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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
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
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
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
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:
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
===2.4.2.3 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
===2.4.3 Silver Composite Materials===
===2.4.3.1 Silver-Nickel (SINIDUR) Materials===
