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<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">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>
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: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. 2.135)--> 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 mixed oxides (silver tungstates – Ag<sub>2</sub> WO<sub>4</sub>) are formed on the Ag/W surface, creating 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.
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 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 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 19 wt% WC and 3 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: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)-->).
<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 GrafitGraphite</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/WWC</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>
|-
|W/Cu 60/40 <br/>
|58 57 - 6263
|12,9 - 13,3
|1083
