Densification of carbon-rich boron carbide nanopowder compacts

The densification behavior of 20–40-nm graphite-coated B₄C nano-particles was studied using dilatometry, x-ray diffraction, and electron microscopy. The sintering onset temperature was higher than expected from a nanoscale powder (∼1500 °C); remnant B₂O₃ kept particles separated until B₂O₃ volatilization, and the graphite coatings imposed particle-to-particle contact of a substance more refractory than B₄C. Solid-state sintering (1500–1850 °C) was followed by a substantial slowing of contraction rate attributed to the formation of eutectic liquid droplets more than 10× the size of the original nano-particles. These droplets were induced to form well below the B₄C-graphite eutectic temperature by the high surface energy of nanoparticles. They were interpreted to have quickly solidified to form a vast number of voids in particle packing, which in turn, impeded detection of continued solid-state sintering. Starting at 2200 °C, a permanent and interconnected liquid phase formed, which facilitated rapid contraction by liquid phase sintering and/or compact slumping.