Chromium diboride CrB2

spore: profile
id: chromium-boride
spore_version: 0.2
family: carbide-nitride-boride-ceramics
name: Chromium diboride
abbreviation: CrB2
role: hexagonal metal boride ceramic; related hard coating chromium carbide (Cr3C2) included
category: ceramics
type: ultra-high-temperature-ceramics
summary: |
  Chromium diboride (CrB2) is a hexagonal boride (space group P6/mmm, Materials Project). The sourced numbers for the bulk material are thin: a supplier safety data sheet gives a relative density of 5.15 g/cc and a melting point of 1850 degC for the powder, and the Materials Project calculates a density of 5.32 g/cm3, a bulk modulus of 252 GPa and a shear modulus of 183 GPa (density functional theory, not a measurement). Silicon-alloyed CrB2 thin films, deposited by sputtering, formed an oxide scale of about 400 nm on average after 3 h at 1200 degC in air. Chromium carbide (Cr3C2) is covered as a related hard coating: a 2024 study of HVOF Cr3C2-NiCr coatings on ductile cast iron measured a microhardness of 9.72 GPa near the surface, and a 2025 study found the NiCr matrix corrodes selectively in a ferric chloride test. PubChem's aggregated hazard codes for CrB2 (skin sensitisation, eye damage and carcinogenicity) conflict with the supplier SDS, which says the powder is not classified as hazardous.
best_for: Silicon-alloyed CrB2 films that must resist oxidation at 1100 to 1200 degC in air (laboratory films only; the paper gives an oxide scale of about 400 nm)
weakest_at: Few measured bulk properties (most CrB2 values here are calculated or from a powder safety data sheet) and conflicting hazard classifications for CrB2 between PubChem and the supplier
identity:
  formula: CrB2
  cas: 12007-16-8
  crystal_system: hexagonal
  space_group: P6/mmm
  source: espi-crb2
  where: Chromium boride SDS, section 3 (CAS 12007-16-8, formula CrB2); crystal system and space group from Materials Project mp-374. PubChem CID 6336856 (formula B2Cr). Chromium carbide Cr3C2 is CAS 12012-35-0 (ESPI SDS, section 3) and PubChem CID 3650773.
tags:
  environments: []
  stresses:
    - extreme-heat
  applications:
    - heat-shields
    - mechanisms
properties:
  - benchmark: density
    label: Relative density, chromium boride powder (SDS)
    value: 5.15
    unit: g/cc
    scope: material
    conditions:
      form: powder product (supplier SDS)
      basis: relative density (specific gravity) as printed; the SDS does not state whether it is theoretical or measured
    method: Relative Density (Specific Gravity) row of the physical and chemical properties section of the ESPI Metals SDS
    status: ground
    source: espi-crb2
    where: Physical and chemical properties section, Relative Density row
  - benchmark: melting_point
    label: Melting point, chromium boride (SDS)
    value: 1850
    unit: degC
    scope: material
    conditions:
      form: powder product (supplier SDS)
    method: Melting Point row of the physical and chemical properties section of the ESPI Metals SDS
    status: ground
    source: espi-crb2
    where: Physical and chemical properties section, Melting Point row
  - benchmark: density
    label: Density, CrB2 (Materials Project calculated)
    value: 5.32
    unit: g/cm3
    scope: material
    conditions:
      basis: density functional theory calculation of the ideal crystal (hexagonal P6/mmm)
    method: Density field of the Materials Project record, which notes that calculated cell volumes are usually overestimated by about 3 percent
    status: calculated
    source: mp-crb2
    where: Density field, mp-374
  - benchmark: bulk_modulus
    label: Bulk modulus, CrB2 (Materials Project calculated, Voigt-Reuss-Hill)
    value: 252
    unit: GPa
    scope: material
    conditions:
      basis: density functional theory, ideal crystal, Voigt-Reuss-Hill average
    method: Bulk Modulus K VRH field of the Materials Project record (calculated, not measured)
    status: calculated
    source: mp-crb2
    where: Elastic properties, Bulk Modulus K VRH field, mp-374
  - benchmark: shear_modulus
    label: Shear modulus, CrB2 (Materials Project calculated, Voigt-Reuss-Hill)
    value: 183
    unit: GPa
    scope: material
    conditions:
      basis: density functional theory, ideal crystal, Voigt-Reuss-Hill average
    method: Shear Modulus G VRH field of the Materials Project record (calculated, not measured)
    status: calculated
    source: mp-crb2
    where: Elastic properties, Shear Modulus G VRH field, mp-374
  - benchmark: poissons_ratio
    label: Poisson's ratio, CrB2 (Materials Project calculated)
    value: 0.21
    unit: unitless
    scope: material
    conditions:
      basis: density functional theory, ideal crystal
    method: Poisson's Ratio field of the Materials Project record (calculated, not measured)
    status: calculated
    source: mp-crb2
    where: Elastic properties, Poisson's Ratio field, mp-374
  - benchmark: oxide_scale_thickness
    label: Oxide scale thickness, Si-alloyed CrB2 film after oxidation (sample B, 1200 degC, 3 h, air)
    value: 400
    unit: nm
    scope: material
    conditions:
      temperature: 1200 degC
      time: 3 h
      atmosphere: ambient air
      form: sputtered thin film, about 2.4 um thick, on polycrystalline alumina; silicon-alloyed CrB2
      measurement: TEM cross-section
    method: Mean of the TEM thickness range for sample B in the 2022 Molecules paper (range 180 to 750 nm); custom property name, not in the canon file
    status: ground
    note: Thin-film result, not a bulk part. Sample A (1100 degC) is not given a thickness in the paper.
    source: pmc-crb2-ox
    where: Results, paragraph on TEM of sample B (interface and oxide scale thickness)
  - benchmark: density
    label: Density, Cr3C2 powder (SDS)
    value: 6.68
    unit: g/cc
    scope: material
    conditions:
      form: powder product (supplier SDS)
      basis: relative density (specific gravity) as printed
    method: Relative Density (Specific Gravity) row of the physical and chemical properties section of the ESPI Metals chromium carbide SDS
    status: ground
    source: espi-cr3c2
    where: Physical and chemical properties section, Relative Density row
  - benchmark: melting_point
    label: Melting point, Cr3C2 (SDS)
    value: 1980
    unit: degC
    scope: material
    conditions:
      form: powder product (supplier SDS)
    method: Melting Point row of the physical and chemical properties section of the ESPI Metals chromium carbide SDS
    status: ground
    source: espi-cr3c2
    where: Physical and chemical properties section, Melting Point row
  - benchmark: hardness
    label: Microhardness, HVOF Cr3C2-NiCr coating on ductile iron, at 25 um depth
    value: 9.72
    unit: GPa
    scope: material
    conditions:
      coating: Cr3C2-25NiCr, HVOF sprayed on EN-GJS-500-7 ductile iron
      depth: 25 um from the surface
      uncertainty: plus or minus 2.21 GPa as printed
      method_detail: indentation hardness (HIT) on the cross-section, Micro Combi Tester
    method: Results, micromechanical measurements; reported as HIT in GPa (custom property name, not in the canon file)
    status: ground
    note: Indentation hardness of a coating, not the bulk Vickers value. The matrix at 25 um depth gives the lower of the two depths.
    source: pmc-cr3c2-co
    where: Results, micromechanical measurements paragraph
  - benchmark: hardness
    label: Microhardness, HVOF Cr3C2-NiCr coating on ductile iron, at 200 um depth
    value: 12.46
    unit: GPa
    scope: material
    conditions:
      coating: Cr3C2-25NiCr, HVOF sprayed on EN-GJS-500-7 ductile iron
      depth: 200 um from the surface (maximum microhardness depth)
      uncertainty: plus or minus 2.46 GPa as printed
      method_detail: indentation hardness (HIT) on the cross-section
    method: Results, micromechanical measurements paragraph (custom property name, not in the canon file)
    status: ground
    source: pmc-cr3c2-co
    where: Results, micromechanical measurements paragraph
  - benchmark: hardness
    label: Microhardness, HVOF Cr3C2-NiCr+Co composite coating, at 25 um depth
    value: 7.97
    unit: GPa
    scope: material
    conditions:
      coating: Cr3C2-25NiCr with 10 wt% Co, HVOF sprayed on ductile iron
      depth: 25 um from the surface
      uncertainty: plus or minus 0.76 GPa as printed
    method: Results, micromechanical measurements paragraph (custom property name, not in the canon file)
    status: ground
    note: Cobalt lowers the hardness of the carbide coating; the paper says so explicitly.
    source: pmc-cr3c2-co
    where: Results, micromechanical measurements paragraph
  - benchmark: hardness
    label: Microhardness, HVOF Cr3C2-NiCr+Co composite coating, at 200 um depth
    value: 10.09
    unit: GPa
    scope: material
    conditions:
      coating: Cr3C2-25NiCr with 10 wt% Co, HVOF sprayed on ductile iron
      depth: 200 um from the surface
      uncertainty: plus or minus 0.51 GPa as printed
    method: Results, micromechanical measurements paragraph (custom property name, not in the canon file)
    status: ground
    source: pmc-cr3c2-co
    where: Results, micromechanical measurements paragraph
  - benchmark: porosity
    label: Porosity, HVOF Cr3C2-NiCr+Co composite coating
    value: 2.3
    unit: percent
    scope: material
    conditions:
      coating: Cr3C2-25NiCr with 10 wt% Co, HVOF sprayed on ductile iron
      uncertainty: plus or minus 0.6 percent as printed
    method: Results, microstructure paragraph; custom property name, not in the canon file. The paper gives only the composite value.
    status: ground
    source: pmc-cr3c2-co
    where: Results, microstructure paragraph
  - benchmark: cte
    label: Coefficient of thermal expansion, Cr3C2-25NiCr coating (as cited)
    value: 11.1
    unit: 10-6/K
    scope: material
    conditions:
      coating: 75 wt% Cr3C2 with 25 wt% NiCr
      basis: value the paper quotes from its reference 7 for this coating
    method: Results, residual stress paragraph; the paper quotes this figure from an earlier work rather than measuring it here
    status: ground
    source: pmc-cr3c2-co
    where: Results, residual stress paragraph (CTE of the 75Cr3C2-25NiCr coating, citing reference 7)
corrosion:
  - environment: Silicon-alloyed CrB2 thin film in ambient air at 1100 to 1200 degC (3 h)
    headline: Silicon-alloyed CrB2 films form a thin oxide scale of about 400 nm on average at 1200 degC.
    effect: The 2022 Molecules paper oxidised two silicon-alloyed CrB2 sputtered films for 3 h in air at 1100 degC (sample A) or 1200 degC (sample B). TEM of sample B showed an oxide scale 180 to 750 nm thick, 400 nm on average, with a rough interface to the boride. The paper cites an earlier study that finds an outer Cr2O3 layer and an inner silicon-rich layer. The paper calls silicon-alloyed CrB2 oxidation-resistant, and the scale is a film result, not a bulk part result.
    rating: low
    conditions:
      temperature: 1100 to 1200 degC
      time: 3 h
      atmosphere: ambient air
      form: sputtered CrB2 film with silicon, about 2.4 um thick
    source: pmc-crb2-ox
    where: Introduction; Materials and Methods (oxidation treatment); Results (TEM of sample B)
  - environment: Transition metal diborides at elevated temperature in oxidizing atmospheres (general statement)
    headline: The paper says the oxidation resistance of transition metal diborides is inadequate at high temperature, and that silicon additions change this for CrB2.
    effect: The Introduction of the 2022 Molecules paper says that transition metal diborides have fascinating mechanical and thermal properties, but at elevated temperatures and oxidizing atmospheres their use is limited by inadequate oxidation resistance. It names Si-alloyed chromium diboride as a case that overcomes this limit. No bulk CrB2 oxidation rate was read in this run.
    rating: moderate
    conditions:
      basis: general statement about transition metal diborides; not a CrB2 bulk test
    source: pmc-crb2-ox
    where: Introduction, first paragraphs
  - environment: Iron(III) chloride solution, 5 percent FeCl3 with 1 percent NaNO3, 45 degC, 24 h (ASTM A 923 Method C, accelerated test)
    headline: A Cr3C2-NiCr coating lost about half the mass of bare ductile iron in the accelerated test, and the NiCr matrix dissolved selectively.
    effect: 'Mass-loss corrosion rates from Table 12 of the 2025 Materials paper: ductile cast iron 205.65 g/m2 per day (printed as g/m2 doba, doba is Polish for day), Cr3C2-NiCr coating 105.14, and the Cr3C2-NiCr+Ni composite 55.49. The paper says corrosion was selective dissolution of the NiCr matrix. The NiCr-only coating showed pits, cracks perpendicular to the interface, and complete delamination from the substrate; the composite with nickel particles corroded mainly at the nickel particles.'
    rating: moderate
    conditions:
      temperature: 45 degC
      time: 24 h
      electrolyte: 5 percent FeCl3 and 1 percent NaNO3 in distilled water
      substrate: EN-GJS-500-7 ductile cast iron, 15 x 15 mm samples
    source: pmc-cr3c2-ni
    where: Section 2.5 (corrosion test); Section 3.3 and Table 12
  - environment: Contact with moisture or water, and high temperature in air (chromium carbide Cr3C2 powder)
    headline: The ESPI SDS says Cr3C2 may react with moisture to form methane gas, and says to avoid high temperatures in air.
    effect: The chromium carbide SDS lists 'may react with moisture to form methane gas' under possibility of hazardous reactions, and lists high temperatures in air and contact with water as conditions to avoid. Hazardous decomposition products are chromium oxides, carbon oxides and methane. The SDS gives no corrosion rate.
    rating: moderate
    conditions:
      basis: supplier safety data sheet statement; no test read
    source: espi-cr3c2
    where: Section 10 (stability and reactivity)
risks:
  hazards:
    - text: 'PubChem aggregated GHS for chromium boride (CrB2, CID 6336856, 47 company reports, signal word Danger): H317 (93.6 percent of reports), H318 (12.8 percent), H350 (may cause cancer, 93.6 percent), H400 and H410 (very toxic to aquatic life, 93.6 percent). PubChem notes that 4.3 percent (2 of 47) of reports say the substance does not meet GHS hazard criteria.'
      ghs:
        - H317
        - H318
        - H350
        - H400
        - H410
      source: pubchem-crb2
      where: Safety and Hazards, GHS Classification (CID 6336856)
    - text: 'Supplier safety data sheet for chromium boride (ESPI Metals, CrB2): not classified as hazardous under 29 CFR 1910.1200, with no signal word and no hazard statements. The SDS does not give a carcinogenicity classification for the compound; it lists chromium and certain chromium compounds as NTP K (known to be carcinogenic) under the chromium entry.'
      ghs: []
      source: espi-crb2
      where: Section 2 (hazards identification) and Section 11 (carcinogenicity)
    - text: 'Supplier safety data sheet for chromium carbide (ESPI Metals, Cr3C2 powder): not classified as hazardous under 29 CFR 1910.1200. The SDS lists the same chromium entry as NTP K and IARC not identified as carcinogenic.'
      ghs: []
      source: espi-cr3c2
      where: Section 2 (hazards identification) and Section 11 (carcinogenicity)
  failure_risks:
    - mode: Oxidation of the boride at high temperature in air
      trigger: Exposure to air at 1100 to 1200 degC or above
      consequence: The 2022 paper says transition metal diborides have inadequate oxidation resistance in oxidizing atmospheres at high temperature. Silicon-alloyed CrB2 films form a thin oxide scale instead (about 400 nm mean after 3 h at 1200 degC); no bulk CrB2 oxidation data were read.
      severity: moderate
      source: pmc-crb2-ox
      where: Introduction
    - mode: Selective matrix corrosion and delamination of Cr3C2-NiCr coatings
      trigger: Chloride solution (ferric chloride test, 45 degC, 24 h)
      consequence: The NiCr-only coating developed pits, cracks perpendicular to the interface and complete delamination from the ductile iron substrate; the composite with nickel particles showed less damage.
      severity: moderate
      source: pmc-cr3c2-ni
      where: Section 3.3 (corrosion behavior)
    - mode: Gas evolution from carbide reacting with moisture
      trigger: Contact of Cr3C2 powder with moisture or water
      consequence: The SDS says it may react with moisture to form methane gas, which is flammable; the same SDS warns of dust hazards and fire.
      severity: moderate
      source: espi-cr3c2
      where: Section 10 (possibility of hazardous reactions)
  side_effects:
    - text: 'Fire: the chromium boride SDS says the material may emit toxic fumes under fire conditions. Decomposition products are chromium oxides and boron oxides. The SDS calls for full face self-contained breathing apparatus for firefighters.'
      affects: Firefighters and the surrounding area
      source: espi-crb2
      where: Section 5 (firefighting measures) and Section 10 (hazardous decomposition products)
    - text: 'Dust: the chromium carbide SDS says fine dust may be a weak to moderate fire hazard if it accumulates and meets a strong ignition source, and releases chromium oxide fume in a fire.'
      affects: Storage and handling areas
      source: espi-cr3c2
      where: Section 5 (specific hazards arising from the material)
  handling:
    - text: 'Chromium boride: store in a cool, dry area in tightly sealed labelled containers and protect from moisture. Keep it away from oxidizing agents, which the SDS lists as incompatible.'
      source: espi-crb2
      where: Section 7 (storage) and Section 10 (incompatible materials)
    - text: 'Chromium carbide: OSHA PEL for chromium(II) compounds (as Cr) is 0.5 mg/m3, and the ACGIH TLV is not established. The SDS calls for local exhaust ventilation where dust forms and says to avoid breathing dust.'
      source: espi-cr3c2
      where: Section 8 (exposure limits and engineering controls)
  mitigations: []
uses:
  - application: mechanisms
    device: HVOF wear-resistant coatings on machinery and turbine blades
    example: The 2024 Materials paper says HVOF Cr3C2-NiCr coatings shield important machinery and equipment components from wear in harsh, high-temperature conditions, and that industrial turbine blades in thermal and nuclear power plants need protective coatings of this kind.
    source: pmc-cr3c2-co
    where: Abstract and Introduction
  - application: mechanisms
    device: Erosion and corrosion protection coatings on cast iron components
    example: The 2025 Materials paper concludes that Cr3C2-NiCr+Ni coatings on cast iron are an effective solution for applications that need high mechanical strength, resistance to cracking, wear and corrosion. It is a laboratory study on spray-coated samples.
    source: pmc-cr3c2-ni
    where: Conclusions
  - application: heat-shields
    device: Oxidation-protective thin film (laboratory only)
    example: The 2022 Molecules paper studied silicon-alloyed CrB2 sputtered films for oxidation at 1100 to 1200 degC in air. No flight, heat-shield or other service use of CrB2 was read in this run.
    source: pmc-crb2-ox
    where: Introduction and Results
supply:
  - subject: Chromium
    abundance: 'USGS: world chromite resources are greater than 12 billion tons of shipping-grade chromite, 95 percent of them in Kazakhstan and southern Africa. Global mine production was 51,000 thousand tons in 2025 (estimated, chromium ore gross weight).'
    producers:
      - country: South Africa
        share_percent: 45.1
        year: 2025
      - country: Turkey
        share_percent: 17.6
        year: 2025
      - country: Kazakhstan
        share_percent: 13.7
        year: 2025
      - country: India
        share_percent: 5.9
        year: 2025
      - country: Zimbabwe
        share_percent: 3.9
        year: 2025
    critical_lists:
      - list: 'USGS Final 2025 List of Critical Minerals (Table 6, chromium: stainless steel and metallurgy)'
        listed: true
    recycling: 'USGS: recycled chromium in reported stainless-steel scrap receipts accounted for 21 percent of apparent US consumption in 2025.'
    substitutes:
      - 'USGS: chromium has no substitute in stainless steel or superalloys. This is a metal-use statement and does not cover chromium borides or carbides.'
    source: usgs-mcs2026-chromium
    where: 'Salient Statistics, Events Trends and Issues, World Mine Production and Reserves table (2025e column; shares calculated by Space Mushroom against the 51,000 thousand t world total as printed), Recycling and Substitutes sections. Critical list: USGS MCS 2026 Table 6.'
  - subject: Boron
    abundance: 'USGS: 2025 production as printed (thousand tons): Turkey refined borates 1,500, Bolivia ulexite 380, Chile ulexite 300, China boric oxide equivalent 230, Peru crude borates 220, Argentina crude ore 170, Russia datolite ore 80, Germany compounds 40. The world total is withheld (XX), so shares are not calculated.'
    producers: []
    critical_lists:
      - list: USGS Final 2025 List of Critical Minerals (boron added in the final 2025 list)
        listed: true
    recycling: 'USGS: insignificant.'
    substitutes:
      - 'USGS: substitution is possible in detergents, enamels, insulation and soaps. No substitute for boron in boride ceramics is discussed.'
    source: usgs-mcs2026-boron
    where: World Production and Reserves table (2025 column), Events Trends and Issues (critical list paragraph), Recycling and Substitutes sections
media: []
documents:
  - title: 'ESPI Metals safety data sheet: Chromium Boride CrB2 (CAS 12007-16-8)'
    type: sds
    url: https://www.espimetals.com/index.php/msds/521-Chromium%20Boride%20CrB2
    year: 2026
  - title: 'ESPI Metals safety data sheet: Chromium Carbide powder Cr3C2 (CAS 12012-35-0)'
    type: sds
    url: https://www.espimetals.com/index.php/msds/520-Chromium%20Carbide
    year: 2026
  - title: PubChem compound record CID 6336856, Chromium boride (CrB2), GHS classification
    type: database
    url: https://pubchem.ncbi.nlm.nih.gov/compound/6336856
    year: 2026
  - title: PubChem compound record CID 3650773, Chromium carbide (C2Cr3)
    type: database
    url: https://pubchem.ncbi.nlm.nih.gov/compound/3650773
    year: 2026
  - title: USGS Mineral Commodity Summaries 2026, Chromium
    type: report
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-chromium.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: 'USGS Mineral Commodity Summaries 2026 (full report, Table 6: Final 2025 Critical Minerals List)'
    type: report
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026.pdf
    year: 2026
  - title: Materials Project mp-374, CrB2 (hexagonal, P6/mmm), calculated properties
    type: database
    url: https://legacy.materialsproject.org/materials/mp-374/
    year: 2026
  - title: Ksiazek M. et al. 2024, HVOF-sprayed (Cr3C2-NiCr+Co) composite coating on ductile cast iron, Materials 17(7):1484 (open access)
    type: paper
    url: https://pmc.ncbi.nlm.nih.gov/articles/PMC11012888/
    year: 2024
  - title: Ksiazek M. and Boron L. 2025, Microstructure, tribological and corrosion behavior of HVOF-sprayed (Cr3C2-NiCr+Ni) coatings, Materials 18(8):1856 (open access)
    type: paper
    url: https://pmc.ncbi.nlm.nih.gov/articles/PMC12029035/
    year: 2025
  - title: Weiss M. et al. 2022, Quantitative depth profiling by online-LASIL of transition metal borides, Molecules 27(10):3221 (open access)
    type: paper
    url: https://pmc.ncbi.nlm.nih.gov/articles/PMC9147019/
    year: 2022
gaps:
  - 'Bulk mechanical data for CrB2 that are measured (hardness, fracture toughness, flexural strength, elastic modulus): the Sciencedirect spark-plasma-sintering paper and the OSTI hot-pressed oxidation report could not be opened in this run (publisher access blocked and OSTI connection failed). Only calculated bulk moduli are given.'
  - 'CrB (chromium(III) boride, PubChem CID 82788, formula BCr): no property or hazard source read in this run. The PubChem record was confirmed for formula only. The CrB is not profiled.'
  - 'PubChem GHS for chromium carbide Cr3C2 (CID 3650773) and chromium boride CrB (CID 82788): the records have no GHS section, so no hazard classification from PubChem for these two.'
  - 'Cr3C2 bulk properties from a measured source (Vickers hardness, elastic modulus, oxidation rate in air): not read. Cr3C2 microhardness above is for HVOF coatings on ductile iron, not bulk material.'
  - Cr3C2 coating porosity and microhardness values from the 2024 paper are for coatings with and without cobalt. The 2024 paper gives no porosity for the Cr3C2-NiCr coating without cobalt.
  - 'EU critical raw materials list status for chromium and boron: the European Commission page was readable but did not list the materials in text, and the list could not be confirmed in this run.'
  - 'Space, vacuum, radiation and thermal-cycling data for CrB2 and Cr3C2: none read in this run. No NASA, ESA or flight source was found.'
  - 'Designations (UNS, EN, DIN, JIS, AISI and similar codes) for CrB2 or Cr3C2: none printed in a source read in this run.'
  - 'Custom property names used because they are not in spores/property-canon.yaml: hardness, porosity, oxide_scale_thickness.'
  - 'Boron producer shares are not calculated: the 2025 world total is withheld (XX) in the USGS boron chapter.'
  - 'Hot-pressed CrB2 oxidation and thermal-shock behaviour (quench cracking reported in search snippets): the source was not opened in this run, so no value or claim from it is used.'
sources:
  espi-crb2:
    title: ESPI Metals, Safety Data Sheet, Chromium Boride CrB2 (CAS 12007-16-8)
    url: https://www.espimetals.com/index.php/msds/521-Chromium%20Boride%20CrB2
    kind: datasheet
    license: supplier safety data sheet, linked not reproduced
  espi-cr3c2:
    title: ESPI Metals, Safety Data Sheet, Chromium Carbide powder Cr3C2 (CAS 12012-35-0)
    url: https://www.espimetals.com/index.php/msds/520-Chromium%20Carbide
    kind: datasheet
    license: supplier safety data sheet, linked not reproduced
  pubchem-crb2:
    title: PubChem compound CID 6336856, Chromium boride (CrB2), GHS classification (aggregated ECHA notifications, 47 reports)
    url: https://pubchem.ncbi.nlm.nih.gov/compound/6336856
    kind: database
    license: PubChem public database
  pubchem-cr3c2:
    title: PubChem compound CID 3650773, Chromium carbide (C2Cr3)
    url: https://pubchem.ncbi.nlm.nih.gov/compound/3650773
    kind: database
    license: PubChem public database
  usgs-mcs2026-chromium:
    title: USGS Mineral Commodity Summaries 2026, Chromium
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-chromium.pdf
    kind: report
    license: public domain (USGS)
  usgs-mcs2026-boron:
    title: USGS Mineral Commodity Summaries 2026, Boron
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-boron.pdf
    kind: report
    license: public domain (USGS)
  usgs-mcs2026:
    title: USGS Mineral Commodity Summaries 2026 (full report), Table 6 Final 2025 Critical Minerals List
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026.pdf
    kind: report
    license: public domain (USGS)
  nist-atomic-weights:
    title: NIST, Atomic Weights and Isotopic Compositions for All Elements (standard atomic weights for Cr, B and C)
    url: https://physics.nist.gov/cgi-bin/Compositions/stand_alone.pl?ele=Cr&all=all&ascii=html
    kind: database
    license: public domain (NIST)
  mp-crb2:
    title: Materials Project mp-374, CrB2 (hexagonal, P6/mmm, 191), calculated (DFT) properties
    url: https://legacy.materialsproject.org/materials/mp-374/
    kind: database
    license: Materials Project, calculated data
  pmc-cr3c2-co:
    title: Ksiazek M. et al., An HVOF-Sprayed (Cr3C2-NiCr+Co) Composite Coating on Ductile Cast Iron, Materials 17(7), 1484 (2024), doi 10.3390/ma17071484
    url: https://pmc.ncbi.nlm.nih.gov/articles/PMC11012888/
    kind: paper
    license: open access (PMC)
  pmc-cr3c2-ni:
    title: Ksiazek M. and Boron L., Microstructure, Tribological, and Corrosion Behavior of HVOF-Sprayed (Cr3C2-NiCr+Ni) Coatings on Ductile Cast Iron, Materials 18(8), 1856 (2025), doi 10.3390/ma18081856
    url: https://pmc.ncbi.nlm.nih.gov/articles/PMC12029035/
    kind: paper
    license: open access (PMC)
  pmc-crb2-ox:
    title: Weiss M. et al., Quantitative Depth Profiling Using Online-LASIL of Transition Metal Borides (Si-alloyed CrB2 films), Molecules 27(10), 3221 (2022), doi 10.3390/molecules27103221
    url: https://pmc.ncbi.nlm.nih.gov/articles/PMC9147019/
    kind: paper
    license: open access (PMC)
license: CC-BY-4.0
updated: '2026-10-26'
review:
  status: draft
  method: assistant
  checked_values: 15
  unreachable: 3
  caveats: 4
  note: 'Written in this run from two ESPI safety data sheets, PubChem GHS for CrB2, USGS MCS 2026 chromium and boron chapters and the full report critical-list table, Materials Project mp-374, and three open-access papers (Materials 2024, Materials 2025, Molecules 2022). Unreachable: ScienceDirect CrB2 sintering paper, OSTI hot-pressed CrB2 report, NIOSH pocket guide. Not verified: values to be checked by verify-values.'
story:
  recipe:
    line: CrB2 has one chromium and two boron atoms per formula unit (ESPI SDS CAS 12007-16-8; Materials Project P6/mmm). Cr3C2, the related carbide, has three chromium and two carbon per formula unit (ESPI SDS CAS 12012-35-0). Atomic weights are NIST standard values (source nist-atomic-weights); NIST prints boron as an interval, so boron is given to two decimals.
    parts:
      - name: Chromium
        symbol: Cr
        atoms: 1
        atomic_weight: 51.9961
        role: The metal atom in the hexagonal boride. It sets the chemistry of the oxide scale; the 2022 paper's silicon-alloyed films form a chromium oxide layer on top.
      - name: Boron
        symbol: B
        atoms: 2
        atomic_weight: 10.81
        role: Forms the boron layers of the hexagonal structure. The SDS lists boron oxides among the decomposition products in a fire.
  risk_groups:
    - title: For people
      level: 0
      verdict: 'PubChem: skin, eye and cancer codes'
      lead: The aggregated PubChem record for CrB2 gives sensitisation, eye-damage and carcinogenicity statements, but the supplier SDS for the same compound says it is not classified as hazardous. Treat the powder as a possible hazard until the supplier's classification is checked.
      points:
        - label: PubChem
          line: Hazard codes H317 and H350 each appear in 93.6 percent of the 47 company reports, H318 in 12.8 percent, and H400 and H410 in 93.6 percent. PubChem says 4.3 percent of reports (2 of 47) do not meet GHS criteria.
        - label: Supplier SDS
          line: The ESPI SDS for CrB2 and for Cr3C2 says not classified as hazardous under 29 CFR 1910.1200, with no signal word. Both SDSs list chromium and certain chromium compounds as NTP K.
        - label: Dust
          line: Cr3C2 dust may form a weak to moderate fire hazard, and the OSHA PEL for chromium(II) compounds (as Cr) is 0.5 mg/m3.
    - title: If a part fails
      lead: These are laboratory and calculated results, not field data.
      points:
        - label: Oxidation
          line: Silicon-alloyed CrB2 films formed a scale about 400 nm thick after 3 h at 1200 degC in air. The 2022 paper says ordinary transition metal diborides have inadequate oxidation resistance at high temperature.
        - label: Chloride
          line: In an accelerated ferric chloride test, a Cr3C2-NiCr coating on cast iron lost about half the mass of bare cast iron, but its NiCr matrix dissolved selectively, and the coating without nickel delaminated.
        - label: Moisture
          line: The Cr3C2 SDS says the powder may react with moisture to form methane gas.
    - title: Handling and storage
      points:
        - label: Storage
          line: Store CrB2 in a cool, dry area in sealed containers, away from oxidizing agents (ESPI SDS).
        - label: Ventilation
          line: Use local exhaust ventilation where dust forms and avoid breathing dust (ESPI SDS, Cr3C2).
  why: 'Chromium diboride is a hexagonal boride. This profile is thinner than the titanium diboride profile: the bulk numbers come from a supplier safety data sheet and from a density functional calculation, not from a measured mechanical test. The calculated bulk modulus is 252 GPa and the melting point given by the supplier is 1850 degC. Thin silicon-alloyed films of CrB2 are the most specific evidence on oxidation. Cr3C2 is the carbide used in wear-resistant thermal-spray coatings, and its coating studies give the strongest numbers on hardness and corrosion in this profile.'
  uses:
    - where: Wear-resistant coatings on machinery and turbine blades (Cr3C2)
      line: A 2024 study describes HVOF Cr3C2-NiCr coatings as a way to shield machinery and equipment from wear in harsh, high-temperature conditions. It names turbine blades in thermal and nuclear power plants as a place that needs protective coatings.
    - where: Cast iron protection (Cr3C2-NiCr+Ni)
      line: A 2025 study concludes that the nickel-modified Cr3C2-NiCr coating suits applications that need strength, resistance to cracking, wear and corrosion. It is a laboratory study on spray-coated samples.
    - where: Oxidation-protective film (CrB2, laboratory only)
      line: Silicon-alloyed CrB2 films were studied for oxidation at 1100 to 1200 degC in air. No flight, heat-shield or service use of CrB2 was read in this run.
  can_do:
    - label: Stiffness
      benchmark: bulk_modulus
      line: The Materials Project calculates a bulk modulus of 252 GPa and a shear modulus of 183 GPa for the ideal CrB2 crystal. These are calculated values, not measured ones.
    - label: Melting
      benchmark: melting_point
      value: 1850 degC
      line: The chromium boride SDS gives a melting point of 1850 degC for the powder.
    - label: Coating hardness
      benchmark: hardness
      value: 9.72 GPa
      line: In HVOF Cr3C2-NiCr coatings on ductile iron, indentation hardness is 9.72 GPa at 25 um depth and 12.46 GPa at 200 um depth.
  compare: CrB2 is a boride and Cr3C2 is a carbide, so they are not interchangeable. The supplier data put CrB2 at 5.15 g/cc and 1850 degC, and Cr3C2 at 6.68 g/cc and 1980 degC. The calculated CrB2 density is 5.32 g/cm3. No hardness was read for bulk CrB2, so no hardness comparison between the two is made.
  compare_rows:
    - label: Density
      benchmark: density
    - label: Melting point
      benchmark: melting_point
    - label: Coating microhardness
      benchmark: hardness
  handling:
    - Keep the powder dry and in a closed container, away from oxidizing agents (ESPI SDS).
    - Use local exhaust ventilation where dust forms. Chromium(II) compounds have an OSHA PEL of 0.5 mg/m3 as Cr (ESPI SDS, Cr3C2).
    - Cr3C2 may react with moisture to form methane gas (ESPI SDS).
  good_for:
    - Oxidation-protective films. Silicon-alloyed CrB2 films formed a scale about 400 nm thick after 3 h at 1200 degC in air.
    - Wear-resistant thermal-spray coatings. Cr3C2-NiCr coatings on cast iron showed microhardness near 10 GPa, with less corrosion mass loss than bare cast iron.
  watch_out:
    - Hazard data conflict. PubChem gives sensitisation, eye-damage and carcinogenicity codes for CrB2 in most reports, while the supplier SDS says not classified.
    - Bulk CrB2 mechanical data are calculated (Materials Project) or missing. Measured hardness, toughness and oxidation rate for bulk CrB2 were not read in this run.
    - The nickel-free Cr3C2-NiCr coating shows selective corrosion and delamination in chloride solution.
  verdict: CrB2 is a boride with a supplier melting point of 1850 degC and a calculated bulk modulus of 252 GPa. Its best evidence is for oxidation-protective films, which are laboratory results. The hazard picture is unresolved between PubChem and the supplier SDS, so check the supplier's classification before handling the powder.