B4C
Boron carbide · tetraboron carbide, Tetrabor, Norbide
B4C is boron carbide, a very hard covalent ceramic made of boron and carbon and a thermodynamically stable phase of the boron–carbon system. It belongs to the carbide hard-materials family and combines very high hardness, low density and strong neutron absorption. Electronically it is a p-type semiconductor.
About Boron carbide
What is B4C and what structure does it have?
Boron carbide is a boron-rich covalent carbide with a rhombohedral crystal structure. It is built from twelve-atom icosahedral clusters (ideally B11C) that bond directly to each other and are bridged by three-atom linear chains (ideally C–B–C). It is not a single fixed-composition compound: it has a broad homogeneity range in which the boron-to-carbon ratio varies while the icosahedral framework stays the same. B4C is the nominal formula, close to the carbon-rich end of that range. This rigid, strongly bonded framework is what gives it its extreme hardness.
What is boron carbide used for?
Boron carbide is used where extreme hardness and low density matter most. Its best-known use is as a ceramic strike face in lightweight body armor and vehicle armor. Because it resists wear, it is also made into grit-blasting and water-jet cutting nozzles, wire-drawing dies, hard ceramic bearings, and grinding and lapping powders. The boron-10 isotope absorbs neutrons strongly, so boron carbide is widely used in nuclear reactors for control rods and neutron shielding.
How stable and synthesizable is B4C?
Boron carbide is a thermodynamically stable phase and is made on an industrial scale. The standard method is carbothermal reduction: boric acid or boron oxide is reacted with carbon in electric furnaces at high temperature. It can also be made by reducing boron oxide with magnesium in the presence of carbon, or by reacting the elements directly. Strong directional covalent bonds slow down diffusion, so making fully dense parts usually needs hot pressing, hot isostatic pressing, or sintering aids.
Is B4C a conductor, semiconductor, or insulator?
Boron carbide is a semiconductor across its whole composition range, and it is normally p-type. Its electrical conduction is unusual: charge moves by thermally activated hopping of self-trapped carriers (small polarons or bipolarons), not by ordinary band transport. Its high-temperature thermoelectric behaviour has attracted interest for this reason. Refractory transition-metal carbides such as HfC and ZrC are quite different, because they conduct electricity like metals.
How does B4C compare to SiC, HfC, ZrC, and related carbides?
Compared with silicon carbide, boron carbide is harder and noticeably less dense. Silicon carbide is generally cheaper to produce and process. Hafnium carbide and zirconium carbide are metallic conductors with ultra-high melting points, unlike the covalently bonded, semiconducting boron carbide.
Key Properties
Cross-validated computational properties for Boron carbide, aggregated across 5 databases.
Band GapEnergy needed to move an electron from the valence band to the conduction band. Lower or zero values tend to behave more metallic; larger gaps are more insulating or semiconducting.
Energy Above HullThermodynamic distance from the most stable set of competing phases. 0 eV/atom is on the convex hull; small positive values may still be experimentally accessible.
StabilityA plain-language summary of the best reported energy-above-hull result. It reflects whether the lowest-energy structure is on, near, or far from the stability hull.
StructuresCount of reported calculated crystal structures for this formula, including alternate polymorphs, source databases, and observed space groups.
Cross-Source DFT Agreement
How well independent DFT databases agree on the thermodynamics of B4C. Tight agreement means computed properties can be trusted without re-running calculations.
Agreement ScoreA normalized confidence score summarizing how closely independent DFT databases agree. Higher scores mean tighter cross-source agreement.
Hull SpreadDifference between the highest and lowest energy-above-hull values reported by comparable sources. Smaller spread means less thermodynamic disagreement.
Sources ComparedNumber and names of computational sources with comparable entries for this formula.
Space Group ConsensusWhether independent sources predict the same crystal symmetry for the lowest-energy structure.
Reported Structures
Lowest-energy structures reported for B4C, ranked by energy above hull.
| Space GroupSymmetry classification of the crystal arrangement. The number is the international space-group index. | Crystal SystemBroad lattice family, such as cubic, tetragonal, monoclinic, or triclinic, derived from unit-cell symmetry. | Band Gap (eV)Electronic gap calculated for this specific reported structure, measured in electronvolts. | E above hull (eV/atom)Thermodynamic distance from the convex hull for this structure, normalized per atom. Lower is generally more stable. | E/atom (eV)Computed total energy normalized per atom. Use energy above hull, not this value alone, when comparing stability. | Density (g/cm³)Mass per relaxed crystal volume, reported in grams per cubic centimeter. |
|---|---|---|---|---|---|
| Cm (No. 8) | monoclinic | 3.24 | 0.0000 | -7.997 | 2.54 |
| — | — | 1.65 | 0.0366 | -0.031 | — |
| R-3m (No. 166) | trigonal | 1.66 | 0.0369 | -0.054 | — |
| R-3m (No. 166) | trigonal | 1.51 | 0.0812 | -7.916 | 2.49 |
| P1 (No. 1) | triclinic | 0.00 | 0.0973 | -7.900 | 2.43 |
| — | — | 0.00 | 0.8673 | 0.800 | — |
| R-3m (No. 166) | trigonal | 0.00 | 0.9018 | -7.095 | 1.99 |
| R-3m (No. 166) | trigonal | 0.00 | 1.0325 | 0.942 | — |
| R-3m (No. 166) | trigonal | 0.00 | 1.0526 | -6.944 | 1.24 |
| — | — | 0.00 | 1.2691 | 1.201 | — |
| P1 (No. 1) | triclinic | 0.00 | 1.3735 | -6.624 | 2.43 |
| C2 (No. 5) | monoclinic | — | — | — | 2.49 |
Applications
Where Boron carbide is used.
Patent Landscape
6 patents reference B4C or close compositional variants.
| Patent | Title | Assignee | Granted |
|---|---|---|---|
| 7550029 | Recycling method for Al—B4C composite materials | — | — |
| 8377369 | Density and hardness pressureless sintered and post-HIPed B4C | — | — |
| 5039633 | B4C/Al cermets and method for making same | — | — |
| 11906010 | Sacrificial materials to improve chemical vapor infiltration of B4C loaded preforms | — | — |
| 11795111 | Electro-conductive B4C-TiB2 composite ceramic and preparation method thereof | — | — |
| 11364654 | Indirect additive manufacturing process for producing SiC—B4C—Si composites | — | — |
Frequently Asked Questions
Common questions about Boron carbide, answered from cross-validated data and the published literature.
Is boron carbide harder than diamond?
No. It is one of the hardest known materials, but it ranks behind diamond and cubic boron nitride.
Why is B4C used in body armor?
Does boron carbide absorb radiation?
Is B4C electrically conductive?
Can B4C be sintered easily?
What is B4C?
What is B4C used for?
What is the band gap of B4C?
Is B4C a metal, semiconductor, or insulator?
Is B4C thermodynamically stable?
What is the crystal structure of B4C?
How many polymorphs of B4C are known?
What elements does B4C contain?
Where does the data for B4C come from?
Related Compounds
Other Carbide Hard Materials in the database.
Data sources & attribution
- materials_project — Data from the Materials Project (materialsproject.org). Cite: Jain et al., APL Materials 1, 011002 (2013). (CC-BY-4.0)
- oqmd — Data from the OQMD (oqmd.org). Cite: Saal et al., JOM 65, 1501 (2013); Kirklin et al., npj Comp. Mater. 1, 15010 (2015). (CC-BY-4.0)
- jarvis — Data from JARVIS (jarvis.nist.gov), NIST. Cite: Choudhary et al., npj Comp. Mater. 6, 173 (2020). (LicenseRef-US-Gov-PD)
- mpaloe — Data from MP-ALOE. Cite: Kuner et al., npj Comput. Mater. (2025), doi:10.1038/s41524-025-01834-9.
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