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===Tungsten and Molybdenum (Pure Metals)===
Tungsten is characterized by its advantageous properties of high melting and boiling points, sufficient electrical and thermal conductivity and high hardness and density (<xr id="tab:tab2.35Mechanical_Properties_of_Tungsten_and_Molybdenum"/> <!--(Table 2.35)-->). It is mainly used in the form of brazed contact tips for switching duties , that require a rapid switching sequence , such as horn contacts for cars and trucks.
Molybdenum has a much lesser importance as a contact material since it is less resistant against oxidation than tungsten.
Both metals are however used in large amounts as components in composite materials with silver and copper.
<figtable id="tab:tab2.35Mechanical_Properties_of_Tungsten_and_Molybdenum"><caption>'''<!--Table 2.35: -->Mechanical Properties of Tungsten and Molybdenum'''</caption><table borderclass="1" cellspacing="0" style="border-collapse:collapsetwocolortable"><tr><tdth><p class="s12">Material</p></tdth><tdth><p class="s12">Micro Structure Condition</p></tdth><tdth><p class="s12">Vickers</p><p class="s12">Hardness HV 10</p></tdth><tdth><p class="s12">Tensile Strength</p><p class="s12">[MPa]</p></tdth></tr><tr><td><p class="s12">WolframTungsten</p></td><td><p class="s12">Lightly worked structure</p><p class="s12">(wire and strip > 1.0 mm thick)</p><p class="s12">Strongly worked structure</p><p class="s12">(wire and strip < 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">MolybdänMolybdenum</p></td><td><p class="s12">Lightly worked structure</p><p class="s12">(wire and strip > 1.0 mm thick)</p><p class="s12">Strongly worked structure</p><p class="s12">(wire and strip < 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 thermal conductivity 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 typically 50-80 wt% tungsten is performed by the powder metallurgical processes ofliquid 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:fig2.134Micro structure of Ag W 25 75"/> <!--(Fig. 2.134 and )-->, <xr id="fig:fig2.135Micro structure of Ag WC 50 50"/> <!--(Fig. 2.135, )--> and <xr id="tab:tab2.37Physical Properties of Contact Materials Based on Silver-Tungsten, Silver-Tungsten Carbide and Silver Molybdenum"/> (Table 2.37)''.
During repeated switching under arcing loads , tungsten oxides and mixed oxides (silver tungstates – Ag<sub>2</sub> WO<sub>4</sub> ) are formed on the Ag/W surface , creating 2 4 poorly conducting layers which increase the contact resistance and by this the temperature rise during current carrying. Because of this fact the Ag/W is paired in 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 for the ease of attachment with a fine silver backing layer and quite often an additional thin layer of a brazing alloy. The attachment to contact carriers is usually done by brazing, but also by direct resistance welding for smaller tips.
Ag/W materials are mostly used as the arcing contacts in disconnect switches for higher loads and as the main contacts in small and medium duty power
switches and industrial circuit breakers (<xr id="tab:tab2.38Contact 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 america they are also used in large volumes in miniature circuit breakers of small to medium current ratings in domestic wiring as well as for commercial power distribution.
<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 very hard Tungsten Carbide and erosion wear resistant tungsten carbide and the high conductivity silver Silver Molybdenum"><xr id="fig:fig2.135"/caption> (Fig. 2.135) (Table 2.36)''. Compared to Ag/W the Ag/WC (SIWODUR C) materials exhibit a higher resistance against contact welding '<xr id="tab:tab2.37"/> (!--Table 2.37). The rise in contact resistance experienced with Ag/W is less pronounced in Ag/WC because during arcing a protective gas layer 7:-->Physical Properties of CO is formed which limits the reaction of oxygen Contact Materials Based on the contact surface Silver-Tungsten, Silver-Tungsten Carbide and therefore the formation of metal oxides.Silver Molybdenum'''</caption>
=== 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:fig2.134Micro structure of Ag WC 50 50"/> <!--(Fig. 2.134135)--> and <xr id="tab: Micro structure 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 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)-->). The rise in contact resistance experienced with Ag/W 25is less pronounced in Ag/75WC 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.
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 for example 19 wt% WC and 1 wt% graphite or 16 wt% WC and 2 wt% graphite are manufactured using the single tip press-sinter-repress (PSR) process (<xr id="fig:fig2.135Micro structure of -Ag WC 27 C3"/> <!--(Fig. 2.135: Micro structure of Ag/WC 50/50136)-->).
The applications of Ag/WC contacts are similar to those for Ag/W (<xr id="figtab:fig2.136Contact and Switching Properties of Contact Materials Based on Silver – Tungsten (SIWODUR), Silver–Tungsten Carbide (SIWODUR C) and Silver Molybdenum (SILMODUR)"/> Fig<!--(Table 2. 238)-->).136: Micro structure of Ag/WC27/C3
=== Silver–Molybdenum Materials===Ag/Mo materials with typically 50-70 wt% molybdenum are usually produced by the powder metallurgical infiltration process (<xr id="fig:fig2.137Micro 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)-->). 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/Mo 35however 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)"/65><!--(Table 2.38)-->).
<div class="multiple-images">
<figure id="fig:fig2.134Micro structure of Ag W 25 75">
[[File:Micro structure of Ag W 25 75.jpg|left|thumb|<caption>Micro structure of Ag/W 25/75</caption>]]
</figure>
<figure id="fig:fig2.135Micro structure of 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:fig2.136Micro 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:fig2.137Micro 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 class="clear"></div>
</figtable>
<figtable id="tab:tab2.38Contact and Switching Properties of Contact Materials Based on Silver – Tungsten (SIWODUR), Silver–Tungsten Carbide (SIWODUR C) and Silver Molybdenum (SILMODUR)"><caption>'''<!--Table 2.38: Application Examples -->Contact and Forms Switching Properties of Supply for Contact Materials Based on Silver–Tungsten (SIWODUR)Silver – Tungsten, Silver–Tungsten Carbide (SIWODUR C) and Silver Molybdenum (SILMODUR)'''</caption>
{| class="twocolortable" style="text-align: left; font-size: 12px"
!Form of Supply
|-
|Ag/W<br />SIWODUR
|Circuit breakers (not current limiting)
|rowspan="3" | Contact tips, brazed and welded<br />contact parts
|-
|Ag/W<br />SIWODUR<br />Ag/WC<br />SIWODUR C<br />Ag/WC/C<br />SIWODUR C/C
|(Main) Power switches<br /> paired with Ag/C)<br />
Fault current circuit breakers<br />(paired with Ag/C)
|-
|Ag/Mo<br />SILMODUR
|Device protection switches
|}
</figtable>
=== Copper–Tungsten (CUWODUR) Materials===Copper–tungsten (CUWODUR) materials with typically 50-85 wt% tungsten are produced by the infiltration process with the tungsten particle size selectedaccording to the end application [[#figures4|(Figs. 5 – 86)]] <!--(Figs. 2.138 – 2.141) --> and (<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.
<div class="multiple-images">
<figure id="fig:fig2.139"> [[File:Micro structure of W Cu 70 30 G.jpg|left|thumb|<caption>Micro structure of W/Cu 70/30 G</caption>]]</figure> <figure id="fig:fig2.140">[[File:Micro structure of W Cu 70 30 HG.jpg|left|thumb|<caption>Micro structure of W/Cu 70/30 H(coarse)</caption>]]
</figure>
<figure id="fig:fig2.138Micro structure of W Cu 70 30 F">[[File:Micro structure of W Cu 70 30 F.jpg|left|thumb|<caption>Micro structure of W/Cu 70/30 F(fine)</caption>]]
</figure>
</div>
<div class="clear"></div>
==References==
[[Contact Materials for Electrical Engineering#References|References]]
[[de:Werkstoffe_auf_Wolfram-_und_Molybdän-Basis]]