FNC vs nitriding — stop confusing them

Forge dies need a deep diffusion zone (0.4-0.8 mm) carrying the contact load and a thin or absent compound layer that will not spall under impact and thermal shock. FNC delivers neither. The compound layer is thicker than impact tooling tolerates, and the cycle is too short to build the deep diffusion zone an H13 forge die wants. Specify straight gas or plasma nitriding for forge work (Lessons 3 and 5).

Plastic mold cavities polished to a mirror finish are also wrong for FNC. The polish operation cuts through the compound layer in the polished regions and leaves it intact in the unpolished regions, producing a surface that wears unevenly and discolors over the run. Worse, the polished regions lose the corrosion resistance that the compound layer was supposed to provide. Spec straight plasma nitriding at low Kn for polished molds, with a compound layer thin enough to remove uniformly or absent entirely.

PVD substrate prep is the third wrong-fit case. PVD coatings bond to a hard, clean, metallic substrate. The FNC compound layer is porous on the outer surface, contains substantial carbon and nitrogen, and shears cohesively at the layer-substrate interface under coating stress. Lesson 7 covers the failure mode. The fix is to nitride (or FNC) first, then polish or lap off the compound layer down to the diffusion zone, then coat. A combined FNC+PVD process with no compound-layer removal step is a predictable delamination failure.