The bonding state of segregated elements on α‐iron surfaces and in their iron compounds studied by electron spectroscopy

Abstract

A combined electron spectroscopic study by means of x-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and Auger electron spectroscopy (AES) was performed to characterize the bonding state of the nonmetal elements S, C and N segregated on the a-iron(100) surface as a saturated overlayer with c(2 X 2) structure. Solid nonmetal iron compounds were prepared for comparative measurements:The segregation of solute impurities in iron and low alloyed steels leads to compositional changes of grain boundaries and free surfaces. Surface equilibrium segregation influences surface properties such as surface reaction kinetics, friction and adhesion, surface diffusion and sintering and corrosion resistance. Grain boundary segregation affects materials performance and may lead to phenomena such as temper embrittlement, creep embrittlement, stress corrosion cracking and intergranular corrosion.Sulphur is known to cause embrittlement of iron, i.e. a strong tendency to intergranular fracture, whereas carbon and nitrogen will reduce embrittlement. The embrittlement by sulphur has been explained by a weakening o f metalmetal bonds in lattice planes adjoining the grain boundary. This hypothesis was supported by cluster calculations,' which indicate that sulphur attracts negative charge and will draw electrons from surrounding metal atoms.The x-ray photoemission spectra of the core lines S 2 S 2p, C Is and N 1s from the segregated nonmetal exhibit characteristic shifts to lower binding energies in comparison to corresponding lines from appropriate reference samples. The latter samples are solid sulphur Sx. graphite and molecular nitrogen adsorbed on iron at low temperatures. These references represent a homopolar bond of the nonmetal under investigation, so that the chemical shift reveals the polarity of the iron-non-metal bond.FeS and FeS? show S 2p binding energies which are 0.3 eV and 0.5 eV below the one of sulphur segregated on Fe( 100) of 162.2 eV, i.e. the charge is higher on sulphide sulphur atoms. This value is found also for the sulphur enrichment on the grain boundaries of an in situ broken sample. The chemical shift related to the binding energy of 164.0 eV of the reference sample of solid S8 indicates an almost ionic bond character for the iron-sulphur bond, as it was predicted by cluster calculations. In the spectra of segregated carbon only two lines appear separated by 2.0 eV. The one with higher binding energy of 284.6 eV corresponds to a graphitic precipitation below the temperature Ts of the solid solubility limit of carbon in airon. XPS binding energy and Auger lineshape of this carbon species equal those of graphite. The other line is ascribed to chemisorbed carbon, since it is observable only above T,, when surface graphite is dissolved into the bulk.Around 7' s a sharp peak arises in the Auger spectrum of the CKI.I, transition at the high energy side o f the KL23LZJ line. As this peak represents an interatomic transition carbon core hole, the origin of the Auger electron is the v...