Reactive spark plasma sintering of diamond-B4C-SiC composites: Effect of diamond size on microstructural evolution

Abstract

Diamond-B₄C-SiC composites were synthesized at 1600 °C via reactive spark plasma sintering of SiB₆ and 30 wt% diamond powder blends. This processing route leverages the surface graphitization of metastable diamond to provide a free carbon source for the in-situ formation of SiC and B₁₂ (C,Si,B)₃ reaction products. The investigation focused on the role of diamond particle size (D₅₀ = 2.8, 12.4, 32.4, and 188.8 μm) in governing phase evolution, densification and porosity, and residual thermal cracking. Finer diamond particles (≤12.4 μm) promoted nearly complete consumption of the SiB₆ matrix due to high surface area and short inter-particle distances but resulted in elevated porosity (~4%). This behavior is attributed to rapid reaction-driven shrinkage and limited matrix flow. Conversely, the 188.8 μm diamond composite exhibited limited reaction formation and developed through-thickness macro-cracks upon cooling due to severe thermal-expansion mismatch between diamond and the SiB₆-rich matrix, which drives a size-dependent stress intensity factor that exceeds the matrix fracture toughness in coarse-grained microstructures. Optimal densification was identified for the 32.4 μm diamond composite that exhibits 0.43% porosity due to the larger inter-particle distances and increased matrix flow. This composite also fully transformed SiB₆ into B₁₂(C,Si,B)₃ and SiC with negligible residual Si.