Wear-Resistant Nylon: Fillers and Lubrication Design

Nylon earns its place in moving parts because of an unusual combination: it is tough, it resists abrasion, and it self-lubricates well enough that many gears and bearings run dry for years. It also absorbs moisture, grows with temperature, and will melt its own sliding surface if the pressure-times-velocity product exceeds what the grade can handle.

Figure 1. PV limit curve for dry sliding nylon with wear additive comparison table.

Wear in a nylon part is never a material property alone. It is a system property: nylon against a specific counter-surface, at a specific load, speed, temperature and clearance. This guide covers the additives that improve it, the design variables that decide it, and how to validate a wear claim.

Nylon as a wear material

• PA66 — the highest strength, stiffness and wear resistance of the common nylons; also the most moisture-sensitive (dimensions and properties shift noticeably with humidity).
• PA6 — slightly tougher and easier to process than PA66, with similar moisture behavior.
• PA12, PA612, PA11 — lower moisture absorption, better dimensional stability and better performance in cold and wet conditions; lower strength and stiffness.
• PA46, PA4T, PA6T — high-temperature nylons with high crystallinity and excellent wear and creep resistance at elevated temperature.
• Cast (MC) nylon — very high molecular weight, excellent abrasion resistance and impact, used for large bearings, sheaves and wear pads where the part can be machined.
• Aromatic / semi-aromatic grades — higher stiffness and heat resistance for demanding bearing applications.

The universal nylon trade-off: more strength and wear resistance usually comes with more moisture sensitivity. Specify the resin against the environment, not just the load.

Wear additives and what they actually do

• PTFE (typically 10–20%) — lowers the coefficient of friction and reduces the transfer film, the thin layer of polymer that forms on the metal counter-surface and largely controls whether dry sliding is stable. The most common addition for low-friction, low-noise parts.
• Molybdenum disulfide (MoS₂) — a solid lubricant that performs well under high load and in dry conditions; often combined with PTFE.
• Graphite — effective in humid environments and at higher temperatures; less effective in dry, cold air.
• Silicone — improves slip and reduces break-in friction, but can be harder to paint or bond afterwards.
• Aramid fiber (e.g. Kevlar) — improves abrasion resistance at higher loads and temperatures; it also improves strength and creep.
• Carbon fiber — raises stiffness, creep resistance and thermal conductivity while helping to dissipate frictional heat, which is often the real limit; it is also thermally conductive in the wrong direction if the surrounding structure is sensitive to heat.
• Glass fiber — increases strength, stiffness and creep resistance substantially, and improves wear against hard counter-surfaces. The trade-off: glass-filled nylon is abrasive to a soft or unhardened metal counter-surface, so the mating part must be hardened or the wear will simply move.
• Impact modifiers — keep toughness when fillers are added; without them, a heavily filled nylon becomes brittle.

The practical rule: lubricant additives (PTFE, MoS₂, graphite, silicone) reduce friction and stabilize dry sliding; reinforcing fibers (glass, carbon, aramid) raise stiffness, creep resistance and load capacity. High-load parts usually need both.

The counter-surface decides the wear rate

Wear is a two-body phenomenon. For nylon running against metal:

• Hardness — the metal should be harder than the polymer by a wide margin, and hard-chromed or hardened surfaces last far longer.
• Roughness — a very smooth counter-surface risks adhesion and stick-slip; a very rough one abrades the polymer. A controlled roughness in the region of Ra 0.2–0.4 µm is a good starting point.
• Material pairing — hardened steel, hard-anodized aluminum, stainless and chromed surfaces are common; soft aluminum and mild steel wear quickly against filled nylon.
• Against another polymer — nylon against nylon or another polymer can wear well or catastrophically; get data or test.
• Transfer film — much of the performance depends on whether a stable polymer transfer film forms on the metal. Fillers change how quickly and how stably that film develops, which is why wear data must come from tests that run long enough to reach steady state.

Designing with PV limits

The controlling parameter for dry sliding is the product of contact pressure (P) and sliding velocity (V).

• Every material has a limiting PV, a limiting P at low speed (compressive/creep limited) and a limiting V at low pressure (heat limited).
• The practical limit is usually thermal: frictional heat has nowhere to go in a polymer with low thermal conductivity, so surface temperature rises until the material softens, smears or fails.
• Design to about 50–70% of the catalog limiting PV, because real parts have edge loading, contamination, misalignment and humidity that laboratory tests do not.

Practical methods to stay inside the limit: increase contact area, reduce velocity, improve heat dissipation (metal backing, thermally conductive filler, air flow), reduce coefficient of friction with a lubricant additive, and add an external lubricant where the application permits.

Clearances and moisture

• Allow running clearance for thermal growth and moisture swelling. PA66 can take up roughly 2–2.5% dimensional change in humid or wet conditions; PA12 and cast nylon are much more stable.
• Design the joint with backlash, not with a snug fit. A bearing or gear bore that starts tight will seize as the part absorbs moisture.
• Avoid fully constrained assemblies. Clamp a nylon part between rigid walls and the swelling becomes stress; stress plus heat equals creep and cracking.
• Compensate for wear-in. Allow a wear allowance in the running clearance so that the part still functions after the surface has bedded in.
• Condition before measurement. A dimension taken on a freshly molded PA66 part is not the production dimension. Measure at controlled humidity after equilibrium, or specify the conditioning state on the drawing.

Lubrication: sometimes needed anyway

• Dry running is a genuine benefit of nylon, and many applications should stay dry. Grease plus a dusty environment can create an abrasive paste.
• Some lubrication still helps at high load, in high-temperature applications, in low-speed high-torque gears, and where initial break-in would otherwise damage the surface. A compatible grease, a silicone-free lubricant for painting processes, or a filled grade with an internal lubricant are the options.
• Check compatibility. Some lubricants attack nylon or extract additives. Verify compatibility at temperature, and verify that the grease does not migrate.

Validating wear claims

• Thrust washer wear test — a standard tribology test for sliding wear at controlled pressure and velocity.
• Component-level testing — the only test that represents the real part. Run a gear pair or bearing at representative load, speed and temperature, for long enough to reach steady-state wear, and measure wear rate, temperature rise, noise and backlash growth.
• Thermal measurement — measure the sliding-surface temperature, not the ambient. Most wear failures are thermal failures first.
• Environmental conditioning — test after moisture conditioning and at the temperature extremes the application will see.
• Field correlation — compare accelerated results against existing field data before extrapolating a lifetime.

Figure 2. Quick reference: the key checks and reference values from this guide.

Checklist

• Resin chosen for the moisture and temperature environment, not only the load
• Lubricant additive specified where friction, noise or stick-slip matter
• Fiber reinforcement specified where stiffness, creep and load capacity matter
• Counter-surface defined: material, hardness and roughness
• Glass-filled grade paired with a hardened counter-surface
• Application PV calculated and compared to the limiting PV with a safety factor
• Heat dissipation path considered; frictional heat not trapped
• Running clearance allows for thermal growth and moisture swelling
• Bearing or gear bore starts with designed backlash, not a snug fit
• Lubrication decision made deliberately, with compatibility verified if used
• Dimensions specified and measured in a defined conditioned state
• Component-level wear test run to steady state with surface temperature logged

Wear-resistant nylon is a system design, not a material swap. Choose the grade for the environment, add the right lubricant and reinforcement package, pair it with a hard and correctly finished counter-surface, stay below the PV limit with margin for real-world conditions, and allow the clearance the material's own moisture behavior demands. Then test the actual part at load, speed and temperature long enough for steady-state wear to appear — because that is the only number worth designing to.