The effect of voltage on microstructure of iron by nitrogen plasma immersion ion implantation

The influence of the applied voltage on the microstructure of nitrogen implanted iron by plasma immersion ion implantation at 360 °C was studied. Glancing X-ray diffraction shows that the amount of nitride decreases with increase of the applied voltage. In the surface layer of the iron samples, ε and Fe2N nitrides are formed at the surface, and γ′ nitrides are formed in the sub-layer of the surface. There is only a small fraction of ε nitride at the surface for the applied voltage up to 45 kV, which may be the result of so-called ‘outer diffusion’ of nitrogen near the surface. The high-energy ion-bombardment might cause a high density of defects at the sample surface, and thus the defects promote the nitrogen diffusion. As a result, the formation of ‘high concentration’ nitride (such as ε phase) might be restrained. Even though a nitrided layer with the thickness of 150 μm is obtained by our process owing to the high dose rate. The observation of scanning electron microscopy (SEM) shows that the thickness of the nitrided layer is not homogenous at the applied voltage less than 15 kV, which may result from the preferential diffusion of nitrogen along the grain boundaries. It is also found that long needle-like γ′ nitride appears as a parallel distribution in the diffusion zone. The surface SEM morphology at different voltages shows that the formation of nitrides in different zone is not homogenous.