Silver Based Materials

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