Changes

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

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''' From this group silver has the greatest importance for switching devices in the higherenergy technology. Other precious metals such as gold and platinum are only used inapplications for the information technology in the form of thin surface layers. As a nonpreciousmetal tungsten is used for some special applications such as for example asautomotive horn contacts. In some rarer cases pure copper is used but mainly pairedto a silver-based contact material. *'''Alloys'''

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.

*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 typically

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_s = 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. ''Sintering with liquid phase'' has the advantage of shorter process times due tothe accelerated diffusion and also results in near-theoretical densities of thefigure>

FigDuring ''sintering without a liquid phase'' (left side of schematic) the powder mix is first densified by pressing, then undergoes a heat treatment (sintering), and eventually is re-pressed again to further increase the density. 2The sintering atmosphere depends on the material components and later application; a vacuum is used for example for the low gas content material Cu/Cr.1: PowderThis process is used for individual contact parts and also termed press-metallurgical manufacturing of composite sinterrepress (PSR). For materials with high silver content the starting point at pressing is most a larger block (schematicor billet)T = Melting point which is then after sintering hot extruded into wire, rod, or strip form. The extrusion further increases the density of the lower melting componentthese 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.

''Sintering with liquid phase'' has the advantage of shorter process times due to the accelerated diffusion and also results in near-theoretical densities of the composite material. To ensure the shape stability during the sintering process it

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 after sinteringthe porous skeleton is infiltrated with liquid metal of the second material. Thefilling up of the pores happens through capillary forces. This process reachesafter 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.

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 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]]

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 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 significantlyincrease in contact resistance. Therefore Pt and Pd are typically used as alloys andnot in their pure form for electrical contact applications.

Main ArticelArticle: [[Platinum Metal Based Materials| Platinum Metal Based Materials]]

Table 2.31Main Article: Application Examples of Silver–Metal Oxide [[Silver Based Materials ====Silver–Graphite (GRAPHOR)-| Silver Based Materials====Ag/C (GRAPHOR) contact materials are usually produced by powder metallurgywith graphite contents of 2 – 5 wt% ''(Table 2.32)''. The earlier typicalmanufacturing process of single pressed tips by pressing - sintering - repressing(PSR) has been replaced in Europe for quite some time by extrusion. In NorthAmerica and some other regions however the PSR process is still used to someextend mainly for cost reasons. The extrusion of sintered billets is now the dominant manufacturing method forsemi-finished AgC materials ''(Figs. 2.126 – 2.129)''. The hot extrusion processresults in a high density material with graphite particles stretched and oriented inthe extrusion direction ''(Figs. 2.130 – 2.133)''. Depending on the extrusionmethod in either rod or strip form the graphite particles can be oriented in thefinished contact tips perpendicular (GRAPHOR) or parallel (GRAPHOR D) to theswitching contact surface ''(Figs. 2.131 and 2.132)''. Since the graphite particles in the Ag matrix of Ag/C materials prevent contacttips from directly being welded or brazed, a graphite free bottom layer isrequired. This is achieved by either burning out (de-graphitizing) the graphiteselectively on one side of the tips or by compound extrusion of a Ag/C billetcovered with a fine silver shell. Ag/C contact materials exhibit on the one hand an extremely high resistance tocontact welding but on the other have a low arc erosion resistance. This iscaused by the reaction of graphite with the oxygen in the surroundingatmosphere at the high temperatures created by the arcing. The weld resistanceis especially high for materials with the graphite particle orientation parallel to thearcing contact surface. Since the contact surface after arcing consists of puresilver the contact resistance stays consistently low during the electrical life of thecontact parts. A disadvantage of the Ag/C materials is their rather high erosion rate. In materialswith parallel graphite orientation this can be improved if part of the graphite isincorporated into the material in the form of fibers (GRAPHOR DF), ''(Fig. 2.133)''.The weld resistance is determined by the total content of graphite particles. Ag/C tips with vertical graphite particle orientation are produced in a specificsequence: Extrusion to rods, cutting of double thickness tips, burning out ofgraphite to a controlled layer thickness, and a second cutting to single tips.Such contact tips are especially well suited for applications which require both,a high weld resistance and a sufficiently high arc erosion resistance ''(Table 2.33)''.For attachment of Ag/C tips welding and brazing techniques are applied. welding the actual process depends on the material's graphite orientation. ForAg/C tips with vertical graphite orientation the contacts are assembled withsingle tips. For parallel orientation a more economical attachment starting withcontact material in strip or profile tape form is used in integrated stamping andwelding operations with the tape fed into the weld station, cut off to tip form andthen welded to the carrier material before forming the final contact assemblypart. For special low energy welding the Ag/C profile tapes GRAPHOR D and DFcan be pre-coated with a thin layer of high temperature brazing alloys such asCuAgP. In a rather limited way, Ag/C with 2 – 3 wt% graphite can be produced in wireform and headed into contact rivet shape with low head deformation ratios. The main applications for Ag/C materials are protective switching devices suchas miniature molded case circuit breakers, motor-protective circuit breakers,and fault current circuit breakers, where during short circuit failures highestresistance against welding is required ''(Table 2.34)''. For higher currents the lowarc erosion resistance of Ag/C is compensated by asymmetrical pairing withmore erosion resistant materials such as Ag/Ni and Ag/W. Fig. 2.126:Strain hardeningof Ag/C 96/4 Dby cold working Fig. 2.127:Softening of Ag/C 96/4 D afterannealing Fig. 2.128: Strain hardeningof Ag/C DF by cold working Fig. 2.129: Softeningof Ag/C DF after annealing Fig. 2.130: Micro structure of Ag/C 97/3: a) perpendicular to extrusion directionb) parallel to extrusion direction, 1) Ag/C contact layer, 2) Ag backing layer Fig. 2.131: Micro structure of Ag/C 95/5: a) perpendicular to extrusion directionb) parallel to extrusion direction, 1) Ag/C contact layer, 2) Ag backing layer Fig. 2.132: Micro structure of Ag/C 96/4 D: a) perpendicular to extrusion directionb) parallel to extrusion direction, 1) Ag/C contact layer, 2) Ag backing layer Fig. 2.133: Micro structure of Ag/C DF: a) perpendicular to extrusion directionb) parallel to extrusion direction, 1) Ag/C contact layer, 2) Ag/Ni 90/10 backing layer Table 2.32: Physical Properties of Silver–Graphite (GRAPHOR) Contact Materials Table 2.33: Contact and Switching properties of Silver–Graphite (GRAPHOR) Contact Materials Table 2.34: Application Examples and Forms of Supply of Silver–Graphite (GRAPHOR) Contact Materials Pre-Production of Contact Materials(Bild)]]

===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 contact tips forswitching duties that require a rapid switching sequence such as horn contactsfor cars and trucks. 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 2.35Main Article: 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 – Ag₂ WO₄ ) 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 2.37: 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]]

===Low Gas Content Materials Based on Refractory Metals===Contact materials The trade name VAKURIT is assigned to a family of W/Cu, W/Ag, WC/Ag, or Mo/Cu can be used in vacuumswitches if their total low gas content does not exceed approximately 150 ppm. Incontact materials developed for the low gas content W/Cu use in vacuum switching devices [[Special_Contact_Materials_(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._for_Vacuum_Switches|Table 1]]

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 Main Article: [[Special Contact 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 Vacuum Switches| Special Contact 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]]