dry-lubrication

name	dry-lubrication
description	Dry lubrication — solid lubricants (MoS2, PTFE, graphite, WS2, h-BN), coating methods, friction coefficient, PV limit, wear rate, DLC coatings, space/vacuum applications, ASTM G99/G77, tribological testing.
metadata	{"priority":7,"promptSignals":{"phrases":["dry lubrication","solid lubricant","MoS2 coating","PTFE lubricant","DLC coating tribology","dry film lubricant"],"minScore":3}}

Dry Lubrication — Complete Skill

Solid Lubricant Mechanisms

Principle: lamellar crystal structure → low shear strength between basal planes → easy sliding → low friction Transfer film: material transfers from lubricant to counterface → smooth, lubricated surface pair

Lubricant layers: soft solid between sliding surfaces → 3-body contact; lower effective friction No fluid required: operate in vacuum, cryogenic T, high T, radiation environments where oils fail

Solid Lubricant Properties

MoS₂ (Molybdenum Disulfide)

Structure: S-Mo-S sandwich layers; van der Waals forces between layers → easy basal plane shear Friction coefficient: In vacuum / dry nitrogen: μ = 0.01–0.03 (very low; best solid lubricant at low humidity) In humid air: μ = 0.1–0.2 (water adsorbs → disrupts transfer film) In oxygen-rich environments: oxidizes at T > 350°C → MoO₃ (abrasive!) → AVOID above 350°C in air

Applications: space mechanisms, dry-film coatings, self-lubricating bearings (PMC), cutting tools (burnished MoS₂ coatings) Load capacity: PV limit up to 35 MPa·m/s (bonded coating)

PTFE (Polytetrafluoroethylene)

Friction coefficient: μ = 0.04–0.10 (very low; even without bonding to surface) Mechanism: smooth molecular surface; banded structure at interface; transfer film formation on metal counterface Temperature range: -200°C to +260°C (continuous); to +300°C intermittent Wear rate: high vs. metals (0.1–1 × 10⁻⁴ mm³/N·m); fillers reduce wear: 25% glass fiber + 5% MoS₂ = wear rate ×0.01

PTFE composites:

Filler	%	Wear improvement	μ
None	0	Baseline	0.04–0.08
Bronze	40%	×3	0.10–0.15
Glass fiber	25%	×100	0.08–0.12
Carbon	15–25%	×50	0.06–0.10
MoS₂	5–15%	×5–20	0.04–0.08

Graphite

Friction in air/humid: μ = 0.1–0.15 (needs moisture/adsorbed gas for lubrication!) In vacuum/dry conditions: μ INCREASES (not a good vacuum lubricant; unlike MoS₂) Temperature range: to 600°C in air; 450°C continuous service Applications: high-T lubricant (furnace conveyors, exhaust system components), electrical brushes Graphite vs. MoS₂: graphite needs moisture; MoS₂ best in vacuum/dry → choose accordingly

WS₂ (Tungsten Disulfide)

Structure: similar to MoS₂; lamellar disulfide Advantages over MoS₂: Higher oxidation temperature: T_ox = 450°C (vs. 350°C for MoS₂) Similar low friction: μ = 0.03–0.05 in dry conditions Applications: slightly higher-T applications than MoS₂; space mechanism bearings

h-BN (Hexagonal Boron Nitride)

Structure: layered hexagonal (isostructural with graphite); "white graphite" Friction: μ = 0.15–0.30 (moderate; worse than MoS₂) Temperature: stable to 900°C (air); 1000°C (inert); much better than MoS₂ or graphite at high T Applications: metalworking lubricant (forging, drawing); glass processing; thermal interface

DLC (Diamond-Like Carbon) Coatings

Structure: amorphous carbon with sp³ (diamond-like) and sp² (graphite-like) bonds Types:

• a-C:H (hydrogenated): μ = 0.05–0.15 vs. steel; moderate hardness 15–25 GPa
• ta-C (tetrahedral amorphous carbon): μ = 0.03–0.10; highest hardness 40–80 GPa
• Metal-doped (W-C:H, Cr-C:H): improved adhesion; μ = 0.1–0.2; better thermal stability

DLC friction coefficient (tribo-conditions): Dry nitrogen: μ = 0.001–0.01 (superlubricity for ta-C/ta-C pair!) Humid air: μ = 0.05–0.15 (dependent on H content, humidity) Oil: μ = 0.04–0.10 (boundary lubrication improvement)

Temperature limit: a-C:H degrades at > 300°C (H loss); ta-C: 400°C in air Thickness: 1–5 μm (a-C:H); 0.5–2 μm (ta-C) — hard, brittle, must be thin Stress: high compressive residual stress in ta-C (1–10 GPa) → adhesion challenge on thick coatings

Deposition Methods

Burnished films (MoS₂, graphite): powder rubbed onto surface; low adhesion; limited life Bonded coatings: binder (resin, sodium silicate) + lubricant powder; spray or dip; bake at 200°C Sputtered coatings (MoS₂, WS₂, DLC): PVD; excellent adhesion; 0.1–5 μm; highest performance PVD TiN + MoS₂ composite: dual target sputtering; TiN matrix + MoS₂ → hard and self-lubricating CVD DLC: plasma-enhanced CVD; large area possible; conformal coating

PV Limit

PV limit: maximum product of pressure P [MPa] and sliding velocity V [m/s] before excessive wear/failure Material PV limits:

Material	PV limit [MPa·m/s]
PTFE (unfilled)	0.05
PTFE + 25% glass	0.35
PTFE + bronze	0.50
Nylon 66	0.10
PEEK	0.35
Acetal (Delrin)	0.10
Sintered bronze (oil-impreg.)	1.5
MoS₂ bonded film	35
DLC coating	50–100

Safety factor on PV: operate at PV ≤ 0.5 × PV_limit

Tribological Testing Standards

ASTM G99: pin-on-disk; measure friction coefficient and wear rate; most common Wear rate k = V / (F × d) [mm³/N·m; V = volume lost; F = normal load; d = sliding distance]

ASTM G77: block-on-ring; for evaluating solid lubricant coatings under line contact ASTM G133: oscillating (fretting) test; for fretting-type applications ASTM D2714: thrust washer test; for bulk materials under high contact stress

Specific wear rate k (typical values): MoS₂ bonded (dry): k = 10⁻⁶–10⁻⁵ mm³/N·m DLC a-C:H on steel: k = 10⁻⁷–10⁻⁶ mm³/N·m ta-C on ta-C (dry): k = 10⁻⁸–10⁻⁷ mm³/N·m (near superlubricity) PTFE: k = 10⁻⁴–10⁻³ mm³/N·m (high wear)

Standards

Standard	Scope
ASTM G99	Wear testing by pin-on-disk
ASTM G77	Wear testing by block-on-ring
ASTM G133	Oscillating friction (fretting)
MIL-L-8937	Solid lubricant coatings, MoS₂-based
AMS 2526	Solid film lubricant (MoS₂) specification
ECSS-Q-ST-70-71	Space tribology — ESA standard

Output

Provide: solid lubricant type (MoS₂/PTFE/graphite/WS₂/DLC), deposition method (sputtered/bonded/burnished), friction coefficient μ [dry/humid], environment (vacuum/air/humid) and why lubricant appropriate, operating PV [MPa·m/s] vs. PV limit, specific wear rate k [mm³/N·m], coating thickness [μm], temperature range [°C], transfer film formation mechanism, DLC type (a-C:H/ta-C) and hardness [GPa], compressive stress [GPa] (if DLC), test standard (ASTM G99 conditions), and applicable specification (MIL-L-8937, AMS 2526, ECSS-Q-ST-70-71).