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Created page with "=== 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 ..."
=== 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
===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)''.
====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.
====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:
Softening of AgCu3
after annealing for 1 hr
after 80% cold working
Fig. 2.56:
Strain hardening of AgCu5 by cold
working
Fig. 2.57:
Softening of AgCu5 after
annealing for 1 hr after 80% cold
working
Fig. 2.58:
Strain hardening of AgCu 10
by cold working
Fig. 2.59:
Softening of AgCu10 after
annealing for 1 hr after 80% cold
working
Fig. 2.60:
Strain hardening of AgCu28 by
cold working
Fig. 2.61:
Softening of AgCu28
after annealing for 1 hr after
80% cold working
Fig. 2.62:
Strain hardening of AgNi0.15
by cold working
Fig. 2.63:
Softening of AgNi0.15
after annealing for 1 hr after 80%
cold working
Fig. 2.64:
Strain hardening of
ARGODUR 27
by cold working
Fig. 2.65:
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
Table 2.16: Application Examples and Forms of Supply for Silver and Silver Alloys
====Silver-Palladium Alloys====
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
material transfer under DC loads ''(Table 2.19)''. On the other hand the electrical
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
(>60V, >2A) as shown in Table 2.20. Because of the high palladium price these
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
Fig. 2.67:
Strain hardening
of AgPd30 by cold working
Fig. 2.68:
Strain hardening
of AgPd50 by cold working
Fig. 2.69:
Strain hardening
of AgPd30Cu5
by cold working
Fig. 2.70:
Softening of AgPd30, AgPd50,
and AgPd30Cu5 after annealing of 1 hr
after 80% cold working
Table 2.17: Physical Properties of Silver-Palladium Alloys
Table 2.18: Mechanical Properties of Silver-Palladium Alloys
Table 2.19: Contact and Switching Properties of Silver-Palladium Alloys
Table 2.20: Application Examples and Forms of Suppl for Silver-Palladium Alloys
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
===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)''.
====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.
====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:
Softening of AgCu3
after annealing for 1 hr
after 80% cold working
Fig. 2.56:
Strain hardening of AgCu5 by cold
working
Fig. 2.57:
Softening of AgCu5 after
annealing for 1 hr after 80% cold
working
Fig. 2.58:
Strain hardening of AgCu 10
by cold working
Fig. 2.59:
Softening of AgCu10 after
annealing for 1 hr after 80% cold
working
Fig. 2.60:
Strain hardening of AgCu28 by
cold working
Fig. 2.61:
Softening of AgCu28
after annealing for 1 hr after
80% cold working
Fig. 2.62:
Strain hardening of AgNi0.15
by cold working
Fig. 2.63:
Softening of AgNi0.15
after annealing for 1 hr after 80%
cold working
Fig. 2.64:
Strain hardening of
ARGODUR 27
by cold working
Fig. 2.65:
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
Table 2.16: Application Examples and Forms of Supply for Silver and Silver Alloys
====Silver-Palladium Alloys====
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
material transfer under DC loads ''(Table 2.19)''. On the other hand the electrical
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
(>60V, >2A) as shown in Table 2.20. Because of the high palladium price these
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
Fig. 2.67:
Strain hardening
of AgPd30 by cold working
Fig. 2.68:
Strain hardening
of AgPd50 by cold working
Fig. 2.69:
Strain hardening
of AgPd30Cu5
by cold working
Fig. 2.70:
Softening of AgPd30, AgPd50,
and AgPd30Cu5 after annealing of 1 hr
after 80% cold working
Table 2.17: Physical Properties of Silver-Palladium Alloys
Table 2.18: Mechanical Properties of Silver-Palladium Alloys
Table 2.19: Contact and Switching Properties of Silver-Palladium Alloys
Table 2.20: Application Examples and Forms of Suppl for Silver-Palladium Alloys
