H. Viefhaus

Equilibrium surface segregation of dissolved nonmetal atoms on iron(100) faces

At elevated temperatures equilibria of surface segregation X (dissolved) = X (adsorbed) have been studied for the nonmetal atoms X = C, N and S. Iron single crystals with (100)orientation have been doped with different concentrations of solute atoms (in the range about 10–100 wt ppm). The samples were introduced into the UHV chamber, cleaned and then heated to temperatures in the α-solid solution range. The surface concentration of the segregated nonmetal atoms was observed by AES for different bulk concentrations in dependence of the temperature. The LEED pattern was also observed during segregation equilibrium at temperatures up to about 750° C. The LEED patterns indicate a c(2 × 2) structure for carbon and nitrogen as well as for sulfur. The temperature dependence of the surface concentration for carbon on Fe(100) can be described by a Langmuir-McLean equation, an average segregation enthalpy of −85 kJ/mol°C is obtained. Since N2 desorption occurs the nitrogen segregation is in virtual equilibrium only at temperatures <500°C. The equilibrium surface concentration of sulfur on α-iron is virtually independent of the solute concentration and the temperature: there is always a saturated layer of sulfur on the (100) faces, even at small bulk concentrations. Since the thermodynamic activity of the nonmetal atoms is well defined in the segregation studies (except nitrogen at higher temperatures) , the results can be correlated with studies in gas atmospheres at atmospheric pressure. The relations to the kinetics of the carburization and the nitrogenation of iron are discussed and the influence of sulfur on these reactions.

Segregation of sulfur during growth of oxide scales

Abstract There is no equilibrium segregation of sulfur to the intact metal/ oxide interface, as was confirmed by taking AES sputter profiles of the chromia layer on an Fe-Cr alloy and the alumina layer on the intermetallic phase β-NiAl after short term oxidation (10 min) and segregation anneal at high temperatures. After prolonged oxidation (> 3 h) a monolayer of sulfur was detected in regions where the scale had cracked and spalled, however, in this state large cavities had formed beneath the scale and the sulfur obviously had segregated to the free metal surface of these cavities. The adverse effect of sulfur on scale adherence in the oxidation of high-temperature materials containing some ppm sulfur can be explained by the reduction of surface energy caused by sulfur segregation which favors the formation of voids and accelerates their growth to cavities.

Surface segregation of Si on Fe single crystal surfaces and interaction with carbon

Abstract AES and LEED are applied to the study of silicon surface segregation on Fe−3wt%Si single crystals between 400 and 900°C. The variations of the Si coverage with time at a given temperature or with the temperature at equilibrium show a strong orientation dependence. For the (100) orientation, the diffusivity and segregation enthalpy of silicon on iron could be derived from the measurements. Lateral interactions between segregated atoms are better observed in the presence of a third element. The study of the Fe-Si-C ternary system thus confirmed the weakness of the Si-Si lateral interactions. It also showed that the repelling of Si by C on the surface is mainly due to a site competition reaction, whereby the strong Si-C repulsive interaction plays a minor role.

Electron spectroscopic studies of tin surface segregation on iron single crystal surfaces

Abstract AES, ELS, LEED and XPS investigations of the surface segregation of tin dissolved in a Fe-4wt%Sn alloy were performed in ultra-high vacuum at elevated temperatures. The three low indexed surface orientations (100), (110) and (111) were studied. In all cases, no dependence of the maximum tin surface coverage on temperature was detected within the temperature range from 450 to 650°C. An order-disorder transition was observed by LEED, AES and XPS for the (100) oriented surface during tin segregation. The binding state for the segregated tin atoms abruptly changes at the order-disorder transition as determined by XPS. Similar results were obtained for the (111) surface. A deviating behaviour was observed for the (110) surface orientation, where two different ordered hexagonal surface structures were detected by LEED during tin surface enrichment. The first structure is similar to the diamond structure of pure tin, and the second one corresponds to the formation of a thin layer of the intermetallic compound FeSn on the (110) surface. The electron binding energies of the segregated tin atoms determined by XPS increase with increasing tin coverage on the (110) oriented surface. ELS studies on (100) and (111) oriented surfaces saturated with segregated tin show in comparison with literature data of pure tin a surface plasmon loss peak but no signal for the corresponding bulk loss. An energy loss signal found only for the (110) surface at Sn saturation coverage seems to be characteristic of an intermetallic FeSn surface phase.

Formation of CrN surface compounds and surface precipitates on Fe-15%Cr-N single crystals

Abstract AES, LEED and XPS were applied to the study of chromium and nitrogen surface segregation on Fe-15%Cr-N (100) oriented single crystals in the temperature range from 490 to 750°C. At low temperatures up to about 560°C (depending on the nitrogen content) continuous precipitation of cubic CrN on the surface was observed. Between about 600 and 670°C a monolayer of a surface compound CrN was stabilized by epitaxial growth. In this range no further growth of CrN occurs beyond the monolayer coverage. At higher temperatures the coverage decreases by desegregation of Cr and N and by desorption of N2.

Influence of H2S on metal dusting

Presence of H 2 S in a carburizing atmosphere causes S-adsorption which retards carbon transfer and deposition and can suppress metal dusting of iron and steels. In the latter process cementite Fe 3 C is an intermediate, graphite deposition would initiate its decomposition but graphite nucleation is prevented by adsorbed sulfur. Thus continued Fe 3 C growth can be observed in the presence of H 2 S. Thermogravimetric studies in CO-H 2 -H 2 O-H 2 S mixtures have been conducted at 500 °C at various carbon activities a C and H 2 S/H 2 -ratios. With increasing a C higher H 2 S/H 2 -ratios are needed to suppress metal dusting, with increasing H 2 S/H 2 -ratio the kinetics of Fe 3 C growth change from diffusion controlled parabolic kinetics to linear carbon transfer controlled kinetics. At very high a C ≥ 1000 besides Fe 3 C also the Hagg carbide Fe 5 C 2 was observed as an outer layer on the cementite.

Initial stages of oxidation of a 9CrMoV-steel: role of segregation and martensite laths

Abstract The initial stages of oxidation of steel P91 were studied in a UHV system at oxygen partial pressures ranging from 10 −8 mbar up to 10 −5 mbar. Experiments were conducted at 600–650°C for heating times 5–120 min. The oxide scales were analyzed by means of Auger electron spectroscopy (AES), scanning Auger microscopy (SAM), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The very initial stages up to growth of the first few oxide layers correspond to a complex surface situation and are influenced by Cr-nitride formation, P segregation and SiO 2 formation. After the first monolayers of oxides have grown the process becomes simpler and different oxidation kinetics accounts for the further growth. The microstructure of steel P91, tempered martensite along with chromium carbide precipitates leads to preferential chromium-rich oxide formation along martensite laths. The results are also discussed in view of diffusion data taking into account material transport by bulk and fast diffusion paths.

Surface segregation on Fe3%Si0.04%VC(100) single crystal surfaces

Abstract Surface segregation phenomena on (100) oriented single crystal surfaces of the ferritic Fe-3%Si-0.04%V-C alloy were investigated by AES and LEED. At temperatures below 635 °C vanadium and carbon cosegregation is observed after prolonged heating. At thermodynamic equilibrium the substrate surface is saturated with the binary surface compound VC. The two-dimensional VC is epitaxially arranged on the substrate surface as indicated by LEED investigations. Its structure corresponds to the (100) plane of the three-dimensional VC with rocksalt structure. Sharp above 635 °C the surface compound VC is dissolved into the bulk. At higher temperatures the substrate surface is covered with segregated silicon forming a c(2 × 2) structure. This surface phase transition is reversible. Because of the low concentration and slow diffusion of vanadium, non-equilibrium surface states are formed as intermediates upon segregation of silicon and carbon. Below 500 °C a disordered graphite layer with a characteristical asymmetrical C Auger peak is observed on the substrate surface. Above 500 °C carbon segregation leads to the formation of an ordered c(2 × 2) structure with a symmetrical C Auger peak being characteristic for carbidic or atomically adsorbed species. At increasing temperatures silicon segregation takes place leading to a c(2 × 2) structure. Between silicon and carbon site competition is effective.

Orientation dependence of surface structures during antimony segregation to crystallographic low-index iron planes

Abstract Surface segregation of antimony in a binary FeSb alloy of ≈ 4wt% Sb content was studied by electron spectroscopic methods. AES, LEED and XPS investigations for samples with low-indexed surface orientations (100), (110) and (111) were performed at 450 to 650°C. A (1 × 1) structure was found for the (111) surface at Sb saturation which occurs at all temperatures. Calibrated Auger measurements show that at this state one atomic monolayer of antimony has segregated to this surface. An imperfect c(2 × 2) structure was observed for a (100) oriented sample. Calibration of the maximum Sb peak height ratio shows that this state corresponds to half a monolayer within a 10% deviation (θ = 0.55). The (110) surface reconstructs into (111) and (11 1 ) facets after Sb saturation is attained, as an analysis of the irregular electron energy dependence of the observed LEED beams shows. Further, XPS measurements show that the Sb(3d) binding energies for the facetted state and the (111) plane at saturation are the same, and that the Sb(3d) shift observed during segregation can be explained in terms of segregant-segregant and segregant-substrate interaction.

Surface cosegregation and surface precipitation on Fe-3%V-C,N(100) single crystals

Abstract Surface cosegregation and surface precipitation has been studied on (100) oriented surfaces of Fe-3%V-C,N and Fe-3%V-C single crystals by means of AES and LEED. Between 450 and 750°C cosegregation leads to the formation of the two-dimensional surface compounds V(C,N) and VC, which are both epitaxially arranged on the substrate surface as indicated by LEED. The stoichiometry of the surface compounds, determined by quantitative evaluation of Auger spectra, is VC 1.2 and V(C x N y ). x + y = 1.2, the composition of the latter varies between V(C 0.6 N 0.6 ) at 450°C and V(C 0.2 N 1.0 ) at 650°C. Saturation is obse to 650°C, at higher temperatures the total surface coverage decreases gradually. Ar + sputtering and room temperature oxidation experiments indicate that the surface compounds form a homogeneous layer on the substrates, the layer thickness is estimated to be less than 2 atomic layers. Upon quenching V(C,N) and VC surface precipitates are growing on clean and sulphur saturated substrate surfaces to a thickness of the precipitates of about 50 A. Sulphur does not prevent the formation of surface precipitates upon quenching but effectively suppresses the formation of V(C,N) and VC surface compounds.