Nickel–Boron Nanolayer‐Coated Boron Carbide Pressureless Sintering

Abstract

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.

Microstructural Characterization of a Commercial Hot‐Pressed Boron Carbide Armor Plate

“…The second‐phase precipitates observed in the commercial plate are different from those found in lab‐consolidated boron carbide . Firstly, the chemistry of the second phases are different: graphite inclusions were the most prominent second phase in the commercial sample, but they are rarely found in laboratory samples.…”

“…Secondly, the distributions of the second phases are different: both intragranular and intergranular AlN and h‐BN precipitates were observed in the commercial samples, while the second phases in lab‐consolidated samples are generally intergranular, filling up micrometer‐size pores. Thirdly, the sizes of the second phases are different: AlN nanoprecipitates were observed in about 10% of the grains in the commercial boron carbide plate, but no nanoprecipitates have been reported for lab‐consolidated boron carbide specimens . Lastly, the effects of second phases on mechanical properties are different: the effect of the second phases on mechanical properties in the commercial samples is less well understood and could be either beneficial or detrimental.…”

“…Thirdly, the sizes of the second phases are different: AlN nanoprecipitates were observed in about 10% of the grains in the commercial boron carbide plate, but no nanoprecipitates have been reported for lab-consolidated boron carbide specimens. 2,[4][5][6][9][10][11][12][14][15][16][17] Lastly, the effects of second phases on mechanical properties are different: the effect of the second phases on mechanical properties in the commercial samples is less well understood and could be either beneficial or detrimental. In contrast, almost all the second phases observed in the lab-consolidated samples were reported to toughen the boron carbide by improving densification and causing crack deflection.…”

“…In the literature, the vast majority of investigations of processing-microstructure-property relations in boron carbide have focused on lab-consolidated samples. 2, [5][6][7][8][9][10][11][12][13] In these studies, sintering aides (e.g., oxides, 5,6,10,13,14 metals, 6,7,[9][10][11][12] carbides, 7,12,15 and borides 16,17 ) have been shown to improve the densification 18 and toughness 2,4-6,9-12,14-17 of laboratorysized samples of boron carbide. However, commercial boron carbide plates may have different sintering aids and processing parameters, and their detailed microstructural characterization has been rather limited.…”

Studies on the effect of processing parameters on electroless coating of copper on boron carbide particles

“…Its wide spread structural applicability is limited due to its low boron and carbon diffusion mobility resulting from covalent bonding, low fracture toughness fracture, high resistance to grain boundary sliding, low sinterability and low superficial tension produce by low self diffusion coefficient and also the presence B 2 O 3 on surface in solid state [14][15][16][17][18]. The fabrication of fully densified B 4 C without sintering requires hot pressing above 2373K with the application of load of more than 30MPa in uniaxial direction [19,20].…”