PEEK friction pad swelling testing measures the dimensional and mass changes of PEEK-based friction materials after immersion in transmission fluids and other media at elevated temperatures. This test is essential for verifying clearance stability, drag torque behavior, and long-term reliability of wet clutch and brake systems. This article explains the sample types, test items, measurement methods, acceptance criteria, and typical applications of PEEK friction pad expansion detection.
Technical Overview
PEEK (polyetheretherketone) is a semi-crystalline, high-performance engineering thermoplastic with a glass-transition temperature of approximately 143 °C and a melting point of approximately 343 °C. Because of its outstanding thermal stability, chemical resistance, mechanical strength, and tribological performance, PEEK and PEEK-based compounds are widely used as friction materials in wet clutches, wet brakes, and power-shift transmissions that operate while continuously immersed in automatic transmission fluid (ATF) or other lubricants.
When a PEEK friction pad is exposed to hot fluid, small amounts of the fluid diffuse into the polymer matrix and interact with the resin and its additives. The result is swelling — a measurable increase in thickness, diameter, volume, and mass. A limited degree of swelling is normal and is often accounted for in the design clearance of a clutch pack. Excessive or non-uniform swelling, however, reduces running clearance, increases drag torque and heat generation, shifts the friction coefficient, and can eventually cause plate distortion, bond-line stress, or engagement failure. PEEK friction pad swelling testing therefore quantifies how much the material grows under defined fluid, temperature, and time conditions, providing objective data for material qualification, batch release, and design validation.
Typical test programs combine immersion aging with precise dimensional metrology, mass and volume determination, hardness measurement, and — where required — pre- and post-immersion friction characterization on an SAE No. 2 or equivalent wet friction test machine. The methodology is generally adapted from standards such as ASTM D471 (effect of liquids on polymeric materials), ASTM D570, ISO 62, and GB/T 1034 (water absorption of plastics), and ISO 291 (standard atmospheres for conditioning and testing), supplemented by OEM-specific technical specifications.
Testing Samples
Production Clutch Plates and Brake Discs
Complete wet friction plates with PEEK-based linings bonded or sintered to a steel core are tested in their as-manufactured state, including the groove pattern, because groove geometry and the bond line influence local swelling and distortion. Full-size parts allow evaluation of flatness, waviness, and thickness distribution across the entire friction surface.
Segmented Pads and Coupon Specimens
Individual friction segments, or coupon specimens such as 25 mm diameter buttons or strips cut from production linings, are used for screening tests, fluid-compatibility studies, and swelling-kinetics measurements. Coupons allow several replicate specimens per plate so that statistically valid averages and scatter can be reported.
Molded Plaques and Reference Compounds
For material development and incoming-material control, plaques compression- or injection-molded from the PEEK friction compound are tested. Plaque results isolate the intrinsic behavior of the compound from the effects of part processing, bonding, and geometry.
Testing Items
A comprehensive swelling evaluation program typically includes the following test items:
- Thickness swelling rate at multiple defined measuring points, before and after immersion
- Mass change rate (fluid uptake) determined gravimetrically
- Outer diameter and inner diameter growth of the friction plate
- Volume swelling rate, determined geometrically or by hydrostatic weighing
- Flatness, warpage, and taper of the plate after immersion
- Surface hardness change, for example Shore D or ball indentation hardness
- Recovery behavior: dimensional reversion after drying or re-conditioning
- Swelling kinetics: time-dependent swell curves at 24 h, 72 h, 168 h, 500 h, and 1000 h
- Temperature dependence, for example immersion at 80 °C, 100 °C, 120 °C, and 150 °C
- Media dependence: ATF (Dexron-, Mercon-, and low-viscosity types), DCT fluid, hydraulic oil, engine oil, brake fluid, and water-glycol mixtures
Testing Methods
Sample Conditioning and Initial Metrology
Samples are conditioned in a standard laboratory atmosphere of 23 ± 2 °C and 50 ± 5 % relative humidity (ISO 291) until stable. Measuring positions are marked — typically 6 to 12 points distributed around the circumference and across the friction face — and initial thickness, diameters, flatness, mass, and volume are recorded. Thickness gauges use flat, low-pressure anvils to avoid indenting the friction surface; a non-contact laser or optical measurement system is preferred for soft or textured surfaces.
Immersion Aging
Samples are fully immersed in the specified test fluid and held at constant temperature in a ventilated oven or oil-aging bath. Following practice adapted from ASTM D471, the fluid volume is kept sufficiently large relative to the sample surface area (commonly at least 20:1), the fluid is refreshed or circulated for long-duration tests, and duplicate samples are included. The key parameters — fluid type, temperature, and duration — are selected to reproduce the actual service condition or a validated accelerated equivalent. At each interval, samples are removed, drained, and blotted with lint-free cloth without squeezing, and all measurements are completed within a defined time window (commonly 15 to 30 minutes) to avoid drying artifacts.
Measurement and Calculation
Thickness is re-measured at the marked points, diameters are checked on a coordinate measuring machine or optical comparator, and mass is determined on an analytical balance with 0.1 mg resolution. Volume change is determined by hydrostatic (Archimedes) weighing or calculated from geometry. The thickness swelling rate is calculated as:
St = (t1 − t0) / t0 × 100 %, where t0 is the initial thickness and t1 is the thickness after immersion. Mass and volume change rates are calculated analogously. The test report states the mean value, the maximum value, and the point-to-point distribution, because a locally high swell concentration can be more damaging to clutch performance than the average value.
Post-Immersion Evaluation
After measurement, samples may be dried to constant mass at moderate temperature to separate reversible swelling (fluid desorption) from irreversible swelling caused by plasticization, additive extraction, or changes in crystallinity. Hardness testing and surface examination under optical microscopy, optionally with cross-sectioning, complete the evaluation. Where the specification requires it, friction and wear characteristics are re-tested after immersion to correlate the swelling data with functional performance.
Applications
PEEK friction pad swelling testing is applied across all industries that use oil-immersed friction components:
- Automatic transmission (AT) clutch packs and torque-converter lock-up clutches
- Dual-clutch transmission (DCT) wet clutches and synchronizer components
- Hybrid and electric drive modules with oil-immersed disconnect clutches
- Motorcycle and powersport wet clutches
- Wet brakes on agricultural and construction machinery
- Industrial clutches and brakes, marine gearboxes, and heavy-duty power-shift transmissions
- Aerospace hydraulic and actuation systems using PEEK-based friction elements
In these applications, the swelling data directly supports the definition of design clearances, material and fluid selection, supplier qualification, and incoming batch quality control, ensuring that clutch engagement behavior and drag torque remain within specification over the full service life.
FAQ
What swelling level is acceptable for a PEEK friction pad?
Acceptance limits are defined by the OEM or product specification. As general industry practice, thickness swelling of PEEK-based wet friction materials after hot ATF immersion (for example 168 h at 100–120 °C) is commonly required to remain within approximately 1–3 %, with tight additional limits on flatness change. The applicable specification must always be consulted for the binding values.
Why does PEEK swell less than many other polymers?
PEEK is a semi-crystalline polymer with a crystallinity of typically 30–35 %. The tightly packed crystalline regions exclude fluid molecules, so overall fluid uptake remains low, and the material resists oils, hydraulic media, and hydrolysis far better than amorphous thermoplastics or elastomeric binders.
Which fluids are used in the test?
The fluid the component will encounter in service is used: ATFs of the relevant specification (Dexron-, Mercon-, or low-viscosity hybrid types), DCT fluids, hydraulic oils, engine oils, brake fluids, and water-glycol coolant mixtures, depending on the application.
How long does a swelling test take?
Screening tests of 24–72 h give a rapid indication, formal qualification commonly uses 168 h, and long-term aging may extend to 500 or 1000 h to establish the saturation plateau of the swell curve and confirm dimensional stability.
What is the difference between reversible and irreversible swelling?
Reversible swelling disappears when the absorbed fluid desorbs during drying. Irreversible swelling reflects permanent changes such as plasticization, extraction of additives, or altered crystallinity, and it is a stronger indicator of long-term performance risk.
Can swelling testing be combined with friction testing?
Yes. A common qualification sequence first measures friction characteristics on an SAE No. 2 wet friction machine, then immerses the plate in hot ATF, and finally repeats the friction test to verify that the swelled material still meets the friction level, engagement time, and durability requirements.