Plasma Nitriding

**Plasma nitriding** (also called **ion nitriding** or **glow-discharge nitriding**) is a vacuum-based surface-hardening process in which nitrogen ions are accelerated onto a workpiece surface by a DC electric field in a partial-vacuum chamber. It is the modern industrial replacement for toxic cyanide-salt Salt Bath Nitriding. ## Process 1. Place workpiece in a sealed vacuum chamber. 2. Pump down, then backfill with a controlled nitrogen + hydrogen mixture at **50-600 Pa** (~0.4-4.5 Torr). 3. Heat workpiece to **480-580°C**. 4. Apply several hundred to 1000 V DC (often pulsed) between the chamber wall and workpiece. 5. The electric field ionises the gas, producing a visible glow discharge (plasma). 6. Nitrogen ions accelerate toward the negatively-charged workpiece, embed in the surface, react with iron to form nitrides. 7. Hold for several hours depending on desired case depth. 8. Cool in vacuum or gas. ## The pressure window is critical Plasma nitriding works only within **50-600 Pa**. Outside this range: - **Too low pressure (deep vacuum)**: electrons and ions travel long mean free paths before interacting. Plasma fills the entire chamber rather than concentrating near the workpiece. Energy dissipates into chamber walls, workpiece gets relatively little treatment. - **Too high pressure**: short mean free paths, plasma density increases, current and heat rise. Thermal load can exceed chamber cooling capacity. Also, surface bombardment gets too intense, damaging the workpiece. ### Visual diagnosis (Justin Atkin's useful tip) - **Plasma fills entire chamber**: pressure too low. Retro-fill with more gas. - **Plasma hugs the workpiece as a bright conformal glow**: pressure in correct range. This is what you want. Justin's home plasma-nitriding attempt failed because his vacuum chamber was designed for **sputter deposition** — a process that wants much deeper vacuum (below 1 Pa). When operated at the higher pressure plasma nitriding requires, his system couldn't handle the thermal load from the higher plasma density and current. ## Gas composition Industrial plasma nitriding uses **N₂ + H₂** gas mix, typically in a 3:1 to 1:3 ratio depending on desired nitrided phase composition. Hydrogen: - Reduces native iron oxide on the workpiece surface (removing the passive layer that blocks nitrogen uptake). - Stabilizes the plasma. - Scavenges residual oxygen from the chamber. Some processes switch to O₂ at the end for black oxide finish (corrosion resistance + distinctive dark appearance). ## Advantages over salt bath - **Clean**: no toxic waste salts, no HCN or NOx fumes. - **Glossy surface finish**: much nicer than the rough salt-bath surface. - **Precise process control**: gas mix, temperature, voltage, time all tunable. - **Selective masking**: areas mechanically covered don't get nitrided. Salt bath nitrides everything. - **No quenching**: like salt bath nitriding, hardness develops from the nitride formation itself. - **No environmental disposal problem**: used gases vent, no hazardous waste. ## Disadvantages vs salt bath - **Much higher capital cost**: vacuum chamber, vacuum pumps, HV power supply, gas supply, temperature control. A basic industrial plasma nitriding furnace starts in the low six figures; salt bath kit can be assembled for under $1000. - **More complex to operate**: requires process engineering, not just 'drop in and wait.' - **Not home-accessible**: very few hobbyists have the full equipment stack. ## Industrial use Plasma nitriding is now the dominant industrial process for: - Automotive crankshafts, gears, cams - Injection molding dies - Hydraulic components - Firearm barrels - Many high-value wear components It has largely replaced cyanide soft nitriding in automotive manufacturing specifically because of the toxicity history. ## Home-accessible alternatives For hobbyist-scale work: - Salt Bath Nitriding (KNO₃/KCl, 650°C, alumina crucible) is the most accessible nitriding option. - Case Hardening and Cementation Steel works for carbon-steel and mild-steel parts at the kiln-and-charcoal level. - Gas nitriding with ammonia is technically home-possible but logistically harder than either of the above. ## Related - Steel Crystal Structures: Austenite, Ferrite, Martensite — the phase-diagram context. - Case Hardening and Cementation Steel — carbon-based alternative for non-stainless steels.