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Contact Materials for Electrical Engineering

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The contact parts are important components in switching devices. They have tomaintain their function from the new state until the end of the functional life of thedevices.
The requirements on contacts are rather broad. Besides typical contact propertiessuch as
*High arc erosion resistance
*Good arc extinguishing capability
they They have to exhibit physical, mechanical, and chemical properties like high electricaland thermal conductivity, high hardness, high corrosion resistance, etc . and besidesthis , should have good mechanical workability, and also be suitable for good weld andbrazing attachment to contact carriers. In addition they must be made fromenvironmentally friendly materials.
Materials suited for use as electrical contacts can be divided into the following groupsbased on their composition and metallurgical structure:
*Pure metals
*Alloys
*Composite materials
*Pure metals
From this group silver has the greatest importance for switching devices in the higher
energy technology. Other precious metals such as gold and platinum are only used in
applications for the information technology in the form of thin surface layers. As a nonprecious
metal tungsten is used for some special applications such as for example as
automotive horn contacts. In some rarer cases pure copper is used but mainly paired
to a silver-based contact material.
*'''Pure metals''' Within this group, silver has the greatest importance for switching devices in the higher energy technology. Other precious metals such as gold and platinum are only used in applications for the information technology in the form of thin surface layers. As a nonprecious metal, tungsten is used for some special applications such as, for example, automotive horn contacts. In some rarer cases, pure copper is used, but mainly paired to a silver-based contact material. '''Alloys''' Besides these few pure metals, a larger number of alloy materials made by melt technology are available for the use as contacts. An alloy is characterized by the fact, that its components are completely or partially soluble in each other in the solid state. Phase diagrams for multiple metal compositions show the number and type of the crystal structure as a function of the temperature and composition of the alloying components.
Besides these few pure metals a larger number They indicate the boundaries of liquid and solid phases and define the parameters of alloy materials made by meltsolidification.technology are available for Alloying allows to improve the use as contacts. An alloy is characterized by properties of one material at the factthat its components are completely or partially soluble in each other in cost of changing them for the solid statesecond material.Phase diagrams for multiple metal compositions show As an example, the number and type hardness of a base metal may be increased while at thecrystal structure as a function of same time the temperature and composition electrical conductivity decreases with even small additions of the second alloying componentscomponent.
They indicate the boundaries of liquid and solid phases and define theparameters of solidification.Alloying allows to improve the properties of one material at the cost of changingthem for the second material. As an example, the hardness of a base metal maybe increased while at the same time the electrical conductivity decreases witheven small additions of the second alloying component.'''Composite Materials'''
*Composite Materialsmaterials are a material group whose properties are of great importance for electrical contacts that are used in switching devices for higherelectrical currents.
Composite materials are a material group whose properties are of greatimportance for electrical contacts that are used in switching devices for higherelectrical currents.Those used in electrical contacts are heterogeneous materials , composed of twoor more uniformly dispersed components , in which the largest volume portionconsists of a metal.The properties of composite materials are determined mainly independent fromeach other by the properties of their individual components. Therefore it is forexample possible to combine the high melting point and arc erosion resistanceof tungsten with the low melting and good electrical conductivity of copper, orthe high conductivity of silver with the weld resistant metalloid graphite.
Figure 2The properties of composite materials are determined mainly independent from each other by the properties of their individual components.1 Therefore it is, for example, possible to combine the high melting point and arc erosion resistance of tungsten with the low melting and good electrical conductivity of copper or the high conductivity of silver with the weld resistant metalloid graphite. <xr id="fig:Powder metallurgical manufacturing of composite materials (schematic)"/> shows the schematic manufacturing processes from powderblending to contact material. Three basic process variations are typicallyapplied:
*Sintering without liquid phase (Press-Sinter-Repress, PSR)
*Infiltration (Press-Sinter-Infiltrate, PSI)
During ''sintering without a liquid phase'' <figure id="fig:Powder metallurgical manufacturing of composite materials (left side of schematic) the powder mix is">first densified by pressing, then undergoes a heat treatment [[File:Powder metallurgical manufacturing of composite materials (sinteringschematic), andeventually is re-pressed again to further increase the density. The sinteringatmosphere depends on the material components and later application; avacuum is used for example for the low gas content material Cu/Cr. Thisprocess is used for individual contact parts and also termed pressjpg|thumb|<caption>Powder-sinterrepressmetallurgical manufacturing of composite materials (PSRschematic). For materials with high silver content T<sub>s</sub> = Melting point of the starting point atpressing is most a larger block (or billetlower melting component) which is then after sintering hotextruded into wire, rod, or strip form. The extrusion further increases the density</caption>]]of these composite materials and contributes to higher arc erosion resistance.Materials such as Ag</Ni, Ag/MeO, and Ag/C are typically produced by thisprocess.figure>
During ''Sintering with sintering without a liquid phase'' has (left side of schematic), the advantage of shorter process times due powder mix is first densified by pressing, then undergoes a heat treatment (sintering) and eventually is re-pressed again tofurther increase the accelerated diffusion density. The sintering atmosphere depends on the material components and later application; a vacuum is used for example for the low gas content material Cu/Cr. This process is used for individual contact parts and also results in neartermed press-theoretical densities sinter-repress (PSR). For materials with high silver content, the starting point before pressing is mostly a large block (or billet) which is then, after sintering, hot extruded into wire, rod or strip form. The extrusion further increases the density of thethese composite materials and contributes to higher arc erosion resistance. Materials such as Ag/Ni, Ag/MeO and Ag/C are typically produced by this process.
Fig. 2.1: Powder''Sintering with liquid phase'' has the advantage of shorter process times due to the accelerated diffusion and also results in near-metallurgical manufacturing of composite materials (schematic)T = Melting point theoretical densities of the lower melting component composite material. To ensure the shape stability during the sintering process , it
is however necessary to limit the volume content of the liquid phase material.
As opposed to the liquid phase sintering , which has limited use for electricalcontact manufacturing, the ''Infiltration process'' as shown on the right side of theschematic , has a broad practical range of applications. In this process thepowder of the higher melting component , sometimes also as a powder mix witha small amount of the second material , is pressed into parts and . Then, right after sintering, the porous skeleton is infiltrated with liquid metal of the second material. Thefilling fill-up process of the pores happens through capillary forces. This process reaches, after the infiltration , near-theoretical density without subsequent pressing and iswidely used for Ag- and Cu-refractory contacts. For Ag/W or Ag/WC contacts,controlling the amount or excess on the bottom side of the contact of theinfiltration metal Ag , results in contact tips that can be easily attached to theircarriers by resistance welding. For larger Cu/W contacts , additional machining isoften used to obtain the final shape of the contact component.
==Gold Based Materials==
Pure Gold is besides Platinum the chemically most stable of all precious metals.In its pure form , it is not very suitable for use as a contact material in electromechanical devices because of its tendency to stick and cold-weld at even low contact forces. In addition , it is not hard or strong enough to resistmechanical wear and exhibits high materials material losses under electrical arcingloads. This limits its use in form of thin electroplated or vacuum deposited layers.
Main ArticelArticle: [[Gold Based Materials| Gold Based Materials]]
==Platinum Metal Based Materials==
The platinum group metals include the elements Pt, Pd, Rh, Ru, Ir, and Os ''([[Platinum_Metal_Based_Materials|Table 1]]<!--(Table2.6)''-->). For electrical contacts , platinum and palladium have practical significanceas base alloy materials and ruthenium and iridium are used as alloying components.Pt and Pd have similar corrosion resistance as gold but because of due to theircatalytical properties , they tend to polymerize adsorbed organic vapors on contactsurfaces. During frictional movement between contact surfaces , the polymerizedcompounds known as “brown powder” are formed , which can lead to significantlya significant increase in contact resistance. Therefore Pt and Pd are typically used as alloys andare rather not used in their pure form for electrical contact applications.
Main ArticelArticle: [[Platinum Metal Based Materials| Platinum Metal Based Materials]]
==Silver Based Materials==
Pure Silver, Silver Alloys, Silver Composite Materials
 
Main Articel: [[Silver Based Materials| Silver Based Materials]]
Main Article: [[Silver Based Materials| Silver Based Materials]]
==Tungsten and Molybdenum Based Materials==
Main Article: [[Tungsten and Molybdenum (Pure Metals), Silver–Tungsten (SIWODUR) Based Materials, Silver–Tungsten Carbide (SIWODUR C) Materials, Silver–Molybdenum (SILMODUR) Materials, Copper–Tungsten (CUWODUR) | Tungsten and Molybdenum Based Materials]]
Main Articel: [[Tungsten and Molybdenum Based ==Contact Materials| Tungsten and Molybdenum Based Materials]]for Vacuum Switches==
===Tungsten and Molybdenum (Pure Metals)===Tungsten is characterized by its advantageous properties of high melting andboiling points, sufficient electrical and thermal conductivity and high hardnessand density ''(Table 2.35)''. It is mainly used in the form of brazed The low gas content contact tips materials are developed forswitching duties that require a rapid the use in vacuum switching sequence such as horn contactsfor cars and trucksdevices.
Molybdenum has a much lesser importance as a contact material since it is lessresistant against oxidation than tungsten.Both metals are however used in large amounts as components in compositematerials with silver and copper. Table 2.35: Mechanical Properties of Tungsten and Molybdenum === Silver–Tungsten (SIWODUR) Materials===Ag/W (SIWODUR) contact materials combine the high electrical and thermalconductivity of silver with the high arc erosion resistance of the high meltingtungsten metal ''(Table 2.36)''. The manufacturing of materials with typically50-80 wt% tungsten is performed by the powder metallurgical processes ofliquid phase sintering or by infiltration. Particle size and shape of the startingpowders are determining the micro structure and the contact specific propertiesof this material group ''(Figs. 2.134 and 2.135) (Table 2.37)''. During repeated switching under arcing loads tungsten oxides and mixedoxides (silver tungstates – Ag<sub>2</sub> WO<sub>4</sub> ) are formed on the Ag/W surface creating 2 4poorly conducting layers which increase the contact resistance and by this thetemperature rise during current carrying. Because of this fact the Ag/W is pairedin many applications with Ag/C contact parts. Silver–tungsten contact tips are used in a variety of shapes and are produced forthe ease of attachment with a fine silver backing layer and quite often anadditional thin layer of a brazing alloy. The attachment to contact carriers isusually done by brazing, but also by direct resistance welding for smaller tips. Ag/W materials are mostly used as the arcing contacts in disconnect switchesfor higher loads and as the main contacts in small and medium duty powerswitches and industrial circuit breakers ''(Table 2.38)''. In north and south americathey are also used in large volumes in miniature circuit breakers of small tomedium current ratings in domestic wiring as well as for commercial powerdistribution. === Silver–Tungsten Carbide (SIWODUR C) Materials===This group of contact materials contains the typically 40-65 wt-% of the veryhard and erosion wear resistant tungsten carbide and the high conductivity silver''(Fig. 2.135) (Table 2.36)''. Compared to Ag/W the Ag/WC (SIWODUR C)materials exhibit a higher resistance against contact welding ''(Table 2.37)''. Therise in contact resistance experienced with Ag/W is less pronounced in Ag/WCbecause during arcing a protective gas layer of CO is formed which limits thereaction of oxygen on the contact surface and therefore the formation of metaloxides. Higher requirements on low temperature rise can be fulfilled by adding a smallamount of graphite which however increases the arc erosion. Silver–tungstencarbide–graphite materials with for example 27 wt% WC and3 wt% graphite or 16 wt% WC and 2 wt% graphite are manufactured using thesingle tip press-sinter-repress (PSR) process ''(Fig. 2.136)''. The applications of Ag/WC contacts are similar to those for Ag/W ''(Table 2.38)''. === Silver–Molybdenum (SILMODUR) Materials===Ag/Mo materials with typically 50-70 wt% molybdenum are usually produced bythe powder metallurgical infiltration process ''(Fig. 2.137) (Table 2.36)''. Theircontact properties are similar to those of Ag/W materials ''(Table 2.37)''. Since themolybdenum oxide is thermally less stable than tungsten oxide the self-cleaningeffect of Ag/Mo contact surface during arcing is more pronounced and thecontact resistance remains lower than that of Ag/W. The arc erosion resistanceof Ag/Mo however is lower than the one for Ag/W materials. The mainapplications for Ag/Mo contacts are in equipment protecting switching devices''(Table 2.38)''. Fig. 2.134: Micro structure of Ag/W 25/75 Fig. 2.135: Micro structure of Ag/WC 50/50 Fig. 2.136: Micro structure of Ag/WC27/C3 Fig. 2.137: Micro structure of Ag/Mo 35/65 Table 2.36: Physical Properties of Contact Materials Based on Silver–Tungsten (SIWODUR),Silver–Tungsten Carbide (SIWODUR C) and Silver Molybdenum (SILMODUR) Table 2.37Main Article: Contact and Switching Properties of [[Contact Materials Based on Silver – Tungsten(SIWODUR), Silver–Tungsten Carbide (SIWODUR C)and Silver Molybdenum (SILMODUR) Table 2.38: Application Examples and Forms of Supply for Contact Materials Basedon Silver–Tungsten (SIWODUR), Silver–Tungsten Carbide (SIWODUR C)and Silver Molybdenum (SILMODUR) ==== Copper–Tungsten (CUWODUR) Materials====Copper–tungsten (CUWODUR) materials with typically 50-85 wt% tungsten areproduced by the infiltration process with the tungsten particle size selectedaccording to the end application ''(Figs. 2.138 – 2.141) (Table 2.39)''. To increasethe wettability of the tungsten skeleton by copper a small amount of nickel< 1 wt% is added to the starting powder mix. W/Cu materials exhibit a very high arc erosion resistance ''(Table 2.40)''.Compared to silver–tungsten materials they are however less suitable to carrypermanent current. With a solid tungsten skeleton as it is the case for W/C infiltrated materials with70-85 wt% tungsten the lower melting component copper melts and vaporizesin the intense electrical arc. At the boiling point of copper (2567°C) the still solidtungsten is efficiently “cooled” and remains pretty much unchanged. During very high thermal stress on the W/Cu contacts, for example during shortcircuit currents > 40 kA the tungsten skeleton requires special high mechanicalstrength. For such applications a high temperature sintering of tungsten fromselected particle size powder is applied before the usual infiltration with copper(example: CUWODUR H). For high voltage load switches the most advantageous contact system consistsof a contact tulip and a contact rod. Both contact assemblies are made usuallyfrom the mechanically strong and high conductive CuCrZr material and W/Cu asthe arcing tips. The thermally and mechanically highly stressed attachmentbetween the two components is often achieved by utilizing electron beamwelding or capacitor discharge percussion welding. Other attachment methodsinclude brazing and cast-on of copper followed by cold forming steps toincrease hardness and strength. The main application areas for CUWODUR materials are as arcing contacts inload and high power switching in medium and high voltage switchgear as wellas electrodes for spark gaps and over voltage arresters ''(Table 2.41)''. Table 2.39: Physical Properties of Copper–Tungsten (CUWODUR) Contact Materials Fig. 2.139: Micro structure of W/Cu 70/30 G Fig. 2.140: Micro structure of W/Cu 70/30 H Fig. 2.138: Micro structure of W/Cu 70/30 F Fig. 2.141: Micro structure of W/Cu 80/20 H Manufacturing of Contact Parts forMedium and High Voltage Switchgear Table 2.40: Contact and Switching Properties of Copper–Tungsten(CUWODUR) Contact Materials Table 2.41: Application Examples and Forms of Supply for Tungsten–Copper (CUWODUR) Contact Materials ==Special Contact Materials (VAKURIT) for Vacuum Switches==The trade name VAKURIT is assigned to a family of low gas content contactmaterials developed for the use in vacuum switching devices ''(Table 2.42)''. ===Low Gas Content Materials Based on Refractory Metals===| Contact materials of W/Cu, W/Ag, WC/Ag, or Mo/Cu can be used in vacuumswitches if their total gas content does not exceed approximately 150 ppm. Inthe low gas content W/Cu (VAKURIT) material mostly used in vacuum contactorsthe high melting W skeleton is responsible for the high erosion resistance whencombined with the high conductivity copper component which evaporatesalready in noticeable amounts at temperatures around 2000 °C. Since there is almost no solubility of tungsten, tungsten carbide, or molybdenumin copper or silver the manufacturing of these material is performed powdermetallurgically.The W, WC, or Mo powders are pressed and sintered and theninfiltrated with low gas content Cu or Ag. The content of the refractory metals istypically between 60 and 85 wt% ''(Figs. 2.142 and 2.143)''. By adding approximately 1 wt% antimony the chopping current, i.e. the abruptcurrent decline shortly before the natural current-zero, can be improved forW/Cu (VAKURIT) materials ''(Table 2.43)''.The contact components mostly used in vacuum contactors are usually shapedas round discs. These are then attached by brazing in a vacuum environment totheir contact carriers ''(Table 2.44)''. ===Low Gas Content Materials Based on Copper-Chromium===As contact materials in vacuum interupters in medium voltage devices low gasmaterials based on Cu/Cr have gained broad acceptance. The typical chromiumcontents are between 25 and 55 wt% ''(Figs. 2.144 and 2.145)''. During thepowder metallurgical manufacturing a mix of chromium and copper powders ispressed into discs and subsequently sintering in a reducing atmosphere orvacuum below the melting point of copper. This step is followed by cold or hotre-pressing. Depending on the composition the Cu/Cr (VAKURIT) materialscombine a relatively high electrical and thermal conductivity with high dielectricstability. They exhibit a low arc erosion rate and good resistance against weldingas well as favorable values of the chopping current in medium voltage loadswitches, caused by the combined effects of the two components, copper andchromium ''(Table 2.43)''. The switching properties of Cu/Cr (VAKURIT) materials are dependent on thepurity of the Cr metal powders and especially the type and quantity of impuritiescontained in the chromium powder used. Besides this the particle size anddistribution of the Cr powder are of high importance. Because of the getteractivity of chromium a higher total gas content of up to about 650 ppmcompared to the limits in refractory based materials can be tolerated in theseCu/Cr contact materials. Besides the more economical sinter technology alsoinfiltration and vacuum arc melting are used to manufacture these materials.Cu/Cr contacts are supplied in the shape of discs or rings which often alsocontain slots especially for vacuum load switches in medium voltage devices''(Table 2.44)''. Increased applications of round discs can also be observed for lowvoltage vacuum contactors. Table 2.42: Physical Properties of the Low Gas Materials (VAKURIT) for Vacuum Switches Fig. 2.142: Micro structure of W/Cu 30Sb1– low gas Fig. 2.143: Micro structure of WC/Ag 50/50– low gas Fig. 2.144: Micro structure of Cu/Cr 75/25– low gas Fig. 2.145: Micro structure of Cu/Cr 50/50– low gas Table 2.43: Contact and Switching Properties of VAKURIT Materials Table 2.44: Application Examples and Form of Supply for VAKURIT Materials]]
==References==
Manufacturing Equipment for Semi-Finished Materials
(Bild)
 
[[de:Kontaktwerkstoffe_für_die_Elektrotechnik]]

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