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===2.1 Introduction===
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 2.1 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)
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
Fig. 2.1: Powder-metallurgical manufacturing of composite materials (schematic)
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.
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.
===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
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 2.1 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)
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
Fig. 2.1: Powder-metallurgical manufacturing of composite materials (schematic)
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.
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.
===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
