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

Duplex (nitride plus PVD) is the correct technical answer on three classes of forge work. First, aluminum forging, where adhesive wear and aluminum pickup are the dominant failure modes and AlTiN cuts pickup by 5-10x over nitride alone. Second, hot-trim dies on scaled billet, where the cutting edge sees sliding wear against oxide debris and the coating extends edge life 3-5x. Third, plastic injection of glass-filled or mineral-filled compounds, where the abrasive filler tears unprotected nitride surfaces fast.

Duplex is the wrong tool on most mid-volume hot-forge steel work. An H13 die at 48 HRC with a 0.40 mm plasma nitride case running 800-1200°C steel billet fails by thermal-fatigue heat checking, not by adhesive wear. PVD does not prevent thermal cycling cracks. The cracks initiate in the substrate and propagate up through the coating, sometimes faster than they would in nitride alone because the coating constrains thermal expansion at the surface and adds tensile stress on cooling. More layers do not change the failure mode the die is actually dying from.

The decision rule is failure-mode-first. If the cause of scrap is wear or pickup, PVD targets that mode and duplex extends life. If the cause of scrap is heat checking, gross cracking, or plastic deformation, PVD adds process steps without changing the failure mode and standard nitride is the right stop.