Papers by Author: Xiang Yu Zhang

Abstract: C/SiC composites were prepared by liquid silicon infiltration process, and their microstructure, mechanical and tribological properties were studied in this paper. The results showed that the composites demonstrated non-catastrophic fracture behavior with long fibers pulled out and about 115 MPa flexural strength. The stability of coefficient of friction (COF) was improved and wear rate demonstrated a linear variation with the increasing of rotation speeds. However, wear rate exhibited a higher value (~3.6 μg/m) under a higher load (~150 N), even though the COF curves for C/SiC composites showed no significant changes.

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: The microstructure and microwave dielectric properties of inhomogeneous SiC-AlN solid solutions, hot pressed under a pressure of 40 MPa, have been investigated. Microstructure analysis detected the existence of BN in the resulted samples, which may be a reason that homogeneous solid solution was not formed for the samples with AlN content higher than 20%. The dc resistivity of the solid solutions at room temperature varies from 8 × 103 Ω m to 1 × 109 Ω m. Ion jump and dipole relaxation losses are the main mechanism of dielectric losses.

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.