Microstructural characterization of spark plasma sintered boron carbide ceramics

Fully dense boron carbide specimens were fabricated by the spark plasma sintering (SPS) technology in the absence of any sintering additives. Densification starts at 1500 °C and the highest densification rate is reached at about 1900 °C. The microstructure of the ceramic sintered at 2200 °C, with heating rates in the 50–400 °C/min range, displays abnormal grain growth, while for a 600 °C/min heating rate a homogeneous distribution of finely equiaxed grains with 4.05 ± 1.62 μm average size was obtained. TEM analysis revealed the presence of W-based amorphous and of crystalline boron-rich B50N2 secondary phases at triple-junctions. No grain-boundary films were detected by HRTEM. The formation of a transient liquid alumino-silicate phase stands apparently behind the early stage of densification.