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. 2018 Jul 25;11(8):1279.
doi: 10.3390/ma11081279.

Computational Predictions and Microwave Plasma Synthesis of Superhard Boron-Carbon Materials

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Free PMC article

Computational Predictions and Microwave Plasma Synthesis of Superhard Boron-Carbon Materials

Paul A Baker et al. Materials (Basel). .
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Abstract

Superhard boron-carbon materials are of prime interest due to their non-oxidizing properties at high temperatures compared to diamond-based materials and their non-reactivity with ferrous metals under extreme conditions. In this work, evolutionary algorithms combined with density functional theory have been utilized to predict stable structures and properties for the boron-carbon system, including the elusive superhard BC₅ compound. We report on the microwave plasma chemical vapor deposition on a silicon substrate of a series of composite materials containing amorphous boron-doped graphitic carbon, boron-doped diamond, and a cubic hard-phase with a boron-content as high as 7.7 at%. The nanoindentation hardness of these composite materials can be tailored from 8 GPa to as high as 62 GPa depending on the growth conditions. These materials have been characterized by electron microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, X-ray diffraction, and nanoindentation hardness, and the experimental results are compared with theoretical predictions. Our studies show that a significant amount of boron up to 7.7 at% can be accommodated in the cubic phase of diamond and its phonon modes and mechanical properties can be accurately modeled by theory. This cubic hard-phase can be incorporated into amorphous boron-carbon matrices to yield superhard materials with tunable hardness values.

Keywords: ab initio calculations; boron-carbon compound; chemical vapor deposition; superhard materials.

Conflict of interest statement

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

Figures

Figure 1
Figure 1
The measured Raman spectrum from a lightly boron-doped diamond (sample LBDD) recorded with the 532 nm laser excitation. A zone-center Raman mode at 1327.6 cm−1 is accompanied by weak bands attributed to the Fano effect.
Figure 2
Figure 2
(a) Raman spectrum from sample SBCC showing “D” and “G” bands attributed to microcrystalline boron-doped graphite, and amorphous carbon (AC); (b) Raman spectrum from sample HBCC showing a hard cubic-phase. The vertical bars show the location of phonon modes in cubic diamond as determined by neutron scattering experiments [25]. The observed Raman modes in HBCC sample are considerably shifted from the cubic diamond positions.
Figure 3
Figure 3
The X-ray Photoelectron Spectroscopy (XPS) analysis of the two boron-carbon films where Raman spectra have been presented in Figure 2. The composition of the film is determined from the intensities of B 1s and C1s emission intensities. (a) HBDD sample and (b) HBCC sample. The satellite (sat) peaks associated with C1s emission are also labeled.
Figure 4
Figure 4
The load displacement curves for the two samples to a depth of 400 nm. (a) SBCC sample showing considerable plastic deformation and yielding a hardness value of H = 7.8 GPa and Elastic Modulus of E = 174 GPa based on two such indents and (b) HBCC sample showing minimal plastic deformation and yielding a hardness value of H = 61.7 GPa and Elastic Modulus of E = 532 GPa based on seven such indents.
Figure 5
Figure 5
High resolution X-ray diffraction pattern recorded on various B-C films using a hybrid monochromator showing increased splitting of the cubic-diamond (111) diffraction peak with increasing boron-content. The boron-doped diamond peak shift to lower 2θ angles, as indicated by arrows, showing an increase in lattice parameter with increasing boron content.
Figure 6
Figure 6
Scanning Electron Micrograph (SEM) of various boron-carbon composites synthesized in this study. (a) SEM of the HBDD sample (2.9 at% boron) showing (100) morphology; (b) SEM of the SBCC sample containing microcrystalline boron-doped graphite; (c) SEM of the HBCC sample.
Figure 7
Figure 7
The lowest-energy structure of superhard BC5 (containing two formula units) predicted by the evolutionary algorithm as implemented in USPEX [13,14,15]. The unit cell is orthorhombic with Pmma symmetry. The lattice parameters are described in the text. The predicted hardness is 63 GPa.
Figure 8
Figure 8
Theoretical X-ray diffraction patterns with 1.5406 wavelength angstrom (Cu K-alpha1) for orthorhombic BC5 with Pmma symmetry compared to the experimental X-ray diffraction pattern for the HBCC sample. Each spectrum is normalized by its highest peak intensity.
Figure 9
Figure 9
Phonon dispersion and density of states (DOS) of orthorhombic BC5 with Pmma symmetry. The phonon DOS of cubic diamond is also shown. Each phonon DOS spectrum is normalized by its peak intensity.
Figure 10
Figure 10
Comparison of the Raman spectrum for the cubic-diamond phase without boron incorporation and the orthorhombic BC5 with Pmma symmetry.
Figure 11
Figure 11
Plot of DFT calculation showing lattice parameters for cubic diamond structure as a function of the boron content. The linear behavior is predicted by Vegard′s Law. The inset structure shows a 2 × 2 × 2 supercell of boron-doped cubic diamond with a boron content of 9.375 at%.

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