Surface nanocrystallization of pure iron achieved by anodization-reduction treatment and its effect on lower-temperature nitriding behaviors

Abstract

In this work, a novel method named anodization-reduction composite treatment was designed to achieve surface nanocrystallization on pure iron materials. Subsequently, this method was employed as pretreatment for gas nitriding of pure iron and its effect was studied systematically. The experimental data illustrated that a 1.3 μm thick Fe2O3 layer with nanoporous structure was successfully fabricated by anodization treatment in solutions containing 0.1 M NH4F and 1 M H2O. After hydrogen reduction at 400 °C, the nanoporous Fe2O3 layer was transformed to pure layer with nanoparticles stacked structure. Additionally, each particle is composed of several pure iron grains with size of 2–10 nm. After lower-temperature gas nitriding at 400 °C, a dense compound layer composed of Fe3N and Fe4N phases with thickness of 3 μm was formed on surface of anodization-reduced sample. The anodization-reduced nitrided sample show the higher hardness, lower friction coefficient and improved wear resistance than those of original nitrided sample. The acceleration of nitriding process may be attributed to the increased adsorption area of nitrogen atoms, rapid diffusion effect in nanocrystalline and more nucleation sites for nitrides. This paper provides a novel surface nanocrystallization method for iron to reduce its nitriding temperature or duration.