Hafnium carbide HfC

spore: profile
id: hafnium-carbide
spore_version: 0.2
family: carbide-nitride-boride-ceramics
name: Hafnium carbide
abbreviation: HfC
role: very high melting point ultra-high-temperature ceramic (carbide), a candidate for hypersonic leading edges and high-temperature coatings
category: ceramics
type: carbides
summary: |
  Hafnium carbide is a hard, dense ceramic with a rock-salt (NaCl-type, face-centred cubic) structure. The supplier safety data sheet for a powder gives a density of 12.2 g/cm3 at 20 degC and a melting point of 3890 degC. Zhang and McMahon (2020) give an experimental melting point of 4255 plus or minus 30 K, and experimental elastic constants C11 of 500 GPa and C44 of 180 GPa at 298 K. The main chemical limit is oxidation in air: a 2022 paper reports that HfC is intensively oxidized in air at 400 to 500 degC. A 1960 NASA Lewis note measured a linear expansion coefficient for hot-pressed HfC, and a 2015 NASA Ames arc-jet test of a carbon-carbon nozzle coating that contains HfC showed no erosion at 3,100 F.
best_for: Very high melting point ceramic parts and coatings where a protective layer keeps air away from the surface, such as hypersonic leading edges and nozzle liners
weakest_at: Oxidation in air from 400 to 500 degC (Gaydaychuk et al. 2022), and low toughness, with no measured fracture toughness for HfC found in this run
identity:
  formula: HfC, rock-salt hafnium monocarbide, usually carbon deficient (HfCx with x from 0.5 to 1.0)
  cas: 12069-85-1
  crystal_system: cubic
  space_group: Fm3m
  source: zeng2013
  where: Introduction and structure section, rocksalt-type (B1) HfC, space group F m3 bar m (also NaCl-type in zhang2020, Section on lattice parameters). CAS from the supplier SDS, Section 3, and PubChem synonyms.
tags:
  environments: []
  stresses:
    - extreme-heat
  applications:
    - heat-shields
    - propulsion-and-tanks
properties:
  - benchmark: density
    label: Density, hafnium carbide powder (supplier SDS, 20 degC)
    value: 12.2
    unit: g/cm3
    scope: material
    conditions:
      temperature: 20 degC
      form: powder, supplier product
    method: Section 9 of the supplier safety data sheet, printed as Density at 20 degC (68 degF) 12.2 g/cm3
    status: ground
    source: sds-sama
    where: Section 9 (Physical and chemical properties), density row
  - benchmark: density
    label: Density, hafnium carbide (Goodfellow datasheet)
    value: 12.2
    unit: g/cm3
    scope: material
    conditions:
      temperature: not stated on the page
      form: powder, supplier product
    method: Physical Properties table on the product page, printed as Density 12.2
    status: ground
    source: goodfellow-hfc
    where: Physical Properties table, Density row
  - benchmark: density
    label: Theoretical density, hot-pressed HfC (1960 NASA note)
    value: 12.1
    unit: g/cm3
    scope: material
    conditions:
      basis: theoretical density from a chemical analysis and a unit-cell dimension of 4.63 (X-ray, NaCl-type)
      unit_in_source: gm. per ml.
    method: Text of the 1960 note, which gives a theoretical density of 12.1 gm. per ml. (equal to g/cm3)
    status: calculated
    source: grisaffe1960
    where: Text, paragraph on specimen density (Section Experimental)
  - benchmark: melting_point
    label: Melting point, HfC (supplier SDS)
    value: 3890
    unit: degC
    scope: material
    conditions:
      form: powder, supplier product
    method: Section 9 of the supplier safety data sheet, printed as 3890 degC (7034 degF)
    status: ground
    source: sds-sama
    where: Section 9 (Physical and chemical properties), melting point row
  - benchmark: melting_point
    label: Melting point, HfC (Goodfellow datasheet)
    value: 3890
    unit: degC
    scope: material
    conditions:
      form: powder, supplier product
    method: Thermal Properties table on the product page, printed as Melting point 3890. The same page's product description says above 3900 degC, which is not used.
    status: ground
    note: The product description on the same page says above 3900 degC. The table value is kept.
    source: goodfellow-hfc
    where: Thermal Properties table, Melting point row
  - benchmark: melting_point
    label: Melting point, HfC (Zhang and McMahon, introduction)
    value: 4255
    unit: K
    scope: material
    conditions:
      uncertainty: plus or minus 30 K, as printed
    method: Introduction, first paragraph, printed as 4255 plus or minus 30 K for HfC, citing an earlier melting study
    status: ground
    note: A second-hand figure from a 2020 paper that cites earlier work for it. It is not the same as the 3890 degC supplier value.
    source: zhang2020
    where: Introduction, paragraph beginning "Among UHTCs"
  - benchmark: melting_point
    label: Melting point, HfC (1960 NASA note, introduction)
    value: 7029
    unit: degF
    scope: material
    conditions:
      basis: stated in the introduction of a 1960 note on hot-pressed HfC
    method: Introduction, first sentence, printed as HfC has a melting point of 7029 degF
    status: ground
    note: Historical figure from a 1960 note. Kept as printed in degF.
    source: grisaffe1960
    where: Introduction, first sentence
  - benchmark: elastic_constant_c11
    label: Elastic constant C11, HfC (experimental, 298 K)
    value: 500
    unit: GPa
    scope: material
    conditions:
      temperature: 298 K
    method: Zhang and McMahon text, comparing with experiment (their ref 58), printed as C11 = 500 GPa at 298 K
    status: ground
    source: zhang2020
    where: Results, paragraph comparing HfC elastic constants with experiment (ref 58)
  - benchmark: elastic_constant_c11
    label: Elastic constant C11, HfC (quasi-harmonic, 0 K, calculated)
    value: 500.9
    unit: GPa
    scope: material
    conditions:
      temperature: 0 K
      method_detail: quasi-harmonic approximation, first principles
    method: Results, paragraph on HfC at 0 K, printed as 500.9 GPa for C11 from QHA
    status: calculated
    source: zhang2020
    where: Results, paragraph beginning "For HfC (at 0 K)"
  - benchmark: elastic_constant_c12
    label: Elastic constant C12, HfC (quasi-harmonic, 0 K, calculated)
    value: 104.1
    unit: GPa
    scope: material
    conditions:
      temperature: 0 K
      method_detail: quasi-harmonic approximation, first principles
    method: Results, paragraph on HfC at 0 K, printed as 104.1 GPa for C12 from QHA. The paper notes C12 is not experimentally available for HfC.
    status: calculated
    source: zhang2020
    where: Results, paragraph beginning "For HfC (at 0 K)"
  - benchmark: elastic_constant_c44
    label: Elastic constant C44, HfC (experimental, 298 K)
    value: 180
    unit: GPa
    scope: material
    conditions:
      temperature: 298 K
    method: Zhang and McMahon text, comparing with experiment (their ref 58), printed as C44 = 180 GPa at 298 K
    status: ground
    source: zhang2020
    where: Results, paragraph comparing HfC elastic constants with experiment (ref 58)
  - benchmark: elastic_constant_c44
    label: Elastic constant C44, HfC (quasi-harmonic, 0 K, calculated)
    value: 175.5
    unit: GPa
    scope: material
    conditions:
      temperature: 0 K
      method_detail: quasi-harmonic approximation, first principles
    method: Results, paragraph on HfC at 0 K, printed as 175.5 GPa for C44 from QHA
    status: calculated
    source: zhang2020
    where: Results, paragraph beginning "For HfC (at 0 K)"
  - benchmark: hardness
    label: Vickers hardness, HfC (calculated)
    value: 32.95
    unit: GPa
    scope: material
    conditions:
      basis: calculated from elastic constants with the hardness model used in the paper
    method: Mechanical properties section, paragraph after Table II, printed as Vickers hardness of HfC 32.95 GPa. The paper computes it; no measured HfC hardness was read.
    status: calculated
    note: Calculated, not measured. The paper gives 22.28 GPa for Hf3C2 and 30.91 GPa for Hf6C5 in the same sentence.
    source: zeng2013
    where: Mechanical properties section, paragraph after Table II
  - benchmark: lattice_constant
    label: Lattice parameter, HfC (first principles, 298 K)
    value: 4.646
    unit: Angstrom
    scope: material
    conditions:
      temperature: 298 K
    method: Structure section, printed as the optimized lattice parameter of HfC at 298 K
    status: calculated
    source: zhang2020
    where: Structure section, paragraph beginning "Both polycrystalline ZrC and HfC"
  - benchmark: lattice_constant
    label: Lattice parameter, HfC (experimental, as cited)
    value: 4.644
    unit: Angstrom
    scope: material
    conditions:
      basis: experimental value for HfC as quoted by the paper (its ref 50)
    method: Structure section, printed as 4.644 Angstrom for HfC in the experimental comparison
    status: ground
    source: zhang2020
    where: Structure section, same paragraph as the calculated value
  - benchmark: cte
    label: Coefficient of thermal expansion, hot-pressed HfC (1960 NASA note)
    value: 6.59
    unit: 10-6/K
    scope: material
    conditions:
      temperature: 25 to 612 degC
      uncertainty: plus or minus 0.04, as printed
      form: hot-pressed HfC, 96.2 to 99.2 percent of theoretical density
    method: Text after Figures 1 and 2, printed as a HfC = 6.59 plus or minus 0.04 X 10-6 per degC from 25 to 612 degC. The same sentence gives 3.66 plus or minus 0.02 X 10-6 per degF from 77 to 1133 degF, which is not used here.
    status: ground
    note: A 1960 measurement by dilatometry on hot-pressed cones. No modern CTE value for HfC was read in this run.
    source: grisaffe1960
    where: Text, paragraph beginning "A coefficient of thermal expansion was calculated"
  - benchmark: oxidation_resistance_temp
    label: Oxidation temperature range of HfC in air (review of earlier work)
    value: 400
    value_high: 500
    bound: range
    unit: degC
    scope: material
    conditions:
      atmosphere: air
      basis: stated as the range where HfC is intensively oxidized in air, citing an earlier study
    method: Introduction, printed as HfC is intensively oxidized in air, even at 400 to 500 degC
    status: ground
    note: A statement in the introduction of a 2022 paper, citing earlier work. Not a measured onset for bulk HfC.
    source: tomsk-hfc-al-2022
    where: Introduction, paragraph beginning "However, one of the main disadvantages of HfC"
corrosion:
  - environment: Air at 400 to 500 degC
    headline: HfC is intensively oxidized in air from about 400 to 500 degC.
    effect: The paper says a porous HfO2 layer forms on the surface, which promotes intense oxygen diffusion, so the material oxidizes further. The statement is an introduction summary of earlier work, and no test is described for it.
    rating: severe
    conditions:
      temperature: 400 to 500 degC
      atmosphere: air
    source: tomsk-hfc-al-2022
    where: Introduction, and the oxidation mechanism paragraph in the discussion
  - environment: Air at 600 and 800 degC, 10 micrometre magnetron-sputtered HfC film, compared with Hf-Al-C films
    headline: Adding aluminium cut the mass gain of HfC films by 44.3 percent at 600 degC and by 22.5 percent at 800 degC.
    effect: The paper reports that pristine HfC films oxidized through, because the HfO2 scale does not slow oxygen diffusion, and that gaseous CO and CO2 made the scale porous. It also says pure HfC films protect only up to about 400 degC at the same thickness. The Hf-Al-C films protected up to 800 degC.
    rating: severe
    conditions:
      temperature: 600 and 800 degC
      time: 140 min at 800 degC (Figure 4)
      atmosphere: air
      form: thin film, 10 micrometre thick, magnetron sputtered
    source: tomsk-hfc-al-2022
    where: Results, weight-gain figure (Figure 3) and cross-section figure (Figure 4); discussion of the oxidation mechanism
risks:
  hazards:
    - text: 'Supplier SDS (Stanford Advanced Materials, revised 27/11/2022): classified under Regulation (EC) No 1272/2008 as Flam. Sol. 2, H228 Flammable solid, with signal word Warning and pictogram GHS02. Section 9 also lists the powder as highly flammable.'
      ghs:
        - H228
      source: sds-sama
      where: Section 2 (Hazards identification), label elements; Section 9 (Flammability)
    - text: The same SDS says hafnium and its compounds may irritate the skin and eyes, and that there are reports of liver damage from hafnium exposure. The acute and chronic toxicity of this substance is not fully known.
      ghs: []
      source: sds-sama
      where: Section 11 (Toxicological information), subacute to chronic toxicity
    - text: 'Exposure limit printed in the SDS for hafnium, metal and compounds as Hf: ACGIH TLV 0.5 mg/m3, USA PEL 0.5 mg/m3, and several European limits of 0.5 mg/m3.'
      ghs: []
      source: sds-sama
      where: Section 8 (Exposure controls), control parameters
  failure_risks:
    - mode: Oxidation of HfC in air
      trigger: Air at 400 to 500 degC
      consequence: A porous HfO2 layer forms and lets oxygen through, so the material oxidizes intensively. Pure HfC thin films oxidized through in the 2022 test at 600 and 800 degC.
      severity: high
      source: tomsk-hfc-al-2022
      where: Introduction; Results, Figure 3 and Figure 4; discussion of the oxidation mechanism
    - mode: Low fracture toughness of monolithic HfC
      trigger: Mechanical load on a monolithic part
      consequence: The paper states that Hf-Al-C bulk composites have much higher strength and fracture toughness than HfC. No fracture-toughness value for HfC was read in this run.
      severity: moderate
      source: tomsk-hfc-al-2022
      where: Introduction, paragraph on bulk composites
  side_effects:
    - text: During oxidation, gaseous CO and CO2 form, which makes the oxide scale porous and adds mass to the sample.
      affects: The oxide scale and the sample mass gain
      source: tomsk-hfc-al-2022
      where: Discussion of the oxidation mechanism
  handling:
    - text: Keep the container tightly sealed in a cool, dry place with good ventilation, protect against electrostatic charges, and store away from oxidizing agents (SDS Section 7).
      source: sds-sama
      where: Section 7 (Handling and storage)
    - text: 'Fire: use special powder for metal fires and do not use water. Hazardous decomposition products are carbon monoxide, carbon dioxide and metal oxide fume (SDS Sections 5 and 10).'
      source: sds-sama
      where: Section 5 (Firefighting measures); Section 10 (Stability and reactivity)
  mitigations:
    - text: SDS Section 8 advises a chemical fume hood with an average face velocity of at least 100 feet per minute, impervious gloves, safety glasses and protective work clothing, and a respirator when high concentrations are present.
      source: sds-sama
      where: Section 8 (Exposure controls and personal protection)
uses:
  - application: propulsion-and-tanks
    device: Carbon-carbon nozzle extension coating with embedded HfC and ZrB2 (NASA SBIR Phase 2, arc-jet test)
    example: A test in NASA's Arc Jet Complex at 3,100 F, heat flux 198 BTU per sq ft per second and pressure 0.265 atm, over four cycles totaling 28 minutes, showed enhanced high-temperature durability and no erosion. HfC is one of two carbides in this coating, so the result is not for HfC alone.
    source: ntrs-ccat-hfc
    where: Abstract and description paragraph (NTRS 20160005370)
  - application: heat-shields
    device: Hypersonic flight and gas-turbine thermal barrier coatings (proposed)
    example: Zhang and McMahon say the potential applications of ZrC and HfC go well beyond hypersonic flight, and that both could be used as thermal barrier coatings for gas turbines. This is a proposal in the paper, not a flight or test record.
    source: zhang2020
    where: Introduction, first paragraph
supply:
  - subject: Hafnium (the hafnium half of HfC; hafnium is recovered as a co-product of zirconium production)
    abundance: 'USGS: world primary hafnium production data and quantitative estimates of hafnium reserves were not available. Hafnium resources are associated with zircon and baddeleyite, with no quantitative estimate. U.S. unwrought hafnium price was 3,800 dollars per kilogram in 2025 (estimated).'
    producers: []
    critical_lists: []
    recycling: 'USGS: hafnium metal recycling was minimal.'
    substitutes:
      - 'USGS: boron or cadmium-silver-indium alloys can substitute for hafnium metal in nuclear control rods.'
    source: usgs-mcs2026-zirconium-hafnium
    where: 'Salient Statistics (price, 2021 to 2025e); World Mine Production and Reserves; World Resources; Recycling; Substitutes. Import sources for unwrought hafnium (2021 to 24): Germany 54 percent, China 21 percent, France 12 percent, United Kingdom 8 percent, other 5 percent. These are import shares, not production shares.'
media: []
documents:
  - title: Zhang J and McMahon JM, Temperature-dependent mechanical properties of ZrC and HfC from first principles, arXiv:2002.07903 (2020)
    type: paper
    url: https://arxiv.org/pdf/2002.07903
    year: 2020
  - title: 'Zeng Q et al., Prediction of stable hafnium carbides: their stoichiometries, mechanical properties, and electronic structure, arXiv:1309.6516 (2013)'
    type: paper
    url: https://arxiv.org/pdf/1309.6516
    year: 2013
  - title: Stanford Advanced Materials, Safety Data Sheet, Hafnium Carbide Powder, CAS 12069-85-1 (revised 27/11/2022)
    type: sds
    url: https://www.samaterials.com/tds/1697119761-1390-hafnium-carbide-hfc-powder-sds.pdf
    year: 2022
  - title: Goodfellow, Hafnium carbide powder group, product datasheet page
    type: datasheet
    url: https://www.goodfellow.com/usa/hafnium-carbide-powder-group
    year: 2026
  - title: PubChem compound CID 16212551, Hafnium(IV) carbide (formula CHf, molecular weight 190.50)
    type: database
    url: https://pubchem.ncbi.nlm.nih.gov/compound/16212551
    year: 2026
  - title: Grisaffe SJ, Thermal Expansion of Hafnium Carbide, Journal of The American Ceramic Society 43(9), September 1960 (NASA Lewis reprint, NTRS 20160000488)
    type: report
    url: https://ntrs.nasa.gov/citations/20160000488
    year: 1960
  - title: Thompson J, High-Melt Carbon-Carbon Coating for Nozzle Extensions, Carbon Carbon Advanced Technologies, NASA NTRS 20160005370 (2015)
    type: report
    url: https://ntrs.nasa.gov/citations/20160005370
    year: 2015
  - title: Gaydaychuk A, Linnik S, Mitulinsky A and Zenkin S, Effect of Al Addition on the Oxidation Resistance of HfC Thin Films, Coatings (2022)
    type: paper
    url: https://earchive.tpu.ru/bitstream/11683/70753/1/reprint-nw-38399.pdf
    year: 2022
  - title: USGS Mineral Commodity Summaries 2026, Zirconium and Hafnium
    type: report
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-zirconium-hafnium.pdf
    year: 2026
gaps:
  - Thermal conductivity of dense HfC was not found in any source read in this run. Searches returned ZrC, HfB2 and HfCxN1-x values only, which are not HfC.
  - Measured Vickers hardness, indentation hardness and fracture toughness of HfC were not found; the hardness here is a calculated value from Zeng et al.
  - Young modulus and Poisson ratio for polycrystalline HfC were not read in this run. Zhang and McMahon give single-crystal style elastic constants (C11, C12, C44) only.
  - Oxidation rate constants, weight gain and activation energy for bulk HfC at 1000 degC were not found. The MDPI Coatings 2017 paper on TaC-HfC solid solutions returned 403 Forbidden, and the 1997 OSTI paper on HfC single-crystal oxidation could not be reached.
  - Corrosion in hot hydrogen, water vapour, nitrogen or molten salts was not found in a source read in this run.
  - Ageing and long-term stability data for HfC were not found.
  - 'Supply: no world production data or producer shares for hafnium were printed by USGS (see the supply entry). Import shares for unwrought hafnium are noted as import shares only.'
  - 'Critical mineral status: the USGS zirconium-hafnium chapter read in this run does not print a critical-list status for hafnium, and the EU critical raw materials list was not read.'
  - 'Designations: no UNS, EN, DIN, JIS, AISI or ASTM code for HfC was printed in a source read in this run. The SDS lists EC number 235-114-1, which is not one of the designation systems used here, so it is not listed.'
  - 'Space use: no NASA, ESA or flight record for HfC was found. The NASA Ames coating test (NTRS 20160005370) is a ground arc-jet test of a coating that contains HfC and ZrB2.'
  - 'Environment tag: the sources read say hypersonic flight, not re-entry, so no environment tag is set. Add re-entry only if a source says so.'
  - PubChem record CID 16212551 was read through its JSON record. It gives formula CHf and molecular weight 190.50, but no GHS classification and no experimental-property section.
  - 'Custom property names used because they are not in spores/property-canon.yaml: hardness, elastic_constant_c11, elastic_constant_c12, elastic_constant_c44, lattice_constant, oxidation_resistance_temp. These should be added to the canon file.'
  - Wikipedia was used for composition only (HfCx, x from 0.5 to 1.0) and for the synthesis route by reduction of hafnium oxide with carbon. Its oxidation onset of 430 degC and its hardness and density claims are not used.
  - 'Melting point conflict: 3890 degC (SDS and Goodfellow), 4255 plus or minus 30 K (Zhang and McMahon) and 7029 degF (1960 note). All are kept with sources named; none has been reconciled.'
  - 'Density conflict: 12.2 g/cm3 (SDS and Goodfellow) against a theoretical 12.1 g/mL in the 1960 note, which is a different basis.'
sources:
  zhang2020:
    title: Zhang J and McMahon JM, Temperature-dependent mechanical properties of ZrC and HfC from first principles, arXiv:2002.07903 (2020)
    url: https://arxiv.org/pdf/2002.07903
    kind: paper
    license: arXiv preprint
  zeng2013:
    title: 'Zeng Q et al., Prediction of stable hafnium carbides: their stoichiometries, mechanical properties, and electronic structure, arXiv:1309.6516 (2013)'
    url: https://arxiv.org/pdf/1309.6516
    kind: paper
    license: arXiv preprint
  sds-sama:
    title: Stanford Advanced Materials, Safety Data Sheet, Hafnium Carbide Powder, CAS 12069-85-1 (revised 27/11/2022)
    url: https://www.samaterials.com/tds/1697119761-1390-hafnium-carbide-hfc-powder-sds.pdf
    kind: datasheet
    license: supplier document, cited not reproduced
  goodfellow-hfc:
    title: Goodfellow, Hafnium carbide powder group, product datasheet page
    url: https://www.goodfellow.com/usa/hafnium-carbide-powder-group
    kind: datasheet
    license: supplier page, cited not reproduced
  pubchem-hfc:
    title: PubChem compound CID 16212551, Hafnium(IV) carbide (formula CHf, CAS 12069-85-1 in synonyms)
    url: https://pubchem.ncbi.nlm.nih.gov/compound/16212551
    kind: database
    license: PubChem public database
  grisaffe1960:
    title: Grisaffe SJ, Thermal Expansion of Hafnium Carbide, Journal of The American Ceramic Society 43(9), September 1960 (NASA Lewis reprint, NTRS 20160000488)
    url: https://ntrs.nasa.gov/citations/20160000488
    kind: report
    license: US government work, public domain
  ntrs-ccat-hfc:
    title: Thompson J, High-Melt Carbon-Carbon Coating for Nozzle Extensions, NASA NTRS 20160005370 (2015)
    url: https://ntrs.nasa.gov/citations/20160005370
    kind: report
    license: US government work, public domain
  tomsk-hfc-al-2022:
    title: Gaydaychuk A, Linnik S, Mitulinsky A and Zenkin S, Effect of Al Addition on the Oxidation Resistance of HfC Thin Films, Coatings (2022)
    url: https://earchive.tpu.ru/bitstream/11683/70753/1/reprint-nw-38399.pdf
    kind: paper
    license: CC BY 4.0 (as published by MDPI, reprint from Tomsk Polytechnic University archive)
  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
  wiki-hfc:
    title: Wikipedia, Hafnium carbide (composition and synthesis route only)
    url: https://en.wikipedia.org/wiki/Hafnium_carbide
    kind: web
    license: CC BY-SA 4.0
license: CC-BY-4.0
updated: '2026-10-26'
review:
  status: draft
  method: assistant
  checked_values: 17
  unreachable: 3
  caveats: 4
  note: Written by an AI assistant on 2026-10-26 from the arXiv Zhang and McMahon paper, the arXiv Zeng et al. paper, the Stanford Advanced Materials SDS (2022), the Goodfellow product page, the PubChem JSON record, the 1960 NASA Lewis HfC thermal expansion note, the 2015 NASA CCAT coating record, the 2022 Gaydaychuk et al. paper and the USGS MCS 2026 zirconium and hafnium chapter. The MDPI 2017 TaC-HfC paper returned 403, the OSTI 1997 paper could not be reached, and the PubChem HTML page was behind a browser check (its JSON record was used).
story:
  recipe:
    line: HfC is a rock-salt compound of hafnium and carbon with one hafnium atom and one carbon atom in the ideal formula. Real material is usually carbon deficient, written HfCx with x from 0.5 to 1.0 (Wikipedia). Powder is made by carbothermal reduction of hafnium(IV) oxide, which takes a long time to remove oxygen (Wikipedia). The 1960 NASA note used hot-pressed HfC specimens.
    parts:
      - name: Hafnium
        symbol: Hf
        role: The metal atom in the rock-salt lattice. Its heavy atoms give the high density (12.2 g/cm3, SDS) and very high melting point.
      - name: Carbon
        symbol: C
        role: The non-metal atom in the lattice. Carbon deficiency is common (Wikipedia), so the carbon content varies with x from 0.5 to 1.0.
  risk_groups:
    - title: For people
      level: 0
      verdict: Flammable solid (H228), with irritation risk
      lead: The supplier SDS classifies HfC powder as H228, flammable solid, and says hafnium compounds may irritate the skin and eyes. Hazard data are thin.
      points:
        - label: Flammability
          line: Classified H228 (flammable solid), signal word Warning. Keep away from heat, sparks and open flames.
        - label: Dust and exposure
          line: The SDS lists 0.5 mg/m3 as the ACGIH and USA PEL limit for hafnium, metal and compounds, as Hf. Use a fume hood and a respirator when dust levels are high.
        - label: Liver
          line: The SDS notes reports of liver damage from hafnium exposure.
    - title: If a part fails
      lead: These results come from one 2022 thin-film test and an introduction summary, not from field incidents.
      points:
        - label: Oxidation
          line: The 2022 paper states HfC is intensively oxidized in air at 400 to 500 degC. Pure HfC thin films oxidized through in its 600 and 800 degC test.
        - label: Toughness
          line: The same paper says bulk Hf-Al-C composites are much tougher than HfC. No fracture-toughness value for HfC was read.
  why: Hafnium carbide is a rock-salt ceramic with a very high melting point and high density. Its supplier data give a melting point of 3890 degC and a density of 12.2 g/cm3. Elastic constants and hardness from first-principles calculations give it high stiffness, and a 1960 measurement gives a linear expansion coefficient of 6.59 x 10-6 per degC from 25 to 612 degC. Those properties are why it is studied for hypersonic flight and for coatings. Its main limit is oxidation in air above about 400 degC, which the sources say needs a protective layer or alloying.
  uses:
    - where: Carbon-carbon nozzle extension coating
      line: A NASA Ames arc-jet test of a carbon-carbon coating that embeds HfC and ZrB2 showed enhanced high-temperature durability and no erosion at 3,100 F over four cycles totaling 28 minutes. The coating has two carbides, so the result is not for HfC alone.
    - where: Hypersonic flight and gas-turbine coatings (proposed)
      line: Zhang and McMahon say ZrC and HfC could be used beyond hypersonic flight and as gas-turbine thermal barrier coatings. This is a proposal in the paper, not a flight or test record.
  can_do:
    - label: Stiffness
      benchmark: elastic_constant_c11
      value: 500 GPa (C11)
      line: Experimental C11 is 500 GPa and C44 is 180 GPa at 298 K, as quoted by Zhang and McMahon. Their calculated quasi-harmonic values at 0 K are 500.9 GPa for C11 and 175.5 GPa for C44.
    - label: Melting
      benchmark: melting_point
      value: 3890 degC
      line: The supplier SDS and the Goodfellow datasheet both give 3890 degC. Zhang and McMahon quote 4255 plus or minus 30 K from an earlier study, and the 1960 note gives 7029 degF.
    - label: Hardness
      benchmark: hardness
      value: 32.95 GPa (calculated)
      line: Zeng et al. calculate a Vickers hardness of 32.95 GPa for HfC. This is a model result, not a measurement.
    - label: Heat expansion
      benchmark: cte
      value: 6.59 x 10-6/K
      line: The 1960 NASA note gives a linear expansion coefficient of 6.59 plus or minus 0.04 x 10-6 per degC from 25 to 612 degC for hot-pressed HfC.
  compare: The sources read here give HfC's density, melting point, elastic constants and hardness, but they give no side-by-side numbers against carbon-carbon composites or other carbides in the same test conditions, so no ranking is claimed. Within these sources, HfC is a very dense, very high melting point ceramic whose main weakness is oxidation in air from 400 to 500 degC.
  compare_rows:
    - label: Density
      benchmark: density
    - label: Melting point
      benchmark: melting_point
    - label: Hardness
      benchmark: hardness
  handling:
    - 'Powder: the SDS lists H228 (flammable solid). Keep it away from heat and ignition sources, and control dust with a fume hood.'
    - 'Storage: keep the container tightly sealed in a cool, dry, well-ventilated place, and store away from oxidizing agents.'
    - 'Fire: use special powder for metal fires and do not use water. Decomposition products include carbon monoxide, carbon dioxide and metal oxide fume.'
    - 'Oxidation: above about 400 degC in air, HfC needs a protective coating or alloying (Gaydaychuk et al. 2022).'
  good_for:
    - Very high melting point parts and coatings, where the melting point is the design driver (SDS, Zhang and McMahon).
    - Carbon-carbon nozzle structures with a protective coating (NASA Ames arc-jet test, 2015).
  watch_out:
    - Oxidation in air from 400 to 500 degC (Gaydaychuk et al. 2022). Pure HfC thin films oxidized through at 600 and 800 degC.
    - 'Melting point differs between sources: 3890 degC (SDS and Goodfellow), 4255 plus or minus 30 K (Zhang and McMahon), and 7029 degF (1960 note).'
    - 'Low fracture toughness: no value for HfC was read, and the paper says bulk Hf-Al-C composites are much tougher.'
    - Hazard data come from one supplier SDS. The PubChem record read in this run has no GHS classification.
  verdict: Hafnium carbide is a dense, very high melting point rock-salt ceramic. Its supplier density and melting point are ground values, and its elastic constants and hardness are calculated or quoted from experiment. Its main limit is oxidation in air from about 400 degC, so it needs a protective layer or alloying. The space-related uses found are a ground arc-jet test of a carbon-carbon nozzle coating that contains HfC, and proposals for hypersonic and gas-turbine coatings. No flight record was found.