Contact Materials for Electrical Engineering

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

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

2.4.1 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