Dynamic Friction Coefficient Testing of Different Material Pairings

Rubber and Elastomer Pairings

Elastomeric materials such as NBR, EPDM, silicone rubber, and thermoplastic elastomers are paired against metal shafts, glass, plastic housings, or other rubbers to simulate seals, gaskets, tires, belts, and grippers. Because rubber friction is highly sensitive to normal load and sliding speed, test parameters are selected to reflect actual service conditions, and specimens are conditioned before testing to stabilize their mechanical state.

Coated, Treated, and Lubricated Surfaces

Surface engineering layers such as anodizing, electroplating, PVD coatings, DLC coatings, thermal spray layers, and polymer-based dry lubricant films are tested against selected counterfaces to verify friction reduction and durability. Tests may be conducted under dry sliding conditions or with a defined lubricant, grease, or oil film so that the contribution of the lubricating medium to the overall friction response can be quantified.

Sample Preparation and Conditioning

Proper preparation is essential for reliable results. Specimens are cleaned with suitable solvents to remove mold release agents, machining oils, and handling contamination, then dried and conditioned in a standard atmosphere, typically 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 40 hours, unless otherwise specified. Sample dimensions, thickness, surface roughness, and cleaning procedure are recorded in the final test report to ensure traceability and repeatability.

Testing Items

A comprehensive dynamic friction coefficient test program for paired materials typically includes the following measurable items:

• Dynamic (kinetic) coefficient of friction under dry sliding conditions
• Static (breakaway) coefficient of friction, measured as a reference value immediately before motion begins
• Dynamic coefficient of friction under lubricated or greased conditions
• Friction behavior as a function of normal load, sliding speed, and test duration
• Friction performance at elevated or sub-ambient temperatures
• Combined friction and wear evaluation, including wear scar dimensions, wear volume, and mass loss
• Force-displacement (friction trace) curves showing stick-slip tendency and friction stability
• Statistical reporting of average, maximum, and minimum μk values across multiple replicate tests

Testing Methods

Horizontal Sled Method (ASTM D1894 / ISO 8295)

The sled method is the standard approach for films, sheeting, and many rigid plastic pairings. A sled of defined mass and contact area is placed on the specimen, and a motorized drive pulls the sled across the counterface at a constant speed while a calibrated load cell records the friction force. The dynamic coefficient of friction is calculated from the average force measured during the steady sliding portion of the travel, excluding the initial static peak. Typical parameters include a 200 g sled and a sliding speed of 150 mm/min, though loads and speeds can be adjusted to simulate specific applications.

Pin-on-Disk Rotary Method (ASTM G99)

In the pin-on-disk configuration, a stationary pin, ball, or coupon is pressed against a rotating disk under a constant normal load. The friction force is measured continuously as the pair slides along a circular wear track. This method is well suited to metal, ceramic, coating, and lubricant evaluations, allowing long-duration tests in which the evolution of the dynamic coefficient of friction with sliding distance can be monitored, together with wear of both the pin and the disk.

Linear Reciprocating Method (ASTM G133)

The reciprocating method slides a specimen or ball back and forth against a flat counterface at a controlled frequency and stroke length. It is particularly appropriate for simulating oscillating service conditions such as seals on shafts, connectors, medical device components, and lubricated contacts. The dynamic coefficient of friction is derived from the friction force recorded during the steady-state portion of each stroke, in both forward and reverse directions.

Incline and Other Supplementary Methods

For certain packaging and paper-based applications, inclined-plane methods may be used to determine the angle at which sliding initiates, providing a complementary measure of frictional behavior. Selection among methods depends on the material form, the intended service motion, and the governing specification.

Procedure and Data Evaluation

All tests are performed on calibrated instruments with verified load cells and velocity control. After mounting and preconditioning the specimen pair, the normal load and sliding parameters are set, and the friction force is recorded continuously. The dynamic coefficient of friction is computed as the ratio of the mean steady-state friction force to the normal load, typically averaged over at least three to five replicates per pairing. The final report includes the test standard, environment, specimen description, surface preparation, test parameters, individual and mean μk values, and representative friction traces.

Applications

Dynamic friction coefficient data for material pairings directly support engineering decisions across many industries:

• Packaging: verifying that films and laminates run reliably on high-speed form-fill-seal and bag-making lines without jamming or excessive drag
• Automotive: evaluating brake materials, wiper blades, seals on shafts, interior trim pairings, and sliding seat and door components
• Medical devices: confirming low and consistent friction for catheters, syringes, stopcocks, and device housing interfaces
• Industrial machinery: selecting bearing, guide, and bushing pairings, and validating the effect of coatings and lubricants on friction
• Consumer electronics: tuning the tactile feel of buttons, hinges, and slider mechanisms by controlling friction between mating parts
• Footwear and flooring: assessing relative sliding behavior between sole materials and floor surfaces
• Research and failure analysis: comparing candidate materials, diagnosing friction-related failures, and validating surface treatments

FAQ

What is the difference between static and dynamic coefficient of friction? The static coefficient describes the resistance at the moment motion starts, while the dynamic coefficient describes the resistance during continued sliding. In most pairings the dynamic value is somewhat lower than the static value, and both are reported together to characterize stick-slip tendency.

Why must friction be tested on the actual material pairing? Friction arises from the interaction of two surfaces and cannot be predicted reliably from the properties of either material alone. Surface finish, oxide layers, transfer films, and counterface hardness all contribute, so only paired testing yields design-valid data.

Which standard should be chosen for my samples? Flexible films and sheets are normally tested to ASTM D1894 or ISO 8295, general rigid pairings and coatings to ASTM G99 or ASTM G133, and reporting follows the guidance of ASTM G115. The intended service motion and material form are the primary selection criteria.

How many samples are required? Typically, at least three to five valid replicate tests per material pairing are recommended to account for surface variability. Specimen dimensions depend on the method, and laboratories can advise on cutting plans to minimize material consumption.

How do temperature and humidity affect the results? Rising temperature can soften polymers, change lubricant viscosity, or alter surface oxide films, all of which shift the measured μk. Humidity affects hydrophilic surfaces and some elastomers significantly, which is why conditioning and testing in a controlled environment are standard practice.

What typical values can be expected? PTFE-based pairings often show dynamic coefficients around 0.05 to 0.20, unlubricated steel-on-steel commonly falls between 0.4 and 0.8, and polymer films used in packaging are frequently specified to remain below approximately 0.3 for good machinability. Exact values, however, must always be confirmed by testing the specific pairing under the relevant conditions.