• Free electrons with energy depending on their translational speed or equivalent temperature designated in electron volts. One electron volt is equivalent to 11,600 degrees.
• Ions, molecules or atoms, which have lost one or more electrons. There can also be negative ions, formed by electrons attaching themselves to the atoms of electronegative gases.
• Atoms and molecules, which consist of nuclei with positive charge, surrounded by bound electrons rotating around the nuclei. These can be in their ground state or excited states.
• Photons created within the plasma by interactions between the plasma species.

The charged particles collectively can respond to external electromagnetic fields and transport energy. The fluid properties are enhanced by the particles setting up internal self-consistent electric and magnetic fields, resulting in collective effects like flows, waves, instabilities and self-organization. The internal energy is composed of thermal, electric, magnetic and radiation fields, whose relative magnitudes allow the plasma state to exist in an extended, multi-dimensional parameter space.

The energetic electrons in plasmas interact effectively with electron systems of atoms and molecules, thereby imparting to these plasmas remarkable properties for inducing chemical reactions. When materials are exposed to plasmas, a variety of physical, chemical and metallurgical transformation of the material takes place.

The type of plasma formed is characterized by voltage current relation. The abnormal glow, in which the current increases with the voltage, is used for plasma nitriding.

Nitriding Plasma
The abnormal glow is a low pressure gas discharge characterized by a small degree of ionization less than 0.01 percent. This discharge is primarily produced by electron impact ionization and dissociation. Average energies of ions and neutrals in the plasma are considerably less (about 0.1 eV). The energetic electrons obtain their energy from local electric fields within the plasma or at plasma-electrode boundary. These electrons then transfer energy to the gas atoms and molecules via inelastic collisions that can produce a wide variety of ionic and radical species within a molecular gas resulting in chemical reactions. These species make the plasma highly reactive and can be effectively utilized for material surface modifications like nitriding by using appropriate gas mixture.

This discharge uniformly envelops the cathode surface (the sheath region where most of the cathode potential is dropped) providing uniform current density. The sheath thickness and its properties are determined by parameters like external applied voltage and plasma density. The charge particles are accelerated near the sheath region. The energy of the accelerating ions can be suitably modified to provide sputtering action on the surface. The sputtering can be utilized for removal of impurities and barrier layers (oxide layer).

In plasma nitriding arcing is a usual phenomena. The operating regime of plasma nitriding is on the positive slope of voltage current characteristics. As the current density increases beyond certain value the discharge can run into the plateau region which is called the arc region where the current densities are much higher compared to abnormal glow discharge regime. This phenomenon has to be avoided, by having appropriate control on the discharge current, otherwise it can also damage the cathode surface. The increase in current density is a result of enhanced ionization in the discharge. This enhanced ionization can be due a number of reasons like localized overheating resulting in evaporation of material and subsequent ionization.

Usually the nitriding plasmas are made in a mixture of nitrogen and hydrogen in suitable ratio based on the nitriding properties desired. The plasma of this gas mixture results in formation of ions and radicals corresponding to nitrogen, hydrogen and molecular species of NHx type.

Process
The process of plasma nitriding involves diffusion of nitrogen in metal surface at elevated temperature in the presence of a nitrogen bearing gas mixture. The diffused nitrogen reacts with the metal and forms metal nitrides to impart hardness on the surface upto a certain depth determined by the metal properties.

A high voltage DC is applied between the cathode (which is the base plate on which workpieces are placed) and the anode (usually the chamber wall) in a vacuum vessel with low pressure mixture of nitrogen and hydrogen. A glow discharge is formed and the current is sustained by the discharge in the form of ions flowing through the cathode sheath and electrons to the anode wall. The voltage is increased till one reaches abnormal glow regime where current density is uniform on the workpieces and cathode surface.

Plasma nitriding process involves a number of stages which includes heating, sputter cleaning, nitriding and cooling. The heating is usually done using external heating till all the volatile compounds in the vacuum chamber are removed. This is followed by sputter cleaning of the workpiece surface and simultaneous heating, through ion bombardment, in addition to external heating. The sputter cleaning is done at a lower pressure than the nitriding pressure. Typical operating parameters in sequence of operation cycle are listed in table 1. The actual nitriding process is started after achieving the nitriding temperature and it can run from few hours to several tens of hours depending on the material being nitrided and the properties required.

The key parameters for control of resulting nitrided properties in any method are temperature, time and gas composition. These parameters can be independently varied to achieve desired metallurgical results. Even in conventional techniques of nitriding it is well understood that the temperature can play a crucial role in determining the resulting metallurgical properties. One of the important flexibility provided in plasma nitriding is that the ability to vary gas composition in terms of nitrogen percentage. It is found that varying nitrogen percentage in gas mixture will result in variation in nitrogen concentration. This can alter the metallurgical properties of the case specifically that of the compound zone. It is observed that lower percentage of nitrogen (less than 20%) can eliminate white layer completely. The combination Fe₄N.’ and Fe₃N layers can be tailored to suit a particular requirement. Further one can add a small percentage of carbon into the gas mixture to form a pure Fe₃N layer. The properties of the diffusion layer in any case are determined by the alloying elements.

Another significant characteristic of plasma nitriding is the ability to reduce the cycle time compared to conventional techniques. Several theories have been proposed to understand the actual mechanism in plasma nitriding. Some assume the formation of iron nitrides in the gas phase as the precursor while some others assumes nitrogen adsorption, penetration and reaction to form stable nitrides. In third approach, ionic nitrogen dominates the process.

According to Kobel’s hypothesis sputtered iron reacts with nitrogen in the gas phase to form unstable FeN, which condenses on the surface and dissolve into stable iron nitrides like the Fe₃N and Fe₄N'.

Heterogeneous reactions of NHx with iron leading to formation of iron nitride in the boundary layer which decompose to release nitrogen is considered possible. Hydrogen in the gas mixture plays a crucial role of activating the surface (because it has better sticking coefficient on metal surface) for capture of nitrogen species from the gas phase into an adsorbed state.