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.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 density. The sintering atmosphere depends on the material components and later application; a vacuum is used for example for the accelerated diffusion low gas content material Cu/Cr. This process is used for individual contact parts and also results in neartermed press-theoretical densities sinterrepress (PSR). For materials with high silver content the starting point at pressing is most a larger 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

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]]

=== Pure Silver===Pure silver (also called fine silver) exhibits the highest electrical and thermalconductivity of all metals. It is also resistant against oxidation. Major disadvantagesare its low mechanical wear resistance, the low softening temperature,and especially its strong affinity to sulfur and sulfur compounds. In the presenceof sulfur and sulfur containing compounds brownish to black silver sulfide layerare formed on its surface. These can cause increased contact resistance oreven total failure of a switching device if they are not mechanically, electrically,or thermally destroyed. Other weaknesses of silver contacts are the tendency toweld under the influence of over-currents and the low resistance againstmaterial transfer when switching DC loads. In humid environments and underthe influence of an electrical field silver can creep (silver migration) and causeelectrical shorting between adjacent current paths. Table 2.11 shows the typically available quality grades of silver. In certaineconomic areas, i.e. China, there are additional grades with varying amounts ofimpurities available on the market. In powder form silver is used for a widevariety of silver based composite contact materials. Different manufacturingprocesses result in different grades of Ag powder as shown in Table 2.12.additional properties of silver powders and their usage are describedin chapter 8.1.Semi-finished silver materials can easily be warm or cold formed and can beclad to the usual base materials. For attachment of silver to contact carriermaterials welding of wire or profile cut-offs and brazing are most widely applied.Besides these mechanical processes such as wire insertion (wire staking) andthe riveting (staking) of solid or composite contact rivets are used in themanufacture of contact components. Contacts made from fine silver are applied in various electrical switchingdevices such as relays, pushbuttons, appliance and control switches forcurrents < 2 A ''(Table 2.16)''. Electroplated silver coatings are widely used toreduce the contact resistance and improve the brazing behavior of other contactmaterials and components. Table 2.11: Overview of the Most Widely Used Silver Grades Table 2.12: Quality Criteria of Differently Manufactured Silver Powders Fig. 2.45:Strain hardeningof Ag 99.95 by cold working Fig. 2.46:Softening of Ag 99.95after annealing for 1 hr after differentdegrees of strain hardening ===Silver Alloys===To improve the physical and contact properties of fine silver melt-metallurgicalproduced silver alloys are used ''(Table 2.13)''. By adding metal components themechanical properties such as hardness and tensile strength as well as typicalcontact properties such as erosion resistance, and resistance against materialtransfer in DC circuits are increased ''(Table 2.14)''. On the other hand however,other properties such as electrical conductivity and chemical corrosionresistance can be negatively impacted by alloying ''(Figs. 2.47 and 2.48)''. ====Fine-Grain Silver====Fine-Grain Silver (ARGODUR-Spezial) is defined as a silver alloy with an additionof 0.15 wt% of Nickel. Silver and nickel are not soluble in each other in solidform. In liquid silver only a small amount of nickel is soluble as the phase diagram''(Fig. 2.51)'' illustrates. During solidification of the melt this nickel addition getsfinely dispersed in the silver matrix and eliminates the pronounce coarse graingrowth after prolonged influence of elevated temperatures ''(Figs. 2.49 and 2.50)''. Fine-grain silver has almost the same chemical corrosion resistance as finesilver. Compared to pure silver it exhibits a slightly increased hardness andtensile strength ''(Table 2.14)''. The electrical conductivity is just slightly decreasedby this low nickel addition. Because of its significantly improved contactproperties fine grain silver has replaced pure silver in many applications. ====Hard-Silver Alloys====Using copper as an alloying component increases the mechanical stability ofsilver significantly. The most important among the binary AgCu alloys is that ofAgCu3, known in europe also under the name of hard-silver. This material stillhas a chemical corrosion resistance close to that of fine silver. In comparison topure silver and fine-grain silver AgCu3 exhibits increased mechanical strengthas well as higher arc erosion resistance and mechanical wear resistance''(Table 2.14)''. Increasing the Cu content further also increases the mechanical strength ofAgCu alloys and improves arc erosion resistance and resistance againstmaterial transfer while at the same time however the tendency to oxide formationbecomes detrimental. This causes during switching under arcing conditions anincrease in contact resistance with rising numbers of operation. In specialapplications where highest mechanical strength is recommended and a reducedchemical resistance can be tolerated, the eutectic AgCu alloy with 28 wt% ofcopper ''(Fig. 2.52)'' is used. AgCu10 also known as coin silver has beenreplaced in many applications by composite silver-based materials while sterlingsilver (AgCu7.5) has never extended its important usage from decorative tablewear and jewelry to industrial applications in electrical contacts. Besides these binary alloys, ternary AgCuNi alloys are used in electrical contactapplications. From this group the material ARGODUR 27, an alloy of 98 wt% Agwith a 2 wt% Cu and nickel addition has found practical importance close to thatof AgCu3. This material is characterized by high resistance to oxidation and lowtendency to re-crystallization during exposure to high temperatures. Besideshigh mechanical stability this AgCuNi alloy also exhibits a strong resistanceagainst arc erosion. Because of its high resistance against material transfer thealloy AgCu24.5Ni0.5 has been used in the automotive industry for an extendedtime in the North American market. Caused by miniaturization and the relatedreduction in available contact forces in relays and switches this material hasbeen replaced widely because of its tendency to oxide formation. The attachment methods used for the hard silver materials are mostly close tothose applied for fine silver and fine grain silver. Hard-silver alloys are widely used for switching applications in the informationand energy technology for currents up to 10 A, in special cases also for highercurrent ranges ''(Table 2.16)''. Dispersion hardened alloys of silver with 0.5 wt% MgO and NiO (ARGODUR 32)are produced by internal oxidation. While the melt-metallurgical alloy is easy tocold-work and form the material becomes very hard and brittle after dispersionhardening. Compared to fine silver and hard-silver this material has a greatlyimproved temperature stability and can be exposed to brazing temperatures upto 800°C without decreasing its hardness and tensile strength.Because of these mechanical properties and its high electrical conductivity Table 2.13: Physical Properties of Silver and Silver Alloys ARGODUR 32 is mainly used in the form of contact springs that are exposed tohigh thermal and mechanical stresses in relays, and contactors for aeronauticapplications. Fig. 2.47:Influence of 1-10 atom% of differentalloying metals on the electrical resistivity ofsilver Fig. 2.48:Electrical resistivity pof AgCu alloys with 0-20 weight% Cuin the soft annealedand tempered stagea) Annealed and quenchedb) Tempered at 280°C Fig. 2.49: Coarse grain micro structureof Ag 99.97 after 80% cold workingand 1 hr annealing at 600°C Fig. 2.50: Fine grain microstructureof AgNi0.15 after 80% cold workingand 1 hr annealing at 600°C Fig. 2.51:Phase diagramof silver-nickel Fig. 2.52:Phase diagramof silver-copper Fig. 2.53:Phase diagram ofsilver-cadmium Table 2.14: Mechanical Properties of Silver and Silver Alloys Fig. 2.54:Strain hardeningof AgCu3by cold working Fig. 2.55:Softening of AgCu3after annealing for 1 hrafter 80% cold working Fig. 2.56:Strain hardening of AgCu5 by coldworking Fig. 2.57:Softening of AgCu5 afterannealing for 1 hr after 80% coldworking Fig. 2.58:Strain hardening of AgCu 10by cold working Fig. 2.59:Softening of AgCu10 afterannealing for 1 hr after 80% coldworking Fig. 2.60:Strain hardening of AgCu28 bycold working Fig. 2.61:Softening of AgCu28after annealing for 1 hr after80% cold working Fig. 2.62:Strain hardening of AgNi0.15by cold working Fig. 2.63:Softening of AgNi0.15after annealing for 1 hr after 80%cold working Fig. 2.64:Strain hardening ofARGODUR 27by cold working Fig. 2.65:Softeningof ARGODUR 27 after annealingfor 1 hr after 80% cold working Table 2.15: Contact and Switching Properties of Silver and Silver Alloys Table 2.16: Application Examples and Forms of Supply for Silver and Silver Alloys ====Silver-Palladium Alloys====The addition of 30 wt% Pd increases the mechanical properties as well as theresistance of silver against the influence of sulfur and sulfur containingcompounds significantly ''(Tables 2.17 and 2.18)''.Alloys with 40-60 wt% Pd have an even higher resistance against silver sulfideformation. At these percentage ranges however the catalytic properties ofpalladium can influence the contact resistance behavior negatively. Theformability also decreases with increasing Pd contents. AgPd alloys are hard, arc erosion resistant, and have a lower tendency towardsmaterial transfer under DC loads ''(Table 2.19)''. On the other hand the electricalconductivity is decreased at higher Pd contents. The ternary alloy AgPd30Cu5has an even higher hardness which makes it suitable for use in sliding contactsystems. AgPd alloys are mostly used in relays for the switching of medium to higher loads(>60V, >2A) as shown in Table 2.20. Because of the high palladium price theseformerly solid contacts have been widely replaced by multi-layer designs suchas AgNi0.15 or AgNi10 with a thin Au surface layer. A broader field of applicationfor AgPd alloys remains in the wear resistant sliding contact systems. Fig. 2.66: Phase diagram of silver-palladium Fig. 2.67:Strain hardeningof AgPd30 by cold working Fig. 2.68:Strain hardeningof AgPd50 by cold working Fig. 2.69:Strain hardeningof AgPd30Cu5by cold working Fig. 2.70:Softening of AgPd30, AgPd50,and AgPd30Cu5 after annealing of 1 hrafter 80% cold working Table 2.17: Physical Properties of Silver-Palladium Alloys Table 2.18: Mechanical Properties of Silver-Palladium Alloys Table 2.19: Contact and Switching Properties of Silver-Palladium Alloys Table 2.20: Application Examples and Forms of Suppl for Silver-Palladium Alloys ===Silver Composite Materials=== ====Silver-Nickel (SINIDUR) Materials====Since silver and nickel are not soluble in each other in solid form and in the liquidphase have only very limited solubility silver nickel composite materials withhigher Ni contents can only be produced by powder metallurgy. During extrusionof sintered Ag/Ni billets into wires, strips and rods the Ni particles embedded inthe Ag matrix are stretched and oriented in the microstructure into a pronouncedfiber structure ''(Figs. 2.75. and 2.76)'' The high density produced during hot extrusion aids the arc erosion resistanceof these materials ''(Tables 2.21 and 2.22)''. The typical application of Ag/Nicontact materials is in devices for switching currents of up to 100A ''(Table 2.24)''.In this range they are significantly more erosion resistant than silver or silveralloys. In addition they exhibit with nickel contents <20 wt% a low and over theiroperational lifetime consistent contact resistance and good arc movingproperties. In DC applications Ag/Ni materials exhibit a relatively low tendencyof material transfer distributed evenly over the contact surfaces ''(Table 2.23)''. Typically Ag/Ni (SINIDUR) materials are usually produced with contents of 10-40wt% Ni. The most widely used materials SINIDUR 10 and SINIDUR 20- and alsoSINIDUR 15, mostly used in north america-, are easily formable and applied bycladding ''(Figs. 2.71-2.74)''. They can be, without any additional welding aids,economically welded and brazed to the commonly used contact carriermaterials.The (SINIDUR) materials with nickel contents of 30 and 40 wt% are used inswitching devices requiring a higher arc erosion resistance and where increasesin contact resistance can be compensated through higher contact forces. The most important applications for Ag/Ni contact materials are typically inrelays, wiring devices, appliance switches, thermostatic controls, auxiliaryswitches, and small contactors with nominal currents >20A ''(Table 2.24)''. Table 2.21: Physical Properties of Silver-Nickel (SINIDUR) Materials Table 2.22: Mechanical Properties of Silver-Nickel (SINIDUR) Materials Fig. 2.71:Strain hardeningof Ag/Ni 90/10 by cold working Fig. 2.72:Softening of Ag/Ni 90/10after annealingfor 1 hr after 80% cold working Fig. 2.73:Strain hardeningof Ag/Ni 80/20 by cold working Fig. 2.74:Softening of Ag/Ni 80/20after annealingfor 1 hr after 80% cold working Fig. 2.75: Micro structure of Ag/Ni 90/10 a) perpendicular to the extrusion directionb) parallel to the extrusion direction Fig. 2.76: Micro structure of Ag/Ni 80/20 a) perpendicular to the extrusion directionb) parallel t o the extrusion direction Table 2.23: Contact and Switching Properties of Silver-Nickel (SINIDUR) Materials Table 2.24Main Article: Application Examples and Forms of Supplyfor Silver-Nickel (SINIDUR) Materials ==== [[Silver-Metal Oxide Based Materials Ag/CdO, Ag/SnO₂, Ag/ZnO====The family of silver-metal oxide contact materials includes the material groups:silver-cadmium oxide (DODURIT CdO), silver-tin oxide (SISTADOX), and silverzincoxide (DODURIT ZnO). Because of their very good contact and switchingproperties like high resistance against welding, low contact resistance, and higharc erosion resistance, silver-metal oxides have gained an outstanding positionin a broad field of applications. They mainly are used in low voltage electricalswitching devices like relays, installation and distribution switches, appliances,industrial controls, motor controls, and protective devices ''(Table 2.13)''. *Silver-cadmium oxide (DODURIT CdO) materials Silver-cadmium oxide (DODURIT CdO) materials with 10-15 wt% are producedby both, internal oxidation and powder metallurgical methods ''(Table 2.25)''. The manufacturing of strips and wires by internal oxidation starts with a moltenalloy of silver and cadmium. During a heat treatment below it's melting point in aoxygen rich atmosphere in such a homogeneous alloy the oxygen diffuses fromthe surface into the bulk of the material and oxidizes the Cd to CdO in a more orless fine particle precipitation inside the Ag matrix. The CdO particles are ratherfine in the surface area and are becoming larger further away towards the centerof the material ''(Fig. 2.83)''. During the manufacturing of Ag/CdO contact material by internal oxidation theprocesses vary depending on the type of semi-finished material.For Ag/CdO wires a complete oxidation of the AgCd wire is performed, followedby wire-drawing to the required diameter ''(Figs. 2.77 and 2.78)''. The resultingmaterial is used for example in the production of contact rivets. For Ag/CdO stripmaterials two processes are commonly used: Cladding of an AgCd alloy stripwith fine silver followed by complete oxidation results in a strip material with asmall depletion area in the center of it's thickness and a Ag backing suitable foreasy attachment by brazing (sometimes called “Conventional Ag/CdO”). Usinga technology that allows the partial oxidation of a dual-strip AgCd alloy materialin a higher pressure pure oxygen atmosphere yields a composite Ag/CdO stripmaterial that has besides a relatively fine CdO precipitation also a easily brazableAgCd alloy backing ''(Fig. 2.85)''. These materials (DODURIT CdO ZH) are mainlyused as the basis for contact profiles and contact tips. During powder metallurgical production the powder mixed made by differentprocesses are typically converted by pressing, sintering and extrusion to wiresand strips. The high degree of deformation during hot extrusion produces auniform and fine dispersion of CdO particles in the Ag matrix while at the sametime achieving a high density which is advantageous for good contact properties''(Fig. 2.84)''. To obtain a backing suitable for brazing, a fine silver layer is appliedby either com-pound extrusion or hot cladding prior to or right after the extrusion''(Fig. 2.86)''. For larger contact tips, and especially those with a rounded shape, the single tipPress-Sinter-Repress process (PSR) offers economical advantages. Thepowder mix is pressed in a die close to the final desired shape, the “green” tipsare sintered, and in most cases the repress process forms the final exact shapewhile at the same time increasing the contact density and hardness. Using different silver powders and minor additives for the basic Ag and CdOstarting materials can help influence certain contact properties for specializedapplications. Fig. 2.77:Strain hardening of internally oxidizedAg/CdO 90/10 by cold working Fig. 2.78:Softening of internally oxidizedAg/CdO 90/10 after annealingfor 1 hr after 40% cold working Table 2.25: Physical and Mechanical Properties as well as Manufacturing Processes andForms of Supply of Extruded | Silver Cadmium Oxide(DODURIT CdO) Contact Materials Fig. 2.79:Strain hardening ofAg/CdO 90/10 P by cold working Fig. 2.80: Softeningof Ag/CdO 90/10 P after annealingfor 1 hr after 40% cold working Fig. 2.81:Strain hardeningof Ag/CdO 88/12 WP Fig. 2.82:Softening of Ag/CdO 88/12WP after annealingfor 1 hr after different degrees ofcold working Fig. 2.83: Micro structure of Ag/CdO 90/10 i.o. a) close to surfaceb) in center area Fig. 2.84: Micro structure of Ag/CdO 90/10 P:a) perpendicular to extrusion directionb) parallel to extrusion direction Fig. 2.85:Micro structure of Ag/CdO 90/10 ZH:1) Ag/CdO layer2) AgCd backing layer Fig. 2.86: Micro structure of AgCdO 88/12 WP: a) perpendicular to extrusion directionb) parallel to extrusion direction *Silver–tin oxide(SISTADOX)materialsOver the past years, many Ag/CdO contact materials have been replaced byAg/SnO₂ based materials with 2-14 wt% SnO₂ because of the toxicity ofCadmium. This changeover was further favored by the fact that Ag/SnO₂contacts quite often show improved contact and switching properties such aslower arc erosion, higher weld resistance, and a significant lower tendencytowards material transfer in DC switching circuits ''(Table 2.30)''. Ag/SnO₂materials have been optimized for a broad range of applications by other metaloxide additives and modification in the manufacturing processes that result indifferent metallurgical, physical and electrical properties ''(Table 2.29)''. Manufacturing of Ag/SnO₂ by ''internal oxidation'' is possible in principle, butduring heat treatment of alloys containing > 5 wt% of tin in oxygen, dense oxidelayers formed on the surface of the material prohibit the further diffusion ofoxygen into the bulk of the material. By adding Indium or Bismuth to the alloy theinternal oxidation is possible and results in materials that typically are rather hardand brittle and may show somewhat elevated contact resistance and is limitedto applications in relays. To make a ductile material with fine oxide dispersion(SISTADOX TOS F) ''(Fig. 2.114)'' it is necessary to use special process variationsin oxidation and extrusion which lead to materials with improved properties inrelays. Adding a brazable fine silver layer to such materials results in a semifinishedmaterial suitable for the manufacture as smaller weld profiles(SISTADOX WTOS F) ''(Fig. 2.116)''. Because of their resistance to materialtransfer and low arc erosion these materials find for example a broaderapplication in automotive relays ''(Table 2.31)''. ''Powder metallurgy'' plays a significant role in the manufacturing of Ag/SnO₂contact materials. Besides SnO₂ a smaller amount (<1 wt%) of one or moreother metal oxides such as WO₃, MoO₃, CuO and/or Bi₂O₃ are added. Theseadditives improve the wettability of the oxide particles and increase the viscosityof the Ag melt. They also provide additional benefits to the mechanical andarcing contact properties of materials in this group ''(Table 2.26)''. In the manufacture the initial powder mixes different processes are appliedwhich provide specific advantages of the resulting materials in respect to theircontact properties ''(Figs. 2.87 – 2.119)''. Some of them are described here asfollows::'''a) Powder blending from single component powders''' <br> In this common process all components including additives that are part of the powder mix are blended as single powders. The blending is usually performed in the dry stage in blenders of different design. :'''b) Powder blending on the basis of doped powders''' <br> For incorporation of additive oxides in the SnO₂ powder the reactive spray process (RSV) has shown advantages. This process starts with a waterbased solution of the tin and other metal compounds. This solution is nebulized under high pressure and temperature in a reactor chamber. Through the rapid evaporation of the water each small droplet is converted into a salt crystal and from there by oxidation into a tin oxide particle in which the additive metals are distributed evenly as oxides. The so created doped AgSnO2 powder is then mechanically mixed with silver powder. :'''c) Powder blending based on coated oxide powders''' <br> In this process tin oxide powder is blended with lower meting additive oxides such as for example Ag₂ MoO₄ and then heat treated. The SnO₂ particles are coated in this step with a thin layer of the additive oxide. :'''d) Powder blending based on internally oxidized alloy powders''' <br> A combination of powder metallurgy and internal oxidation this process starts with atomized Ag alloy powder which is subsequently oxidized in pure oxygen. During this process the Sn and other metal components are transformed to metal oxide and precipitated inside the silver matrix of each powder particle. :'''e) Powder blending based on chemically precipitated compound powders''' <br> A silver salt solution is added to a suspension of for example SnO₂ together with a precipitation agent. In a chemical reaction silver and silver oxide respectively are precipitated around the additive metal oxide particles who act as crystallization sites. Further chemical treatment then reduces the silver oxide with the resulting precipitated powder being a mix of Ag and SnO₂. Further processing of these differently produced powders follows theconventional processes of pressing, sintering and hot extrusion to wires andstrips. From these contact parts such as contact rivets and tips aremanufactured. To obtain a brazable backing the same processes as used forAg/CdO are applied. As for Ag/CdO, larger contact tips can also bemanufactured more economically using the press-sinter-repress (PSR) process''(Table 2.27).'' Fig. 2.87:Strain hardening ofAg/SnO₂ 92/8 PE by cold working Fig. 2.88:Softening ofAg/SnO₂ 92/8 PE after annealingfor 1 hr after 40% cold working Table 2.26: Physical and Mechanical Properties as well as Manufacturing Processes andForms of Supply of Extruded Silver-Tin Oxide (SISTADOX) Contact Materials Fig. 2.89:Strain hardening ofAg/SnO₂ 88/12 PE by cold working Fig. 2.90:Softening of Ag/SnO₂ 88/12 PEafter annealing for1 hr after 40% cold working Fig. 2.91:Strain hardening of oxidizedAg/SnO₂ 88/12 PW4 by cold working Fig. 2.92:Softening of Ag/SnO₂ 88/12 PW4 afterannealing for 1 hrafter 30% cold working Fig. 2.93:Strain hardening ofAg/SnO₂ 98/2 PXby cold working Fig. 2.94:Softening ofAg/SnO₂ 98/2 PXafter annealingfor 1 hr after 80%cold working Fig 2.95:Strain hardeningof Ag/SnO₂ 92/8 PXby cold working Fig. 2.96:Softening ofAg/SnO₂ 92/8 PXafter annealing for 1 hrafter 40% cold working Fig. 2.97:Strain hardening of internallyoxidizedAg/SnO₂ 88/12 TOS Fby cold working Fig. 2.98:Softening ofAg/SnO₂ 88/12 TOS F afterannealing for 1 hr after 30%cold working Fig. 2.99:Strain hardening ofinternally oxidizedAg/SnO₂ 88/12Pby cold working Fig. 2.100:Softening ofAg/SnO₂ 88/12Pafter annealing for 1 hr after40% cold working Fig. 2.101:Strain hardening ofAg/SnO₂ 88/12 WPCby cold working Fig. 2.102:Softening of Ag/SnO₂ 88/12 WPC after annealingfor 1 hr after different degrees of cold working Fig. 2.103:Strain hardening ofAg/SnO₂ 86/14 WPCby cold working Fig. 2.104:Softening of Ag/SnO₂ 86/14 WPC after annealingfor 1 hr after different degrees of cold working Fig. 2.105:Strain hardening ofAg/SnO₂ 88/12 WPDby cold working Fig. 2.106:Softening of Ag/SnO₂ 88/12 WPD afterannealing for 1 hr after different degreesof cold working Fig. 2.108:Softening of Ag/SnO₂ 88/12 WPX afterannealing for 1 hr after different degreesof cold working Fig. 2.107:Strain hardening ofAg/SnO₂ 88/12 WPXby cold working Fig. 2.109: Micro structure of Ag/SnO₂ 92/8 PE: a) perpendicular to extrusion directionb) parallel to extrusion direction Fig. 2.110: Micro structure of Ag/SnO₂ 88/12 PE: a) perpendicular to extrusion directionb) parallel to extrusion direction Fig. 2.111: Micro structure of Ag/SnO₂ 88/12 PW: a) perpendicular to extrusion directionb) parallel to extrusion direction Fig. 2.112: Micro structure of Ag/SnO₂ 98/2 PX: a) perpendicular to extrusion directionb) parallel to extrusion direction Fig. 2.113: Micro structure of Ag/SnO₂ 92/8 PX: a) perpendicular to extrusion directionb) parallel to extrusion direction Fig. 2.114: Micro structure of Ag/SnO₂ 88/12 TOS F: a) perpendicular to extrusion directionb) parallel to extrusion direction Fig. 2.115: Micro structure of Ag/SnO₂ 86/14 WPC: a) perpendicular to extrusion directionb) parallel to extrusion direction, 1) AgSnO₂ contact layer, 2) Ag backing layer Fig. 2.116: Micro structure of Ag/SnO₂ 92/8 WTOS F: a) perpendicular to extrusion directionb) parallel to extrusion direction,1) AgSnO₂ contact layer, 2) Ag backing layer Fig. 2.117: Micro structure ofAg/SnO₂ 88/12 WPD: parallel to extrusion direction1) AgSnO₂ contact layer, 2) Ag backing layer Fig. 2.118: Micro structure ofAg/SnO₂ 88/12 WPX:parallel to extrusion direction1) AgSnO₂ contact layer, 2) Ag backing layer Fig. 2.119: Micro structure of Ag/SnO₂ 86/14 WPX: a) perpendicular to extrusion directionb) parallel to extrusion direction, 1) AgSnO₂ contact layer, 2) Ag backing layer Table 2.27: Physical Properties of Powder Metallurgical Silver-Metal Oxide Materialswith Fine Silver Backing Produced by the Press-Sinter-Repress Process *'''Silver–zinc oxide (DODURIT ZnO) materials'''Silver zinc oxide (DODURIT ZnO) contact materials with mostly 6 - 10 wt% oxidecontent including other small metal oxides are produced exclusively by powdermetallurgy ''(Figs. 2.120 – 2.125)'' ''(Table 2.28)''. Adding Ag₂WO₄ in the process b)as described in the preceding chapter on Ag/SnO₂ has proven most effectivefor applications in AC relays, wiring devices, and appliance controls. Just likewith the other Ag metal oxide materials, semi-finished materials in strip and wireform are used to manufacture contact tips and rivets.Because of their high resistance against welding and arc erosion Ag/ZnOmaterials present an economic alternative to Cd free Ag-tin oxide contactmaterials ''(Tables 2.30 and 2.31)''. Table 2.28: Physical and Mechanical Properties as well as Manufacturing Processes andForms of Supply of Extruded Silver-Zinc Oxide (DODURIT ZnO) Contact Fig. 2.120: Strain hardening ofAg/ZnO 92/8 PW25 by cold working Fig. 2.121: Softening of Ag/ZnO 92/8 PW25after annealing for 1 hr after 30% cold working Fig. 2.122: Strain hardening ofAg/ZnO 92/8 WPW25by cold working Fig. 2.123: Softening ofAg/ZnO 92/8 WPW25 after annealing for1hr after different degrees of cold working Fig. 2.115: Micro structure of Ag/ZnO 92/8 Pw25: a) perpendicular to extrusion directionb) parallel to extrusion direction Fig. 2.116: Micro structure of Ag/ZnO 92/8 WPW25:a) perpendicular to extrusion directionb) parallel to extrusion direction, 1) Ag/ZnO contact layer, 2) Ag backing layer Table 2.29: Optimizing of Silver–Tin Oxide Materials Regarding their SwitchingProperties and Forming Behavior Table 2.30: Contact and Switching Properties of Silver–Metal Oxide Materials Table 2.31: Application Examples of Silver–Metal Oxide Materials ====Silver–Graphite (GRAPHOR)-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 Based 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]]

Fig. 2.145: Micro structure The trade name VAKURIT is assigned to a family of Cu/Cr 50/50– low gascontent contact materials developed for the use in vacuum switching devices [[Special_Contact_Materials_(VAKURIT)_for_Vacuum_Switches|Table 1]]

Table 2.43Main Article: [[Special Contact and Switching Properties of Materials (VAKURIT ) for Vacuum Switches| Special Contact Materials Table 2.44: Application Examples and Form of Supply (VAKURIT) for VAKURIT MaterialsVacuum Switches]]