The. Gelcasting Reaction Sintered Silicon Carbide Preparation Process

The reaction-bonded silicon carbide (RBSC), which is widely used in metallurgy, architecture, chemistry and national defence, has excellent performance such as high temperature resistance, oxidation resistance, standing wear and tear, corrosion resistance, thermal shock resistance, high hardness and high heat conductivity. Presently, the traditional slip casting is introduced by most manufacturers of RBSC. As a result, the cracks of complicated products will occur because of density asymmetry of the slip cast bodies. Gel-casting is a new net-shape processing technology which is based on the slip casting technology and the theory of high polymer chemistry, and the slurry is solidified by the polyreaction of organic monomer. The bodies prepared by gel-casting are homogeneous, high dense and hard enough to machining. In this thesis, SiC/carbon black slurry with high solid loading and low viscosity was prepared successfully. The processing parameters of gel-casting were studied and the RBSC bodies with homogeneous density and high density were obtained. The mechanical properties of green bodies and sintered bodies were also tested and analyzed. Furthermore, processing and mechanical properties of short carbon fibers reinforced RBSC matrix composites prepared by gel-casting were discussed.This paper studied the effects of SiC particle size distribution, milling time, dispersant, pH value and the content of AM on the viscosity of suspension. A SiC/carbon black suspension with high solid loading (60vol%) and low viscosity was successfully prepared. The effect of vacuum-pumping time on the water absorption of body was also discussed and the result shows that the best vacuum-pumping time is 10-15 minutes. The optimal idle time of polyreaction and gel time can be determined by changing solid loading, amount of initiator and temperature to make sure the casting process can be done successfully.The research result shows that solid loading and the AM content are the key factors influencing the properties of green bodies and sintered bodies of RBSC. To some extent, the bending strength of bodies goes up with the increasing of AM content. The bending strength of the green body is 25MPa when the solid loading is 55vol% and the amount of AM is 4.0wt% of total powders, while the bending strength of bodies will fall down when AM content keep on increasing. Meanwhile, the bending strength and density of RBSC slightly increase with the increasing of AM content. The shrinkage of green bodies is reduced, and by improving the solid loading the density of green bodies and sintered bodies can be increased. The bending strength of green bodies and sintered bodies are as high as 27MPa and 480MPa respectively when the solid loading is 58vol% and the AM content is 3.5wt%. Therefore, the bodies prepared by gel-casting are hard enough to machining. The microscopic examination shows the green body and sintered sample are homogeneous and dense. Fine newly formedβ-SiC can be found around theα-SiC. The principle phases of RBSC are SiC and Si, which were investigated by XRD and the un-reacted carbon black was not found.The preparation process for short carbon fibers reinforced RBSC matrix composites was initially studied, which is based on the gel-casting of RBSC. In this study, the effect of amount of carbon fibers on properties of RBSC was discussed. The results show that, to some extent, both bending strength and fracture toughness can be improved with the introduction of short carbon fibers. When the solid loading is 55vol%, bending strength of composite is 475MPa with 4vol% carbon fibers content and the highest fracture toughness value is 4.67MPa·m^1/2 with 6vol% carbon fibers content, which is improved 30% in comparison to the RBSC. It can be observed from the SEM images that short carbon fibers distribute randomly in the RBSC matrix and the main reinforcement mechanism is the pull out and bridging of short fibers.