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

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.