Reaction Sintering Of Boron Carbide / Silicon Carbide Composites

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. Although the performances of RBSC ceramics are good,the mechanical properties of RBSC such as fracture toughness and hardness are still inferior to those of conventional sintered products due to free silicon in the final products.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,the reaction sintering B₄C/SiC composites were prepared by gelcasting with the addition of boron carbide.B₄C/C/SiC slurry with high solid loading and low viscosity was prepared successfully.The processing parameters of gel-casting were studied and the reaction bonded B₄C/SiC ceramics with homogeneous density and high density were obtained. The mechanical properties of green bodies and sintered bodies were also tested and analyzed.This paper studied the effects of SiC particle size distribution,B₄C content,milling time,pH value and solid loading on the viscosity of suspension.A B₄C/C/SiC suspension with high solid loading(55vol%)and low viscosity was successfully prepared when the B₄C content is 20wt%.The optimal idle time of polyreaction and gel time can be determined by changing initiator content,temperature and acetylacetone content to make sure the casting process can be done successfully.The results show that that the shrinkage of green bodies is reduced with the increasing of solid loading and B₄C content.The bending strength of the green body is 24.3MPa when the solid loading is 55vol%.The density of sintered bodies is reduced by improving B₄C content.To some extent,fracture toughness can be improved increasing with the content of B₄C.When the content of B₄C is 10wt%,the maximum fracture toughness of composite is 5.07MPa·m^1/2.The hardness of sintered bodies increases with the increasing of B₄C content.The hardness is as high as 94.5HRA when the content of B₄C is 20wt%.The microscopic examination shows the green body and sintered sample are homogeneous and dense.The principle phases of RBSC are SiC and Si,which were investigated by XRD and the un-reacted carbon black was not found.