Richard W. Vook

Surface segregation from gold alloys

Among the properties of alloys affected by surface segregation are corrosion resistance and strength. In this paper, the surface segregation phenomenon is reviewed, with particular attention given to gold-containing alloys. A brief discussion of the various thermodynamic models for surface segregation is provided. The results of experimental investigations of surface segregation for gold-containing alloys are summarized and explained in terms of some of the thermodynamic models.

Nucleation And Growth Of Thin Films

There are three basic thin-film growth modes: Volmer-Weber (island), Frank and van der Merwe (layer), and Stranski-Krastanov (layer followed by island). Examples of these growth modes and variations thereof are presented, and the theories governing the initial and later stages of growth in each case are reviewed. Some useful applications of the theories are also given.

Microstructural characterization of rotating Cu-Cu electrical contacts in vacuum and wet CO2 environments☆

Abstract The chemical, electrical and wear properties of the rotating interface between OFHC copper slip rings and two high purity copper wire brushes were investigated in situ in ultrahigh vacuum (UHV) and in 1 atm wet CO 2 . The chemical composition of the slip ring surface was determined by Auger electron spectroscopy. The contact resistance was measured by a potentiometric four-point probe technique while the wear properties of the interface and the morphology of the debris were studied by frictional force, scanning electron microscopy (SEM), transmission electron diffraction (TED) and X-ray diffraction (XRD) measurements. Rotation in UHV of a conventionally cleaned (CC) slip ring produced a much cleaner surface. The contact resistance of both brush interfaces decreased and the frictional force increased with increasing number of revolutions. After many revolutions the brush and slip ring welded. The decrease in contact resistance with the number of slip ring revolutions more or less paralleled the decrease in total impurities. Rotation in wet CO 2 of a CC slip ring and brushes also produced much cleaner surfaces. In contrast, initially argon ion sputter-cleaned surfaces became slightly contaminated (mainly carbon and sulfur) when rotated. The contact resistance at both interfaces and the coefficient of friction decreased with increasing number of slip ring revolutions, finally reaching steady state values. After each experiment, SEM examination of vacuum rotated surfaces showed deep ridges and broken pieces of material on the slip ring surface and badly deformed brush wire ends. Wet CO 2 rotated surfaces were relatively smooth and shallow ridges were seen. SEM examination of wear particles collected during rotation indicated that they may have come from both the slip ring and brush wire materials and were rolled in the regions between the brush and slip ring. XRD and TED from individual particles showed a randomly oriented polycrystalline microstructure. The particles collected from the wet CO 2 experiments were much smaller in size than those collected in vacuum experiments. In wet CO 2 , the contact resistance was interpreted as being predominantly due to an electron tunneling mechanism through the CO 2 -H 2 O molecular layer at the interface. As expected, the thickness of the layer appeared to vary with the contact pressure. Friction would then arise largely when the molecular layer was occasionally broken, allowing intimate contact and temporary welding of the brush and slip ring surfaces. Subsequent fracture of these welds during continued rotation would initiate the formation of wear particles.

In Situ AES Characterization of Rotating Electrical Contacts

The electrical contact resistance and surface chemical composition of a rotating copper slip ring in contact with two wire brushes were investigated in situ under ultrahigh vacuum (UHV) conditions as a function of the number of revolutions of the slip ring. The initial surface of the copper slip ring was examined by Auger electron spectroscopy (AES) techniques and found to be almost completely covered by surface impurities largely consisting of carbon. As the slip ring was rotated in contact with the brushes, the impurity concentration declined sharply to less than ten at% after several hundred revolutions. A corresponding decrease in electrical contact resistance and a sharp increase in friction and wear were also observed. Regardless of polarity, the brush with the higher contact pressure had the lower contact resistance. The in situ experiments were terminated when the motor turning the slip ring could no longer overcome the adhesive forces between the brushes and slip ring. Subsequent scanning electron microscopy observations of the brush and slip ring surfaces gave supplementary information on the mechanisms of friction, wear and surface cleaning during rotation. For a given experiment the change in contact resistance during rotation divided by the change in impurity concentration on the surface of the slip ring is approximately the same for the positive and negative interfaces. This parallelism of contact resistance and surface impurity concentration suggests that the former is caused to a significant extent by the latter. Since the surface impurities are mostly carbon atoms, it is concluded that carbon impurities are largely responsible for the observed contact resistance.

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

Abstract 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.

Electromigration studies using in situ TEM electrical resistance measurements

Abstract An in situ transmission electron microscopy (TEM) method has been devised for the measurement of electrical resistance as a function of temperature and/or time for thin film stripes. A custom TEM sample holder was fabricated through which up to eight leads may be passed. These leads provide currents to the heater and allow for the measurement of hot stage temperature and the electrical resistance of the sample. This configuration enables one to correlate changes in resistance of the stripe during electromigration experiments to the microstructural changes occurring at the same time. The time-dependent microstructural phenomena are recorded with a camcorder on video tape, a capability which greatly enhances the subsequent data analysis. Preliminary experiments have been performed using the Temperature-ramp Resistance Analysis to Characterize Electromigration (TRACE) method in order to determine the kinetic parameters for electromigration for pure aluminum films.

The epitaxy of gold

Previous studies of the epitaxial growth of gold by vapor deposition on a variety of substrates have been compiled and listed according to the type of substrate. A short introduction describing the epitaxy process has been included for general reference purposes and some highly promising applications of thin film technology are discussed.

In situ auger electron spectroscopy characterization of wet-CO2-lubricated sliding copper electrical contact

Abstract The electrical contact resistance, elemental surface composition and friction of an OFHC copper slip ring rotating in contact with two high purity copper wire brushes on different tracks were investigated in situ for heavy and light normal contact forces under a wet CO 2 environment at atmospheric pressure. Scanning electron microscopy was also used to characterize the slip ring and brush surfaces. Previous work in ultrahigh vacuum showed that, as rotation proceeded, interfacial impurities were almost totally removed and the electrical contact resistance decreased until cold welding occurred. In the present work, the slip ring surface was sputter cleaned (more than 95% Cu) before contact rotation and was only slightly contaminated after rotating in wet CO 2 . Both the contact resistance and the friction decreased quickly and reached steady state values almost simultaneously in the early stages of rotation. Also, cold welding phenomena did not occur. Scanning electron micrographs taken after each experiment showed that the surfaces of the slip ring tracks and the brush wire ends were much rougher when heavy normal contact forces were used than in the light normal force condition. All these results confirm that wet CO 2 is an effective lubricant for Cu-Cu electrical sliding contacts.

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.

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.