Contact Materials for Electrical Engineering

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.

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.

Main Articel: Gold Based Materials

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.

Main Articel: Platinum Metal Based Materials

Silver Based Materials

Pure Silver, Silver Alloys, Silver Composite Materials

Main Articel: Silver Based Materials

Tungsten and Molybdenum Based Materials

Tungsten and Molybdenum (Pure Metals)

Tungsten is characterized by its advantageous properties of high melting and boiling points, sufficient electrical and thermal conductivity and high hardness and density (Table 2.35). It is mainly used in the form of brazed contact tips for switching duties that require a rapid switching sequence such as horn contacts for cars and trucks.

Molybdenum has a much lesser importance as a contact material since it is less resistant against oxidation than tungsten. Both metals are however used in large amounts as components in composite materials with silver and copper.

Table 2.35: Mechanical Properties of Tungsten and Molybdenum

Silver–Tungsten (SIWODUR) Materials

Ag/W (SIWODUR) contact materials combine the high electrical and thermal conductivity of silver with the high arc erosion resistance of the high melting tungsten metal (Table 2.36). The manufacturing of materials with typically 50-80 wt% tungsten is performed by the powder metallurgical processes of liquid phase sintering or by infiltration. Particle size and shape of the starting powders are determining the micro structure and the contact specific properties of this material group (Figs. 2.134 and 2.135) (Table 2.37).

During repeated switching under arcing loads tungsten oxides and mixed oxides (silver tungstates – Ag₂ WO₄ ) are formed on the Ag/W surface creating 2 4 poorly conducting layers which increase the contact resistance and by this the temperature rise during current carrying. Because of this fact the Ag/W is paired in many applications with Ag/C contact parts.

Silver–tungsten contact tips are used in a variety of shapes and are produced for the ease of attachment with a fine silver backing layer and quite often an additional thin layer of a brazing alloy. The attachment to contact carriers is usually done by brazing, but also by direct resistance welding for smaller tips.

Ag/W materials are mostly used as the arcing contacts in disconnect switches for higher loads and as the main contacts in small and medium duty power switches and industrial circuit breakers (Table 2.38). In north and south america they are also used in large volumes in miniature circuit breakers of small to medium current ratings in domestic wiring as well as for commercial power distribution.

Silver–Tungsten Carbide (SIWODUR C) Materials

This group of contact materials contains the typically 40-65 wt-% of the very hard and erosion wear resistant tungsten carbide and the high conductivity silver (Fig. 2.135) (Table 2.36). Compared to Ag/W the Ag/WC (SIWODUR C) materials exhibit a higher resistance against contact welding (Table 2.37). The rise in contact resistance experienced with Ag/W is less pronounced in Ag/WC because during arcing a protective gas layer of CO is formed which limits the reaction of oxygen on the contact surface and therefore the formation of metal oxides.

Higher requirements on low temperature rise can be fulfilled by adding a small amount of graphite which however increases the arc erosion. Silver–tungsten carbide–graphite materials with for example 27 wt% WC and 3 wt% graphite or 16 wt% WC and 2 wt% graphite are manufactured using the single tip press-sinter-repress (PSR) process (Fig. 2.136).

The applications of Ag/WC contacts are similar to those for Ag/W (Table 2.38).

Silver–Molybdenum (SILMODUR) Materials

Ag/Mo materials with typically 50-70 wt% molybdenum are usually produced by the powder metallurgical infiltration process (Fig. 2.137) (Table 2.36). Their contact properties are similar to those of Ag/W materials (Table 2.37). Since the molybdenum oxide is thermally less stable than tungsten oxide the self-cleaning effect of Ag/Mo contact surface during arcing is more pronounced and the contact resistance remains lower than that of Ag/W. The arc erosion resistance of Ag/Mo however is lower than the one for Ag/W materials. The main applications for Ag/Mo contacts are in equipment protecting switching devices (Table 2.38).

Fig. 2.134: Micro structure of Ag/W 25/75

Fig. 2.135: Micro structure of Ag/WC 50/50

Fig. 2.136: Micro structure of Ag/WC27/C3

Fig. 2.137: Micro structure of Ag/Mo 35/65

Table 2.36: Physical Properties of Contact Materials Based on Silver–Tungsten (SIWODUR), Silver–Tungsten Carbide (SIWODUR C) and Silver Molybdenum (SILMODUR)

Table 2.37: Contact and Switching Properties of Contact Materials Based on Silver – Tungsten (SIWODUR), Silver–Tungsten Carbide (SIWODUR C) and Silver Molybdenum (SILMODUR)

Table 2.38: Application Examples and Forms of Supply for Contact Materials Based on Silver–Tungsten (SIWODUR), Silver–Tungsten Carbide (SIWODUR C) and Silver Molybdenum (SILMODUR)

Copper–Tungsten (CUWODUR) Materials

Copper–tungsten (CUWODUR) materials with typically 50-85 wt% tungsten are produced by the infiltration process with the tungsten particle size selected according to the end application (Figs. 2.138 – 2.141) (Table 2.39). To increase the wettability of the tungsten skeleton by copper a small amount of nickel < 1 wt% is added to the starting powder mix.

W/Cu materials exhibit a very high arc erosion resistance (Table 2.40). Compared to silver–tungsten materials they are however less suitable to carry permanent current.

With a solid tungsten skeleton as it is the case for W/C infiltrated materials with 70-85 wt% tungsten the lower melting component copper melts and vaporizes in the intense electrical arc. At the boiling point of copper (2567°C) the still solid tungsten is efficiently “cooled” and remains pretty much unchanged.

During very high thermal stress on the W/Cu contacts, for example during short circuit currents > 40 kA the tungsten skeleton requires special high mechanical strength. For such applications a high temperature sintering of tungsten from selected particle size powder is applied before the usual infiltration with copper (example: CUWODUR H).

For high voltage load switches the most advantageous contact system consists of a contact tulip and a contact rod. Both contact assemblies are made usually from the mechanically strong and high conductive CuCrZr material and W/Cu as the arcing tips. The thermally and mechanically highly stressed attachment between the two components is often achieved by utilizing electron beam welding or capacitor discharge percussion welding. Other attachment methods include brazing and cast-on of copper followed by cold forming steps to increase hardness and strength.

The main application areas for CUWODUR materials are as arcing contacts in load and high power switching in medium and high voltage switchgear as well as electrodes for spark gaps and over voltage arresters (Table 2.41).

Table 2.39: Physical Properties of Copper–Tungsten (CUWODUR) Contact Materials

Fig. 2.139: Micro structure of W/Cu 70/30 G Fig. 2.140: Micro structure of W/Cu 70/30 H

Fig. 2.138: Micro structure of W/Cu 70/30 F Fig. 2.141: Micro structure of W/Cu 80/20 H

Manufacturing of Contact Parts for Medium and High Voltage Switchgear

Table 2.40: Contact and Switching Properties of Copper–Tungsten (CUWODUR) Contact Materials

Table 2.41: Application Examples and Forms of Supply for Tungsten– Copper (CUWODUR) Contact Materials

Special Contact Materials (VAKURIT) for Vacuum Switches

The trade name VAKURIT is assigned to a family of low gas content contact materials developed for the use in vacuum switching devices (Table 2.42).

Low Gas Content Materials Based on Refractory Metals

Contact materials of W/Cu, W/Ag, WC/Ag, or Mo/Cu can be used in vacuum switches if their total gas content does not exceed approximately 150 ppm. In the low gas content W/Cu (VAKURIT) material mostly used in vacuum contactors the high melting W skeleton is responsible for the high erosion resistance when combined with the high conductivity copper component which evaporates already in noticeable amounts at temperatures around 2000 °C.

Since there is almost no solubility of tungsten, tungsten carbide, or molybdenum in copper or silver the manufacturing of these material is performed powdermetallurgically. The W, WC, or Mo powders are pressed and sintered and then infiltrated with low gas content Cu or Ag. The content of the refractory metals is typically between 60 and 85 wt% (Figs. 2.142 and 2.143).

By adding approximately 1 wt% antimony the chopping current, i.e. the abrupt current decline shortly before the natural current-zero, can be improved for W/Cu (VAKURIT) materials (Table 2.43). The contact components mostly used in vacuum contactors are usually shaped as round discs. These are then attached by brazing in a vacuum environment to their contact carriers (Table 2.44).

Low Gas Content Materials Based on Copper-Chromium

As contact materials in vacuum interupters in medium voltage devices low gas materials based on Cu/Cr have gained broad acceptance. The typical chromium contents are between 25 and 55 wt% (Figs. 2.144 and 2.145). During the powder metallurgical manufacturing a mix of chromium and copper powders is pressed into discs and subsequently sintering in a reducing atmosphere or vacuum below the melting point of copper. This step is followed by cold or hot re-pressing. Depending on the composition the Cu/Cr (VAKURIT) materials combine a relatively high electrical and thermal conductivity with high dielectric stability. They exhibit a low arc erosion rate and good resistance against welding as well as favorable values of the chopping current in medium voltage load switches, caused by the combined effects of the two components, copper and chromium (Table 2.43).

The switching properties of Cu/Cr (VAKURIT) materials are dependent on the purity of the Cr metal powders and especially the type and quantity of impurities contained in the chromium powder used. Besides this the particle size and distribution of the Cr powder are of high importance. Because of the getter activity of chromium a higher total gas content of up to about 650 ppm compared to the limits in refractory based materials can be tolerated in these Cu/Cr contact materials. Besides the more economical sinter technology also infiltration and vacuum arc melting are used to manufacture these materials. Cu/Cr contacts are supplied in the shape of discs or rings which often also contain slots especially for vacuum load switches in medium voltage devices (Table 2.44). Increased applications of round discs can also be observed for low voltage vacuum contactors.

Table 2.42: Physical Properties of the Low Gas Materials (VAKURIT) for Vacuum Switches

Fig. 2.142: Micro structure of W/Cu 30Sb1 – low gas

Fig. 2.143: Micro structure of WC/Ag 50/50 – low gas

Fig. 2.144: Micro structure of Cu/Cr 75/25 – low gas

Fig. 2.145: Micro structure of Cu/Cr 50/50 – low gas

Table 2.43: Contact and Switching Properties of VAKURIT Materials

Table 2.44: Application Examples and Form of Supply for VAKURIT Materials

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Manufacturing Equipment for Semi-Finished Materials (Bild)