It has been determined that applying direct plasma to the parts to be nitrided was not really
necessary. It is sufficient to generate plasma somewhere in the vacuum chamber and create
a gentle flow to bring the ions or excited neutrons over the workload. In active screen
plasma nitriding process, the plasma glow discharge is moved from the components to be
treated onto the metal screen (active screen) surrounding the entire workload. The
worktable and the parts are placed in a floating potential or subject to a relatively low
negative voltage.
This screen operates as a cathode and on the other hand, the furnace wall serves as an
anode. Under these conditions, the plasma is produced on the screen. The active screen with
the glowing plasma on the one hand produces a mixture of active species required for the
nitriding process; on the other hand it serves as a hot wall, which transfers the heat
produced by the plasma discharge to the workload resulting in a uniform temperature
distribution over the parts even with highly different geometry. Due to replacement of the
glow discharge from the components surface many possible damages caused by hollowcathode
and edge effects, arcing and non-uniform nitriding results owing to unequal
temperature distribution can be avoided [4].
Uniform nitriding response was achieved on an industrial ASPN set-up only when
additional energetic activation in terms of cathodic bias power to the component parts to be
treated was applied. The amount of bias power was tuned to the value of 10-15% of the
actual active screen power. The nitriding efficiency for component parts set under floating
potential is strongly reduced with increasing distance from the active screen. Therefore, the
application of bias activation in industrial scale furnaces provides successful nitriding
results offering more active nitrogen species at the component’s surfaces. For a uniform
nitriding a sufficient bias activation is necessary. The latter depends on the size of furnace
and on the process pressure. The application of the bias power above 15% did not show any
significant improvement of nitriding results, based on the data of edge hardness tests and
nitriding depth profiles [4].