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

A typical compound layer on a forge-die nitride cycle runs 5-10 µm. The white layer is the load-bearing wear surface for most of die life. Polishing after nitriding removes material from that layer fast. A 6 µm diamond paste lap at moderate pressure pulls 2-5 µm per pass. Three passes and the compound layer is gone.

The build has to budget for this. If the cycle delivers 7 µm of compound layer and polish step two removes 3 µm, the die enters service with 4 µm of compound layer, which is the working spec. Specify the as-delivered compound layer 2-3 µm above the working target. Specify the polish removal limit on the routing in microns, not as "polish to remove glare." A polisher who removes 8 µm because the print said "polish smooth" has burned through the compound layer into the diffusion zone. The PVD then deposits on diffusion zone in some areas and bare substrate in others, and the coating adheres unevenly.

For dies that do not get PVD, polish step two is often skipped entirely. The compound layer goes straight into service. For dies that do get PVD, the polish is required to thin the compound layer to where it will not crack under the PVD interface load. Compound layers above 8-10 µm tend to crack under PVD deposition stress and during early service, taking the coating with them.