79
© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021
L. Lamberson et al. (eds.), Dynamic Behavior of Materials, Volume 1, Conference Proceedings of the Society
for Experimental Mechanics Series, https://doi.org/10.1007/978-3-030-59947-8_15
Chapter 15
Static and Dynamic Mechanical Characterization of a Spark
Plasma Sintered B 6 O–B 4 C Composite
Kimia Ghaffari, Salil Bavdekar, and Ghatu Subhash
Abstract The microstructure and mechanical properties of a 70 wt.% B 6 O–30 wt.% B 4 C composite are compared to a
monolithic B 6 O, both prepared by spark plasma sintering (SPS). Optical and scanning electron micrographs showed significant phase segregation in the composite material along with extensive porosity in regions containing larger amounts of
B 6 O. Raman spectroscopy and energy dispersive spectroscopy (EDS) indicated the presence of distinct B 4 C-dominant and
B 6 O-dominant phases as well as near- homogeneous matrix interphase. The porous structure of the B 6 O resulted in a lower
hardness of the B 6 O-dominant phase (11.3 GPa) and matrix interphase (16.7 GPa) as compared to the B 4 C-dominant phase
(29.8 GPa) and the monolithic B 6 O sample (34.3 GPa). Contrary to theoretical predictions made elsewhere in the literature,
the current mechanical testing of both samples indicated no improvement in hardness or compressive strength in the composite material as compared to the monolithic B 6 O material; however, the incomplete sintering and resulting porosity in the
B 6 O phase of the composite had significant deleterious effects on its properties.
Keywords Hardness · Mechanical properties · Composites · Carbides
15.1 Introduction
Structural ceramics are one of the hardest materials which also exhibit several other desirable properties such as low density,
high strength, high impact resistance, and good wear resistance [1, 2]. One of the hardest, naturally occurring ceramics is
diamond with hardness in excess of 80 GPa [3]. However, the diamond structure is metastable at high temperatures, decomposing into graphite and thereby losing its desirable properties [4]. Furthermore, because of this instability, production of
diamond requires the use of high- pressure sintering, which increases manufacturing costs and limits sample size [5, 6].
A promising, thermally stable alternative to diamond are icosahedron-based ceramics, such as boron carbide (BC) and
boron suboxide (B 6 O). In the past few decades, the use of BC has become more prevalent as it has shown promise as an armor
material due to its inertness at high temperatures, economic manufacturability, high Hugoniot elastic limit (HEL) up to
20 GPa, impressive compressive strength up to 3.6 GPa, high hardness of 25–40 GPa and low density of 2520 kg/m
3
[7–12].
BC with space group 166( R m
3 ) has a rhombohedral structure comprised of a boron-rich icosahedra and a three-atom chain
[13]. However, due to the size similarity between boron and carbon, it can exist in many different polymorphs as the three
carbon atoms can occupy different positions in the crystal. The most abundant polymorph is (B 11 C)CBC, consisting of one
carbon atom in the icosahedra and a boron atom occupying the position at the center of the chain [13–20]. BC also exhibits
stoichiometric variability with carbon content varying from 13 to 20%, therefore it can exist in both boron-rich compositions
(B 13 C 2 ) and carbon- rich compositions (B 11 C 4 ). The boron-rich icosahedra in BC exhibit extreme resistance to deformation
however the presence of the three-atom chain has been shown to introduce significant structural vulnerability [21].
K. Ghaffari
University of Florida Material Science and Engineering, Gainesville, FL, USA
S. Bavdekar · G. Subhash (*)
University of Florida Mechanical and Aerospace Engineering, Gainesville, FL, USA
e-mail: Kimia.gh@ufl.edu
© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021
L. Lamberson et al. (eds.), Dynamic Behavior of Materials, Volume 1, Conference Proceedings of the Society
for Experimental Mechanics Series, https://doi.org/10.1007/978-3-030-59947-8_15
Chapter 15
Static and Dynamic Mechanical Characterization of a Spark
Plasma Sintered B 6 O–B 4 C Composite
Kimia Ghaffari, Salil Bavdekar, and Ghatu Subhash
Abstract The microstructure and mechanical properties of a 70 wt.% B 6 O–30 wt.% B 4 C composite are compared to a
monolithic B 6 O, both prepared by spark plasma sintering (SPS). Optical and scanning electron micrographs showed significant phase segregation in the composite material along with extensive porosity in regions containing larger amounts of
B 6 O. Raman spectroscopy and energy dispersive spectroscopy (EDS) indicated the presence of distinct B 4 C-dominant and
B 6 O-dominant phases as well as near- homogeneous matrix interphase. The porous structure of the B 6 O resulted in a lower
hardness of the B 6 O-dominant phase (11.3 GPa) and matrix interphase (16.7 GPa) as compared to the B 4 C-dominant phase
(29.8 GPa) and the monolithic B 6 O sample (34.3 GPa). Contrary to theoretical predictions made elsewhere in the literature,
the current mechanical testing of both samples indicated no improvement in hardness or compressive strength in the composite material as compared to the monolithic B 6 O material; however, the incomplete sintering and resulting porosity in the
B 6 O phase of the composite had significant deleterious effects on its properties.
Keywords Hardness · Mechanical properties · Composites · Carbides
15.1 Introduction
Structural ceramics are one of the hardest materials which also exhibit several other desirable properties such as low density,
high strength, high impact resistance, and good wear resistance [1, 2]. One of the hardest, naturally occurring ceramics is
diamond with hardness in excess of 80 GPa [3]. However, the diamond structure is metastable at high temperatures, decomposing into graphite and thereby losing its desirable properties [4]. Furthermore, because of this instability, production of
diamond requires the use of high- pressure sintering, which increases manufacturing costs and limits sample size [5, 6].
A promising, thermally stable alternative to diamond are icosahedron-based ceramics, such as boron carbide (BC) and
boron suboxide (B 6 O). In the past few decades, the use of BC has become more prevalent as it has shown promise as an armor
material due to its inertness at high temperatures, economic manufacturability, high Hugoniot elastic limit (HEL) up to
20 GPa, impressive compressive strength up to 3.6 GPa, high hardness of 25–40 GPa and low density of 2520 kg/m
3
[7–12].
BC with space group 166( R m
3 ) has a rhombohedral structure comprised of a boron-rich icosahedra and a three-atom chain
[13]. However, due to the size similarity between boron and carbon, it can exist in many different polymorphs as the three
carbon atoms can occupy different positions in the crystal. The most abundant polymorph is (B 11 C)CBC, consisting of one
carbon atom in the icosahedra and a boron atom occupying the position at the center of the chain [13–20]. BC also exhibits
stoichiometric variability with carbon content varying from 13 to 20%, therefore it can exist in both boron-rich compositions
(B 13 C 2 ) and carbon- rich compositions (B 11 C 4 ). The boron-rich icosahedra in BC exhibit extreme resistance to deformation
however the presence of the three-atom chain has been shown to introduce significant structural vulnerability [21].
K. Ghaffari
University of Florida Material Science and Engineering, Gainesville, FL, USA
S. Bavdekar · G. Subhash (*)
University of Florida Mechanical and Aerospace Engineering, Gainesville, FL, USA
e-mail: Kimia.gh@ufl.edu