Thickness dependent phase transformations in implanted iron

The dependence of the iron nitride formation and the phase transformations in the nitrogen implanted layer on the thickness of the implanted material is investigated by Conversion Electron Mössbauer Spectroscopy (CEMS). Phase transformations in nitrogen-implanted iron layers of various thickness ranging from 60 nm to 870 nm deposited on SiO2 substrate by rf sputtering technique have been studied. Such prepared samples were implanted at room temperature with nitrogen ions with the energy of 50 keV with ion doses ranging from 1x1017 at. N/cm2 to 5x1017 at. N/cm2. After each implantation step the samples were characterized by CEMS using the gas flow electron counter with He-6%CH4 gas. The implanted samples were also investigated by the Grazing Angle X-ray Diffraction (GXRD). The results revealed that the nitride formation is strongly enhanced in thin films as compared to thicker layers or bulk iron samples. In thin iron films a given nitride phases (‘/”, -Fe3N, -Fe2N) are formed at lower nitrogen doses and the transformation of the original iron layer into iron nitrides is more complete than in the bulk -Fe. In bulk iron samples about 20 % of Fe atoms remain in the -Fe phase even for the highest implantation doses (5x1017 at. N/cm2) whereas the complete transformation of thin 60 - 230 nm iron layer into Fe nitrides has been observed.
The observed effects are not fully understood yet. The GXRD experiments performed for the same samples revealed that the stress in the interfacial region of Fe layer and the substrate is significantly larger than in the top (free) surface of the layer. The higher stress may favor the formation of iron nitrides. On the other hand the presence of the interface between Fe film and the substrate may block the in-bulk diffusion of N atoms leading to the higher nitrogen concentration in thin films as compared to the bulk.
The strong dependence of the nitride formation due to N-implantation on the thickness of the implanted material is important for implantation metallurgy, especially for implantation of thin coatings often used for the extension of lifetime of precise tools and details. The phase transformations in thin films may occur in significantly different way as compared to the bulk materials leading to the formation of new phases characterized by extremely high impurity concentration.