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Table 2.34: Application Examples and Forms of Supply of Silver–
Graphite (GRAPHOR) Contact Materials
Pre-Production of Contact Materials
(Bild)
===2.5 Tungsten and Molybdenum Based Materials===
===2.5.1 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 ''(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.
Both metals are however used in large amounts as components in composite
materials with silver and copper.
Table 2.35: Mechanical Properties of Tungsten and Molybdenum
===2.5.2 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 ''(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 ''(Figs. 2.134 and 2.135) (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 ''(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.
===2.5.3 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
''(Fig. 2.135) (Table 2.36)''. Compared to Ag/W the Ag/WC (SIWODUR C)
materials exhibit a higher resistance against contact welding ''(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 ''(Fig. 2.136)''.
The applications of Ag/WC contacts are similar to those for Ag/W ''(Table 2.38)''.
===2.5.4 Silver–Molybdenum (SILMODUR) Materials===
Ag/Mo materials with typically 50-70 wt% molybdenum are usually produced by
the powder metallurgical infiltration process ''(Fig. 2.137) (Table 2.36)''. Their
contact properties are similar to those of Ag/W materials ''(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
''(Table 2.38)''.
Fig. 2.134: Micro structure of Ag/W 25/75
Fig. 2.135: Micro structure of Ag/WC 50/50
Fig. 2.136: Micro structure of Ag/WC27/C3
Fig. 2.137: Micro structure of Ag/Mo 35/65
Table 2.36: Physical Properties of Contact Materials Based on Silver–Tungsten (SIWODUR),
Silver–Tungsten Carbide (SIWODUR C) and Silver Molybdenum (SILMODUR)
Table 2.37: Contact and Switching Properties of Contact Materials Based on Silver – Tungsten
(SIWODUR), Silver–Tungsten Carbide (SIWODUR C)
and Silver Molybdenum (SILMODUR)
Table 2.38: Application Examples and Forms of Supply for Contact Materials Based
on Silver–Tungsten (SIWODUR), Silver–Tungsten Carbide (SIWODUR C)
and Silver Molybdenum (SILMODUR)
===2.5.5 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 ''(Figs. 2.138 – 2.141) (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 ''(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.
During very high thermal stress on the W/Cu contacts, for example during short
circuit currents > 40 kA the tungsten skeleton requires special high mechanical
strength. For such applications a high temperature sintering of tungsten from
selected particle size powder is applied before the usual infiltration with copper
(example: CUWODUR H).
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 ''(Table 2.41)''.
Table 2.39: Physical Properties of Copper–Tungsten (CUWODUR) Contact Materials
Fig. 2.139: Micro structure of W/Cu 70/30 G Fig. 2.140: Micro structure of W/Cu 70/30 H
Fig. 2.138: Micro structure of W/Cu 70/30 F Fig. 2.141: Micro structure of W/Cu 80/20 H
