Main Articel: [[Tungsten and Molybdenum Based Materials| Tungsten and Molybdenum Based Materials]]
=== 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)
==== 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
Manufacturing of Contact Parts for
Medium and High Voltage Switchgear
Table 2.40: Contact and Switching Properties of Copper–Tungsten
(CUWODUR) Contact Materials
Table 2.41: Application Examples and Forms of Supply for Tungsten–
Copper (CUWODUR) Contact Materials
==Special Contact Materials (VAKURIT) for Vacuum Switches==
