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Volume 6
Research & Reviews: Journal of Material Sciences
ISSN: 2321-6212
Ceramics 2018
May 14-15, 2018
Page 58
conference
series
.com
May 14-15, 2018 | Rome, Italy
4
th
International Conference and Expo on
Ceramics and Composite Materials
Ghatu Subhash, Res. Rev. J Mat. Sci. 2018, Volume 6
DOI: 10.4172/2321-6212-C1-013
Bonding structure and deformation mechanisms in ultra-hard ceramics
U
ltrahard ceramics mainly stem from two structural forms: diamond-like and icosahedral. While diamond-like structures
(e.g., naturally occurring diamond and c-BN) hold the microhardness record at over 100 GPa, icosahedral boron-rich solids
have attracted considerable attention in recent years due to their strong thermal/chemical stability and an excellent combination
of high hardness (~40 GPA) and low mass density (~2.5 g/cm
3
). Boron carbide (B
4
C) and boron suboxide (B
6
O) are two popular
ceramics in this class of materials and are preferred candidates for impact and wear resistance applications. While their structure
mainly consists of a 12-atom icosahedron and few atoms that bond to the icosahedron, the arrangement of these atoms and their
chemical nature controls their deformation mechanisms. For e.g., B
4
C has a 3-atom chain attached to the equatorial atoms of the
icosahedron, B
6
O does not have a chain but one oxygen atom bonded on each side of the icosahedron. Due to the closeness in
atomic radii of boron and carbon, B
4
C exhibits polymorphisms where a carbon can substitute for boron and potentially yieldmore
than 200 polymorphs. On the other hand, B
6
O has no polymorphs. Thus B
6
O is structurally more homogeneous than B
4
C. These
structural differences influence their properties and deformation mechanisms. Both materials have high hardness (>30 GPa), low
density (2.52 g/cm
3
for B
4
C and 2.6 g/cm
3
for B
6
O), high compressive strength (up to 5 GPa), moderate fracture toughness (3.4
MPa·m
1/2
for B
4
C and 4.2 MPa·m
1/2
for B
6
O) and exhibit amorphization (localized crystal structure collapse) under high pressure
loads. But, amorphization in B
4
C can be detected in Raman spectroscopy (through appearance peaks beyond 1200 cm
-1
), B
6
O does
not show any new peaks due to amorphization. While the 3-atom chain bending has been proposed as the main mechanism for
amorphization in B
4
C, the lack of chain structure in B
6
O raises new questions as the root-cause of amorphization in this material.
Similarly, B
4
C has been found to occasionally undergo deformation twinning but B
6
O has been shown to undergo ‘nanotwinning’
(twin spacing of nm scale) even in virgin state and has been theorized to provide extremely high hardness if the entire specimen
undergoes nanotwinning at critical twin spacing of two atomic planes. In this research, a coordinated experimental, spectroscopic,
microscopic, and quantummechanical investigations are performed to provide fundamental insight into the above issues. Finally,
implications of these structural, behavioral and bonding differences during high pressure dynamic deformation will be discussed.
The long-term goal of this research is to identify novel avenues for designing of ultrahard materials with tailored properties.
Biography
Ghatu Subhash conducts multidisciplinary research at the intersection of solid mechanics, materials science, and biomechanics.Subhash’s research has resulted in
invention disclosures and provisional patents for a novel method for rapid testing of flexible package sealing, impact absorbing cushions for preventing head injury, rapid
processing of UO
2
fuels, and development of ultra-hard boron carbide which has numerous applications as armor, hard abrasive, and thermoelectric.
subhash@ufl.eduGhatu Subhash
University of Florida, USA