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±0.6×1011cm−3 in N2+H2 mixed gases with N2 content changed from 92.5% to 32.5%. After nitriding at 450°C for 120min, ~5-μm thick compound layers formed on the sample surface of pure iron with diffusion zone as deep as 0.4mm, leading to 7.5–8.5GPa 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 ~2.5μm. After nitriding for 120min at temperatures of 330–500°C, the high-density plasma flow could produce a compound layer of 4–5μm with hardness of 10GPa. 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.