Effect Of Particle Size Distribution On Microstructure And Properties Of SiC/B₄C Composite

Boron carbide (B4C) ceramics possesses excellent physical and mechanical properties of high modulus and hardness, low density, good abrasion resistance, excellent to chemical agents as well as good neutron absorption ability. So it has been used in various industrial fields. However, the widespread application of B4C ceramics has been restricted because of the low fracture toughness and the poor sinterability due to a low self-diffusion coefficient. The optimum method to modify the properties is using sintering aids or second phases to B4C ceramics. Therefore, in the present study, based on the principle and process of reaction sintered SiC, we find that preparing reaction sintered SiC/B4C ceramics composites is a good idea. It will have a very important significance to develop the application of B4C ceramics. The grain size shows important influence on Si infiltration, density and properties of reaction sintered SiC/B4C composite. Therefore, this study was focused on the effect of particle size distribution of B4C on the phase compositions, microstructure, density and mechanical properties of reaction sintered SiC/B4C.The experimental results were shown as follows.(1) SiC/B4C ceramic composite has been successfully fabricated by Si infiltration into a porous preform containing B4C and C. The sintering temperature was at1350℃holding1hour in vacuum. XRD analyses showed that the phases of this composite consisted of B11.15C2.85, B12.97Si0.03C2, SiC and Si. During the infiltration process, SiC was formed by the reaction between Si and C; SiC, B11.15C2.85and B12.97Si0.03C2phases were detected resulted from the reaction between some B4C particles and Si. The content of residual Si could be controlled less than10wt%in the composite.(2) SiC and free Si filled in the continuous gaps among B4C grains. The rim-core structure could be observed in B4C grains with large particle size. Moreover, observed by TEM, dislocations were observed inside the B4C grain, and the reaction synthesized SiC was mainly in layer structure.(3) Based on the analysis of orthogonal experiments, it could be concluded that when the particle size distribution of B4C was7:1:2, the SiC/B4C showed the optimum properties. The open porosity, volume density, Vickers-hardness, bending strength and fracture toughness were0.114%,2.618g/cm3,25.6GPa,170.2MPa and3.60MPa·m1/2, respectively. It was found that the SiC/B4C ceramics composite showed inter/transgranular fracture mode.(4) According to the thermodynamic calculation results, the mechanism of Si infiltration process was determined. It could be concluded that during the process of Si infiltration, besides the reaction between Si and C, Si could also reacted with B4C particles.