The boron-rich boron carbide materials have been traditionally synthesized by adding boron powder to B4C material and subjecting it to hot pressing sintering for materials composition containing 8.8–20 at. % carbon in boron (composition range of B10.4C to B4C). Our study explores a synthesis route for B10C starting from high-purity boron and carbon and direct conversion under high pressure and high temperature (HPHT) conditions of 2000 °C and 6–8 GPa. Synthesis was verified via x-ray diffraction analysis, showing the conversion of the high-purity boron and carbon powder mixture into a hexagonal B10C structure (R-3m space group) with lattice parameters of a = b = 5.6115 Å and c = 12.197 Å. The concentration of boron was measured through x-ray photoelectron spectroscopy, confirming the B10C ratio. The measured nanoindentation mean hardness of B10C was 40 GPa. Raman spectroscopy of the HPHT synthesized sample shows characteristic vibrational breathing modes of boron icosahedron and an additional intense band at a vibrational frequency of 380 cm−1. This Raman band, which appears notably weaker in earlier studies and B4C samples, is assigned to the linear chain of B–B–B and attributed to the maximal incorporation of boron within the hexagonal structure.

Boron carbide, often referred to as B4C, is a ceramic material that has many appealing properties, like high Vickers hardness (29.1 GPa),1 excellent thermal oxidization resistance, high neutron absorption(600 barn),2 high wear resistance, and very low density (2.52 gm·cm−3).1 It is the third hardest material after diamond and cubic boron nitride. These properties make boron carbides useful in a wide range of industries, ranging from ballistic armor and heat shields to nuclear applications.2,3 These characteristics depend on the stoichiometric ratio of boron to carbon, which can range from 8.8 at % to 20 at % carbon in boron (Refs. 4 and 5). Elemental boron exhibits diverse base structures, including B12 icosahedra, B6 octahedra, linear atomic chains, and atomic clusters within a three-dimensional network.6–9 Various configurations of these icosahedra can arise, along with other structural components, during the processing of the materials.10,11 Boron carbide commonly exhibits stoichiometries such as B4C, B13C2, and B12C3.10,12 Other boron carbide crystal structures, such as B50C2, B50C, B48C3, B51C, and B49C3, are predominantly tetragonal.13,14 Both theoretical and experimental studies have demonstrated significant alterations in atomic bonding, electron density, mechanical properties, and lattice constants as the boron-to-carbon ratio varies.15–19 From a thermodynamic perspective, the β-tetragonal and rhombohedral crystal structures are the most stable forms of boron carbide.15,20 The most generic form of boron carbide, often called B4C, is made up of 20% carbon. Efforts to synthesize boron carbide outside the 8%–20% carbon range result in a material containing residual phases of boron or carbon, depending on whether the initial material is boron or carbon-rich. Boron carbide B4C is made up of an icosahedron consisting of 12 atoms, which are linked together by a 3-atom chain to form a rhombohedral crystal structure. With the boron carbide with a stoichiometric ratio of 20% carbon having been extensively explored, it is of interest to investigate the boron-rich composition. Usually, boron-rich boron carbide is synthesized by B4C with excess boron,6 but the objective of this study was to synthesize boron-rich boron carbide with boron and carbon powders only. In this experimental work, B10C is synthesized via high-pressure, high-temperature methods, employing high-purity boron and carbon powders as starting materials.

Pure boron powder, 99.99%, was bought from SkySpring Nanomaterials, and graphite powder, 99.9999%, was bought from Alfa Aesar. These powders were mixed in a ratio of 8.8 at. % carbon, then ball milled in a high energy ball mill (Spex 8000 M) with a tungsten carbide medium for 6 h. After mixing, the powder was passed through a 400-mesh sieve. Synthesis was conducted at the High-Pressure Collaborative Access Team (HPCAT), located at the Advanced Photon Source. Due to the APS being in the middle of an upgrade, HPCAT’s Paris-Edinburgh (PE) press was utilized in an offline beam configuration, and the pressures were based on earlier calibrations with MgO standards. The PE press was used for its capability of synthesizing a relatively large sample under high pressure of 6–8 GPa8,9 at a temperature of 2000 °C.

The sample was compressed into a pellet with dimensions of 1.5 mm in diameter and 2 mm in height using a tungsten carbide press and then placed inside a hBN capsule, which was then loaded into the standard PE press cell. The cell was then placed into the PE press cell and compressed to an applied pressure of 12000 psi (sample pressure of 6–8 GPa), at which point the temperature was slowly ramped up to 2000 °C through the use of a graphite heater element present inside the standard PE press cell. More detail on the PE press setup is given in previous studies.21,22 The sample was left to soak at max pressure and temperature; after 30 min, the power to the heater was shut off and the sample was quenched. The PE press cell has a measured cooling rate of ∼35° C/s. Once the PE press was cool enough to handle, the sample was decompressed and recovered for confirmation of synthesis and characterization.

X-ray diffraction (XRD) analysis was utilized to investigate the synthesized material’s crystal structure and phase composition, as illustrated in Fig. 1. Rietveld refinement revealed the sample is predominantly composed of boron carbide (98.3%), and a residual amount of tungsten boride (0.8%) and graphite (0.9%) is also present. The measured lattice parameters of the major boron carbide phase are a = b = 5.6115 Å and c = 12.197 Å (R-3m space group).

FIG. 1.

The top panel shows the measured x-ray diffraction spectrum with wavelength λ = 1.5418 Å. The structural refinement results in the identification of the three phases present in the sample, with a majority of 98.3% hexagonal phases of B10C and weak residual phases of graphite and tungsten boride.

FIG. 1.

The top panel shows the measured x-ray diffraction spectrum with wavelength λ = 1.5418 Å. The structural refinement results in the identification of the three phases present in the sample, with a majority of 98.3% hexagonal phases of B10C and weak residual phases of graphite and tungsten boride.

Close modal

XPS of the HTHP synthesized boron-rich boron carbide sample consists of 89.0% boron, 9.0% carbon, and 2% oxygen (relative atomic percentages), with no other elements detected. The experimental survey spectra are very close to the stoichiometric ratio for B10C [Fig. 2(a)]. The high-resolution XPS spectra of B1s and C1s show B–B, B–C, C–B, and C–C bonding on the surface. In Fig. 2(b), the high-resolution B1s scan indicates that 92% of the boron is bonded to boron, and the remaining 8% participates in boron–carbon bonding. Figure 2(c) exhibits the high-resolution C1s scan, revealing that 50% of the carbon is bonded to boron (B–C), and the other 50% is bonded to carbon (data summarized in Table I).

FIG. 2.

(a) Survey spectra and high-resolution XPS spectra of (b) B1s and (c) C1s. Hi-res spectra show B–B, B–C, C–B, and C–C bonding in the sample.

FIG. 2.

(a) Survey spectra and high-resolution XPS spectra of (b) B1s and (c) C1s. Hi-res spectra show B–B, B–C, C–B, and C–C bonding in the sample.

Close modal
TABLE I.

Compositional analysis and fitted parameters of B1s and C1s by x-ray photoelectron spectroscopy.9,23

PeaksBinding energy (eV)Peak area (%)Assignment
B1s 187 92 B–B 
B1s 189.2 B–C 
C1s 281.4 50 C–B 
C1s 283.2 50 C–C 
PeaksBinding energy (eV)Peak area (%)Assignment
B1s 187 92 B–B 
B1s 189.2 B–C 
C1s 281.4 50 C–B 
C1s 283.2 50 C–C 

The Raman measurements were obtained utilizing a micro-Raman spectrometer (Dilor XY, Lille, France) featuring a 1200 groove/mm grating, a 100× microscope objective, and laser excitation at a wavelength of 532 nm. Figure 3(a) shows the Raman spectrum of the commercial B4C powder, and Fig. 3(b) shows the HPHT synthesized boron-rich boron carbide sample. The spectrum of the commercial B4C powder displays distinctive Raman peaks at 272, 324, 481, 534, 728, 828, 998, and 1090 cm−1 assigned to crystalline B4C.24 The boron-rich boron carbide Raman spectra reveal distinct features unlike the crystalline B4C.25 The icosahedra breathing mode (IBM) at 1090 cm−1 also downshifted to 1065 cm−1 and a new peak appears at 380 cm−1.25 

FIG. 3.

Raman spectra of (a) commercial B4C and (b) synthesized boron-rich boron carbide samples in the 300–1300 cm−1 range.

FIG. 3.

Raman spectra of (a) commercial B4C and (b) synthesized boron-rich boron carbide samples in the 300–1300 cm−1 range.

Close modal

Nanoindentation hardness was determined using an Agilent Nano Indenter G200 employing the continuous stiffness measurement (CSM) method, equipped with a Berkovich diamond tip with a nominal radius of 50 nm. To ensure precision, calibration involved measuring a fused silica reference with an accepted Young’s modulus value of 72 ± 3 GPa before and after indenting our sample. The consistent Young’s modulus range observed in the silica standard, both pre- and post-indentation of our sample, confirms the preservation of tip geometry throughout testing. All indentations, including those on silica, were conducted using the continuous stiffness measurement (CSM) method with a maximum depth of 750 nm. In Fig. 4(a), the CSM hardness values are depicted against the displacement into the surface. Hardness values were calculated within the depth range of 100–700 nm. Hardness remained consistent across all depths, yielding a mean value of 40 GPa with a standard deviation of 1 GPa. Figure 4(b) illustrates the CSM modulus values as a function of displacement into the surface. Similar to hardness, Young’s modulus of the sample was calculated within the depth range of 100–700 nm and exhibited minimal variation across the tested depth range. The mean modulus value for boron-rich boron carbide was determined to be 446 GPa, with a standard deviation of 13 GPa. The load-displacement curve of 14 indents in the boron-rich boron carbide sample is shown in Fig. 4(c). All the data points in the different regions of the sample exhibit identical load-displacement curves, suggesting the uniformity of the sample. The relative impact of elastic and plastic deformation can be obtained by examining the ultimate unloading depth of the load-displacement curves. The unloading data indicate a substantial elastic recovery, ∼50% of which is evident in the boron-rich boron carbide sample.

FIG. 4.

(a) Continuous stiffness measurement hardness data from 14 indents, displaying the mean hardness value of 40 GPa. (b) Continuous stiffness measurement modulus data from 14 indents, showing the mean modulus value of 446 GPa. (c) The nanoindentation load-displacement curve was obtained from 14 indents, revealing a mean hardness of 40 GPa at a depth of 300 nm.

FIG. 4.

(a) Continuous stiffness measurement hardness data from 14 indents, displaying the mean hardness value of 40 GPa. (b) Continuous stiffness measurement modulus data from 14 indents, showing the mean modulus value of 446 GPa. (c) The nanoindentation load-displacement curve was obtained from 14 indents, revealing a mean hardness of 40 GPa at a depth of 300 nm.

Close modal

In this study, boron-rich B10C was synthesized using high-purity boron and carbon powders. Analysis of the resulting crystal structure revealed lattice parameters of a = b = 5.6115 Å and c = 12.197 Å. A comparison with the lattice parameters of conventional B4C, where a = b = 5.855 Å and c = 12.06 Å, highlights a difference: our synthesized B10C exhibits larger lattice parameters. This expansion of lattice parameters with increasing boron content aligns with findings from prior research, confirming the phenomenon.25 Boron carbides exhibit a single-phase structure within the carbon concentration range of ∼8–20 at. %.6,7 However, the atomic structures of boron carbide, particularly within the range below ∼13.3 at. % carbon, are known as boron–very rich boron carbide (BvrBC).11,13,26 BvrBC is relatively unexplored in the scientific literature due to the complicated complexities of its structure and bonding. Based on x-ray diffraction (XRD) data, a proposed model suggests the composition containing 20 atomic percent carbon comprises B12 icosahedra intertwined with carbon–carbon–carbon (CCC) chains.27 Alternatively, additional models suggest that a carbon atom can also be integrated into the icosahedral framework (B11C), contributing to forming carbon–boron–carbon (CBC) chains.28,29 Furthermore, as the carbon atomic percentage decreases below 20%–∼13.3%, there is a notable preference for boron to supplant carbon within the icosahedral lattice. Carbon atoms persist solely within the icosahedral chain when their atomic percentage approaches ∼13.3%.30–32 Based on our x-ray photoelectron spectroscopy (XPS) findings, our sample comprises just 9 at. % carbon. Furthermore, within this carbon content, 50% is observed to be bonded with boron, equating to only 4.5 at. % of carbon forming bonds with boron. This observation suggests the absence of carbon within the icosahedral structure. When more boron is added, carbon is also replaced by the boron in the chain. We do not observe 481 and 534 cm−1 peaks (for the CBC chain) and peaks at 400 cm−1 (CBB chain) in our synthesized boron-rich born carbide sample.6 Instead, we observed a band appearing in a low-frequency region centered at 380 cm−1. This band arises from the vibration mode of Boron–Boron–Boron (BBB) chains, where the boron atom replaces the carbon atom in the chain Ref. 25. The icosahedra breathing mode (IBM) at 1090 cm−1 also downshifted to 1065 cm−1, indicating more boron incorporation in the lattice and an anticipated lattice expansion.33 The lattice expansion of our synthesized B10C sample is also confirmed by our XRD analysis. Raman measurements indicate that the carbon atoms are not located in icosahedral structures or chains. The carbon atom may occupy a different interstitial position in the unit cell. While x-ray diffraction (XRD) and Raman spectra offer valuable insights, they do not provide precise identification of the specific occupancies of carbon atoms in the lattices. Verification of this could be pursued through additional theoretical calculations, although such analyses are beyond the scope of this present study. The average hardness of the synthesized born-rich boron carbide sample is higher than that of the B4C and other variants of boron carbide with different boron-to-carbon ratios, which is a desirable outcome and the goal of this study.

In this study, we present the direct synthesis of B10C, a superhard boron-rich boron carbide material, achieved through high-pressure, high-temperature conditions using a ball-milled mixture of high-purity boron and carbon powders. The B10C material synthesized at a pressure of 6–8 GPa and a temperature of 2000 °C has a hexagonal structure (R-3m space group). The x-ray photoelectron spectroscopy confirmed the boron-to-carbon ratio in the synthesized material. The XRD and Raman spectra indicate maximal boron incorporation in the hexagonal lattice. The observation of an intense Raman band at a vibrational frequency of 380 cm−1 confirms the occurrence of a B–B–B linear chain stabilized by the direct synthesis approach utilizing high-pressures and high-temperatures. The mean nanoindentation hardness of the B10C sample is 40 GPa with a mean Young’s modulus of 446 GPa, and this neutron absorbing material can be used in several applications under extreme pressures, temperatures, and high radiation environments.

This material is based on work supported by the Department of Energy - National Nuclear Security Administration under Award No. DE-NA0004090. The portions of this work were performed at HPCAT (Sector 16), Advanced Photon Source (APS), and Argonne National Laboratory. HPCAT operations are supported by DOE-NNSA’s Office of Experimental Sciences. The Advanced Photon Source is a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06C.H.11357.

The authors have no conflicts to disclose.

Seth Iwan: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Methodology (equal); Writing – original draft (equal); Writing – review & editing (equal). Kallol Chakrabarty: Data curation (equal); Formal analysis (equal); Methodology (equal); Writing – review & editing (equal). Paul A. Baker: Formal analysis (equal); Methodology (equal). Yogesh K. Vohra: Funding acquisition (lead); Methodology (equal); Project administration (lead); Writing – review & editing (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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