Formation and growth of iron nitrides during ion-nitriding

Abstract

In order to clarify the formation and growth kinetics of iron nitrides, Fe₂N (ζ), Fe_2–3N (ɛ) and Fe₄N (γ′), on the surface of iron during ion-nitriding, and their contribution to the mechanism of ion-nitriding coarse-grained specimens (2 to 6 mm) were ion-nitrided in pure nitrogen and nitrogen-hydrogen (20%–80%) plasma at temperatures between 500 and 600°C. Reflection electron diffraction (RED) showed immediate formation of the nitrides. Growth of these phases in latter stages of ion-nitriding was studied using optical microscopy and X-ray diffraction. The mechanism of the nitride layer formation is discussed and compared with the existing gas nitriding data. In the case region, Fe₄N (γ′) and Fe₁₆N₂ (α″) precipitation was observed to occur under all experimental conditions studied. Case depth is seen to be parabolic with time and nitriding rate increases slightly when nitrogen-hydrogen plasma is used. Discussions are given to explain the difference in the nitriding efficiency under two different plasma compositions.