Polymer-derived SiC ceramics from polycarbosilane/boron mixtures densified by SPS

Spark plasma sintering (SPS) is considered as an efficient method for the densification of advanced ceramics. Since commercially available prepyrolized polycarbosilane could not be densified by SPS even at 2050 °C, the addition of amorphous boron was investigated. Aiming to control dispersion of boron in the powder mixture, the preceramic polymer was mixed either before or after pyrolysis. Boron worked as a sintering additive resulting in dense monoliths, with a good combination of Vickers hardness and indentation fracture toughness. Toughening mechanisms were mainly attributed to crack deflection of finely distributed boron carbide phase. The boron mixing before pyrolysis resulted in optimum dispersion of B-rich secondary phase which resulted in Vickers hardness as high as 25.8 GPa and fracture toughness of 4.7 MPa m0.5.