Low-pressure ion nitriding of AISI 304 austenitic stainless steel with an intensified glow discharge

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Abstract
Low-pressure ion nitriding of AISI 304 austenitic stainless steel was conducted in the present study by intensifying the glow discharge. Plasma intensification was produced by thermionic emission and by utilizing a triode glow discharge system. The results showed that under the intensified plasma conditions, effective nitriding can be accomplished at relatively low temperatures. The nitride layer had a dense structure, high purity and an ultrafine microstructure with a grain (nucleus) size less than 80 Å. The nitriding treatment produced a thin layer of high nitrides (FeN and Fe2N) followed by a diffusion controlled layer of (Fe,Cr)3N. X-ray and electron-diffraction analysis showed that all nitrides were hexagonal in crystal structure. The intensified plasma treatment caused a significant increase in the nitride layer growth kinetics. The enhancement of the nitrogen diffusion was attributed to the higher surface concentration of vacancies due to the energetic particle bombardment and to grain boundary diffusion in the nitride phase. A simplified diffusion model is developed to estimate the effective nitrogen diffusivity under the intensified plasma ion nitriding.
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