Plasma nitriding

Plasma nitriding

Plasma nitriding or 'ion' nitriding (sometimes also called plasma ion nitriding) or glow-discharge nitriding, is an industrial surface hardening treatment for metallic materials.

A plasma is the 'fourth' state of matter, the other three being solid, liquid and gaseous states. In the plasma state, matter exists in its excited form, also called the 'ionized' form, meaning the electrons in the outermost orbit have been knocked off. Thus a plasma usually is an ionized gas, and a mixture of neutral as well as charged particles.

There are hot plasmas typified by plasma jets used for metal cutting, welding, cladding or spraying. There are also cold plasmas, usually generated inside vacuum chambers, at low pressure regimes. Here the high temperature characteristics of the ionized gases are not used, but the electronic properties become more useful. Thus an ionized gas like nitrogen in such a low pressure regime becomes much more reactive. Thus surface treatment using the ionized nitrogen results in hardening of metals by two mechanisms :

* by diffusion, since the diffiusivity of the excited nitrogen atoms is higher -can be up to ten times faster compared with gas nitriding, forming the Diffusion Zone where precipitation hardening is present, and
* by thermo-chemical reactions which yield a thin layer of very hard iron and alloy nitrides, named Compound Layer or White Layer.

Usually steels, alloy steels etc. are very beneficially treated with plasma nitriding. Plasma nitriding advantage is related to the close control of the nitrided microstructure, allowing nitriding with or without compound layer formation. Not only the performance of metal parts gets enhanced but working lifespan gets boosted. So does the strain limit, and the fatigue strength of the metals being treated.

A plasma nitrided part is usually ready for use. It calls for no machining, or polishing or any other post-nitriding operations. Thus the process is user-friendly, saves energy since it works fastest, and causes little or no distortion. This process was invented by Dr. Bernhardt Berghaus of Germany who later settled in Zurich to escape persecution of his community by the Nazis in 1939. It was only after his death in late 1960s that the process was acquired by Klockner group and popularized world over.

Plasma nitriding is often coupled with PVD physical vapor deposition process and labelled Duplex Treatment, to avail of immensely enhanced benefits. Many users prefer to have a plasma oxidation step combined at the last phase of processing to generate a smooth jetblack layer of oxides which is very resistant to not only wear but corrosion.

Advantages of Plasma Nitriding
  • low temperature processing
  • shorter cycle time
  • better control on compound layer metallurgy and diffusion depth
  • uniform properties independent of component geometry and shape
  • low operating cost
  • clean and non-polluting

Process Capabilities
  • High Quality Nitriding
  • Nitrocarburising
  • Post Oxidation

Typical Components Processed
  • Crank shafts
  • Plastic injection machinery
  • Automobile parts

Plasma nitriding (also known as glow discharge or Ion nitriding) is a low temperature, low distortion surface engineering process. A glow discharge plasma is used to transfer nitrogen (and in the case of plasma nitrocarburising nitrogen and carbon) to the surface of the components undergoing treatment. Operating at temperatures between 400°C and 750°C these plasma processes can produce high surface hardnesses and hardened depths up to 0.8mm.

The plasma nitriding facilities at Telford comprise 2 hot wall pulse power plasma furnaces and 2 cold wall D.C. plasma furnaces. Capable of processing up to 1.8m and 2.8m high the furnaces are used to treat volume product whose requirements are not easily met by the conventional gaseous processes.

The components are made the cathode of an electrical circuit whilst the chamber becomes the anode, application of a voltage of 400V between the 2 electrodes at a pressure of 1-10 mbar establishes a current intensive glow discharge.This anomalous glow discharge covers all of the cathode supplying heat to the surface of the parts and a supply of nitrogen. Nitrogen is able to diffuse into the surface where it combines with nitride forming elements such as chromium, and aluminium forming alloy nitrides, which significantly strengthen the surface.

In addition to the normal range of low alloy nitriding steels, plasma nitriding also offers the possibility of processing other materials such as:
  • Powder metallurgy materials
  • Cast irons
  • Stainless steels
  • Titanium alloys