Difference between revisions of "Contact Materials for Electrical Engineering"

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===2.1 Introduction===
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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.
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
+
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
+
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
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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 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)
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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
+
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.
the accelerated diffusion and also results in near-theoretical densities of the
 
  
Fig. 2.1: Powder-metallurgical manufacturing of composite materials (schematic)
+
''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
T = Melting point of the lower melting component
 
 
 
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.
 
 
 
===2.2 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.
 
 
 
For most electrical contact applications gold alloys are used. Depending on the
 
alloying metal the melting is performed either under in a reducing atmosphere or
 
in a vacuum. The choice of alloying metals depends on the intended use of the
 
resulting contact material. The binary Au alloys with typically <10 wt% of other
 
precious metals such as Pt, Pd, or Ag or non-precious metals like Ni, Co, and
 
Cu are the more commonly used ones (Table 2.2). On one hand these alloy
 
additions improve the mechanical strength and electrical switching properties
 
but on the other hand reduce the electrical conductivity and chemical corrosion
 
resistance (Fig. 2.2) to varying degrees.
 
 
 
Under the aspect of reducing the gold content ternary alloys with a gold content
 
of approximately 70 wt% and additions of Ag and Cu or Ag and Ni resp., for
 
example AuAg25Cu5 or AuAg20Cu10 are used which exhibit for many
 
applications good mechanical stability while at the same time have sufficient
 
resistance against the formation of corrosion layers (Table 2.3). Other ternary
 
alloys based on the AuAg system are AuAg26Ni3 and AuAg25Pt6. These alloys
 
are mechanically similar to the AuAgCu alloys but have significantly higher
 
oxidation resistance at elevated temperatures (Table 2.4).
 
 
 
Caused by higher gold prices over the past years the development of alloys with
 
further reduced gold content had a high priority. The starting point has been the
 
AuPd system which has continuous solubility of the two components. Besides
 
the binary alloy of AuPd40 and the ternary one AuPd35Ag9 other multiple
 
component alloys were developed. These alloys typically have < 50 wt% Au and
 
often can be solution hardened in order to obtain even higher hardness and
 
tensile strength. They are mostly used in sliding contact applications.
 
 
 
Gold alloys are used in the form of welded wire or profile (also called weldtapes),
 
segments, contact rivets, and stampings produced from clad strip
 
materials. The selection of the bonding process is based on the cost for the
 
joining process, and most importantly on the economical aspect of using the
 
least possible amount of the expensive precious metal component.
 
 
 
Besides being used as switching contacts in relays and pushbuttons, gold
 
alloys are also applied in the design of connectors as well as sliding contacts for
 
potentiometers, sensors, slip rings, and brushes in miniature DC motors
 
(Table 2.5).
 
 
 
Table 2.3: Mechanical Properties of Gold and Gold-Alloys
 
 
 
Table 2.1: Commonly Used Grades of Gold
 
 
 
Table 2.2: Physical Properties of Gold and Gold-Alloys
 
 
 
Fig. 2.2:
 
Influence of 1-10 atomic% of different
 
alloying metals on the electrical resistivity of gold
 
(according to J. O. Linde)
 
 
 
Fig. 2.3:
 
Phase diagram
 
of goldplatinum
 
 
 
Fig. 2.4:
 
Phase diagram
 
of gold-silver
 
 
 
Fig. 2.5:
 
Phase diagram
 
of gold-copper
 
 
 
Fig. 2.6: Phase diagram of gold-nickel
 
 
 
Fig. 2.7: Phase diagram of gold-cobalt
 
 
 
Fig. 2.8:
 
Strain hardening
 
of Au by cold working
 
 
 
Fig. 2.9:
 
Softening of Au after annealing
 
for 0.5 hrs after 80%
 
cold working
 
 
 
Fig. 2.10:
 
Strain hardening of
 
AuPt10 by cold working
 
 
 
Fig. 2.11:
 
Strain hardening
 
of AuAg20 by cold working
 
 
 
Fig. 2.12:
 
Strain hardening of
 
AuAg30 by cold working
 
 
 
Fig. 2.13:
 
Strain hardening of AuNi5
 
by cold working
 
 
 
Fig. 2.14:
 
Softening
 
of AuNi5 after annealing
 
for 0.5 hrs after 80%
 
cold working
 
 
 
Fig. 2.15:
 
Strain hardening
 
of AuCo5 by cold working
 
 
 
Fig. 2.16:
 
Precipitation hardening of
 
AuCo5 at 400°C hardening
 
temperature
 
 
 
Fig. 2.17:
 
Strain hardening
 
of AuAg25Pt6 by cold working
 
 
 
Fig. 2.18:
 
Strain hardening
 
of AuAg26Ni3 by cold working
 
 
 
Fig. 2.19:
 
Softening
 
of AuAg26Ni3 after
 
annealing for 0.5 hrs
 
after 80% cold
 
working
 
 
 
Fig. 2.20:
 
Strain hardening of
 
AuAg25Cu5
 
by cold working
 
 
 
Fig. 2.21:
 
Strain hardening of
 
AuAg20Cu10
 
by cold working
 
 
 
Fig. 2.22:
 
Softening
 
of AuAg20Cu10 after
 
annealing for 0.5 hrs
 
after 80% cold working
 
 
 
Fig. 2.23:
 
Strain hardening of
 
AuCu14Pt9Ag4
 
by cold working
 
 
 
Fig. 2.24:
 
Precipitation
 
hardening of
 
AuCu14Pt9Ag4
 
at different
 
hardening
 
temperatures
 
after 50%
 
cold working
 
 
 
Table 2.4: Contact and Switching Properties of Gold and Gold Alloys
 
 
 
Table 2.5: Application Examples and Forms of Gold and Gold Alloys
 
 
 
===2.3 Platinum Metal Based Materials===
 
 
 
The platinum group metals include the elements Pt, Pd, Rh, Ru, Ir, and Os (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.
 
 
 
Rhodium is not used as a solid contact material but is applied for example as a
 
electroplated layer in sliding contact systems. Ruthenium is mostly used as an alloying
 
component in the material PdRu15. The metals osmium and iridium have no practical
 
applications in electrical contacts.
 
 
 
Since Pd was for the longest time rather stable in price it was looked at as a substitute
 
for the more expensive gold. This was followed by a steep increase in the Pd price
 
which caused a significant reduction in its use in electrical contacts. Today (2011) the
 
Pd price again is lower than that of gold.
 
 
 
Alloys of Pt with Ru, Ir, Ni, and W were widely used in electromechanical components
 
in the telecommunication industry and in heavy duty automotive breaker points (Table
 
2.7). Today these components have been replaced in many applications by solid
 
state technology and the usage of these materials is greatly reduced. Pd alloys
 
however have a more significant importance. PdCu15 is widely used for example in
 
automotive flasher relays. Because of their resistance to sulfide formation PdAg alloys
 
are applied in various relay designs. The ability to thermally precipitation harden some
 
multi component alloys based on PdAgAuPt they find special usage in wear resistant
 
sliding contact applications. Pd44Ag38Cu15PtAuZn is a standard alloy in this group.
 
 
 
Platinum and palladium alloys are mainly used similar to the gold based materials in
 
the form of welded wire and profile segments but rarely as contact rivets. Because of
 
the high precious metal prices joining technologies are used that allow the most
 
economic application of the contact alloy in the area where functionally needed.
 
Because of their resistance to material transfer they are used for DC applications and
 
due to their higher arc erosion resistance they are applied for medium electrical loads
 
up to about 30W in relays and switches (Table 2.10). Multi-component alloys based
 
on Pd with higher hardness and wear resistance are mainly used as spring arms in
 
sliding contact systems and DC miniature motors.
 
 
 
Table 2.6: Properties, Production Processes, and Application Forms for Platinum Metals
 
 
 
Table 2.7: Physical Properties of the Platinum Metals and their Alloys
 
 
 
Table 2.8: Mechanical Properties of the Platinum Metals and their Alloys
 
 
 
Fig. 2.25:
 
Influence of 1-
 
20 atom% of
 
different additive
 
metals on the
 
electrical
 
resistivity p of
 
platinum
 
(Degussa)
 
 
 
Fig. 2.26:
 
Influence of 1-22 atom% of different
 
additive metals on the electrical
 
resistivity
 
p of palladium
 
 
 
Fig. 2.27:
 
Phase diagram of
 
platinum-iridium
 
 
 
Fig. 2.28:
 
Phase diagram of
 
platinum-nickel
 
 
 
Fig. 2.29:
 
Phase diagram
 
of platinum-tungsten
 
 
 
Fig. 2.30:
 
Phase diagram of
 
palladium-copper
 
 
 
Fig. 2.31:
 
Strain
 
hardening
 
of Pt by cold
 
working
 
 
 
Fig. 2.32:
 
Softening of Pt after
 
annealing for 0.5 hrs
 
after 80%
 
cold working
 
 
 
Fig. 2.33:
 
Strain hardening of PtIr5
 
by cold working
 
 
 
Fig. 2.34:
 
Softening of PtIr5 after annealing for 1 hr
 
after different degrees of cold working
 
 
 
Fig. 2.35:
 
Strain hardening
 
of PtNi8 by cold working
 
 
 
Fig. 2.36:
 
Softening of PtNi8 after
 
annealing
 
for 1 hr after
 
80% cold working
 
 
 
Fig. 2.37:
 
Strain hardening
 
of PtW5 by cold working
 
 
 
Fig. 2.38:
 
Softening
 
of PtW5 after
 
annealing for 1hr
 
after 80% cold
 
working
 
 
 
Fig. 2.39:
 
Strain hardening
 
of Pd 99.99 by cold working
 
 
 
Fig. 2.40:
 
Strain hardening
 
of PdCu15 by cold working
 
 
 
Fig. 2.41:
 
Softening
 
of PdCu15 after
 
annealing
 
for 0.5 hrs
 
 
 
Fig. 2.42:
 
Strain hardening
 
of PdCu40 by cold working
 
 
 
Fig. 2.43:
 
Softening
 
of PdCu40
 
after annealing
 
for 0.5 hrs after 80%
 
cold working
 
 
 
Fig. 2.44:
 
Electrical resistivity p
 
of PdCu alloys with and without an
 
annealing step for forming an ordered
 
phase
 
 
 
Table 2.9: Contact and Switching Properties
 
of the Platinum Metals and their Alloys
 
 
 
Table 2.10: Application Examples and Form
 
of Supply for Platinum Metals and their Alloys
 
 
 
===2.4 Silver Based Materials===
 
 
 
===2.4.1 Pure Silver===
 
Pure silver (also called fine silver) exhibits the highest electrical and thermal
 
conductivity of all metals. It is also resistant against oxidation. Major disadvantages
 
are its low mechanical wear resistance, the low softening temperature,
 
and especially its strong affinity to sulfur and sulfur compounds. In the presence
 
of sulfur and sulfur containing compounds brownish to black silver sulfide layer
 
are formed on its surface. These can cause increased contact resistance or
 
even total failure of a switching device if they are not mechanically, electrically,
 
or thermally destroyed. Other weaknesses of silver contacts are the tendency to
 
weld under the influence of over-currents and the low resistance against
 
material transfer when switching DC loads. In humid environments and under
 
the influence of an electrical field silver can creep (silver migration) and cause
 
electrical shorting between adjacent current paths.
 
 
 
Table 2.11 shows the typically available quality grades of silver. In certain
 
economic areas, i.e. China, there are additional grades with varying amounts of
 
impurities available on the market. In powder form silver is used for a wide
 
variety of silver based composite contact materials. Different manufacturing
 
processes result in different grades of Ag powder as shown in Table 2.12.
 
additional properties of silver powders and their usage are described
 
in chapter 8.1.
 
Semi-finished silver materials can easily be warm or cold formed and can be
 
clad to the usual base materials. For attachment of silver to contact carrier
 
materials welding of wire or profile cut-offs and brazing are most widely applied.
 
Besides these mechanical processes such as wire insertion (wire staking) and
 
the riveting (staking) of solid or composite contact rivets are used in the
 
manufacture of contact components.
 
 
 
Contacts made from fine silver are applied in various electrical switching
 
devices such as relays, pushbuttons, appliance and control switches for
 
currents < 2 A (Table 2.16). Electroplated silver coatings are widely used to
 
reduce the contact resistance and improve the brazing behavior of other contact
 
materials and components.
 
 
 
Table 2.11: Overview of the Most Widely Used Silver Grades
 
 
 
Table 2.12: Quality Criteria of Differently Manufactured Silver Powders
 
 
 
Fig. 2.45:
 
Strain hardening
 
of Ag 99.95 by cold working
 
 
 
Fig. 2.46:
 
Softening of Ag 99.95
 
after annealing for 1 hr after different
 
degrees of strain hardening
 
 
 
===2.4.2 Silver Alloys===
 
To improve the physical and contact properties of fine silver melt-metallurgical
 
produced silver alloys are used (Table 2.13). By adding metal components the
 
mechanical properties such as hardness and tensile strength as well as typical
 
contact properties such as erosion resistance, and resistance against material
 
transfer in DC circuits are increased (Table 2.14). On the other hand however,
 
other properties such as electrical conductivity and chemical corrosion
 
resistance can be negatively impacted by alloying (Figs. 2.47 and 2.48).
 
 
 
===2.4.2.1 Fine-Grain Silver===
 
Fine-Grain Silver (ARGODUR-Spezial) is defined as a silver alloy with an addition
 
of 0.15 wt% of Nickel. Silver and nickel are not soluble in each other in solid
 
form. In liquid silver only a small amount of nickel is soluble as the phase diagram
 
(Fig. 2.51) illustrates. During solidification of the melt this nickel addition gets
 
finely dispersed in the silver matrix and eliminates the pronounce coarse grain
 
growth after prolonged influence of elevated temperatures (Figs. 2.49 and 2.50.
 
 
 
Fine-grain silver has almost the same chemical corrosion resistance as fine
 
silver. Compared to pure silver it exhibits a slightly increased hardness and
 
tensile strength (Table 2.14). The electrical conductivity is just slightly decreased
 
by this low nickel addition. Because of its significantly improved contact
 
properties fine grain silver has replaced pure silver in many applications.
 
 
 
===2.4.2.2 Hard-Silver Alloys===
 
Using copper as an alloying component increases the mechanical stability of
 
silver significantly. The most important among the binary AgCu alloys is that of
 
AgCu3, known in europe also under the name of hard-silver. This material still
 
has a chemical corrosion resistance close to that of fine silver. In comparison to
 
pure silver and fine-grain silver AgCu3 exhibits increased mechanical strength
 
as well as higher arc erosion resistance and mechanical wear resistance
 
(Table 2.14).
 
 
 
Increasing the Cu content further also increases the mechanical strength of
 
AgCu alloys and improves arc erosion resistance and resistance against
 
material transfer while at the same time however the tendency to oxide formation
 
becomes detrimental. This causes during switching under arcing conditions an
 
increase in contact resistance with rising numbers of operation. In special
 
applications where highest mechanical strength is recommended and a reduced
 
chemical resistance can be tolerated, the eutectic AgCu alloy with 28 wt% of
 
copper (Fig. 2.52) is used. AgCu10 also known as coin silver has been
 
replaced in many applications by composite silver-based materials while sterling
 
silver (AgCu7.5) has never extended its important usage from decorative table
 
wear and jewelry to industrial applications in electrical contacts.
 
 
 
Besides these binary alloys, ternary AgCuNi alloys are used in electrical contact
 
applications. From this group the material ARGODUR 27, an alloy of 98 wt% Ag
 
with a 2 wt% Cu and nickel addition has found practical importance close to that
 
of AgCu3. This material is characterized by high resistance to oxidation and low
 
tendency to re-crystallization during exposure to high temperatures. Besides
 
high mechanical stability this AgCuNi alloy also exhibits a strong resistance
 
against arc erosion. Because of its high resistance against material transfer the
 
alloy AgCu24.5Ni0.5 has been used in the automotive industry for an extended
 
time in the North American market. Caused by miniaturization and the related
 
reduction in available contact forces in relays and switches this material has
 
been replaced widely because of its tendency to oxide formation.
 
 
 
The attachment methods used for the hard silver materials are mostly close to
 
those applied for fine silver and fine grain silver.
 
 
 
Hard-silver alloys are widely used for switching applications in the information
 
and energy technology for currents up to 10 A, in special cases also for higher
 
current ranges (Table 2.16).
 
 
 
Dispersion hardened alloys of silver with 0.5 wt% MgO and NiO (ARGODUR 32)
 
are produced by internal oxidation. While the melt-metallurgical alloy is easy to
 
cold-work and form the material becomes very hard and brittle after dispersion
 
hardening. Compared to fine silver and hard-silver this material has a greatly
 
improved temperature stability and can be exposed to brazing temperatures up
 
to 800°C without decreasing its hardness and tensile strength.
 
Because of these mechanical properties and its high electrical conductivity
 
 
 
Table 2.13: Physical Properties of Silver and Silver Alloys
 
 
 
ARGODUR 32 is mainly used in the form of contact springs that are exposed to
 
high thermal and mechanical stresses in relays, and contactors for aeronautic
 
applications.
 
 
 
Fig. 2.47:
 
Influence of 1-10 atom% of different
 
alloying metals on the electrical resistivity of
 
silver
 
 
 
Fig. 2.48:
 
Electrical resistivity p
 
of AgCu alloys with 0-20 weight% Cu
 
in the soft annealed
 
and tempered stage
 
a) Annealed and quenched
 
b) Tempered at 280°C
 
 
 
Fig. 2.49: Coarse grain micro structure
 
of Ag 99.97 after 80% cold working
 
and 1 hr annealing at 600°C
 
 
 
Fig. 2.50: Fine grain microstructure
 
of AgNi0.15 after 80% cold working
 
and 1 hr annealing at 600°C
 
 
 
Fig. 2.51:
 
Phase diagram
 
of silver-nickel
 
 
 
Fig. 2.52:
 
Phase diagram
 
of silver-copper
 
 
 
Fig. 2.53:
 
Phase diagram of
 
silver-cadmium
 
 
 
Table 2.14: Mechanical Properties of Silver and Silver Alloys
 
 
 
Fig. 2.54:
 
Strain hardening
 
of AgCu3
 
by cold working
 
  
Fig. 2.55:
+
==Gold Based Materials==
Softening of AgCu3
 
after annealing for 1 hr
 
after 80% cold working
 
  
Fig. 2.56:
+
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.
Strain hardening of AgCu5 by cold
 
working
 
  
Fig. 2.57:
+
Main Article: [[Gold Based Materials| Gold Based Materials]]
Softening of AgCu5 after
 
annealing for 1 hr after 80% cold
 
working
 
  
Fig. 2.58:
+
==Platinum Metal Based Materials==
Strain hardening of AgCu 10
 
by cold working
 
  
Fig. 2.59:
+
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.
Softening of AgCu10 after
 
annealing for 1 hr after 80% cold
 
working
 
  
Fig. 2.60:
+
Main Article: [[Platinum Metal Based Materials| Platinum Metal Based Materials]]
Strain hardening of AgCu28 by
 
cold working
 
  
Fig. 2.61:
+
==Silver Based Materials==
Softening of AgCu28
 
after annealing for 1 hr after
 
80% cold working
 
  
Fig. 2.62:
+
Main Article: [[Silver Based Materials| Silver Based Materials]]
Strain hardening of AgNi0.15
 
by cold working
 
  
Fig. 2.63:
+
==Tungsten and Molybdenum Based Materials==
Softening of AgNi0.15
 
after annealing for 1 hr after 80%
 
cold working
 
  
Fig. 2.64:
+
Main Article: [[Tungsten and Molybdenum Based Materials| Tungsten and Molybdenum Based Materials]]
Strain hardening of
 
ARGODUR 27
 
by cold working
 
  
Fig. 2.65:
+
==Contact Materials for Vacuum Switches==
Softening
 
of ARGODUR 27 after annealing
 
for 1 hr after 80% cold working
 
  
Table 2.15: Contact and Switching Properties of Silver and Silver Alloys
+
The low gas content contact materials are developed for the use in vacuum switching devices.  
  
Table 2.16: Application Examples and Forms of Supply for Silver and Silver Alloys
+
Main Article: [[Contact Materials for Vacuum Switches| Contact Materials for Vacuum Switches]]
  
===2.4.2.3 Silver-Palladium Alloys===
+
==References==
The addition of 30 wt% Pd increases the mechanical properties as well as the
 
resistance of silver against the influence of sulfur and sulfur containing
 
compounds significantly (Tables 2.17 and 2.18).
 
Alloys with 40-60 wt% Pd have an even higher resistance against silver sulfide
 
formation. At these percentage ranges however the catalytic properties of
 
palladium can influence the contact resistance behavior negatively. The
 
formability also decreases with increasing Pd contents.
 
  
AgPd alloys are hard, arc erosion resistant, and have a lower tendency towards
+
Vinaricky, E.(Hrsg.): Elektrische Kontakte, Werkstoffe und Anwendungen.
material transfer under DC loads (Table 2.19). On the other hand the electrical
+
Springer-Verlag, Berlin, Heidelberg etc. 2002
conductivity is decreased at higher Pd contents. The ternary alloy AgPd30Cu5
 
has an even higher hardness which makes it suitable for use in sliding contact
 
systems.
 
  
AgPd alloys are mostly used in relays for the switching of medium to higher loads
+
Lindmayer, M.: Schaltgeräte-Grundlagen, Aufbau, Wirkungsweise.
(>60V, >2A) as shown in Table 2.20. Because of the high palladium price these
+
Springer-Verlag, Berlin, Heidelberg, New York, Tokio, 1987
formerly solid contacts have been widely replaced by multi-layer designs such
 
as AgNi0.15 or AgNi10 with a thin Au surface layer. A broader field of application
 
for AgPd alloys remains in the wear resistant sliding contact systems.
 
  
Fig. 2.66: Phase diagram of silver-palladium
+
Rau, G.: Metallische Verbundwerkstoffe. Werkstofftechnische
 +
Verlagsgesellschaft, Karlsruhe 1977
  
Fig. 2.67:
+
Schreiner, H.: Pulvermetallurgie elektrischer Kontakte. Springer-Verlag
Strain hardening
+
Berlin, Göttingen, Heidelberg, 1964
of AgPd30 by cold working
 
  
Fig. 2.68:
+
Hansen. M.; Anderko, K.: Constitution of Binary Alloys. New York:
Strain hardening
+
Mc Graw-Hill, 1958
of AgPd50 by cold working
 
  
Fig. 2.69:
+
Shunk, F.A.: Constitution of Binary Alloy. 2 Suppl. New York; Mc Graw-Hill, 1969
Strain hardening
 
of AgPd30Cu5
 
by cold working
 
  
Fig. 2.70:
+
Edelmetall-Taschenbuch. ( Herausgeber Degussa AG, Frankfurt a. M.),
Softening of AgPd30, AgPd50,
+
Heidelberg, Hüthig-Verlag, 1995
and AgPd30Cu5 after annealing of 1 hr
 
after 80% cold working
 
  
Table 2.17: Physical Properties of Silver-Palladium Alloys
+
Rau, G.: Elektrische Kontakte-Werkstoffe und Technologie. Eigenverlag G. Rau
 +
GmbH & Co., Pforzheim, 1984
  
Table 2.18: Mechanical Properties of Silver-Palladium Alloys
+
Heraeus, W. C.: Werkstoffdaten. Eigenverlag W.C. Heraeus, Hanau, 1978
  
Table 2.19: Contact and Switching Properties of Silver-Palladium Alloys
+
Linde, J.O.: Elektrische Widerstandseigenschaften der verdünnten Legierungen
 +
des Kupfers, Silbers und Goldes. Lund: Hakan Ohlsson, 1938
  
Table 2.20: Application Examples and Forms of Suppl for Silver-Palladium Alloys
+
Engineers Relay Handbook, RSIA, 2006
  
===2.4.3 Silver Composite Materials===
+
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
  
===2.4.3.1 Silver-Nickel (SINIDUR) Materials===
+
Gehlert, B.: Edelmetall-Legierungen für elektrische Kontakte.
Since silver and nickel are not soluble in each other in solid form and in the liquid
+
Metall 61 (2007) H. 6, 374-379
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
+
Aldinger, F.; Schnabl, R.: Edelmetallarme Kontakte für kleine Ströme.
of these materials (Tables 2.21 and 2.22). The typical application of Ag/Ni
+
Metall 37 (1983) 23-29
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
+
Bischoff, A.; Aldinger, F.: Einfluss geringer Zusätze auf die mechanischen
wt% Ni. The most widely used materials SINIDUR 10 and SINIDUR 20- and also
+
Eigenschaften von Au-Ag-Pd-Legierungen. Metall 36 (1982) 752-765
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
+
Wise, E.M.: Palladium, Recovery, Properties and Uses. New York, London:
relays, wiring devices, appliance switches, thermostatic controls, auxiliary
+
Academic Press 1968
switches, and small contactors with nominal currents >20A (Table 2.24).
 
  
Table 2.21: Physical Properties of Silver-Nickel (SINIDUR) Materials
+
Savitskii, E.M.; Polyakova, V.P.; Tylina, M.A.: Palladium Alloys, Primary Sources.
 +
New York: Publishers 1969
  
Table 2.22: Mechanical Properties of Silver-Nickel (SINIDUR) Materials
+
Gehlert, B.: Lebensdaueruntersuchungen von Edelmetall Kontaktwerkstoff-
 +
Kombinationen für Schleifringübertrager. VDE-Fachbericht 61, (2005) 95-100
  
Fig. 2.71:
+
Holzapfel,C.: Verschweiß und elektrische Eigenschaften von
Strain hardening
+
Schleifringübertragern. VDE-Fachbericht 67 (2011) 111-120
of Ag/Ni 90/10 by cold working
 
  
Fig. 2.72:
+
Schnabl, R.; Gehlert, B.: Lebensdauerprüfungen von Edelmetall-
Softening of Ag/Ni 90/10
+
Schleifkontaktwerkstoffen für Gleichstrom Kleinmotoren.
after annealing
+
Feinwerktechnik & Messtechnik (1984) 8, 389-393
for 1 hr after 80% cold working
 
  
Fig. 2.73:
+
Kobayashi, T.; Koibuchi, K.; Sawa, K.; Endo, K.; Hagino, H.: A Study of Lifetime
Strain hardening
+
of Au-plated Slip-Ring and AgPd Brush System for Power Supply.
of Ag/Ni 80/20 by cold working
+
th Proc. 24 Int. Conf. on Electr. Contacts, Saint Malo, France 2008, 537-542
  
Fig. 2.74:
+
Harmsen, U.; Saeger K.E.: Über das Entfestigungsverhalten von Silber
Softening of Ag/Ni 80/20
+
verschiedener Reinheiten. Metall 28 (1974) 683-686
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
+
Behrens, V.; Michal, R.; Minkenberg, J.N.; Saeger, K.E.: Abbrand und
b) parallel to the extrusion direction
+
Kontaktwiderstandsverhalten von Kontaktwerkstoffen auf Basis von Silber-
 +
Nickel. e.& i. 107. Jg. (1990), 2, 72-77
  
Fig. 2.76: Micro structure of Ag/Ni 80/20 a) perpendicular to the extrusion direction
+
Behrens, V.: Silber/Nickel und Silber/Grafit- zwei Spezialisten auf dem Gebiet
b) parallel t o the extrusion direction
+
der Kontaktwerkstoffe. Metall 61 (2007) H.6, 380-384
  
Table 2.23: Contact and Switching Properties of Silver-Nickel (SINIDUR) Materials
+
Rieder, W.: Silber / Metalloxyd-Werkstoffe für elektrische Kontakte,
 +
VDE - Fachbericht 42 (1991) 65-81
  
Table 2.24: Application Examples and Forms of Supply
+
Harmsen,U.: Die innere Oxidation von AgCd-Legierungen unter
for Silver-Nickel (SINIDUR) Materials
+
Sauerstoffdruck.
 +
Metall 25 (1991), H.2, 133-137
  
===2.4.3.2: Silver-Metal Oxide Materials Ag/CdO, Ag/SnO , Ag/ZnO===
+
Muravjeva, E.M.; Povoloskaja, M.D.: Verbundwerkstoffe Silber-Zinkoxid und
The family of silver-metal oxide contact materials includes the material groups:
+
Silber-Zinnoxid, hergestellt durch Oxidationsglühen.
silver-cadmium oxide (DODURIT CdO), silver-tin oxide (SISTADOX), and silverzinc
+
Elektrotechnika 3 (1965) 37-39
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
+
Behrens, V.; Honig Th.; Kraus, A.; Michal, R.; Saeger, K.-E.; Schmidberger, R.;
 +
Staneff, Th.: Eine neue Generation von AgSnO<sub>2</sub> -Kontaktwerkstoffen.
 +
VDE-Fachbericht 44, (1993) 99-114
  
Silver-cadmium oxide (DODURIT CdO) materials with 10-15 wt% are produced
+
Braumann, P.; Lang, J.: Kontaktverhalten von Ag-Metalloxiden für den Bereich
by both, internal oxidation and powder metallurgical methods (Table 2.25).
+
hoher Ströme. VDE-Fachbericht 42, (1991) 89-94
  
The manufacturing of strips and wires by internal oxidation starts with a molten
+
Hauner, F.; Jeannot, D.; Mc Neilly, U.; Pinard, J.: Advanced AgSnO Contact 2
alloy of silver and cadmium. During a heat treatment below it's melting point in a
+
th Materials for High Current Contactors. Proc. 20 Int. Conf. on Electr. Contact
oxygen rich atmosphere in such a homogeneous alloy the oxygen diffuses from
+
Phenom., Stockholm 2000, 193-198
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
+
Wintz, J.-L.; Hardy, S.; Bourda, C.: Influence on the Electrical Performances of
processes vary depending on the type of semi-finished material.
+
Assembly Process, Supports Materials and Production Means for AgSnO<sub>2</sub> .
For Ag/CdO wires a complete oxidation of the AgCd wire is performed, followed
+
Proc.24<sub>th</sub> Int. Conf. on Electr. Contacts, Saint Malo, France 2008, 75-81
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
+
Behrens, V.; Honig, Th.; Kraus, A.; Michal, R.: Schalteigenschaften von
processes are typically converted by pressing, sintering and extrusion to wires
+
verschiedenen Silber-Zinnoxidwerkstoffen in Kfz-Relais. VDE-Fachbericht 51
and strips. The high degree of deformation during hot extrusion produces a
+
(1997) 51-57
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
+
Schöpf, Th.: Silber/Zinnoxid und andere Silber-Metalloxidwerkstoffe in
Press-Sinter-Repress process (PSR) offers economical advantages. The
+
Netzrelais. VDE-Fachbericht 51 (1997) 41-50
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
+
Schöpf, Th.; Behrens, V.; Honig, Th.; Kraus, A.: Development of Silver Zinc
starting materials can help influence certain contact properties for specialized
+
th Oxide for General-Purpose Relays. Proc. 20 Int. Conf. on Electr. Contacts,
applications.
+
Stockholm 2000, 187-192
  
Fig. 2.77:
+
Braumann, P.; Koffler, A.: Einfluss von Herstellverfahren, Metalloxidgehalt und
Strain hardening of internally oxidized
+
Wirkzusätzen auf das Schaltverhalten von Ag/SnO in Relais. 2
Ag/CdO 90/10 by cold working
+
VDE-Fachbericht 59, (2003) 133-142
  
Fig. 2.78:
+
Kempf, B.; Braumann, P.; Böhm, C.; Fischer-Bühner, J.: Silber-Zinnoxid-
Softening of internally oxidized
+
Werkstoffe: Herstellverfahren und Eigenschaften. Metall 61(2007) H. 6, 404-408
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
+
Lutz, O.; Behrens, V.; Finkbeiner, M.; Honig, T.; Späth, D.: Ag/CdO-Ersatz in
Forms of Supply of Extruded Silver Cadmium Oxide
+
Lichtschaltern. VDE-Fachbericht 61, (2005) 165-173
(DODURIT CdO) Contact Materials
 
  
Fig. 2.79:
+
Lutz, O.; Behrens, V.; Wasserbäch, W.; Franz, S.; Honig, Th.; Späth,
Strain hardening of
+
D.; Heinrich, J.: Improved Silver/Tin Oxide Contact Materials for Automotive
Ag/CdO 90/10 P by cold working
+
th Applications. Proc.24 Int. Conf. on Electr. Contacts, Saint Malo, France 2008,
 +
88-93
  
Fig. 2.80: Softening
+
Leung, C.; Behrens, V.: A Review of Ag/SnO Contact Materials and Arc Erosion. 2
of Ag/CdO 90/10 P after annealing
+
th Proc.24 Int. Conf. on Electr. Contacts, Saint Malo, France 2008, 82-87
for 1 hr after 40% cold working
 
  
Fig. 2.81:
+
Chen, Z.K.; Witter, G.J.: Comparison in Performance for Silver–Tin–Indium
Strain hardening
+
Oxide Materials Made by Internal Oxidation and Powder Metallurgy.
of Ag/CdO 88/12 WP
+
th Proc. 55 IEEE Holm Conf. on Electrical Contacts, Vancouver, BC, Canada,
 +
(2009) 167 – 176
  
Fig. 2.82:
+
Roehberg, J.; Honig, Th.; Witulski, N.; Finkbeiner, M.; Behrens, V.: Performance
Softening of Ag/CdO 88/12WP after annealing
+
of Different Silver/Tin Oxide Contact Materials for Applications in Low Voltage
for 1 hr after different degrees of
+
th Circuit Breakers. Proc. 55 IEEE Holm Conf. on Electrical Contacts, Vancouver,
cold working
+
BC, Canada, (2009) 187 – 194
  
Fig. 2.83: Micro structure of Ag/CdO 90/10 i.o. a) close to surface
+
Muetzel, T.; Braumann, P.; Niederreuther, R.: Temperature Rise Behavior of
b) in center area
+
th Ag/SnO Contact Materials for Contactor Applications. Proc. 55 IEEE Holm 2
 +
Conf. on Electrical Contacts, Vancouver, BC, Canada, (2009) 200 – 205
  
Fig. 2.84: Micro structure of Ag/CdO 90/10 P:
+
Lutz, O. et al.: Silber/Zinnoxid – Kontaktwerkstoffe auf Basis der Inneren
a) perpendicular to extrusion direction
+
Oxidation fuer AC – und DC – Anwendungen.
b) parallel to extrusion direction
+
VDE Fachbericht 65 (2009) 167 – 176
  
Fig. 2.85:
+
Harmsen, U.; Meyer, C.L.: Mechanische Eigenschaften stranggepresster Silber-
Micro structure of Ag/CdO 90/10 ZH:
+
Graphit-Verbundwerkstoffe. Metall 21 (1967), 731-733
1) Ag/CdO layer
 
2) AgCd backing layer
 
  
Fig. 2.86: Micro structure of AgCdO 88/12 WP: a) perpendicular to extrusion direction
+
Behrens, V.: Mahle, E.; Michal, R.; Saeger, K.E.: An Advanced Silver/Graphite
b) parallel to extrusion direction
+
th Contact Material Based on Graphite Fibre. Proc. 16 Int. Conf. on Electr.
 +
Contacts, Loghborough 1992, 185-189
  
*Silver–tin oxide(SISTADOX)materials
+
Schröder, K.-H.; Schulz, E.-D.: Über den Einfluss des Herstellungsverfahrens
Over the past years, many Ag/CdO contact materials have been replaced by
+
th auf das Schaltverhalten von Kontaktwerkstoffen der Energietechnik. Proc. 7 Int.
Ag/SnO<sub>2</sub> based materials with 2-14 wt% SnO<sub>2</sub> because of the toxicity of
+
Conf. on Electr. Contacts, Paris 1974, 38-45
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
+
Mützel, T.: Niederreuther, R.: Kontaktwerkstoffe für Hochleistungsanwendungen.
during heat treatment of alloys containing > 5 wt% of tin in oxygen, dense oxide
+
VDE-Bericht 67 (2011) 103-110
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>
+
Lambert, C.; Cambon, G.: The Influence of Manufacturing Conditions and
contact materials. Besides SnO<sub>2</sub> a smaller amount (<1 wt%) of one or more
+
Metalurgical Characteristics on the Electrical Behaviour of Silver-Graphite
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
+
th Contact Materials. Proc. 9 Int. Conf.on Electr. Contacts,
additives improve the wettability of the oxide particles and increase the viscosity
+
Chicago 1978, 401-406
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
+
Vinaricky, E.: Grundsätzliche Untersuchungen zum Abbrand- und
which provide specific advantages of the resulting materials in respect to their
+
Schweißverhalten von Ag/C-Kontaktwerkstoffen. VDE-Fachbericht 47 (1995)
contact properties ''(Figs. 2.87 – 2.119)''. Some of them are described here as
+
159-169
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.
+
Agte, C.; Vacek, J.: Wolfram und Molybdän. Berlin: Akademie-Verlag 1959
  
:'''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.
+
Keil, A.; Meyer, C.-L.: Der Einfluß des Faserverlaufes auf die elektrische
 +
Verschleißfestigkeit von Wolfram-Kontakten. ETZ 72, (1951) 343-346
  
:'''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.
+
Slade, P. G.: Electric Contacts for Power Interruption. A Review. Proc. 19 Int.
 +
Conf. on Electric Contact Phenom. Nuremberg (Germany) 1998, 239-245
  
:'''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>.
+
Slade, P. G.: Variations in Contact Resistance Resulting from Oxide Formation
 +
and Decomposition in AgW and Ag-WC-C Contacts Passing Steady Currents
 +
for Long Time Periods. IEEE Trans. Components, Hybrids and Manuf. Technol.
 +
CHMT-9,1 (1986) 3-16
  
Further processing of these differently produced powders follows the
+
Slade, P. G.: Effect of the Electric Arc and the Ambient Air on the Contact
conventional processes of pressing, sintering and hot extrusion to wires and
+
Resistance of Silver, Tungsten and Silver-Tungsten Contacts.
strips. From these contact parts such as contact rivets and tips are
+
J.Appl.Phys. 47, 8 (1976) 3438-3443
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:
+
Lindmayer, M.; Roth, M.: Contact Resistance and Arc-Erosion of W-Ag and
Strain hardening of
+
WC-Ag. IEEE Trans components, Hybrids and Manuf. Technol.
Ag/SnO<sub>2</sub> 92/8 PE by cold working
+
CHMT-2, 1 (1979) 70-75
  
Fig. 2.88:
+
Leung, C.-H.; Kim, H.J.: A Comparison of Ag/W, Ag/WC and Ag/Mo Electrical
Softening of
+
Contacts. IEEE Trans. Components, Hybrids, Manuf. Technol.,
Ag/SnO<sub>2</sub> 92/8 PE after annealing
+
Vol. CHMT-7, 1 (1984) 69-75
for 1 hr after 40% cold working
 
  
Table 2.26: Physical and Mechanical Properties as well as Manufacturing Processes and
+
Allen, S.E.; Streicher, E.: The Effect of Microstructure on the Electrical
Forms of Supply of Extruded Silver-Tin Oxide (SISTADOX) Contact Materials
+
th Performance of Ag-WC-C Contact Materials. Proc. 44 IEEE Holm Conf. on Electr.
 +
Contacts, Arlington, VA, USA (1998), 276-285
  
Fig. 2.89:
+
Haufe, W.; Reichel, W.; Schreiner H.: Abbrand verschiedener W/Cu-Sinter-
Strain hardening of
+
Tränkwerkstoffe an Luft bei hohen Strömen. Z. Metallkd. 63 (1972) 651-654
Ag/SnO<sub>2</sub> 88/12 PE by cold working
 
  
Fig. 2.90:
+
Althaus, B.; Vinaricky, E.: Das Abbrandverhalten verschieden hergestellter
Softening of Ag/SnO<sub>2</sub> 88/12 PE
+
Wolfram-Kupfer-Verbundwerkstoffe im Hochstromlichtbogen.
after annealing for
+
Metall 22 (1968) 697-701
1 hr after 40% cold working
 
  
Fig. 2.91:
+
Gessinger, G.H.; Melton, K.N.: Burn-off Behaviour of WCu Contact Materials in an
Strain hardening of oxidized
+
Electric Arc. Powder Metall. Int. 9 (1977) 67-72
Ag/SnO<sub>2</sub> 88/12 PW4 by cold working
 
  
Fig. 2.92:
+
Magnusson, M.: Abbrandverhalten und Rißbildung bei WCu-Tränkwerkstoffen
Softening of Ag/SnO<sub>2</sub> 88/12 PW4 after
+
unterschiedlicher Wolframteilchengröße. ETZ-A 98 (1977) 681-683
annealing for 1 hr
 
after 30% cold working
 
  
Fig. 2.93:
+
Heitzinger, F.; Kippenberg, H.; Saeger, K.E.; Schröder, K.H.: Contact Materials for
Strain hardening of
+
Vacuum Switching Devices. Proc. XVth ISDEIV, Darmstadt 1992, 273-278
Ag/SnO<sub>2</sub> 98/2 PX
 
by cold working
 
  
Fig. 2.94:
+
Grill, R.; Müller, F.: Verbundwerkstoffe auf Wolframbasis für
Softening of
+
Hochspannungsschaltgeräte. Metall 61 (2007) H. 6, 390-393
Ag/SnO<sub>2</sub> 98/2 PX
 
after annealing
 
for 1 hr after 80%
 
cold working
 
  
Fig 2.95:
+
Slade, P.: G.: The Vacuum Interrupter- Theory; Design; and Application. CRC
Strain hardening
+
Press, Boca Raton, FL (USA), 2008
of Ag/SnO<sub>2</sub> 92/8 PX
 
by cold working
 
  
Fig. 2.96:
+
Frey, P.; Klink, N.; Saeger, K.E.: Untersuchungen zum Abreißstromverhalten von
Softening of
+
Kontaktwerkstoffen für Vakuumschütze. Metall 38 (1984) 647-651
Ag/SnO<sub>2</sub> 92/8 PX
 
after annealing for 1 hr
 
after 40% cold working
 
  
Fig. 2.97:
+
Frey, P.; Klink, N.; Michal, R.; Saeger, K.E.: Metallurgical Aspects of Contact
Strain hardening of internally
+
Materials for Vacuum Switching Devices. IEEE Trans. Plasma Sc. 17, (1989) 743-
oxidized
+
740
Ag/SnO<sub>2</sub> 88/12 TOS F
 
by cold working
 
  
Fig. 2.98:
+
Slade, P.: Advances in Material Development for High Power Vacuum Interrupter
Softening of
+
th Contacts. Proc.16 Int. Conf. on Electr. Contact Phenom.,
Ag/SnO<sub>2</sub> 88/12 TOS F after
+
Loughborough 1992,1-10
annealing for 1 hr after 30%
 
cold working
 
  
Fig. 2.99:
+
Behrens, V.; Honig, Th.; Kraus, A.; Allen, S.: Comparison of Different Contact
Strain hardening of
+
th Materials for Low Voltage Vacuum Applications. Proc.19 Int. Conf. on Electr.
internally oxidized
+
Contact Phenom., Nuremberg 1998, 247-251
Ag/SnO<sub>2</sub> 88/12P
 
by cold working
 
  
Fig. 2.100:
+
Rolle, S.; Lietz, A.; Amft, D.; Hauner, F.: CuCr Contact Material for Low Voltage
Softening of
+
th Vacuum Contactors. Proc. 20 int. Conf. on Electr. Contact. Phenom. Stockholm
Ag/SnO<sub>2</sub> 88/12P
+
2000, 179-186
after annealing for 1 hr after
 
40% cold working
 
  
Fig. 2.101:
+
Kippenberg, H.: CrCu as a Contact Material for Vacuum Interrupters.
Strain hardening of
+
th Proc.13 Int. Conf. on Electr. Contact Phenom. Lausanne 1986, 140-144
Ag/SnO<sub>2</sub> 88/12 WPC
 
by cold working
 
  
Fig. 2.102:
+
Hauner, F.; Müller, R.; Tiefel, R.: CuCr für Vakuumschaltgeräte-
Softening of Ag/SnO<sub>2</sub> 88/12 WPC after annealing
+
Herstellungsverfahren, Eigenschaften und Anwendung.
for 1 hr after different degrees of cold working
+
Metall 61 (2007) H. 6, 385-389
  
Fig. 2.103:
+
Manufacturing Equipment for Semi-Finished Materials
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
 
 
 
===2.4.3.3 Silver–Graphite (GRAPHOR)-Materials===
 
Ag/C (GRAPHOR) contact materials are usually produced by powder metallurgy
 
with graphite contents of 2 – 5 wt% ''(Table 2.32)''. The earlier typical
 
manufacturing process of single pressed tips by pressing - sintering - repressing
 
(PSR) has been replaced in Europe for quite some time by extrusion. In North
 
America and some other regions however the PSR process is still used to some
 
extend mainly for cost reasons.
 
 
 
The extrusion of sintered billets is now the dominant manufacturing method for
 
semi-finished AgC materials ''(Figs. 2.126 – 2.129)''. The hot extrusion process
 
results in a high density material with graphite particles stretched and oriented in
 
the extrusion direction ''(Figs. 2.130 – 2.133)''. Depending on the extrusion
 
method in either rod or strip form the graphite particles can be oriented in the
 
finished contact tips perpendicular (GRAPHOR) or parallel (GRAPHOR D) to the
 
switching contact surface ''(Figs. 2.131 and 2.132)''.
 
 
 
Since the graphite particles in the Ag matrix of Ag/C materials prevent contact
 
tips from directly being welded or brazed, a graphite free bottom layer is
 
required. This is achieved by either burning out (de-graphitizing) the graphite
 
selectively on one side of the tips or by compound extrusion of a Ag/C billet
 
covered with a fine silver shell.
 
 
 
Ag/C contact materials exhibit on the one hand an extremely high resistance to
 
contact welding but on the other have a low arc erosion resistance. This is
 
caused by the reaction of graphite with the oxygen in the surrounding
 
atmosphere at the high temperatures created by the arcing. The weld resistance
 
is especially high for materials with the graphite particle orientation parallel to the
 
arcing contact surface. Since the contact surface after arcing consists of pure
 
silver the contact resistance stays consistently low during the electrical life of the
 
contact parts.
 
 
 
A disadvantage of the Ag/C materials is their rather high erosion rate. In materials
 
with parallel graphite orientation this can be improved if part of the graphite is
 
incorporated into the material in the form of fibers (GRAPHOR DF), ''(Fig. 2.133)''.
 
The weld resistance is determined by the total content of graphite particles.
 
 
 
Ag/C tips with vertical graphite particle orientation are produced in a specific
 
sequence: Extrusion to rods, cutting of double thickness tips, burning out of
 
graphite to a controlled layer thickness, and a second cutting to single tips.
 
Such contact tips are especially well suited for applications which require both,
 
a high weld resistance and a sufficiently high arc erosion resistance ''(Table 2.33)''.
 
For attachment of Ag/C tips welding and brazing techniques are applied.
 
 
 
welding the actual process depends on the material's graphite orientation. For
 
Ag/C tips with vertical graphite orientation the contacts are assembled with
 
single tips. For parallel orientation a more economical attachment starting with
 
contact material in strip or profile tape form is used in integrated stamping and
 
welding operations with the tape fed into the weld station, cut off to tip form and
 
then welded to the carrier material before forming the final contact assembly
 
part. For special low energy welding the Ag/C profile tapes GRAPHOR D and DF
 
can be pre-coated with a thin layer of high temperature brazing alloys such as
 
CuAgP.
 
 
 
In a rather limited way, Ag/C with 2 – 3 wt% graphite can be produced in wire
 
form and headed into contact rivet shape with low head deformation ratios.
 
 
 
The main applications for Ag/C materials are protective switching devices such
 
as miniature molded case circuit breakers, motor-protective circuit breakers,
 
and fault current circuit breakers, where during short circuit failures highest
 
resistance against welding is required ''(Table 2.34)''. For higher currents the low
 
arc erosion resistance of Ag/C is compensated by asymmetrical pairing with
 
more erosion resistant materials such as Ag/Ni and Ag/W.
 
 
 
Fig. 2.126:
 
Strain hardening
 
of Ag/C 96/4 D
 
by cold working
 
 
 
Fig. 2.127:
 
Softening of Ag/C 96/4 D after
 
annealing
 
 
 
Fig. 2.128: Strain hardening
 
of Ag/C DF by cold working
 
 
 
Fig. 2.129: Softening
 
of Ag/C DF after annealing
 
 
 
Fig. 2.130: Micro structure of Ag/C 97/3: a) perpendicular to extrusion direction
 
b) parallel to extrusion direction, 1) Ag/C contact layer, 2) Ag backing layer
 
 
 
Fig. 2.131: Micro structure of Ag/C 95/5: a) perpendicular to extrusion direction
 
b) parallel to extrusion direction, 1) Ag/C contact layer, 2) Ag backing layer
 
 
 
Fig. 2.132: Micro structure of Ag/C 96/4 D: a) perpendicular to extrusion direction
 
b) parallel to extrusion direction, 1) Ag/C contact layer, 2) Ag backing layer
 
 
 
Fig. 2.133: Micro structure of Ag/C DF: a) perpendicular to extrusion direction
 
b) parallel to extrusion direction, 1) Ag/C contact layer, 2) Ag/Ni 90/10 backing layer
 
 
 
Table 2.32: Physical Properties of Silver–Graphite (GRAPHOR) Contact Materials
 
 
 
Table 2.33: Contact and Switching properties of Silver–Graphite (GRAPHOR) Contact Materials
 
 
 
Table 2.34: Application Examples and Forms of Supply of Silver–
 
Graphite (GRAPHOR) Contact Materials
 
 
 
Pre-Production of Contact Materials
 
 
(Bild)
 
(Bild)
  
===2.5 Tungsten and Molybdenum Based Materials===
+
[[de:Kontaktwerkstoffe_für_die_Elektrotechnik]]
 
 
===2.5.1 Tungsten and Molybdenum (Pure Metals)===
 
Tungsten is characterized by its advantageous properties of high melting and
 
boiling points, sufficient electrical and thermal conductivity and high hardness
 
and density ''(Table 2.35)''. It is mainly used in the form of brazed contact tips for
 
switching duties that require a rapid switching sequence such as horn contacts
 
for cars and trucks.
 
 
 
Molybdenum has a much lesser importance as a contact material since it is less
 
resistant against oxidation than tungsten.
 
Both metals are however used in large amounts as components in composite
 
materials with silver and copper.
 
 
 
Table 2.35: Mechanical Properties of Tungsten and Molybdenum
 
 
 
===2.5.2 Silver–Tungsten (SIWODUR) Materials===
 
Ag/W (SIWODUR) contact materials combine the high electrical and thermal
 
conductivity of silver with the high arc erosion resistance of the high melting
 
tungsten metal ''(Table 2.36)''. The manufacturing of materials with typically
 
50-80 wt% tungsten is performed by the powder metallurgical processes of
 
liquid phase sintering or by infiltration. Particle size and shape of the starting
 
powders are determining the micro structure and the contact specific properties
 
of this material group ''(Figs. 2.134 and 2.135) (Table 2.37)''.
 
 
 
During repeated switching under arcing loads tungsten oxides and mixed
 
oxides (silver tungstates – Ag<sub>2</sub> WO<sub>4</sub> ) are formed on the Ag/W surface creating 2 4
 
poorly conducting layers which increase the contact resistance and by this the
 
temperature rise during current carrying. Because of this fact the Ag/W is paired
 
in many applications with Ag/C contact parts.
 
 
 
Silver–tungsten contact tips are used in a variety of shapes and are produced for
 
the ease of attachment with a fine silver backing layer and quite often an
 
additional thin layer of a brazing alloy. The attachment to contact carriers is
 
usually done by brazing, but also by direct resistance welding for smaller tips.
 
 
 
Ag/W materials are mostly used as the arcing contacts in disconnect switches
 
for higher loads and as the main contacts in small and medium duty power
 
switches and industrial circuit breakers ''(Table 2.38)''. In north and south america
 
they are also used in large volumes in miniature circuit breakers of small to
 
medium current ratings in domestic wiring as well as for commercial power
 
distribution.
 
 
 
===2.5.3 Silver–Tungsten Carbide (SIWODUR C) Materials===
 
This group of contact materials contains the typically 40-65 wt-% of the very
 
hard and erosion wear resistant tungsten carbide and the high conductivity silver
 
''(Fig. 2.135) (Table 2.36)''. Compared to Ag/W the Ag/WC (SIWODUR C)
 
materials exhibit a higher resistance against contact welding ''(Table 2.37)''. The
 
rise in contact resistance experienced with Ag/W is less pronounced in Ag/WC
 
because during arcing a protective gas layer of CO is formed which limits the
 
reaction of oxygen on the contact surface and therefore the formation of metal
 
oxides.
 
 
 
Higher requirements on low temperature rise can be fulfilled by adding a small
 
amount of graphite which however increases the arc erosion. Silver–tungsten
 
carbide–graphite materials with for example 27 wt% WC and
 
3 wt% graphite or 16 wt% WC and 2 wt% graphite are manufactured using the
 
single tip press-sinter-repress (PSR) process ''(Fig. 2.136)''.
 
 
 
The applications of Ag/WC contacts are similar to those for Ag/W ''(Table 2.38)''.
 
 
 
===2.5.4 Silver–Molybdenum (SILMODUR) Materials===
 
Ag/Mo materials with typically 50-70 wt% molybdenum are usually produced by
 
the powder metallurgical infiltration process ''(Fig. 2.137) (Table 2.36)''. Their
 
contact properties are similar to those of Ag/W materials ''(Table 2.37)''. Since the
 
molybdenum oxide is thermally less stable than tungsten oxide the self-cleaning
 
effect of Ag/Mo contact surface during arcing is more pronounced and the
 
contact resistance remains lower than that of Ag/W. The arc erosion resistance
 
of Ag/Mo however is lower than the one for Ag/W materials. The main
 
applications for Ag/Mo contacts are in equipment protecting switching devices
 
''(Table 2.38)''.
 
 
 
Fig. 2.134: Micro structure of Ag/W 25/75
 
 
 
Fig. 2.135: Micro structure of Ag/WC 50/50
 
 
 
Fig. 2.136: Micro structure of Ag/WC27/C3
 
 
 
Fig. 2.137: Micro structure of Ag/Mo 35/65
 
 
 
Table 2.36: Physical Properties of Contact Materials Based on Silver–Tungsten (SIWODUR),
 
Silver–Tungsten Carbide (SIWODUR C) and Silver Molybdenum (SILMODUR)
 
 
 
Table 2.37: Contact and Switching Properties of Contact Materials Based on Silver – Tungsten
 
(SIWODUR), Silver–Tungsten Carbide (SIWODUR C)
 
and Silver Molybdenum (SILMODUR)
 
 
 
Table 2.38: Application Examples and Forms of Supply for Contact Materials Based
 
on Silver–Tungsten (SIWODUR), Silver–Tungsten Carbide (SIWODUR C)
 
and Silver Molybdenum (SILMODUR)
 
 
 
===2.5.5 Copper–Tungsten (CUWODUR) Materials===
 
Copper–tungsten (CUWODUR) materials with typically 50-85 wt% tungsten are
 
produced by the infiltration process with the tungsten particle size selected
 
according to the end application ''(Figs. 2.138 – 2.141) (Table 2.39)''. To increase
 
the wettability of the tungsten skeleton by copper a small amount of nickel
 
< 1 wt% is added to the starting powder mix.
 
 
 
W/Cu materials exhibit a very high arc erosion resistance ''(Table 2.40)''.
 
Compared to silver–tungsten materials they are however less suitable to carry
 
permanent current.
 
 
 
With a solid tungsten skeleton as it is the case for W/C infiltrated materials with
 
70-85 wt% tungsten the lower melting component copper melts and vaporizes
 
in the intense electrical arc. At the boiling point of copper (2567°C) the still solid
 
tungsten is efficiently “cooled” and remains pretty much unchanged.
 
 
 
During very high thermal stress on the W/Cu contacts, for example during short
 
circuit currents > 40 kA the tungsten skeleton requires special high mechanical
 
strength. For such applications a high temperature sintering of tungsten from
 
selected particle size powder is applied before the usual infiltration with copper
 
(example: CUWODUR H).
 
 
 
For high voltage load switches the most advantageous contact system consists
 
of a contact tulip and a contact rod. Both contact assemblies are made usually
 
from the mechanically strong and high conductive CuCrZr material and W/Cu as
 
the arcing tips. The thermally and mechanically highly stressed attachment
 
between the two components is often achieved by utilizing electron beam
 
welding or capacitor discharge percussion welding. Other attachment methods
 
include brazing and cast-on of copper followed by cold forming steps to
 
increase hardness and strength.
 
 
 
The main application areas for CUWODUR materials are as arcing contacts in
 
load and high power switching in medium and high voltage switchgear as well
 
as electrodes for spark gaps and over voltage arresters ''(Table 2.41)''.
 
 
 
Table 2.39: Physical Properties of Copper–Tungsten (CUWODUR) Contact Materials
 
 
 
Fig. 2.139: Micro structure of W/Cu 70/30 G Fig. 2.140: Micro structure of W/Cu 70/30 H
 
 
 
Fig. 2.138: Micro structure of W/Cu 70/30 F Fig. 2.141: Micro structure of W/Cu 80/20 H
 
 
 
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
 
 
 
===2.6 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)''.
 
 
 
===2.6.1 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)''.
 
 
 
===2.6.2 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===
 
 
 
<ref>Vinaricky, E.(Hrsg.): Elektrische Kontakte, Werkstoffe und Anwendungen.
 
Springer-Verlag, Berlin, Heidelberg etc. 2002</ref>
 

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

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Manufacturing Equipment for Semi-Finished Materials (Bild)