In the present research, Ti3SiC2 powders used as inert fillers were fabricated by the in-situ reaction between Ti and polycarbosilane mixtures. The purity of Ti3SiC2 powders analyzed by XRD was determined by RIR method, which is a semi-quantitative XRD analysis. The results showed that the purity of Ti3SiC2 powders is about 96%. Cf/Ti3SiC 2-SiC composites are obtained by polymer infiltration and pyrolysis process using Ti3SiC2 powders as the inert fillers. The bending strength of Cf/Ti3SiC2-SiC composites was about 160 MPa.

Abstract: ZrC-SiC powders were fabricated by means of liquid precursor conversion method, using Zr containing polymer precursor and polycarbosilane. The effects of staring reagents and the pyrolysis temperature on the fabrication of ZrC-SiC powders were studied. Results show that ZrC-SiC powders with different ZrC/SiC ratio could be formed when the staring reagents were different. Pyrolysis temperature affects the pyrolysed product. When temperature was lower, less amount of ZrC was formed in the powder. The size of crystallite and morphology of the synthesized powders were characterized by transmission electron microscopy and scanning electron microscopy.

Abstract: SiC/SiC composites were fabricated by hot pressing (HP) via liquid phase sintering (LPS) using carbon coated 2D woven Tyranno SA fabrics as reinforcement. Both nano-SiC and micro-SiC powders with sintering additives were used for matrix. The effects of preparation conditions on the microstructure and mechanical properties of the composites were characterized. Highly densified composite was obtained at 1780°C under 20MPa with nano-SiC particles. The strength and elastic modulus of the composite were enhanced. When micro-SiC powder was used, higher strength revealed for the composite sintered at 1780°C under 15MPa, although it was not densified enough. Higher sintering temperature (1800°C) is beneficial for the densification of the composite, but is not obvious for the improvement of mechanical properties.

Abstract: To obtain high performance ceramic matrix composites (CMCs), fiber coatings are often fabricated as the interphase between fiber and matrix. The SiC coating was synthesized at low temperature and reduced pressure in the present experiment. SiC was derived from a gaseous methyltrichlorosilane (MTS)/H2 precursor by chemical vapor infiltration (CVI). The thickness of the coating was inspected by SEM. The correlation between the coating thickness and the depositing conditions, i.e. the deposition temperature, the pressure, the deposition time per pulse and the pulse number were investigated. Based on these work, the C/SiC double-layer coating was fabricated.

Abstract: 3-D braided C fiber preform was used to reinforce SiC matrix by polymer infiltration and pyrolysis (PIP). The effect of PCS pyrolysis process on the uncoated carbon fiber was studied. During the pyrolysis, amorphous SiCxOy and some free silicon yielded. The Si element diffused into the C fiber from the matrix because of the concentration gradient at high temperature and destroyed the intrinsic structure of the uncoated C fiber. At the same time, the free Si reacted with the uncoated C fiber. Thus, strong bonding between the fiber and matrix was formed. As a result, bending strength of the composite was decreased.

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: 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: Ultra-fine titanium diboride (TiB2) powders have been prepared by carbothermal reduction reactions in TiO2-B2O3-C system using tetrabutyl titanate, boric acid and phenolic resin as the solution-derived precursors. The reactions were substantially completed at relatively lower temperature (<1400°C) and the resulting products had a smaller average crystallite size (< 200 nm). However, below 1100°C, titanium carbide was the predominant phase and the relative content decreased with the rise of temperature. The thermodynamic change in TiO2-B2O3-C system was mainly studied by TG-DTA and the mechanism of synthesis of TiB2 was discussed. The crystallite size and morphology of the synthesized powders were characterized by SEM and TEM.