ABSTRACT
Notwithstanding its rapid development and numerous advantages over traditional nitriding methods, the uptake of plasma nitriding technology into industrial surface engineering has recently slowed down. In fact, in some countries today as much as 65% of commercial nitriding is still performed using gas or salt bath technology. This is in spite of the significant benefits that plasma nitriding offers in terms of reduced gas and energy consumption and the complete removal of any environmental hazard. Difficulties in maintaining a uniform chamber temperature, particularly in full workloads and damage caused to parts by arcing have been contributory factors in this.
Recent reports indicate that, contrary to earlier assertions, it is not ions bombarding the metal surface but neutral particles contained in the plasma that are responsible for the nitriding effect. It is therefore not necessary for the plasma to form directly on the parts to be treated. This is the important finding that forms the basis for through cage (TC), or active screen plasma nitriding.
A large screen or cage surrounds the entire workload and it is on this cage, and not the parts to be treated, that the plasma is formed. The formation of the glow discharge on the screen has two functions:
To heat up the workload by radiation, analogous to cold wall vacuum furnace heating methods, providing a very uniform temperature throughout the entire load.
To generate active particles. These are then encouraged to flow through the cage and over the workload by the direction of gas flow.
The gas mixture has been specifically designed to ensure that the active particles generated have a sufficient lifetime.By controlling the gas mixture the classical nitrided structure is obtained in steels, either with a γ′ or ε compound layer and a deep diffusion zone supporting it. Very complex shapes can be treated successfully and the active species can even enter blind holes, producing completely uniform modified layers on all parts in a heavily loaded chamber.
