Metal borides (transition-metal diborides and boride layers) TM borides

spore: profile
id: metal-borides
spore_version: 0.2
family: carbide-nitride-boride-ceramics
name: Metal borides (transition-metal diborides and boride layers)
abbreviation: TM borides
role: 'family of hard transition-metal borides: ZrB2, HfB2, NbB2, TaB2, WB, Co2B/CoB and the FeB/Fe2B boride layers on steel'
category: ceramics
type: ultra-high-temperature-ceramics
summary: |
  This family profile covers the transition-metal borides that are not yet profiled on their own: zirconium diboride (ZrB2), hafnium diboride (HfB2), niobium diboride (NbB2), tantalum diboride (TaB2), tungsten boride (WB), cobalt borides (Co2B and CoB) and the iron borides (FeB and Fe2B) that form on steel during boriding. Sani et al. (2016) give the best sourced set: dense ZrB2 has a density of 6.119 g/cm3, a melting temperature of 3245 degC, a Young's modulus of 489 GPa and a hardness of 23 GPa, while dense TaB2 is much denser (12.568 g/cm3), melts at 3037 degC and is softer (hardness 19.6 GPa, modulus 248.2 GPa). Birmingham's reaction spark plasma sintering study gives NbB2 a hardness of 20.25 GPa, an elastic modulus of 539 GPa and a fracture toughness of 4 MPa m1/2 at 97.7 percent of theoretical density. For the iron borides, a 2011 electrochemical boriding study grew a single-phase Fe2B layer of 75 micrometres with a microhardness of about 16 GPa, and reports that FeB is harder but fractures more easily. Hafnium diboride and tungsten boride have no measured density, hardness or modulus in a source read for this profile, and the cobalt borides have only a nanocrystal density (7.61 to 7.78 g/cm3, Co2B).
best_for: Hard, high-melting diboride parts and coatings where ZrB2 or TaB2 is needed, and boride layers that give steel a hard, wear-resistant surface (Fe2B)
weakest_at: Brittleness and few measured bulk values for most borides (only ZrB2, TaB2 and NbB2 have sourced bulk values here, and FeB fractures easily under load)
identity:
  formula: MB2 for ZrB2, HfB2, NbB2 and TaB2 (AlB2-type hexagonal); WB; Co2B and CoB; FeB and Fe2B (boride layers on steel)
  cas: 12045-64-6 (ZrB2); 12007-23-7 (HfB2); 12007-35-1 (TaB2); 12007-09-9 (WB, as listed on PubChem); 12006-84-7 (FeB)
  crystal_system: hexagonal (ZrB2, TaB2, AlB2-type)
  space_group: P6/mmm (ZrB2 and TaB2, AlB2 prototype)
  source: sani-2016
  where: Table 1 of Sani et al. 2016 (crystal system, space group and AlB2 prototype rows); CAS numbers from the PubChem records listed under documents
tags:
  environments: []
  stresses:
    - extreme-heat
  applications:
    - heat-shields
    - thermal-control
    - mechanisms
properties:
  - benchmark: density
    label: Density, zirconium diboride (ZrB2)
    value: 6.119
    unit: g/cm3
    scope: material
    conditions:
      basis: Table 1 literature value for ZrB2 (cited to ref. 6 in the paper); not the paper's own samples
      process: Sani et al. section 2 used Zr powder below 44 micrometres (98.5 percent purity) and amorphous boron of at least 95 percent purity
    method: Density (g/cm3) row of Table 1 in Sani et al. 2016
    status: ground
    source: sani-2016
    where: Table 1, Density row, ZrB2 column
  - benchmark: melting_point
    label: Melting temperature, zirconium diboride (ZrB2)
    value: 3245
    unit: degC
    scope: material
    conditions:
      basis: Table 1 literature value (ref. 6 in the paper)
    method: Melting Temperature row of Table 1 in Sani et al. 2016
    status: ground
    source: sani-2016
    where: Table 1, Melting Temperature row, ZrB2 column
  - benchmark: cte
    label: Coefficient of thermal expansion, ZrB2
    value: 5.9
    unit: 10-6/K
    scope: material
    conditions:
      basis: Table 1 literature value; temperature not stated in the table
    method: Coefficient of Thermal Expansion row of Table 1 (printed as 5.9 x 10-6 K-1)
    status: ground
    source: sani-2016
    where: Table 1, Coefficient of Thermal Expansion row, ZrB2 column
  - benchmark: thermal_conductivity
    label: Thermal conductivity, ZrB2
    value: 60
    unit: W/(m·K)
    scope: material
    conditions:
      basis: Table 1 literature value; temperature not stated in the table
    method: Thermal conductivity row of Table 1 in Sani et al. 2016
    status: ground
    source: sani-2016
    where: Table 1, Thermal conductivity row, ZrB2 column
  - benchmark: elastic_modulus
    label: Young's modulus, ZrB2
    value: 489
    unit: GPa
    scope: material
    conditions:
      basis: Table 1 literature value; temperature not stated in the table
    method: Young's Modulus row of Table 1 in Sani et al. 2016
    status: ground
    source: sani-2016
    where: Table 1, Young's Modulus row, ZrB2 column
  - benchmark: hardness
    label: Hardness, ZrB2
    value: 23
    unit: GPa
    scope: material
    conditions:
      basis: Table 1 literature value; test method and load not stated in the table
    method: Hardness row of Table 1 in Sani et al. 2016
    status: ground
    source: sani-2016
    where: Table 1, Hardness row, ZrB2 column
  - benchmark: enthalpy_of_formation
    label: Enthalpy of formation at 25 degC, ZrB2
    value: -322.6
    unit: kJ/mol
    scope: material
    conditions:
      temperature: 25 degC
    method: Enthalpy of Formation row of Table 1 in Sani et al. 2016 (custom property name, not in the canon file)
    status: ground
    source: sani-2016
    where: Table 1, Enthalpy of Formation at 25 degC row, ZrB2 column
  - benchmark: molar_heat_capacity
    label: Heat capacity at 25 degC, ZrB2
    value: 48.2
    unit: J/(mol·K)
    scope: material
    conditions:
      temperature: 25 degC
    method: Heat Capacity row of Table 1 in Sani et al. 2016 (molar basis as printed; custom property name)
    status: ground
    source: sani-2016
    where: Table 1, Heat Capacity at 25 degC row, ZrB2 column
  - benchmark: lattice_parameter_a
    label: Lattice parameter a, ZrB2
    value: 3.17
    unit: Å
    scope: material
    conditions:
      basis: hexagonal P6/mmm cell, as printed in Table 1
    method: a (Å) row of Table 1 in Sani et al. 2016 (custom property name)
    status: ground
    source: sani-2016
    where: Table 1, a (Å) row, ZrB2 column
  - benchmark: lattice_parameter_c
    label: Lattice parameter c, ZrB2
    value: 3.53
    unit: Å
    scope: material
    conditions:
      basis: hexagonal P6/mmm cell, as printed in Table 1
    method: c (Å) row of Table 1 in Sani et al. 2016 (custom property name)
    status: ground
    source: sani-2016
    where: Table 1, c (Å) row, ZrB2 column
  - benchmark: density
    label: Density, tantalum diboride (TaB2)
    value: 12.568
    unit: g/cm3
    scope: material
    conditions:
      basis: Table 1 literature value for TaB2 (cited to ref. 6 in the paper); not the paper's own samples
    method: Density (g/cm3) row of Table 1 in Sani et al. 2016
    status: ground
    source: sani-2016
    where: Table 1, Density row, TaB2 column
  - benchmark: melting_point
    label: Melting temperature, tantalum diboride (TaB2)
    value: 3037
    unit: degC
    scope: material
    conditions:
      basis: Table 1 literature value (ref. 6 in the paper)
    method: Melting Temperature row of Table 1 in Sani et al. 2016
    status: ground
    source: sani-2016
    where: Table 1, Melting Temperature row, TaB2 column
  - benchmark: cte
    label: Coefficient of thermal expansion, TaB2
    value: 8.2
    unit: 10-6/K
    scope: material
    conditions:
      basis: Table 1 literature value; temperature not stated in the table
    method: Coefficient of Thermal Expansion row of Table 1 (printed as 8.2 x 10-6 K-1)
    status: ground
    source: sani-2016
    where: Table 1, Coefficient of Thermal Expansion row, TaB2 column
  - benchmark: thermal_conductivity
    label: Thermal conductivity, TaB2
    value: 16
    unit: W/(m·K)
    scope: material
    conditions:
      basis: Table 1 literature value; temperature not stated in the table
    method: Thermal conductivity row of Table 1 in Sani et al. 2016
    status: ground
    source: sani-2016
    where: Table 1, Thermal conductivity row, TaB2 column
  - benchmark: elastic_modulus
    label: Young's modulus, TaB2
    value: 248.2
    unit: GPa
    scope: material
    conditions:
      basis: Table 1 literature value; temperature not stated in the table
    method: Young's Modulus row of Table 1 in Sani et al. 2016
    status: ground
    source: sani-2016
    where: Table 1, Young's Modulus row, TaB2 column
  - benchmark: hardness
    label: Hardness, TaB2
    value: 19.6
    unit: GPa
    scope: material
    conditions:
      basis: Table 1 literature value; test method and load not stated in the table
    method: Hardness row of Table 1 in Sani et al. 2016
    status: ground
    source: sani-2016
    where: Table 1, Hardness row, TaB2 column
  - benchmark: enthalpy_of_formation
    label: Enthalpy of formation at 25 degC, TaB2
    value: -209.2
    unit: kJ/mol
    scope: material
    conditions:
      temperature: 25 degC
    method: Enthalpy of Formation row of Table 1 in Sani et al. 2016 (printed as -209.200; custom property name)
    status: ground
    source: sani-2016
    where: Table 1, Enthalpy of Formation at 25 degC row, TaB2 column
  - benchmark: molar_heat_capacity
    label: Heat capacity at 25 degC, TaB2
    value: 33.92
    unit: J/(mol·K)
    scope: material
    conditions:
      temperature: 25 degC
    method: Heat Capacity row of Table 1 in Sani et al. 2016 (molar basis as printed; custom property name)
    status: ground
    source: sani-2016
    where: Table 1, Heat Capacity at 25 degC row, TaB2 column
  - benchmark: lattice_parameter_a
    label: Lattice parameter a, TaB2
    value: 3.088
    unit: Å
    scope: material
    conditions:
      basis: hexagonal P6/mmm cell, as printed in Table 1 (3.0880)
    method: a (Å) row of Table 1 in Sani et al. 2016 (custom property name)
    status: ground
    source: sani-2016
    where: Table 1, a (Å) row, TaB2 column
  - benchmark: lattice_parameter_c
    label: Lattice parameter c, TaB2
    value: 3.241
    unit: Å
    scope: material
    conditions:
      basis: hexagonal P6/mmm cell, as printed in Table 1 (3.2410)
    method: c (Å) row of Table 1 in Sani et al. 2016 (custom property name)
    status: ground
    source: sani-2016
    where: Table 1, c (Å) row, TaB2 column
  - benchmark: density
    label: Density, ZrB2 with 20 volume percent SiC (composite, not pure ZrB2)
    value: 5.56
    unit: g/cm3
    scope: material
    conditions:
      basis: hot-pressed ZrB2 plus 20 vol percent SiC particulate, above 98 percent of theoretical density (NASA TPSX description)
      unit_in_source: kg/m3, SI column of the NASA table (5.56 x 10^3 kg/m3)
    method: Density row of the Properties at Standard Conditions table; converted from kg/m3 to g/cm3 here. The composite is not pure ZrB2, so it is not compared with the 6.119 value.
    status: ground
    source: nasa-tpsx-zrb2
    where: Properties at Standard Conditions table, Density row (SI units); Description
  - benchmark: hardness
    label: Hardness, NbB2 (reaction SPS, 97.7 percent dense)
    value: 20.25
    unit: GPa
    scope: material
    conditions:
      temperature: room temperature (not stated)
      basis: dense NbB2 at 97.7 percent of theoretical density, reaction spark plasma sintering at 1900 degC for 15 min
    method: Hardness value stated in the abstract (test method and load not stated in the abstract)
    status: ground
    source: sairam-nbb2-2014
    where: Abstract, final sentence
  - benchmark: elastic_modulus
    label: Elastic modulus, NbB2 (reaction SPS, 97.7 percent dense)
    value: 539
    unit: GPa
    scope: material
    conditions:
      basis: dense NbB2 at 97.7 percent of theoretical density, reaction spark plasma sintering at 1900 degC for 15 min
    method: Elastic modulus stated in the abstract
    status: ground
    source: sairam-nbb2-2014
    where: Abstract, final sentence
  - benchmark: fracture_toughness
    label: Indentation fracture toughness, NbB2 (reaction SPS)
    value: 4
    unit: MPa·m1/2
    scope: material
    conditions:
      basis: dense NbB2 at 97.7 percent of theoretical density; indentation method
    method: Indentation fracture toughness stated in the abstract (custom property name)
    status: ground
    source: sairam-nbb2-2014
    where: Abstract, final sentence
  - benchmark: relative_density
    label: Relative density, NbB2 after reaction SPS at 1900 degC
    value: 97.7
    unit: '%'
    scope: material
    conditions:
      temperature: 1900 degC
      time: 15 min holding period
      basis: percent of theoretical density (the abstract prints it as about 97.7 percent of rho-th)
    method: Abstract, reaction spark plasma sintering from elemental Nb and B (two-stage heat treatment, 1200 degC synthesis, 1700 to 1900 degC densification)
    status: ground
    source: sairam-nbb2-2014
    where: Abstract, second sentence
  - benchmark: density
    label: Density, Co2B nanocrystals (specific density, measured)
    value: 7.61
    value_high: 7.78
    bound: range
    unit: g/cm3
    scope: material
    conditions:
      basis: Co2B nanocrystals from mechanochemical synthesis, not bulk Co2B
    method: Specific density range printed in the abstract of the 2016 Bulletin of Materials Science paper
    status: ground
    source: co2b-2016
    where: Abstract, sentence on specific density
  - benchmark: saturation_magnetization
    label: Saturation magnetization, Co2B nanocrystals
    value: 35
    value_high: 50
    bound: range
    unit: emu/g
    scope: material
    conditions:
      basis: Co2B nanocrystals from mechanochemical synthesis
    method: Saturation magnetization range printed in the abstract (custom property name)
    status: ground
    source: co2b-2016
    where: Abstract, sentence on saturation magnetization
  - benchmark: hardness
    label: Hardness, Fe2B single-phase layer on low-carbon steel (electrochemical boriding)
    value: 16
    unit: GPa
    scope: material
    conditions:
      basis: single-phase Fe2B layer of 75 micrometres after 15 min boriding in a borax electrolyte at 950 degC plus 45 min homogenisation at rest
      test: microhardness, load not stated in the abstract
    method: Microhardness value stated in the abstract
    status: ground
    source: kartal-2011
    where: Abstract, final part (microhardness of about 16GPa)
  - benchmark: layer_thickness
    label: Boride layer thickness, single-phase Fe2B after phase homogenisation
    value: 75
    unit: micrometre
    scope: material
    conditions:
      basis: electrochemical boriding at 950 degC, 15 min, then 45 min at rest without polarisation
    method: Thickness stated in the abstract (custom property name)
    status: ground
    source: kartal-2011
    where: Abstract, second half (thickness increased to 75 micrometres)
  - benchmark: hardness
    label: Hardness, WB3 (predicted structure, DFT, not WB)
    value: 38.3
    unit: GPa
    scope: material
    conditions:
      basis: hP24-WB3 structure predicted by density functional theory; a different phase from WB
    method: Vickers hardness stated in the abstract as a property of the predicted structure (calculated, not measured)
    status: calculated
    note: Predicted for WB3, not for tungsten boride (WB). Shown only as the one tungsten boride hardness read in this run.
    source: wb3-2012
    where: Abstract, first paragraph
corrosion:
  - environment: Oxidising air at high temperature (ZrB2 with 20 volume percent SiC, hot pressed)
    headline: NASA TPSX says SiC improves the oxidation resistance of the composite compared with pure ZrB2. No numbers are given.
    effect: The NASA Ames TPS database says studies indicate that the SiC particulate improves the oxidation resistance of ZrB2 compared with pure ZrB2. The page gives no oxidation rate, scale thickness or temperature limit, so the rating is left as unknown.
    rating: unknown
    conditions:
      form: hot-pressed composite, ZrB2 plus 20 vol percent SiC
      basis: qualitative statement in the Description
    source: nasa-tpsx-zrb2
    where: Description paragraph of the Zirconium Diboride (ZrB2/SiC) entry
risks:
  hazards:
    - text: 'PubChem aggregated GHS for zirconium diboride (CID 9812765, signal word Warning): H302 harmful if swallowed, H312 harmful in contact with skin and H332 harmful if inhaled, each in 85.2 percent of the 54 company reports. 7.4 percent (4 of 54) of reports say it does not meet GHS hazard criteria.'
      ghs:
        - H302
        - H312
        - H332
      source: pubchem-zrb2
      where: Safety and Hazards, GHS Classification
    - text: 'PubChem aggregated GHS for hafnium diboride (CID 6336857, signal word Warning): H315 causes skin irritation, H319 causes serious eye irritation and H335 may cause respiratory irritation, each in 98.6 percent of 212 reports. 1.4 percent (3 of 212) of reports say it does not meet GHS hazard criteria.'
      ghs:
        - H315
        - H319
        - H335
      source: pubchem-hfb2
      where: Safety and Hazards, GHS Classification
    - text: 'PubChem aggregated GHS for tantalum diboride (CID 9815613, signal word Warning): H315, H319 and H335 in 99.5 percent of 210 reports. 0.5 percent (1 of 210) of reports say it does not meet GHS hazard criteria.'
      ghs:
        - H315
        - H319
        - H335
      source: pubchem-tab2
      where: Safety and Hazards, GHS Classification
    - text: PubChem lists tungsten boride (CID 82793) as Not Classified, reported as not meeting GHS hazard criteria by 1 of 1 company. This is one notification, so it is weak evidence.
      ghs: []
      source: pubchem-wb
      where: Safety and Hazards, GHS Classification
    - text: PubChem lists iron boride (FeB, CID 82789) as not meeting GHS hazard criteria for 100 percent (5 of 5) of reports. No supplier safety data sheet for any boride in this family was read in this run.
      ghs: []
      source: pubchem-feb
      where: Safety and Hazards, GHS Classification
  failure_risks:
    - mode: Brittle fracture of dense NbB2 ceramic
      trigger: Tensile or bending load on a flaw
      consequence: Indentation fracture toughness of 4 MPa·m1/2 for dense NbB2 (reaction SPS, 97.7 percent dense), so a flaw can grow at a stress well below the hardness limit
      severity: moderate
      source: sairam-nbb2-2014
      where: Abstract, final sentence
    - mode: Delamination or fracture of FeB-rich boride layer
      trigger: High normal or tangential load on the boride layer on steel
      consequence: FeB is much harder than Fe2B but has very poor fracture toughness, so it can fracture or delaminate easily under high normal or tangential loading (Kartal et al.)
      severity: moderate
      source: kartal-2011
      where: Abstract, opening sentences
  side_effects: []
  handling:
    - text: 'Powder of HfB2 and TaB2: PubChem lists H315, H319 and H335, so avoid skin and eye contact and do not breathe the dust.'
      source: pubchem-hfb2
      where: Safety and Hazards, GHS Classification
    - text: Zirconium and tantalum borides are hard to consolidate by pressure-assisted sintering, so spark plasma sintering or reactive sintering is used (Sani et al. 2016, introduction).
      source: sani-2016
      where: Introduction, paragraph on consolidation
  mitigations: []
uses:
  - application: heat-shields
    device: Thermal protection system material for sharp leading edges
    example: NASA Ames lists hot-pressed ZrB2 with 20 volume percent SiC for thermal protection, such as sharp leading edges on next-generation reusable launch vehicles, and notes that the material can be machined by EDM.
    source: nasa-tpsx-zrb2
    where: Description paragraph of the Zirconium Diboride (ZrB2/SiC) entry
  - application: thermal-control
    device: Solar thermal absorber for concentrating solar power
    example: Sani et al. studied the optical properties of dense monophasic ZrB2 and TaB2 for the first time, because they may suit solar absorbers that work at very high temperature.
    source: sani-2016
    where: Abstract and Introduction
  - application: mechanisms
    device: Boride layer on low-carbon steel for hard, wear-resistant surfaces
    example: Electrochemical boriding at 950 degC for 15 min in a borax electrolyte gives a boride layer, and a further 45 min at rest turns it into a single-phase Fe2B layer with a microhardness of about 16 GPa (Kartal et al.).
    source: kartal-2011
    where: Abstract
supply:
  - subject: Boron (common to all the diborides and the boride layers)
    abundance: 'USGS: borates occur in volcanic and arid deposits. The 2025 world total is withheld (XX). 2025 production by country, in thousand metric tons and as printed: Turkey 1,500 (refined borates), Bolivia 380 (ulexite), Chile 300 (ulexite), China 230 (boric oxide equivalent), Peru 220 (crude borates), Argentina 170 (crude ore), Russia 80 (datolite ore), Germany 40 (compounds).'
    producers: []
    critical_lists:
      - list: USGS 2025 Final List of Critical Minerals (boron added in 2025, per the USGS boron chapter)
        listed: true
      - list: EU critical raw materials list (Boron/Borate)
        listed: true
    recycling: 'USGS: insignificant.'
    substitutes:
      - 'USGS: substitution is possible in detergents, enamels, insulation and soaps. No substitute for boron in a boride is discussed.'
    source: usgs-mcs2026-boron
    where: World Production and Reserves table, 2025 column (world total XX); Events, Trends and Issues (critical list paragraph); Recycling; Substitutes
  - subject: Iron (the metal in FeB and Fe2B)
    abundance: 'USGS: the only source of primary iron is iron ore. World usable ore production was 2,600,000 thousand metric tons in 2024 and an estimated 2,600,000 in 2025, with an iron content of 1,600,000 thousand metric tons in both years. 2025 usable ore by country, in thousand metric tons: Australia 980,000, Brazil 420,000, India 310,000, China 290,000.'
    producers: []
    critical_lists:
      - list: EU critical raw materials list (iron is not named)
        listed: false
    recycling: 'USGS: none for iron ore; see the iron and steel scrap chapter (not read).'
    substitutes:
      - 'USGS: the only source of primary iron is iron ore, used directly as direct-shipping ore or converted to pig iron.'
    source: usgs-mcs2026-iron
    where: World Mine Production and Reserves table (usable ore, 2025e column); Substitutes; the critical list paragraph in Events, Trends and Issues
  - subject: Zirconium (the metal in ZrB2)
    abundance: 'USGS: world mine production of zirconium mineral concentrates was about 1,200 thousand metric tons gross weight in 2025 (1,300 in 2024). 2025 by country, in thousand metric tons: Australia 400, South Africa 270, Mozambique 160, Senegal 70, China 100, Indonesia 52, Madagascar 26, Sierra Leone 25. Zirconium reserves are more than 70,000 thousand metric tons ZrO2 content.'
    producers: []
    critical_lists:
      - list: EU critical raw materials list (zirconium is not named)
        listed: false
    recycling: 'USGS: companies in Oregon and Utah recycled zirconium from new scrap and from post-commercial old scrap; foundry mould cores and spent zirconia refractories are often recycled but cannot be quantified.'
    substitutes:
      - 'USGS: chromite and olivine can substitute for zircon in some foundry applications, and niobium, stainless steel and tantalum in some nuclear applications.'
    source: usgs-mcs2026-zirconium-hafnium
    where: World Mine Production and Reserves table; Recycling; Substitutes
  - subject: Hafnium (the metal in HfB2)
    abundance: 'USGS: world primary hafnium production data and quantitative hafnium reserve estimates were not available. Germany supplied 54 percent of unwrought hafnium imports in 2021 to 2024 (USGS import sources); the leading global exporters of unwrought hafnium were China and Germany.'
    producers: []
    critical_lists:
      - list: EU critical raw materials list (hafnium is named)
        listed: true
    recycling: 'USGS: hafnium metal recycling was minimal.'
    substitutes: []
    source: usgs-mcs2026-zirconium-hafnium
    where: World Mine Production and Reserves text; Import Sources (2021 to 2024); Recycling
  - subject: Niobium (the metal in NbB2)
    abundance: 'USGS: world mine production of niobium was 112,000 t in 2024 and 2025 (rounded total). Brazil produced 104,000 t in 2024 and 2025e, and Canada 6,900 t in 2024 and 6,000 t in 2025e. U.S. net import reliance was 100 percent.'
    producers:
      - country: Brazil
        share_percent: 93
        year: 2025
      - country: Canada
        share_percent: 5
        year: 2025
    critical_lists:
      - list: EU critical raw materials list (Niobium)
        listed: true
    recycling: 'USGS: niobium was recycled with niobium-bearing steels and superalloys, but scrap recovery specifically for niobium content was negligible.'
    substitutes:
      - 'USGS: ceramic matrix composites, molybdenum, tantalum and tungsten in superalloys, and molybdenum, tantalum and titanium as alloying elements in steels, can substitute with a performance loss or higher cost.'
    source: usgs-mcs2026-niobium
    where: Events, Trends and Issues (Brazil about 93 percent, Canada 5 percent); World Mine Production and Reserves table; Salient Statistics (net import reliance); Substitutes
  - subject: Tantalum (the metal in TaB2)
    abundance: 'USGS: tantalum has not been mined in the United States since 1959, and U.S. net import reliance was 100 percent. World mine production was 2,500 t in 2024 and 2025e (rounded). Congo (Kinshasa) produced 1,270 t in 2024 and 1,300 t in 2025e, Rwanda 374 t and 400 t, Nigeria 390 t in both years.'
    producers: []
    critical_lists:
      - list: EU critical raw materials list (Tantalum)
        listed: true
    recycling: 'USGS: tantalum was recycled mostly from new scrap of tantalum electronic components and from cemented carbide and superalloy scrap; recycling may account for as much as 30 percent of consumption by domestic primary processors.'
    substitutes:
      - 'USGS: niobium and tungsten in carbides; aluminium, ceramics and niobium in capacitors; hafnium, iridium, molybdenum, niobium, rhenium and tungsten in high-temperature applications.'
    source: usgs-mcs2026-tantalum
    where: Salient Statistics (net import reliance); World Mine Production and Reserves table; Recycling; Substitutes
  - subject: Tungsten (the metal in WB)
    abundance: 'USGS: tungsten has not been mined commercially in the United States since 2015. World mine production was 82,000 t in 2024 and 85,000 t in 2025 (rounded). China produced 67,000 t in both years. U.S. net import reliance was more than 50 percent.'
    producers: []
    critical_lists:
      - list: EU critical raw materials list (Tungsten)
        listed: true
    recycling: 'USGS: the quantity of secondary tungsten and the amount consumed from secondary sources were withheld to protect company data.'
    substitutes:
      - 'USGS: molybdenum carbide, niobium carbide or titanium carbide cemented carbides, ceramics, cermets and tool steels can substitute for cemented tungsten carbide, but most of these reduce rather than replace the tungsten used.'
    source: usgs-mcs2026-tungsten
    where: Domestic Production and Use; Salient Statistics (net import reliance); World Mine Production and Reserves table; Substitutes
  - subject: Cobalt (the metal in Co2B and CoB)
    abundance: 'USGS: Congo (Kinshasa) was the world''s leading source of mined cobalt, with an estimated 73 percent of the 2025 world total, followed by Indonesia with 14 percent. World mine production was 302,000 t in 2024 and 310,000 t in 2025 (rounded). U.S. net import reliance was 79 percent in 2025e.'
    producers:
      - country: Congo (Kinshasa)
        share_percent: 73
        year: 2025
      - country: Indonesia
        share_percent: 14
        year: 2025
    critical_lists:
      - list: EU critical raw materials list (Cobalt)
        listed: true
    recycling: 'USGS: cobalt content of purchased scrap represented 25 percent of estimated U.S. cobalt consumption in 2025.'
    substitutes:
      - 'USGS: barium or strontium ferrites, neodymium-iron-boron alloys, nickel-iron alloys in magnets; iron, nickel, cermets or ceramics in cutting and wear-resistant materials.'
    source: usgs-mcs2026-cobalt
    where: Events, Trends and Issues (Congo 73 percent, Indonesia 14 percent); World Mine Production and Reserves table; Salient Statistics; Recycling; Substitutes
media: []
documents:
  - title: Sani E. et al. 2016, Optical properties of dense zirconium and tantalum diborides for solar thermal absorbers, Renewable Energy 91, 340-346 (arXiv preprint copy read)
    type: paper
    url: https://arxiv.org/pdf/1803.06528
    year: 2016
  - title: Sairam K. et al. 2014, Reaction spark plasma sintering of niobium diboride, Int. J. Refract. Met. Hard Mater. 43, 259-262 (abstract read)
    type: paper
    url: https://research.birmingham.ac.uk/en/publications/reaction-spark-plasma-sintering-of-niobium-diboride/
    year: 2014
  - title: Kartal G. et al. 2011, The growth of single Fe2B phase on low carbon steel via phase homogenization in electrochemical boriding (PHEB), Surface and Coatings Technology 206(7), 2005-2011 (abstract read)
    type: paper
    url: https://research.itu.edu.tr/en/publications/the-growth-of-single-fesub2subb-phase-on-low-carbon-steel-via-pha/
    year: 2011
  - title: Mechanochemical synthesis and characterization of pure Co2B nanocrystals, Bulletin of Materials Science 39(4), 1119-1126 (2016), abstract read
    type: paper
    url: https://avesis.hacettepe.edu.tr/yayin/8d53ea59-7ba3-464f-9afa-322edb325d5c/mechanochemical-synthesis-and-characterization-of-pure-co2b-nanocrystals
    year: 2016
  - title: Liang Y. et al., Novel Tungsten Triboride (arXiv 1210.3777, DFT prediction of WB3)
    type: paper
    url: https://arxiv.org/pdf/1210.3777
    year: 2012
  - title: NASA Ames Thermal Protection Materials Database, Zirconium Diboride (ZrB2/SiC)
    type: report
    url: https://tpsx.arc.nasa.gov/Material?id=28
    year: 2001
  - title: PubChem compound record CID 9812765, Zirconium diboride (GHS classification and melting point)
    type: database
    url: https://pubchem.ncbi.nlm.nih.gov/compound/9812765
    year: 2026
  - title: PubChem compound record CID 6336857, Hafnium boride (HfB2) (GHS classification)
    type: database
    url: https://pubchem.ncbi.nlm.nih.gov/compound/6336857
    year: 2026
  - title: PubChem compound record CID 9815613, Tantalum boride (TaB2) (GHS classification)
    type: database
    url: https://pubchem.ncbi.nlm.nih.gov/compound/9815613
    year: 2026
  - title: PubChem compound record CID 82793, Tungsten boride (GHS classification)
    type: database
    url: https://pubchem.ncbi.nlm.nih.gov/compound/82793
    year: 2026
  - title: PubChem compound record CID 82789, Iron boride (FeB) (GHS classification)
    type: database
    url: https://pubchem.ncbi.nlm.nih.gov/compound/82789
    year: 2026
  - title: USGS Mineral Commodity Summaries 2026, Niobium (Columbium)
    type: report
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-niobium.pdf
    year: 2026
  - title: USGS Mineral Commodity Summaries 2026, Tantalum
    type: report
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-tantalum.pdf
    year: 2026
  - title: USGS Mineral Commodity Summaries 2026, Tungsten
    type: report
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-tungsten.pdf
    year: 2026
  - title: USGS Mineral Commodity Summaries 2026, Cobalt
    type: report
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-cobalt.pdf
    year: 2026
  - title: USGS Mineral Commodity Summaries 2026, Zirconium and Hafnium
    type: report
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-zirconium-hafnium.pdf
    year: 2026
  - title: USGS Mineral Commodity Summaries 2026, Iron Ore
    type: report
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-iron-ore.pdf
    year: 2026
  - title: USGS Mineral Commodity Summaries 2026, Boron
    type: report
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-boron.pdf
    year: 2026
  - title: European Commission, Critical raw materials list (Regulation (EU) 2024/1252)
    type: standard
    url: https://single-market-economy.ec.europa.eu/sectors/raw-materials/areas-specific-interest/critical-raw-materials_en
    year: 2024
gaps:
  - 'Hafnium diboride: no density, melting point, hardness or modulus was read in a source for this run. The Gurcan (ITU/Anadolu) and Akcamli SPS papers were unreachable or gave only search snippets, and the HfB2 values in Wikipedia and vendor pages are not used.'
  - 'Niobium diboride: no density or melting point was read in a source for this run (the Wikipedia and webqc values were not read on a primary page). Only the Birmingham abstract (hardness, modulus, fracture toughness, relative density) is used.'
  - 'Tungsten boride (WB): no measured density, hardness or modulus was read. PubChem''s density (15.2 at 25 degC) has no source stated and is not used. The WB3 hardness is a DFT prediction for a different phase and is not a WB value. W2B5 identity is disputed in the literature (USPEX 2018 snippet), so it is not profiled.'
  - 'Cobalt boride: CoB has no value, and the Henschel 2019 Dalton paper on Co2B and CoB layers was blocked by a browser challenge. The PubChem record for cobalt boride (CID 121233614) has a malformed formula and was not used. No GHS entry for Co2B or CoB was read.'
  - Iron boride density (Fe2B and FeB) was not found. The MDPI Coatings 2023 paper returned HTTP 403, and only search snippets gave hardness (about 1600 HV Fe2B, about 2000 HV FeB) which are not used as values. FeB hardness as a number was not read in a fetched page.
  - 'Oxidation or corrosion data for NbB2, TaB2, HfB2, WB, Co borides and Fe borides: none read in this run. The only corrosion entry is NASA''s qualitative statement for ZrB2 with SiC.'
  - 'Space use: the only space-related source is NASA TPSX (ZrB2 with SiC for thermal protection of sharp leading edges, ground-test material). No flight or lunar source for any boride in this family was read.'
  - 'U.S. 2025 Final List of Critical Minerals status: the boron chapter confirms boron was added, but the niobium, tantalum, tungsten, cobalt, zirconium and hafnium chapters do not state their list status. Only the EU list is recorded for those metals.'
  - 'Designations (UNS, EN, DIN, JIS, AISI and similar codes) for any boride in this family: none printed in a source read in this run.'
  - 'Custom property names used because they are not in spores/property-canon.yaml: hardness, fracture_toughness, relative_density, enthalpy_of_formation, molar_heat_capacity, lattice_parameter_a, lattice_parameter_c, layer_thickness, saturation_magnetization.'
  - Sani et al. electrical conductivity values (printed as 1.0 x 10^7 and 3.03 x 10^6 S/m) are left out because the exponent is not printed in plain form. The molar heat capacities are kept in J/(mol·K) as printed.
  - Hardness test method and load for ZrB2, TaB2 and NbB2 are not stated in the Table 1 or abstract that gives them, so the values are reported without a method.
  - 'Source disagreement: ZrB2 density is 6.119 g/cm3 for dense pure ZrB2 (Sani) and 5.56 g/cm3 for a ZrB2 plus 20 vol percent SiC composite (NASA). They are different materials and are not compared as one value.'
  - 'Story compare paragraph: the comparison is inside this family only. No family or industry comparison set is sourced for HfB2, NbB2 or the cobalt and iron borides.'
sources:
  sani-2016:
    title: Sani E. et al., Optical properties of dense zirconium and tantalum diborides for solar thermal absorbers, Renewable Energy 91 (2016) 340-346, doi 10.1016/j.renene.2016.01.068 (arXiv preprint 1803.06528)
    url: https://arxiv.org/pdf/1803.06528
    kind: paper
    license: arXiv preprint, read for its Table 1 values
  sairam-nbb2-2014:
    title: Sairam K. et al., Reaction spark plasma sintering of niobium diboride, Int. J. Refract. Met. Hard Mater. 43 (2014) 259-262, doi 10.1016/j.ijrmhm.2013.12.011
    url: https://research.birmingham.ac.uk/en/publications/reaction-spark-plasma-sintering-of-niobium-diboride/
    kind: paper
    license: University of Birmingham repository page (abstract)
  kartal-2011:
    title: Kartal G. et al., The growth of single Fe2B phase on low carbon steel via phase homogenization in electrochemical boriding (PHEB), Surface and Coatings Technology 206(7) (2011) 2005-2011, doi 10.1016/j.surfcoat.2011.08.049
    url: https://research.itu.edu.tr/en/publications/the-growth-of-single-fesub2subb-phase-on-low-carbon-steel-via-pha/
    kind: paper
    license: Istanbul Technical University repository page (abstract)
  co2b-2016:
    title: Mechanochemical synthesis and characterization of pure Co2B nanocrystals, Bulletin of Materials Science 39(4) (2016) 1119-1126, doi 10.1007/s12034-016-1231-x
    url: https://avesis.hacettepe.edu.tr/yayin/8d53ea59-7ba3-464f-9afa-322edb325d5c/mechanochemical-synthesis-and-characterization-of-pure-co2b-nanocrystals
    kind: paper
    license: repository listing (abstract)
  wb3-2012:
    title: Liang Y. et al., Novel Tungsten Triboride (arXiv 1210.3777), density functional theory prediction of hP24-WB3
    url: https://arxiv.org/pdf/1210.3777
    kind: paper
    license: arXiv
  nasa-tpsx-zrb2:
    title: NASA Ames Thermal Protection Materials Database, Zirconium Diboride (ZrB2/SiC) entry
    url: https://tpsx.arc.nasa.gov/Material?id=28
    kind: database
    license: NASA public domain
  pubchem-zrb2:
    title: PubChem compound CID 9812765, Zirconium diboride (GHS aggregated notifications, melting point)
    url: https://pubchem.ncbi.nlm.nih.gov/compound/9812765
    kind: database
    license: PubChem public database
  pubchem-hfb2:
    title: PubChem compound CID 6336857, Hafnium boride (HfB2), GHS aggregated notifications
    url: https://pubchem.ncbi.nlm.nih.gov/compound/6336857
    kind: database
    license: PubChem public database
  pubchem-tab2:
    title: PubChem compound CID 9815613, Tantalum boride (TaB2), GHS aggregated notifications
    url: https://pubchem.ncbi.nlm.nih.gov/compound/9815613
    kind: database
    license: PubChem public database
  pubchem-wb:
    title: PubChem compound CID 82793, Tungsten boride, GHS classification
    url: https://pubchem.ncbi.nlm.nih.gov/compound/82793
    kind: database
    license: PubChem public database
  pubchem-feb:
    title: PubChem compound CID 82789, Iron boride (FeB), GHS classification
    url: https://pubchem.ncbi.nlm.nih.gov/compound/82789
    kind: database
    license: PubChem public database
  usgs-mcs2026-boron:
    title: USGS Mineral Commodity Summaries 2026, Boron
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-boron.pdf
    kind: report
    license: US government work, public domain
  usgs-mcs2026-iron:
    title: USGS Mineral Commodity Summaries 2026, Iron Ore
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-iron-ore.pdf
    kind: report
    license: US government work, public domain
  usgs-mcs2026-zirconium-hafnium:
    title: USGS Mineral Commodity Summaries 2026, Zirconium and Hafnium
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-zirconium-hafnium.pdf
    kind: report
    license: US government work, public domain
  usgs-mcs2026-niobium:
    title: USGS Mineral Commodity Summaries 2026, Niobium (Columbium)
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-niobium.pdf
    kind: report
    license: US government work, public domain
  usgs-mcs2026-tantalum:
    title: USGS Mineral Commodity Summaries 2026, Tantalum
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-tantalum.pdf
    kind: report
    license: US government work, public domain
  usgs-mcs2026-tungsten:
    title: USGS Mineral Commodity Summaries 2026, Tungsten
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-tungsten.pdf
    kind: report
    license: US government work, public domain
  usgs-mcs2026-cobalt:
    title: USGS Mineral Commodity Summaries 2026, Cobalt
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-cobalt.pdf
    kind: report
    license: US government work, public domain
  ec-crm:
    title: European Commission, Critical raw materials (list under Regulation (EU) 2024/1252)
    url: https://single-market-economy.ec.europa.eu/sectors/raw-materials/areas-specific-interest/critical-raw-materials_en
    kind: web
    license: European Commission web page
license: CC-BY-4.0
updated: '2026-10-26'
review:
  status: draft
  method: assistant
  checked_values: 30
  unreachable: 4
  caveats: 4
  note: 'Written in this run from Sani et al. 2016 (Table 1), Sairam et al. 2014 (abstract), Kartal et al. 2011 (abstract), the Co2B 2016 abstract, NASA TPSX (ZrB2/SiC), PubChem GHS for five borides, USGS MCS 2026 chapters and the EU critical list. Unreachable or snippet-only: MDPI Coatings 2023 (403), Henschel 2019 Dalton (browser challenge), Gurcan HfB2 and Akcamli HfB2 papers, Sonber ZrB2 paper. Values are not yet checked by verify-values.'
story:
  recipe:
    line: These are hexagonal metal borides. ZrB2 and TaB2 take the AlB2-type structure (space group P6/mmm, Sani 2016 Table 1). Sani et al. made dense ZrB2 and TaB2 by reactive spark plasma sintering from metal and amorphous boron, or by spark plasma sintering of powders first made by self-propagating high-temperature synthesis. Sairam et al. made dense NbB2 (97.7 percent) from elemental niobium and boron, synthesised at 1200 degC and then densified at 1700 to 1900 degC by reaction spark plasma sintering. The iron borides form as layers on steel during electrochemical boriding in a borax electrolyte at 950 degC (Kartal 2011).
    parts:
      - name: Boron
        symbol: B
        share: two boron per metal in MB2 diborides (AlB2-type)
        role: Forms the boron layers in the hexagonal structure. In Sani et al., amorphous boron of at least 95 percent purity was used as one reactant.
      - name: Zirconium
        symbol: Zr
        share: one zirconium per two boron in ZrB2
        role: Metal in ZrB2, the most-studied diboride in this family; Sani et al. used Zr powder below 44 micrometres.
      - name: Tantalum
        symbol: Ta
        share: one tantalum per two boron in TaB2
        role: Metal in TaB2, which is denser than ZrB2 (12.568 g/cm3 against 6.119 g/cm3).
  risk_groups:
    - title: For people
      level: 0
      verdict: Irritant or harmful; GHS data are partial
      lead: PubChem aggregated notifications give harmful-category codes (H302, H312, H332) for ZrB2 and irritation codes (H315, H319, H335) for HfB2 and TaB2. Most reports agree, but a small share say the material is not classified. No supplier SDS was read for these borides.
      points:
        - label: Dust
          line: Control dust when handling powders of any boride. PubChem lists irritation of skin, eyes and the respiratory tract for HfB2 and TaB2.
        - label: Ingestion and skin
          line: PubChem lists H302, H312 and H332 (harmful if swallowed, in contact with skin or inhaled) for ZrB2 in 85.2 percent of its 54 reports.
    - title: If a part fails
      lead: These are the numbers read for dense material at room temperature.
      points:
        - label: Brittleness
          line: Dense NbB2 has an indentation fracture toughness of 4 MPa·m1/2, so a flaw can start a crack well below the hardness limit.
        - label: Boride layer
          line: The FeB layer is much harder than Fe2B but fractures or delaminates easily under high load (Kartal 2011).
  why: Transition-metal borides are hard, high-melting ceramics or layers. Dense ZrB2 has a melting temperature of 3245 degC and a hardness of 23 GPa (Sani 2016), and dense NbB2 has a hardness of 20.25 GPa and an elastic modulus of 539 GPa (Sairam 2014). Steel boriding gives a surface boride layer of about 16 GPa microhardness. The sourced uses are heat shields for launch vehicles (NASA), solar absorbers (Sani) and wear layers on steel (Kartal).
  uses:
    - where: Heat-shield material for sharp leading edges
      line: NASA Ames lists hot-pressed ZrB2 with 20 vol percent SiC for thermal protection, such as sharp leading edges on next-generation reusable launch vehicles.
    - where: Solar thermal absorbers
      line: Sani et al. studied dense ZrB2 and TaB2 for the first time as solar absorbers that work at very high temperature.
    - where: Wear-resistant boride layers on steel
      line: A single-phase Fe2B layer of 75 micrometres, grown by electrochemical boriding at 950 degC, has a microhardness of about 16 GPa (Kartal 2011).
  can_do:
    - label: Density
      benchmark: density
      value: 6.119 g/cm3
      line: Dense ZrB2 has a density of 6.119 g/cm3, and TaB2 is much denser at 12.568 g/cm3 (Sani 2016).
    - label: Heat resistance
      benchmark: melting_point
      value: 3245 degC
      line: The melting temperature is 3245 degC for ZrB2 and 3037 degC for TaB2 (Sani 2016).
    - label: Hardness
      benchmark: hardness
      value: 23 GPa
      line: ZrB2 has a hardness of 23 GPa and TaB2 19.6 GPa (Sani 2016). NbB2 is 20.25 GPa at 97.7 percent density (Sairam 2014).
    - label: Stiffness
      benchmark: elastic_modulus
      value: 539 GPa
      line: NbB2 has an elastic modulus of 539 GPa (Sairam 2014). ZrB2 is 489 GPa and TaB2 248.2 GPa (Sani 2016).
  compare: 'Within this family, ZrB2 is the lightest and hardest of the diborides with sourced bulk values: 6.119 g/cm3, 23 GPa and a melting temperature of 3245 degC. TaB2 is about twice as dense and has a lower melting point (3037 degC), hardness (19.6 GPa) and modulus (248.2 GPa). NbB2 has a hardness of 20.25 GPa and a modulus of 539 GPa at 97.7 percent density, so its stiffness is the highest of the sourced diborides. Hafnium diboride, tungsten boride and the cobalt and iron borides lack sourced bulk values here.'
  compare_rows:
    - label: Density
      benchmark: density
    - label: Hardness
      benchmark: hardness
    - label: Melting temperature
      benchmark: melting_point
  handling:
    - PubChem lists H315, H319 and H335 for HfB2 and TaB2, so avoid skin and eye contact and do not breathe the dust.
    - Zirconium and tantalum borides are hard to sinter. Sani et al. used spark plasma sintering to reach dense material.
    - 'Oxidation in air at high temperature: NASA says SiC improves the oxidation resistance of ZrB2, so pure ZrB2 is less resistant. No rate data were read.'
  good_for:
    - Hard, stiff parts made from ZrB2 or NbB2. The sourced values are 23 GPa for ZrB2 and 20.25 GPa and 539 GPa for NbB2.
    - Hard wear layers on steel from boriding, with a single-phase Fe2B layer of about 16 GPa microhardness.
  watch_out:
    - 'Brittleness: NbB2 has an indentation fracture toughness of 4 MPa·m1/2 (Sairam 2014). FeB fractures easily under high load (Kartal 2011).'
    - 'Thin data: hafnium diboride and tungsten boride have no sourced density or hardness here, and Co and Fe boride bulk values are missing.'
    - 'Mixed basis: the ZrB2 density 6.119 g/cm3 is for pure ZrB2, while the NASA ZrB2 density of 5.56 g/cm3 is for a SiC composite.'
  verdict: The sourced family is a set of hard, high-melting ceramics and steel boride layers. ZrB2 and NbB2 have the best bulk values here, TaB2 is much denser, and the cobalt, iron, hafnium and tungsten borides still need measured data before they can be compared properly.