Diffusion-controlled microstructure evolution during the reaction of liquid silicon with B₄C

The reaction mechanism between B₄C and liquid silicon was systematically studied in this work. By controlling the infiltration time of liquid silicon, the evolution of phase composition and microstructure was elucidated. The resulting ceramics comprised primarily the ternary phase Si-containing B₄C-type phase, SiC, SiB_x, residual Si, and minor impurity phases. Unlike conventional Si activated systems dominated by rapid interfacial reactions, the Si-B₄C system exhibits a diffusion-controlled transformation. Liquid Si diffuses into the B₄C lattice and substitutes for carbon to form Si-containing B₄C-type phase, while the released carbon reacts with Si to produce SiC. The microstructural evolution exhibits two distinctive features, with Si-containing B₄C-type phase grains first coarsening and then refining, and SiC transforming from large, anisotropic plate-like crystals into fine, equiaxed grains. These transformations are governed by coupled diffusion, dissolution-reprecipitation, and cooling-driven solidification processes. The sample prepared with a holding time of 20 min exhibits the highest flexural strength of 228 ± 6 MPa, attributed to the larger grain size of Si-containing B₄C-type phase and improved interfacial bonding between Si-containing B₄C-type phase and residual Si.