80
Specifically, the presence of chain has been linked to a crystal collapse phenomenon known as amorphization [21–26]. This
phenomenon was first observed as post-yield softening in particle velocity measurements during plate impact experiments,
in direct contrast with the post-yield hardening observed in other ceramics such as SiC [9]. Raman spectroscopy of regions
beneath indentations revealed several new bands in the spectra of BC, indicating a structural change and/or phase transformation in the material [27]. Ballistic experiments on BC targets showed a sharp increase in fine fragmentation above a certain
impact velocity and TEM observations of fine particles confirmed the presence of localized amorphized regions in the material [23]. This unique deleterious mechanism limits the applicability of BC to low-velocity armor applications, despite its
high strength and hardness. Though the cause of this loss of strength has been confirmed to be the aforementioned collapse
of the crystal structure, the exact mechanism for this collapse is yet to be determined [9, 23, 24, 28–31].
Efforts to mitigate amorphization and its deleterious effects have focused on (1) the role of stoichiometry [17, 31], (2)
introducing foreign elements as dopants into the BC structure [32], (3) reduction in grain size [33], and finally (4) the formation of composites with other materials such as carbon nanotubes (CNTs) and B 6 O [33, 34]. Density functional theory studies
conducted on the propensity for amorphization of different BC polymorphs indicated that the (B 12 ) CCC conformation is the
most susceptible to structural collapse [17]. Conversely, the (B 11 C p ) CBC polymorph exhibited a significantly higher degree
of structural stability. Unfortunately, the ability to produce BC with a single polymorph is not yet available due to the similarity in the Gibbs free energies for various conformations. Experimental studies have shown that doping BC with silicon atoms
can increase the pressure required for amorphization by nearly twofold [32, 35]. An investigation into the relationship
between grain size and the propensity for amorphization determined that a 300 nm grain size material was significantly less
likely to amorphize as compared to a 10 μm grain size material. Finally, the formation of composites with BC and CNT
showed an improvement in toughness, but no reduction in the material’s tendency to amorphize as well as a dramatic
decrease in hardness as compared to the monolithic material [33].
Boron suboxide (B 6 O) is another superhard ceramic that belongs to icosahedral family and can be produced through the
oxidation of boron with either B 2 O 3 or metal oxides [36, 37]. The crystal structure of B 6 O is similar to that of BC in that it
consists of a network of covalently bonded 12-atom boron icosahedra. However, it lacks the three-atom chain present in the
B 4 C structure, instead consists of two oxygen atoms on either side of the icosahedra to form B 12 OO or B 6 O [38]. Shear deformation simulations using DFT have shown that the three-atom chain in BC is significantly weaker and more brittle than the
oxygen arrangement in B 6 O [39]. Experimental studies have also revealed that B 6 O has superior mechanical properties as
compared to BC, with a compressive strength of up to 5 GPa (vs. 3.5 for BC) and a hardness of 34–45 GPa (vs. 25–35 for
BC), along with a comparable density of 2600 kg/m
3
(vs. 2520 kg/m
3
for BC) [37, 40–43]. An additional unique feature of
B 6 O is the formation of energetically favorable periodic twins with nanoscale spacing, termed as “nanotwins” [41, 44].
Large- scale nanotwinning has been shown to increase the hardness and fracture toughness of several materials, including
diamond and cubic boron nitride [40–42]. Studies have determined that with proper processing conditions, a fully nanotwinned B 6 O structure with a critical nanotwin spacing of 0.89 nm can be achieved, which may further improve its already
impressive hardness [41, 45].
This combination of attractive properties and features observed in monolithic B 6 O makes it a promising material for
structural and armor applications alongside BC. However, quantum mechanical simulations have indicated a potential
improvement in fracture toughness and ductility of a composite through the microalloying of boron suboxide and boron
carbide [34, 41, 43, 46] While it has previously been shown experimentally that B 6 O performs well as a monolithic material
[37, 41, 45, 47–51], little experimental studies into the behavior of a B 6 O–B 4 C composite are currently available to confirm
the above theoretical findings. With this motivation, the current investigation aims to evaluate the properties and characterize
a B 6 O–B 4 C composite along with a monolithic B 6 O material, making comparisons between their performance under static
and dynamic loading conditions.
15.2 Materials and Methods
Two material compositions were analyzed in this study: (1) B 6 O + 30 wt.% B 4 C and (2) 100% B 6 O, both produced by spark
plasma sintering (SPS). SPS employs the use of a high pulsed direct current and uniaxial pressure in order to expedite the
consolidation of hard-to-sinter ceramic powders [52–54]. In contrast to traditional hot pressing, SPS is able to achieve heating rates of up to 1000 °C/min, which has been shown to improve densification even without the application of pressure [53].
Before loading the powders into a die, a thin graphite foil coated with a boron nitride suspension was placed inside the die
cavity to prevent interaction between the ceramic powders and the die wall [34]. Punches are then inserted into the die cavity
and the assembly is placed in the SPS chamber. The powders were then consolidated using SPS for 30 min at 1950 °C under
K. Ghaffari et al.
Specifically, the presence of chain has been linked to a crystal collapse phenomenon known as amorphization [21–26]. This
phenomenon was first observed as post-yield softening in particle velocity measurements during plate impact experiments,
in direct contrast with the post-yield hardening observed in other ceramics such as SiC [9]. Raman spectroscopy of regions
beneath indentations revealed several new bands in the spectra of BC, indicating a structural change and/or phase transformation in the material [27]. Ballistic experiments on BC targets showed a sharp increase in fine fragmentation above a certain
impact velocity and TEM observations of fine particles confirmed the presence of localized amorphized regions in the material [23]. This unique deleterious mechanism limits the applicability of BC to low-velocity armor applications, despite its
high strength and hardness. Though the cause of this loss of strength has been confirmed to be the aforementioned collapse
of the crystal structure, the exact mechanism for this collapse is yet to be determined [9, 23, 24, 28–31].
Efforts to mitigate amorphization and its deleterious effects have focused on (1) the role of stoichiometry [17, 31], (2)
introducing foreign elements as dopants into the BC structure [32], (3) reduction in grain size [33], and finally (4) the formation of composites with other materials such as carbon nanotubes (CNTs) and B 6 O [33, 34]. Density functional theory studies
conducted on the propensity for amorphization of different BC polymorphs indicated that the (B 12 ) CCC conformation is the
most susceptible to structural collapse [17]. Conversely, the (B 11 C p ) CBC polymorph exhibited a significantly higher degree
of structural stability. Unfortunately, the ability to produce BC with a single polymorph is not yet available due to the similarity in the Gibbs free energies for various conformations. Experimental studies have shown that doping BC with silicon atoms
can increase the pressure required for amorphization by nearly twofold [32, 35]. An investigation into the relationship
between grain size and the propensity for amorphization determined that a 300 nm grain size material was significantly less
likely to amorphize as compared to a 10 μm grain size material. Finally, the formation of composites with BC and CNT
showed an improvement in toughness, but no reduction in the material’s tendency to amorphize as well as a dramatic
decrease in hardness as compared to the monolithic material [33].
Boron suboxide (B 6 O) is another superhard ceramic that belongs to icosahedral family and can be produced through the
oxidation of boron with either B 2 O 3 or metal oxides [36, 37]. The crystal structure of B 6 O is similar to that of BC in that it
consists of a network of covalently bonded 12-atom boron icosahedra. However, it lacks the three-atom chain present in the
B 4 C structure, instead consists of two oxygen atoms on either side of the icosahedra to form B 12 OO or B 6 O [38]. Shear deformation simulations using DFT have shown that the three-atom chain in BC is significantly weaker and more brittle than the
oxygen arrangement in B 6 O [39]. Experimental studies have also revealed that B 6 O has superior mechanical properties as
compared to BC, with a compressive strength of up to 5 GPa (vs. 3.5 for BC) and a hardness of 34–45 GPa (vs. 25–35 for
BC), along with a comparable density of 2600 kg/m
3
(vs. 2520 kg/m
3
for BC) [37, 40–43]. An additional unique feature of
B 6 O is the formation of energetically favorable periodic twins with nanoscale spacing, termed as “nanotwins” [41, 44].
Large- scale nanotwinning has been shown to increase the hardness and fracture toughness of several materials, including
diamond and cubic boron nitride [40–42]. Studies have determined that with proper processing conditions, a fully nanotwinned B 6 O structure with a critical nanotwin spacing of 0.89 nm can be achieved, which may further improve its already
impressive hardness [41, 45].
This combination of attractive properties and features observed in monolithic B 6 O makes it a promising material for
structural and armor applications alongside BC. However, quantum mechanical simulations have indicated a potential
improvement in fracture toughness and ductility of a composite through the microalloying of boron suboxide and boron
carbide [34, 41, 43, 46] While it has previously been shown experimentally that B 6 O performs well as a monolithic material
[37, 41, 45, 47–51], little experimental studies into the behavior of a B 6 O–B 4 C composite are currently available to confirm
the above theoretical findings. With this motivation, the current investigation aims to evaluate the properties and characterize
a B 6 O–B 4 C composite along with a monolithic B 6 O material, making comparisons between their performance under static
and dynamic loading conditions.
15.2 Materials and Methods
Two material compositions were analyzed in this study: (1) B 6 O + 30 wt.% B 4 C and (2) 100% B 6 O, both produced by spark
plasma sintering (SPS). SPS employs the use of a high pulsed direct current and uniaxial pressure in order to expedite the
consolidation of hard-to-sinter ceramic powders [52–54]. In contrast to traditional hot pressing, SPS is able to achieve heating rates of up to 1000 °C/min, which has been shown to improve densification even without the application of pressure [53].
Before loading the powders into a die, a thin graphite foil coated with a boron nitride suspension was placed inside the die
cavity to prevent interaction between the ceramic powders and the die wall [34]. Punches are then inserted into the die cavity
and the assembly is placed in the SPS chamber. The powders were then consolidated using SPS for 30 min at 1950 °C under
K. Ghaffari et al.