The coefficient of friction (COF) of materials rarely stays constant as temperature changes, which is why friction-temperature relationship testing is essential for packaging lines, brake systems, polymer bearings, and metal-forming operations. This guide explains how static and kinetic COF values are measured across sub-ambient to high temperatures using sled, pin-on-disc, and reciprocating tribometer methods in accordance with ASTM D1894, ISO 8295, ASTM G99, and related standards, and how accredited third-party laboratories report the resulting friction-temperature curves.
Technical Overview
The coefficient of friction is defined as the ratio of the tangential friction force to the normal load applied between two surfaces in contact. Two values are conventionally reported: the static coefficient of friction, which corresponds to the force required to initiate sliding, and the kinetic coefficient of friction, which corresponds to the force required to sustain sliding. Because friction is a system property rather than an intrinsic material constant, both values can change dramatically with temperature.
Several physical mechanisms drive the temperature dependence of friction, and identifying them is a key objective of COF-temperature testing:
- Viscoelastic transitions in polymers near the glass transition or melting point alter the deformation component of friction.
- Thermal softening increases the real contact area between mating surfaces, typically raising the friction force.
- Lubricant viscosity decreases with temperature, causing transition from hydrodynamic to boundary lubrication and film breakdown.
- Thermal expansion and surface roughness evolution change the geometry of asperity contacts.
- Oxidation and tribochemical reactions at elevated temperature form transfer films that can either increase or reduce friction.
- Migration of slip and anti-block agents in polyolefin films changes surface composition as temperature rises.
Friction-temperature testing quantifies these effects by measuring static and kinetic COF at controlled temperature set points inside an environmental chamber or furnace, producing a friction-temperature curve. The measurement framework is supported by ASTM G115, the general guide for measuring and reporting friction coefficients, together with material-specific methods such as ASTM D1894, ISO 8295, GB/T 10006, and JIS K 7125 for plastic films, and ASTM G99, ASTM G133, ASTM D3702, and ISO 6601 for bulk sliding couples.
Testing Samples
Polymer Films and Flexible Packaging Laminates
LDPE, LLDPE, CPP, BOPP, BOPET, PA, and metallized or coated films used in bags, pouches, and lids are tested film-to-film or film-to-stainless steel to verify machinability across the temperature window experienced on packaging machines, including elevated temperatures near sealing jaws.
Elastomers and Seals
NBR, FKM, EPDM, and silicone compounds are evaluated as plaques, O-ring segments, or custom coupons sliding against steel or anodized aluminum counterfaces to predict seal friction from cold-start to hot-running conditions.
Engineering Polymers and Self-Lubricating Composites
PTFE, PEEK, POM, PA66, and glass- or carbon-fiber-reinforced grades used for gears, bushings, and wear rings are tested as pins or washers against steel counterparts to build friction maps for design calculations.
Metals, Ceramics, Coatings, and Friction Materials
Steel-on-steel couples, tool steels, nitrided surfaces, DLC and PVD coatings, brake lining formulations, clutch facings, and rail steels are tested at elevated temperature to assess friction stability, fade, and recovery.
Lubricants and Process Fluids
Greases, oils, and metalworking fluids are evaluated under boundary-lubricated contact at controlled temperatures to rank additive packages and define operating windows for hot forming and stamping operations.
Testing Items
A comprehensive COF-temperature test program typically delivers the following results:
- Static coefficient of friction as a function of temperature, from the first force peak at each set point.
- Kinetic coefficient of friction as a function of temperature, averaged over the steady-state sliding segment.
- A complete friction-temperature curve with identified transition points, correlated where possible with thermal analysis data such as glass transition and melting temperatures.
- Stick-slip tendency and friction force oscillation amplitude at each temperature.
- Interfacial or near-contact temperature measurements compared with the nominal chamber temperature.
- Companion wear results, including wear scar dimensions and specific wear rate at each temperature, when tribometer methods are used.
- Statistical data treatment, including replicate means, standard deviations, and repeatability, reported in line with ASTM G115 requirements.
The final deliverable is normally a graphical and tabular summary that designers can use directly to define safe friction windows for their application.
Testing Methods
Specimen Preparation and Conditioning
Film and polymer specimens are conditioned at 23 °C and 50% relative humidity for at least 40 hours in accordance with ISO 291 or ASTM D618 unless the governing product standard specifies otherwise. Metal counterfaces are cleaned with isopropanol or heptane and dried. Surface roughness, film thickness, and sliding direction are recorded before testing because these parameters strongly influence the measured coefficient of friction.
Apparatus and Calibration
Force sensors are calibrated with traceable reference weights, and chamber temperature is verified with calibrated thermocouples or an infrared pyrometer, with uniformity typically maintained within ±2 °C of the set point. For tests below ambient temperature, a dry nitrogen purge is used to prevent condensation and frost on the sliding surfaces, while inert atmospheres are recommended above approximately 300 °C to limit uncontrolled oxidation.
Sled-on-Plane Method for Films
Based on ASTM D1894, ISO 8295, GB/T 10006, and JIS K 7125, this method draws a weighted sled, commonly 200 g with a face of approximately 63.5 mm square, across a specimen mounted on a horizontal temperature-controlled plate at a speed of 100 to 150 mm/min. The typical procedure is:
- Mount the film flat on the heated or cooled plate and attach a matching film or counterface strip to the sled.
- Set the chamber to the first temperature and allow the specimens to dwell until the contact zone is stabilized, typically 15 to 30 minutes.
- Run the sled through its travel, recording the force-distance trace.
- Determine the static COF from the maximum static force peak and the kinetic COF from the average force over the plateau region of the trace.
- Repeat at each subsequent set point, for example -20, 0, 23, 40, 60, 80, and 100 °C, using a fresh contact area and at least three replicates per condition.
Rotary Pin-on-Disc Method
Following ASTM G99 and ISO 6601, a stationary ball or pin, commonly a 6 mm diameter steel, alumina, or counterpart-material pin, is pressed against a rotating disc under loads typically between 1 N and 50 N at sliding speeds of 0.05 to 0.5 m/s. A resistance furnace or environmental enclosure extends the range from room temperature to approximately 600 °C, with specialized systems reaching 1000 °C. The friction coefficient is computed continuously as the measured tangential force divided by the normal force, and each temperature step uses a fresh track radius to avoid interference from previously formed transfer films.
Reciprocating and Thrust-Washer Methods
ASTM G133 covers linearly reciprocating ball-on-flat testing with strokes of 2 to 10 mm at 1 to 20 Hz, which reproduces the oscillating contact found in seals, sliders, and joint implants. ASTM D3702 uses a thrust-washer geometry that loads an annular face at pressures of roughly 0.5 to 5 MPa and is preferred for self-lubricated bearing grades under sustained face loading at temperature.
Temperature Profile Strategies
Two profiling approaches are used. The isothermal step method soaks the specimen at each set point until the contact region is within ±1 °C of target before sliding, giving the most reliable data. The continuous ramp method, with ramps not exceeding about 2 °C/min, captures dynamic friction behavior such as brake fade but requires careful correction for thermal lag. Heating and cooling legs are often both recorded to reveal hysteresis caused by transfer film formation or slip-agent migration.
Data Reduction and Reporting
Reports prepared in accordance with ASTM G115 include the materials and counterfaces, surface roughness values, normal load, sliding speed, atmosphere and humidity, the method used to establish interfacial temperature, static and kinetic COF values with mean and standard deviation at each temperature, the full friction-temperature curve, and observations such as stick-slip, transfer film morphology, and wear scar appearance. Correlating friction transitions with calorimetric transitions measured by DSC materially strengthens the interpretation of the curve.
Applications
Friction-temperature relationship testing supports material selection, quality control, and failure analysis across many industries:
- Packaging: confirming that film COF stays within the machinability window at sealing-jaw temperatures on vertical form-fill-seal lines.
- Automotive: characterizing friction fade and recovery of brake and clutch materials from cold conditions to 350 °C and beyond.
- Tire and rubber products: ranking tread compounds on ice and wet surfaces at sub-zero temperatures.
- Polymer machinery components: generating friction input data for gear, bearing, and conveyor design calculations.
- Metal forming: qualifying lubricants for hot stamping and forging where contact temperatures reach several hundred degrees Celsius.
- Quality assurance: supplier qualification, incoming inspection, and comparative benchmarking of competitive materials at accredited third-party laboratories.
In every case, the friction-temperature curve allows engineers to define operating windows in which the coefficient of friction remains predictable, and to identify temperatures at which redesign or material substitution is required.
FAQ
What temperature range can be tested?
Film testing by the sled method is typically performed between -40 °C and 150 °C. Pin-on-disc and thrust-washer tribometry commonly covers room temperature to 600 °C, and specialized high-temperature systems reach approximately 1000 °C for ceramics and hot-forming applications.
Why does the coefficient of friction of polyolefin films increase between 40 °C and 60 °C?
Around this range the film surface begins to soften, which enlarges the real contact area under the sled, while slip agents such as erucamide migrate and redistribute. The combined effect commonly produces a marked rise in both static and kinetic COF, which is exactly the behavior packaging engineers need to anticipate.
Which standards should be specified for a test request?
For plastic films and sheeting, specify ASTM D1894, ISO 8295, GB/T 10006, or JIS K 7125 together with the required temperature points. For bulk material couples, specify ASTM G99 for pin-on-disc, ASTM G133 for reciprocating contact, or ASTM D3702 for thrust-washer testing, and request reporting aligned with ASTM G115. ISO 6601 provides guidance on identifying test parameters for plastics.
How is the temperature at the sliding interface determined?
The chamber or plate temperature alone does not equal the interface temperature because frictional heating generates a flash temperature at the contact. Best practice is to embed a thermocouple close to the contact zone, cross-check with an infrared pyrometer, and report both the nominal bulk temperature and the estimated interface temperature.
What specimen sizes are required?
For film testing, sled-facing specimens of approximately 63.5 mm square and plane specimens roughly 250 mm by 130 mm are typical. Pin-on-disc testing usually requires discs of 30 to 60 mm diameter and 3 to 8 mm thickness, or a small bulk sample from which pins can be machined. Exact dimensions should be confirmed against the governing standard and fixture geometry.
How long does a temperature-sweep test take?
A six- to eight-point isothermal sweep with conditioning, thermal dwell, and replicate runs usually takes one to two working days. Continuous ramp tests are faster but are normally supplemented by isothermal verification at critical points on the curve.
Can tests be performed under controlled humidity or inert atmosphere?
Yes. Chambers with humidity control between roughly 10% and 90% relative humidity allow the combined effects of moisture and temperature to be separated, and nitrogen or argon purges are used below ambient temperature to prevent condensation and above 300 °C to control oxidation of the sliding surfaces.