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>

=== 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 ''(<xr id="tab:Physical Properties of Contact Materials Based on Silver-Tungsten, Silver-Tungsten Carbide and Silver Molybdenum"/><!--(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<xr id="fig:Micro structure of Ag W 25 75"/><!--(Fig. 2.134 and )-->, <xr id="fig:Micro structure of Ag WC 50 50"/><!--(Fig. 2.135) (Table 2.37--> and <xr id="tab:Physical Properties of Contact Materials Based on Silver-Tungsten, Silver-Tungsten Carbide and Silver Molybdenum"/>)''.

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 or Ag/WC/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 ''(<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)''-->). 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.

<figtable id=== Silver–Tungsten Carbide (SIWODUR C) "tab:Physical Properties of Contact Materials===This group of contact materials contains the typically 40Based on Silver-65 wtTungsten, Silver-% of the veryhard and erosion wear resistant tungsten carbide Tungsten Carbide and the high conductivity silverSilver Molybdenum"><caption>'''(Fig. 2.135) (<!--Table 2.36)7:-->Physical Properties of Contact Materials Based on Silver-Tungsten, Silver-Tungsten Carbide and Silver Molybdenum''. 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.caption>

Higher requirements on low temperature rise can be fulfilled by adding a small{| class="twocolortable" style="text-align: left; font-size: 12px"|-amount of graphite which however increases the arc erosion. Silver–tungsten!Materialcarbide–graphite materials with for example 27 !Silver<br/>Content<br/>[wt.% ]!Density<br/>[g/cm³]!Electrical<br/>Conductivity<br/>[MS/m]!Vickers<br/>Hardness<br/>[HV5]|-|Ag/W 50/50 [[#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 and40/60 [[#text-reference|¹]]|37 - 43|12,5 - 13,3 wt% graphite or 16 wt% |18 - 25|230 - 340|-|Ag/WC 80/16C2 [[#text-reference|²]]|80 - 84|9,2 - 9,9|30 - 38|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 and 70/28C2 [[#text-reference|^{2 wt% graphite are}]]|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|-|}<div id="text-reference">₁manufactured using theby infiltration</div>single tip <div id="text-reference">₂ manufactured by press-sinter-repress (PSR) process ''(Fig. 2.136)''.</div></figtable>

The applications === Silver–Tungsten Carbide Materials===This group of contact materials contains typically 40-65 wt-% of the very hard and erosion wear resistant tungsten carbide and the high conductivity silver (<xr id="fig:Micro structure of AgWC 50 50"/><!--(Fig. 2.135)--> and <xr id="tab:Physical Properties of Contact Materials Based on Silver-Tungsten, Silver-Tungsten Carbide and Silver Molybdenum"/WC contacts are similar ><!--(Table 2.36)-->). Compared to those for Ag/W ''the Ag/WC 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 2.3837)-->)''. The rise in contact resistance experienced with Ag/W is less pronounced in Ag/WC because during arcing, a protective gas layer of CO is formed, which limits the reaction of oxygen on the contact surface and therefore the formation of metal oxides.

=== Silver–Molybdenum (SILMODUR) Materials===Ag/Mo Higher requirements on low temperature rise can be fulfilled by adding a small amount of graphite, which however increases the arc erosion. Silver–tungsten-carbide–graphite materials with typically 50-70 for example 19 wt% WC and 1 wt% graphite or 16 wt% WC and 2 wt% molybdenum graphite are usually produced bymanufactured using the powder metallurgical infiltration process ''single tip press-sinter-repress (Fig. 2.137PSR) process (Table 2.36)''. Theircontact properties are similar to those <xr id="fig:Micro structure of -AgWC 27 C3"/W materials ''><!--(Table Fig. 2.37136)''. 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 The applications of Ag/WC contacts are similar to those for Ag/W 25(<xr id="tab:Contact and Switching Properties of Contact Materials Based on Silver – Tungsten (SIWODUR), Silver–Tungsten Carbide (SIWODUR C) and Silver Molybdenum (SILMODUR)"/75><!--(Table 2.38)-->).

=== Silver–Molybdenum Materials===Ag/Mo materials with typically 50-70 wt% molybdenum are usually produced by the powder metallurgical infiltration process (<xr id="fig:Micro structure of Ag Mo 35 65"/><!--(Fig. 2.135137)--> and <xr id="tab:Physical Properties of Contact Materials Based on Silver-Tungsten, Silver-Tungsten Carbide and Silver Molybdenum"/><!--(Table 2.36)-->). Their contact properties are similar to those of Ag/W materials (<xr id="tab: Micro structure 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 the molybdenum oxide is thermally less stable than tungsten oxide, the self-cleaning effect of Ag/Mo contact surface during arcing is more pronounced and the contact resistance remains lower than that of Ag/W. The arc erosion resistance of Ag/WC 50Mo however is lower than the one for Ag/W materials. The main applications 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)"/50><!--(Table 2.38)-->).

Table 2<figure id="fig:Micro structure of Ag W 25 75"> [[File:Micro structure of Ag W 25 75.36jpg|left|thumb|<caption>Micro structure of Ag/W 25/75</caption>]]</figure><figure id="fig: Physical Properties Micro structure of Contact Materials Based on Silver–Tungsten (SIWODUR),Ag WC 50 50"> [[File:Micro structure of Ag WC 50 50.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>Silver–Tungsten Carbide (SIWODUR C) and Silver Molybdenum (SILMODUR)<div class="clear"></div>

Table 2.37<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 2.37:-->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 Ag enriched 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></table></figtable>

<figtable id=== Copper–Tungsten "tab:Contact and Switching Properties of Contact Materials Based on Silver – Tungsten (SIWODUR), Silver–Tungsten Carbide (CUWODURSIWODUR C) Materials===Copper–tungsten and Silver Molybdenum (CUWODURSILMODUR) materials with typically 50-85 wt% tungsten are">produced by the infiltration process with the tungsten particle size selectedaccording to the end application <caption>'''(Figs. <!--Table 2.138 38:-->Contact and Switching Properties of Contact Materials Based on Silver – 2.141) (Table 2.39)Tungsten, Silver–Tungsten Carbide and Silver Molybdenum'''. To increasethe wettability of the tungsten skeleton by copper a small amount of nickel< 1 wt% is added to the starting powder mix./caption>

{| class="twocolortable" style="text-align: left; font-size: 12px"|-!Material !Application Examples!Form of Supply|-|Ag/W<br /Cu materials exhibit a very high arc erosion resistance ''>|Circuit breakers (Table 2.40not current limiting)''.Compared to silver–tungsten materials they are however less suitable to carry|rowspan="3" | Contact tips, brazed and welded<br />contact partspermanent |-|Ag/W<br /><br />Ag/WC<br /><br />Ag/WC/C<br /> |(Main) Power switches<br /> paired with Ag/C)<br />Fault current.circuit breakers<br />(paired with Ag/C)|-|Ag/Mo<br />|Device protection switches|}</figtable>

With a solid tungsten skeleton as it is the case for W/C infiltrated === Copper–Tungsten Materials===Copper–tungsten materials with70typically 50-85 wt% tungsten are produced by the infiltration process with the lower melting component copper melts and vaporizesin tungsten particle size selected according to the intense electrical arcend application [[#figures4|(Figs. 5 – 6)]] <!--(Figs. 2.138 – 2. At the boiling point 141)--> and (<xr id="tab:Physical properties of copper -tungsten materials"/><!--(2567°CTable 2.39) -->). To increase the wettability of the still solidtungsten skeleton by copper a small amount of nickel < 1 wt% is efficiently “cooled” and remains pretty much unchangedadded to the starting powder mix.

During very high thermal stress on the W/Cu contacts, for example during short<figtable id="tab:Physical properties of copper-tungsten materials">circuit currents <caption> 40 kA the tungsten skeleton requires special high mechanicalstrength. For such applications a high temperature sintering '''Physical properties of copper-tungsten fromselected particle size powder is applied before the usual infiltration with copper(example: CUWODUR H).materials'''</caption>

For high voltage load switches the most advantageous contact system consists{| class="twocolortable" style="text-align: left; font-size: 12px"|-!Materialof a contact tulip and a contact rod!Tungsten<br/>Content<br/>[wt. Both contact assemblies are made usually%]!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]|-from the mechanically strong and high conductive CuCrZr material and |W/Cu as60/40 <br/>|57 - 63the arcing tips. The thermally and mechanically highly stressed attachment|12,9 - 13,3|1083|3,85 - 4,55|38 - 45|22 - 26|150 - 200|-|W/Cu 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|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 - 34between the two components is often achieved by utilizing electron beam|16 - 20welding or capacitor discharge percussion welding. Other attachment methods|180 - 280include brazing and cast|-on of copper followed by cold forming steps toincrease hardness and strength.|}</figtable>

The main application areas for CUWODUR W/Cu materials are as arcing contacts inload and high power switching in medium and exhibit a very high voltage switchgear as wellas electrodes for spark gaps and over voltage arresters ''(Table 2arc erosion resistance.41)''Compared to silver–tungsten materials, they are however less suitable to carry permanent current.

Table 2With a solid tungsten skeleton, as it is the case for W/C infiltrated materials with 70-85 wt% tungsten, the lower melting component copper melts and vaporizes in the intense electrical arc.39: Physical Properties At the boiling point of Copper–Tungsten copper (CUWODUR2567°C) Contact Materials, the still solid tungsten is efficiently “cooled” and remains pretty much unchanged.

Fig. 2.139: Micro structure of During very high thermal stress on the W/Cu 70/30 G Figcontacts, for example during short circuit currents > 40 kA, the tungsten skeleton requires special high mechanical strength. 2For such applications, a high temperature sintering of tungsten from selected particle size powder is applied before the usual infiltration with copper.140: Micro structure of W/Cu 70/30 H

FigFor high voltage load switches, the most advantageous contact system consists of a contact tulip and a contact rod. 2.138: Micro structure of Both contact assemblies are usually made from the mechanically strong and high conductive CuCrZr material and W/Cu 70/30 F Figas the arcing tips. 2The thermally and mechanically highly stressed attachment between the two components is often achieved by utilizing electron beam welding or capacitor discharge percussion welding.141: Micro structure Other attachment methods include brazing and cast-on of W/Cu 80/20 Hcopper, followed by cold forming steps to increase hardness and strength.

Manufacturing of Contact Parts The main application areas forMedium W/Cu materials are as arcing contacts in load and High Voltage Switchgearhigh power switching, in medium and high voltage switchgear as well as electrodes for spark gaps and over voltage arresters.

Table 2.40<div class="multiple-images"><figure id="fig: Contact and Switching Properties Micro structure of Copper–TungstenW Cu 70 30 G"> [[File:Micro structure of W Cu 70 30 G.jpg|left|thumb|<caption>Micro structure of W/Cu 70/30 (CUWODURcoarse) Contact Materials</caption>]]</figure>

Table 2.41<figure id="fig: Application Examples and Forms Micro structure of Supply for Tungsten–W Cu 70 30 F">Copper [[File:Micro structure of W Cu 70 30 F.jpg|left|thumb|<caption>Micro structure of W/Cu 70/30 (CUWODURfine) Contact Materials</caption>]]</figure> </div><div class="clear"></div>