Tungsten is characterized by its advantageous properties of high melting andboiling points, sufficient electrical and thermal conductivity and high hardnessand density ''(<xr id="tab:Mechanical_Properties_of_Tungsten_and_Molybdenum"/><!--(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.

<figtable id="tab:Mechanical_Properties_of_Tungsten_and_Molybdenum"><caption>'''<!--Table 2.35: -->Mechanical Properties of Tungsten and Molybdenum'''</caption><table class="twocolortable"><tr><th><p class="s12">Material</p></th><th><p class="s12">Micro Structure Condition</p></th><th><p class="s12">Vickers</p><p class="s12">Hardness HV 10</p></th><th><p class="s12">Tensile Strength</p><p class="s12">[MPa]</p></th></tr><tr><td><p class="s12">Tungsten</p></td><td><p class="s12">Lightly worked structure</p><p class="s12">(wire and strip &gt; 1.0 mm thick)</p><p class="s12">Strongly worked structure</p><p class="s12">(wire and strip &lt; 1.0 mm thick)</p><p class="s12">Re-crystallized structure</p></td><td><p class="s12">300 - 500</p><p class="s12">500 - 750</p><p class="s12">360</p></td><td><p class="s12">1000 - 1800</p><p class="s12">1500 - 5000</p><p class="s12">1000 - 1200</p></td></tr><tr><td><p class="s12">Molybdenum</p></td><td><p class="s12">Lightly worked structure</p><p class="s12">(wire and strip &gt; 1.0 mm thick)</p><p class="s12">Strongly worked structure</p><p class="s12">(wire and strip &lt; 1.0 mm thick)</p><p class="s12">Re-crystallized structure</p></td><td><p class="s12">140 - 320</p><p class="s12">260 - 550</p><p class="s12">140 - 160</p></td><td><p class="s12">600 - 1100</p><p class="s12">800 - 2500</p><p class="s12">600 - 900</p></td></tr></table></figtable>

[[File=== Silver–Tungsten Materials===Ag/W contact materials combine the high electrical and thermal conductivity of silver with the high arc erosion resistance of the high melting tungsten metal (<xr id="tab:Mechanical Physical Properties of Contact Materials Based on Silver-Tungsten, Silver-Tungsten Carbide and Silver Molybdenum"/><!--(Table 2.36)-->). The manufacturing of materials with typically 50-80 wt% tungsten is performed by the powder metallurgical processes of liquid phase sintering or by infiltration. Particle size and shape of the starting powders are determining the micro structure and the contact specific properties of this material group (<xr id="fig:Micro structure of Ag W 25 75"/><!--(Fig. 2.134)-->, <xr id="fig:Micro structure of Ag WC 50 50"/><!--(Fig.jpg|right|thumb|Mechanical 2.135)--> and <xr id="tab:Physical Properties of Contact Materials Based on Silver-Tungsten , Silver-Tungsten Carbide and Silver Molybdenum]]"/>).

=== Silver–Tungsten During repeated switching under arcing loads, tungsten oxides and mixed oxides (SIWODURsilver tungstates – Ag₂ WO₄) Materials===are formed on the Ag/W (SIWODUR) surface, creating poorly conducting layers which increase the contact materials combine the high electrical resistance and thermalconductivity of silver with by this the high arc erosion resistance temperature rise during current carrying. Because of this fact the high meltingtungsten metal ''(Table 2.36)''. The manufacturing of materials Ag/W is paired in many applications with typically50-80 wt% tungsten is performed by the powder metallurgical processes ofliquid phase sintering Ag/C or by infiltration. Particle size and shape of the startingpowders are determining the micro structure and the Ag/WC/C contact specific propertiesof this material group ''(Figs. 2.134 and 2.135) (Table 2.37)''parts.

[[File:Contact Silver–tungsten contact tips are used in a variety of shapes and Switching Properties are produced for the ease of Contact Materials Based on.jpg|right|thumb|Contact attachment with a fine silver backing layer and Switching Properties quite often an additional thin layer of Contact Materials Based on Silver – Tungsten (SIWODUR)a brazing alloy. The attachment to contact carriers is usually done by brazing, Silver–Tungsten Carbide (SIWODUR C) and Silver Molybdenum (SILMODUR)]]but also by direct resistance welding for smaller tips.

During repeated switching under Ag/W materials are mostly used as the arcing contacts in disconnect switches for higher loads tungsten oxides and mixedas the main contacts in small and medium duty poweroxides switches and industrial circuit breakers (silver tungstates <xr id="tab:Contact and Switching Properties of Contact Materials Based on Silver – Ag<sub>2<Tungsten (SIWODUR), Silver–Tungsten Carbide (SIWODUR C) and Silver Molybdenum (SILMODUR)"/sub> WO<sub>4</sub!--(Table 2.38)--> ) . In north and south america they are formed on the Ag/W surface creating 2 4poorly conducting layers which increase the contact resistance and by this thetemperature rise during also used in large volumes in miniature circuit breakers of small to medium current carrying. Because of this fact the Ag/W is pairedratings in many applications with Ag/C contact partsdomestic wiring as well as for commercial power distribution.

Silver–tungsten contact tips are used in a variety <figtable id="tab:Physical Properties of shapes Contact Materials Based on Silver-Tungsten, Silver-Tungsten Carbide and are produced forSilver Molybdenum">the ease <caption>'''<!--Table 2.7:-->Physical Properties of attachment with a fine silver backing layer Contact Materials Based on Silver-Tungsten, Silver-Tungsten Carbide 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.Silver Molybdenum'''</caption>

{| class="twocolortable" style="text-align: left; font-size: 12px"|-!Material!Silver<br/>Content<br/>[wt.%]!Density<br/>[g/cm³]!Electrical<br/>Conductivity<br/>[MS/m]!Vickers<br/>Hardness<br/>[HV5]|-|Ag/W materials are mostly used as the arcing contacts in disconnect switches50/50 [[#text-reference|¹]]<br/>|47 - 53|12,9 - 13,9|29 - 38|110 - 175|-|Ag/W 40/60 [[#text-reference|¹]]|37 - 43|13,9 - 14,5|21 - 32|150 - 240|-|Ag/W 35/65 [[#text-reference|¹]]|32 - 38|14,1 - 15,1|21 - 31|160 - 260|-|Ag/W 32/68 [[#text-reference|¹]]|29 - 35|14,3 - 15,2|21 - 30|180 - 265|-|Ag/WC 60/40 [[#text-reference|¹]]|57 - 63|11,6 - 12,2|21 - 29|140 - 200|-|Ag/WC 40/60 [[#text-reference|¹]]for higher loads and as the main contacts in small and medium duty power|37 - 43switches and industrial circuit breakers ''(Table |12,5 - 13,3|18 - 25|230 - 340|-|Ag/WC 80/16C2 [[#text-reference|^2.]]|80 - 84|9,2 - 9,9|30 - 38)''. In north and south americathey are also used in large volumes in miniature circuit breakers of small to|35 - 55|-|Ag/WC 80/17C3 [[#text-reference|²]]|78 - 82|9,1 - 9,8|23 - 33|35 - 55|-|Ag/WC 80/19C1 [[#text-reference|²]]|78 - 82|9,5 - 10,5|28 - 43|40 - 60|-|Ag/WC 70/28C2 [[#text-reference|²]]|68 - 72|9,6 - 10,3|24 - 32|35 - 55|-|Ag/Mo 65/35 [[#text-reference|¹]]|62 - 68|9,9 - 10,9|16 - 28|140 - 130|-|}medium current ratings in domestic wiring as well as for commercial power<div id="text-reference">₁manufactured by infiltration</div>distribution.<div id="text-reference">₂ manufactured by press sinter-repress</div></figtable>

=== 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''(<xr id="fig:Micro structure of Ag WC 50 50"/><!--(Fig. 2.135) --> and <xr id="tab:Physical Properties of Contact Materials Based on Silver-Tungsten, Silver-Tungsten Carbide and Silver Molybdenum"/><!--(Table 2.36)''-->). Compared to Ag/W the Ag/WC (SIWODUR C)materials exhibit a higher resistance against contact welding ''(<xr id="tab:Contact and Switching Properties of Contact Materials Based on Silver – Tungsten (SIWODUR), Silver–Tungsten Carbide (SIWODUR C) and Silver Molybdenum (SILMODUR)"/><!--(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–tungsten-carbide–graphite materials with for example 27 19 wt% WC and3 1 wt% graphite or 16 wt% WC and 2 wt% graphite are manufactured using thesingle tip press-sinter-repress (PSR) process ''(<xr id="fig:Micro structure of -Ag WC 27 C3"/><!--(Fig. 2.136)''-->).

The applications of Ag/WC contacts are similar to those for Ag/W ''(<xr id="tab:Contact and Switching Properties of Contact Materials Based on Silver – Tungsten (SIWODUR), Silver–Tungsten Carbide (SIWODUR C) and Silver Molybdenum (SILMODUR)"/><!--(Table 2.38)''-->).

=== Silver–Molybdenum (SILMODUR) Materials===Ag/Mo materials with typically 50-70 wt% molybdenum are usually produced bythe powder metallurgical infiltration process ''(<xr id="fig:Micro structure of Ag Mo 35 65"/><!--(Fig. 2.137) --> and <xr id="tab:Physical Properties of Contact Materials Based on Silver-Tungsten, Silver-Tungsten Carbide and Silver Molybdenum"/><!--(Table 2.36)''-->). Theircontact properties are similar to those of Ag/W materials ''(<xr id="tab:Contact and Switching Properties of Contact Materials Based on Silver – Tungsten (SIWODUR), Silver–Tungsten Carbide (SIWODUR C) and Silver Molybdenum (SILMODUR)"/><!--(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''(<xr id="tab:Contact and Switching Properties of Contact Materials Based on Silver – Tungsten (SIWODUR), Silver–Tungsten Carbide (SIWODUR C) and Silver Molybdenum (SILMODUR)"/><!--(Table 2.38)''-->).

Fig<figure id="fig:Micro structure of Ag W 25 75"> [[File:Micro structure of Ag W 25 75. 2jpg|left|thumb|<caption>Micro structure of Ag/W 25/75</caption>]]</figure><figure id="fig:Micro structure of Ag WC 50 50"> [[File:Micro structure of Ag WC 50 50.135: jpg|left|thumb|<caption>Micro structure of Ag/WC 50/50</caption>]]</figure><figure id="fig:Micro structure of -Ag WC 27 C3"> [[File:Micro structure of -Ag WC 27 C3.jpg|left|thumb|<caption>Micro structure of Ag/WC27/C3</caption>]]</figure><figure id="fig:Micro structure of Ag Mo 35 65"> [[File:Micro structure of Ag Mo 35 65.jpg|left|thumb|<caption>Micro structure of Ag/Mo 35/65</caption>]]</figure></div><div class="clear"></div>

Fig. <figtable id="tab:Contact and Switching Properties of Contact Materials Based on Silver – Tungsten (SIWODUR), Silver–Tungsten Carbide (SIWODUR C) and Silver Molybdenum (SILMODUR)"><caption>'''<!--Table 2.13637: Micro structure -->Contact and Switching Properties of Contact Materials Based on Silver – Tungsten, Silver–Tungsten Carbide and Silver Molybdenum'''</caption><table class="twocolortable"><tr><th><p class="s12">Material</p></th><th><p class="s12">Properties</p></th></tr><tr><td><p class="s12">Silver-Tungsten</p><p class="s12"></p><p class="s12">Silver-tungsten carbide</p></td><td><p class="s12">Tendency to weld at high make currents in symmetrical pairing,</p><p class="s12">Higher contact resistance and higher temperature rise over increased number of operations through tungsten oxide and tungstate formation, especially for Ag/W,</p><p class="s12">High welding tendency of closed contacts during short circuit,</p><p class="s12">Very high arc erosion resistance, poor arc moving properties, High hardness and low formability,</p><p class="s12">Easy to braze and weld through Agenriched backing layer</p></td></tr><tr><td><p class="s12">Silver-Tungsten Carbide plus Graphite</p></td><td><p class="s12">Low contact resistance and low temperature rise through graphite addition,</p><p class="s12">Lower tendency to contact welding, Lower arc erosion resistance than Ag/WC</p></td></tr><tr><td><p class="s12">Silver-Molybdenum</p><p class="s12"></p></td><td><p class="s12">Better contact resistance stability due to less stable surface layers,</p><p class="s12">Lower arc erosion resistance than Ag/W</p></td></tr></WC27table></C3figtable>

Table 2.36<figtable id="tab: Physical Contact and Switching Properties of Contact Materials Based on Silver–Tungsten Silver – Tungsten (SIWODUR),Silver–Tungsten Carbide (SIWODUR C) and Silver Molybdenum (SILMODUR)"><caption>'''<!--Table 2.38:-->Contact and Switching Properties of Contact Materials Based on Silver – Tungsten, Silver–Tungsten Carbide and Silver Molybdenum'''</caption>

Table 2.37{| class="twocolortable" style="text-align: Contact and Switching Properties left; font-size: 12px"|-!Material !Application Examples!Form of Contact Materials Based on Silver – TungstenSupply|-|Ag/W<br />|Circuit breakers (SIWODURnot current limiting)|rowspan="3" | Contact tips, Silver–Tungsten Carbide brazed and welded<br />contact parts|-|Ag/W<br /><br />Ag/WC<br /><br />Ag/WC/C<br /> |(SIWODUR Main) Power switches<br /> paired with Ag/C)<br />and Silver Molybdenum Fault current circuit breakers<br />(SILMODURpaired with Ag/C)|-|Ag/Mo<br />|Device protection switches|}</figtable>

Table 2.38: Application Examples and Forms of Supply for Contact === Copper–Tungsten Materials Based===on Silver–Tungsten Copper–tungsten materials with typically 50-85 wt% tungsten are produced by the infiltration process with the tungsten particle size selected according to the end application [[#figures4|(SIWODURFigs. 5 – 6), Silver–Tungsten Carbide ]] <!--(SIWODUR CFigs. 2.138 – 2.141)--> and Silver Molybdenum (SILMODUR<xr id="tab:Physical properties of copper-tungsten materials"/><!--(Table 2.39)-->). To increase the wettability of the tungsten skeleton by copper a small amount of nickel < 1 wt% is added to the starting powder mix.

<figtable id=== Copper–Tungsten (CUWODUR) Materials===Copper–tungsten (CUWODUR) materials with typically 50"tab:Physical properties of copper-85 wt% tungsten arematerials">produced by the infiltration process with the tungsten particle size selectedaccording to the end application <caption>''(Figs. 2.138 – 2.141) (Table 2.39)''. To increasethe wettability Physical properties of the copper-tungsten skeleton by copper a small amount of nickelmaterials'''< 1 wt% is added to the starting powder mix./caption>

{| class="twocolortable" style="text-align: left; font-size: 12px"|-!Material!Tungsten<br/>Content<br/>[wt.%]!Density<br/>[g/cm³]!Melting Point<br/>[°C]!Electrical<br/>Resistivity<br/>[µΩ*cm]!Electrical<br/>Conductivity<br/>[% IACS]!Electrical<br/>Conductivity<br/>[MS/m]!Vickers<br/>Hardness<br/>[HV10]|-|W/Cu 60/40 <br/>|57 - 63|12,9 - 13,3|1083|3,85 - 4,55|38 - 45|22 - 26|150 - 200|-|W/Cu materials exhibit a very high arc erosion resistance ''(Table 65/35|63 - 67|13,6 - 14,0|1083|4,17 - 5,0|34 - 41|20 - 24|160 - 210|-|W/Cu 70/30 |68 - 72|13,9 - 14,4|1083|3,85 - 5,56|31 - 38|18 - 22|160 - 230|-|W/Cu 75/25 |73 - 77|14,6 - 15,2.40)''.Compared to silver–tungsten materials they are however less suitable to carry|1083|4,76 - 5,88|29 - 36|17 - 21|180 - 210|-|W/Cu 80/20|78 - 82|15,3 - 15,9|1083|5,0 - 6,25|28 - 34|16 - 20|180 - 280|-|}permanent current.</figtable>

With a solid tungsten skeleton as it is the case for W/C infiltrated Cu materials with70-85 wt% tungsten the lower melting component copper melts and vaporizesin the intense electrical exhibit a very high arcerosion resistance. At the boiling point of copper (2567°C) the still solidtungsten is efficiently “cooled” and remains pretty much unchangedCompared to silver–tungsten materials, they are however less suitable to carry permanent current.

During very high thermal stress on With a solid tungsten skeleton, as it is the case for W/Cu contactsC infiltrated materials with 70-85 wt% tungsten, for example during shortcircuit currents > 40 kA the tungsten skeleton requires special high mechanicalstrengthlower melting component copper melts and vaporizes in the intense electrical arc. For such applications a high temperature sintering At the boiling point of tungsten fromselected particle size powder is applied before the usual infiltration with copper(example: CUWODUR H2567°C), the still solid tungsten is efficiently “cooled” and remains pretty much unchanged.

For During very high voltage load switches thermal stress on 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 ascontacts, for example during short circuit currents > 40 kA, the arcing tipstungsten skeleton requires special high mechanical strength. The thermally and mechanically highly stressed attachmentbetween For such applications, a high temperature sintering of tungsten from selected particle size powder is applied before the two components is often achieved by utilizing electron beamwelding or capacitor discharge percussion welding. Other attachment methodsinclude brazing and cast-on of usual infiltration with copper followed by cold forming steps toincrease hardness and strength.

The main application areas for CUWODUR materials For high voltage load switches, the most advantageous contact system consists of a contact tulip and a contact rod. Both contact assemblies are as arcing contacts inload usually made from the mechanically strong and high power switching in medium conductive CuCrZr material and high voltage switchgear W/Cu as wellas electrodes for spark gaps the arcing tips. The thermally and over voltage arresters ''(Table 2mechanically highly stressed attachment between the two components is often achieved by utilizing electron beam welding or capacitor discharge percussion welding.41)''Other attachment methods include brazing and cast-on of copper, followed by cold forming steps to increase hardness and strength.

Table 2The main application areas for W/Cu materials are as arcing contacts in load and high power switching, in medium and high voltage switchgear as well as electrodes for spark gaps and over voltage arresters.39: Physical Properties of Copper–Tungsten (CUWODUR) Contact Materials

Fig. 2.139<div class="multiple-images"><figure id="fig:Micro structure of W Cu 70 30 G"> [[File: Micro structure of W/Cu 70/30 G Fig. 2.140: jpg|left|thumb|<caption>Micro structure of W/Cu 70/30 H(coarse)</caption>]]</figure>

Fig. 2.138<figure id="fig:Micro structure of W Cu 70 30 F">[[File: Micro structure of W/Cu 70/30 F Fig. 2.141: jpg|left|thumb|<caption>Micro structure of W/Cu 8070/30 (fine)</caption>]]</20 Hfigure>

Manufacturing of Contact Parts for</div>Medium 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<div class="clear"></div>