Gold-coated contacts, surface

Fretting corrosion (36,37) can lead to high contact resistance of base metal contacts, such as tin plate in electronic connectors. Small cycHcal displacements of the connector halves occur because of external vibration or differential thermal expansion and contraction of the mating contacts. The wear debris that is formed remains in the contact zone. The accumulation of oxide debris in the contact region leads to increased contact resistance. Solutions to this problem are stmctures that do not permit movement of contact surfaces with respect to one another, the use of gold as a contact finish, and the appHcation of thick coatings of contact lubricants and greases, which reduce the rate of wear and restrict access of air to the contact surfaces. 

Fig. 2.2. Mycelium of Hebeloma crustuliniforme after colonization of a potassium feldspar surface for seven months. The sample was prepared by fixation and critical point drying followed by gold coating and analysis by scanning electron microscopy. Hyphae (H) and bacteria (B) are visible. Scale bar = 10 pm. The hyphal surface contact is mediated by a fdm of extracellular mucilage (arrow) and bacteria are seen in the mucilage.

In their experiments the authors worked with gold-coated silica spheres, with self-assembled monolayers of mixtures of undecanethiols and co-hydroxy undecane thiols, to vaiy the contact angle between 20 and 100°. The results were compared with sessile drop measurements on the same system. A systematic difference up to 20° for the receding angles) between the two sets of results was observed, but since these differences depend on the nature of the solid surface it was not possible to pinpoint the origin definitely. A remarkable feature was that in the AFM method, hysteresis was very small, if not absent. Anyhow, this appears to be a promising technique. 

This method is used for bonding two silicon surfaces. A thin layer of gold is coated onto the surface of one of the wafers, or a thin sheet of gold-siiicon or gold-tin mixture is placed between the two surfaces (a preform). The assembled device is then clamped together and heated in a vacuum chamber above the eutectic temperature for the particular materials (e.g., eutectic temperature for Si/Au is 36 °C). The preform melts and a diffusion process (gold into the silicon surface) leads to the formation of a silicon alloy between the contacted surfaces. As this cools, a bond is formed between the two surfaces. ... 

Microcantilevers are associated with AFM, which basically are gold-coated surfaces based on a silicon core, which are associated with nanomechanics for biomolecular recognition (1,2,20,25). In AFM technology, a cantilever is in direct contact with the sample surface then the bending of the cantilever is determined by optical detection of the position of a laser beam (33). In fact, this method is a versatile tool for surface characterization and provides information concerning topological variations at the molecular level. 

Fig. 4.2 Schematic of alkanethiol monolayer-modified gold-coated AFM tip used for CFM and interactions between exposed functional groups on the contacting surfaces (right chemically modified AFM tip and functionalized surface. Different functionalities, and interactions are indicated by different gray scales and arrows, respectively). For details on the self-assembled monolayer-based tip functionalization, see text (adapted/reproduced with permission from [11])...

In previous studies [3, 4], we had pointed out that the interphase formation mechanisms result from dissolution of the metallic surface layers, concomitantly with ion diffusion through the liquid prepolymer. In order to detect the dissolution phenomenon, pure amine (either DETA or IPDA) was previously applied to chemically etched metallic sheets (either A1 or Ti alloys were used, and had hydroxidic surfaces). After 3 h, the metallic surfaces were scraped with a PTFE spatula. The modified amine (i.e., the amine reacted with the metal) was analyzed. Whatever the natures of the amine and the metal were, metal ions were detected in the modified amines by ICP analysis and new peaks were detected by infrared spectroscopy [5]. To indicate hydroxide dissolution, a very thin layer of liquid amine was applied to chemically etched aluminum, and Infrared Reflection - Absorption Spectroscopy (IRRAS) spectra were recorded every 5 min (the hydroxide band intensity variation at ca. 3430 cm was followed). The OH group peak intensity decreased when the amine-metal contact time increased [5]. Conversely, if pure DGEBA monomer was apphed to the metal surfaces, even after 3 h in contact with the metallic surfaces, no metal ion was detected by ICP in the DGEBA recovered, and the infrared spectra remained identical before and after the contact with the metal. Finally, if pure amine monomer was applied to gold-coated substrates, no chemical reaction was observed (by either IGP or FTIR analyses). 

The correlation of pull-off forces with the cosine of contact angles measured with water clearly showed the applicability of this approach for various polar pol3uners (66,85). For apolar pol5miers, for example, the Lifshitz theory can be successfully applied to predict the chemical contrast. Feldman and co-workers demonstrated this powerful quantitative approach for different pol5miers probed with gold-coated and silicon oxide coated tips in perfluorodecalin (86), in which the dispersive van der Waals interactions are selectively amplified (see section Surface Forces and Energies). 

Two types of specular references are used as standard materials. The first type, called first-surface mirrors, reflect incident radiation off the primary contact surface. These are typically vapor deposited or electroplated metals—gold, silver, rhodium, or aluminium on metallic or nonmetaUic (glass, quartz, or some other material that is thermally and mechanically stable) substrates. An important consideration is the flatness of the surface to be coated. The material must have minimal roughness and the surface must be as flat as possible to avoid measurement error. 

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