Electrical Contact Applications Research Articles

Electrical, Mechanical and Microstructural Properties of Copper-Chromium-Silver-Alumina Composites Obtained by Powder Metallurgy

It has been studied the correlation of the obtained microstructures in cermets of Cu-Cr-Ag-Al2O3with the physical properties and the fabrication method varying the components proportions to obtain by powder metallurgy a material with good electrical conductivity and also good mechanical strength compared with pure copper allied to efficient fabrication and economical energetically process. The electrical conductivity data correspond to 40% IACS indicating that the utilized thermal treatments were relatively good for electrical contact applications and also mechanically.

AgMgAl metallic glassy and intermetallic thin films for electric contact applications

The AgMgAl thin films, in an attempt in replacing the expensive pure Au contact films, are prepared by co-sputtering. The surface morphology, roughness, amorphous or crystalline atomic structure, grain size, and electric resistivity are systematically examined. Depending on the film compositions, the films can be fully amorphous or fully nanocrystalline, or a composite with the mixture of nanocrystalline phases dispersed in the amorphous matrix. Under the as-sputtered condition, the crystalline group has the lowest resistivity, ranging from 27 to 37 μΩ·cm, the composite group lies in the middle, 31–70 μΩ·cm, and the fully amorphous group possesses the highest resistivity, 87–122 μΩ·cm. Appropriate short thermal annealing for the amorphous films can drastically lower the resistivity down to as low as 9 μΩ·cm, already compatible to pure Au (3–7 μΩ·cm). This study demonstrates the feasibility of the AgMgAl films in replacing the pure Au.

Growth and characterization of chromium carbide films deposited by high rate reactive magnetron sputtering for electrical contact applications

Chromium carbide films with different phase contents were deposited at 126±26°C by industrial high rate reactive magnetron sputtering, using both direct current magnetron sputtering (DCMS) and high power impulse magnetron sputtering (HiPIMS). Film structure and properties were studied by SEM, XRD, TEM, XPS, NRA, Raman spectroscopy, nanoindentation, unlubricated reciprocating sliding experiments, and a laboratory setup to measure electrical contact resistance. The films consisted of amorphous a-CrCy, a nanocrystalline minority phase of metastable cubic nc-CrCx, and a hydrogenated graphite-like amorphous carbon matrix (a-C:H). The DCMS and HiPIMS processes yielded films with similar phase contents and microstructures, as well as similar functional properties. Low elastic modulus, down to 66GPa, indicated good wear properties via a hardness/elastic modulus (H/E) ratio of 0.087. Unlubricated steady-state friction coefficients down to 0.13 were obtained for films with 69 at.% carbon, while the electrical contact resistance could be reduced by two orders of magnitude by addition of a-C:H phase to purely carbidic films. The present films are promising candidates for sliding electrical contact applications.

Reactive sputtering of NbCx-based nanocomposite coatings: An up-scaling study

Nanocomposite Nb-C coatings, with a C/Nb ratio of 0.93–1.59, have been deposited by reactive sputtering in a commercial sputtering system where the C is supplied from an acetylene gas at deposition rates of up to 200nm/min. The coatings are compared to non-reactively sputtered Nb-C coatings deposited from Nb and C targets in lab-scale equipment at deposition rates two orders of magnitude lower. X-ray diffraction, X-ray photoelectron spectroscopy, and electron microscopy are used to conclude that all coatings consist of nanocrystalline NbCx grains (nc-NbCx) embedded in a matrix of amorphous C (a-C). The coating performance was evaluated in terms of their mechanical, tribological, and electrical properties. The chemical stability of the coatings was evaluated by exposure to a flowing mixture of corrosive gases. It is found that the coatings have comparable microstructure and performance to the coatings deposited by non-reactive sputtering. The high deposition rate and presence of different C-radicals on the coating surface during film growth for the reactively sputtered coatings are believed to result in a smaller NbCx grain size compared to the non-reactively sputtered coatings (reactive process: 10–3nm, non-reactive process: ~75–3nm). This difference results in a thinner a-C matrix of about 0.2nm, which is not varying with C content for the reactively sputtered coatings. The thinner a-C matrix is reflected in coating properties, with a higher conductivity and slightly higher hardness. The coating richest in C content (C/Nb ratio 1.59) shows the lowest friction (0.23), wear rate (0.17×10−6mm3/mN), and contact resistance before (11mΩ at 10N) and after (30mΩ at 10N) the chemical stability test. These results imply that nc-NbCx/a-C coatings of this composition are a good candidate for electrical contact applications, and that up-scaling of the process is achievable.

Nb-B-C thin films for electrical contact applications deposited by magnetron sputtering

The high wear resistance, high chemical inertness, and high electrical conductivity of magnetron-sputtered transition metal diborides make them a candidate material for sliding electrical contacts. However, their high hardness makes it difficult to penetrate surface oxides, resulting in a high electrical contact resistance. In this study, the authors have investigated how the contact resistance can be improved by the formation of softer Nb-B-C films. The Nb-B-C films were deposited by magnetron sputtering and shown to exhibit a nanocomposite microstructure consisting of nanocrystalline NbB2−x grains with a solid solution of C separated by an amorphous BCx phase. The formation of the BCx phase reduces the hardness from 41 GPa for the NbB2−x film to 19 GPa at 36 at. % C. As a consequence the contact resistance is drastically reduced and the lowest contact resistance of 35 mΩ (contact force 5 N) is achieved for a film containing 30 at. % C. However, crack formation and subsequent delamination and fragmentation is observed for the C-containing Nb-B-C films in tribology tests resulting in high friction values for these films.

Synthesis of Ag–ZnO powders by means of a mechanochemical process

Nowadays, Ag–CdO alloys are widely used in electrical contact applications, because of their good electrical and thermal conductivity is as well as high resistance to arc erosion and contact welding. Considering the restricted use of Cd due to its toxicity, it is necessary to find a material that can replace those alloys. The objective of this work was to study the possibility of obtaining an Ag–ZnO alloy from an Ag–Zn solid solution powders by means of a mechanochemical method. The mechanochemical process was carried out in a SPEX 8000D mill, under air and with ethanol as a reaction agent. Based on the results obtained, it can be concluded that an Ag–ZnO alloy with a fine and uniform ZnO distribution in the Ag matrix can be obtained by applying the mechanochemical process for 25 h.

Production and characterization of nanocomposite thin films based on Ni matrix reinforced with SnO2 single-crystalline nanowires for electrical contact applications

Nanocomposite thin films based on electrodeposited Ni matrix reinforced with SnO2 single-crystalline nanowires grown onto Si substrates by chemical vapor deposition were produced. The composites were characterized by means of scanning and transmission electron microscopy (for imaging, selected area diffraction and transmission Kikuchi diffraction), atomic force microscopy (for 3D surface profiling and roughness evaluation) and 4-point probe electrical resistivity measurements. The Ni matrices obtained were nanocrystalline in nature (41nm crystallite mean size) even though low direct current electrodeposition was used. The topography and roughness of the samples were strongly affected by the presence of the nanowires as so was the electrical resistivity, which could be improved by Ag-coating the nanowires. A comparison with pure Ni produced in the same way is presented for determining the effects of the SnO2 nanowires.

Pulse Electrodeposition of Nanostructured Silver-Tungsten-Cobalt Oxide Composite from a Non-Cyanide Plating Bath

A new cyanide-free “self-lubricated” silver alloy electroplating bath was formulated for silver-based contact finishes for electrical contacts applications. The formulated bath deposited nanostructured silver-tungsten-cobalt oxide composite through pulse-electroplating process. Recipirocation sliding wear, hardness, surface morphology, composition, grain size, and thickness of the deposits were measured. The deposited material displayed exceptional galling resistivity compared to standard silver and any commercially available electroplated silver alloys such as Ag, and Ag-W. This new alloy of silver has potential to replace standard silver or silver-alloy plating of electrical contacts for obtaining better performance with regard to galling or adhesive wear resistivity.

Effect of microwave sintering over vacuum and conventional sintering of Cu based nanocomposites

The blend compositions of Cu99Cr1 (All the compositions are in atom% until otherwise mentioned.), Cu94Cr6, Cu99Cr1–4wt.% SiC (average particle size ∼30nm) and Cu94Cr6–4wt.% SiC were ball-milled for 50h in a stainless steel grinding media. The structural investigation and phase evolution during milling of the same compositions have already been reported in elsewhere ([28]). In the present study, we are reporting effect of microwave sintering on mechanical properties and electrical conductivity over vacuum and conventional sintering carried out at 900°C. Relative densification and grain coarsening characteristics were investigated and compared. The microstructural features were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM). The mechanical properties, namely, Vickers hardness and wear resistance, and electrical conductivity were studied to investigate the suitability of the materials for electrical contact applications. The best combination of mechanical properties and electrical conductivity was obtained for microwave sintered specimens. This is possibly due to the enhanced densification and better grain size distribution achieved in microwave sintering technique. Difference in the properties is discussed in the light of electron scattering factor and densification in presence of ultra fine SiC particles in the nanocomposites.

Electroless deposition of a Ag matrix on semiconducting one-dimensional nanostructures

Composite materials consisting of one-dimensional semiconducting nanostructures embedded in a Ag matrix were produced. The regarded nanostructures are hydrothermally grown TiO2 nanorods and SnO2 nanowires obtained by chemical vapor deposition with a vapor–liquid–solid growth process. The Ag was deposited by means of electroless plating, for which AgNO3 was employed as the Ag+ source and a glucose-based aqueous solution as the reducing agent. Cross-section micrographs obtained through scanning electron microscopy show homogeneous and compact Ag coatings, which penetrated down to the substrate level filling the space between the oxide structures. Additionally, the SnO2 composite was analyzed by focused ion beam nanotomography with a measured porosity of 0.1%. The obtained materials are potential candidates for electrical contacts applications.

Friction, wear and tribofilm formation on electrical contact materials in reciprocating sliding against silver-graphite

In this study nanocomposite coatings mating silver-graphite were investigated for sliding electrical contact applications with the aim to optimize tribological and electrical properties. Apart from two different Ti–Ni–C nanocomposite coatings, brass, steel and TiN were also tested against a commercial silver-graphite at varying load and current. Friction, wear and contact resistance were measured in reciprocating sliding in ambient air. It was concluded that the wear of the silver-graphite was increased by current, for TiN and steel as much as four times, at a 5N load. A tribofilm, with properties differing from the silver-graphite, formed on the coating/metal surface in all cases. This resulted in a very similar coefficient of friction, 0.3, for all mating materials. However, different load and current gave rise to slightly different thickness and morphology. A too low load was detrimental, as the coating became damaged, while a too high load was not favorable for tribofilm formation. In tests with varying current, a specific current could be identified that best governed the build-up of a well conducting and stable tribofilm. The largest differences were observed in the initial stages of testing, since once the tribofilm was built up, the contact resistance approached that of self mated silver-graphite, 40mΩ. Experiments showed that the load and the current can be optimized to minimize wear of the silver-graphite which in turn would allow for lower maintenance costs.

Effect of sintering temperature on the preparation of Cu–Ti3SiC2 metal matrix composite

Ti3SiC2 has the potential to replace graphite as reinforcing particles in Cu matrix composites for applications in brush, electrical contacts and electrode materials. In this paper the fabrication of Cu–Ti3SiC2 metal matrix composites prepared by warm compaction powder metallurgy forming and spark plasma sintering (SPS) was studied. The stability of Ti3SiC2 at different sintering temperatures was also studied. The present experimental results indicate that the reinforcing particles in Cu–Ti3SiC2 composites are not stable at and above 800°C. The decomposition of Ti3SiC2 will lead to the formation of TiC and/or other carbides and TiSi2. If purity is the major concern, the processing and servicing temperatures of the Cu–Ti3SiC2 composite should be limited to 750°C or lower. The composites prepared by warm compaction forming and SPS sintering at 750°C have lower density when compared with the composites prepared by SPS sintering at 950°C, but their electrical resistivity values are very close to each other and even lower.

Preparation and characterization of nanostructured silver particles by one-step spray pyrolysis

Abstract Recently nanostructured silver particles have received much attention due to their wide applications in conductors, electrical contacts, catalysts and wound dressing. Although various methods have been applied to produce silver particles, the spray pyrolysis (SP) process has an advantage in its low cost and feasibility for mass production. The majority of the previous studies using the SP process focused on particle size control by adjusting concentration of silver nitrate salts. However there are few reports investigating the correlation between particle morphologies and precursor types. Here we have prepared silver particles from silver acetate salts using the SP process. The morphology and size of the synthesized silver particles were characterized. Two types of the silver particles, a mixture of nano and submicron particles, and nano-convex particles, were obtained through the control of the calcination temperatures. Finally, a formation mechanism of nanostructured silver particles using SP process was proposed.

Electrical and tribological properties of a Ni–18%Ru alloy for contact applications

A study of the microstructure, development of contact resistance during oxidation, and abrasive wear behavior for a Ni–18 at.%Ru alloy is presented in this article. It is shown that the alloy can be solutionized and aged, resulting in a fine lamellar mixture of FCC α-Ni and HCP β-Ru phases. Upon oxidation in air for 400 h, the measured contact resistance of the alloy is two orders of magnitude lower than that of pure Ni after 400-h oxidation. This behavior results from the formation of a low-resistivity rutile RuO2 scale on the β phase lamellae, which gives conducting pathways through the insulating NiO scale that forms on the α phase. After an initial run-in period, the steady-state abrasive wear rate measured for the Ni–Ru alloy is an order of magnitude less than that of pure Ni. Since the micro-cutting and flaking wear mechanisms are the same, the differences in the wear rates are ascribed to the presence of the well-dispersed hard Ru-rich β phase. The combination of a low-resistivity self-healing native oxide scale and good wear properties makes the alloy an excellent candidate for electrical contact applications.

Structural, mechanical and electrical-contact properties of nanocrystalline-NbC/amorphous-C coatings deposited by magnetron sputtering

Niobium-carbide nanocomposite coatings with a carbon content varying from 43 to 64at.% were deposited by dual DC magnetron sputtering. X-ray diffraction, x-ray photoelectron spectroscopy and electron microscopy showed that all coatings consisted of nanometer sized NbC grains embedded in a matrix of amorphous carbon. Mechanical properties and electrical resistivity showed a strong dependency on the amount of amorphous carbon (a-C) and NbC grain size in the coating. The highest hardness (23GPa), elastic modulus (295GPa) and the lowest resistivity (260μΩcm) were measured for the coating with about 15% of a-C phase. Contact resistance measurements using a crossed cylinder set-up showed lowest contact resistance for the coating containing 33% a-C (140μΩ at a contact force of 100N), which is comparable to a Ag reference (45μΩ at a contact force of 100N). Comparison with TiC-based nanocomposites studied under similar conditions showed that the NbC system has less tendency to form a-C and that lowest contact resistance is obtained at comparable amounts of a-C phase in both material systems (33% for NbC compared to 35% for TiC). With these good electrical contact properties, the NbC nanocomposites can be considered as a potential material for electrical contact applications.

Fretting behaviour of various intermetallic compounds in electrical contacts: Influence on reliability

Abstract Many connectors are made of cuprous substrates with electroplated final surfaces such as tin and nickel. The main failure mechanism of such devices has been shown to be fretting. Intermetallic compounds (IMC) formed due to the ageing of tin coatings have been shown to be an aggravating factor. Much work is still dedicated to better understand the failure mechanisms involved in tin fretting. The aim of this work was to elaborate improved coatings for better reliability of electronic devices. The work has been performed on tin layers electrodeposited on cuprous substrates such as those used in the electronic industry. Sine displacement cycles were imposed to ball/plane samples in “controlled displacement” fretting” tests. Experiments were performed for samples with underlayers of different types and thickness both as received and after heat treatment. The number of cycles for the contact resistance to exceed the threshold value of 10 mΩ increased with thin underlayers of 0.1 μm. The heat treated tin layer with a 0.1 μm cobalt underlayer showed a low and stable Rc value for a highly increased number of cycles. Various physicochemical techniques were used to analyse the composition profiles of the samples and to measure the thickness of the intermetallic compounds. Thin underlayers allow copper to diffuse; intermetallic compounds of different composition and with different structures are formed: Cu6Sn5, Ni3Sn4 or unidentified (Cu–Sn–Ni) and (Cu–Sn–Co) compounds. The electrical and tribological behaviours of bare IMC were investigated after stripping the free tin; sticking phenomena could account for the lower values of resistance recorded for the (Cu–Sn–Ni) and (Cu–Sn–Co) compounds. The particular needle shape of the intermetallic compound formed with the thin cobalt underlayer could cause different fretting conditions leading to lower values of contact resistance for a longer time. Despite the complex phenomena involved, indications are given for the elaboration of coatings with improved fretting behaviour in electrical contacts applications.

Relationship between composition, structure, and mechanical properties of a condensed composite of copper–tungsten system

For a copper–tungsten microlayered composite material for electrical contact applications, which is prepared by electron-beam evaporation-condensation, the changes in its structure, conductivity, hardness, and mechanical properties in tension at room temperature and elevated temperatures are studied versus the tungsten content and heat treatment conditions. New morphological features of the condensed composite and the related changes in the material properties have been revealed. The conditions for the formation of structural defects and their influence on mechanical properties and fracture behavior of the material in tensile tests have been investigated. A relationship has been established between the tungsten content of the composite, its structure, strength, and hardness.

Investigation of the Arc Erosion Behaviour of W-Cu Composites

In practical applications, silver-based composite materials have been used for high load bearing or high class contact materials. However, composites such as CdO-Ag and silver-based SnO2, TiO2 and ZnO are not economically feasible due to toxicity concerns and cost effectiveness. On the other hand, copper matrices are widely used for electrical contact applications in a diverse range of fields. In particular, W-Cu alloys are considered as an alternative for contact materials and arcing resistance electrodes due to the high thermal and electrical conductivity of copper and the high arc erosion resistance and low thermal expansion coefficient of tungsten. However, the contacts are vulnerable to thermal damage due to the high temperature and mass flow on the local surfaces of the contacts during the make-and-break of the contacts. In this study, particle-reinforced copper (W-Cu and TiC-Cu) composite powders were fabricated using a novel electroless copper plating process. Electrical contacts made of pure Cu and the Cu matrix powders were employed for dynamic electrode tests. The specimens were subjected to 1,000 and 10,000 arc breakings to study the effects of arc erosion on the contacts as well as their make-and-break life. Results show that the contacts made of pure Cu had the greatest loss of mass and the shortest life while the W-Cu composites preserved most of their mass; in addition, the W-Cu composites with finer W particles (1 μm) outperformed the rest of the composites.RésuméDans les applications pratiques, on a utilisé les matériaux composites à base d'argent dans les matériaux porteurs de charge élevée ou dans les matériaux de contact de qualité supérieure. Cependant, les composites tels que CdO-Ag ou SnO2, TiO2 et ZnO à base d'argent ne sont pas économiquement réalisables à cause de problèmes de toxicité et de souci d'économie. D'un autre côté, les matrices de cuivre sont largement utilisées dans les contacts électriques dans une grande gamme de champs d'application. En particulier, les alliages de W-Cu sont considérés comme solution de rechange pour les matériaux de contact et pour les électrodes de résistance à la formation d'arc grâce à la conductivité thermique et électrique élevée du cuivre et grâce à la résistance élevée à l'érosion de l'arc et au faible coefficient d'expansion thermique du tungstène. Cependant, les contacts sont vulnérables à l'endommagement thermique à cause de la température élevée et du haut débit sur les surfaces locales des contacts lors de l'enclenchementdéclenchement des contacts. Au cours de cette étude, on a fabriqué des poudres composites de cuivre renforcées de particules (W-Cu et TiC-Cu) en utilisant un nouveau procédé de placage chimique du cuivre. On a utilisé des contacts électriques faits de cuivre pur ou de poudres à matrice de cuivre pour des essais dynamiques d'électrode. On a soumis les échantillons à 1,000 et 10,000 ruptures d'arc pour étudier l'effet d'érosion de l'arc sur les contacts ainsi que leur durée de vie d'enclenchement-déclenchement. Les résultats montrent que les contacts faits de cuivre pur avaient la plus grande perte de masse et la plus courte durée de vie alors que les composites de W-Cu préservaient presque toute leur masse; de plus, les composites de W-Cu avec les particules de W les plus fines (1 μm) surpassaient le reste des composites.

Improving the contact resistance at low force using gold coated carbon nanotube surfaces

Investigations to determine the electrical contact performance under repeated cycles at low force conditions for carbon-nanotube (CNT) coated surfaces were performed. The surfaces under investigation consisted of multi-walled CNT synthesized on a silicon substrate and coated with a gold film. These planar surfaces were mounted on the tip of a PZT actuator and contacted with a plated Au hemispherical probe. The dynamic applied force used was 1 mN. The contact resistance (Rc) of these surfaces was investigated with the applied force and with repeated loading cycles performed for stability testing. The surfaces were compared with a reference Au–Au contact under the same experimental conditions. This initial study shows the potential for the application of gold coated CNT surfaces as an interface in low force electrical contact applications.

The Relationship Between Contact Resistance and Contact Force on Au-Coated Carbon Nanotube Surfaces Under Low Force Conditions

Carbon nanotube (CNT)-coated surfaces are investigated to determine the electrical contact performance under low force conditions. The surfaces under investigation are vertically aligned multiwalled CNTs formed on a silicon substrate and coated with an Au film. These planar surfaces are mated with a hemispherical Au plated probe mounted in a nanoindentation apparatus. The maximum contact force used is 1 mN. The contact resistance of these surfaces is investigated as a function of the applied force and is also studied under repeated loading cycles. The surfaces are compared with a reference Au-Au contact under the same experimental conditions and the results compared to established contact theory. The results show that the vertically aligned multiwalled CNT surface provides a stable contact resistance. This paper shows the potential for the application of CNT surfaces as an interface in low force electrical contact applications.