Microwave plasma nitrided austenitic AISI-304 stainless steel

A microwave (f = omega/2 pi = 2.45 GHz) electron cyclotron resonance (ECR) plasma source was employed for low-temperature (350 degrees C) and low-pressure (3-4 x 10(-4) Torr) nitriding of AISI-304 stainless steel. Even for nitriding times as short as 40 min, the surface hardness was found to increase by up to six times the initial value, and the nitrogen concentration and penetration depth were typically up to 27 at.% and 15 mu m, respectively. These quantities were determined by measurements of the nitrogen concentration profiles obtained by nuclear reaction analysis (NRA) and by means of energy-dispersive X-ray spectroscopy (EDX) analysis of a surface perpendicular to that which received the nitrogen flux. Three different mixtures of N-2/H-2 were used as working gases with an electron plasma density that was greater than the critical value. This density was achieved by lowering the external magnetic field to almost half of its resonant value (B-x = 875 G-resonant magnetic field). In this way, the samples underwent enhanced surface ion bombardment during the treatment. The plasma specifications were determined using electric probes. (C) 1998 Elsevier Science S.A.