Failure modes — white-layer spall, network cracking, distortion, embrittlement

The most common nitriding failure on impact and high-contact tooling. Visual: micro-flakes departing the surface in the highest-stress zones, often following the contact pattern. Under 10-30× optical, the flaked area shows a clean step from compound layer down to diffusion zone, with the flake itself (when recovered) showing iron-nitride on one face and a slightly oxidized fracture on the other.

Microsection: cracks running parallel to the surface within the compound layer, or detaching cleanly at the compound-layer/diffusion-zone interface. Etched with 2-3% Nital the compound layer reads white, the diffusion zone reads gray, and the crack plane is unmistakable.

Three root causes drive almost every white-layer spall.

Compound layer too thick for the load. On impact tooling, a compound layer above roughly 15 µm becomes brittle enough to crack under shock and shed. The Lee et al. thermal-fatigue study on plasma-nitrided H13 quantified this directly: thicker compound layers initiated surface cracks faster than thin or compound-layer-free cases.

Wrong phase. A mixed ε/γ' compound layer with a sharp internal phase boundary cracks along that boundary, because ε and γ' have different lattice parameters and thermal expansion coefficients. The Mittemeijer group's work on compound-zone phase composition and the Surface and Coatings Technology paper on phase/diffusion-zone interactions both show this clearly. A single-phase compound layer (either all γ' or all ε) survives where a mixed layer fails.

Porous channels in an ε-rich outer zone acting as crack initiators. The porous zone is normal in ε formation, but on impact tooling the pores are stress concentrators. Specify a denser γ'-dominant layer, or cap the porous-zone depth at 30% of total compound thickness.

The fix is rarely "more nitride." It is the opposite: thinner compound layer (5-10 µm), single-phase target, FNC if ε is genuinely wanted for sliding wear, γ' if not. The trade-off has been studied in Advanced Materials Research on plasma-nitrided H13 wear, which found that thin multi-phase white layers wore worse than thick single-phase layers at both ambient and elevated temperature.