Plasma nitriding (plasma nitriding) is a plasma-assisted thermochemical hardening process that improves wear resistance, surface hardness, and fatigue strength by forming a hard layer containing compressive stresses. Another interesting solution to consider is plasma Ion Nitriding. This is a method that guarantees the even formation of nitrides that reflect the shape of the workpiece without a „white” layer.
Benefits of plasma nitriding
The advantages of the gas nitriding process can be surpassed by plasma nitriding. Especially for high-alloy steels, plasma nitriding provides high surface hardness, thus improving resistance to wear, abrasion, galling and galling. The increase in strength is mainly due to the generation of surface compressive stresses. Plasma nitriding is a good choice when a part must have both nitrided and softened areas. The ability to create a diffusion-free composite layer is often used in plasma nitriding before applying PVD or CVD coatings. This is because customized layers and hardness profiles can be obtained.
Plasma nitriding – applications
Typical applications include gears, crankshafts, camshafts, cam lifters, valve parts, extruder screws, die casting tools, forging dies, cold forming tools, injectors, plastic forming tools, long shafts, axles, couplings, and engine parts. Plasma nitriding and plasma nitriding are generally superior to corresponding gas processes requiring masking. Plasma nitriding is suitable for all ferrous materials but also for sintered steels, cast ion nitriding, and high-alloy tool steels with high porosity, even with chromium contents above 12%. Stainless steels and nickel-based alloys can be plasma nitrided and retain most of their corrosion resistance at low temperatures. Plasma nitriding of titanium and aluminum alloys is a unique application. For heavy loads on large machine parts, such as shafts and spindles, nitriding with special chromium and aluminum steels is highly advantageous, as plasma nitriding produces surface hardnesses over 1,000 HV.
Details of the plasma nitriding process
Plasma nitriding is a modern thermochemical process in a mixture of gases that releases nitrogen, hydrogen, and (optionally) carbon. During this low-pressure process, stress is applied between the charge and the furnace walls. A high ionization glow discharge (plasma) is generated around the component. On the surfaces where the ions are directly charged, nitrogen-rich nitrides are formed and decompose, releasing reactive nitrogen to the surface. This mechanism allows shielding to be easily achieved by covering the relevant area with a metal blanket. Plasma nitriding makes it possible to modify the surface according to the desired properties. By adjusting the gas mixture, customized layers and hardness distributions can be obtained: from surfaces without low-nitrogen composite layers up to 20 microns thick to carbon gas composite layers with high nitrogen content (plasma nitriding). The wide temperature range used makes many applications beyond the capabilities of gas or salt bath processes. One of the most significant advantages of plasma heat treatment over controlled atmosphere heat treatment is its lower environmental impact. For example, ammonia is commonly used for nitriding in controlled atmosphere furnaces. In plasma nitriding, on the other hand, steel can be nitrided with nitrogen and hydrogen. In addition, plasma nitriding heats only the workpiece and does not require heating the entire interior of the furnace, which is necessary for an atmosphere-controlled furnace.
Advantages of plasma nitriding
1. Plasma nitriding is much faster than other conventional nitriding techniques.
2. Proper control of temperature, atmosphere composition, and discharge parameters can result in excellent microstructure and better control of the final product’s surface composition, structure, and properties.
3. Plasma nitriding is harmless to the environment.
4. Unlike conventional nitriding methods, the process can be carried out at temperatures up to 350°C. Low-temperature nitriding makes it possible to achieve high surface hardness while maintaining the high core strength of low-temperature hardened steel. In addition, processing at such low temperatures minimizes distortion.
There are several disadvantages to the plasma nitriding process:
– cleanliness of component surfaces is critical to prevent the formation of unstable arcs during heating cycles,
– parts require repair to avoid overheating,
– due to the power/area relationship, components of similar size cannot be plasma nitrided in one batch,
– high initial plasma cost.
Plasma nitriding: the process
Plasma nitriding (also known as plasma nitriding, pulsed plasma nitriding, cold nitriding, or plasma hardening) is a thermochemical heat treatment process used to improve the reliability and wear resistance of mechanically stressed metal parts. Treating the surface in a particularly gentle manner improves the fatigue strength and corrosion protection of the material. Under the influence of heat, plasma nitriding causes a chemical transformation of the surface layer through the diffusion of nitrogen, which forms nitrides with the workpiece material. This increases surface hardness and a significant improvement in wear resistance. Compared to the conventional hardening process, the workpiece is machined at a much lower temperature, which ensures high dimensional accuracy in this heat treatment. As a result, the costly post-processing of carburized workpieces is no longer necessary or can be reduced to a minimum, so plasma nitriding can further save the process chain. Raw materials can often be produced to the final size in a soft state and made after plasma heat treatment with little or no post-processing. In addition, heat-treated steels with shallow tempering temperatures can be processed without loss of core strength. In principle, various processes can be used for nitriding. In addition to plasma nitriding, bath nitriding and gas nitriding are well known. Among quenching processes, plasma nitriding is unique because of its reproducibility, environmental friendliness, and energy efficiency.
The physical principle of plasma nitriding
Plasma nitriding is a vacuum-assisted process. The workpiece forms the cathode, and the furnace wall forms the anode. After emptying the charge tank, an electric field is applied between the workpiece and the furnace wall. The supplied process gas is cracked and ionized in the electric field. It forms a conductive gas – plasma. When the current flows to the cathode, the nitrogen ions therein are accelerated and strike the surface of the workpiece with high energy. The effects of the above process are as follows:
– the surface of the atoms is thoroughly cleaned,
– passivation layers (e.g., on stainless steel or stainless steel and titanium) are dissolved,
– surface activation occurs,
– heating of the nitriding charge occurs,
– nitrogen diffuses to the surface of the workpiece.
Once the processing temperature is reached, the holding time begins. This depends on the type of material and the required depth of hardness for nitriding. For plasma nitriding, this time is usually 12-50 hours. Compared to gas nitriding, plasma nitriding requires only about half the holding time. After a suitable processing time, the pressure is equalized by filling with gas. The charge is then cooled and controlled, and the finished part can be removed at a low temperature or in salt bath nitriding. Analyzing the above processes, let’s keep in mind what Ion Bombardment is, which avoids changes in the structure of the polymer in the mass by causing changes only in its surface layer, but also causes surface hardening.