Effect of particle dispersion on microstructure and strength of reaction-bonded silicon carbide

In a binary system of silicon carbide (SiC) and carbon black (C), various green bodies having different porosity and pore size distribution were prepared from suspensions with different dispersion conditions controlled by varying pH, and by the type and concentration of polyelectrolyte dispersant. The green bodies showing different green microstructures were fabricated by slip casting, and the reaction bonding process was then carried out on these green bodies at 1600 °C for 20 min under vacuum atmosphere using the infiltration process of molten silicon. SEM analysis was applied to investigate the effect of green microstructure on that of reaction-bonded silicon carbide (RBSC) and subsequently of which strength was measured. It was found that green microstructure was highly dependent on the dispersibility of starting powders, which influenced sintered microstructure as well as strength of RBSC. RBSC prepared from the well-dispersed suspension showed an average four-point flexure strength of 310±40 MPa, compared to the RBSC from the relatively agglomerated suspension showing that of 260±50 MPa. The bimodal microstructure, which is frequently seen in a typical RBSC, was not observed in the present study, indicating that the ratio between SiC and C was responsible for the formation of bimodal microstructure.