The role of hydrogen in plasma nitriding: Hydrogen behavior in the titanium nitride layer

This paper describes the role of hydrogen in titanium nitriding using an N₂–H₂ gas mixture plasma, which is empirically known to enhance the rate of nitriding. The nitrided titanium specimen was characterized by SIMS, ERD, XRD, SEM, and hardness measurements. Optical emission spectroscopy was conducted for plasma diagnostics of the N₂–H₂ plasma with various partial pressures of H₂ gas. The rate of nitriding was most enhanced when a 60 vol.%N₂–40 vol.%H₂ gas mixture plasma was applied. The hydrogen content of the titanium nitride layer was in the order of 10 wt.ppm and increased with increasing partial pressure of H₂ gas in the working gas. In N₂–H₂ plasma, NH, N₂, N and H radicals were detected as reactive species. The amount of NH radicals was greatest when 30–40 vol.%H₂ gas was added to plasma, while the amount of H radical increased monotonously with the ratio of H₂ gas. Based on the experimental results, we propose that H radicals pick up hydrogen from NH radicals to produce active nitrogen and thus enhance the rate of nitriding.