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*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
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
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
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
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
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
===2.3 Platinum Metal Based Materials===
The platinum group metals include the elements Pt, Pd, Rh, Ru, Ir, and Os ''(Table2.6)''. 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
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 ''(Table2.7)''. 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
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.
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.
===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===
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.
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
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
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)
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
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
of sintered Ag/Ni billets into wires, strips and rods the Ni particles embedded in
the Ag matrix are stretched and oriented in the microstructure into a pronounced
fiber structure ''(Figs. 2.75. and 2.76)''
The high density produced during hot extrusion aids the arc erosion resistance
of these materials ''(Tables 2.21 and 2.22)''. The typical application of Ag/Nicontact materials is in devices for switching currents of up to 100A ''(Table 2.24)''.
In this range they are significantly more erosion resistant than silver or silver
alloys. In addition they exhibit with nickel contents <20 wt% a low and over their
operational lifetime consistent contact resistance and good arc moving
properties. In DC applications Ag/Ni materials exhibit a relatively low tendency
of material transfer distributed evenly over the contact surfaces ''(Table 2.23)''.
Typically Ag/Ni (SINIDUR) materials are usually produced with contents of 10-40
wt% Ni. The most widely used materials SINIDUR 10 and SINIDUR 20- and also
SINIDUR 15, mostly used in north america-, are easily formable and applied by
cladding ''(Figs. 2.71-2.74)''. They can be, without any additional welding aids,
economically welded and brazed to the commonly used contact carrier
materials.
The most important applications for Ag/Ni contact materials are typically in
relays, wiring devices, appliance switches, thermostatic controls, auxiliary
switches, and small contactors with nominal currents >20A ''(Table 2.24)''.
Table 2.21: Physical Properties of Silver-Nickel (SINIDUR) Materials
for Silver-Nickel (SINIDUR) Materials
===2.4.3.2: Silver-Metal Oxide Materials Ag/CdO, Ag/SnO <sub>2</sub>, Ag/ZnO===
The family of silver-metal oxide contact materials includes the material groups:
silver-cadmium oxide (DODURIT CdO), silver-tin oxide (SISTADOX), and silverzinc
in a broad field of applications. They mainly are used in low voltage electrical
switching devices like relays, installation and distribution switches, appliances,
industrial controls, motor controls, and protective devices ''(Table 2.13)''.
*Silver-cadmium oxide (DODURIT CdO) materials
Silver-cadmium oxide (DODURIT CdO) materials with 10-15 wt% are produced
by both, internal oxidation and powder metallurgical methods ''(Table 2.25)''.
The manufacturing of strips and wires by internal oxidation starts with a molten
less fine particle precipitation inside the Ag matrix. The CdO particles are rather
fine in the surface area and are becoming larger further away towards the center
of the material ''(Fig. 2.83)''.
During the manufacturing of Ag/CdO contact material by internal oxidation the
processes vary depending on the type of semi-finished material.
For Ag/CdO wires a complete oxidation of the AgCd wire is performed, followed
by wire-drawing to the required diameter ''(Figs. 2.77 and 2.78)''. The resulting
material is used for example in the production of contact rivets. For Ag/CdO strip
materials two processes are commonly used: Cladding of an AgCd alloy strip
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.
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
