Determination Methods for the Dynamic Friction Coefficient of Coatings

The dynamic coefficient of friction (COF) is a critical performance parameter for coated materials, governing how coated surfaces behave during forming, stacking, transport, and end use. This article explains the principle of dynamic friction coefficient testing for coatings, the applicable international standards such as ASTM D1894 and ISO 8295, typical sample types, test procedures, calculation methods, and practical applications in quality control.

Technical Overview

The dynamic (kinetic) coefficient of friction, usually denoted as μd, quantifies the resistance to sliding between two surfaces that are already in relative motion. For coated products, this property directly determines processability and functional performance in service. A coating with an excessively high dynamic COF may cause blocking, scratching, feeding failures, or excessive wear on processing equipment, while an extremely low value may cause slippage problems during stacking or transport.

By definition, the dynamic coefficient of friction is the ratio of the tangential friction force measured during steady sliding to the normal force pressing the two surfaces together, expressed as μd = Fd / N, where Fd is the mean dynamic friction force and N is the normal force (typically the weight of the sled). The result is a dimensionless value, commonly ranging from about 0.10 to 0.80 for organic coatings. The measured value depends on the coating formulation (for example, wax or silicone additives, matting agents), surface roughness, degree of cure, the counterface material, and the specified test conditions.

Because COF is not an intrinsic material constant but a system property, standardized test geometries, loads, speeds, and conditioning environments are essential to obtain comparable results. Accredited third-party laboratories typically perform these tests in accordance with recognized standards and report the complete test conditions together with the numerical result.

Testing Samples

Coil-Coated Metal Panels

Prepainted galvanized steel or aluminum panels coated with polyester, polyurethane, PVDF, epoxy, or plastisol systems are the most common samples. These are tested in both the rolling direction and transverse direction, since coil coating processes can create direction-dependent friction behavior.

Coatings on Flexible Substrates

Coatings applied to plastic films, foils, or paper are tested as coated film-to-film or coated film-to-metal combinations. This category includes packaging laminates, release coatings, heat-seal coatings, and overprint varnishes.

Powder-Coated and Liquid-Coated Components

Flat test panels prepared with powder coatings, solvent-borne, water-borne, or radiation-cured coatings can be evaluated, provided the coated surface is sufficiently flat and uniform for the sled to make full contact.

Functional and Anti-Slip Coatings

Floor coatings, deck coatings, and textured architectural coatings may be tested, often under wet or contaminated conditions, to assess slip resistance and safety-related performance.

Sample Preparation and Conditioning

Specimens should be free of defects such as drips, orange peel, scratches, and contamination in the test area. Unless otherwise specified, samples are conditioned at 23 ± 2 °C and 50 ± 5 % relative humidity for a minimum of 40 hours in accordance with ISO 291 or ASTM D618. The test surfaces must not be touched by hand after cleaning with a dry, lint-free cloth.

Testing Items

Typical test items and reported parameters include the following:

• Dynamic (kinetic) coefficient of friction between the coating and a specified counterface, measured during steady sliding
• Static coefficient of friction, often captured in the same test run from the initial breakaway force
• Friction force versus displacement curves, revealing stick-slip behavior, first-slip peaks, and sliding stability
• Coating-to-coating and coating-to-metal (or other counterface) friction comparisons
• Directional comparison of COF relative to rolling or brushing direction
• Optional durability checks, such as COF after abrasion, weathering, humidity exposure, or accelerated aging

Testing Methods

Sled-on-Flat Horizontal Plane Method

This is the primary and most widely used method, based on the principles of ASTM D1894, ISO 8295, and the equivalent GB/T 10006. A sled of known mass (commonly 200 g) fitted with the specified contact surface is placed on the coated specimen secured to a flat, level bed. The sled is attached to a load cell and drawn across the surface at a constant speed, typically 100 mm/min or 150 mm/min. The mean force recorded during the sliding phase divided by the normal force gives the dynamic coefficient of friction. Depending on the specification, the sled face may carry a second piece of the coated specimen (coating-to-coating test) or a defined counterface such as polished stainless steel, rubber, or uncoated substrate (coating-to-counterface test).

Inclined Plane Method

In this simpler configuration, the coated specimen rests on a plane that is gradually tilted until sliding begins. The tangent of the angle at the moment of movement gives the coefficient of friction. This method is primarily suited to static COF and is often used as a quick screening technique or for quality comparison, rather than for precise determination of dynamic friction.

Instrumented Sliding and Tribological Methods

For research and development purposes, sliding ball-on-plane or pin-on-disk configurations, conducted in line with the general guidance of ASTM G115, can characterize friction over repeated cycles, varying loads, and varying speeds. These methods provide deeper insight into coating durability, transfer films, and running-in behavior.

Wet and Contaminated Condition Testing

For floor and safety coatings, dynamic COF may be measured under wet conditions, following the philosophy of ANSI A137.1 slip-resistance testing, or with reference to ASTM D2047 for polished floor surfaces. The test liquid, contact material, and speed must be strictly controlled because water dramatically alters friction behavior.

General Test Procedure

• Cut specimens to the required size, mark the test direction, and condition them in the standard atmosphere
• Clean the test area and the counterface with a dry, lint-free cloth; do not use solvents unless specified
• Mount the specimen flat on the test bed, ensuring full contact and no wrinkles or air pockets
• Place the calibrated sled gently onto the surface without impact and connect it to the load cell
• Start the drive at the specified constant speed and record the complete force-displacement curve
• Calculate μd as the mean dynamic friction force divided by the normal force, and report the average of at least three valid measurements per direction
• Report the standard used, counterface material, sled mass, speed, temperature, humidity, and any deviation from the standard

Factors Affecting Results

• Normal load and sled contact area
• Sliding speed, which can shift the balance between adhesive and deformable friction mechanisms
• Counterface material, hardness, and roughness
• Temperature and relative humidity during conditioning and testing
• Surface cleanliness and handling contamination such as fingerprints
• Coating cure degree, thickness, surface texture, and additive distribution
• Test direction relative to coil rolling or application direction

Applications

Dynamic COF testing of coatings supports a wide range of industrial activities:

• Quality control of coil coating production lines and incoming inspection at roll-forming and profiling plants, where consistent friction ensures smooth feeding and bending of prepainted metal
• Manufacture of food and beverage cans, closures, and easy-open ends, where lubricity of the coating affects high-speed stamping and tool wear
• Packaging machinability, including film transport, folding, gluing, and stacking operations on high-speed filling and packing lines
• Forming simulation and process approval for bending, deep drawing, and profiling of coated sheet metal
• Printing and lamination processes, where controlled friction prevents misfeeds and surface damage
• Safety assessment of coated flooring and pedestrian surfaces under dry and wet conditions
• Research and development of coating formulations, including optimization of wax, silicone, and matting additives for targeted slip performance
• Verification of coating performance after weathering, humidity aging, or abrasion in durability studies

FAQ

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

The static coefficient of friction describes the force needed to initiate sliding between two surfaces at rest, calculated from the initial breakaway peak. The dynamic coefficient of friction describes the force needed to maintain sliding, calculated from the mean force during steady motion. Many testers record both from a single run, but the dynamic value is the key parameter for continuous processing operations.

Which standards are most commonly used for coated materials?

ASTM D1894 and ISO 8295 (with its national equivalent GB/T 10006) are the most widely applied sled-type methods, originally developed for plastic films and routinely adapted to coated films, coated papers, and coated metal. ASTM G115 provides general guidance for measuring and reporting friction coefficients, and ASTM D2047 addresses polished floor coatings. For coated coils, the specific test conditions are normally agreed between supplier and customer based on these frameworks.

What dynamic COF value is considered acceptable?

There is no universal pass or fail limit. Coil coatings intended for roll forming are often specified in the range of approximately 0.15 to 0.35 against steel, while packaging and flooring applications have their own requirements. Acceptance criteria should always be established between the interested parties with reference to the actual counterface and processing conditions.

How many measurements are required?

A minimum of three valid measurements per direction is standard practice, and the report should include the average value, the range, and any observed stick-slip or instability. More repetitions are recommended when results are inconsistent or when certification is involved.

Can the test be performed after aging or environmental exposure?

Yes. The same method can be applied to specimens after accelerated weathering, humidity exposure, heat aging, or abrasion, allowing the friction stability of the coating over its service life to be evaluated.

Why must test conditions be reported along with the result?

Because friction is a system property rather than an intrinsic material constant, the measured μd is meaningful only when the sled mass, sliding speed, counterface, temperature, humidity, and test direction are documented. Comparing results obtained under different conditions can lead to serious misinterpretation.