Surface Segregation Of Sulfur Research Articles

The effect of cold work on surface segregation of sulfur on oxygen-free high conductivity copper

An investigation of thermally enhanced sulfur segregation along wear tracks made on annealed oxygen-free high conductivity (OFHC) copper and on the surfaces of differently polished unannealed OFHC copper specimens was carried out. Sulfur diffusivity in the cold-worked regions of the specimens was 1–2 orders of magnitude higher than the rate of diffusion in annealed, undeformed regions. Sulfur surface segregation was rapid at low annealing temperatures because of high densities of dislocations in the plastically deformed regions. Dips in the enhanced diffusivities at several annealing temperatures were observed for almost all specimens. These dips were interpreted as being associated with various annealing stages of the cold worked samples. They were centered at approximately 330, 350 and 390 °C. The observed declines in diffusivity were attributed to the rearrangement of dislocations to lower diffusivity configurations and the reduction in dislocation density and grain boundary length as the specimens were annealed at progressively higher temperatures. Evidence of grain growth was obtained by transmission electron microscopy analysis of the polished specimens. X-ray diffraction traces indicated that annealing of samples with polished surfaces also resulted in a strong (100) preferred orientation.

Non-equilibrium surface segregation of sulfur during recrystallization of nickel

Non-equilibrium surface segregation of sulfur during recrystallization of nickel

Influence of surface segregation of sulphur on the corrosion resistance of 270 nickel in acidic and alkaline media

270 Nickel has a sulphur content lower than 1 p.p.m. However, surface concentrations close to saturation values can be obtained after several hours of cold-hammering followed by annealing procedures. The effects of this segregation on the electrochemical behaviour of nickel are analysed using current-potential curves and by examining the corrosion features via scanning electron microscopy (SEM). An overall increase in dissolution for the nickel with surface segregation of sulphur with respect to the homogeneous metal is observed in acidic (1m H2SO4) as well as in alkaline medium [(K2SO4+LiOH) at pH 9.5]. The reaction model proposed here, which depicts the dissolution kinetics, allows specification of the depassivating role of sulphur.

Kinetics of sulfur surface segregation in Fe-6at%Si

The kinetics of Si and S segregation to the (100) surface in Fe-6at%Si single crystals has been measured by Auger electron spectroscopy for the temperature range from 770 to 900 K. A rather unexpected behaviour for sulfur was found which was not controlled by bulk diffusion. Sulfur segregation kinetics can be explained by taking into account both sulfide formation as well as pipe diffusion. At higher sulfur surface coverage the reaction rate drops. The latter may be a consequence of a surface phase transition. Moreover, the model is extended to such cases where the history of the sample should also be considered. Thus, the experiments of the segregation sequence can be described.

XPS study of the segregation of sulphur on Ni-21Cr-8Fe (100) alloy

The surface composition and structure of a Ni-21Cr-8Fe (100) single crystal were studied by angle-resolved XPS, AES and LEED. A detailed calibration was done by XPS with the individual metals. The composition of the clean surface is identical to the composition of the bulk alloy and a (1 × 1) structure is observed. The surface segregation of sulphur during annealing sulphur-doped alloys in UHV was studied in the temperature range 600 to 850°C. At saturation a c(2 × 2) structure is observed by LEED. The segregation of sulphur induces a marked cosegregation of chromium: when the surface is covered by a complete monolayer of sulphur, the surface concentration of chromium is ~ 94%. The free enthalpy of surface segregation of sulphur (Δ H S seg) was derived from the plot of the surface coverage of sulphur versus temperature. A value Δ H S seg= −105 kJ mol −1 is obtained. The segregation kinetics (sulphur coverage versus time) were investigated at 550, 600, 620 and 750°C and a diffusion coefficient D = 0.1 exp(−175 × 10 3 [Jmol −1]/ RT) cm 2s −1 is derived from the experimental results. The adsorption of sulphur was studied by exposing the alloy surface to H 2S-H 2 gas mixtures of well controlled H 2S partial pressure, at 970°C. The alloy surface is saturated by adsorbed sulphur in the range of p(H 2S)/ p(H 2)= 7 × 10 −5 to 1.8 × 10 −4. The sulphur coverage, measured with radioactive sulphur ( 48(± 3) × 10 −9 g cm 2 , which corresponds to a ratio of sulphur atoms to metal atoms on the surface of 0.56. For partial pressures H 2S above 1.8 × 10 −4, the sulphidation of the alloy is observed, which indicates that the alloy does not behave as an ideal solid solution.

Surface segregations on Invar alloy

Equilibrium segregation of sulphur on thefree surface of an Fe–36Ni (wt-%) Invar alloy was studied using Auger electron spectroscopy. It was found that sulphur segregates strongly in the temperature range 600–1100°C. The equilibrium segregation of sulphur can be described using the Fowler equation and a value of the free energy can be determined: −114Z·87 kJ mol−1. At high temperatures (1100°C), sulphur disappears completely from the surface leaving the phosphorus to form presumably a phosphide layer on which sulphur cannot segregate. At lower temperatures (≤700°C), phosphorus cosegregates temporarily with sulphur, then reappears in the alloy matrix. The compound BN precipitates on the surface only if the material is both prior annealed at a temperature higher than the precipitation temperature and ion bombarded at room temperature. In spite of the strong surface segregation of sulphur, the Fe–36Ni alloy is never brittle at room temperature. It is highly probable that manganese drastically decreases the solubility of sulphur.MST/1134

Adsorption and surface segregation of sulphur on Fe17Cr12.6Ni(100) and Fe17Cr14.5Ni2.3Mo(100) alloys

Auger electron spectroscopy (AES) and low energy electron diffraction (LEED) were used to study the surface compositions and structures of Fe17Cr12.6Ni(100) and Fe17Cr14.5Ni2.3Mo(100) alloys with or without segregated sulphur. Ion etching of Fe17Cr12.6Ni(100) alloy produces a surface composition identical to the matrix composition. On this alloy, segregation of sulphur at high temperature (∼800°C) induces the cosegregation of chromium. The cosegregation of nitrogen and chromium was also observed at 600°C, leading to the multilayer growth of chromium nitride. Ion etching of Fe17Cr14.5Ni2.3Mo(100) alloy leads to surface enrichment of molybdenum. The thermal segregation of sulphur on this alloy surface does not induce any chromium cosegregation. On both alloys, the LEED patterns show that the monolayer of segregated sulphur has a c(2 x 2) structure. The chemisorption of sulphur at 970°C in H 2S ∗-H 2 was investigated using the 35S radioisotope. A complete monolayer of sulphur was observed in the investigated range of partial pressures, 1.9 x 10 -5 < p(H 2S) p(H 2) ⩽ 1.5 x 10 -4 . The measured coverages of sulphur were 39(±-3)x10 -9 and 37(±-4)x10 -9 g/cm 2 on Fe17Cr12.6Ni(100) and on Fe17Cr14.5Ni2.3Mo(100), respectively. These measured coverages are consistent with the c(2 x 2) structure observed with segregated sulphur.

Influence of surface melting treatment by laser on the segregation of sulphur, nitrogen and chromium on the surface of a single‐crystal FeCrNi alloy

AbstractAfter melting the (100) surface of a FeCrNi single crystal by a CWCO2 laser, a quasi‐epitaxial regrowth of the melted zone takes place2 within a misorientation of ∼4°. Three distinct areas are obtained: unaffected, heat affected and melted surfaces. On all areas, the segregation of sulphur alone occurs at high temperature, while there is a cosegregation of sulphur and nitrogen at T &lt; 700°C. The thermodynamic interpretation of the segregation results is reported.For the surface segregation of sulphur, no interactions are noted with other alloy elements, except with chromium on the unaffected surface. The difference between the free enthalpies of segregation of sulphur on the untreated and treated surfaces is associated with the evolution of the defect content.When sulphur and nitrogen cosegregate on the surface of the untreated alloy, a repulsive coefficient of interaction between sulphur and nitrogen (i.e. βSN) of the order of 20 ± 5 kJ mol−1 is calculated. For the laser‐treated surfaces, the chromium content increases on the surface. Then, the regular solid solution model is applied to the CrNS ternary system. The calculated βSN coefficient is now 45 ± 5 kJ mol−1 and agrees with the experimental data obtained from the experimental evolution of ΔG versus YN (with ΔG = free energy of segregation of S and data YN = nitrogen concentration on the surface). More complex interactions are also discussed, especially for nitrogen, on account of the site competition between N and S and the influence of the vacancy content on the treated surfaces.

Enhanced surface segregation in sliding wear tracks

The effect of sliding wear on the rate of surface segregation of sulfur from an oxygen-free, high-conductivity (OFHC) copper sample was investigated. The study was carried out in an ultra-high-vacuum system having a residual gas pressure of 5*10/sup -11/ torr. Wear tracks were formed on OFHC Cu that had previously been annealed and argon-ion sputter cleaned. A bent pin (palladium-based alloy, ASTM B540) was made to slide across the surface with a contact force of 25 g for 9000 cycles. Auger electron spectroscopy was used to characterize the composition of the surface on and off the wear track. No surface segregation was observed to occur as a result of forming the wear track. Subsequent in situ isothermal annealing between 310 degrees C and 470 degrees C produced S enhancement on the specimen surface. It was observed that the rate of S segregation on the wear track was much faster than off track up to approximately 390 degrees C, where this rate decreased significantly. At higher temperatures, the rate of S segregation on-track approximately equaled the off-track rate. The drop in the rate of S segregation on the track of 390 degrees C is attributed to annealing-out of the short-circuit diffusion paths associated with the defects formed when the wear track was made. The subsequent increase in S concentration at higher temperatures is due to the normal surface segregation phenomenon which occurs in annealed samples.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

AES study of aluminum and sulfur surface segregation in the Ni - 9%Al solid-solution

AES study of aluminum and sulfur surface segregation in the Ni - 9%Al solid-solution

Influence of laser melting treatment on surface composition of Fe–17Cr–13Ni single crystal alloy Part 2 Surface segregation of nitrogen and sulphur

An Fe–17Cr–13Ni (wt-%) single crystal alloy was melted using a continuous wave (CW) CO2 laser. The surface segregation of sulphur and nitrogen at high temperature in vacuum is investigated on the various surface zones (base alloy, heat affected zone, and melted zone) using Auger electron spectroscopy (AES). Below ~700°C, cosegregation of sulphur and nitrogen is observed, while above 700°C, only sulphur segregates on the surface on all three zones. The segregation of sulphur or the cosegregation of sulphur and nitrogen induce, on the three zones, the segregation of chromium in the first atomic plane. The enthalpy for the surface segregation of sulphur ∆HsegS is found to be −164±10 kJ mol−1 for the base alloy. The values of ∆HsegS are about 40% lower for the treated zones. The result for the base alloy is in agreement with the segregation enthalpy calculated from the reported data on adsorption and dissolution of sulphur, i.e. ∆HsegS ≍−157 kJ mol−1. The pseudodiffusion parameters for sulphur and nitrogen are determined. The activation energy for nitrogen diffusion is the same for all three zones and is in agreement with literature data (~−180 kJ mol−1). For sulphur, the energies for diffusion are highly dependent on the laser treatment. The activation energy for diffusion is maximum in the melted zone: ~ 146 kJ mol−1; the values in the untreated and heat affected zones are 75 and 85 kJ mol−1, respectively.MST/1175

Influence of laser melting treatment on surface composition of Fe–17Cr–13Ni single crystal alloy Part 2 Surface segregation of nitrogen and sulphur

An Fe–17Cr–13Ni (wt-%) single crystal alloy was melted using a continuous wave (CW) CO2 laser. The surface segregation of sulphur and nitrogen at high temperature in vacuum is investigated on the various surface zones (base alloy, heat affected zone, and melted zone) using Auger electron spectroscopy (AES). Below ~700°C, cosegregation of sulphur and nitrogen is observed, while above 700°C, only sulphur segregates on the surface on all three zones. The segregation of sulphur or the cosegregation of sulphur and nitrogen induce, on the three zones, the segregation of chromium in the first atomic plane. The enthalpy for the surface segregation of sulphur ∆HsegS is found to be −164±10 kJ mol−1 for the base alloy. The values of ∆HsegS are about 40% lower for the treated zones. The result for the base alloy is in agreement with the segregation enthalpy calculated from the reported data on adsorption and dissolution of sulphur, i.e. ∆HsegS ≍−157 kJ mol−1. The pseudodiffusion parameters for sulphur and nitrogen are determined. The activation energy for nitrogen diffusion is the same for all three zones and is in agreement with literature data (~−180 kJ mol−1). For sulphur, the energies for diffusion are highly dependent on the laser treatment. The activation energy for diffusion is maximum in the melted zone: ~ 146 kJ mol−1; the values in the untreated and heat affected zones are 75 and 85 kJ mol−1, respectively.MST/1175

Adherence of Al 2O 3 coating layers to steels: The detrimental effect of sulphur and the beneficial effect of rare earth metals

Based on our previous work of scale spalling during cyclic oxidation and the surface segregation of sulphur, we considered that the adherence of an Al 2O 3 coating layer to substrate steel will be weakened by the segregation of sulphur at the Al 2O 3-steel interface. In this work, the surface segregation rate of sulphur was measured on type 304 steels with different sulphur contents, type 310S steels with and without rare earth metal (REM) addition and MA6000 alloy with Y 2O 3 dispersion. The same types of steel were then coated with Al 2O 3 and the spalling of the Al 2O 3 layer during cyclic oxidation examined. It was found that the extent of spalling was large on steels having a large surface segregation rate of sulphur and vice versa. Thus we concluded that sulphur segregation is detrimental to Al 2O 3-metal adherence, and that the beneficial effect of REM is attributed to the suppression of sulphur segregation through trapping it as a stable sulphide in the matrix.

Sliding and thermal effects on sulfur segregation to gold alloy contact surfaces

Thermally induced surface segregation of sulfur has been measured on a gold alloy (ASTM B541). Comparisons were made among ‘‘as‐received,’’ abraded, and wear track surfaces that were ion‐sputter cleaned before heating. The wear tracks were formed by oscillatory sliding at room temperature in a pin‐on‐plate configuration; the pin was a palladium alloy (ASTM B540). Wear tracks and heating were accomplished in situ in an ultrahigh vacuum system. The elemental compositions were measured with Auger electron spectroscopy. Previous conclusions were confirmed that sulfur segregates to wear tracks due to rubbing alone and that the source of the sulfur was the bulk material. It was found that thermal sulfur segregation was significantly greater in wear tracks than on abraded surfaces, and both wear tracks and abraded surfaces had greater thermal sulfur segregation than as‐received surfaces. In the case of as‐received surfaces, a highly anisotropic distribution of sulfur was observed and was attributed to enhanced diffusion of sulfur along grain boundaries and/or defects in the near‐surface material. Thermal segregation was strongly influenced by the thermal treatment of the sample.

Sulphur restraint of surface sites in Co-5.4at.%Ru alloy

Using Auger electron spectroscopy, site competition in surface segregation of sulphur and phosphorus in a polycrystalline Co-5.4at.%Ru alloy was investigated. Annealing of the sample at 1250 K results in sulphur desorption. Then the phosphorus segregation starts to increase. A model of the surface which explains the observed phenomenon is presented.

Surface segregation of sulfur and silicon in an Fe-6 at% Si alloy

Etude, par spectrometrie Auger, de la segregation du soufre entre 750C et 850 C et du silicium a la surface (100) de l'alliage contenant 60% at. p.p.m. C et 3% at. p.p.m. S. Analyse des interactions entre Si et S dans la matrice et sur la surface. Interpretation des resultats par le modele de Guttmann dans les solutions ternaires de substitution

Effect of surface impurities and argon sputtering on deuterium permeation of nickel

To clarify the effect of surface impurities upon hydrogen isotope permeation through nickel, the deuterium permeation rate of variously treated nickel was measured in the pressure range of 3 × 10 −4 to 2 × 10 −1 Pa at 673 K. Surface segregation of sulfur proceeded during a thermal anneal at ∼ 673 K as was identified with in-situ AES analysis. Even in this low pressure range, the permeation rate of nickel obeyed Richardson's equation: the rate-limiting step is bulk diffusion. Neither carbon nor sulfur on the surface significantly affected the steady state permeation rate, which is different from the case for vanadium. On the other hand, argon ion sputtering decreased the deuterium permeation rate of nickel. This decrease in permeation rate can be recovered to some extent by an anneal at ∼ 673 K. Such behavior is probably attributable to some sort of irradiation damage produced during argon ion sputtering.

UHV-AES Investigation of Sulfur Surface Segregation in Precious Metal Wear Tracks

Cyclic sliding experiments of a palladium-base (ASTM B540) alloy pin On a gold-base (ASTM B541) alloy plate have been completed in an ultrahigh vacuum (UHV) system that was backfilled to one atmosphere with chromatographic-grade helium. The elemental concentrations in the wear tracks were measured in situ by Auger electron spectroscopy (AES) before and after several thousand cycles were performed. The concentration of sulfur in the wear tracks showed an · appreciable increase (a factor of 2.5-6.4) relative to the off-track regions. The sulfur enhancement was dependent on the number Of sliding cycles. Residual gas analysis of the vacuum-system atmosphere before introduction of the helium and analysis of the chromatographic-grade helium detected no sulfur-containing gases; it is estimated that the partial pressure of sulfur-containing gases was less than - 1 x 10-11torr for all experiments. These results complement an earlier study involving Cyclic sliding of the same alloy couple which also showed that sulfur enhancemerit occurred in the wear tracks. The earlier study was completed in a dry box using controlled environments of helium, carbon dioxide, and oxygen; however, the partial pressure of sulfur-bearing gases could have been orders of magnitude larger than those in the present study. It is concluded that the sulfur originates in the bulk of the alloys rather than in the various environments in which the oscillations were carried out.

Effect of Surface Segregated Sulphur upon Hydrogen Permeation of Vanadium*

Surface segregation of sulphur and its effect on hydrogen permeation through vanadium have been studied by the use of in-situ sputter cleaning and in-situ AES analysis of specimen surfaces. Rates of hydrogen permeation in vanadium decrease to nearly one-tenth as the result of surface coverage by sulphur up to saturation. The surface segregation of sulphur hardly affects either the temperature or pressure dependency of hydrogen permeability. A new atomistic model has been developed to explain effects of surface segregated sulphur on dissociative hydrogen adsorption and on hydrogen permeation rates.

ORDERED STRUCTURE FORMED BY THE SEGREGATION OF SULFUR ON THE Mo(111) SURFACE

The segregation kinetics and surface structures of sulfur on the Mo(111) surface have been studied by means of AES and LEED in the temperature range between 700℃ and 800℃. When the sample was heated to 800℃, sulfur segregated rapidly and progressively replaced the carbon on the surface. However, as heated to 700℃, no significant sulfur segregation has been observed. An AES analysis also showed that sulfur segregation may be controlled by its bulk diffusion rate and the presence of carbon on the surface might inhibit the surface segregation of sulfur. LEED patterns of new surfacestructures, namely Mo(111) (31/2×31/2)R30°-S and Mo(lll) l/3(99-44)-Shave beenobserved respectively.