Nitriding Techniques (ion nitriding)
Ion nitriding is a process of surface hardening of metallic materials that incorporates nitrogen into the surface of the treated parts. Due to the particular nature of the nitrogen incorporation, the ion nitriding of the steel and grey cast iron is, in many cases, far superior to the simple nitriding processes previously discovered. This technique improves mechanical properties such as wear resistance, fatigue resistance, resilience, etc.
The metallurgical causes of this improvement to the surface characteristics are associated with the particular qualities of the surface layers it is possible to achieve with this process.
In nitriding many different kinds of mediums are used. With regard to the different states of matter (solid, liquid or gaseous), a distinction is made between powder, bath and gas nitriding. Besides the three states of matter already mentioned, another is known in physics that is considerably more active: PLASMA.
A plasma is composed of charged particles, i.e. electrons and ions. If we want to use heat to move from a gaseous to a plasma state, the gas must be heated to tens of thousands of degrees and even to more than 100,000 degrees.
Ion nitriding uses this plasma state, meaning the process can also be called plasma nitriding, by analogy with the other types of nitriding.
The process called glow-discharge nitriding (a term that often appears in technical literature from Germany) is the same thing, and ignores the fundamental nature of the process.
How does ion nitriding work and how can we create the plasma state? The best way to find out is to look first at the structure of the ion nitriding equipment.
This equipment comprises three basic units:
- the vacuum furnace
- the gas metering device
- the electrical installation
In the vacuum furnace, the parts to be treated are hung or placed such that they are electrically isolated. The gas metering device can, together with a vacuum pump, feed the necessary treatment gas, for example ammonia, into the vacuum furnace and regulate the pressure of this gas between 0.1 and 10 torr.
The treatment gas passes into a plasma state with the help of the electrical installation. The latter applies a continuous current of several hundreds of volts to about 1500 V between the parts being treated and the furnace wall, such that the part to be treated is negative (cathode) relative to the furnace walls, which represent the anode.
The charge carriers, accelerated by the voltage drop between the cathode and the anode, dissociate the treatment gas molecules by a process of inelastic collisions, while they excite and ionize the atoms and molecules. This has the effect of continually issuing new charge carriers: electrons are accelerated towards the kiln wall (anode) and the positive ions toward the part to be treated, which constitutes the cathode.
The voltage applied does not decreases in linear fashion between the furnace wall and the part, as might be the case in a strong vacuum, but undergoes great changes in the load space created near the part, so that virtually all of the applied voltage drops only a few millimetres above the surface of the part. This voltage drop near the surface of the cathode (the part to be treated) is also called cathode fall.
E= first cathodic edge
H= Hittorf dark space
F= Faraday dark space
S= glowing edges
B= anodic sheath
Discharge zones and drop in potential according to W. WEIZEL.
Thus, all electric shocks in ion nitriding are limited to this narrow zone of cathode fall. The perfect state of the treatment gas plasma only appears during the cathode fall, i.e. in the immediate vicinity of the surface of the part, while the rest of the treatment chamber of the vacuum furnace is full of normal treatment gas enriched with charge carriers. Therefore, glow discharge surface treatment is practically independent of the distance between the part and the furnace wall. The phenomena of cathode shock, here referred to as glowing edges, are limited to a narrow zone around the part. This represents the inside of a vacuum furnace loaded with various parts for ion nitriding treatment. The glowing edge follows the contours of all surfaces of each piece. This assumes that all surfaces (including the notches and bores) are subjected to a uniform ion bombardment which leads to a uniform hardening.