Determination of dry friction coefficient variation over time is a specialized tribological test that tracks how the static and kinetic friction coefficients of a material evolve during sustained dry sliding contact. This testing is essential for packaging films, rubber components, textiles, flooring, and engineering polymers, where time-dependent friction behavior such as running-in, transfer film formation, and wear-induced drift directly affects product performance and processability. Accredited third-party laboratories perform these measurements in accordance with international standards such as ASTM D1894, ISO 8295, ASTM G99, and ASTM G115 to deliver reproducible friction-versus-time data for quality control and product development.
Technical Overview
The dry friction coefficient (COF) is the ratio of the tangential force resisting relative sliding between two surfaces to the normal force pressing them together, measured under unlubricated (dry) contact conditions. While many routine quality-control tests report only a single static or kinetic COF value at the start of sliding, real tribological systems rarely exhibit a perfectly constant friction coefficient. Instead, the measured COF typically varies as a function of sliding time, sliding distance, and accumulated thermal and mechanical history.
Determining the dry friction coefficient as a function of time involves continuously recording the friction force throughout a defined test duration and converting the resulting force-time trace into a COF-versus-time curve. This curve reveals characteristic stages of friction evolution that a single-point measurement cannot capture, including:
- Initial static breakaway friction and the transition to kinetic sliding
- A running-in (break-in) period during which asperities flatten, real contact area grows, and the COF rises or falls toward a characteristic value
- A steady-state regime in which the kinetic COF stabilizes, governed by equilibrium between surface smoothing and wear debris generation
- Long-term drift, in which progressive wear, transfer film formation or removal, thermal softening, or surface oxidation causes the COF to increase or decrease over extended durations
- Stick-slip oscillations, in which the friction force alternates between static and kinetic values, producing periodic time-dependent fluctuations
Understanding this time-dependent behavior is critical because many field failures and processing problems, such as film jams on packaging lines, squealing brake components, or loss of slip resistance on flooring, occur not at the initial COF value but after a period of service. Time-resolved COF testing therefore provides a far more realistic picture of tribological performance than a conventional single-value determination.
Testing Samples
Plastic Films and Sheeting
Flexible packaging films made of polyethylene (PE), polypropylene (PP), PET, PA, and multilayer laminates are among the most frequently tested materials. These products often contain slip additives such as erucamide or oleamide that migrate to the surface over time, causing the COF to change both during storage and during sustained sliding on filling and sealing machinery.
Rubber and Elastomeric Products
Seals, gaskets, tires, conveyor belts, and damping components are tested to evaluate how the dry friction of elastomeric compounds evolves with sliding time, heat build-up, and surface degradation of the rubber.
Textiles and Nonwovens
Fabrics, interlinings, geotextiles, and hygiene nonwovens are evaluated for friction against themselves or against counter-surface materials, since fiber orientation, finishing agents, and compression-set behavior produce measurable time-dependent COF changes.
Flooring and Walkway Surfaces
Resilient flooring, ceramic tiles, laminates, and coated concrete are tested to assess how repeated dry foot traffic and polishing or wear of the surface affect slip resistance over the service life of the floor.
Engineering Polymers, Composites and Coatings
Bearing materials, sliding guides, PTFE-filled compounds, and low-friction coatings are characterized by their running-in behavior and steady-state friction, which differ substantially from their initial COF values.
Paper, Board and Converted Packaging
Carton board, corrugated materials, and printed packaging are tested for time-dependent friction that influences stacking stability and machine feedability.
Metals, Ceramics and Surface-Treated Components
Dry sliding pairs used in mechanisms, fasteners, and consumer hardware are evaluated to quantify friction evolution caused by oxide layer growth, wear, and surface finish changes.
Testing Items
A comprehensive determination of dry friction coefficient variation over time typically includes the following measured and derived quantities:
- Static coefficient of friction at breakaway, measured at defined time intervals after contact establishment
- Kinetic (dynamic) coefficient of friction recorded continuously as a function of sliding time or sliding distance
- Running-in time and running-in sliding distance required to reach steady-state friction
- Steady-state kinetic COF, calculated as the mean value over the stabilized portion of the curve
- COF drift rate, expressed as the change in coefficient per unit time or per unit sliding distance
- Peak COF and minimum COF observed during the test duration
- Stick-slip amplitude (difference between static and kinetic peaks) and oscillation frequency
- Time to threshold, defined as the elapsed sliding time until the COF exceeds or falls below a specified acceptance limit
- Repeatability and specimen-to-specimen variability of the friction-time curve
- Correlation of friction evolution with mass loss, wear scar dimensions, and contact temperature rise
Testing Methods
Horizontal Plane (Sled) Method with Continuous Recording
The most widely used approach for films, sheets, and textiles follows ASTM D1894, ISO 8295, and the equivalent GB/T 10006. A standard sled with a defined mass (typically 200 g in ASTM D1894, or 400 g for ISO 8295 film testing) and defined contact face is placed on the specimen, and the plane is driven at a constant speed (commonly 150 mm/min) while a load cell continuously records the friction force. Unlike a routine single-value test, the data acquisition system samples the force signal throughout the entire traverse, and in extended-duration protocols the specimen is subjected to repeated passes or prolonged sliding to construct the full COF-versus-time profile.
Pin-on-Disc Method
For rigid materials, composites, coatings, and bearing compounds, ASTM G99 describes a rotating disc configuration in which a stationary pin or ball is loaded against a rotating specimen under a constant normal force. The tangential force is recorded continuously over a specified number of revolutions or test duration, directly yielding the friction-time trace. This method is well suited to studying running-in, transfer film formation, and long-term friction drift under controlled load and speed.
Linear Reciprocating Method
ASTM G133 and related reciprocating procedures use a ball or flat counter-specimen that slides back and forth across the test surface at controlled frequency and stroke. The oscillating motion reproduces the intermittent contact found in many service conditions, and the continuous force record captures both forward and reverse stroke friction as a function of accumulated cycles.
Inclined Plane (Ramp) Method for Time-Staged Static COF
For static friction specifically, an inclined-plane apparatus tilts the contact at a slow, constant rate until slip occurs, and the tangent of the slip angle equals the static COF. To evaluate time dependence, the measurement is repeated after defined dwell or rest periods between sliding episodes, revealing how static friction builds with contact time, a phenomenon known as stiction or rising static friction.
Test Condition Control and Specimen Conditioning
Because friction is highly sensitive to surface contamination, temperature, and humidity, specimens are conditioned prior to testing in a standard atmosphere, commonly 23 ± 2 °C and 50 ± 5% relative humidity in accordance with ISO 291 or ASTM D618. Test variables that must be documented and held constant include normal load, sliding speed, contact geometry, counter-surface material and roughness, ambient temperature, and relative humidity.
Data Analysis and Reporting
The raw force-time signal is converted to a COF-time curve by dividing the friction force by the constant normal force. Key parameters are then extracted: the initial static peak, the running-in segment, the steady-state mean COF calculated over a defined averaging window, and any long-term trend. Results are reported as individual curves plus summary statistics, typically the mean and standard deviation from at least three to five replicate specimens, with the test direction, contact area, and full condition history clearly documented in accordance with the reporting guidance of ASTM G115.
Applications
Time-resolved dry friction testing supports a wide range of industrial and regulatory applications:
- Flexible packaging: verifying that slip-agent migration stabilizes machine-direction COF for reliable operation of form-fill-seal and horizontal wrapping equipment
- Automotive engineering: characterizing running-in and friction fade or recovery in dry clutches, brake friction materials, and interior trim sliding pairs
- Medical devices: qualifying the long-term sliding consistency of syringe plungers, catheter surfaces, and device mechanisms that must remain lubricious without liquid lubricants
- Flooring safety: demonstrating that slip resistance under dry conditions is maintained after accelerated surface wear, supporting compliance with safety expectations and standards-based floor testing programs
- Polymer processing and conveying: selecting materials for chutes, guides, and conveyor systems where friction must not rise to a level that causes jamming or product damage
- Textiles and hygiene products: ensuring consistent hand feel, layer separation, and converting performance as finishes and fiber finishes redistribute over time
- Research and development: comparing candidate formulations, surface treatments, and coatings by their full friction-time signatures rather than a single point value
- Quality control and supplier qualification: using steady-state COF and drift-rate acceptance limits in incoming inspection and lot-release testing
FAQ
Why does the dry friction coefficient change over time?
The COF evolves because sliding itself modifies the contact. During running-in, surface asperities deform and the real contact area changes; subsequently, transfer films form or wear away, frictional heat softens or reorganizes near-surface material, additive molecules migrate to or from the surface, and oxidative or tribochemical reactions alter the chemistry of the sliding interface. Each mechanism can raise or lower the COF, so the direction and magnitude of change are material- and condition-specific.
How long should a friction-versus-time test run?
The duration is chosen to cover the regime of interest. For films, a single traverse under ASTM D1894 or ISO 8295 may suffice to capture initial and average kinetic COF, while running-in and drift studies may require minutes to hours of continuous sliding or thousands of reciprocating cycles. A useful criterion is that the test should continue until a clearly defined steady-state or a specified endurance limit is reached.
What is the difference between static and kinetic COF in a time-based test?
The static COF is the ratio at the instant sliding begins and is often the first peak of the force record; it can increase with dwell time between sliding episodes. The kinetic COF is the ratio during continuous sliding and is read from the sustained portion of the record. In stick-slip systems, the signal alternates between the two, and both quantities are extracted from the same time record.
Which counter-surface should be used?
Whenever possible, the counter-surface should replicate the intended service pairing, for example the same film against itself or a specific stainless-steel grade. Standards such as ASTM D1894 permit a specified steel or glass reference surface when a material-versus-material pair is not defined, and the choice must always be reported with the results.
How are results made reproducible?
Reproducibility depends on rigorous control of normal load, speed, geometry, counter-surface finish, cleanliness, and conditioning atmosphere, combined with adequate replication. Testing at accredited third-party laboratories that operate calibrated friction apparatus under ISO, ASTM, or EN procedures provides traceable, defensible friction-time data suitable for regulatory submissions and supplier agreements.
What does a typical test report include?
A complete report identifies the test standard, material and specimen preparation, conditioning atmosphere, apparatus and calibration status, normal load, sliding speed and path, counter-surface description, the full COF-versus-time curve for each specimen, derived parameters such as static peak, running-in duration, steady-state COF, drift rate, and stick-slip characteristics, together with mean values, standard deviations, and the number of replicates.