MoS2 dry-film lubricant coatings (bonded solid-lubricant coatings) are functional films, typically 5–25 µm thick, in which flaky molybdenum disulfide (MoS₂) and similar solid lubricants are anchored to the part surface by an inorganic or organic binder (sodium silicate, phosphate, silane, epoxy), often with zinc/aluminium flake added for corrosion protection. Cured by dip-spin or spray application and baking, they carry three jobs in oil-free, dust-sensitive or vacuum-duty service: prevent galling, stabilise friction, and control the assembly torque factor.
TL;DR — the one-minute version
- Mechanism: the S–Mo–S layers of MoS₂ sit 0.62 nm apart (d002 ≈ 6.15 Å) and are held only by van der Waals forces, so they shear easily and smear into a transfer film on the mating surface; a few microns of coating bring the friction coefficient into the 0.05–0.15 range.
- Environment decides everything: sputtered MoS₂ measured µ = 0.04 in vacuum or inert gas, while in humid air µ runs about 3× higher and wear life falls to roughly one tenth; graphite behaves the opposite way — it needs adsorbed water vapour and rises above µ 0.5 in high vacuum.
- Temperature ceiling: MoS₂ oxidises above about 350–400 °C in air to hard, abrasive MoO₃, and most binders stop between 150 and 330 °C — the system is rated by its weaker half.
- Fasteners are the core market: test to ISO 16047 (µtot, µth, µb and torque factor K in T = K·F·d), hold K at roughly 0.12–0.16 with batch scatter near ±0.02, or assemblies end up under- or over-tightened.
- Workhorse architecture: a chromium-free zinc-flake base coat (8–15 µm, cured 280–330 °C) plus a low-cure lubricant top coat (5–20 µm) passes 720–1,000 h neutral salt spray and ELV/RoHS/REACH hex-chromium limits — the default route for automotive, wind and rail fasteners.
Key numbers at a glance
- Lamellar geometry: 2H-MoS₂ interlayer spacing d002 ≈ 0.615 nm (c-axis ≈ 1.23 nm); shear happens between layers.
- Friction: sputtered MoS₂ films down to µ 0.04 in vacuum/inert atmosphere; Ti-doped films µ 0.05–0.08 in air with more than 10× the wear life of pure MoS₂ (Guangdong Vacuum Society annual meeting, 2015).
- Humidity cost: µ in humid air ≈ 3× the vacuum value and life ≈ 1/10; MoS₂/Pb films shift from mechanics-dominated (13% RH) to oxidation-dominated behaviour (75% RH) (Chinese Academy of Sciences thesis measurements).
- Assembly mechanics: ISO 16047:2005 covers M3–M39 at 10–35 °C, driving speeds 10–40 r/min for M3–M16; about 90% of input torque is consumed by thread and bearing friction, only ~10% becomes clamp force.
- Process window: zinc-flake base: pre-dry 80–120 °C, cure 280–330 °C × 20–40 min, 8–15 µm; lubricant top coat 5–20 µm, organic binders curing at 100–180 °C (compatible with electrodeprimer).
- Compliance floor: ELV 2000/53/EC and RoHS 2011/65/EU cap lead, cadmium, mercury and hexavalent chromium at 1,000 mg/kg per homogeneous material; REACH Annex XVII entry 47 restricts Cr(VI) compounds.
1. Mechanism: lamellar shear and the transfer film
Dry-film lubrication does not rely on oil. It relies on a solid shear layer that keeps feeding — and re-healing — the interface.
1.1 Why layered crystals slide
MoS₂ is stacked S–Mo–S tri-atomic planes: strong intra-layer bonding, weak van der Waals coupling between planes. Under the high pressures and flash temperatures of asperity contact, the flakes are rolled out and align basal-plane-parallel to the surface (basal texture), so real shear occurs at interlayer strength rather than at the metal. Flake aspect ratio matters industrially: common powders are 2–10 µm platelets with aspect ratio above 10, and dispersion quality directly sets film performance.
1.2 Half the lubricant lives on the counterface
In a working system much of the MoS₂ ends up transferred onto the mating surface; the running-in period, where µ falls and settles, is the transfer film covering the contact. When supply fails — overload, or oxidation to MoO₃ in humid heat — the film starves and the pair drops back to boundary contact. That is why dry-film failure is a cliff, not a slope.
1.3 The binder does not lubricate; it anchors
Inorganic binders (silicate/phosphate) tolerate higher temperature and resist polish wear but are brittle and need 200 °C-plus bakes; organic binders (epoxy, silane, urethane) cure at 100–180 °C and match existing primer stacks. Film thickness is the lubricant inventory: sliding distance spends it, and re-coating intervals should be designed around it.
2. Five solid lubricants compared
| System | Friction (typical) | Air/oxidation limit | Vacuum behaviour | Humidity behaviour | Duty |
|---|---|---|---|---|---|
| MoS₂ | air 0.05–0.15; vacuum down to 0.04 | oxidises from ~350–400 °C | generally better than humid air; transfer-film conditions change, doped grades used | wet air raises µ, cuts life | fasteners, cold forming, general |
| WS₂ | similar class, strong vacuum pedigree | slightly better oxidation stability | route of choice in space-mechanism literature | environment-sensitive too | space mechanisms, UHV hardware |
| Graphite | ~0.1 in humid air | oxidises above ~450 °C in air | µ climbs past 0.5 without adsorbed vapour | needs water vapour films | foundry, hot non-vacuum duties |
| PTFE | 0.04–0.12 initial | continuous service ~260 °C | insensitive | chemically inert | release/anti-stick; watch creep |
| h-BN | ~0.2–0.3 at room temperature | stable near 900 °C in air | stronger interlayer coupling, higher shear | insensitive | hot + insulating contacts |
Figures condense public literature and manufacturer data; load, speed, atmosphere and counterface can shift results by an order of magnitude — always qualify under simulated service conditions.
3. The humidity–vacuum–temperature window
3.1 Humidity is a double-edged sword
Trace water vapour and oxygen help MoS₂ run at low friction and wear; sustained high humidity accelerates oxidation and hydration of the film. Measured sputtered coatings: µ in humid air about 3× vacuum and life about one tenth; in 13–75% RH cycling, unmodified films drift while oxidation-stabilised grades repeat their friction curves. Practical rule: qualify a humidity spectrum, not one laboratory point.
3.2 Vacuum: opportunity and trap
Space mechanisms are the classic proving ground — the 1989 Galileo high-gain-antenna deployment anomaly (NASA, 1994 symposium record) taught the industry redundancy in solid lubrication; MoS₂–Ta co-sputtered grades and self-aligning bearings carry the field forward (Serles et al., Adv. Funct. Mater. 2022; Lince, Lubricants 2020; review in Front. Mech. Eng. 2023, DOI 10.1007/s11465-023-0755-1). But low friction in vacuum does not automatically mean low wear: transfer-film adhesion and debris compaction differ from air, so keep margin when extrapolating ground tests.
3.3 Temperature: the binder usually surrenders first
MoS₂ oxidises around 350–400 °C, yet zinc-flake stacks cure at ~300 °C and organic-binder top coats stop at 150–250 °C. Rate the system by the lowest of lubricant, binder and substrate-temper thresholds — and check the cold end too: frost, condensation and thermal cycling crack thin films and eat adhesion (combine with GB/T 10125 / ISO 9227 salt-spray and damp-heat regimes).
4. Application practice: film, thickness, cure
Dip-and-spin dominates fastener volume work (thread penetration, thickness uniformity); spraying suits large or selective parts; brush touch-up is maintenance-only. The standard duplex: clean/blast → zinc-flake base → high-temperature cure → low-cure lubricant top coat. Total build enlarges screw thread pitch diameter, so 8–15 µm management plus post-coat tapping or allowance grades keeps go-gauges honest. Curing above 300 °C would scorch an electrodeprimer stack, which is exactly why the lubricant top coat must cure at 100–180 °C. Steep pre-dry ramps trap bubbles; peak temperature and dwell set ceramic conversion, adhesion (GB/T 9286 cross-cut) and µ repeatability — record them batch by batch.
5. Fastener verification: ISO 16047 and K control
Torque–clamp behaviour follows T = K·F·d. ISO 16047:2005 resolves friction with the Kellermann–Klein relationship µtot = (T/F − P/(2πd))/(0.577d + 0.5D_b), separating thread friction µth from bearing friction µb — telling you which interface to fix. Bare dry steel scatters around K 0.15–0.25; lubricated zinc-flake product targets K ≈ 0.13 (band 0.12–0.16, µtot 0.10–0.15, ±0.02 control). Because roughly 90% of torque burns in friction, small K drifts become large preload errors — accept batches on K distributions, not one-off µ readings.
| Symptom | Likely cause | Check & fix |
|---|---|---|
| Torque ok, clamp low | high µ; film punched through | µth/µb split per ISO 16047; thicker top coat or lower-µ grade |
| Bolts yield/snapped | µ too low; mixed batches | incoming K audit; oxidation-stabilised lubricant grade |
| Go-gauge rejects threads | pitch diameter growth | post-coat tapping; allowance class; thickness cap 8–15 µm |
| White/red rust at edges | thin edge coverage | centrifuge/drain parameters; edge-thickness inspection |
| Cracking, peeling | fast pre-dry ramp; oily substrate | gentler cure profile; degrease bath control |
6. Selection checklist
- Define the duty spectrum: temperature and RH bands, vacuum exposure, load, sliding distance, salt-spray coexistence.
- Choose the lubricant phase: MoS₂ for general duties; Ti/Ta-stabilised or WS₂ for humid/hot or vacuum; PTFE for release; h-BN for hot insulation.
- Match binder and cure chain to the primer stack (electrodeprimer → low-cure top coat only).
- Pre-define the torque target: K band, sampling plan and µth/µb split per ISO 16047.
- Plan thread allowance and post-coat tapping before the drawing freezes.
- Design life: film thickness = lubricant inventory; set recoat intervals; no partial touch-up on preload-critical joints.
- Verify compliance: ELV/RoHS/REACH Cr(VI) and heavy metals, third-party reports on file.
FAQ
Q1: How is a dry film different from MoS₂ grease or paste?
A cured film neither bleeds nor attracts dust and repeats its torque factor; pastes ride on an oil carrier that migrates, pumps out and evaporates. Cleanrooms, semiconductor tools and torque-critical fasteners choose dry film.
Q2: The datasheet says µ = 0.05 but our assembly measures 0.13 — why?
Datasheets quote a standard rig at one load/speed/atmosphere/counterface; an assembly mixes thread and bearing contact, plus coating thickness and batch variation. Report both as separate accounts and test per ISO 16047 under simulated conditions.
Q3: How do I survive humid service?
Use oxidation-stabilised (doped) grades, cure to full density, and build the duplex: corrosion base coat carries the salt, lubricant top coat is treated as a consumable with planned re-coat intervals.
Q4: For 10.9/12.9 bolts, is friction scatter or hydrogen embrittlement the bigger worry?
Both, from different sources: electroplated high-strength parts need a ~200 °C bake within four hours of plating against HE; plating-free zinc-flake plus dry film removes the hydrogen source altogether, while K audits control friction.
Q5: Does the top coat affect electrical grounding?
Yes — inorganic flake systems are poor conductors. Mask grounding faces or use a conductive stack; public zinc-flake guides carry the same non-conductive caution.
Q6: Can a scratched part be touched up locally?
For low-load slideways, brush touch-up works if parameters are logged. For preload-critical fasteners, recoat fully or replace — local repair cannot reproduce dip-spin density or µ.
Last updated: 2026-10-18 ｜ Prepared by the technical team of Kexin New Materials (Guangdong) Co., Ltd.
References: ISO 16047:2005 Fasteners — Torque/clamp force testing; Wei Chunbei et al., Magnetron-sputtered solid-lubricant MoS₂ films, Guangdong Vacuum Society Annual Meeting, 2015; Chinese Academy of Sciences thesis on MoS₂-based films (humidity/vacuum/salt-spray behaviour); J. Mech. Eng. 2017, friction of MoS₂-coated CF170 space-machine parts in vacuum; Johnson M R., The Galileo High Gain Antenna Deployment Anomaly, 28th Aerospace Mechanisms Symposium, NASA 1994; Serles P. et al., Adv. Funct. Mater. 2022, 32(20):2110429; Lince J R., Lubricants 2020, 8(7):74; Application of MoS₂ in the space environment: a review, Front. Mech. Eng. 2023, DOI 10.1007/s11465-023-0755-1; public zinc-flake (Dacromet-type) process data; EU 2000/53/EC, 2011/65/EU, REACH Annex XVII entry 47; GB/T 10125; GB/T 9286.