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materials present an economic alternative to Cd free Ag-tin oxide contact
materials ''(Tables 2.30 and 2.31)''.
Table 2.28: Physical and Mechanical Properties as well as Manufacturing Processes and
Forms of Supply of Extruded Silver-Zinc Oxide (DODURIT ZnO) Contact
Fig. 2.120: Strain hardening of
Ag/ZnO 92/8 PW25 by cold working
Fig. 2.121: Softening of Ag/ZnO 92/8 PW25
after annealing for 1 hr after 30% cold working
Fig. 2.122: Strain hardening of
Ag/ZnO 92/8 WPW25
by cold working
Fig. 2.123: Softening of
Ag/ZnO 92/8 WPW25 after annealing for
1hr after different degrees of cold working
Fig. 2.115: Micro structure of Ag/ZnO 92/8 Pw25: a) perpendicular to extrusion direction
b) parallel to extrusion direction
Fig. 2.116: Micro structure of Ag/ZnO 92/8 WPW25:a) perpendicular to extrusion direction
b) parallel to extrusion direction, 1) Ag/ZnO contact layer, 2) Ag backing layer
Table 2.29: Optimizing of Silver–Tin Oxide Materials Regarding their Switching
Properties and Forming Behavior
Table 2.30: Contact and Switching Properties of Silver–Metal Oxide Materials
Table 2.31: Application Examples of Silver–Metal Oxide Materials
===2.4.3.3 Silver–Graphite (GRAPHOR)-Materials===
Ag/C (GRAPHOR) contact materials are usually produced by powder metallurgy
with graphite contents of 2 – 5 wt% ''(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 ''(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 ''(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 ''(Figs. 2.131 and 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 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 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), ''(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 ''(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 GRAPHOR D and DF
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 ''(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 and Ag/W.
Fig. 2.126:
Strain hardening
of Ag/C 96/4 D
by cold working
Fig. 2.127:
Softening of Ag/C 96/4 D after
annealing
Fig. 2.128: Strain hardening
of Ag/C DF by cold working
Fig. 2.129: Softening
of Ag/C DF after annealing
Fig. 2.130: Micro structure of Ag/C 97/3: a) perpendicular to extrusion direction
b) parallel to extrusion direction, 1) Ag/C contact layer, 2) Ag backing layer
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
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
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
Table 2.32: Physical Properties of Silver–Graphite (GRAPHOR) Contact Materials
Table 2.33: Contact and Switching properties of Silver–Graphite (GRAPHOR) Contact Materials
Table 2.34: Application Examples and Forms of Supply of Silver–
Graphite (GRAPHOR) Contact Materials
