Compare materials and pick the pair that won't gall

Adhesive wear — galling — isn't the surface slowly wearing away. It's cold welding: under pressure, two metal surfaces of similar composition bond at the contact point. When motion continues, metal is torn from one and migrates to the other.

The result is that deep, shiny score no polishing removes, and which only gets worse — the transferred burr becomes the tool that tears the rest.

It's the most expensive confusion in this field. Two H13 at 52 HRC gall against each other just as readily as two H13 at 30 HRC — perhaps more, because contact pressure is higher before any deformation relieves it.

What decides is the chemistry: identical atomic structures weld, different ones don't. That's why a bronze bushing against a steel shaft has worked for a century, with no treatment on either side.

The toolroom reference for a sliding surface is 10 HRC of difference. Looser specifications accept 2 to 4 HRC when one side is nitrided.

The reason isn't hardness in itself: with equal hardness both surfaces yield at the same instant and key into each other. With a difference, the harder one wears the softer — and the softer becomes the sacrificial part, wearing predictably and getting replaced.

Worth insisting on, because it's where most people go wrong. Nitriding hardens a great deal — past 1000 HV — but the surface is still steel. Against the same steel, the chemical affinity remains untouched.

A ceramic PVD (CrN, TiN, DLC) does something else: the surface stops being metal. With no metal-on-metal pair, the adhesive mechanism has nowhere to start. It's the strongest mitigation there is for this problem — and coating one side usually does it.

In nitriding, the white compound layer is hard and excellent against wear. And it's brittle. Where there is thermal fatigue or impact — an insert that cycles, an aluminium die casting mold — it cracks and flakes, and the crack carries on inward.

When ordering nitriding, say whether you want the white layer or only the diffusion zone. The heat treater does both; they just don't know which is your case.

It's the most common confusion about expansion, and it inverts the whole design. A hole in a heated body grows, as if it were solid material: the metal around it pushes away from the centre. The clearance opens when you heat only the outer part.

Designing on the belief that the bore closes ends up leaving too much clearance — and the ejector guides poorly, makes flash and goes out of line. The error is the mirror image of seizing, and harder to diagnose.

A 4 µm CrN on the radius takes 8 µm off the diameter of each coated side. If both sides are coated, that's 16 µm — in a 20 µm clearance, almost all of it.

The number disappears from the design of anyone who treats expansion and coating as separate subjects. Either the clearance is opened up beforehand, or the part is machined undersize by the layer; what doesn't work is coating afterwards and hoping it fits.

Contact pressure, speed, lubrication, alignment, working temperature and clearance. None of it goes in here — and all of it weighs on the real outcome.

What the tool does is what an experienced designer does from memory and forgets when in a hurry: check that both sides aren't the same steel, that there's enough hardness difference, and that someone broke the affinity. It's a weighted checklist, not a simulation.