Hollow cathode effect when plasma nitriding complex-shaped objects with deep holes,
especially blind holes which prevent gas movement through them. This can lead to overionization
inside the hole where gas is entrapped, and can lead to uncontrolled heating. The
temperatures reached are not usually high enough for melting but they can cause localized
structure and/or property alterations that affect the performance of a part in service. Thus,
hollow cathode effects raise major reproducibility questions and are of major concern in
industrial treatment. The size of the hole, the processing gas pressure and the plasma
current density all influence the occurrence of a hollow cathode.