Tungsten disulfide WS2

spore: profile
id: tungsten-disulphide
spore_version: 0.2
family: lubricant
name: Tungsten disulfide
abbreviation: WS2
role: dry film lubricant
category: tribology
type: dry-lubricants
summary: |
  Tungsten disulfide is a layered sulfide, like molybdenum disulfide. Its sheets slide over one another because they are held together only by weak van der Waals forces. It is used as a powder, a sputtered or laser-deposited film, and as nanotubes and two-dimensional layers. In NASA tests in ultrahigh vacuum a laser-deposited film ran at a friction below 0.1 and wore at a rate 30 times lower after annealing. In humid air the same family of films lasted about 9,000 passes, against more than 500,000 in vacuum. NASA describes WS2 as resisting oxidation in air better than MoS2, by about 100 degC.
best_for: 'Dry sliding contacts in vacuum where oil or grease cannot be used (NASA: sputtered film friction 0.055 and over one million cycles in vacuum)'
weakest_at: Humid air and atmosphere, where friction rises to 0.22 and sputtered film life falls to 1,500 cycles (NASA, 1970)
identity:
  formula: WS2
  cas: 12138-09-9
  crystal_system: molybdenite type (layered)
  source: wiki-ws2
  where: Infobox, Crystal structure (Molybdenite); formula from SDS section 3 (WS2, molecular weight 247.98 g/mol)
tags:
  environments: []
  stresses: []
  applications:
    - mechanisms
    - batteries
    - electronics-and-chips
properties:
  - benchmark: density
    label: Relative density
    value: 7.5
    unit: g/cm3
    scope: material
    conditions:
      basis: crystalline WS2 powder, supplier SDS (printed as 7.500 g/cm3)
    method: Relative density as printed in the supplier SDS
    status: ground
    source: sds-ws2
    where: Section 9.1, item m) Relative density
  - benchmark: density
    label: Density (Wikipedia infobox)
    value: 7.5
    unit: g/cm3
    scope: material
    conditions:
      basis: WS2, solid, as printed in the infobox
    method: Infobox value as printed
    status: ground
    source: wiki-ws2
    where: Infobox, Density 7.5 g/cm3, solid
  - benchmark: melting_point
    label: Melting point or range
    value: 1200
    unit: degC
    scope: material
    conditions:
      basis: crystalline WS2 powder, supplier SDS
    method: Melting point/range as printed in the SDS (1,200 degC, 2,192 degF)
    status: ground
    source: sds-ws2
    where: Section 9.1, item e) Melting point/range
  - benchmark: decomposition_temperature
    label: Decomposition temperature
    value: 1250
    unit: degC
    scope: material
    conditions:
      basis: WS2 heated without oxygen; the article says it decomposes to tungsten and sulfur at this temperature
    method: Infobox value as printed (1,250 degC, 2,280 degF, 1,520 K, decomposes)
    status: ground
    source: wiki-ws2
    where: Infobox, Melting point row
  - benchmark: coefficient_of_friction
    label: Friction in ultrahigh vacuum, sputtered film
    value: 0.055
    unit: dimensionless
    scope: material
    conditions:
      basis: dc-sputtered WS2 film on Ni, Ni-Cr or Nb disk, sliding against a metal rider, ultrahigh vacuum; load not stated in the text
    method: Average coefficient of friction over the test, as printed in the text of section VII
    status: ground
    source: nasa-tm-x52819
    where: Section VII, paragraph beginning When the experiments were conducted in vacuum
  - benchmark: coefficient_of_friction
    label: Friction in atmosphere, sputtered film
    value: 0.22
    unit: dimensionless
    scope: material
    conditions:
      basis: the same dc-sputtered WS2 film tested under atmospheric conditions (humidity not stated in the text)
    method: Coefficient of friction as printed; the report says it rose immediately on test start
    status: ground
    source: nasa-tm-x52819
    where: Section VII, paragraph beginning When the experiments were conducted in vacuum
  - benchmark: coefficient_of_friction
    label: Friction in ultrahigh vacuum, laser-annealed film
    value: 0.1
    unit: dimensionless
    scope: material
    conditions:
      basis: pulsed-laser-deposited WS2 coating, about 1 um thick, laser-annealed, on 440C stainless steel, 6 mm 440C ball, ultrahigh vacuum 7 x 10-7 Pa, 0.98 N (100 g), room temperature
    method: Steady-state friction; the summary says the friction was less than 0.1, so 0.1 is the upper bound as printed
    status: ground
    source: nasa-tm-107342
    where: Summary and Concluding Remarks (annealed WS2 coating, coefficient of friction less than 0.1)
  - benchmark: endurance_life
    label: Endurance life in atmosphere, sputtered film
    value: 1500
    unit: cycles
    scope: material
    conditions:
      basis: the same dc-sputtered WS2 film in atmospheric conditions; film failure reported after this many cycles
    method: Cycles to failure as printed
    status: ground
    source: nasa-tm-x52819
    where: Section VII, paragraph beginning When the experiments were conducted in vacuum
  - benchmark: endurance_life
    label: Endurance life in ultrahigh vacuum, untreated film
    value: 50000
    unit: passes
    scope: material
    conditions:
      basis: pulsed-laser-deposited WS2 on 440C, ultrahigh vacuum 7 x 10-7 Pa, 0.98 N; life is the number of passes at which the friction reaches 0.3
    method: Wear life as printed (around 50,000 passes)
    status: ground
    source: nasa-tm-107342
    where: Wear Life section (untreated coating in ultrahigh vacuum)
  - benchmark: endurance_life
    label: Endurance life in ultrahigh vacuum, laser-annealed film
    value: 500000
    unit: passes
    scope: material
    conditions:
      basis: pulsed-laser-deposited WS2 on 440C, laser-annealed, ultrahigh vacuum 7 x 10-7 Pa, 0.98 N; life is the number of passes at which the friction reaches 0.3
    method: Lower bound as printed (greater than 500,000 passes)
    status: ground
    source: nasa-tm-107342
    where: Wear Life section (laser-annealed coating in ultrahigh vacuum)
  - benchmark: endurance_life
    label: Endurance life in humid air, both films
    value: 9000
    unit: passes
    scope: material
    conditions:
      basis: pulsed-laser-deposited WS2 on 440C, humid air at 15 to 40 percent relative humidity, 0.98 N; life is the number of passes at which the friction reaches 0.8 in humid air
    method: Around 9,000 passes as printed, for both untreated and annealed coatings
    status: ground
    source: nasa-tm-107342
    where: Wear Life section (humid air, around 9000 passes)
  - benchmark: film_thickness
    label: Maximum film thickness, Dicronite coating
    value: 0.5
    unit: um
    scope: material
    conditions:
      basis: Dicronite dry-film WS2 coating, precision film
    method: Maximum as printed (0.5 microns, 0.000020 inches)
    status: ground
    source: precision-dicronite
    where: Section Key to Dicronite dry lubrication applications, precision film thickness
  - benchmark: elastic_modulus
    label: Young's modulus, WS2 nanotubes
    value: 160
    value_high: 170
    unit: GPa
    scope: material
    conditions:
      basis: WS2 nanotubes, as stated in the review; whether measured or calculated is not stated in the sentence
    method: Range as printed in the review (Young's modulus of 160 to 170 GPa)
    status: ground
    source: yadgarov-tenne-2024
    where: Mechanical, thermal and electrical properties section
  - benchmark: thermal_conductivity
    label: Thermal conductivity, single WS2 layer
    value: 32
    unit: W/(m·K)
    scope: material
    conditions:
      basis: single layer of WS2 at room temperature, as printed in the review
    method: Value as printed (32 W mK-1 at room temperature)
    status: ground
    source: yadgarov-tenne-2024
    where: Mechanical, thermal and electrical properties section
  - benchmark: band_gap_optical_direct
    label: Direct optical band gap, monolayer
    value: 2.05
    unit: eV
    scope: material
    conditions:
      basis: WS2 monolayer, optical direct gap; printed with a tilde (approximate)
    method: Infobox value as printed (about 2.05 eV)
    status: ground
    source: wiki-ws2
    where: Band gap section (optical, direct, monolayer)
  - benchmark: band_gap_optical_indirect
    label: Indirect optical band gap, bulk
    value: 1.35
    unit: eV
    scope: material
    conditions:
      basis: bulk WS2, optical indirect gap; printed with a tilde (approximate)
    method: Band gap section value as printed (about 1.35 eV)
    status: ground
    source: wiki-ws2
    where: Band gap section (optical, indirect, bulk)
corrosion:
  - environment: Air or atmosphere during sliding, dc-sputtered WS2 film
    headline: Atmosphere cut sputtered WS2 life from over one million cycles in vacuum to 1,500 cycles.
    effect: In vacuum the film averaged 0.055 friction and lasted over a million cycles. In atmosphere the friction rose to 0.22 at once and the film failed after 1,500 cycles. The report explains the failure by sulfuric acid formed on the surface, which has a corrosive effect.
    rating: severe
    conditions:
      basis: dc-sputtered WS2 film, atmospheric conditions (humidity not stated)
    source: nasa-tm-x52819
    where: Section VII, paragraph on atmospheric tests
  - environment: Humid air at 15 to 40 percent relative humidity, room temperature, pin on disk
    headline: Humid air reduces the wear life of pulsed-laser WS2 coatings by a factor of more than 50 compared with ultrahigh vacuum.
    effect: Both untreated and laser-annealed coatings lasted about 9,000 passes in humid air. The annealed coating lasted more than 500,000 passes in ultrahigh vacuum. The report ranks the environments by life as humid air, then ultrahigh vacuum, then dry nitrogen.
    rating: severe
    conditions:
      basis: pulsed-laser-deposited WS2 on 440C, 0.98 N, 6 mm ball, life defined as friction reaching 0.8 in humid air and 0.3 in vacuum
    source: nasa-tm-107342
    where: Wear Life section
  - environment: Dry nitrogen below 1 percent relative humidity, room temperature
    headline: Dry nitrogen gives the longest life of the three environments tested.
    effect: The coating lasted more than 2 million passes in dry nitrogen, against about 9,000 in humid air. Friction in dry nitrogen decreased with the number of passes, while it rose in humid air.
    rating: low
    conditions:
      basis: pulsed-laser-deposited WS2 on 440C, 0.98 N; life defined as friction reaching 0.15 in dry nitrogen
    source: nasa-tm-107342
    where: Friction Behavior and Wear Life sections
  - environment: Heating in air
    headline: NASA reports that WS2 resists oxidation in air about 100 degC better than MoS2, but gives no absolute oxidation temperature.
    effect: The report says WS2 has an advantage of about 100 degC over MoS2 in oxidation resistance and thermal stability. The report gives no absolute onset temperature for oxidation in air.
    rating: unknown
    conditions:
      basis: relative comparison only; no absolute temperature given in the sources opened
    source: nasa-tm-107342
    where: Introduction
risks:
  hazards:
    - text: The supplier SDS classifies WS2 powder as skin irritant (Category 2, H315), eye irritant (Category 2A, H319) and a single-exposure respiratory irritant (Category 3, H335), with the Warning signal word.
      ghs:
        - H315
        - H319
        - H335
      source: sds-ws2
      where: Section 2.1 and 2.2, GHS classification and label elements
    - text: PubChem records H315, H319 and H335 in 57.2 percent of 138 company notifications, and no GHS hazard for the other 42.8 percent of reports (59 of 138).
      ghs:
        - H315
        - H319
        - H335
      source: pubchem-ws2
      where: Safety and Hazards, GHS Classification (CID 82938)
    - text: The SDS lists no IARC or NTP carcinogen at 0.1 percent or more, and does not classify the substance as carcinogenic.
      ghs: []
      source: sds-ws2
      where: Section 11.1, Carcinogenicity
  failure_risks:
    - mode: Atmospheric exposure during sliding
      trigger: Sputtered film run in air instead of ultrahigh vacuum
      consequence: Friction rose to 0.22 at once and the film failed after 1,500 cycles, against more than one million cycles in vacuum. NASA attributes the failure to sulfuric acid formed on the surface.
      severity: high
      source: nasa-tm-x52819
      where: Section VII, atmospheric tests
    - mode: Humid-air wear
      trigger: Pulsed-laser WS2 coating run at 15 to 40 percent relative humidity
      consequence: Wear life of about 9,000 passes, against more than 500,000 for the annealed coating in ultrahigh vacuum.
      severity: high
      source: nasa-tm-107342
      where: Wear Life section
    - mode: Humid storage of a pressed WS2 compact
      trigger: Cold-pressed compact wrapped in paper and stored in atmospheric conditions
      consequence: The report says the humidity affected the compact over several weeks; no number is given, and the result is shown only in a figure.
      severity: moderate
      source: nasa-tm-x52819
      where: Section VII, paragraph on the stored compact (Figure 19)
  side_effects:
    - text: In a fire, WS2 decomposes to sulphur oxides and tungsten oxide. The SDS gives no other decomposition data.
      affects: Responders and the surrounding materials in a fire
      source: sds-ws2
      where: Section 10.6, hazardous decomposition products
  handling:
    - text: The SDS says to keep the container tightly closed in a dry and well-ventilated place, and to store under nitrogen. It classes the material as air-, heat- and moisture-sensitive.
      source: sds-ws2
      where: Section 7.2, conditions for safe storage
    - text: Avoid dust formation and provide exhaust ventilation where dust forms. Further processing of the solid may produce combustible dust.
      source: sds-ws2
      where: Sections 7.1 and 7.2 (handling and dust)
    - text: Keep WS2 away from strong acids and strong oxidizing agents.
      source: sds-ws2
      where: Section 10.5, incompatible materials
    - text: The Dicronite coating is applied near ambient temperature, at a maximum of 35 degC.
      source: precision-dicronite
      where: Section Key to Dicronite dry lubrication applications, application temperature
  mitigations:
    - text: Laser annealing reduced the wear rate of the coating by a factor of 30 in ultrahigh vacuum and gave more than 500,000 passes against about 50,000 for the untreated coating.
      source: nasa-tm-107342
      where: Summary and Concluding Remarks
uses:
  - application: mechanisms
    device: Sliding contacts in vacuum, such as bearings and gears where oil or grease cannot be used
    example: NASA describes WS2 as an effective inorganic lubricant in a vacuum, behaving much like MoS2, and tested pulsed-laser WS2 coatings on 440C stainless steel for sliding life in ultrahigh vacuum.
    source: nasa-tm-107342
    where: Introduction and Summary
  - application: electronics-and-chips
    device: Field-effect transistor made from small-diameter WS2 nanotubes
    example: The review reports p-type mobility of 1.7 plus or minus 0.5 cm2 V-1 s-1 and n-type mobility of 0.26 plus or minus 0.05 cm2 V-1 s-1 for nanotubes of about 20 nm diameter used in transistors.
    source: yadgarov-tenne-2024
    where: Mechanical, thermal and electrical properties section (transistor paragraph)
  - application: batteries
    device: Electrode material in a battery test using WS2 nanotubes
    example: The review reports a capacity of 371.9 mAh/g and 62 percent retention after 500 cycles at 1000 mA/g for a WS2 nanotube electrode.
    source: yadgarov-tenne-2024
    where: Applications, battery section
supply:
  - subject: Tungsten (feedstock for WS2)
    producers:
      - country: China
        tonnes: 67000
        share_percent: 78.8
        year: 2025
      - country: Vietnam
        tonnes: 3000
        share_percent: 3.5
        year: 2025
      - country: Kazakhstan
        tonnes: 2400
        share_percent: 2.8
        year: 2025
      - country: Russia
        tonnes: 2000
        share_percent: 2.4
        year: 2025
      - country: Korea North
        tonnes: 2000
        share_percent: 2.4
        year: 2025
    reserves: 'USGS 2026: reserves in tonnes of tungsten content: China 2,500,000; Russia 400,000; Australia 570,000; Vietnam 170,000; world total greater than 4,700,000.'
    critical_lists: []
    recycling: 'USGS: the amount of secondary tungsten produced and consumed in 2025 was withheld to avoid disclosing proprietary data. No recycled share is given.'
    export_controls: 'USGS: in February 2025 China implemented new export controls on selected tungsten items. Rotterdam prices rose from $266 to $551 per metric ton unit for 65 percent concentrate and from $331 to $675 for APT.'
    source: usgs-mcs2026-tungsten
    where: World Mine Production and Reserves table (2025 column, world total 85,000 t rounded); Events, Trends and Issues. Shares calculated by Space Mushroom as tonnes divided by the 85,000 t world total.
media: []
documents:
  - title: Tungsten disulfide, Crystal, 99.995 percent, safety data sheet (Stanford Advanced Materials, version 3.0)
    type: sds
    url: https://www.samaterials.com/pdf/Tungsten-Disulfide-(WS2)-Powder-sds.pdf
    year: 2017
  - title: Friction characteristics of sputtered solid film lubricants (NASA TM X-52819, Spalvins)
    type: report
    url: https://ntrs.nasa.gov/citations/19700020846
    year: 1970
  - title: Surface Chemistry, Friction, and Wear Properties of Untreated and Laser-Annealed Surfaces of Pulsed-Laser-Deposited WS2 Coatings (NASA TM 107342)
    type: report
    url: https://ntrs.nasa.gov/citations/19970009635
    year: 1996
  - title: 'USGS Mineral Commodity Summaries 2026: Tungsten'
    type: report
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-tungsten.pdf
    year: 2026
  - title: Nanotubes from Transition Metal Dichalcogenides, Recent Progress in the Synthesis, Characterization and Electrooptical Properties (Yadgarov and Tenne, Small)
    type: paper
    url: https://pmc.ncbi.nlm.nih.gov/articles/PMC12272021/
    year: 2024
  - title: 'PubChem: Tungsten disulfide (CID 82938), GHS classification'
    type: database
    url: https://pubchem.ncbi.nlm.nih.gov/compound/82938
    year: 2026
gaps:
  - The NASA 1970 report prints the vacuum life as 'over a million cycles', and the NASA 1996 report prints the dry-nitrogen life as 'greater than 2 million passes'. Both are written in words, so no numeric value was entered for them (the verifier needs the printed number).
  - The Dicronite distributor page (precisioncoatings.com) gives a functional span of 188 to 538 degC in air and up to 1316 degC in vacuum. Its raw HTML does not print these figures, so they are not entered as values. The Lubrication Sciences International datasheet was not opened.
  - No space flight, low Earth orbit or atomic oxygen result for WS2 was found in the sources opened. All friction and life numbers are ground tests.
  - Hardness of the WS2 film was not found in any opened source.
  - Friction in humid air for the laser-coated film is given only qualitatively in the text (friction rises in air); the numeric values are in a figure that was not extracted.
  - The Dicronite friction range of about 0.03 to 0.07 quoted by distributors was not read on a fetched page. The precision coatings page gives only ultra-low friction, with no number. The Lubrication Sciences International datasheet was not opened.
  - The absolute oxidation onset of WS2 in air was not found. The 1996 report gives only an advantage of about 100 degC over MoS2.
  - The PubChem density and melting point were not read; the page was truncated before its experimental properties section. Density and melting point are taken from the supplier SDS and Wikipedia.
  - The Wikipedia optical indirect bulk gap of about 1.35 eV disagrees with the review's bulk material gap of 2.01 to 2.05 eV. Neither has been reconciled; both are recorded with their sources.
  - The review gives no method (measured or calculated) for the nanotube Young's modulus, and no oxidation or humidity numbers for WS2 nanotubes.
  - The 1970 report does not state the test load, the humidity of the atmospheric tests, or the film thickness in the text (figure caption gives about 2,500 angstrom for one wear track).
  - Sulfur's standard atomic weight is printed by NIST as an interval, 32.059 to 32.076. The value 32.07 used here is the rounded midpoint, and the recipe role notes this.
  - Crustal abundance of tungsten and any critical-mineral list status were not found in opened text; the USGS 2026 tungsten extract does not mention a critical list.
  - 'Benchmark names not in spores/property-canon.yaml and used here: coefficient_of_friction, endurance_life, wear_rate, decomposition_temperature, film_thickness, band_gap_optical_direct, band_gap_optical_indirect. Check them against the canon before import.'
  - Ossila WS2 properties page returned HTTP 429 and was not read.
sources:
  wiki-ws2:
    title: Tungsten disulfide
    id: Wikipedia, English edition
    url: https://en.wikipedia.org/wiki/Tungsten_disulfide
    kind: web
    license: CC BY-SA 4.0
  pubchem-ws2:
    title: Tungsten disulfide (CID 82938)
    id: PubChem, NIH National Library of Medicine
    url: https://pubchem.ncbi.nlm.nih.gov/compound/82938
    year: 2026
    kind: database
    license: public domain (NLM)
  sds-ws2:
    title: Tungsten disulfide, Crystal, 99.995% safety data sheet (Stanford Advanced Materials, version 3.0)
    id: SAM product WS2 powder, revision date 09/04/2017
    url: https://www.samaterials.com/pdf/Tungsten-Disulfide-(WS2)-Powder-sds.pdf
    year: 2017
    kind: datasheet
    license: supplier SDS (cited, not reproduced)
  nasa-tm-x52819:
    title: Friction characteristics of sputtered solid film lubricants
    id: NASA TM X-52819 (Spalvins, Lewis Research Center, 1970)
    url: https://ntrs.nasa.gov/citations/19700020846
    year: 1970
    kind: report
    license: NASA public use
  nasa-tm-107342:
    title: Surface Chemistry, Friction, and Wear Properties of Untreated and Laser-Annealed Surfaces of Pulsed-Laser-Deposited WS2 Coatings
    id: NASA TM 107342 (Miyoshi, Wheeler, Zabinski, December 1996)
    url: https://ntrs.nasa.gov/citations/19970009635
    year: 1996
    kind: report
    license: NASA public use
  usgs-mcs2026-tungsten:
    title: 'Mineral Commodity Summaries 2026: Tungsten'
    id: USGS, February 2026
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-tungsten.pdf
    year: 2026
    kind: report
    license: public domain (USGS)
  precision-dicronite:
    title: Dicronite dry film lubrication (distributor page)
    id: Precision Coatings, Dicronite page
    url: https://precisioncoatings.com/dicronite/
    kind: web
    license: web page (cited, not reproduced)
  yadgarov-tenne-2024:
    title: Nanotubes from Transition Metal Dichalcogenides, Recent Progress in the Synthesis, Characterization and Electrooptical Properties
    id: Yadgarov and Tenne, Small, 2024 (PubMed Central PMC12272021)
    url: https://pmc.ncbi.nlm.nih.gov/articles/PMC12272021/
    year: 2024
    kind: paper
    license: open access (PMC)
  nist-atomic-weights:
    title: Atomic weights of the elements (NIST, tungsten and sulfur)
    id: NIST Physical Measurement Laboratory, Atomic Weights and Isotopic Compositions
    url: https://physics.nist.gov/cgi-bin/Compositions/stand_alone.pl?ele=W&all=all
    kind: database
    license: public domain (NIST)
license: CC-BY-4.0
updated: '2026-10-26'
review:
  status: draft
  method: assistant
  checked_values: 16
  unreachable: 2
  caveats: 3
  note: Written 2026-10-26 from NASA TM X-52819 (1970), NASA TM 107342 (1996), the USGS 2026 tungsten chapter, the supplier SDS, PubChem, Wikipedia, the Dicronite distributor page, the Yadgarov and Tenne 2024 review and NIST. Hardness, space-flight data and the Dicronite friction number are not found; see gaps.
story:
  recipe:
    line: WS2 is a layered compound. Each formula unit combines one tungsten atom with two sulfur atoms, in trigonal prismatic coordination. The review says WS2 nanotubes are made by sulfiding tungsten oxide precursors with H2S at high temperature. The NASA films were made by sputtering and by pulsed-laser deposition.
    parts:
      - name: Tungsten
        symbol: W
        atoms: 1
        atomic_weight: 183.84
        role: The metal centre. Each tungsten sits between two layers of sulfur, and the sulfur sheets are the sliding planes.
      - name: Sulfur
        symbol: S
        atoms: 2
        atomic_weight: 32.07
        role: Forms the top and bottom layers of each sheet. Weak van der Waals bonds link the sheets, which is why they slide. The sulfur atomic weight is the rounded midpoint of the NIST interval.
  risk_groups:
    - title: For people
      level: 0
      verdict: Irritant hazard, not carcinogenic in the sources
      lead: WS2 powder is classed as an irritant for skin, eyes and the respiratory tract. The SDS lists no carcinogen.
      points:
        - label: Irritation
          line: The SDS classes WS2 for skin irritation (H315), serious eye irritation (H319) and respiratory irritation (H335), with the Warning signal word.
        - label: Most reports
          line: PubChem records the same three codes in 57.2 percent of 138 company notifications. The rest find no GHS hazard criteria met.
        - label: Cancer
          line: The SDS lists no IARC or NTP carcinogen at 0.1 percent or more.
        - label: Dust
          line: Dust should be avoided, with exhaust ventilation where dust forms. Further processing may make combustible dust.
    - title: Handling the coating
      points:
        - label: Application
          line: The Dicronite coating is applied near ambient temperature, at a maximum of 35 degC, and is a precision film of up to 0.5 micron.
        - label: Storage
          line: Store in a dry, well-ventilated place under nitrogen. The SDS calls the material air-, heat- and moisture-sensitive.
  why: Tungsten disulfide (WS2) is a layered lubricant that works in the same way as molybdenum disulfide. Its layers slide with little resistance, so it lubricates where oil or grease cannot. NASA's ultrahigh-vacuum tests gave a friction of 0.055 for a sputtered film and over a million cycles of life, and a laser-annealed film ran at a friction below 0.1. Like MoS2 it loses life in humid air or atmosphere. WS2 is chosen where vacuum friction and oxidation resistance matter, and where the humidity history of the part is controlled.
  uses:
    - where: Vacuum sliding contacts
      line: NASA describes WS2 as an effective inorganic lubricant in a vacuum. It tested pulsed-laser WS2 coatings on 440C stainless steel for sliding life in ultrahigh vacuum.
    - where: Nanotube transistors
      line: The review reports a p-type mobility of 1.7 plus or minus 0.5 cm2 V-1 s-1 and an n-type mobility of 0.26 plus or minus 0.05 cm2 V-1 s-1 for WS2 nanotubes of about 20 nm diameter used in transistors.
    - where: Battery electrodes
      line: The review reports a WS2 nanotube electrode with a capacity of 371.9 mAh/g and 62 percent retention after 500 cycles at 1000 mA/g.
  can_do:
    - label: Friction in vacuum
      value: '0.055'
      line: Sputtered WS2 averaged 0.055 friction in ultrahigh vacuum (NASA TM X-52819). The laser-annealed coating was below 0.1 (NASA TM 107342).
    - label: Friction in air
      value: '0.22'
      line: The same sputtered film rose to 0.22 in atmosphere and failed after 1,500 cycles (NASA TM X-52819).
    - label: Endurance life
      value: More than 1 million cycles
      line: In ultrahigh vacuum the sputtered film lasted over a million cycles. The laser-annealed coating lasted more than 500,000 passes (NASA TM 107342).
    - label: Temperature limit
      value: Rated by the distributor
      line: The distributor's Dicronite page gives a wide functional temperature span, higher in vacuum than in air. Its figures could not be checked against the page text, so they are not entered as values in this profile.
  compare: WS2 and MoS2 work by the same layered sliding mechanism, and NASA says WS2 behaves much like MoS2 in vacuum. WS2 is the more oxidation-resistant of the two, and its Dicronite dry film is rated higher in air than the MoS2 product in the MoS2 profile. The endurance numbers are not directly comparable. The MoS2 profile's NASA tests used a higher load and different films from the WS2 tests here. Both lose most of their life in humid air.
  compare_rows:
    - scope: material
      label: Friction in vacuum
      benchmark: coefficient_of_friction
    - scope: material
      label: Endurance life
      benchmark: endurance_life
    - scope: material
      label: Melting point (decomposes above it)
      benchmark: melting_point
  handling:
    - Read the supplier safety data sheet first. The SDS classes WS2 as an irritant and says it is air-, heat- and moisture-sensitive.
    - Store the powder in a closed container, under nitrogen, in a dry and well-ventilated place. Keep it away from strong acids and oxidizers.
    - Avoid dust. Use exhaust ventilation where dust forms.
    - Keep coatings out of humid storage. NASA found that humidity affected a pressed WS2 compact over several weeks.
  good_for:
    - Dry sliding in vacuum. A sputtered film averaged 0.055 friction and lasted more than a million cycles in NASA's ultrahigh-vacuum tests.
    - Oxidation resistance. NASA reports that WS2 resists oxidation in air about 100 degC better than MoS2, and the sources give a melting point of 1200 degC.
  watch_out:
    - Atmosphere and humidity. A sputtered film failed after 1,500 cycles in atmosphere, and laser-coated films lasted about 9,000 passes in humid air.
    - Sulfuric acid. NASA attributes the atmospheric failure to acid formed on the surface.
    - Limited data. No flight data, no LEO data and no hardness value were found in the sources opened.
  verdict: WS2 is a vacuum lubricant of the same family as MoS2. Its strongest points are the vacuum friction and a higher oxidation resistance than MoS2, but the humidity penalty is severe and the sources hold few numbers for humid service.