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→Silver–Graphite (GRAPHOR)-Materials
Ag/C (GRAPHOR) contact materials are usually produced by powder metallurgy with graphite contents of 2 – 5 wt% <xr id="tab:tab2.32"/> (Table 2.32). The earlier typical manufacturing process of single pressed tips by pressing - sintering - repressing (PSR) has been replaced in Europe for quite some time by extrusion. In North America and some other regions however the PSR process is still used to some extend mainly for cost reasons.
The extrusion of sintered billets is now the dominant manufacturing method for semi-finished AgC materials <!--[[#figures3|(Figs. 82 – 85)]](Figs. 2.126 – 2.129)-->. The hot extrusion process results in a high density material with graphite particles stretched and oriented in the extrusion direction [[#figures4|(Figs. 86 – 89)]](Figs. 2.130 – 2.133)''. Depending on the extrusion method in either rod or strip form the graphite particles can be oriented in the finished contact tips perpendicular (GRAPHOR) or parallel (GRAPHOR D) to the switching contact surface <xr id="fig:fig2.131Micro structure of Ag C 95 5"/> (Fig. 2.131) and <xr id="fig:fig2.132Micro structure of Ag C 96 4 D"/> (Fig. 2.132).
Since the graphite particles in the Ag matrix of Ag/C materials prevent contact tips from directly being welded or brazed, a graphite free bottom layer is required. This is achieved by either burning out (de-graphitizing) the graphite selectively on one side of the tips or by compound extrusion of a Ag/C billet covered with a fine silver shell.
Ag/C contact materials exhibit on the one hand an extremely high resistance to contact welding but on the other have a low arc erosion resistance. This is caused by the reaction of graphite with the oxygen in the surrounding atmosphere at the high temperatures created by the arcing. The weld resistanceis especially high for materials with the graphite particle orientation parallel to the arcing contact surface. Since the contact surface after arcing consists of pure silver the contact resistance stays consistently low during the electrical life of the contact parts.
A disadvantage of the Ag/C materials is their rather high erosion rate. In materials with parallel graphite orientation this can be improved if part of the graphite is incorporated into the material in the form of fibers (GRAPHOR DF), <xr id="fig:fig2.133Micro structure of Ag C DF"/> (Fig. 2.133). The weld resistance is determined by the total content of graphite particles.
Ag/C tips with vertical graphite particle orientation are produced in a specific sequence: Extrusion to rods, cutting of double thickness tips, burning out of graphite to a controlled layer thickness, and a second cutting to single tips. Such contact tips are especially well suited for applications which require both, a high weld resistance and a sufficiently high arc erosion resistance <xr id="tab:tab2.33"/> (Table 2.33). For attachment of Ag/C tips welding and brazing techniques are applied.
b) parallel to extrusion direction, 1) Ag/C contact layer, 2) Ag backing layer
<xr id="fig:fig2.131Micro structure of Ag C 95 5"/> Fig. 2.131: Micro structure of Ag/C 95/5: a) perpendicular to extrusion direction
b) parallel to extrusion direction, 1) Ag/C contact layer, 2) Ag backing layer
<xr id="fig:fig2.132Micro structure of Ag C 96 4 D"/> Fig. 2.132: Micro structure of Ag/C 96/4 D: a) perpendicular to extrusion direction
b) parallel to extrusion direction, 1) Ag/C contact layer, 2) Ag backing layer
<xr id="fig:fig2.133Micro structure of Ag C DF"/> Fig. 2.133: Micro structure of Ag/C DF: a) perpendicular to extrusion direction
b) parallel to extrusion direction, 1) Ag/C contact layer, 2) Ag/Ni 90/10 backing layer
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<figure id="fig:fig2.131Micro structure of Ag C 95 5">
[[File:Micro structure of Ag C 95 5.jpg|left|thumb|<caption>Micro structure of Ag/C 95/5: a) perpendicular to extrusion direction b) parallel to extrusion direction, 1) Ag/C contact layer, 2) Ag backing layer</caption>]]
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<figure id="fig:fig2.132Micro structure of Ag C 96 4 D">
[[File:Micro structure of Ag C 96 4 D.jpg|left|thumb|<caption>Micro structure of Ag/C 96/4 D: a) perpendicular to extrusion direction b) parallel to extrusion direction, 1) Ag/C contact layer, 2) Ag backing layer</caption>]]
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<figure id="fig:fig2.133Micro structure of Ag C DF">
[[File:Micro structure of Ag C DF.jpg|left|thumb|<caption>Micro structure of Ag/C DF: a) perpendicular to extrusion direction b) parallel to extrusion direction, 1) Ag/C contact layer, 2) Ag/Ni 90/10 backing layer</caption>]]
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