Polyoxymethylene (POM) Coefficient of Friction Against Steel: What the Number Really Measures

Polyoxymethylene (POM), supplied as either acetal homopolymer or copolymer, is a semicrystalline engineering thermoplastic used in sliding machine elements such as gears, cams, and bearing retainers. The data-sheet value frequently cited as “POM Coefficient of Friction Against Steel: What the Number Really Measures” is not a single interface constant. Under the most commonly referenced tribological test configurations—ASTM G99 pin-on-disc, ASTM D3702 thrust-washer, and ISO 7148-2 polymer bearing screening—the reported coefficient is the steady-state ratio of tangential friction force to applied normal load at the sliding interface. For unfilled POM against dry ground steel, commercial datasheets typically report a dynamic coefficient of friction in the range of 0.20 to 0.35, with static values from 0.25 to 0.40. Those ranges are meaningful only when accompanied by counterface roughness, pressure, speed, and run-in state. The number therefore measures the shear resistance of the developed interface, not the bulk polymer alone.

Does the Coefficient Reported on a POM Data Sheet Represent an Intrinsic Material Property?

It does not. A friction coefficient is a system response. The same unfilled POM homopolymer can return a dynamic coefficient below 0.20 after a stable transfer film has formed on a hardened low-roughness steel counterface, yet exceed 0.35 during low-speed stick-slip or against a roughened stainless steel surface. The polymer composition, crystallinity, and additives determine only part of the response; the remainder is set by the steel alloy, hardness, roughness, surface oxide, adsorbed moisture, contact pressure, sliding velocity, and interfacial temperature. Data sheets that present a single coefficient without specifying these conditions offer a comparator for material screening, not a design allowable. ASTM D3702 and ISO 7148-2 define standardized configurations but do not eliminate contact-geometry effects; a thrust-washer value cannot be transferred directly to a helical gear flank or an injection-molded sleeve bearing.

In a pin-on-disc apparatus operating with an unfilled POM pin against a through-hardened 100Cr6 or AISI 52100 steel disc at 0.1 m/s to 1.0 m/s and 0.5 MPa to 1.0 MPa nominal contact pressure, the friction trace typically passes through a high run-in phase before reaching a plateau. During the first several hundred metres of sliding, the friction force may fluctuate because the steel asperities plough through the POM surface and fragment polymer debris. The steady-state value is not simply POM-on-steel friction; it includes the force required to shear a thin POM transfer layer adhered to the steel counterface and to deform the near-surface polymer. If the counterface roughness falls below approximately Ra 0.05 µm, adhesion can increase and the trace may exhibit intermittent motion even at moderate speeds. If roughness exceeds Ra 0.4 µm, abrasive ploughing raises the reported coefficient and accelerates wear. The practical window for low and stable friction in dry POM against steel is therefore typically in the Ra 0.1 µm to Ra 0.2 µm range, although published data for specific grades can vary.

The ranges above are representative commercial datasheet values under dry, unlubricated conditions. They are not design inputs without the associated pressure, speed, roughness, and run-in state documented in the source test report.

A Transfer-Film Shear Measurement, Not a Bulk Polymer Constant

The steady-state dynamic coefficient for POM against steel is dominated by the cohesion and shear behavior of the POM transfer film. As sliding proceeds, oriented POM wear particles compact onto the steel surface and form a film that can range from submicrometer to several micrometers in thickness. The measured friction force then reflects the work required to shear this film at a rate controlled by sliding velocity and interface temperature. Because the film is polymer-rich, the reported value often resembles an internal POM shearing process after run-in. This explains why unfilled acetal, although not as low-friction as PTFE, can still reach a dynamic coefficient near 0.20 when the counterface is hard enough to avoid abrasive rupture of the film. If the steel is soft or the surface is roughened, the film cannot stabilize; the coefficient rises and the wear mechanism shifts from adhesive transfer to abrasion. The number, therefore, is a process indicator for transfer-film stability under the specific test geometry, not a fundamental surface-energy property of POM.

Filled POM grades change the reported coefficient mostly by altering the transfer film or the polymer shear strength. A 10–20 wt% PTFE modification reduces the dynamic coefficient to roughly 0.10–0.20 because PTFE forms a low-shear film. A 2–5 wt% molybdenum disulfide addition lowers friction moderately but can alter wear debris. Glass-fiber reinforcement increases the dynamic coefficient and can accelerate steel wear; its value in a bearing application is therefore not captured by a low friction figure alone. Silicone-modified grades migrate lubricant to the surface and can suppress initial stick-slip, but the effect depends on continuous migration and is temperature-sensitive. The bulk modulus of unfilled POM is approximately 2600–3200 MPa per ISO 527-2, but the friction coefficient is not proportional to modulus. Glass-fiber reinforcement raises flexural modulus and can also raise friction and wear of the steel counterface, breaking the intuitive assumption that a stiffer material always slides more easily.

On production injection molding lines, the frictional response of POM can shift between batches even when the base resin is unchanged. A mold temperature set at 90 °C to 100 °C with a melt temperature of 190 °C to 230 °C for copolymer grades produces a more uniform crystalline skin than rapid quenching in a 40 °C mold. Thinner frozen layers and high orientation near the gate can produce local variations in surface hardness and run-in friction. Mold release agents that remain on the part can temporarily reduce friction but are not a reliable design input. Compounding lines running acetal on a 40:1 L/D twin-screw extruder must maintain vent-port vacuum and avoid residence times that generate formaldehyde; thermal degradation not only weakens the bulk part but also changes friction by producing a brittle surface that wears rapidly. Melt flow rate, measured per ISO 1133-1:2022 at 190 °C with 2.16 kg load, is a processing indicator, not a friction predictor. Lower MFR acetal grades, typically 2–4 g/10 min for extruded rod and machined parts, may show better wear resistance due to higher molecular weight, but the friction coefficient under steady-state sliding often falls in the same range as higher MFR injection grades.

When Sliding Speed Moves the Interface Out of the Reported Regime

If a designer uses a dynamic coefficient of 0.20 to 0.30 from a room-temperature data sheet at 0.1 m/s and then applies it to a low-speed positioning mechanism operating below 0.01 m/s, the result can be misleading. At very low speeds, POM can exhibit stick-slip motion, and the instantaneous force peak may be closer to the static value or higher because interfacial junctions have time to strengthen. At high speeds, frictional heating can raise interface temperature beyond the POM melting onset and produce transient melt lubrication, which lowers the coefficient but also accelerates surface damage. The coefficient is therefore velocity-dependent. Reported values should be tied to a speed–pressure map, not a single point. ASTM G99 permits speed and load variation; unless the data sheet provides the full matrix, the reported coefficient cannot be extrapolated to boundary conditions outside the test window.

When the sliding speed is increased from 0.1 m/s to 1.0 m/s at constant load, the measured friction force for unfilled POM may pass through a maximum and then decline as the interface temperature rises. This is not a material improvement; it is a transition toward surface softening. Wear rate data collected under the same conditions often show a nonlinear increase above a temperature threshold. The coefficient alone cannot distinguish a stable transfer film from incipient melting. For that reason, laboratory programs that report friction without corresponding wear depth profiles or surface temperature logs provide incomplete tribological data. Designers of POM gear pairs and cams should record the friction trace over time, not the minimum plateau value, because the initial peak and the end-of-test stability are often more relevant to field behavior.

Pressure-Velocity Limits and Counterface Hardness in Acetal Bearing Design

The practical design boundary for dry unfilled POM sliding against steel is commonly expressed as a pressure-velocity limit. Published unfilled acetal data place the continuous dry PV limit near 0.07 MPa·m/s to 0.14 MPa·m/s at 20 °C to 25 °C; for short-cycle operation with frequent cooling, higher values may be accepted but at reduced life. The combination of high load and high speed creates frictional heating that can exceed the thermal stability of the polymer, causing surface melting, formaldehyde emission, and rapid loss of dimension. Counterface hardness is equally important. A through-hardened AISI 52100 or 100Cr6 steel at 58–62 HRC with roughness controlled to Ra 0.1–0.2 µm is typically used for stable transfer-film formation. Austenitic stainless steels can generate higher friction and galling because of lower thermal conductivity and passive oxide behavior; published data for POM against stainless steel is more variable than against hardened bearing steel. Lubrication with light mineral oil or synthetic grease can reduce the dynamic coefficient below 0.10, but the resulting system behavior is then dominated by the lubricant film thickness, not the dry polymer-to-steel value.

Moisture adsorption in POM is low, typically below 0.3% at 23 °C and 50% RH, but humid environments can still alter the friction trace by changing adsorbed water at the steel oxide layer. Acidic media at pH < 4 can hydrolyze acetal and produce roughened, high-friction surfaces. The friction number should not be used as a chemical compatibility indicator. A reported POM coefficient of friction against steel measures only the force ratio under a constrained tribological condition; operational limits for pressure, speed, counterface alloy, and chemical exposure must be evaluated separately.