Nickel–Boron Nanolayer-Coated Boron Carbide Pressureless Sintering

Sintering of pure B₄C and Ni₂B nanolayer-coated B₄C was studied from 1300° to 1600°C, with the holding time at the peak temperatures being 2 or 10 h. Compacts were made by uniaxial die compaction and combustion-driven compaction. Pure B₄C sample shows less sintering at all conditions. Ni₂B-coated B₄C sample shows more extensive densification, neck formation, and grain shape accommodation. The combustion driven compaction process accelerates sintering by offering higher green density to start with. The Ni₂B species on the B₄C particle surfaces melts into a nickel–boron-containing liquid phase during heating, remains as liquid during sintering, and then transforms into Ni₄B₃ and NiB during cooling. High-resolution composition analysis shows that there is no nickel diffusion into bulk B₄C during the sintering process. However, there is boron diffusion into the Ni₂B coating layer. Carbon diffusion cannot be directly measured but is believed to be a simultaneous process as boron diffusion. A multievent sintering process has been proposed to explain the observations.