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→Silver–Graphite Materials
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 resistance is 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 consistantly 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 a part of the graphite is incorporated into the material (GRAPHOR DF) in the form of fibers (GRAPHOR DF), <xr id="fig:Micro 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.
Welding the actual process depends on the material's graphite orientation. For Ag/C tips with vertical graphite orientation the contacts are assembled with single tips. For parallel orientation a more economical attachment starting with contact material in strip or profile tape form is used in integrated stamping and welding operations with the tape fed into the weld station, cut off to tip form and then welded to the carrier material before forming the final contact assembly part. For special low energy welding, the Ag/C profile tapes can be pre-coated with a thin layer of high temperature brazing alloys such as CuAgP.
In a rather limited way, Ag/C with 2 – 3 wt% graphite can be produced in wire form and headed into contact rivet shape with low head deformation ratios.
The main applications for Ag/C materials are protective switching devices such as miniature molded case circuit breakers, motor-protective circuit breakers, and fault current circuit breakers, where during short circuit failures, highest resistance against welding is required (<xr id="tab:tab2.34"/>)<!--(Table 2.34)-->. For higher currents the low arc erosion resistance of Ag/C is compensated by asymmetrical pairing with more erosion resistant materials such as Ag/Ni, Ag/W and Ag/WC.
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