High-Temperature Dynamic Friction Coefficient Testing of Seals: Methods, Standards, and Applications

High-temperature dynamic friction coefficient testing measures how sealing materials behave under sliding contact at elevated temperatures, providing critical data on energy loss, stick-slip tendency, and wear performance. This article explains the testing principles, sample types, evaluation parameters, recognized standards such as ISO 15113 and ASTM G115, and typical industrial applications for seal friction characterization. Understanding these procedures helps engineers select reliable sealing materials for demanding thermal service conditions.

Technical Overview

The dynamic friction coefficient (COF) of a seal is the dimensionless ratio of the tangential friction force resisting sliding motion to the normal load pressing the seal against its mating surface (μ = F/N). In real service, seals in engines, hydraulic cylinders, valves, and rotating equipment must maintain acceptable friction behavior not only at room temperature but across a wide thermal range. As temperature rises, elastomers soften, thermal expansion increases contact pressure, fillers and lubricating additives migrate to or degrade at the surface, and lubricant films thin out—all of which can cause the friction coefficient to change dramatically.

High-temperature dynamic friction testing quantifies these effects under controlled conditions. A tribometer or seal friction rig applies a defined normal load and sliding velocity between the seal specimen and a countersurface inside an environmental chamber held at a set temperature. Load cells and torque transducers continuously record the friction force or torque, from which the dynamic COF is calculated. The resulting data are essential for predicting seal life, estimating actuation forces and energy consumption, diagnosing stick-slip and jerky motion risks, and validating material formulations before production release.

Testing Samples

Elastomeric Seals

Compression-molded or injection-molded seals in common elastomers such as NBR, HNBR, FKM, FFKM, EPDM, VMQ (silicone), and polyurethane. FKM and FFKM grades are of particular interest because they retain useful friction and wear properties at continuous service temperatures from 200 °C up to approximately 300–327 °C.

Thermoplastic and High-Performance Polymer Seals

PTFE-based seals (including filled grades with bronze, carbon, or glass), PEEK components, UHMWPE elements, and thermoplastic polyurethane wipers. These materials are frequently tested at elevated temperature to evaluate their low-friction advantage over unfilled elastomers.

Finished Seal Configurations

• O-rings per ISO 3601 dimensional classes
• Rotary shaft lip seals and bearing seals
• Hydraulic piston and rod seals (U-cups, buffer seals, wipers)
• Gaskets, valve stem packing rings, and custom-machined test buttons or plaques

Specimens may be tested as complete finished seals mounted in realistic hardware, or as standardized coupons (buttons, blocks, flats) machined or molded from the same compound, depending on the test geometry selected.

Testing Items

Typical parameters evaluated during high-temperature dynamic friction testing include:

• Dynamic (kinetic) friction coefficient as a function of temperature, typically from ambient up to 250–300 °C
• Breakaway friction and starting torque after dwell at elevated temperature
• Friction force stability over extended sliding cycles and time at temperature
• Stick-slip tendency and friction-force oscillation amplitude
• Wear rate, wear scar dimensions, and mass/volume loss correlated with friction behavior
• Comparison of dry versus lubricated friction conditions (engine oil, hydraulic fluid, grease)
• Friction coefficient retention after thermal aging or compression set exposure
• Effect of mating surface finish and material (steel, stainless steel, hard chrome, anodized aluminum)

Testing Methods

Test Geometries

• Linear reciprocating tribometry (block-on-flat, ring-on-flat): best suited to simulating hydraulic rod and pneumatic seal motion
• Rotating configurations (pin-on-disc, thrust washer, rotary seal rig): suited to shaft seals and rotary applications
• Full-scale seal bench testing in actual cylinders or housings with torque and pressure instrumentation

Reference Standards

While no single standard covers all high-temperature seal friction scenarios, testing is commonly performed in accordance with or adapted from the following frameworks:

• ISO 15113 — Rubber: Determination of frictional properties
• ASTM G115 — Standard Guide for Measuring and Reporting Friction Coefficients
• ASTM G99 — Wear Testing with a Pin-on-Disk Apparatus
• ASTM G133 — Linearly Reciprocating Ball-on-Flat Sliding Wear
• ASTM D3702 — Wear and Friction of Materials in a Thrust Washer Testing Machine
• ISO 6601 and GB/T 3960 — Determination of friction and wear properties of plastics under sliding contact
• ISO 23529 — General procedures for conditioning and preparation of rubber test pieces

Test Procedure

Specimens are conditioned at standard laboratory atmosphere before being mounted in the heated chamber. The countersurface is brought to the target temperature and allowed to stabilize, with thermocouples verifying both bulk and interface temperatures. A defined normal load and sliding speed are applied—covering low-speed/high-pressure conditions known to promote stick-slip as well as representative service velocities—and friction force or torque is recorded continuously until a steady-state coefficient is achieved or a specified number of cycles is completed. Tests are commonly repeated at several temperatures (for example 23 °C, 100 °C, 150 °C, 200 °C, and 250 °C) to generate a friction-versus-temperature profile. Post-test examination includes wear measurement, surface microscopy, and hardness change to connect friction behavior with material degradation.

Quality Assurance

Accredited third-party laboratories typically perform these tests under ISO/IEC 17025-accredited quality systems, with calibrated load cells, verified temperature control, and traceable reference materials, ensuring that reported friction coefficients are reproducible and defensible for design, qualification, and regulatory submission purposes.

Applications

• Automotive engineering: engine, transmission, valve stem, turbocharger, and brake component seals where friction stability at 150–250 °C affects efficiency and durability
• Aerospace: hydraulic actuator and landing gear seals requiring predictable breakaway and running friction across extreme thermal envelopes
• Oil and gas: downhole tool and valve seals exposed to high temperature combined with pressure and aggressive media
• Industrial hydraulics and pneumatics: cylinder and rod seal selection to minimize stick-slip, wear, and energy loss
• Semiconductor and process equipment: perfluoroelastomer seals in plasma and high-temperature chambers where particle generation from friction-driven wear is critical
• Materials research and quality control: comparing friction performance of new compounds, fillers, surface treatments, and lubrication strategies

FAQ

What temperature range can be tested?

Standard environmental chambers typically cover ambient to 300 °C, which is suitable for FKM, FFKM, PTFE, and PEEK materials. Higher temperatures may be accommodated with specialized furnaces and ceramic fixtures depending on the laboratory's equipment.

Should testing be done dry or lubricated?

Both conditions are valuable. Dry testing isolates the intrinsic material friction behavior, while lubricated testing with the actual service fluid reproduces boundary and mixed-film conditions that dominate in real machinery. Many qualification programs include both.

How much specimen material is required?

For coupon-based tribometry, finished seals or a small quantity of the same compound (enough to mold several test buttons) is generally sufficient. Full-scale bench testing requires complete seal assemblies plus the mating cylinder or housing hardware.

How is stick-slip evaluated?

Stick-slip is identified from oscillations in the recorded friction force trace during steady sliding, particularly at low speeds and high loads. It is reported as the friction-force fluctuation amplitude and its frequency relative to the sliding cycle.

What constitutes an acceptable friction coefficient?

Acceptance limits are application-specific and normally defined by the end user or product specification. Typical qualified dynamic COF values for well-formulated seals range from approximately 0.1 (filled PTFE, lubricated conditions) to 0.5 or higher for unfilled elastomers under dry sliding at elevated temperature.