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

Gas nitriding gives the operator two real knobs: temperature and atmosphere (Kn, set by NH₃/H₂). Plasma gives four.

Temperature. 380-560°C, set by the bias-induced ion bombardment and supplemental resistance heat. Lower temperature means less thermal distortion and a wider margin below the substrate temper. The case takes longer to grow at the low end; this is a time trade, not a hardness trade.

Gas ratio. N₂:H₂ from 1:1 to roughly 1:5. Hydrogen-rich mixes (more H₂) suppress compound-layer growth and shift the phase toward γ' or pure diffusion zone. Nitrogen-rich mixes grow ε. Methane or CO₂ can be added in trace amounts for plasma nitrocarburizing.

Bias voltage. 300-800 V. Higher bias accelerates ions harder, increases sputter rate, raises part temperature, and densifies the case. Modern pulsed plasma cycles the bias on and off in microsecond windows to control heat input on complex geometries.

Time. 10-30 hours typical. Cycles are shorter than gas because diffusion at the surface is driven by an active ion flux rather than a thermal-cracking equilibrium.

Each lever moves the result without forcing the others to follow. Lower the temperature 30°C, raise the H₂ fraction, hold time constant, and the compound layer thins without losing diffusion-zone depth. The same change in a gas cycle would require running the Kn control loop off baseline and would not deliver the same independence.