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Plasma nitriding or ion nitriding is a plasma supported thermochemical case hardening process that increases surface hardness, wear resistance, and steel fatigue resistance.It also improves the sliding properties of the surface. Steel fatigue increases mainly due to the development of superficial compressive stress.
The option of forming a diffusion layer without a compound layer is often used in plasma nitriding prior to the application of PVD or CVD coatings or on die-casting tools where thermal fatigue of the material is present. During nitriding in plasma, the diffusion and compound layer can be controlled very precisely, as the proportion of the individual phases of the layer depends on the composition of the gas mixture in the nitriding phase.
Applications
Typical applications include gears, crankshafts, camshafts, valve parts, die-casting tools, forging tools, cold-forming tools, mould-forming tools, long shafts, axles, engine parts.
The process of nitro carburisation increases surface hardness and improves wear resistance. As the process takes place at temperatures below 590°C, it allows for little to no dimensional distortion after the process and an unchanged condition of the improved core structure.
The thin layer on the surface of the nitrided part is called a compound or white layer. It consists of intermetallic compounds of nitrogen and iron nitride ε and nitride γ`; its thickness can range from 0.005 mm to 0.025 mm, depending on the selected cycle parameters and the base material, and its hardness from 600 to 800 HV05, depending on the material being treated.
High-alloy steel with nitride-forming elements such as Cr, Mo, Ti, V, and Al will achieve a greater hardness of the nitrided surface than ordinary carbon and low-alloy steel.
Applications
Post-oxidation is a combination of plasma nitro carburisation and oxidation. This process enables good corrosion protection along with good friction and sliding properties of tools and components. Combined treatment is an environmentally friendly alternative to conventional corrosion protection procedures.
Plasma is very effective in removing the passive layer of chromium oxide that naturally forms on the surface of stainless steel – it is crucial to select a process in which steel does not lose its stainless properties by preventing the formation of CrN and CrC. The process for hardening the surface can be done by cementing or nitriding the expanded austenite S-phase or C-phase.
The steel surface will be saturated only with nitrogen or carbon and will form an expanded austenitic layer with greater corrosion resistance and a thickness from 2 to 45 micrometres with extremely high hardness (more than 1000 Vickers). Resistant to cavitation and erosion as well as to tribological or corrosive interactions. Suitable for the energy and nautical industry; the surface treatment is extremely beneficial as it helps to prevent corrosion.
| Layer properties | Plasma nitriding | Nitrocarburisation |
|---|---|---|
| Temperature | 300-450°C | 300-550°C |
| Layer thickness | 2-15µm | 5-45µm |
| Layer homogeneity | Poor | Very good |
| Layer structure | S-phase | C-avstenit |
| Corrosion resistance | Good | Good |
| Surface hardness | 1200-1600 HV02 | 900-1100 HV02 |
| Hardness profile | Sudden reduction | Gradual reduction |
| Abrasion resistance | Very good | Good |
| Layer toughness | Poorer | Very good |
