tribological-testing

name	tribological-testing
description	Tribological testing — pin-on-disk (ASTM G99), block-on-ring (G77), four-ball (G99/D2266), linear reciprocating (G133), ball-on-flat, fretting, dry sand/rubber wheel (G65), high-speed high-stress abrasion (G105), friction coefficient measurement, wear rate calculation (volume loss, Archard wear coefficient k), Stribeck curve measurement, contact pressure and Hertz stress, traceability and uncertainty, and selection of test method to simulate specific tribological mechanism.
metadata	{"priority":7,"promptSignals":{"phrases":["tribological testing","wear testing","pin-on-disk","friction test","Archard wear coefficient","Stribeck curve"],"minScore":3}}

Tribological Testing — Complete Skill

Tribological Testing Fundamentals

Purpose and Selection

Tribological tests: measure friction coefficient µ and wear rate under controlled conditions Goal: rank materials/coatings/lubricants; determine Archard wear coefficient k; simulate service conditions Key principle: no universal test — always match dominant wear mechanism and contact geometry to service

Wear mechanisms to identify: Abrasive: hard particles scratch surface (mining, tillage, machining chips) Adhesive: cold welding and transfer between surfaces (unlubricated metal/metal) Oxidative: oxide film forms and removes cyclically (mild steel fretting) Fatigue (delamination): subsurface crack propagation under cyclic contact (rolling bearings) Erosive: particle/droplet impact (turbine blades, pump impellers) Fretting: small-amplitude oscillatory contact (bolted joints, press fits, aircraft fasteners)

Friction Measurement

Friction Coefficient

Amonton's law: F_friction = µ × F_normal [µ = kinematic or static friction coefficient; Amontons' approximation] µ depends on: materials, surface roughness, lubrication, temperature, speed, contact pressure µ is NOT a material constant; always specify test conditions

Typical µ values:

Interface	Condition	µ (static)	µ (kinetic)
Steel/steel	Dry	0.6–0.8	0.4–0.6
Steel/steel	Lubricated (mineral oil)	0.08–0.15	0.06–0.12
Steel/PTFE	Dry	0.04–0.10	0.04–0.08
Al/steel	Dry	0.5–0.7	0.4–0.6
Rubber/asphalt	Dry	0.7–1.0	0.6–0.9
Rubber/asphalt	Wet	0.4–0.7	0.3–0.6
DLC/steel	Dry (N₂ atmosphere)	0.01–0.05	0.01–0.04
Ti6Al4V/Ti6Al4V	Dry	0.5–0.7	0.4–0.6

Stribeck Curve

Stribeck curve: friction coefficient vs. Hersey number (η × n / p) η = lubricant dynamic viscosity; n = speed (rev/s or m/s); p = contact pressure

Lubrication regimes: Boundary: Hersey << 1 × 10⁻⁹; µ = 0.08–0.15; metal-to-metal contact; extreme pressure additives active Mixed: transition zone; partial fluid film; µ varies rapidly with Hersey number Elastohydrodynamic (EHD): fluid film established; µ = 0.04–0.08 Hydrodynamic (HD): full fluid film; µ = 0.001–0.01; dominated by viscous shear; increases with speed

Stribeck measurement: Step-change speed at fixed load and lubricant; record µ at each speed → plot vs. Hersey number ASTM G181: standard for Stribeck curve with four-ball or pin-on-disk

Pin-on-Disk Test (ASTM G99)

Test Setup and Procedure

Geometry: cylindrical or spherical pin (ball) stationary against rotating disk Normal force: 1–100 N (pneumatic or deadweight loading) Disk speed: 0.01–3 m/s (linear sliding speed at track radius) Track radius r: 5–30 mm; sliding distance L = 2π × r × revolutions

Procedure (ASTM G99):

1. Clean pin and disk with solvent; weigh (balance ± 0.01 mg)
2. Set F_N, speed, temperature; run 500–5,000 m sliding distance
3. Measure friction continuously (load cell)
4. Weigh pin and disk after test (mass loss Δm)
5. Calculate wear volume: W_V = Δm / ρ [ρ = density]
6. Or measure wear track width and depth: optical profilometry

Archard wear equation: W_V = k × F_N × L / H [k = specific wear coefficient [m²/N]; F_N = normal force [N]; L = sliding distance [m]; H = hardness [Pa]] Or simplified: W_V = K × F_N × L [K = Archard wear coefficient [mm³/(N·m)]; most commonly reported] W_rate = W_V / L = K × F_N [mm³/m → wear per unit distance; normalized by load: K = W_V/(F_N × L)]

Typical K values (pin-on-disk, dry):

Pair	K [mm³/(N·m)]
Hardened steel / hardened steel	10⁻⁴ to 10⁻³
Steel / brass	10⁻³ to 10⁻²
Steel / PTFE	10⁻⁶ to 10⁻⁵
WC-Co (HVOF) / hardened steel	10⁻⁷ to 10⁻⁶
DLC / steel	10⁻⁸ to 10⁻⁷
Alumina / hardened steel	10⁻⁶ to 10⁻⁵

Hertz Contact Stress

Ball (radius R) on flat: a = (3 × F_N × R / (4 × E*))^(1/3) [contact radius] p_0 = (3 × F_N) / (2 × π × a²) [peak Hertz pressure] 1/E* = (1−ν₁²)/E₁ + (1−ν₂²)/E₂ [reduced modulus]

Example: Steel ball R = 3 mm; F_N = 10 N; E* for steel/steel = E/(2(1−ν²)) = 200/(2×0.91) = 110 GPa a = (3 × 10 × 0.003 / (4 × 110×10⁹))^(1/3) = (9×10⁻²/(4.4×10¹¹))^(1/3) = (2.05×10⁻¹³)^(1/3) = 5.9×10⁻⁵ m = 59 μm p_0 = 3×10/(2π×(5.9×10⁻⁵)²) = 30/(2.19×10⁻⁸) = 1.37 GPa

Plasticity index: Ψ = σ_s/H × √(E*/H × (σ/β)) [Greenwood-Williamson; σ_s = rms roughness; β = asperity radius; H = hardness] Ψ > 1: plastic asperity contact (adhesive/abrasive wear likely); Ψ < 0.6: elastic (fatigue wear)

Block-on-Ring (ASTM G77)

Block stationary; ring rotates; block pressed on OD of ring Normal force: 50–1,000 N; ring speed: 0.5–3 m/s; sliding distance up to 10 km Higher F_N than pin-on-disk → better for boundary lubrication, galling tests Contact: rectangular; p_avg = F_N / (b × l) where b = hertzian contact width; l = block length

Applications: gear lubricant evaluation; cutting fluid assessment; journal bearing material comparison Scoring test: increase F_N until scuffing (sudden µ increase) → determines galling threshold

Four-Ball Test (ASTM D2266 / ASTM D2783 / ASTM D2596)

Geometry: three balls in cradle (stationary); fourth ball rotates on top Standard balls: 12.7 mm steel balls (52100 steel; HRC 64–66)

D2266 (wear): 40 kgf (392 N); 1,200 RPM; 60 min; measure wear scar diameter (WSD) on stationary balls D2783 (extreme pressure - EP): increasing load until welding → load wear index (LWI); weld point [kgf] D2596 (grease EP): same as D2783 but for greases

Wear scar diameter: measure with optical microscope; average of 3 balls; report WSD [mm] at 40 kgf/60 min Compare lubricant formulations; lower WSD = better anti-wear performance Baseline: non-additive base oil WSD ≈ 1.0–1.5 mm; ZDDP (zinc dialkyldithiophosphate) additive WSD ≈ 0.4–0.7 mm

Linear Reciprocating Test (ASTM G133)

Small oscillating stroke: 6 mm amplitude; 15 Hz typical; ball on flat Simulates fretting and reciprocating sliding (hydraulic seals, valve stems, prosthetics) Contact: ball (6 mm) on flat; F_N = 10–500 N Output: µ vs. cycles; wear volume; wear track profile

Fretting wear (micro-oscillation): Stroke: 2–100 μm amplitude; high frequency (20–100 Hz) Critical in: aerospace fasteners, press fits, power connectors Fretting wear rate much higher than continuous sliding due to debris accumulation and fresh surface exposure

Dry Sand/Rubber Wheel Abrasion (ASTM G65)

Principle: silica sand (212–300 μm) fed between rubber wheel and specimen Load: 130 N (Procedure A/B); speed: 200 RPM; revolutions: 2,000 (A) or 6,000 (B) Measures: mass loss → convert to volume loss using density

Abrasion resistance (volume loss ranking): Soft steel (HB 140): 300–500 mm³ (poor) Hardened steel (HRC 50): 50–100 mm³ (moderate) HVOF WC-Co (HV 1,000): 5–15 mm³ (excellent) Al₂O₃ ceramic: 10–30 mm³ (good)

High-stress abrasion (ASTM G105): rubber-wheel test at higher sand F_N → simulates crushing wear

Uncertainty and Reproducibility

Inter-laboratory variability: Pin-on-disk wear coefficient: ±50–100% (factor 2–3) between labs; high variability Four-ball WSD: ±0.1–0.3 mm between labs (less variable) Key sources: cleanliness, surface finish, humidity, running-in procedure, measurement technique

Reporting requirements: Specify ALL test parameters: F_N, speed, distance, material, surface prep, temperature, humidity, lubricant Report K with units [mm³/(N·m)] and uncertainty; report µ as mean ± standard deviation Compare within same test series only — never across different geometries without validated correlation

Correlation to service: Tribological tests are accelerated and simplified; must validate correlation to service wear Wear map approach: identify wear mechanism in service → select test that produces same mechanism

Standards and References

Standard	Scope
ASTM G99	Pin-on-disk test
ASTM G77	Block-on-ring test
ASTM G133	Linear reciprocating test
ASTM G65	Dry sand/rubber wheel abrasion
ASTM D2266	Four-ball wear test (lubricants)
ASTM D2783	Four-ball EP test
ISO 7148-1	Plain bearing tribological testing
Blau (ASTM MNL58)	Friction science and technology

Output

Provide: tribological problem (contact type: sliding/rolling/fretting; dominant mechanism: abrasive/adhesive/fatigue/erosive; materials; environment: dry/lubricated/T/humidity; contact geometry approximation), test method selection (pin-on-disk G99 for sliding wear; G77 block-on-ring for higher load; G65 for abrasion; G133 for fretting; G105 for high-stress abrasion; four-ball D2266 for lubricant comparison; basis for method selection), Hertz contact stress (ball or flat geometry; F_N [N]; R [mm]; E* [GPa]; p_0 [GPa]; a [μm]; plasticity index Ψ — elastic vs. plastic contact), test parameters (F_N [N]; speed [m/s] or [RPM]; sliding distance L [m]; temperature [°C]; lubricant [type/quantity]; environment RH [%]), results (µ_kinetic ± σ [value]; wear volume W_V [mm³]; Archard K = W_V/(F_N × L) [mm³/(N·m)]; wear rate [μm/km]), Stribeck (if lubricant study: regime identified: boundary/mixed/EHD/HD; µ vs. Hersey number plot description; recommendation for lubricant viscosity), comparison ranking (if multiple materials/coatings: table of K values; relative ranking; preferred choice with rationale), correlation to service (estimated service wear at F_N_service and distance_service; confidence in correlation; any mechanism mismatch), and applicable standard (ASTM G99/G65/G77/G133; ISO 7148; ASTM D2266 for lubricant).