Learn how to choose suitable electrical contact materials for your electrical equipments

 

Electrical contact materials are materials used to establish and maintain an electrical connection between two or more conductive surfaces. They are used in various electrical and electronic devices, including switches, relays, connectors, contactors, etc.

In the early stage, metals were used as electrical Contact Materials, such as pure W, pure Mo, pure Cu, and precious metals Ag and Au. Later, Ag-Cu, Pb-Ag, Au-Ag, and other alloys were used further to improve the electrical properties of the contact materials.

After the 1960s, multiple precious metals and various precious metal composites were developed, such as AgNi, AgCdO, etc. Which reduces the usage amount of precious metal silver, and improves the reliability of contact materials. In the early 21st century, the use of toxic materials AgCdO was greatly reduced and gradually replaced by AgSnO2 materials.

 

3 major types of electrical contact materials: metals, alloys, and composites.

 

There are many types of electrical contact materials, including metals, alloys, and composites. The most common metals used as electrical contact materials are copper, silver, and gold. Copper is the most commonly used material due to its high electrical conductivity and low cost. Silver is also a good conductor but is more expensive than copper. Gold is an excellent conductor, but it is also costly and is generally only used in high-end applications where a low-resistance connection is required.

Alloys, such as copper-based alloys and silver-based alloys, are commonly used as electrical contact materials. They offer a combination of good electrical conductivity and mechanical strength.

Composite materials, such as carbides and MAX phase ceramics composites, are also used as electrical contact materials. These materials are often used in high-temperature or high-wear applications, as they have good wear resistance and can withstand high temperatures.

The phenomenon of contact material in the opening and closing process is extremely complex, and there are many influencing factors. The selection of an appropriate electrical contact material depends on the specific application and requirements of the device in which it will be used. Factors such as electrical conductivity, mechanical strength, corrosion resistance, and cost should all be considered when selecting electrical contact materials.

 

An ideal electrical contact material must have the following good properties.

 

A. Physical performance:
Low resistivity and vapor pressure; High thermal conductivity, melting point, and boiling point; Good thermal stability, and heat capacity.

B. Mechanical performance:
High strength at room temperature and high temperature, good hardness, and good plasticity and toughness.

C. Electrical contact performance:
Good arc burn resistance, low and stable contact resistance, low tendency of welding, and material transfer.

D. Chemical properties:
Good corrosion resistance, not easy to get oxidized, carbonized, vulcanized, and form conductive compounds or salt slag film in the atmospheric environment; High electrochemical potential, good chemical corrosion resistance, and low tendency of gas dissolution.

E. Processing and manufacturing performance:
It shall be easily soldered and fixed to the contact base or bridge by brazing or other methods.

 

Is there a formula or standardization for electrical contacts selection?

 

Unfortunately, the answer is no. The exact behavior of electrical contacts within a switch can only be discerned through sampling and testing.

How much heat is expected to be generated within the system and how will it be dissipated?

How many cycles should the electrical contacts last in your electrical equipments?

Can the electrical contacts be designed to meet your specific requirement?

There are many factors you need to consider when selecting electrical contacts for switches. Many of them require significant technical and metallurgical expertise. That’s the reason you shouldn’t go it alone. For best results, discuss these design factors with engineers and suppliers to ensure you get the right electrical contact for your electrical equipment. You need to make sure contact materials are compatible with electrical loads and environmental conditions.

 

The mainstream silver-based contact materials.

 

Precious metal-based electrical contact materials were widely used in the electronics and electrical industry, and silver-based electrical contact materials are the most important among them. The mainstream silver-based contact materials with their characteristics are listed below.

 

Pure Silver Contact Materials

 

Pure silver (Ag) is the contact material with the best electrical and thermal conductivity, and the contact resistance is low and stable. Low strength, hardness, good ductility, excellent machining performance. But Ag has low hardness, low melting point, and wear resistance; Under humidity and high temperature, and under sulfur or sulfide, silver sulfide film will be formed on the surface. Under the condition of a large load, it is easy to form arc and welding.

Widely used in small capacity low-voltage electrical appliances with rated current not exceed 10A, such as universal relays, heat protectors, timers, auxiliary switches, control switches, etc

 

 

FAg Contact Materials

 

By adding a small amount of nickel to silver, FAg greatly refines the microstructure of the material. Under the condition of the same contact resistance, its mechanical strength and heat resistance are higher than that of silver. Therefore, the welding resistance and arc resistance are better than Ag. Widely used in small capacity low-voltage electrical appliances with rated current not exceeding 15A, such as universal relays, heat protectors, timers, auxiliary switches, control switches, etc.

Widely used in small capacity low-voltage electrical appliances with rated current not exceeding 15A, such as universal relays, heat protectors, timers, auxiliary switches, control switches, etc

 

 

AgNi(Silver Nickel) Contact Materials

 

AgNi contact material has the advantages of good arc transfer characteristics, low and stable contact resistance, and excellent machining performance. It has good welding resistance and arc erosion resistance under small and medium currents, and strong material transfer resistance under DC conditions. The increase in nickel content can improve the welding resistance of the material, but slightly increase the contact resistance and temperature rise. However, due to the formation of oxides during on-off, the contact resistance is increased, sensitive to sulfur, and the welding resistance is poor under a large current.

Main products include AgNi10, AgNi15, AgNi20, AgNi30, AgNi40, and widely used in AC, DC relay, command switch, small current contactor, light switch, light control switch, temperature controller and washing machine timer, protection switch, etc.

 

AgC(Silver Graphite) Contact Materials

 

AgC is low voltage contact material with the best welding resistance. There is parallel method (∥), vertical method (⊥), and powder monolithic pressing method for silver graphite sheets. By the sintering extrusion process, the density is up to 99.9%. At the same time, the second phase graphite in silver presents as short fibers, and the direction of the fiber arrangement is vertical or parallel to the surface of the electrical contact product, which greatly improves the physical and mechanical properties of the material. At the same time, the product has better welding resistance, arc erosion resistance, low and stable electrical resistivity, and other electrical properties. But the processing plasticity is poor, the yield is low.

Main products include AgC3, AgC4, AgC5, mainly used for protection switches, such as miniature circuit breakers, leakage protection switches, or engine protection switches, pairing contacts usually silver nickel or copper.

 

AgCdO(Silver Cadmium Oxide) Contact Materials

 

AgCdO materials have good electrical wear resistance, welding resistance, electrical conductivity, thermal conductivity, low and stable contact resistance, and also have good machining performance. Cadmium oxide will decompose and evaporate violently under the action of the arc, this helps arc extinguishing. Therefore, its welding resistance is excellent, and wins the name of “universal contact”.

Cadmium oxide is toxic, the increase of its content can improve the welding resistance of AgCdO material, but will increase the contact resistance and temperature rise, and reduce the machining performance of the material. In the aspect of welding resistance and arc erosion resistance, it is more and more difficult to meet the requirements of miniaturization, high reliability, and long life of the contact material for electrical switches.

Main products include AgCdO8, AgCdO10, AgCdO12, AgCdO15, and AgCdO17, widely used in micro switches, relays, contactors, household switches, all kinds of protection switches, and small and medium capacity circuit breakers.

 

AgSnO2(Silver Tin Oxide) Contact Materials

 

AgSnO2 is the ideal environmentally friendly low-voltage contact material to replace toxic silver cadmium oxide. It has excellent welding resistance and arc erosion resistance, and its material transfer resistance ability is strong in DC switching circuits. The abrasion of AgSnO2 is much less than that of AgCdO, and the performance and life span of some electrical appliances are superior to AgCdO material.  AgSnO2 contact material has high hardness and brittleness and is not easy to process, And the contact resistance and temperature rise are high.

Main products include AgSnO2(8), AgSnO2(10), AgSnO2(12), AgSnO2(14.5), widely used in micro switches, relays, contactors, household switches, all kinds of protection switches, and small and medium capacity circuit breakers.

 

AgSnO2In2O3(Silver Tin Oxide Indium Oxide) Contact Materials

 

By adding In2O3 into AgSnO2, the strength and hardness of the contact material are improved. Compared with AgSnO2, it has stronger resistance to arc burning, welding, and material transfer. The material with 3 ~ 5wt.% indium oxide has better material transfer resistance under DC load, which is suitable for application in automotive relays and other fields. However, the addition of more additives will lead to higher hardness and greater resistance than AgSnO2, and the addition of expensive Indium will lead to higher material costs.

Main products include AgSnO2(5)In2O3(3), AgSnO2(6)In2O3(4), AgSnO2(7)In2O3(3), AgSnO2(8)In2O3(4), AgSnO2(9)In2O3(4), AgSnO2(10)In2O3(4), suitable for medium and large capacity AC contactor (such as CJ20, CJ40, 3TF series, etc.), high power AC switch (more than 50kW), DC contactor, AC/DC power relay, automotive electrical appliances, and small and medium capacity low-voltage circuit breaker.

 

AgZnO(Silver Zinc Oxide) Contact Materials

 

AgZnO contact materials have the characteristics of short arc burning time, high breaking performance, and strong current impact resistance. Its contact resistance is higher than that of AgCdO. Under the condition of breaking current of 3000~5000A, silver zinc oxide has better arc erosion resistance, which is one of the main environmentally friendly low-voltage contact materials to replace toxic silver cadmium oxide.

Main products include AgZnO(8), AgZnO(10), AgSnO2(12), mainly used in small and medium capacity low-voltage circuit breaker with rated current within 200A, and also used for power protection switch, leakage protection switch, AC relay, DZ15L series leakage circuit breaker, etc.

 

AgCuO(Silver Copper Oxide) Contact Materials

 

AgCuO is a new type of environmentally friendly electrical contact material, the material has high welding resistance. The performance of DC contactor is better than that of silver tin oxide and silver nickel. Copper oxide content can be 8% – 15%.

Main products include AgCuO(10), widely used in medium and heavy load low voltage switches, such as air circuit breakers, voltage controllers, contactors, starters, etc. Especially suitable for DC contactors.

 

AgW(Silver Tungsten) Contact Materials

 

AgW contact material not only has good electrical conductivity and machinability of silver but also has the characteristics of high melting point, high hardness, arc erosion resistance, welding resistance, and less material transfer of tungsten. Its biggest characteristic is that it has a strong ability to withstand large current arcs. When the content of W in the material is high, the refractory component is usually made into the skeleton first, and then the Ag is melted into the skeleton to make the almost completely dense sintered body.

AgW loss is mainly caused by Ag loss and volatilization under arc burning. The W or WO3 layer with poor conductivity and thermal conductivity is formed on the contact surface, which leads to the increase of contact resistance and temperature rise. In addition, Ag2WO4 will be formed on the contact surface during operation, and the contact resistance will rise sharply.

Main products include AgW(40), AgW(50), AgW(60), AgW(70), AgW(80), AgW(50)/Cu, widely used in all kinds of air circuit breaker, moduled case circuit breaker, heavy load AC, DC contactor and other switching appliances.

 

AgWC(Silver Tungsten Carbide) Contact Materials

 

The WC in AgWC contact material can enhance the resistance to electrical erosion and acid of the contact. In addition to the characteristics of silver tungsten, silver tungsten carbide also has higher hardness, and is more advantageous in the case of a strong arc. When the content of WC in the material is high, the high content of WC will increase the resistance.

Main products include AgWC(40), AgWC(50), AgWC(60), AgWC(70), AgWC(80), mainly used for switching devices with high current loads, such as moduled case circuit breakers and air circuit breakers. In many cases, it is paired with silver tungsten carbide, silver graphite, or silver tungsten carbide graphite.

 

AgWCC(Silver Tungsten Carbide Graphite) Contact Materials

 

The WC in AgWCC contact material can improve the hardness of the contact, and enhance the resistance to electrical erosion and acid of the contact. Under the action of the arc, graphite will produce carbon dioxide gas, which can play the role of arc extinguishing. Graphite can also prevent contact from welding.

Silver tungsten carbide graphite contacts have good electrical conductivity, welding resistance, electrical wear resistance, and low and stable resistance, and are used as fixed contacts in asymmetric pairing.

Main products include AgWC(12)C(3), AgWC(22)C(3), AgWC(27)C(3), AgWC(22)C(3)/Cu, mainly used for switching devices with high current loads, such as circuit breakers and leakage switches. In many cases, it is paired with silver-tungsten or silver-tungsten carbide.

 

AgNiC(Silver Nickel Graphite) Contact Materials

 

AgNiC contact material is composed of silver, nickel, and graphite. The role of nickel is to refine the grain and improve the hardness, wear resistance, burn resistance, and arc erosion resistance of the contact. Graphite can prevent contact bonding and welding because of its good self-lubrication. AgNiC contact material has the characteristics of good welding resistance, low contact resistance, and stability.

Main products include AgNi(25)C(2), AgNi(25)C(5), AgNi(30)C(3), widely used in moduled case circuit breakers, and air circuit breakers. Usually, it is paired with silver tungsten or silver nickel.

 

 

The Manufacturing Processes of Electrical Contact Materials

 

Electrical Contact Materials are manufactured through different processes, and each has its advantage and disadvantages. The mainstream manufacturing processes of silver alloy contact materials are listed below.

 

Smelting Method

 

Pure Ag and other metals that can dissolve with pure Ag, such as Cu, a small amount of Ni, and Ce, are melted through high-temperature smelting, then cast into an alloy spindle, and finally processed into electrical contacts through extrusion, drawing, and other subsequent processing.

Advantages: Simple process, suitable for processing simple composition silver metal alloy.

Disadvantages: Due to the solid solubility of silver and some second-phase components, only a small amount of second-phase elements can be added, such as FAg; Ag-MeO(silver metal oxide) can not be produced directly, and it requires further oxidation treatment.

 

Solid-Phase Method / Liquid-Phase Method

 

Mix Ag powder with W powder, WC powder, other metal powder, or metal oxide powder in proportion. Then through granulation, initial Shaping, Sintering/ Melt Infiltration, Final Shaping, and other treatments, are processed into electrical contacts, such as AgW(50), AgWC(12)C(3), etc.

Advantages: With significant energy saving, and material saving, can be processed into any silver and other metals or metal oxide components of the contact material, has excellent performance, high precision, and good stability, very suitable for mass production.

Disadvantages: The original powder particle size will directly affect the performance of the contact material; Due to the influence of mixing equipment and powder dispersion, the powder particles of each element have a certain degree of uneven distribution.

 

Conventional Internal Oxidation Method

 

Pure Ag and other metals, such as Sn, Cu, Sn+In, and Cd, are melted through high-temperature smelting, cast into a spindle, and then through extrusion, hot rolling compounding, cold rolling, stamping, and other subsequent processing, finally the products are oxidized at high temperature and high pressure to obtain AgMeO contact materials.

Advantages: High density and hardness of the material, strong bonding force between MeO particles and Ag, good wear resistance, good arc erosion resistance, long electrical life, and low manufacturing cost.

Disadvantages: Coarse grain inside the material, oxide on the grain boundary, there is a lean oxide zone in the core, the surface, and internal oxidation structure is difficult to achieve uniformity, high resistivity, and long oxidation time.

 

Alloy Powder Preoxidation Method

 

Ag and other metals such as Sn, Cu, Sn+In, and Cd are manufactured by high-pressure water atomization, and then AgMeO contact materials are obtained through pre-oxidation, forming, sintering, extrusion, drawing, rolling, and other follow-up processing.

Advantages: MeO particles are highly dispersed in the Ag, have a uniform structure, no component segregation, good wear resistance, welding resistance, and low resistivity.

Disadvantages: the density and hardness of the material are not as good as those made by internal oxidation.

 

Alloy Wire/Sheet Oxidation Method

 

Pure Ag and other metals, such as Sn, Cu, Sn+In, and Cd, are melted and cast into alloy spindles through high-temperature smelting. After subsequent processing such as extrusion, drawing, and rolling, the products are oxidized at high temperatures to obtain AgMeO contact materials.

Advantages: The material structure is uniform. Although there are some problems such as poor oxide zone after oxidation in the alloy, the material structure tends to be uniform through subsequent extrusion processing, with good processability and wide applicability.

Disadvantages: Too many processes, the uniformity of the material is not as good as the powder oxidation products.

 

Powder Mixing Method

 

The powder mixed with Ag and MeO particles and additives was obtained by the powder mixing method. The silver content, MeO, and an additive amount of the composite powder can be accurately prepared according to the required ratio, which solves the problem that the additive is difficult to add. The manufactured contact material has good processing performance and the electrical performance is also greatly improved.

Advantages: It is not limited by the proportion of components, and can be prepared according to any proportion. The microstructure of the final material can be controlled by controlling the particle size and morphology of the original powder. Good plasticity and low hardness.

Disadvantages: the wettability of Ag and MeO particles is poor, and they are easy to separate under the action of an electric arc. Poor resistance to arc erosion.

 

Chemical Coating Method

 

The Ag composite powder coated with SnO2 particles was obtained by a chemical process. This composite powder can improve the sintering properties of the material, and solve the interface problem between Ag and SnO2 in the traditional powder metallurgy process. The contact material has good machining properties, uniform microstructure, and greatly improved electrical properties.

Advantages: The sintering activity of powder is improved, which is conducive to the densification of the material. The binding force between Ag and MeO particles is strong, and the electrical conductivity and processing plasticity of the material is improved.

Disadvantages: cleaning is difficult, there is pollution; The stability of powder made from different batches is difficult to control.

 

 

The first step for electrical contact choosing is to learn the characteristics and applications of each type of electrical contact material. If you are not sure about which type of electrical contact is suitable. We can help you choose the right electrical contact for your electrical equipment. Leave a message, and we will offer our suggestions.