Nickel-Boron Nanolayer-Coated Boron Carbide Pressureless Sintering

Sintering of pure B4C and Ni2B nanolayer-coated B4C was studied from 1300 degrees to 1600 degrees 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.