Papers by Author: Qing Zhou

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: 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: Continuous carbon fiber-reinforced silicon carbide (Cf/SiC) composite was fabricated by hot-pressing, via liquid phase sintering. Sintering conditions strongly affect the densification process, and therefore dominate the mechanical properties and fracture behavior. The composites under the lower sintering temperature behaves less densified matrix and it demonstrates a relatively weak fiber/matrix bonding allowing the longer fibers pull-out. Increasing sintering temperature could accelerate the densified matrix and make fiber/matrix bonding stronger. In this case, the shorter fibers pull-out was predominant fracture behavior and it could improve mechanical properties.

Abstract: Hot pressing is an effective way to densify powder compacts, especially during the inclusion of a second phase, such as particles, whiskers or fibers. In the present study, SiC/SiC and C/SiC composites were prepared by hot pressing via liquid phase sintering. Nano-SiC powder was used for matrix formation with sintering additives. The effects of preparation conditions such as sintering temperature, pressure and matrix composition, on the microstructural evolution and mechanical behaviors were discussed. Using micro-SiC powder and nano-SiC powder for matrix formation, the interaction between fiber and matrix was characterized. Because the matrix compositions can be easily modified in the CMC-processing from powders by hot pressing, the SiC-BN matrix was also studied in the present experiment. The inclusion of BN can either improve the machinability or provide oxidation resistance to the composite. BN was derived through an in-situ reaction between boron acid and urea by hot-pressing. Boric acid and urea were solved into the ethanol and mixed with nano-SiC particles, and then infiltrated into the fiber bundles. Correlations among microstructures, properties and compositions will be discussed.