Maintenance options: polish, repolish, weld repair, re-nitride, re-coat, scrap

Re-coat is the right call when the substrate, diffusion zone, and compound layer are still in good condition, but a PVD overcoat has worn through in localized regions. The pattern is bare nitrided steel visible under the coating wear, no propagating cracks, dimensions in spec, and surface hardness on the still-coated areas above process minimum.

The trap with re-coat is the prep. PVD coatings need a clean, dimensionally correct, metallurgically sound substrate to bond to. On a die coming back from production at 50,000 hits, that substrate is rarely there without engineering effort. Residual coating has to be stripped chemically or by light blast. Surface roughness has to be returned to the coating-vendor's spec, which typically means polish back to roughly Ra 0.2 to 0.4 µm on the contact surfaces. Any localized galling or pickup has to be polished out without removing meaningful dimension. The compound layer, if it is still there, has to be intact or rebuilt.

The engineering effort to prep a die for re-coat often costs more in shop labor and machining time than the coating itself. The coating cycle is hours in a PVD chamber. The prep is days of skilled labor. A re-coat quote that comes in cheap usually means the vendor is skipping prep steps, and the coating fails by delamination within the first 5,000 to 10,000 hits.

Life extension on a properly re-coated die is 30 to 100% of the original PVD's contribution. Variance is driven by how good the prep was and whether the underlying nitride layer is still intact. If the nitride has thinned enough that the substrate cannot support the coating against impact load, re-coat fails the same way new PVD over inadequate nitride fails (the egg-shell-on-pudding failure mode covered in Course 3). In that case the right sequence is re-nitride first, then re-coat on the rebuilt case.