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The effect of plasma in the hardening process
The electrically conductive gas called plasma
Plasma is the term used to describe an electrically conductive gas. For a gas to be conductive, free charge carriers must be available for the transport of electricity. At pressures of > 0.1 bar, this condition is only fulfilled at temperatures above approx. 8000 K. If the pressure is reduced to approx. 1mbar, a plasma can also be generated at much lower temperatures. This effect is utilised in plasma heat treatment. The low-pressure plasma enables "high-temperature surface chemistry at low component temperatures" and opens up unique process technologies for many areas. To generate plasma, a voltage of several hundred volts is applied in a vacuum between the component (cathode) and the container wall (anode). Depending on the conductivity of the gases used, a certain current density results when the voltage is applied.
Plasma nitriding was already being used before the Second World War. Afterwards, at the end of the 1950s, the establishment of an institute of the Gesellschaft zur Förderung der Glimmentladungsforschung (Society for the Promotion of Glow Discharge Research) gave new impetus to the industrial utilisation of the process. These developments are closely linked to the Berghaus name.
Until the beginning of the 1980s, the state of the art was water-cooled systems in which the discharge was fed by a DC voltage (cold wall technology). The decisive disadvantages of this process technology are large temperature differences in a batch and consequently large variations in the treatment result as well as a relatively low charging density, high energy consumption and the close coupling of thermal and chemical processes.
Significant progress has been achieved through the use of pulsed discharge. Pulsing reduces the energy input into the system and improves temperature uniformity in the charge. Today, all industrial systems are equipped with this pulsing technology. Typical values for the pulse duration are 50 to 100 µsec and for the pulse repetition time 100 to 300 µsec.
Alongside gas nitriding,plasma nitriding is a nitriding process in hardening technology. Due to the high energetic effect of the plasma, the process can be carried out at low working temperatures of less than 500°C, making it suitable for materials that are sensitive to distortion. Plasma nitriding is one of the most environmentally friendly methods of surface hardening due to the fact that no toxic gases are used.