Toughening of super-hard ultra-fine grained B4C densified by spark-plasma sintering via SiC addition

Toughening of super-hard B4C ceramics with ultra-fine grained microstructures via the addition of SiC (15 wt.%) or the simultaneous addition of SiC (15 wt.%) and graphite (2 wt.%) is reported. The ultra-fine grained B4C–SiC and B4C–SiC–C composites prepared by spark-plasma sintering from powder mixtures subjected to high-energy co-ball-milling are found to be remarkably tougher (i.e., ∼65% and 50%) than the pure B4C ceramic with a coarsened microstructure. Crack bridging by the homogenously dispersed SiC grains can give an explanation for the improvement in toughness. Also, the addition of SiC to the B4C matrix was found to change the fracture mode from purely transgranular to a mixture of intergranular and transgranular fracture. This is derived from the weakness of the B4C–SiC interfaces due to the existence of residual thermo-elastic stresses. It was also found that despite SiC is softer than B4C, the B4C–SiC are yet extremely hard if densified appropriately, with the hardness even reaching 36 GPa.