PLASMA NITRIDING PROCESS
Plasma nitriding, known also as ion nitriding is a form of case hardening process. It is an extension of conventional nitriding process, utilizing plasma discharge physic to diffuse nitrogen into the surface of a ferrous alloy.
Plasma nitriding can be further branched out into plasma nitrocarburising. In this process, carbon together with nitrogen was introduced into the metal surface. The harden case, which is the nitriding layer is commonly known as ‘diffused case’ or ‘diffusion zone’.
1.1 Advantages of case hardening
- Less distortion compared to through hardening steel
- Fatigue properties of a part can be controlled and frequently improved
- Relatively inexpensive steel can be given wear-resisting properties which would be normally attained through the use of more highly alloyed and more expensive steels
- Hardening of the surface of steels which cannot be normally capable of being hardened to a high degree by altering the surface composition
- Combination of case and core properties can be attained that are not possible with conventional hardening treatment
- Scaling and decarburisation are minimized during surface hardening, offering advantages in producing machined parts
- Can be applied to very large parts, which due to very large mass or because of danger of cracking would be impractical to harden, by conventional heating and quenching.
- Selected area can be hardened on any sized place that are difficult with conventional heating and quenching
- High surface hardness, improve resistance to wear and galling, improve fatigue life, improve corrosion resistance (stainless steel is an exception)
1.2 Types of case hardening methods:
- Nitriding (Gas nitriding, liquid nitriding, plasma nitriding)
- Induction (Flame, magnetic, shell, electron-beam, laser)
- Sprayed metal coating
1.3 Advantages for utilizing plasma nitriding
- Ability to automate the system which gives good reproducibility of results
- Shorter cycle time
- No environmental hazard
- Improve control of case depth
- Ability to select the compound layer type to suit the required usage
- Good friction, wear, and fatigue properties
- High hardness of the treated surface
- Flexibility to nitride stainless steels, titanium alloys
- Possibility to lower nitriding temperature and to limit distortion
1.4 Process Parameters
- Temperature (400-600°C)
- Pressure (0.5 – 7 mbars)
- Gas composition (Ammonia, Nitrogen, Methane Hydrogen and Argon)
- Time (4 - 30 hrs depending on case depth)
Additional process parameters are
- Current intensity
- Pulse duration
- Pulse pause frequency
- Pulse pause duration
Plasma (ionized gas) is the fourth material state. It is a very reactive medium, which contains ions, very energetic neutral particles, as a result of dissociation (separation) of gas molecules in an electric field. The glow layer surrounding the job part is formed by these positive ions.
Plasma nitriding is achieved using a D.C glow discharge technology, whereby the nitrogen gas inside the furnace is converted into nitrogen ions and absorbed by the metal. Molecular nitrogen is first broken into atomic nitrogen through direct plasma dissociation.
N2 + e- → N + N + e-
Atomic nitrogen is then further converted into nitrogen ion through plasma ionization
N + e- → N+ + 2e-
The nitrogen ion, N+, will then diffuse into the metal surface as finely dispersed nitrides, imparting high hardness to the surface. Thus, case hardening is achieved. Fig.1 shows a schematic of the plasma nitriding process. During the plasma nitriding process, the job part and the cathode inside the furnace will be emitting a purple glow. This is because voltages had dropped sharply at these regions. This provided a large amount of discharged energy, which causes the cathode and job part to glow. Fig.2 shows a job part undergoing plasma nitriding treatment.
To put it simply, plasma nitriding utilizes a Direct Current electricity source to break up nitrogen gas (N2) into nitrogen ion (N+). This nitrogen ion is then absorbed by the metal surface to harden it.
3. Typical plasma nitriding process
A typical plasma nitriding process is shown in Fig.3 and the processing parameters are detailed in Table 1.
4. Nitrided layers
There are three types of nitriding layer that could be formed during plasma nitriding treatment are featured in Fig.4. The details are summarized in Tables 2 and 3.