Dry Friction Coefficient Testing of Metallic Materials – Static and Kinetic μ Measurement

Dry friction coefficient testing of metallic materials quantifies the static and kinetic friction behavior of metal-on-metal pairs under unlubricated sliding contact. Using calibrated tribometers in pin-on-disc, ball-on-flat, and block-on-ring configurations, accredited third-party laboratories measure friction coefficients in accordance with ASTM, ISO, DIN, and GB standards, providing essential data for materials selection, fastener design, and forming process development.

Technical Overview

The coefficient of friction (μ) is a dimensionless parameter defined as the ratio of the tangential friction force (F) resisting relative motion to the normal load (N) pressing two surfaces together. In dry friction testing, no lubricant, oil, or grease is applied to the contact interface, so the measured friction reflects the intrinsic interaction between the metallic surfaces, their oxide layers, adsorbed films, and surface topography. This makes dry sliding one of the most demanding yet most informative tribological test conditions for metals.

Two distinct values are generally reported: the static friction coefficient (μs), which describes the force threshold required to initiate sliding, and the kinetic (dynamic) friction coefficient (μk), which describes the resistance during steady-state relative motion. For common engineering pairs such as dry steel-on-steel, μ typically falls between 0.4 and 0.8, whereas the same pair under boundary lubrication may drop to 0.1 or lower. Because friction is a system property rather than a pure material constant, results depend strongly on surface roughness, normal load, sliding speed, temperature, humidity, and the chemical state of the surfaces.

Dry friction coefficient measurements for metals are typically performed on tribometers that continuously record friction force via precision load cells while a known normal load is applied through a dead-weight, pneumatic, or electro-mechanical loading system. Testing and reporting are guided by recognized standards organizations, including ASTM G115 (guide for measuring and reporting friction coefficients), ASTM G99 (pin-on-disc wear and friction), ASTM G133 (reciprocating ball-on-flat sliding), ASTM G77 (block-on-ring), ISO 20808 (ball-on-disc method), DIN 50324, and GB/T 12444 for metallic wear and friction evaluation. For threaded fasteners, ISO 16047 defines the determination of friction coefficients under dry or lubricated conditions.

Testing Samples

Bulk Metal-on-Metal Pairs

Standard material couples machined into pins, balls, blocks, discs, or flats. Typical combinations include carbon steel against carbon steel, stainless steel, cast iron, copper alloys (brass, bronze), aluminum alloys, and tool steels. Samples are ground or polished to a specified surface roughness (commonly Ra 0.05–0.8 μm) and degreased with analytical-grade solvents immediately before testing to remove residual oils and contaminants.

Coated and Surface-Treated Metals

Specimens carrying engineered surface layers, such as electroplated zinc and zinc-nickel coatings, phosphated conversion layers, anodized aluminum, nitrided or carburized steels, thermal spray coatings, and PVD/CVD hard coatings (TiN, CrN, DLC). Dry friction testing evaluates how these treatments modify friction behavior and whether they suppress galling or scuffing of the underlying substrate.

Sheet Metals and Formed Components

Flat coupons cut from cold-rolled or hot-rolled sheet, galvanized automotive sheet, stainless strip, or formed blanks used in stamping and deep-drawing operations. Testing against die steels under dry conditions helps isolate the substrate contribution to friction and assesses the risk of galling in dies when lubrication is marginal.

Fasteners and Threaded Components

Bolts, nuts, screws, and washers tested to determine the total friction coefficient and the thread and bearing-face friction coefficients, which govern the torque–tension relationship during assembly. ISO 16047 specifies the specimen preparation, clamping, and measurement procedure for these tests.

Testing Items

Core measurable quantities include:

• Static friction coefficient (μs) — the peak ratio at the onset of sliding
• Kinetic (dynamic) friction coefficient (μk) — the mean steady-state value after run-in
• Friction force and friction coefficient versus time or sliding distance curves
• Run-in (break-in) transient behavior and time to steady state
• Stick-slip tendency and friction force fluctuation amplitude
• Friction coefficient as a function of normal load, sliding speed, or temperature
• Supplementary wear volume, wear rate, and specific wear rate (mm³/N·m) from profilometry of the wear track
• Post-test surface examination of wear scars, transfer layers, and galling damage

Testing Methods

Pin-on-Disc / Ball-on-Disc Testing (ASTM G99, ISO 20808)

A stationary pin or ball with a defined diameter is pressed against a rotating disc under a constant normal load, typically 1–50 N. The tribometer records tangential force continuously, and the friction coefficient is computed as F/N. Test parameters such as rotation speed (0.01–1.0 m/s sliding speed), track radius, test duration, and stop cycles are selected to simulate the intended service condition. This geometry provides stable, repeatable kinetic friction data and allows long-duration steady-state measurements.

Reciprocating Ball-on-Flat Testing (ASTM G133)

A ball or cylindrical rider oscillates linearly against a flat specimen at a fixed stroke length and frequency. This method is well suited to low-speed, high-contact-pressure dry sliding, to fretting-type motion, and to evaluating thin coatings, since the contact remains within a defined wear scar. It is particularly effective for detecting stick-slip and break-in transients.

Block-on-Ring Testing (ASTM G77)

A rectangular block is loaded against the outer circumference of a rotating ring. The conforming line contact generates high local pressures typical of bushings, guides, and sliding machine elements, and the method ranks material pairs by friction and galling resistance under dry conditions.

Horizontal Plane / Inclined Plane Method for Static Friction

For static friction coefficient determination, a rider of known mass is either pulled horizontally at a very slow, controlled rate on a flat counterface or placed on a plane that is gradually tilted until sliding initiates. The angle of incipient motion gives μs = tan θ. This approach follows the general guidance of ASTM G115 and is appropriate for gravity-loaded and clamped interfaces.

Fastener Friction Testing (ISO 16047)

A bolt-nut assembly is tightened in a calibrated torque-tension device while total torque, thread torque, and clamping force are measured simultaneously. From these data the total friction coefficient as well as the thread and under-head friction coefficients are calculated under dry or as-delivered surface conditions.

Environmental and Parameter Control

All dry friction tests are conducted in temperature- and humidity-controlled laboratories (commonly 23 ± 2 °C and 50 ± 5% relative humidity), because adsorbed moisture films can measurably alter friction of clean metals. Normal load calibration, load-cell verification, and surface cleanliness verification are documented for each test, and a minimum of three replicate runs per condition is recommended to establish statistical confidence.

Applications

Dry friction coefficient data for metals directly support engineering decisions across many industries:

• Mechanical design of unlubricated or marginally lubricated sliding joints, guides, cams, and linkages
• Torque-tension calculations for bolted joints, ensuring correct preload without over-tightening or galling
• Sheet metal forming — evaluation of die materials, coating combinations, and galling risk during stamping
• Bearing and bushing material selection where boundary or dry running can occur
• Development and quality control of surface treatments, conversion coatings, and thin hard coatings
• Verification of friction assumptions used in simulation models for contact, wear, and fastened assemblies
• Comparative benchmarking of supplier materials, heat treatment conditions, or machining finishes
• Failure analysis, where unexpectedly high or unstable friction contributed to seizure, fretting, or wear

FAQ

What is a typical dry friction coefficient for steel on steel? For dry, clean carbon steel sliding against carbon steel, μ generally ranges from about 0.4 to 0.8, depending on roughness, load, and oxide condition. Polished stainless steel pairs can exceed 0.8 and are prone to galling under high pressure. Lubricated or phosphated surfaces typically reduce the value to 0.1–0.2.

Why is the static coefficient usually higher than the kinetic coefficient? At rest, real contact areas grow slightly under load and surface films may be squeezed out, so a larger tangential force is needed to initiate motion. Once sliding begins, shear occurs continuously at asperity junctions, and the sustaining force is typically 10–30% lower.

How is a truly dry interface guaranteed? Specimens are sequentially ultrasonically cleaned in solvents, handled with lint-free gloves or tweezers, and tested promptly in a controlled-humidity environment. Lubricant-free fixtures and calibrated dry-air purging of the test chamber further prevent contamination.

What sample dimensions are required? Pin-on-disc testing typically uses a 3–10 mm diameter ball or a 4–6 mm diameter flat-ended pin and discs of 30–60 mm diameter; reciprocating tests use flats of approximately 20 × 20 mm. Fastener tests require complete bolt-nut-washer assemblies. Exact dimensions are confirmed against the selected standard before testing.

Can friction and wear be measured in the same test? Yes. Pin-on-disc and reciprocating tribometers record friction continuously, and after the run the wear scar is profiled with a 3D profilometer or optical microscope to calculate wear volume and specific wear rate, giving a complete tribological characterization from a single test campaign.

How are results reported? Reports include the test standard, material pairing and hardness, surface roughness, normal load, sliding speed, temperature and humidity, mean static and kinetic friction coefficients with standard deviation, full friction-versus-distance curves, wear data, and photographic documentation of the wear tracks.