IDB-WER-043
Tribology · wear · friction · PV limit
Wear & material pairing
Designing against wear — the wear mechanisms, friction and which material pairs to run together, the PV limit, and the surfaces and coatings that last.
Abstract
Wherever surfaces slide, roll or vibrate against each other, material is lost. Wear is not one thing — it has distinct mechanisms (adhesive, abrasive, fatigue, fretting, corrosive, erosive), and the fix depends on which one you have. The recurring levers are material pairing, hardness, surface finish, contact pressure and lubrication.
Section 1 covers the wear mechanisms. Section 2 is friction and material pairing. Section 3 is the PV limit. Section 4 is wear-resistant materials. Section 5 is coatings and surface engineering. Section 6 is selection and a checklist.
1.Wear mechanisms
Identify the mechanism before choosing a fix — Archard's law (V = k·F·s/H: wear volume ∝ load × sliding distance / hardness) sets the scale, but the type sets the remedy:
| Mechanism | Cause | Guard against it |
|---|---|---|
| Adhesive (galling/scuffing) | like surfaces cold-weld at asperities | dissimilar pair, hardness differential, lubrication, coatings |
| Abrasive | hard particles/asperities plough the surface | harder surface, filtration, exclude debris (seals) |
| Surface fatigue (pitting) | cyclic Hertzian contact (gears, bearings) | harder/cleaner steel, lower contact stress, EHL film |
| Fretting | tiny oscillation at "static" joints | raise preload, lubricate, coat, eliminate micro-motion |
| Corrosive / tribochemical | wear + chemical attack together | compatible materials, inhibitors, coatings |
| Erosion | impinging particles or fluid | harder/tougher material, redirect flow |
2.Friction and material pairing
The single biggest wear lever is what runs against what. Never slide identical metals dry (especially stainless on stainless or aluminium on aluminium) — they gall. Pair dissimilar materials, or create a hardness differential, or add a low-friction phase.
| Sliding pair (dry) | Typical µ | Note |
|---|---|---|
| Steel on steel | ~0.6 | galls — lubricate or change pair |
| Steel on bronze | ~0.3 | classic bearing pair |
| Steel on acetal (POM) | ~0.2 | quiet, self-lubricating-ish |
| Steel on nylon | ~0.3 | absorbs water, dims shift |
| Steel on PTFE | 0.05–0.10 | lowest friction; soft, low load |
| Steel on steel, lubricated | ~0.10 | a film changes everything |
Run a hard part against a softer, sacrificial one so wear is concentrated in the cheap, replaceable component.
3.The PV limit
For plain (sliding) bearings and bushings, frictional heat — not pressure alone — sets the limit, captured by PV (contact pressure × sliding velocity). Stay under the material's PV limit and check P and V individually:
| Material | Max P (MPa) | Max V (m/s) | Note |
|---|---|---|---|
| Oil-impregnated sintered bronze | ~14 | ~6 | self-lubricating |
| PTFE-lined metal (DU) | ~250 static | ~2 | dry/marginal lube |
| Acetal (POM) | ~10 | ~3 | quiet, moulded |
| Nylon (PA) | ~10 | ~3 | tough; water-sensitive |
| UHMWPE | ~7 | ~3 | low friction, chemical resistant |
(Consistent with the Bearing selection reference — use it for rolling-element wear/pitting.)
4.Wear-resistant materials
- Hardened / nitrided steelthe default for loaded, lubricated wear surfaces (shafts, gears, cams).
- Bronze / oil-impregnated bronzebushings, low-speed sliding, conformable.
- Engineering plasticsacetal (gears, low friction), UHMWPE (abrasion, food), PTFE-filled nylon (dry bushings), PEEK (high temp).
- Ceramicsextreme hardness/temperature, low wear, but brittle (pump seals, guides).
- Aim for a hardness differential between the pair and the hardest practical surface on the high-wear part.
5.Coatings and surface engineering
| Surface | Benefit |
|---|---|
| Nitriding | hard (~700–1000 HV), low distortion, fatigue + wear |
| Hard chrome | wear + hardness on shafts/cylinders |
| DLC (diamond-like carbon) | very hard, very low friction, thin |
| PVD (TiN, CrN, TiAlN) | hard tool/decorative coatings, thin |
| Thermal spray (WC-Co, etc.) | thick, rebuildable wear layers |
| Anodize type III (Al hardcoat) | hard, wear-resistant aluminium surface |
A fine surface finish plus a maintained lubricant film beats almost any coating for sliding wear — coat when finish and lube alone can't carry it.
6.Selection and checklist
- Identify the mechanism (table 1)the remedy depends entirely on it.
- Fix the pairdissimilar materials / hardness differential; never like-metals dry.
- Stay under PV for plain bearings; under the contact-stress limit for rolling (see Hertz).
- Finish + lubricatesmooth surfaces and the right lubricant (see Lubrication reference); exclude debris with seals/filtration.
- Add hardness/coating where finish and lube aren't enoughnitride, DLC, hardcoat.
- Make the cheap part wearconcentrate wear in a replaceable bushing/liner, not the expensive housing.