Plasma source ion nitriding of pure iron: Formation of an iron nitride layer and hardened diffusion layer at low temperature

Plasma source ion nitriding is a new low temperature, low pressure nitriding approach for the surface modification of iron and steel. A nitriding apparatus based on an electron cyclotron resonance (ECR) microwave plasma source has been developed. Nitrogen ions are accelerated from the ECR microwave plasma by a pulsed negative bias (typically -- 2 kV) applied directly to the sample, are implanted, and are finally diffused to a depth below the surface at elevated temperatures which are regulated up to 450 degrees C by an auxiliary heater. An experimental investigation of plasma source ion nitriding into pure iron is described. The nitrided samples were characterized using optical metallography, microhardness measurement, glancing angle X-ray diffraction and transmission electron microscopy. The effect of the process temperatures on the surface hardness, the hardness-depth profile and the microstructure of nitrided pure iron has been investigated. On the basis of these findings, it has been established that plasma source ion nitriding of pure iron can produce an iron nitride layer and a hardened diffusion zone at process temperatures from 150 to 450 degrees C.