Difference between revisions of "Contact Materials for Electrical Engineering"

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The contact parts are important components in switching devices. They have to
+
The contact parts are important components in switching devices. They have to maintain their function from the new state until the end of the functional life of the devices.
maintain their function from the new state until the end of the functional life of the
 
devices.
 
  
The requirements on contacts are rather broad. Besides typical contact properties
+
The requirements on contacts are rather broad. Besides typical contact properties such as
such as
 
  
 
*High arc erosion resistance
 
*High arc erosion resistance
Line 12: Line 9:
 
*Good arc extinguishing capability
 
*Good arc extinguishing capability
  
they have to exhibit physical, mechanical, and chemical properties like high electrical
+
They have to exhibit physical, mechanical and chemical properties like high electrical and thermal conductivity, high hardness, high corrosion resistance etc. and besides this, should have good mechanical workability and also be suitable for good weld and brazing attachment to contact carriers. In addition they must be made from environmentally friendly materials.
and thermal conductivity, high hardness, high corrosion resistance, etc and besides
 
this should have good mechanical workability, and also be suitable for good weld and
 
brazing attachment to contact carriers. In addition they must be made from
 
environmentally friendly materials.
 
  
Materials suited for use as electrical contacts can be divided into the following groups
+
Materials suited for use as electrical contacts can be divided into the following groups based on their composition and metallurgical structure:
based on their composition and metallurgical structure:
 
  
 
*Pure metals
 
*Pure metals
 
*Alloys
 
*Alloys
 
*Composite materials
 
*Composite materials
*Pure metals
 
  
From this group silver has the greatest importance for switching devices in the higher
 
energy technology. Other precious metals such as gold and platinum are only used in
 
applications for the information technology in the form of thin surface layers. As a nonprecious
 
metal tungsten is used for some special applications such as for example as
 
automotive horn contacts. In some rarer cases pure copper is used but mainly paired
 
to a silver-based contact material.
 
  
*Alloys
+
'''Pure metals'''
 +
 
 +
Within this group, silver has the greatest importance for switching devices in the higher energy technology. Other precious metals such as gold and platinum are only used in applications for the information technology in the form of thin surface layers. As a nonprecious metal, tungsten is used for some special applications such as, for example, automotive horn contacts. In some rarer cases, pure copper is used, but mainly paired to a silver-based contact material.
 +
 
 +
'''Alloys'''
 +
 
 +
Besides these few pure metals, a larger number of alloy materials made by melt technology are available for the use as contacts. An alloy is characterized by the fact, that its components are completely or partially soluble in each other in the solid state. Phase diagrams for multiple metal compositions show the number and type of the crystal structure as a function of the temperature and composition of the alloying components.
  
Besides these few pure metals a larger number of alloy materials made by melt
+
They indicate the boundaries of liquid and solid phases and define the parameters of solidification.
technology are available for the use as contacts. An alloy is characterized by the fact
+
Alloying allows to improve the properties of one material at the cost of changing them for the second material. As an example, the hardness of a base metal may be increased while at the same time the electrical conductivity decreases with even small additions of the second alloying component.
that its components are completely or partially soluble in each other in the solid state.
 
Phase diagrams for multiple metal compositions show the number and type of the
 
crystal structure as a function of the temperature and composition of the alloying components.
 
  
They indicate the boundaries of liquid and solid phases and define the
+
'''Composite Materials'''
parameters of solidification.
 
Alloying allows to improve the properties of one material at the cost of changing
 
them for the second material. As an example, the hardness of a base metal may
 
be increased while at the same time the electrical conductivity decreases with
 
even small additions of the second alloying component.
 
  
*Composite Materials
+
Composite materials are a material group whose properties are of great importance for electrical contacts that are used in switching devices for higher
 +
electrical currents.
  
Composite materials are a material group whose properties are of great
+
Those used in electrical contacts are heterogeneous materials, composed of two or more uniformly dispersed components, in which the largest volume portion consists of a metal.
importance for electrical contacts that are used in switching devices for higher
 
electrical currents.
 
Those used in electrical contacts are heterogeneous materials composed of two
 
or more uniformly dispersed components in which the largest volume portion
 
consists of a metal.
 
The properties of composite materials are determined mainly independent from
 
each other by the properties of their individual components. Therefore it is for
 
example possible to combine the high melting point and arc erosion resistance
 
of tungsten with the low melting and good electrical conductivity of copper, or
 
the high conductivity of silver with the weld resistant metalloid graphite.
 
  
Figure 2.1 shows the schematic manufacturing processes from powder
+
The properties of composite materials are determined mainly independent from each other by the properties of their individual components. Therefore it is, for example, possible to combine the high melting point and arc erosion resistance of tungsten with the low melting and good electrical conductivity of copper or the high conductivity of silver with the weld resistant metalloid graphite. <xr id="fig:Powder metallurgical manufacturing of composite materials (schematic)"/> shows the schematic manufacturing processes from powder blending to contact material. Three basic process variations are typically applied:
blending to contact material. Three basic process variations are typically
 
applied:
 
  
 
*Sintering without liquid phase (Press-Sinter-Repress, PSR)
 
*Sintering without liquid phase (Press-Sinter-Repress, PSR)
Line 70: Line 42:
 
*Infiltration (Press-Sinter-Infiltrate, PSI)
 
*Infiltration (Press-Sinter-Infiltrate, PSI)
  
During ''sintering without a liquid phase'' (left side of schematic) the powder mix is
+
<figure id="fig:Powder metallurgical manufacturing of composite materials (schematic)">
first densified by pressing, then undergoes a heat treatment (sintering), and
+
[[File:Powder metallurgical manufacturing of composite materials (schematic).jpg|thumb|<caption>Powder-metallurgical manufacturing of composite materials (schematic) T<sub>s</sub> = Melting point of the lower melting component)</caption>]]
eventually is re-pressed again to further increase the density. The sintering
+
</figure>
atmosphere depends on the material components and later application; a
 
vacuum is used for example for the low gas content material Cu/Cr. This
 
process is used for individual contact parts and also termed press-sinterrepress
 
(PSR). For materials with high silver content the starting point at
 
pressing is most a larger block (or billet) which is then after sintering hot
 
extruded into wire, rod, or strip form. The extrusion further increases the density
 
of these composite materials and contributes to higher arc erosion resistance.
 
Materials such as Ag/Ni, Ag/MeO, and Ag/C are typically produced by this
 
process.
 
 
 
''Sintering with liquid phase'' has the advantage of shorter process times due to
 
the accelerated diffusion and also results in near-theoretical densities of the
 
  
Fig. 2.1: Powder-metallurgical manufacturing of composite materials (schematic)
+
During ''sintering without a liquid phase'' (left side of schematic), the powder mix is first densified by pressing, then undergoes a heat treatment (sintering) and eventually is re-pressed again to further increase the density. The sintering atmosphere depends on the material components and later application; a vacuum is used for example for the low gas content material Cu/Cr. This process is used for individual contact parts and also termed press-sinter-repress (PSR). For materials with high silver content, the starting point before pressing is mostly a large block (or billet) which is then, after sintering, hot extruded into wire, rod or strip form. The extrusion further increases the density of these composite materials and contributes to higher arc erosion resistance. Materials such as Ag/Ni, Ag/MeO and Ag/C are typically produced by this process.
T = Melting point of the lower melting component
 
  
composite material. To ensure the shape stability during the sintering process it
+
''Sintering with liquid phase'' has the advantage of shorter process times due to the accelerated diffusion and also results in near-theoretical densities of the composite material. To ensure the shape stability during the sintering process, it
 
is however necessary to limit the volume content of the liquid phase material.
 
is however necessary to limit the volume content of the liquid phase material.
  
As opposed to the liquid phase sintering which has limited use for electrical
+
As opposed to the liquid phase sintering, which has limited use for electrical contact manufacturing, the ''Infiltration process'' as shown on the right side of the schematic, has a broad practical range of applications. In this process the powder of the higher melting component, sometimes also as a powder mix with a small amount of the second material, is pressed into parts. Then, right after sintering, the porous skeleton is infiltrated with liquid metal of the second material. The fill-up process of the pores happens through capillary forces. This process reaches, after the infiltration, near-theoretical density without subsequent pressing and is widely used for Ag- and Cu-refractory contacts. For Ag/W or Ag/WC contacts, controlling the amount or excess on the bottom side of the contact of the infiltration metal Ag, results in contact tips that can be easily attached to their carriers by resistance welding. For larger Cu/W contacts, additional machining is often used to obtain the final shape of the contact component.
contact manufacturing, the ''Infiltration process'' as shown on the right side of the
 
schematic has a broad practical range of applications. In this process the
 
powder of the higher melting component sometimes also as a powder mix with
 
a small amount of the second material is pressed into parts and after sintering
 
the porous skeleton is infiltrated with liquid metal of the second material. The
 
filling up of the pores happens through capillary forces. This process reaches
 
after the infiltration near-theoretical density without subsequent pressing and is
 
widely used for Ag- and Cu-refractory contacts. For Ag/W or Ag/WC contacts,
 
controlling the amount or excess on the bottom side of the contact of the
 
infiltration metal Ag results in contact tips that can be easily attached to their
 
carriers by resistance welding. For larger Cu/W contacts additional machining is
 
often used to obtain the final shape of the contact component.
 
  
 
==Gold Based Materials==
 
==Gold Based Materials==
  
Pure Gold is besides Platinum the chemically most stable of all precious metals.
+
Pure Gold is besides Platinum the chemically most stable of all precious metals. In its pure form, it is not very suitable for use as a contact material in electromechanical devices because of its tendency to stick and cold-weld at even low contact forces. In addition, it is not hard or strong enough to resist mechanical wear and exhibits high material losses under electrical arcing loads. This limits its use in form of thin electroplated or vacuum deposited layers.
In its pure form it is not very suitable for use as a contact material in  
 
electromechanical devices because of its tendency to stick and cold-weld at even  
 
low contact forces. In addition it is not hard or strong enough to resist
 
mechanical wear and exhibits high materials losses under electrical arcing
 
loads. This limits its use in form of thin electroplated or vacuum deposited layers.
 
  
Main Articel: [[Gold Based Materials| Gold Based Materials]]
+
Main Article: [[Gold Based Materials| Gold Based Materials]]
  
 
==Platinum Metal Based Materials==
 
==Platinum Metal Based Materials==
  
The platinum group metals include the elements Pt, Pd, Rh, Ru, Ir, and Os ''(Table
+
The platinum group metals include the elements Pt, Pd, Rh, Ru, Ir and Os ([[Platinum_Metal_Based_Materials|Table 1]]<!--(Table 2.6)-->). For electrical contacts, platinum and palladium have practical significance as base alloy materials and ruthenium and iridium are used as alloying components. Pt and Pd have similar corrosion resistance as gold but due to their catalytical properties, they tend to polymerize adsorbed organic vapors on contact surfaces. During frictional movement between contact surfaces, the polymerized compounds known as “brown powder” are formed, which can lead to a significant increase in contact resistance. Therefore Pt and Pd are typically used as alloys and are rather not used in their pure form for electrical contact applications.
2.6)''. For electrical contacts platinum and palladium have practical significance
 
as base alloy materials and ruthenium and iridium are used as alloying components.
 
Pt and Pd have similar corrosion resistance as gold but because of their
 
catalytical properties they tend to polymerize adsorbed organic vapors on contact
 
surfaces. During frictional movement between contact surfaces the polymerized
 
compounds known as “brown powder” are formed which can lead to significantly
 
increase in contact resistance. Therefore Pt and Pd are typically used as alloys and
 
not in their pure form for electrical contact applications.
 
  
Main Articel: [[Platinum Metal Based Materials| Platinum Metal Based Materials]]
+
Main Article: [[Platinum Metal Based Materials| Platinum Metal Based Materials]]
  
 
==Silver Based Materials==
 
==Silver Based Materials==
Pure Silver, Silver Alloys, Silver Composite Materials
 
 
Main Articel: [[Silver Based Materials| Silver Based Materials]]
 
  
 +
Main Article: [[Silver Based Materials| Silver Based Materials]]
  
 
==Tungsten and Molybdenum Based Materials==
 
==Tungsten and Molybdenum Based Materials==
  
===Tungsten and Molybdenum (Pure Metals)===
+
Main Article: [[Tungsten and Molybdenum Based Materials| Tungsten and Molybdenum Based Materials]]
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
 
 
 
=== 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.
 
 
 
=== 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)''.
 
 
 
=== 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
+
==Contact Materials for Vacuum Switches==
(SIWODUR), Silver–Tungsten Carbide (SIWODUR C)
 
and Silver Molybdenum (SILMODUR)
 
  
Table 2.38: Application Examples and Forms of Supply for Contact Materials Based
+
The low gas content contact materials are developed for the use in vacuum switching devices.  
on Silver–Tungsten (SIWODUR), Silver–Tungsten Carbide (SIWODUR C)
 
and Silver Molybdenum (SILMODUR)
 
  
==== Copper–Tungsten (CUWODUR) Materials====
+
Main Article: [[Contact Materials for Vacuum Switches| Contact Materials for Vacuum Switches]]
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==
 
The trade name VAKURIT is assigned to a family of low gas content contact
 
materials developed for the use in vacuum switching devices ''(Table 2.42)''.
 
 
 
===Low Gas Content Materials Based on Refractory Metals===
 
Contact materials of W/Cu, W/Ag, WC/Ag, or Mo/Cu can be used in vacuum
 
switches if their total gas content does not exceed approximately 150 ppm. In
 
the low gas content W/Cu (VAKURIT) material mostly used in vacuum contactors
 
the high melting W skeleton is responsible for the high erosion resistance when
 
combined with the high conductivity copper component which evaporates
 
already in noticeable amounts at temperatures around 2000 °C.
 
 
 
Since there is almost no solubility of tungsten, tungsten carbide, or molybdenum
 
in copper or silver the manufacturing of these material is performed powdermetallurgically.
 
The W, WC, or Mo powders are pressed and sintered and then
 
infiltrated with low gas content Cu or Ag. The content of the refractory metals is
 
typically between 60 and 85 wt% ''(Figs. 2.142 and 2.143)''.
 
 
 
By adding approximately 1 wt% antimony the chopping current, i.e. the abrupt
 
current decline shortly before the natural current-zero, can be improved for
 
W/Cu (VAKURIT) materials ''(Table 2.43)''.
 
The contact components mostly used in vacuum contactors are usually shaped
 
as round discs. These are then attached by brazing in a vacuum environment to
 
their contact carriers ''(Table 2.44)''.
 
 
 
===Low Gas Content Materials Based on Copper-Chromium===
 
As contact materials in vacuum interupters in medium voltage devices low gas
 
materials based on Cu/Cr have gained broad acceptance. The typical chromium
 
contents are between 25 and 55 wt% ''(Figs. 2.144 and 2.145)''. During the
 
powder metallurgical manufacturing a mix of chromium and copper powders is
 
pressed into discs and subsequently sintering in a reducing atmosphere or
 
vacuum below the melting point of copper. This step is followed by cold or hot
 
re-pressing. Depending on the composition the Cu/Cr (VAKURIT) materials
 
combine a relatively high electrical and thermal conductivity with high dielectric
 
stability. They exhibit a low arc erosion rate and good resistance against welding
 
as well as favorable values of the chopping current in medium voltage load
 
switches, caused by the combined effects of the two components, copper and
 
chromium ''(Table 2.43)''.
 
 
 
The switching properties of Cu/Cr (VAKURIT) materials are dependent on the
 
purity of the Cr metal powders and especially the type and quantity of impurities
 
contained in the chromium powder used. Besides this the particle size and
 
distribution of the Cr powder are of high importance. Because of the getter
 
activity of chromium a higher total gas content of up to about 650 ppm
 
compared to the limits in refractory based materials can be tolerated in these
 
Cu/Cr contact materials. Besides the more economical sinter technology also
 
infiltration and vacuum arc melting are used to manufacture these materials.
 
Cu/Cr contacts are supplied in the shape of discs or rings which often also
 
contain slots especially for vacuum load switches in medium voltage devices
 
''(Table 2.44)''. Increased applications of round discs can also be observed for low
 
voltage vacuum contactors.
 
 
 
Table 2.42: Physical Properties of the Low Gas Materials (VAKURIT) for Vacuum Switches
 
 
 
Fig. 2.142: Micro structure of W/Cu 30Sb1
 
– low gas
 
 
 
Fig. 2.143: Micro structure of WC/Ag 50/50
 
– low gas
 
 
 
Fig. 2.144: Micro structure of Cu/Cr 75/25
 
– low gas
 
 
 
Fig. 2.145: Micro structure of Cu/Cr 50/50
 
– low gas
 
 
 
Table 2.43: Contact and Switching Properties of VAKURIT Materials
 
 
 
Table 2.44: Application Examples and Form of Supply for VAKURIT Materials
 
  
 
==References==
 
==References==
Line 598: Line 329:
 
Manufacturing Equipment for Semi-Finished Materials
 
Manufacturing Equipment for Semi-Finished Materials
 
(Bild)
 
(Bild)
 +
 +
[[de:Kontaktwerkstoffe_für_die_Elektrotechnik]]

Latest revision as of 11:54, 26 January 2023

The contact parts are important components in switching devices. They have to maintain their function from the new state until the end of the functional life of the devices.

The requirements on contacts are rather broad. Besides typical contact properties such as

  • High arc erosion resistance
  • High resistance against welding
  • Low contact resistance
  • Good arc moving properties
  • Good arc extinguishing capability

They have to exhibit physical, mechanical and chemical properties like high electrical and thermal conductivity, high hardness, high corrosion resistance etc. and besides this, should have good mechanical workability and also be suitable for good weld and brazing attachment to contact carriers. In addition they must be made from environmentally friendly materials.

Materials suited for use as electrical contacts can be divided into the following groups based on their composition and metallurgical structure:

  • Pure metals
  • Alloys
  • Composite materials


Pure metals

Within this group, silver has the greatest importance for switching devices in the higher energy technology. Other precious metals such as gold and platinum are only used in applications for the information technology in the form of thin surface layers. As a nonprecious metal, tungsten is used for some special applications such as, for example, automotive horn contacts. In some rarer cases, pure copper is used, but mainly paired to a silver-based contact material.

Alloys

Besides these few pure metals, a larger number of alloy materials made by melt technology are available for the use as contacts. An alloy is characterized by the fact, that its components are completely or partially soluble in each other in the solid state. Phase diagrams for multiple metal compositions show the number and type of the crystal structure as a function of the temperature and composition of the alloying components.

They indicate the boundaries of liquid and solid phases and define the parameters of solidification. Alloying allows to improve the properties of one material at the cost of changing them for the second material. As an example, the hardness of a base metal may be increased while at the same time the electrical conductivity decreases with even small additions of the second alloying component.

Composite Materials

Composite materials are a material group whose properties are of great importance for electrical contacts that are used in switching devices for higher electrical currents.

Those used in electrical contacts are heterogeneous materials, composed of two or more uniformly dispersed components, in which the largest volume portion consists of a metal.

The properties of composite materials are determined mainly independent from each other by the properties of their individual components. Therefore it is, for example, possible to combine the high melting point and arc erosion resistance of tungsten with the low melting and good electrical conductivity of copper or the high conductivity of silver with the weld resistant metalloid graphite. Figure 1 shows the schematic manufacturing processes from powder blending to contact material. Three basic process variations are typically applied:

  • Sintering without liquid phase (Press-Sinter-Repress, PSR)
  • Sintering with liquid phase
  • Infiltration (Press-Sinter-Infiltrate, PSI)
Figure 1: Powder-metallurgical manufacturing of composite materials (schematic) Ts = Melting point of the lower melting component)

During sintering without a liquid phase (left side of schematic), the powder mix is first densified by pressing, then undergoes a heat treatment (sintering) and eventually is re-pressed again to further increase the density. The sintering atmosphere depends on the material components and later application; a vacuum is used for example for the low gas content material Cu/Cr. This process is used for individual contact parts and also termed press-sinter-repress (PSR). For materials with high silver content, the starting point before pressing is mostly a large block (or billet) which is then, after sintering, hot extruded into wire, rod or strip form. The extrusion further increases the density of these composite materials and contributes to higher arc erosion resistance. Materials such as Ag/Ni, Ag/MeO and Ag/C are typically produced by this process.

Sintering with liquid phase has the advantage of shorter process times due to the accelerated diffusion and also results in near-theoretical densities of the composite material. To ensure the shape stability during the sintering process, it is however necessary to limit the volume content of the liquid phase material.

As opposed to the liquid phase sintering, which has limited use for electrical contact manufacturing, the Infiltration process as shown on the right side of the schematic, has a broad practical range of applications. In this process the powder of the higher melting component, sometimes also as a powder mix with a small amount of the second material, is pressed into parts. Then, right after sintering, the porous skeleton is infiltrated with liquid metal of the second material. The fill-up process of the pores happens through capillary forces. This process reaches, after the infiltration, near-theoretical density without subsequent pressing and is widely used for Ag- and Cu-refractory contacts. For Ag/W or Ag/WC contacts, controlling the amount or excess on the bottom side of the contact of the infiltration metal Ag, results in contact tips that can be easily attached to their carriers by resistance welding. For larger Cu/W contacts, additional machining is often used to obtain the final shape of the contact component.

Gold Based Materials

Pure Gold is besides Platinum the chemically most stable of all precious metals. In its pure form, it is not very suitable for use as a contact material in electromechanical devices because of its tendency to stick and cold-weld at even low contact forces. In addition, it is not hard or strong enough to resist mechanical wear and exhibits high material losses under electrical arcing loads. This limits its use in form of thin electroplated or vacuum deposited layers.

Main Article: Gold Based Materials

Platinum Metal Based Materials

The platinum group metals include the elements Pt, Pd, Rh, Ru, Ir and Os (Table 1). For electrical contacts, platinum and palladium have practical significance as base alloy materials and ruthenium and iridium are used as alloying components. Pt and Pd have similar corrosion resistance as gold but due to their catalytical properties, they tend to polymerize adsorbed organic vapors on contact surfaces. During frictional movement between contact surfaces, the polymerized compounds known as “brown powder” are formed, which can lead to a significant increase in contact resistance. Therefore Pt and Pd are typically used as alloys and are rather not used in their pure form for electrical contact applications.

Main Article: Platinum Metal Based Materials

Silver Based Materials

Main Article: Silver Based Materials

Tungsten and Molybdenum Based Materials

Main Article: Tungsten and Molybdenum Based Materials

Contact Materials for Vacuum Switches

The low gas content contact materials are developed for the use in vacuum switching devices.

Main Article: Contact Materials for Vacuum Switches

References

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Manufacturing Equipment for Semi-Finished Materials (Bild)