What is Plasma Nitriding? The Process explained

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 material surface, 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 the process 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, ion 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.

Applications

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, the heat treatment also works with stainless steel, while maintaining most of the corrosion resistance, 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 benefits of plasma nitriding

Compared to gas nitriding, nitriding with plasma offers several advantages. For example, plasma nitriding process makes it possible to create a hard surface layer even on high-alloy steels. This improves the surface properties and 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

Plasma Nitriding vs. Other Surface Hardening Methods

Plasma Nitriding vs. Gas Nitriding

Gas nitriding is another surface hardening technique where ammonia gas decomposes to release nitrogen, which diffuses into the metal surface. While both methods introduce nitrogen, there are key differences:

• The plasma technique offers more precise control over case depth and composition, whereas gas treatment results in deeper but less uniform hardened layers.
• Plasma-assisted processes are conducted at lower temperatures, reducing the risk of part distortion. Gas nitriding typically occurs at higher temperatures, which can lead to dimensional changes.
• Plasma technology is more environmentally friendly, as it does not involve ammonia gas, which produces waste by-products.
• It allows for selective hardening of specific areas by masking parts of the workpiece, a feature not easily achieved with gas methods.

Plasma Nitriding vs. Salt Bath Nitriding

Salt bath nitriding involves immersing components in a molten salt bath containing nitrogen-releasing compounds. While this method provides good wear resistance and corrosion protection, it has several drawbacks:

• Plasma-based processes do not produce toxic salts or hazardous waste, making them the cleaner option.
• Salt bath treatment can lead to excessive surface porosity, requiring additional finishing processes, whereas plasma methods achieve a smooth and uniform surface.
• The plasma approach allows for greater flexibility in treating complex geometries, while salt bath nitriding can have limitations with intricate components.

Plasma Nitriding vs. BorTec Solutions

BorTec provides alternative surface hardening solutions such as Borinox® and Borocoat®, but these are not nitriding methods. Instead, they are diffusion-based processes that improve wear resistance, corrosion protection, or both, depending on the application.

Plasma Nitriding vs. Borinox®

Borinox® is a proprietary diffusion hardening process for stainless steels and nickel-based alloys. Unlike nitriding, which can lead to chromium nitride formation and reduced corrosion resistance, Borinox® maintains or even enhances corrosion protection while increasing hardness.

Key differences:

• The plasma process can reduce the corrosion resistance of stainless steels due to chromium nitride formation. Borinox® hardens stainless steels while preserving their corrosion resistance.
• Plasma nitriding achieves hardness levels of up to 1000 HV, whereas Borinox® can reach up to 1500 HV.
• Borinox® forms a uniform diffusion layer without creating a brittle compound phase, whereas nitrided surfaces may require post-processing to optimize properties.
• Plasma-based methods are ideal for general industrial applications, while Borinox® is better suited for industries requiring both wear resistance and corrosion stability, such as medical and food processing industries.

Plasma Nitriding vs. Borocoat®

Borocoat® is a boronizing process that forms extremely hard boride layers on the surface of steel, stainless steel, and other materials. Unlike nitriding, boronizing introduces boron atoms into the material, creating highly wear-resistant surfaces.

Key differences:

• Plasma methods improve wear resistance but do not match the extreme hardness of Borocoat®, which achieves up to 2600 HV.
• Borocoat® is particularly suited for components subjected to extreme abrasive wear, such as mining tools, cutting edges, and high-friction surfaces. Plasma-based processes are better for components requiring both wear and fatigue resistance.
• Plasma treatment is commonly used on high-alloy steels and tool steels. Borocoat® can be applied to a wide range of materials, including stainless steels, cast irons, and certain nickel and cobalt alloys.
• Unlike nitriding, boronizing forms a boride diffusion layer that offers superior resistance to abrasive wear.

When to Choose Plasma Nitriding?

This process is an excellent choice when components require a balance of hardness, wear resistance, and fatigue strength. It is especially suitable when:

• A controlled hardening process with minimal distortion is needed.
• Selective hardening of specific areas is required.
• The material is a high-alloy or tool steel that needs enhanced wear resistance.
• A clean, environmentally friendly process is preferred.

This technique is widely used in aerospace, automotive, and industrial applications where high surface durability is required.

When to Choose Plasma Nitriding?

This process is an excellent choice when components require a balance of hardness, wear resistance, and fatigue strength. It is especially suitable when:

• A controlled hardening process with minimal distortion is needed.
• Selective hardening of specific areas is required.
• The material is a high-alloy or tool steel that needs enhanced wear resistance.
• A clean, environmentally friendly process is preferred.

This technique is widely used in aerospace, automotive, and industrial applications where high surface durability is required.

Limitations and Alternatives

Although this method is highly effective, it has limitations, particularly when extreme hardness or corrosion resistance is required.

• For stainless steels and nickel-based alloys, Borinox® is a better alternative, as it provides high hardness without reducing corrosion resistance.
• For applications requiring extreme wear resistance, Borocoat® is the superior choice, offering significantly higher hardness and durability against abrasive conditions.

Choosing the right surface hardening process depends on the material, operating environment, and performance requirements. In cases where hardness, corrosion resistance, or extreme wear protection are essential, BorTec’s Borinox® and Borocoat® provide advanced alternatives.

For expert recommendations on the best hardening solution for your application, consult BorTec’s specialists.