Dynamic Friction Coefficient Testing of Friction Materials

Dynamic friction coefficient testing is the core experimental procedure used to evaluate the braking and transmission performance of friction materials such as brake pads, brake linings, and clutch facings. This article explains the testing principles, sample types, testing items, methods based on standards such as GB/T 5763, SAE J661, and ASTM G99, and typical applications across the automotive, railway, and industrial sectors.

Technical Overview

The dynamic friction coefficient is a dimensionless parameter that quantifies the friction force generated between a friction material and its mating surface while the two surfaces are in relative sliding motion. Unlike the static friction coefficient, which applies to the instant before motion begins, the dynamic friction coefficient governs the actual working behavior of braking and clutch systems under operating conditions. It is typically expressed as the ratio of the tangential friction force to the normal applied load.

For friction materials, the dynamic friction coefficient is not a fixed value. It varies with sliding speed, contact pressure, bulk temperature, humidity, and the number of braking or engagement cycles. Therefore, a single-point measurement is insufficient for material qualification. Laboratory experiments are designed to map friction behavior across a full operating envelope, including elevated temperatures where fade phenomena may occur.

Standardized test methods for dynamic friction coefficient measurement are published by recognized standards organizations, including GB/T 5763 and JIS D4411 for automotive brake linings, SAE J661 for friction coefficient evaluation using small-scale test machines, ISO 26867 for full-scale inertia dynamometer assessment of brake friction behavior, and ASTM G99 for pin-on-disk friction and wear testing. Accredited third-party laboratories routinely perform these experiments for product certification, quality control, and research and development purposes.

Testing Samples

Automotive Brake Pads and Brake Linings

Disc brake pads and drum brake linings for passenger cars, commercial vehicles, and motorcycles are the most frequently tested friction materials. Small-scale specimens are usually cut from finished or semi-finished products to fit the sample holder of the friction test machine, and the friction surface is ground flat and degreased before testing to ensure reproducible contact conditions.

Clutch Facings and Friction Plates

Dry clutch facings and wet friction plates used in transmissions, torque converters, and industrial clutches are tested to determine the dynamic friction coefficient during the engagement process. Wet friction materials are evaluated under lubricated conditions with specific automatic transmission fluid to simulate real service environments, since the friction characteristics of wet materials depend strongly on the lubricant and sliding velocity.

Railway Brake Blocks and Industrial Friction Materials

Railway brake blocks, brake shoes, woven or molded friction sheets, and semi-metallic or ceramic friction composites used in cranes, winches, elevators, and mining equipment are also subject to dynamic friction coefficient experiments. These materials are often tested against the actual wheel tread steel or cast iron counter-surface specified by the end user to reflect authentic tribological pairings.

Testing Items

A complete dynamic friction coefficient experiment typically includes the following measurement items:

• Dynamic friction coefficient at room temperature under specified load and speed
• Friction coefficient as a function of temperature, typically from ambient up to 250 °C to 350 °C or higher
• Fade resistance: the reduction of friction coefficient at elevated temperature
• Recovery characteristics: friction coefficient behavior during cooling back to lower temperatures
• Wear rate and thickness loss of the friction material after a defined number of friction cycles
• Friction stability, expressed as the fluctuation or scatter of the friction coefficient over repeated applications
• Sensitivity analysis with respect to sliding speed, contact pressure, and counter-surface roughness

Testing Methods

Small-Scale Friction Test Machine Method

The most widely used procedure follows the principle of GB/T 5763 and SAE J661. A rectangular specimen with a defined contact area, typically 25 mm by 25 mm, is pressed against the inner surface of a rotating friction drum with a constant normal load. The drum rotates at a fixed speed, commonly around 480 rpm, while the tangential friction force is continuously recorded. The dynamic friction coefficient is calculated as the ratio of the measured friction force to the applied normal force. The test is repeated at successive temperature stages, for example 100 °C, 150 °C, 200 °C, 250 °C, and 300 °C, to generate a friction-temperature curve, followed by a recovery stage during cooldown.

Pin-on-Disk Method

In accordance with ASTM G99, a cylindrical pin or button made from the friction material slides against a rotating disk, usually made of cast iron or steel. Load cells measure the friction force and the normal force simultaneously, allowing continuous calculation of the dynamic friction coefficient. This method is preferred for research purposes because sliding speed, contact pressure, temperature, and environment can be controlled with high precision, and because it requires relatively little material.

Full-Scale Inertia Dynamometer Method

For comprehensive performance validation as described in ISO 26867, complete brake or clutch assemblies are mounted on an inertia dynamometer. A flywheel system simulates the kinetic energy of the vehicle, and braking or engagement events are executed under programmed speed, torque, and temperature conditions. The dynamic friction coefficient is derived from the measured braking torque, effective radius, and clamp force. This method captures system-level effects such as bedding behavior, thermal fade, and speed sensitivity.

Typical Test Conditions and Calculation

Regardless of the specific apparatus, the fundamental calculation is identical: the dynamic friction coefficient equals the measured friction force divided by the applied normal load. Test conditions such as specimen size, normal pressure, sliding velocity, temperature ramp, heating mode, and number of test cycles must be documented in the test report. Result classification codes, for example the two-letter friction ratings defined in SAE J866, may be assigned based on the measured coefficient ranges for brake lining materials.

Applications

Dynamic friction coefficient experiments serve a broad range of industries:

• Automotive OEM and aftermarket brake pad manufacturers performing product qualification and batch release testing
• Clutch facing producers validating engagement behavior and judder resistance for manual and automated transmissions
• Railway rolling stock suppliers verifying the friction performance of brake blocks against wheel tread materials
• Motorcycle and off-highway vehicle brake system developers evaluating ceramic, semi-metallic, and organic friction formulations
• Industrial machinery manufacturers specifying friction materials for cranes, presses, wind turbines, elevators, and marine winches
• Research institutions comparing new friction formulations, including low-metallic, NAO, and carbon fiber reinforced composites

The experimental data support product certification against national and international standards, incoming material inspection, failure analysis, and optimization of formulation variables such as binder resin content, reinforcing fibers, and friction modifiers.

FAQ

What is the difference between static and dynamic friction coefficient testing?

Static friction coefficient testing measures the friction force at the threshold of relative motion, whereas dynamic friction coefficient testing measures the friction force while continuous sliding motion is maintained. Dynamic values are generally lower than static values and are the primary parameter for evaluating braking performance, because brakes and clutches operate predominantly in sliding conditions.

Why does the dynamic friction coefficient change with temperature?

As temperature rises, organic binders and friction modifiers in the material undergo physical and chemical changes, the real contact area increases, and transfer films on the counter-surface evolve. These effects can reduce the friction coefficient, a phenomenon known as fade. Measuring the friction coefficient across a temperature range reveals whether the material maintains adequate and stable friction under severe thermal load.

Which standard should be selected for testing?

The choice depends on the product type and target market. GB/T 5763 and JIS D4411 are commonly applied to automotive brake linings, SAE J661 is the reference procedure for Chase-type small-scale machines and SAE J866 friction classification, ISO 26867 covers full-scale inertia dynamometer evaluation, and ASTM G99 is suited to pin-on-disk research measurements. Accredited third-party laboratories can advise on the appropriate standard for a specific application.

How is wear rate evaluated in the same experiment?

Specimen thickness is measured before and after the friction test, and the difference is recorded as thickness loss in millimeters. In pin-on-disk configurations, mass loss may also be measured to calculate a specific wear rate, expressed as volume loss per unit load and sliding distance. Wear data collected together with friction data provide a complete picture of material durability.

What sample preparation is required before testing?

The friction surface should be flat, parallel, and free of oil, dust, and loose particles. Specimens are usually dried to constant mass, conditioned at standard laboratory temperature and humidity, and pre-ground if necessary. Some procedures include a preliminary bedding-in or burnishing stage to stabilize the contact conditions before formal measurements begin.