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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:Mechanical_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.
<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">Wolfram</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">Molybdän</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>
=== 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 melting tungsten metal <xr id="tab:Physical Properties of-Contact Materials Based"/> <!--(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) --> and <xr id="fig:Micro structure of Ag WC 50 50"/> <!--(Fig. 2.135)-->, <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)''-->.
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 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: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 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.
=== Silver–Tungsten Carbide (SIWODUR C) Materials===
This group of contact materials contains the 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 Ag WC 50 50"/> (Fig. 2.135) --> <xr id="tab:Physical Properties of-Contact Materials Based"/> <!--(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)-->. 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.
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 27 wt% WC and 3 wt% graphite or 16 wt% WC and 2 wt% graphite are manufactured using the single 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 by the powder metallurgical infiltration process <xr id="fig:Micro structure of Ag Mo 35 65"/> <!--(Fig. 2.137) --> <xr id="tab:Physical Properties of-Contact Materials Based"/> <!--(Table 2.36)''-->. Their contact 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 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 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)"/> <!--(Table 2.38)-->.
<figtable id="tab:Physical Properties of-Contact Materials Based">
[[File:Physical Properties of-Contact Materials Based.jpg|right|thumb|Physical Properties of Contact Materials Based on Silver–Tungsten (SIWODUR), Silver–Tungsten Carbide (SIWODUR C) and Silver Molybdenum (SILMODUR)]]
</figtable>
<xr id="fig:Micro structure of Ag W 25 75"/> <!--Fig. 2.134: --> Micro structure of Ag/W 25/75
<xr id="fig:Micro structure of Ag WC 50 50"/> <!--Fig. 2.135: --> Micro structure of Ag/WC 50/50
<xr id="fig:Micro structure of -Ag WC 27 C3"/> <!--Fig. 2.136: --> Micro structure of Ag/WC27/C3
<xr id="fig:Micro structure of Ag Mo 35 65"/> <!--Fig. 2.137: --> Micro structure of Ag/Mo 35/65
<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 (SIWODUR), Silver–Tungsten Carbide (SIWODUR C) and Silver Molybdenum (SILMODUR)'''</caption>
<table class="twocolortable">
<tr><th><p class="s12">Material/ DODUCO- Designation</p></th><th><p class="s12">Properties</p></th></tr><tr><td><p class="s12">Silver-Tungsten</p><p class="s12">SIWODUR</p><p class="s12">Silver-tungsten carbide SIWODUR C</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 Grafit SIWODUR C Plus</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/W</p></td></tr><tr><td><p class="s12">Silver-Molybdenum</p><p class="s12">SILMODUR</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 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 2.38: -->Contact and Switching Properties of Contact Materials Based on Silver – Tungsten (SIWODUR), Silver–Tungsten Carbide (SIWODUR C) and Silver Molybdenum (SILMODUR)'''</caption>
{| class="twocolortable" style="text-align: left; font-size: 12px"
=== 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 selected according to the end application [[#figures4|(Figs. 5 – 8)]] <!--(Figs. 2.138 – 2.141) --> <xr id="tab:Physical Properties of Copper Tungsten CUWODUR Contact 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.
W/Cu materials exhibit a very high arc erosion resistance <xr id="tab:Contact and Switching Properties of Copper–Tungsten (CUWODUR) Contact Materials"/> <!--(Table 2.40)-->. Compared to silver–tungsten materials they are however less suitable to carry permanent current.
With 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. At the boiling point of copper (2567°C) the still solid tungsten is efficiently “cooled” and remains pretty much unchanged.
For high voltage load switches the most advantageous contact system consists of a contact tulip and a contact rod. Both contact assemblies are made usually from the mechanically strong and high conductive CuCrZr material and W/Cu as the arcing tips. The thermally and mechanically highly stressed attachment between the two components is often achieved by utilizing electron beam welding or capacitor discharge percussion welding. Other attachment methods include brazing and cast-on of copper followed by cold forming steps to increase hardness and strength.
The main application areas for CUWODUR 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 <xr id="tab:Application Examples and Forms of Supply for Tungsten– Copper (CUWODUR) Contact Materials"/> <!--(Table 2.41)-->.
<figtable id="tab:Physical Properties of Copper Tungsten CUWODUR Contact Materials">
</figtable>
<div id="figures4">
<xr id="fig:Micro structure of W Cu 70 30 G"/> <!--Fig. 2.139: --> Micro structure of W/Cu 70/30 G
<xr id="fig:Micro structure of W Cu 70 30 H"/> <!--Fig. 2.140: --> Micro structure of W/Cu 70/30 H
<xr id="fig:Micro structure of W Cu 70 30 F"/> <!--Fig. 2.138: --> Micro structure of W/Cu 70/30 F
<xr id="fig:Micro structure of W Cu 80 20 H"/> <!--Fig. 2.141: --> Micro structure of W/Cu 80/20 H
</div>
<figtable id="tab:Contact and Switching Properties of Copper–Tungsten (CUWODUR) Contact Materials">
<caption>'''<!--Table 2.40: -->Contact and Switching Properties of Copper–Tungsten (CUWODUR) Contact Materials'''</caption>
<table class="twocolortable">
<tr><th><p class="s12">Material/ DODUCO- Designation</p></th><th><p class="s12">Properties</p></th></tr><tr><td><p class="s12">W/Cu F</p><p class="s12">CUWODUR F</p></td><td><p class="s12">Very high arc erosion resistance,</p><p class="s12">Uniform erosion pattern after high operation frequency, Very high mechanical strength,</p><p class="s12">Highly resistant against thermal and mechanical shock</p></td></tr><tr><td><p class="s12">W/Cu G</p><p class="s12">CUWODUR G</p></td><td><p class="s12">Very high arc erosion resistance, Very high mechanical strength,</p><p class="s12">Highly resistant against thermal and mechanical shock.</p></td></tr><tr><td><p class="s12">W/Cu H</p><p class="s12">CUWODUR H</p></td><td><p class="s12">Very high arc erosion resistance, very high mechanical strength, Especially high resistance against thermal and mechanical shock.</p></td></tr></table>
<figtable id="tab:Application Examples and Forms of Supply for Tungsten– Copper (CUWODUR) Contact Materials">
<caption>'''<!--Table 2.41: -->Application Examples and Forms of Supply for Tungsten– Copper (CUWODUR) Contact Materials'''</caption>
<table class="twocolortable">
<tr><th><p class="s12">Material</p></th><th><p class="s12">Application Examples</p></th><th><p class="s12">Form of Supply</p></th></tr><tr><td><p class="s12">W/Cu F</p></td><td><p class="s12">Transformer tap changers,</p><p class="s12">Medium voltage circuit breakers</p></td><td><p class="s12">Contact tips, formed parts, brazed</p><p class="s12">and welded contact parts</p></td></tr><tr><td><p class="s12">W/Cu G</p></td><td><p class="s12">Overvoltage arresters with spark gap,</p><p class="s12">Medium voltage circuit breakers, Medium voltage power switches, High voltage power switches and circuit breakers</p></td><td><p class="s12">Contact tips, formed parts, brazed</p><p class="s12">and welded contact parts; Contact tulips, rods and tubes</p></td></tr><tr><td><p class="s12">W/Cu H</p></td><td><p class="s12">High voltage power switches and circuit</p><p class="s12">breakers for very high short circuit currents</p></td><td><p class="s12">Welded contact parts; Contact tulips,</p><p class="s12">rods</p></td></tr></table>