ELECTRON-SPECTROSCOPY STUDIES ON CARBON SEGREGATION FROM A MONO-CRYSTALLINE ALPHA-FE(111) SPECIMEN

The segregation properties of carbon segregation on an iron (111) surface using AES, XPS and LEED methods have been studied. In the segregation process the following steps are observed with increasing carbon coverage: adsorption of atomic carbon for coverage theta = 1, formation of chemisorbed molecules C2* for coverage theta = 2 and then formation of a three-dimensional graphite structure. The calculation of energy of different iron and carbon clusters using the EHT (extended Huckel theory) method points at the underlayer adsorption of carbon atoms. For the chemisorbed carbon molecule C2* the following model is proposed: one atom is located under the iron surface and the second one on the iron surface - on the reconstructed C-7 site. The graphite domains on the iron surface are oriented in the manner where that the angle 13.5-degrees between crystallographic orientations [110BAR] Fe and [1010BAR] graphite is formed. If the iron surface is covered by sulphur, the segregation process will result in the localisation of adsorbed carbon atoms under iron and sulphur atoms. Further segregation process in this case leads to the formation of the three-dimensional cementite Fe3C Structure. The experimental results have been discussed taking into account the stable and metastable iron-carbon phase system.