Enhanced Mechanical Properties of Nanocrystalline B4C-SiC Composites by in-situ High Pressure Reactive Sintering

A unique optimized of core-shell structural B4C nanopowder, sintering aid additive of Si, and high-pressure sintering technique has been used to process nanocrystalline B4C-SiC ceramics with enhanced mechanical properties. C-coated B4C nanopowder was initially uniformly mixed with micron Si of different content by ball-milling. B4C-SiC composites with a homogenous distribution of SiC in B4C matrix were subsequently obtained by sintering the mixed powders at 6 GPa and 1600 ºC. The added Si reacted with submicron amorphous carbon layer and amorphous carbon nanoshell to form dispersed SiC nanocrystals and Si-C phase filled at B4C grain boundaries and pores, respectively. The prepared composite had the most outstanding mechanical properties when the Si content in the precursor was 15 wt%, with a hardness reaching 37.8 GPa and a fracture toughness reaching 7.3 MPa·m1/2. Microstructural characterizations indicated that the multi deflection of nanoscale crack caused by intergranular fracture, the covalent bonding of Si-C phase at the grain boundary, and the abundant nanotwin substructure were jointly responsible for the superior performance in hardness and fracture toughness.