EYWORDS: Armour ceramics; Boron carbide; Reactive spark plasma sintering;
Titanium diboride; Mechanical milling
Boron carbide (B4C) is an essential structural ceramic having superior hardness (up to
43 GPa) along with low density (2.52 g.cm -3) and high melting point (2763 °C). The
combination of high hardness with low density makes it a very attractive material for
body armour applications. However, poor sinterability and low fracture toughness (≈
3 MPa m1/2) are the major limitations of B4C. Strong covalent bonds, low self-
diffusion coefficients, and the high melting point of B4C make its sintering very
difficult. Hence, conventional sintering techniques like pressure-less sintering (PLS)
and hot pressing (HP) require temperatures more than 2000 °C for obtaining dense
compacts. Recently, reactive spark plasma sintering (RSPS) has been shown to sinter
B4C composites at low temperatures. The addition of sintering additives like SiC,
ZrB2, TiC, TiB2, Al, and Fe have also proven to aid the densification and improve the
properties.The addition of TiB2 is expected to enhance fracture toughness,
machinability, and electrical conductivity of the B4C composites with less hardness
reduction. In this context, the objective of the present study is to fabricate dense B4C
composites at low temperature using reactive spark plasma sintering using ball-milled
Ti-B powder as a sintering additive, and study the effect of Ti-B addition, milling
time, and applied pressure on densification and microstructure evolution of the B4C
composites, understand densification mechanisms and kinetics of densification, and
finally to study the mechanical properties and ballistic performance of B4C
composites.
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To study the effect of WC contamination on sintering, SPS was carried out on B4C
powders milled for 1 h and 4 h. WC contamination from the wear of the balls and the
vials was found to increase with milling time. However, the sintered compacts of 1 h
and 4 h milled B4C powders without Ti-B addition had a low relative density of 79.7
and 84.1 %, respectively. This showed that WC was less effective as a sintering
additive. Ti and B powders (1:2 atom ratio) were milled for 8 h and added to B4C
powder in proportions of 5 and 10 wt.% and milled for 1h and 4 h to achieve good
dispersion. Dense B4C composite compacts were obtained in the case of 4 h (97.2 %)
milled powders compared to 1 h (86.3 %) milled powders. This showed that a
combination of WC (from contamination) and reactive Ti-B mixture effectively
achieved low-temperature densification. A pore-free microstructure consisting of B4C
grains surrounded with fine grains of (Ti0.9W0.1)B2 and W2B5 was obtained using SPS
at 1400 °C at a pressure of 50 and 80 MPa. The densification curves showed 3 stages,
and maximum densification happened in stages II & III, which are attributed to the
formation of the borides followed by their plastic flow at higher temperature and
pressure. The activation energy for sintering reduced from 234 to 175 kJ.mol-1 with
the increase in the amount of Ti-B mixture from 5 to 10 wt.%. The Vickers hardness
of the B4C composites decreased with the increasing amount of Ti-B. On the other
end, the indentation fracture toughness of the composites increased with Ti-B addition
and sintering pressure. Flexure tests on the B4C composite with 5 wt.% Ti-B addition
(4 h milled) showed a strength of 340±39 MPa and a fracture toughness of 4.2±0.1
MPa m1/2. Ballistic studies on the above composition were carried out using Depth of
penetration (DOP) tests. The results showed that the reactive spark plasma sintered
B4C composite was effective in ballistic applications.