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 N2 + H2 mixture as working gas. The plasma source was demonstrated to be capable of generating a plasma plume with density of N2+ as high as ~ 7.2 ± 0.6 × 1011 cm− 3 in N2 + H2 mixed gases with N2 content changed from 92.5% to 32.5%. After nitriding at 450 °C for 120 min, ~ 5-μm 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 °C, producing a compound layer with a thickness of ~ 2.5 μm. After nitriding for 120 min at temperatures of 330- 500 °C, the high-density plasma flow could produce a compound layer of 4- 5 μm with hardness of 10 GPa. N2+ 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 ~ 300 °C, and thus is acceptable for low-temperature nitriding.