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Lesson 08·Surface treatment

Surface treatment sequencing: nitride, PVD, polish, in what order

The build order that takes an H13 forge die from rough block to coated, polished, ready-to-run, and why getting any step out of sequence is a failure mode rather than a slow ramp to nominal life.

9 min readLesson 8 of 13

Tying it together

What this means on the shop floor

Aluminum forge dies in H13 get the full duplex sequence. Plasma nitride to 0.20-0.30 mm with a compound layer of 6-8 µm. Polish step two takes the compound layer to 3-5 µm. AlTiN deposited at 3-5 µm. Skipping the nitride and going straight to PVD is the most common mistake on aluminum work and the source of the early-spall failure pattern.

Steel hot-forge dies running below 80,000 cycles get nitride only. Premium H13 (ESR) at 48 HRC, plasma nitride to 0.40 mm, compound layer 5-8 µm γ'-dominant, no PVD.

Hot-trim dies on scaled billet get duplex when sliding wear against oxide debris is the binding constraint.

Impact-dominated drop-forge dies get nitride only, thick diffusion zone, thin γ' compound layer, no PVD. The coating chips off under impact regardless of the substrate underneath.

Pushback questions for the vendor

  1. What is the compound layer thickness target on the cavity, and is the polish step two removal budgeted into that target?
  2. If we are quoting duplex, what is the failure mode you are targeting with the PVD layer, and is the substrate hard enough at the coating interface to prevent eggshell collapse?
  3. What is your masking plan for the parting line and reference surfaces, and is masking specified on the drawing or only in the routing?
  4. What is the cleanliness handoff between the nitride house and the coater, and who owns the part during the gap?

Common confusions

PVD without nitride is not a faster duplex. The PVD is sitting on an eggshell and the die fails by spalling, not by wear.

"Polish to mirror finish" is not a nitride spec. Specify Ra targets and material removal limits in microns. A polisher chasing visual finish removes whatever depth the surface needed, which on a nitrided cavity often means breaching the compound layer.

A nitride compound layer is not a coating. It is a metallurgical case grown into the substrate. Treating it as a coating in the routing leads to spec errors, such as "remove the coating before PVD" when the correct call is "polish the compound layer to 3 µm before PVD."

Duplex is not "more is better." It is targeted protection against a specific wear mode. If the failure mode is thermal-fatigue cracking, duplex does not extend life and may shorten it.

Up next: polishing and surface finish.

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