Plasma-sheath ion nitriding: a cost-efficient technology for the surface modification of SS304

Abstract

Conventional ion nitriding is generally performed by treating the sample in an anomalous glow discharge with the sample behaving as the cathode of the discharge. To create the anomalous condition, the discharge is operated at higher pressures (0.5-5 mbar), resulting in significant ion energy loss due to ion-neutral collisions in the cathode sheath. The sample is heated because of ion bombardment. enabling these low-energy ions to diffuse inside the sample and cause improvements in the surface properties. The present paper deals with another process which has the advantages of ion nitriding, but is much faster and hence cost-effective. The experimental system, which consists of a cylindrical chamber, is evacuated to a pressure of 10^-5 mbar and then filled with a nitrogen and hydrogen (N₂:H₂ = 1:3) gas mixture to the typical operating pressure of 10^-4 mbar. Thoriated tungsten filaments placed in the center of the chamber are heated to emit thermionic electrons which are made to collide and ionize the nitrogen molecules forming a plasma. The plasma density is further enhanced by using a single cusped magnetic field to ~ 10¹⁰ cm^-3. The sample, which is made of SS304, is cleaned and immersed in the center of the cusp, where the magnetic field is minimum and the plasma density is maximum. The sample is heated to a temperature of 410°C, and negative pulses of -1 kV magnitude and 10-20 µs duration with a 10 kHz repetition rate are applied to it. Ions are accelerated towards the sample during the pulse and impinge on the sample with an energy of 1 keV. They then undergo diffusion inside the sample because of the sample temperature. After 4 h of treatment, the sample shows an increase in surface hardness. The advantages of this modified form of nitriding are manifold. The sample does not take part in plasma production, and hence the system pressure can be kept low. This low pressure also helps because there are no ion-neutral collisions in the sheath, and so the ions do nor lose energy while traveling to the sample. To aid diffusion of these ions in the sample, the sample temperature can be raised either by heating externally (by filaments placed beneath the sample) or by drawing electron current from the plasma. Electron current can be drawn from the plasma when the sample bias is made positive with respect to the floating potential of the plasma. The hardness increase obtained is also comparable to ion nitriding. This process is called plasma-sheath ion nitriding because it uses an external plasma and accelerates the ions in the sheath to cause the effects of conventional ion nitriding.