Friction Coefficient Determination of Modified PEEK: Standards, Methods, and Applications

Friction coefficient determination of modified PEEK is a critical tribological test used to verify the sliding performance of PTFE-filled, graphite-lubricated, and fiber-reinforced polyetheretherketone grades in bearings, gears, and seals. This article explains the applicable test standards (ASTM G99, ASTM G77, ASTM G115, ASTM G133, ISO 6601, GB/T 3960), specimen preparation, test configurations, data evaluation, and typical friction coefficient values for modified PEEK materials.

Technical Overview

Polyetheretherketone (PEEK) is a semi-crystalline, high-temperature engineering thermoplastic widely used in demanding tribological environments. In its unfilled state, PEEK exhibits a relatively high friction coefficient of approximately 0.30 to 0.45 against steel under dry sliding conditions, which can cause stick-slip behavior, heat generation, and accelerated wear. To overcome these limitations, PEEK is routinely modified with internal lubricants such as polytetrafluoroethylene (PTFE), graphite, and molybdenum disulfide (MoS2), or reinforced with carbon fibers (CF) and glass fibers (GF). Properly formulated compounds reduce the friction coefficient to roughly 0.08 to 0.30 while improving wear resistance, compressive strength, and creep performance.

The friction coefficient is defined as the dimensionless ratio of the tangential friction force to the applied normal load during sliding contact. Both the static friction coefficient, measured at the onset of relative motion, and the kinetic (dynamic) friction coefficient, measured during steady-state sliding, are typically reported. Because friction is a system property rather than an intrinsic material constant, measured values depend strongly on the counterface material and roughness, contact pressure, sliding velocity, temperature, humidity, and running-in behavior. Standardized configurations and strictly controlled conditions are therefore essential for obtaining comparable, reproducible data.

Testing Samples

Test specimens are prepared from injection-molded plaques, compression-molded slabs, or machined rod and tube stock. Common specimen geometries include flat-ended cylindrical pins, rectangular sliding blocks, flat coupons for reciprocating tests, and thrust washers. The grade of modification, filler content, and fiber orientation relative to the sliding direction must be documented, as they directly influence the measured friction coefficient.

PTFE-Filled PEEK

Typically containing 10 to 20 wt% PTFE, this grade develops a thin, self-lubricating transfer film on the steel counterface, achieving a dry kinetic friction coefficient of approximately 0.14 to 0.25. It is widely used for bushings, wear rings, and seals.

Carbon Fiber Reinforced PEEK

With 10 to 30 wt% carbon fiber, these grades combine high stiffness, low creep, and improved thermal conductivity. The dry friction coefficient generally falls between 0.18 and 0.30, and the fibers enhance load-carrying capacity under high contact pressure.

Glass Fiber Reinforced PEEK

Glass fiber loading of 20 to 30 wt% raises strength and dimensional stability, but glass fibers are harder and can be mildly abrasive to the counterface. The friction coefficient typically ranges from 0.25 to 0.35, and these grades are frequently combined with PTFE or graphite to improve sliding behavior.

Graphite and MoS2 Modified PEEK

Graphite performs well under humid or moderately loaded conditions, while MoS2 excels in dry atmospheres and vacuum environments. These fillers are usually added at 5 to 15 wt%, often together with carbon fiber, for high-load, low-speed applications.

Bearing-Grade Multi-Component PEEK

Bearing-grade compounds combine carbon fiber, PTFE, and graphite in a single formulation, delivering the lowest friction coefficients, typically 0.08 to 0.20, together with specific wear rates on the order of 10-7 mm3/(N·m). These grades provide the highest pressure-velocity (PV) capability among modified PEEK materials.

Testing Items

Friction coefficient determination for modified PEEK is generally integrated into a broader tribological characterization program covering the following items:

• Static friction coefficient at the onset of sliding
• Kinetic friction coefficient under steady-state sliding
• Friction coefficient as a function of normal load and sliding velocity
• Friction behavior at elevated temperatures up to the service limit of approximately 250 °C
• Specific wear rate and wear volume, evaluated alongside friction
• Limiting PV value, determined by progressively increasing load and speed until failure or a defined friction threshold is reached
• Counterface transfer film characterization and surface damage assessment

Testing Methods

Applicable Standards

Several internationally recognized standards govern friction and wear testing of polymeric materials, and the appropriate method is selected according to the intended service condition:

• ASTM G115: Standard Guide for Measuring and Reporting Friction Coefficients
• ASTM G99: Standard Test Method for Wear Testing with a Pin-on-Disk Apparatus
• ASTM G77: Standard Test Method for Ranking Resistance of Materials to Sliding Wear Using Block-on-Ring Wear Testing
• ASTM G133: Standard Test Method for Linearly Reciprocating Ball-on-Flat Sliding Wear
• ASTM D3702: Thrust washer testing for self-lubricating rubbing contacts
• ISO 6601: Plastics, sliding friction and wear, identification of test parameters
• GB/T 3960: Chinese standard for the determination of sliding friction and wear properties of plastics

Pin-on-Disk Testing

A flat-ended PEEK pin is pressed against a rotating disk, usually hardened bearing steel or stainless steel with a surface roughness of Ra 0.05 to 0.20 µm. Loads typically range from 1 to 50 N, with sliding speeds of 0.1 to 1.0 m/s and track radii selected to avoid overlap of wear tracks. The friction force is measured continuously with a load cell, and the kinetic friction coefficient is averaged over the steady-state portion of the trace after the running-in period.

Block-on-Ring Testing

A rectangular PEEK block is loaded against a rotating steel ring. This configuration, referenced by ASTM G77 and GB/T 3960, is particularly common for polymer evaluation and simulates line-contact conditions found in bushings. Typical parameters under GB/T 3960 include a 40 mm diameter steel ring, a rotation speed of approximately 200 rpm, a normal load of 196 N, and a test duration of 2 hours under dry sliding. Mass loss measurements allow simultaneous calculation of the specific wear rate.

Reciprocating Ball-on-Flat Testing

A ball or cylindrical rider oscillates against a flat PEEK coupon with strokes of several millimeters at frequencies of 1 to 20 Hz. This method per ASTM G133 reproduces the reversing motion experienced by seals, guide rails, and piston components, and is suitable for evaluating stick-slip tendency through the ratio of static to kinetic friction.

Test Conditions and Data Evaluation

Specimens are conditioned prior to testing in a standard atmosphere of 23 ± 2 °C and 50 ± 10% relative humidity in accordance with ISO 291. Counterfaces are cleaned with acetone or ethanol to remove machining residues and contaminants. Reported results should include the mean, maximum, and minimum steady-state friction coefficients, the complete friction-versus-time trace, all test parameters, and the counterface specification. Because transfer film formation governs the friction of lubricant-modified PEEK, an initial running-in interval of 20 to 60 minutes is customary before steady-state data are collected.

Applications

Verified friction coefficient data enable designers to select the correct modified PEEK grade, predict operating temperatures, and calculate torque losses and PV margins for sliding components. Key application sectors include:

• Plain bearings, bushings, and thrust washers in automotive transmissions and electric drive units
• Gears, cams, and sliding wear pads in machinery and automation equipment
• Wear rings, seal seats, and pump components in fluid handling systems
• Wafer carriers, grippers, and fixtures in semiconductor and vacuum equipment
• Aerospace actuation hardware and lightweight structural bearings
• Oil-free, grease-free bearing surfaces in food and pharmaceutical processing machinery
• Medical device components requiring sterilizable, low-friction sliding surfaces

FAQ

What is the typical friction coefficient of modified PEEK?

Unmodified PEEK slides against polished steel with a friction coefficient of roughly 0.30 to 0.45. PTFE-filled grades typically measure 0.14 to 0.25, carbon fiber reinforced grades 0.18 to 0.30, and multi-component bearing grades 0.08 to 0.20 under dry conditions. Exact values depend on load, speed, and counterface finish.

Which test standard should be chosen?

Pin-on-disk testing per ASTM G99 is suitable for general material screening, block-on-ring testing per ASTM G77 or GB/T 3960 better simulates bushing-type line contact, and reciprocating testing per ASTM G133 reproduces seal-like oscillating motion. ASTM G115 provides overarching guidance for measurement and reporting regardless of configuration.

Why do static and kinetic friction coefficients differ?

The static friction coefficient is measured at the moment motion begins and is usually higher, while the kinetic coefficient is measured during continuous sliding. A large ratio between the two indicates stick-slip tendency, which is undesirable in precision sliding systems and can be mitigated by PTFE or graphite modification.

Why do reported values vary between laboratories?

Friction is a system property sensitive to counterface material and roughness, contact geometry, load, speed, temperature, humidity, and running-in duration. Comparisons are only valid when all of these parameters are matched, which is why full disclosure of test conditions per ASTM G115 is essential.

Can testing be performed at elevated temperature?

Yes. Modified PEEK retains usable mechanical properties up to approximately 250 to 260 °C, and friction testing can be conducted in environmental chambers at 150 to 250 °C to qualify grades for hot-running bearings and sealing applications.

What sample sizes are required?

Typical requirements are flat-ended pins of 4 to 10 mm diameter, blocks of approximately 10 × 10 × 20 mm or per ASTM G77 dimensions, and flat coupons of at least 25 × 25 mm for reciprocating tests. Machined surfaces should be finished to a consistent roughness, and fiber orientation should be recorded for reinforced grades.