Plasma nitriding is a thermochemical heat treatment process aimed at increasing the surface hardness of materials. In this method, nitrogen is diffused into the peripheral zone of iron-based alloys. The process is carried out in an ionized gas atmosphere and leads to increased protection against abrasive, adhesive and corrosive wear as well as an improvement in the fatigue strength of the material.
How plasma nitriding works
Plasma nitriding and plasma nitrocarburizing use a gas mixture of nitrogen, hydrogen and optionally a carbon providing gas as nitriding medium. This modern thermochemical low-temperature process applies a high voltage between the charge and the wall of a vacuum furnace. The nitrogen-hydrogen gas mixture is ionized by means of a glow discharge. A plasma is formed which wraps itself around the component true to contour. When the positively charged ions hit the surface of the component, nitrogen-rich nitrides are formed. As soon as these decompose, the surface of the workpiece is enriched with atomic nitrogen and, depending on the steel grade, diffused up to 0.8 mm deep into the edge zone. If only individual areas of the machined part are to be hardened, the remaining parts can simply be covered.
In contrast to salt bath and gas nitriding, the working temperature of plasma nitriding can be considerably lowered because the plasma has a high energetic effect. Thus, only temperatures between 350 and 600 °C are used. For this reason, plasma nitriding can also be used for distortion-sensitive materials and components. The wide temperature range of plasma nitriding also allows applications in a very wide spectrum. By varying the gas mixture it is possible to process different steel surfaces and achieve varying hardness profiles. In plasma nitriding, it is possible to nitrate without compound layers by varying gas quantities and electrical parameters. In contrast, a carbon-containing gas is added to the process to remove thicker compound layers. This is called plasma nitrocarburizing.
Plasma nitriding can be applied to a very wide range of materials. The process is therefore suitable for all ferrous materials and numerous steels. These include high porosity sintered steels, high-alloy austenitic and martensitic steels for tools containing more than 12% chromium and cast iron. In addition, plasma nitriding also works with stainless steels and nickel-based alloys. Titanium and aluminum alloys can also be treated with the process. The use of nitriding steels is particularly suitable for large machine parts that are exposed to frequent and high loads. These steel grades have a specific chromium or aluminum content and can achieve a surface hardness of over 1000 HV through heat treatment with plasma.
The advantages of the process
Compared to gas nitriding, nitriding with plasma offers several advantages. For example, plasma nitriding makes it possible to create a hard surface layer even on high-alloy steels. This increases the resistance to friction welding and significantly improves the abrasion resistance and wear resistance of the materials. Since parts of the material to be treated can be covered in order to protect it from processing, the process is frequently used for components that are to have nitrided but also untreated areas.
The possibility of forming a diffusion layer without a compound layer is another advantage of plasma nitriding. For this reason, it can be used in advance for PVD/CVD coating. Thus, it is possible to subject the coatings as well as the hardness profiles to individual adaptation.
Further advantages of the process are:
- Due to the low temperatures during the treatment, the diffusion process is suitable for applications where the distortion should be as low as possible
- Plasma nitriding is one of the most environmentally friendly hardening methods as no toxic gases are used in the treatment
- The higher costs for the construction of the required equipment are amortized by the significantly lower process costs compared to gas nitriding