Ion nitriding of pure iron using high-density plasma beam generated by a tubular plasma source

A tubular plasma source was developed to generate high-density plasma beam for nitriding using N-2 + H-2 mixture as working gas. The plasma source was demonstrated to be capable of generating a plasma plume with density of N-2(+) as high as similar to 7.2 +/- 0.6 x 10(11) cm(-3) in N-2 + H-2 mixed gases with N-2 content changed from 92.5% to 32.5%. After nitriding at 450 degrees C for 120 min, similar to 5-mu m thick compound layers formed on the sample surface of pure iron with diffusion zone as deep as 0.4 mm, leading to 7.5- 8.5 GPa hardness. In addition, nitriding of pure iron could be realized at a low temperature as low as 250 degrees C, producing a compound layer with a thickness of-2.5 pm. After nitriding for 120 min at temperatures of 330-500 degrees C, the high-density plasma flow could produce a compound layer of 4- 5 mu m with hardness of 10 GPa. N-2(+) and N+ are suggested to be responsible for the nitriding, of pure iron by the high-density plasma beam. The tubular plasma source allows us using N2 + H2 mixed gas with low hydrogen content for high-efficient nitriding. The gas temperature in plasma plume is similar to 300 degrees C, and thus is acceptable for low-temperature nitriding. (C) 2016 Elsevier B.V. All rights reserved.