Friction and Wear Testing: Dry Friction Coefficient Determination

Dry friction coefficient testing is a fundamental tribological evaluation used to quantify how material pairs behave under unlubricated sliding contact, providing critical data on friction force, wear resistance, and long-term durability. This testing is essential for industries ranging from automotive and machinery to packaging, medical devices, and advanced coatings. This article explains the principles, sample types, test items, standard methods, and typical applications of dry friction coefficient and wear testing.

Technical Overview

Dry friction coefficient testing measures the frictional resistance generated between two solid surfaces sliding against each other under unlubricated conditions. The coefficient of friction (COF, denoted as μ) is a dimensionless value defined as the ratio of the tangential friction force (F) resisting relative motion to the normal load (N) applied perpendicular to the contact surfaces, expressed as μ = F / N.

Two distinct values are typically reported: the static coefficient of friction (μs), which describes the resistance at the onset of motion, and the kinetic or dynamic coefficient of friction (μk), which describes the resistance during sustained sliding. In most material pairs, the static value is equal to or higher than the kinetic value, and the difference between the two governs behaviors such as stick-slip, chatter, and squeal.

Because dry sliding contact involves no lubricant film, the measured friction and wear response depends strongly on the material combination, surface roughness, applied load, sliding speed, ambient temperature, and humidity. Tests are therefore performed on tribometers under tightly controlled laboratory conditions, typically at 23 ± 2 °C and 50 ± 10% relative humidity, in accordance with recognized international standards such as ASTM G115, ASTM G99, ASTM G133, ASTM G77, ASTM D1894, ASTM D3702, ISO 8295, ISO 6601, and the corresponding GB/T series of Chinese national standards.

Testing Samples

Metallic Materials

Steels (carbon, alloy, stainless), cast irons, aluminum alloys, copper alloys, titanium alloys, and bearing metals are tested to evaluate sliding pairs in engines, gears, bushings, and fasteners. Samples are usually machined into pins, discs, blocks, or rings with a controlled surface finish.

Polymers and Engineering Plastics

Nylon (PA), acetal (POM), PTFE, polyethylene (including UHMWPE), PEEK, and other thermoplastics are widely used as self-lubricating sliding components. Dry friction testing characterizes their tribological performance against steel or ceramic counterfaces.

Rubber and Elastomers

Seals, gaskets, tire tread compounds, and footwear sole materials are evaluated for friction behavior under dry sliding, often with a focus on load, speed, and temperature sensitivity.

Ceramics and Hard Materials

Alumina, zirconia, silicon nitride, silicon carbide, and cemented carbides are tested for high-temperature and high-wear applications where dry contact must be tolerated.

Coatings and Surface Treatments

PVD/CVD hard coatings, DLC coatings, thermal spray coatings, electroplated layers, anodized films, and nitrided surfaces are tested to verify that the treatment reduces friction or improves wear life compared with the untreated substrate.

Films and Sheets

Packaging films, laminates, and polymer sheets are tested using the sled method to determine machine-direction and transverse-direction friction properties that govern processing behavior on high-speed packaging lines.

Composites and Friction Materials

Brake pad materials, clutch facings, carbon fiber composites, and glass fiber composites are evaluated as complete friction pairs under representative contact pressures and sliding speeds.

Testing Items

Depending on the selected method and customer requirements, the following parameters are typically measured and reported:

• Static coefficient of friction (μs) at the initiation of sliding
• Kinetic (dynamic) coefficient of friction (μk) during steady-state sliding
• Friction force and COF versus time or sliding distance curves
• Average and steady-state COF values with statistical scatter
• Specific wear rate (mm³ / N·m) derived from wear volume, load, and sliding distance
• Wear volume loss and mass loss of both the specimen and counterface
• Wear scar width, depth, and cross-sectional profile using profilometry
• Surface roughness changes before and after testing
• Optional failure analysis of worn surfaces (microscopy, SEM, EDS)

Testing Methods

Several standardized test configurations are available, each simulating a different contact geometry and motion pattern. The appropriate method is selected based on the intended service condition of the material.

Pin-on-Disc Test (ASTM G99)

A stationary pin or ball is pressed against a rotating disc under a constant normal load. The friction force is recorded continuously, and the wear volume is determined from the wear scar dimensions or mass loss. This is the most widely used method for ranking material pairs under dry sliding.

Reciprocating Ball-on-Flat Test (ASTM G133)

A ball or sphere slides back and forth against a flat specimen in a linear reciprocating motion. This configuration is well suited to materials subjected to oscillating or vibrational contact, such as seals, joint implants, and electrical contacts.

Block-on-Ring Test (ASTM G77)

A rectangular block is loaded against a rotating ring. This method is commonly applied to rank the galling resistance and sliding wear behavior of metals, coatings, and lubricant-free composites.

Thrust Washer Test (ASTM D3702)

A flat washer-shaped specimen slides against a stationary counterface under an annular face-loaded contact. This geometry is representative of thrust bearings and washers operating under dry or marginally lubricated conditions.

Sled Method for Films (ASTM D1894 / ISO 8295)

A sled of specified weight is drawn across a film surface, either against itself or against another material, to determine the static and kinetic COF of packaging films and sheets in both machine and transverse directions.

Inclined Plane Method

The angle at which a specimen begins to slide on an inclined counterface is measured, and the tangent of this angle yields the static coefficient of friction. This simple method is useful for comparative screening.

The general test workflow includes: specimen preparation and cleaning, conditioning at standard laboratory atmosphere, mounting of the specimen and counterface, application of the specified normal load, sliding speed, and duration, continuous acquisition of the friction force signal, and post-test measurement of wear volume and surface condition. A full report documents the test parameters, environmental conditions, raw friction curves, calculated COF and wear rate values, and photographic records of the wear tracks.

Applications

Dry friction coefficient and wear test data support design, material selection, quality control, and failure analysis across many industries:

• Automotive: brake pads, clutch facings, piston rings, cylinder liners, bushings, and interior trim materials
• Industrial machinery: dry-running bearings, slideways, gears, chain guides, and seals
• Packaging and converting: film COF verification for form-fill-seal lines, laminates, and labels
• Medical devices: UHMWPE bearing surfaces in orthopedic implants and catheter materials
• Footwear and flooring: dry slip resistance characterization of sole and floor materials
• Surface engineering: qualification of PVD, DLC, anodized, and thermal spray coatings
• Aerospace and rail: fastener systems, dry contact interfaces, and current collectors
• Research and development: ranking of new polymers, composites, and additives under standardized dry sliding conditions

FAQ

What is the difference between the static and kinetic coefficient of friction?

The static coefficient describes the friction at the moment sliding begins, while the kinetic coefficient describes friction during continuous sliding. A large difference between the two often indicates stick-slip tendency, which is undesirable in precision slides, films, and seals.

What coefficient of friction values are typical for dry sliding?

Bare metal-on-metal dry pairs commonly show μ of 0.4 to 0.8 or higher, self-lubricating polymers such as PTFE-filled materials may show μ of 0.1 to 0.3, and advanced DLC coatings can achieve μ below 0.15. Exact values depend strongly on load, speed, roughness, and environment, so comparison is only valid under identical test conditions.

Which test parameters most influence the results?

Normal load, sliding speed, counterface material and roughness, temperature, humidity, and sliding distance are the dominant factors. All parameters must be reported with the results to ensure comparability between datasets.

What sample dimensions are required?

Requirements vary by method. Pin-on-disc tests typically use 3 to 10 mm diameter pins with disc specimens of 30 to 60 mm diameter; sled tests for films require specimens of approximately 63 mm width or strips per ASTM D1894. Accredited third-party laboratories can also machine customer parts into suitable specimen geometry.

Can testing be performed at elevated temperatures or under controlled humidity?

Yes. Tribometers equipped with environmental chambers allow dry friction and wear testing at elevated temperatures up to several hundred degrees Celsius, or under controlled dry or humid atmospheres, to simulate specific service environments.

How are wear rate and friction coefficient related?

They are related but independent outputs: a low friction coefficient does not automatically guarantee low wear. Complete tribological characterization requires reporting both the COF curve and the specific wear rate, which together define the suitability of a material pair for a dry sliding application.