Shouhong Tan

Preparation of silicon carbide reticulated porous ceramics

Commercial polyurethane sponges with cell size of ≈11 pores per inch were chosen to produce SiC-based reticulated porous ceramic (RPC) by the replication process. The dispersion of mixed powders (SiC, Al2O3, bentonite) in aqueous media using silica sol as a binder was studied by zeta potential and viscosity/rheological measurements. The pH value of the optimum dispersion was found to be around pH 10 for the mixtures. The optimum condition of the slurry suitable for impregnating the polymeric sponge was obtained. Effects of coating parameters including the separation between the rollers and the number of passes on the coating quality and macrostructure of RPCs were studied in detail. Flexural strength data of the sintered bodies were compared to a theoretical model. A statistical evaluation was carried out on the flexural strength data of the sintered bodies using Weibull statistics.

The control of slurry rheology in the processing of reticulated porous ceramics

In this work commercial polyurethane sponges with open porosity of approximately 13 pores per inch were chosen as the templates to produce the SiC RPCs. Effects of the rheological behavior of the slurry on the coating quality and the properties of SiC RPCs such as strength, density, microstructure, were investigated in detail. The coating quality was found to depend strongly on the slurry viscosity and this was improved dramatically extremely as the viscosity increased. The SiC RPCs displayed a remarkable increase in the flexural strength as the solids content increased. In addition, the increase of the polymer content had also a little contribution to the improvement of the strength The optimum solids content and polymer content were found to be 80 and 0.20 wt.%. This study shows that the control of slurry rheology is very important for the processing of RPCs.

Improvement in the strength of reticulated porous ceramics by vacuum degassing

SiC reticulated porous ceramics (RPCs) were fabricated from ceramic slurries by the replication process with polyurethane sponges as the templates. Vacuum degassing was adopted to remove air bubbles in the slurry. Effect of degassing on the rheological behavior of the slurry and the properties of RPCs was investigated. The slurry showed a slight increase in viscosity after degassing, which improved the loading of the slurry on the sponge. The large defects in the struts were significantly reduced by degassing, which was confirmed by scanning electron microscopy (SEM) observation and Hg porosimetry. Flexural strength of RPCs increases from 2.34 to 3.18 MPa after degassing. The results were discussed in terms of an available theoretical model on the mechanical behavior of open cell ceramics.

Preparation and characterization of nano-structured monolithic SiC and Si3N4/SiC composite by hot isostatic pressing

Abstracts are not published in this journal

Surface nitridation of Al2O3 based composite by N2-HIP post-treatment

Surface nitridation of Alumina based composites reinforced with silicon carbide particles and/or whiskers has been studied. The composites processed by hot-pressing were post-treated by HIP process at 1650–1750°C under 150 MPa of nitrogen gas pressure. X-ray diffraction (XRD) analysis indicates that Alumina and silicon carbide on the surface of the composite are converted to aluminum nitride and silicon nitride, respectively, during the post-treatment. Examinations of surface nitrided layer by scanning electron microscopy (SEM) suggest that grain size can be significantly affected by post-treatment condition. Flexure tests indicate that strength increases significantly by the post-treatment. It is discussed that the improvement of mechanical properties included two parts: one came from the densification of sample, the other came from the surface nitrided layer. Specially, the residual compressive stress plays a key role on the improvement of the flexural strength.

Hot isostatic pressing and post-hot isostatic pressing of SiC-β-sialon composites

Abstract SiC-β-sialon two-phase composites with various compositions were formed by hot isostatic pressing (HIP) and treated by post-hot isostatic pressing (post-HIP). The bending strength of the HIP specimens reached the highest values when the Z-value (Si 6−Z Al Z O Z N 8−Z ) of the second phase, β-sialon, was 2.0, while the highest values of hardness were reached at Z = 1.5. Scanning electron microscopy (SEM) observations of the fracture surface revealed that a fine and homogeneous microstructure was obtained by HIP. Surface nitriding of the HIP specimens by post-HIP in a N 2 atmosphere resulted in significant improvement in the bending strength. The maximum value of bending strength reached 950 MPa at Z = 1.0 for β-sialon. The formation of a dense microstructure, with rod-like grains in the surface layer, and other strengthening mechanisms may be responsible for the improvement.

Preparation of nanocrystal SiC powder by chemical vapour deposition

Nanosized silicon carbide powders of high purity and low oxygen content have been prepared by thermal chemical vapour deposition (CVD) of dimethyldichlorosilane at pyrolytic temperatures, 1100–1400 °C. The nanosized silicon carbide particles prepared at 1400 °C consist of small crystallites of β-SiC arranged randomly in the particles. At pyrolytic temperature below 1300 °C, the particles consist of amorphous phase and β-type SiC crystallites. The average particle size changed from 70 nm to 40 nm and the average size of the β-SiC crystallite changed from 7.3 nm to 1.8 nm depending on the pyrolysis conditions. The C/Si molar ratios of the product powders changed from 0.5 to 1.07 with the CVD conditions. The near theoretical values of C/Si molar ratio of the product powders within 0.95–1.05 can be controlled by CVD conditions such as pyrolytic temperature and reactant concentration. Finally, the product powders were characterized by chemical analysis, X-ray diffraction, electron microscopy, and infrared spectroscopy.