Plasma / ion nitriding — control, distortion, and the parts that need it

Parts are loaded into a vacuum vessel at 1-10 mbar, surrounded by a low-pressure mixture of N₂ and H₂ (no NH₃, no salt). A DC or pulsed-DC bias of 300-800 V is applied between the chamber wall (anode) and the part (cathode). The voltage ignites a glow discharge. Positive nitrogen ions accelerate toward the part surface, sputter the topmost atomic layers, and deliver active nitrogen species directly into the steel. Cycle temperatures run 380-560°C, cycle times 10-30 hours, and the operator independently sets four variables that gas and salt cannot independently set at all.

The compound layer comes out anywhere from zero to about 10 µm, by recipe, on the same equipment with the same substrate. Surface microhardness lands at HV 0.05 of 1000-1200 on tool steel, comparable to a good gas nitride. Diffusion zone depth runs 0.10-0.50 mm for typical tool-and-die cycles. The Rolinski analysis in Thermal Processing Magazine characterizes plasma as a low-Kn process by physics: low nitrogen partial pressure plus continuous sputtering naturally limits compound-zone formation, with values typically below 13 µm even before active tuning.