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
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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
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The requirements on contacts are rather broad. Besides typical contact properties such as
such as
 
  
 
*High arc erosion resistance
 
*High arc erosion resistance
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*Good arc extinguishing capability
 
*Good arc extinguishing capability
  
they have to exhibit physical, mechanical, and chemical properties like high electrical
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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
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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
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*'''Pure metals'''
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
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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.
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
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*'''Alloys'''
to a silver-based contact material.
 
  
*Alloys
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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
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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
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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
blending to contact material. Three basic process variations are typically
 
 
applied:
 
applied:
  
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*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-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.
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 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.
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 materials 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 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.
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]]
 
 
==Silver Composite Materials==
 
 
===Silver-Nickel (SINIDUR) Materials===
 
Since silver and nickel are not soluble in each other in solid form and in the liquid
 
phase have only very limited solubility silver nickel composite materials with
 
higher Ni contents can only be produced by powder metallurgy. During extrusion
 
of sintered Ag/Ni billets into wires, strips and rods the Ni particles embedded in
 
the Ag matrix are stretched and oriented in the microstructure into a pronounced
 
fiber structure ''(Figs. 2.75. and 2.76)''
 
 
The high density produced during hot extrusion aids the arc erosion resistance
 
of these materials ''(Tables 2.21 and 2.22)''. The typical application of Ag/Ni
 
contact materials is in devices for switching currents of up to 100A ''(Table 2.24)''.
 
In this range they are significantly more erosion resistant than silver or silver
 
alloys. In addition they exhibit with nickel contents <20 wt% a low and over their
 
operational lifetime consistent contact resistance and good arc moving
 
properties. In DC applications Ag/Ni materials exhibit a relatively low tendency
 
of material transfer distributed evenly over the contact surfaces ''(Table 2.23)''.
 
 
Typically Ag/Ni (SINIDUR) materials are usually produced with contents of 10-40
 
wt% Ni. The most widely used materials SINIDUR 10 and SINIDUR 20- and also
 
SINIDUR 15, mostly used in north america-, are easily formable and applied by
 
cladding ''(Figs. 2.71-2.74)''. They can be, without any additional welding aids,
 
economically welded and brazed to the commonly used contact carrier
 
materials.
 
The (SINIDUR) materials with nickel contents of 30 and 40 wt% are used in
 
switching devices requiring a higher arc erosion resistance and where increases
 
in contact resistance can be compensated through higher contact forces.
 
 
The most important applications for Ag/Ni contact materials are typically in
 
relays, wiring devices, appliance switches, thermostatic controls, auxiliary
 
switches, and small contactors with nominal currents >20A ''(Table 2.24)''.
 
 
Table 2.21: Physical Properties of Silver-Nickel (SINIDUR) Materials
 
 
Table 2.22: Mechanical Properties of Silver-Nickel (SINIDUR) Materials
 
 
Fig. 2.71:
 
Strain hardening
 
of Ag/Ni 90/10 by cold working
 
 
Fig. 2.72:
 
Softening of Ag/Ni 90/10
 
after annealing
 
for 1 hr after 80% cold working
 
 
Fig. 2.73:
 
Strain hardening
 
of Ag/Ni 80/20 by cold working
 
 
Fig. 2.74:
 
Softening of Ag/Ni 80/20
 
after annealing
 
for 1 hr after 80% cold working
 
 
Fig. 2.75: Micro structure of Ag/Ni 90/10 a) perpendicular to the extrusion direction
 
b) parallel to the extrusion direction
 
 
Fig. 2.76: Micro structure of Ag/Ni 80/20 a) perpendicular to the extrusion direction
 
b) parallel t o the extrusion direction
 
 
Table 2.23: Contact and Switching Properties of Silver-Nickel (SINIDUR) Materials
 
 
Table 2.24: Application Examples and Forms of Supply
 
for Silver-Nickel (SINIDUR) Materials
 
 
=== Silver-Metal Oxide Materials Ag/CdO, Ag/SnO<sub>2</sub>, Ag/ZnO===
 
The family of silver-metal oxide contact materials includes the material groups:
 
silver-cadmium oxide (DODURIT CdO), silver-tin oxide (SISTADOX), and silverzinc
 
oxide (DODURIT ZnO). Because of their very good contact and switching
 
properties like high resistance against welding, low contact resistance, and high
 
arc erosion resistance, silver-metal oxides have gained an outstanding position
 
in a broad field of applications. They mainly are used in low voltage electrical
 
switching devices like relays, installation and distribution switches, appliances,
 
industrial controls, motor controls, and protective devices ''(Table 2.13)''.
 
 
*Silver-cadmium oxide (DODURIT CdO) materials
 
 
Silver-cadmium oxide (DODURIT CdO) materials with 10-15 wt% are produced
 
by both, internal oxidation and powder metallurgical methods ''(Table 2.25)''.
 
 
The manufacturing of strips and wires by internal oxidation starts with a molten
 
alloy of silver and cadmium. During a heat treatment below it's melting point in a
 
oxygen rich atmosphere in such a homogeneous alloy the oxygen diffuses from
 
the surface into the bulk of the material and oxidizes the Cd to CdO in a more or
 
less fine particle precipitation inside the Ag matrix. The CdO particles are rather
 
fine in the surface area and are becoming larger further away towards the center
 
of the material ''(Fig. 2.83)''.
 
 
During the manufacturing of Ag/CdO contact material by internal oxidation the
 
processes vary depending on the type of semi-finished material.
 
For Ag/CdO wires a complete oxidation of the AgCd wire is performed, followed
 
by wire-drawing to the required diameter ''(Figs. 2.77 and 2.78)''. The resulting
 
material is used for example in the production of contact rivets. For Ag/CdO strip
 
materials two processes are commonly used: Cladding of an AgCd alloy strip
 
with fine silver followed by complete oxidation results in a strip material with a
 
small depletion area in the center of it's thickness and a Ag backing suitable for
 
easy attachment by brazing (sometimes called “Conventional Ag/CdO”). Using
 
a technology that allows the partial oxidation of a dual-strip AgCd alloy material
 
in a higher pressure pure oxygen atmosphere yields a composite Ag/CdO strip
 
material that has besides a relatively fine CdO precipitation also a easily brazable
 
AgCd alloy backing ''(Fig. 2.85)''. These materials (DODURIT CdO ZH) are mainly
 
used as the basis for contact profiles and contact tips.
 
 
During powder metallurgical production the powder mixed made by different
 
processes are typically converted by pressing, sintering and extrusion to wires
 
and strips. The high degree of deformation during hot extrusion produces a
 
uniform and fine dispersion of CdO particles in the Ag matrix while at the same
 
time achieving a high density which is advantageous for good contact properties
 
''(Fig. 2.84)''. To obtain a backing suitable for brazing, a fine silver layer is applied
 
by either com-pound extrusion or hot cladding prior to or right after the extrusion
 
''(Fig. 2.86)''.
 
 
For larger contact tips, and especially those with a rounded shape, the single tip
 
Press-Sinter-Repress process (PSR) offers economical advantages. The
 
powder mix is pressed in a die close to the final desired shape, the “green” tips
 
are sintered, and in most cases the repress process forms the final exact shape
 
while at the same time increasing the contact density and hardness.
 
 
Using different silver powders and minor additives for the basic Ag and CdO
 
starting materials can help influence certain contact properties for specialized
 
applications.
 
 
Fig. 2.77:
 
Strain hardening of internally oxidized
 
Ag/CdO 90/10 by cold working
 
 
Fig. 2.78:
 
Softening of internally oxidized
 
Ag/CdO 90/10 after annealing
 
for 1 hr after 40% cold working
 
 
Table 2.25: Physical and Mechanical Properties as well as Manufacturing Processes and
 
Forms of Supply of Extruded Silver Cadmium Oxide
 
(DODURIT CdO) Contact Materials
 
 
Fig. 2.79:
 
Strain hardening of
 
Ag/CdO 90/10 P by cold working
 
 
Fig. 2.80: Softening
 
of Ag/CdO 90/10 P after annealing
 
for 1 hr after 40% cold working
 
 
Fig. 2.81:
 
Strain hardening
 
of Ag/CdO 88/12 WP
 
 
Fig. 2.82:
 
Softening of Ag/CdO 88/12WP after annealing
 
for 1 hr after different degrees of
 
cold working
 
 
Fig. 2.83: Micro structure of Ag/CdO 90/10 i.o. a) close to surface
 
b) in center area
 
 
Fig. 2.84: Micro structure of Ag/CdO 90/10 P:
 
a) perpendicular to extrusion direction
 
b) parallel to extrusion direction
 
 
Fig. 2.85:
 
Micro structure of Ag/CdO 90/10 ZH:
 
1) Ag/CdO layer
 
2) AgCd backing layer
 
 
Fig. 2.86: Micro structure of AgCdO 88/12 WP: a) perpendicular to extrusion direction
 
b) parallel to extrusion direction
 
 
*Silver–tin oxide(SISTADOX)materials
 
Over the past years, many Ag/CdO contact materials have been replaced by
 
Ag/SnO<sub>2</sub> based materials with 2-14 wt% SnO<sub>2</sub> because of the toxicity of
 
Cadmium. This changeover was further favored by the fact that Ag/SnO<sub>2</sub>
 
contacts quite often show improved contact and switching properties such as
 
lower arc erosion, higher weld resistance, and a significant lower tendency
 
towards material transfer in DC switching circuits ''(Table 2.30)''. Ag/SnO<sub>2</sub>
 
materials have been optimized for a broad range of applications by other metal
 
oxide additives and modification in the manufacturing processes that result in
 
different metallurgical, physical and electrical properties ''(Table 2.29)''.
 
 
Manufacturing of Ag/SnO<sub>2</sub> by ''internal oxidation'' is possible in principle, but
 
during heat treatment of alloys containing > 5 wt% of tin in oxygen, dense oxide
 
layers formed on the surface of the material prohibit the further diffusion of
 
oxygen into the bulk of the material. By adding Indium or Bismuth to the alloy the
 
internal oxidation is possible and results in materials that typically are rather hard
 
and brittle and may show somewhat elevated contact resistance and is limited
 
to applications in relays. To make a ductile material with fine oxide dispersion
 
(SISTADOX TOS F) ''(Fig. 2.114)'' it is necessary to use special process variations
 
in oxidation and extrusion which lead to materials with improved properties in
 
relays. Adding a brazable fine silver layer to such materials results in a semifinished
 
material suitable for the manufacture as smaller weld profiles
 
(SISTADOX WTOS F) ''(Fig. 2.116)''. Because of their resistance to material
 
transfer and low arc erosion these materials find for example a broader
 
application in automotive relays ''(Table 2.31)''.
 
 
''Powder metallurgy'' plays a significant role in the manufacturing of Ag/SnO<sub>2</sub>
 
contact materials. Besides SnO<sub>2</sub> a smaller amount (<1 wt%) of one or more
 
other metal oxides such as WO<sub>3</sub>, MoO<sub>3</sub>, CuO and/or Bi<sub>2</sub>O<sub>3</sub> are added. These
 
additives improve the wettability of the oxide particles and increase the viscosity
 
of the Ag melt. They also provide additional benefits to the mechanical and
 
arcing contact properties of materials in this group ''(Table 2.26)''.
 
 
In the manufacture the initial powder mixes different processes are applied
 
which provide specific advantages of the resulting materials in respect to their
 
contact properties ''(Figs. 2.87 – 2.119)''. Some of them are described here as
 
follows:
 
:'''a) Powder blending from single component powders''' <br> In this common process all components including additives that are part of the powder mix are blended as single powders. The blending is usually performed in the dry stage in blenders of different design.
 
 
:'''b) Powder blending on the basis of doped powders''' <br> For incorporation of additive oxides in the SnO<sub>2</sub> powder the reactive spray process (RSV) has shown advantages. This process starts with a waterbased solution of the tin and other metal compounds. This solution is nebulized under high pressure and temperature in a reactor chamber. Through the rapid evaporation of the water each small droplet is converted into a salt crystal and from there by oxidation into a tin oxide particle in which the additive metals are distributed evenly as oxides. The so created doped AgSnO2 powder is then mechanically mixed with silver powder.
 
 
:'''c) Powder blending based on coated oxide powders''' <br> In this process tin oxide powder is blended with lower meting additive oxides such as for example Ag<sub>2</sub> MoO<sub>4</sub> and then heat treated. The SnO<sub>2</sub> particles are coated in this step with a thin layer of the additive oxide.
 
 
:'''d) Powder blending based on internally oxidized alloy powders''' <br> A combination of powder metallurgy and internal oxidation this process starts with atomized Ag alloy powder which is subsequently oxidized in pure oxygen. During this process the Sn and other metal components are transformed to metal oxide and precipitated inside the silver matrix of each powder particle.
 
 
:'''e) Powder blending based on chemically precipitated compound powders''' <br> A silver salt solution is added to a suspension of for example SnO<sub>2</sub> together with a precipitation agent. In a chemical reaction silver and silver oxide respectively are precipitated around the additive metal oxide particles who act as crystallization sites. Further chemical treatment then reduces the silver oxide with the resulting precipitated powder being a mix of Ag and SnO<sub>2</sub>.
 
 
Further processing of these differently produced powders follows the
 
conventional processes of pressing, sintering and hot extrusion to wires and
 
strips. From these contact parts such as contact rivets and tips are
 
manufactured. To obtain a brazable backing the same processes as used for
 
Ag/CdO are applied. As for Ag/CdO, larger contact tips can also be
 
manufactured more economically using the press-sinter-repress (PSR) process
 
''(Table 2.27).''
 
 
Fig. 2.87:
 
Strain hardening of
 
Ag/SnO<sub>2</sub> 92/8 PE by cold working
 
 
Fig. 2.88:
 
Softening of
 
Ag/SnO<sub>2</sub> 92/8 PE after annealing
 
for 1 hr after 40% cold working
 
 
Table 2.26: Physical and Mechanical Properties as well as Manufacturing Processes and
 
Forms of Supply of Extruded Silver-Tin Oxide (SISTADOX) Contact Materials
 
 
Fig. 2.89:
 
Strain hardening of
 
Ag/SnO<sub>2</sub> 88/12 PE by cold working
 
 
Fig. 2.90:
 
Softening of Ag/SnO<sub>2</sub> 88/12 PE
 
after annealing for
 
1 hr after 40% cold working
 
 
Fig. 2.91:
 
Strain hardening of oxidized
 
Ag/SnO<sub>2</sub> 88/12 PW4 by cold working
 
 
Fig. 2.92:
 
Softening of Ag/SnO<sub>2</sub> 88/12 PW4 after
 
annealing for 1 hr
 
after 30% cold working
 
 
Fig. 2.93:
 
Strain hardening of
 
Ag/SnO<sub>2</sub> 98/2 PX
 
by cold working
 
 
Fig. 2.94:
 
Softening of
 
Ag/SnO<sub>2</sub> 98/2 PX
 
after annealing
 
for 1 hr after 80%
 
cold working
 
 
Fig 2.95:
 
Strain hardening
 
of Ag/SnO<sub>2</sub> 92/8 PX
 
by cold working
 
 
Fig. 2.96:
 
Softening of
 
Ag/SnO<sub>2</sub> 92/8 PX
 
after annealing for 1 hr
 
after 40% cold working
 
 
Fig. 2.97:
 
Strain hardening of internally
 
oxidized
 
Ag/SnO<sub>2</sub> 88/12 TOS F
 
by cold working
 
 
Fig. 2.98:
 
Softening of
 
Ag/SnO<sub>2</sub> 88/12 TOS F after
 
annealing for 1 hr after 30%
 
cold working
 
 
Fig. 2.99:
 
Strain hardening of
 
internally oxidized
 
Ag/SnO<sub>2</sub> 88/12P
 
by cold working
 
 
Fig. 2.100:
 
Softening of
 
Ag/SnO<sub>2</sub> 88/12P
 
after annealing for 1 hr after
 
40% cold working
 
 
Fig. 2.101:
 
Strain hardening of
 
Ag/SnO<sub>2</sub> 88/12 WPC
 
by cold working
 
 
Fig. 2.102:
 
Softening of Ag/SnO<sub>2</sub> 88/12 WPC after annealing
 
for 1 hr after different degrees of cold working
 
 
Fig. 2.103:
 
Strain hardening of
 
Ag/SnO<sub>2</sub> 86/14 WPC
 
by cold working
 
 
Fig. 2.104:
 
Softening of Ag/SnO<sub>2</sub> 86/14 WPC after annealing
 
for 1 hr after different degrees of cold working
 
 
Fig. 2.105:
 
Strain hardening of
 
Ag/SnO<sub>2</sub> 88/12 WPD
 
by cold working
 
 
Fig. 2.106:
 
Softening of Ag/SnO<sub>2</sub> 88/12 WPD after
 
annealing for 1 hr after different degrees
 
of cold working
 
 
Fig. 2.108:
 
Softening of Ag/SnO<sub>2</sub> 88/12 WPX after
 
annealing for 1 hr after different degrees
 
of cold working
 
 
Fig. 2.107:
 
Strain hardening of
 
Ag/SnO<sub>2</sub> 88/12 WPX
 
by cold working
 
 
Fig. 2.109: Micro structure of Ag/SnO<sub>2</sub> 92/8 PE: a) perpendicular to extrusion direction
 
b) parallel to extrusion direction
 
 
Fig. 2.110: Micro structure of Ag/SnO<sub>2</sub> 88/12 PE: a) perpendicular to extrusion direction
 
b) parallel to extrusion direction
 
 
Fig. 2.111: Micro structure of Ag/SnO<sub>2</sub> 88/12 PW: a) perpendicular to extrusion direction
 
b) parallel to extrusion direction
 
 
Fig. 2.112: Micro structure of Ag/SnO<sub>2</sub> 98/2 PX: a) perpendicular to extrusion direction
 
b) parallel to extrusion direction
 
 
Fig. 2.113: Micro structure of Ag/SnO<sub>2</sub> 92/8 PX: a) perpendicular to extrusion direction
 
b) parallel to extrusion direction
 
 
Fig. 2.114: Micro structure of Ag/SnO<sub>2</sub> 88/12 TOS F: a) perpendicular to extrusion direction
 
b) parallel to extrusion direction
 
 
Fig. 2.115: Micro structure of Ag/SnO<sub>2</sub> 86/14 WPC: a) perpendicular to extrusion direction
 
b) parallel to extrusion direction, 1) AgSnO<sub>2</sub> contact layer, 2) Ag backing layer
 
 
Fig. 2.116: Micro structure of Ag/SnO<sub>2</sub> 92/8 WTOS F: a) perpendicular to extrusion direction
 
b) parallel to extrusion direction,1) AgSnO<sub>2</sub> contact layer, 2) Ag backing layer
 
 
Fig. 2.117: Micro structure of
 
Ag/SnO<sub>2</sub> 88/12 WPD: parallel to extrusion direction
 
1) AgSnO<sub>2</sub> contact layer, 2) Ag backing layer
 
 
Fig. 2.118: Micro structure of
 
Ag/SnO<sub>2</sub> 88/12 WPX:parallel to extrusion direction
 
1) AgSnO<sub>2</sub> contact layer, 2) Ag backing layer
 
 
Fig. 2.119: Micro structure of Ag/SnO<sub>2</sub> 86/14 WPX: a) perpendicular to extrusion direction
 
b) parallel to extrusion direction, 1) AgSnO<sub>2</sub> contact layer, 2) Ag backing layer
 
 
Table 2.27: Physical Properties of Powder Metallurgical Silver-Metal Oxide Materials
 
with Fine Silver Backing Produced by the Press-Sinter-Repress Process
 
 
*'''Silver–zinc oxide (DODURIT ZnO) materials'''
 
Silver zinc oxide (DODURIT ZnO) contact materials with mostly 6 - 10 wt% oxide
 
content including other small metal oxides are produced exclusively by powder
 
metallurgy ''(Figs. 2.120 – 2.125)'' ''(Table 2.28)''. Adding Ag<sub>2</sub>WO<sub>4</sub> in the process b)
 
as described in the preceding chapter on Ag/SnO<sub>2</sub> has proven most effective
 
for applications in AC relays, wiring devices, and appliance controls. Just like
 
with the other Ag metal oxide materials, semi-finished materials in strip and wire
 
form are used to manufacture contact tips and rivets.
 
Because of their high resistance against welding and arc erosion Ag/ZnO
 
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
+
Main Article: [[Silver Based Materials| Silver Based Materials]]
 
 
====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
 
 
 
Pre-Production of Contact Materials
 
(Bild)
 
  
 
==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
 
(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==
 
==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===
+
The trade name VAKURIT is assigned to a family of low gas content contact materials developed for the use in vacuum switching devices [[Special_Contact_Materials_(VAKURIT)_for_Vacuum_Switches|Table 1]]
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
+
Main Article: [[Special Contact Materials (VAKURIT) for Vacuum Switches| Special Contact Materials (VAKURIT) for Vacuum Switches]]
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 1,115: Line 329:
 
Manufacturing Equipment for Semi-Finished Materials
 
Manufacturing Equipment for Semi-Finished Materials
 
(Bild)
 
(Bild)
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[[de:Kontaktwerkstoffe_für_die_Elektrotechnik]]

Revision as of 22:18, 17 September 2014

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

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

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-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 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 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

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 materials 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 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 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

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 1

Main Article: Special Contact Materials (VAKURIT) for Vacuum Switches

References

Vinaricky, E.(Hrsg.): Elektrische Kontakte, Werkstoffe und Anwendungen. Springer-Verlag, Berlin, Heidelberg etc. 2002

Lindmayer, M.: Schaltgeräte-Grundlagen, Aufbau, Wirkungsweise. Springer-Verlag, Berlin, Heidelberg, New York, Tokio, 1987

Rau, G.: Metallische Verbundwerkstoffe. Werkstofftechnische Verlagsgesellschaft, Karlsruhe 1977

Schreiner, H.: Pulvermetallurgie elektrischer Kontakte. Springer-Verlag Berlin, Göttingen, Heidelberg, 1964

Hansen. M.; Anderko, K.: Constitution of Binary Alloys. New York: Mc Graw-Hill, 1958

Shunk, F.A.: Constitution of Binary Alloy. 2 Suppl. New York; Mc Graw-Hill, 1969

Edelmetall-Taschenbuch. ( Herausgeber Degussa AG, Frankfurt a. M.), Heidelberg, Hüthig-Verlag, 1995

Rau, G.: Elektrische Kontakte-Werkstoffe und Technologie. Eigenverlag G. Rau GmbH & Co., Pforzheim, 1984

Heraeus, W. C.: Werkstoffdaten. Eigenverlag W.C. Heraeus, Hanau, 1978

Linde, J.O.: Elektrische Widerstandseigenschaften der verdünnten Legierungen des Kupfers, Silbers und Goldes. Lund: Hakan Ohlsson, 1938

Engineers Relay Handbook, RSIA, 2006

Großmann, H. Saeger, K. E.; Vinaricky, E.: Gold and Gold Alloys in Electrical Engineering. in: Gold, Progress in Chemistry, Biochemistry and Technology. John Wiley & Sons, Chichester etc, (1999) 199-236

Gehlert, B.: Edelmetall-Legierungen für elektrische Kontakte. Metall 61 (2007) H. 6, 374-379

Aldinger, F.; Schnabl, R.: Edelmetallarme Kontakte für kleine Ströme. Metall 37 (1983) 23-29

Bischoff, A.; Aldinger, F.: Einfluss geringer Zusätze auf die mechanischen Eigenschaften von Au-Ag-Pd-Legierungen. Metall 36 (1982) 752-765

Wise, E.M.: Palladium, Recovery, Properties and Uses. New York, London: Academic Press 1968

Savitskii, E.M.; Polyakova, V.P.; Tylina, M.A.: Palladium Alloys, Primary Sources. New York: Publishers 1969

Gehlert, B.: Lebensdaueruntersuchungen von Edelmetall Kontaktwerkstoff- Kombinationen für Schleifringübertrager. VDE-Fachbericht 61, (2005) 95-100

Holzapfel,C.: Verschweiß und elektrische Eigenschaften von Schleifringübertragern. VDE-Fachbericht 67 (2011) 111-120

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Manufacturing Equipment for Semi-Finished Materials (Bild)