PLASMA-SHEATH ION NITRIDING - A COST-EFFICIENT TECHNOLOGY FOR THE SURFACE MODIFICATION OF SS304

Conventional ion nitriding is generally performed by treating the samp le in an anomalous glow discharge with the sample behaving as the cath ode of the discharge. To create the anomalous condition, the discharge is operated at higher pressures (0.5-5 mbar), resulting in significan t ion energy loss due to ion-neutral collisions in the cathode sheath. The sample is heated because of ion bombardment, enabling these low-e nergy 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 cos t-effective. The experimental system, which consists of a cylindrical chamber, is evacuated to a pressure of 10(-5) mbar and then filled wit h a nitrogen and hydrogen (N-2:H-2 = 1:3) gas mixture to the typical o perating pressure of 10(-4) mbar. Thoriated tungsten filaments placed in the center of the chamber are heated to emit thermionic electrons w hich are made to collide and ionize the nitrogen molecules, forming a plasma. The plasma density is further enhanced by using a single cuspe d magnetic field to similar to 10(10) cm(-3). The sample, which is mad e of SS304, is cleaned and immersed in the center of the cusp, where t he magnetic held is minimum and the plasma density is maximum. The sam ple is heated to a temperature of 410 degrees C, and negative pulses o f -1 kV magnitude and 10-20 mu s duration with a 10 kHz repetition rat e are applied to it. Ions are accelerated towards the sample during th e pulse and impinge on the sample with an energy of 1 keV. They then u ndergo diffusion inside the sample because of the sample temperature. After 4 h of treatment, the sample shows an increase in surface hardne ss. The advantages of this modified form of nitriding are manifold. Th e 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 not lo se energy while traveling to the sample. To aid diffusion of these ion s in the sample, the sample temperature can be raised either by heatin g externally (by filaments placed beneath the sample) or by drawing el ectron current from the plasma. Electron current can be drawn from the plasma when the sample bias is made positive with respect to the floa ting potential of the plasma. The hardness increase obtained is also c omparable to ion nitriding. This process is called plasma-sheath ion n itriding because it uses Bn external plasma and accelerates the ions i n the sheath to cause the effects of conventional ion nitriding. (C) 1 998 Elsevier Science S.A.