Surface Segregation Of Sulfur Research Articles

The Surface Segregation of Sulfur During Oscillatory Pin-On- Plate Wear Studies of Precious Metal Electrical Contact Alloys

An oscillatory pin-on-plate experimental configuration was used to investigate friction and electrical contact resistance phenomena of a gold-base (ASTM B541) and a palladium-base (ASTM B540} alloy couple. Measurements were made in controlled environments of dry helium, dry carbon dioxide, and dry air at atmospheric pressure. No lubrication was employed. Friction coefficients were initially high but decreased to a steady value of approximately 0.2 as the number of oscillatory cycles increased for dry carbon dioxide and dry air. The friction results for dry helium were much more erratic. Friction coefficients are much less sensitive to the nature of the contact interface than is electrical contact resistance. The electrical contact resistance always had initial values of 1-10 mµ, but it tended to increase by up to three orders of magnitude in a nonreproducible manner for all three gases. Sharp fluctuations in electrical contact resistance were common. Electrical polarity was reversed, but due to the large variability in contact resistance no effect was identified. The surface compositions and morphologies of wear tracks were characterized using scanning Auger microscopy combined with Ar-ion beam sputtering. The analyses revealed that the wear tracks formed in all three atmospheres contained an approximately 6-rim thick surface film consisting of S, Ag, Cu, C, O, and Pd; the Pd was transferred from the pin as small debris paMicles. The S in this thin surface film may be present as Ag and/or Cu sulfides. The observed increase in the contact resistance and decrease in the coefficient of kinetic friction are attributed to the growth of thin sulfide layers on the wear track surfaces.

金属表面のイオウ偏析におよぼす気体吸着の効果

金属表面のイオウ偏析におよぼす気体吸着の効果

Observation of surface segregation of sulphur in CuNi alloys at elevated temperature with scanning Auger electron microscope

Surface segregations of Cu and S in a CuNi (50wt%) alloy at elevated temperatures of ∼600°C were studied under scanning Auger electron microscope, JAMP-3. Scanning images of S, Cu and Ni Auger signals have revealed that the surface segregations of both Cu and S take place preferentially depending on grains and the contrasts in S and Cu Auger images are complementary. This leads to another confirmation of the previous work, namely, that the surface segregation of Cu is considerably suppressed by the existence of S at the outermost atom layer.

Observation of surface segregation of sulphur in Cu-Ni alloys at elevated temperature with scanning auger electron microscope

Surface segregations of Cu and S in a Cu-Ni (50wt%) alloy at elevated temperatures of ∼600°C were studied under scanning Auger electron microscope, JAMP-3. Scanning images of S, Cu and Ni Auger signals have revealed that the surface segregations of both Cu and S take place preferentially depending on grains and the contrasts in S and Cu Auger images are complementary. This leads to another confirmation of the previous work, namely, that the surface segregation of Cu is considerably suppressed by the existence of S at the outermost atom layer.

Elemental Surface Composition of Slip Ring Copper as a Function of Temperature

The surfaces of four copper samples--oxygen-free high-conductivity (OFHC) (99.98 percent) Cu, 99.99 percent Cu, 99.9999 percent Cu, and a dilute Ag/Cu alloy (0.086 weight percent (wt %) Ag in OFHC Cu)--were examined in an ultrahigh vacuum system by Auger electron spectroscopy as a function of temperature between 25 and 600°C. It was found that the elemental surface composition of each sample varied drastically with temperature. For example, carbon was a surface constituent in amounts ranging from 0 to about 100 at %, depending upon surface treatment. An interesting finding was the rapid surface segregation of sulfur upon heating in all samples, leading to surface concentrations ranging up to 32 at % in some cases. These results suggest that the higher temperatures occurring at the current-carrying surface asperities will alter the composition of the interface. Hence, the electrical resistance of the interface and its friction and wear properties may be grossly affected.

Adsorption and segregation of sulphur, and its influence on the carburization and nitrogenation of iron and steel

The surface segregation of sulphur on iron was investigated by A.e.s. on iron polycrystals in the temperature range 700-1100 °C. Strong surface segregation occurs even at low bulk concentrations of 2—20 ug/g. The enthalpy of segregation for S(dissolved) = S (adsorbed) is about —165 kJ/mole S. By l.e.e.d. studies at 750 °C, ordered adsorption structures were observed on iron single crystal surfaces. In the segregation studies the equilibrium sulphur activity, a s , was determined by the concentration of dissolved sulphur. The results were correlated with the surface reaction kinetics in the carburization and nitrogenation of iron in the presence of adsorbed sulphur. These studies were performed in a flow apparatus at atmospheric pressure, the sulphur activity being established by the H 2 S/H 2 ratio in the gas atmosphere. The effect of adsorbed sulphur was studied for the reactions CH 4 (g) = C (dissolved) + 2H 2 (g) and N 2 (g) = 2N(dissolved) by a resistance relaxation method on thin iron foils at 800—900 °C. Even at bulk concentrations of about 0.1 ug/g S, the surface reactions were strongly retarded. Carbon and nitrogen transfer can only occur on reaction sites free of adsorbed sulphur. It can be seen from the kinetics that two sites are needed for the dissociation of N 2 but one site is sufficient for the decomposition of CH 4 . From the reaction kinetics, the adsorption isotherm for sulphur adsorption and segregation at 850 °C could be derived.

Studies of the influence of sulphur on the passivation of nickel by Auger electron spectroscopy and electron spectroscopy for chemical analysis

During the anodic polarization of sulphur-doped nickel, selective dissolution of nickel and surface segregation of sulphur are observed. These phenomena cause a complete inhibition of passive layer formation when the sulphur content is above a critical concentration. Auger electron spectroscopy and electron spectroscopy for chemical analysis were used to characterize the species formed on the surface during the sulphur enrichment. The spectra were interpreted by comparison with standard spectra from synthesized compounds: NiS, Ni 3S 2, Ni-adsorbed S. A comparison with previous results using radioactive sulphur ( 35S) allowed us to determine the mechanism of the sulphur enrichment and of the consequent inhibition of passivation by the formation of an adsorbed sulphur layer and subsequent growth of a nickel sulphide Ni 3S 2 which attains a stationary thickness of about 30 Å.

AES study of sulfur surface segregation on polycrystalline copper

The surfaces of four polycrystalline copper samples were examined under UHV conditions by AES: OFHC Cu, 99.99% Cu, 99.9999% Cu, and 0.086 wt. % Ag in OFHC Cu. Surface chemical compositions in the range of 25°–600°C were determined. Sulfur concentrations on the surfaces at various temperatures were also measured as a function of time. In almost all cases the sulfur concentration increased as (time)1/2. This process was interpreted in terms of a model based on diffusion controlled ’’outgassing’’ in which the sulfur segregated to the surface instead of vaporizing. The activation energies for this process for the four samples ranged from 0.18 to 0.31 eV/atom. RHEED data from an OFHC Cu sample indicated that sulfur formed an orthorhombic Cu2S structure on the surface. Around 500°C the Ag–Cu alloy experienced a complex, rapid stage of segregation of sulfur. Above this temperture range the surface sulfur concentration dropped rapidly, reaching zero at 600°C.

Some Observations of Surface Segregation by Auger Electron Emission

Surface segregation of sulfur in nickel and steel, and of chromium and antimony in steel have been observed using an Auger electron method of analysis. The method provides a useful technique for studies of diffusion and segregation in solids.