ELECTRICAL CONTACTS BY POWDER METALLURGY

1Submitted by Priyanshu Jain (12518)

OUTLINE

Electrical contacts: Introduction

Desirable Properties of contact material

Contact material selection

Manufacturing of Contact material

Examples (application and microstructure)

References

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WHAT ARE ELECTRICAL CONTACTS?

• An electrical contact is an electrical circuit component composed of

two pieces of electrically conductive metal that pass electrical current

or insulate when the gap between them is closed or open.

• These are found in electrical switches, voltage regulators, arcing tips,

switch gears, contactors, relays and breakers.

• The gap must be an insulating medium of air, vacuum, oil,

SF6 or other electrically insulating fluid.

• Contacts may be operated by humans in pushbuttons and

switches, by mechanical pressure in sensors or machine

cams, and electromechanically in relays.

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Light Switch

with open

contact

DESIRABLE PROPERTIES OF ELECTRICAL CONTACTS?

High electrical and thermal conductivities

Good fusion resistance

High wear resistance

Good Corrosion resistance

Good mechanical workability

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High electrical and thermal conductivities:

Thermal capacity of contact support are small, the area of heat

dissipation is quite limited; thus the electric contact material must have

high to ensure allowable temperature rise during operating. This will

reduce the possibility of any kind of damage to insulation without

affecting electrical performance.

Good fusion resistance:

When two contacts fuse together, the heat produced by contact

resistance and electric arc will rise the surface temperature of electric

contact reaching its melting point and subsequently cause sticking.

Therefore to resist fusion we must have high melting point and low

contact resistance material.

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High wear resistance:

Electric contact material should have appropriate hardness since under

strong current, electric heat and electric arc may cause electric contact

material to melt, spatter and even evaporate.

Good Corrosion resistance:

Contact material must be stable so that even at higher temperatures,

they won’t react with ambient agents such as Oxygen and Sulphur nor

forming a thin layer of such compounds.

Good mechanical workability:

Contact material must be easy to be riveted and welded.

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CLASSIFICATION

based on their composition and metallurgical structure

Pure Metals:

• silver has the greatest importance for switching devices in the higher

energy technology

• gold and platinum are used in applications for the information

technology in the form of thin surface layers.

• tungsten is used for some special applications e.g. automotive horn

contacts.

• rarely pure copper is used but mainly paired to a silver-based

contact material.7

Contact Material

Pure Metals

Alloys Composites

Alloys:

• made by melt technology

• improve the properties of one material at the cost of changing them

for the second material

• E.g. AgCu, AgW

Composites (pseudoalloys):

• properties are of great importance for electrical contacts

• formed by components exhibiting no intermiscibility in either the

liquid or solid state, and heterogeneous systems composed of a

matrix with fine inclusions of another phase which are firmly

embedded in the matrix but do not react with it

• e.g., Ag--CdO, Ag--C, and W--ThO2

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Importance of Composites:

• their heterogeneous composition ensures an additive combination of

properties imparted to the materials by their individual phase

constituents

• e.g., W-Cu composite imparts good melting point and arc erosion

resistance along with good electrical conductivity of copper

• e.g., Ag-CdO composite, cadmium oxide as a disperse phase in a

silver matrix, adds dispersion-hardening effect and an arc-

suppressing abilities to the electrical and thermal conductivities of

silver

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CONTACT MATERIAL SELECTION

There should be a proper balance in material selection in selectingcontact material for specific application. Adjusting material particlesizes, choosing additives, and altering furnace temperatures plays avital role in the final properties of the selected contact material.

Following general guidelines in material selection and contact designshould be considered:

• Contact Resistance is the resistance across a pair of closed contactswhich is in series with the load of the device. High contact resistancein a device can result in nuisance tripping, overheating, and evenwelding of the contacts, therefore it should be avoided

• Contact Erosion is the loss of material due to electrical or mechanicalconditions experienced at the contact-operating surface during theoperation of the device. Excessive contact erosion is detrimental tothe performance of the device.

• Contact Welding is the bonding of the stationary and movingcontacts in a device. It should be avoided to continue operation.

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MANUFACTURING TECHNIQUE

Why Powder Metallurgy ?

It is possible to combine phase components of any nature and structure

in a single dense material by utilizing:

• intermolecular bonding forces in the powder pressing stage and

viscous flow phenomena and capillary effects

• diffusion in the liquid- or solid-phase sintering and infiltration stages

• suitable choice of starting structure of powder particles

Thus, the nature of composite materials and flexibility of processing

operations in powder metallurgy open up great possibilities for the

synthesis of electrical contact composites, possessing predetermined

combinations of properties.

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PROCEDURE

1. Powder and Mixing:

Preparation of powders having the required

composition, structure, and particle-size, distribution,

capillary characteristics and flow characteristics; mixing

is based on engineering specifications for a given material composition

2. Pressing:

Pre mixed powder are fed into a die cavity, punch

presses the powders into compacts of the desired size

and shape

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3. Sintering:

• Conversion of the compacts into dense parts of controlled porosity or

nonporous parts at high temperatures in protective environments

• It allows the creation of metallurgical bonds between the powder

particles and the creation of a "refractory skeleton" that can be

infiltrated or repressed

• It also eliminates the need for expensive machining

since contacts are at final dimensional requirement

{Based on required Composition}

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Sintering Techniques

Press-Sinter-Infiltrate (PSI)

Press-Sinter-Repress (PSR)

Press-Sinter (PS)

3a. Press Sinter Infiltrate:

• The powder of the higher melting component (sometimes mixed 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 and

composite reaches after the infiltration near-theoretical density

without subsequent pressing.

• 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.

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3b. Press Sinter Repress (PSR):

• 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.

• 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 extruded product is further

densified which contributes to higher arc erosion resistance.

• Materials such as Ag/Ni, Ag/MeO, and Ag/C are typically

produced by this process.

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3c. Sintering with liquid phase:

• 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 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.

4. Post Sintering operations (Surface preparation):

• Machining: To design product with "molded-to-size" configuration

• Refractory Etching: Light etch is applied where a highly conductive

surface is desirable, such as in circuit breaker switching duty

requirements. Etching is also helpful in creating a surface that is easy

to attach to when welding or brazing.

• Solder Flushing: applying an appropriate braze alloy material to

the backside of the contact so as to easily attach the contact to their

base16

• Silver Enrichment: Accomplishes many of the same things as

refractory etching but is employed where contact materials typically

contain graphite. Through a carefully controlled thermal operation,

the graphite is removed from the targeted surfaces, leaving behind a

layer of high conductivity material which is easily attached to contact

supports.

• Cleaning/Tumbling: Finally, contacts receive a cleaning prior to use.

Cleaning is accomplished ultrasonically or more typically by tumbling

in a specific soap and water solution with hardened media.

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SOME IMPORTANT CONTACT MATERIALS AND THEIR PROPERTIES

Silver Tungsten (AgW):

• The W-Ag and Mo-Ag electrical contact materials utilized in heavy-

duty applications are produced exclusively by means of powder

metallurgy (PM).

• Composites containing 30-80 wt-% Ag resist arc erosion and possess

good welding resistance and current carrying capacity.

W-Ag composite materials can be produced by two techniques:

• mixing elemental powders, cold pressing and solid state sintering. At

silver contents exceeding 30% the as-sintered parts can be subjected

to final processing by rolling or re-pressing

• infiltrating tungsten, or molybdenum, porous skeletons with liquid

silver. 18

Infiltrated contacts predominate because infiltration process is suitable

only for materials with tungsten content higher than 70%, which lead to

their excellent application properties like high burn resistant

Uniform particle size and distribution of the refractory phase,

homogeneity of microstructure and amount of porosity lead to excellent

electrical, mechanical, and thermo-physical properties of the composite

material19

W-dark

Ag-Gray

Microstructure

of as-infiltrated

W+30%Ag

composites

• AgW is the most popular of the contact material families used in

circuit breakers and other power switching devices

• Tungsten is preferred in car electronics, as contact-breaker points, for

horns, etc

• Mo has lower density than tungsten therefore sometime used in place

of W, when the mass of moving parts of the contact must be

minimized.

• Molybdenum-base contacts are used for breaking lower currents and

in high voltage electronics

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Silver Tungsten Carbide (AgWC):

• excellent material because of its ability to resist contact “sticking” or

welding and also because of its resistance to oxidation

• Since harder material than

AgW, more resistant to arc

erosion and contact wear

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Copper Tungsten (CuW):

• CuW and CuWC offer low-cost alternatives to AgW and AgWC

when used in non-oxidizing conditions.

• Apart from food electrical and thermal properties it has excellent arc

resistance, high strength, good machinability, low thermal expansion

• Frequently used for arcing contact where contact force is sufficient to

break through the oxides that are inevitably formed

• Used as circuit breakers as well

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Silver Tungsten Carbide Graphite (AgWC):

• Used in many applications as a replacement for silver graphite.

• Less costly than comparable silver graphite materials and exhibits the

positive characteristics of both silver graphite and silver tungsten

carbide.

• When used as a stationary contact, AgWC has good thermal and

electrical properties and has minimal contact erosion with low

chopping current.

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Copper Chromium (CuCr):

• Copper ensures the excellent electrical and thermal conductivity of

the contact material

• Combination has Good mechanical strength, Low gas content,

Excellent arc erosion resistance, Very good dissipation of short circuit

currents and Low welding tendency

• Economical production of near net shapes

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• Fine grained and homogeneous microstructure lead to high ignition

voltages and uniform arc distribution over the smooth surfaces of

these materials during the switching operation

• By repressing, the electrical and thermal conductivity can be

increased and reduction in porosity also leads to an improvement in

breaking capacity.

Silver Graphite (AgC):

• Commonly used as stationary contact material and is typically paired

with AgW or AgWC.

• Mostly used in circuit breaker and switch grades

• Has superior anti-welding characteristics and therefore is a good

choice when tack welding is an issue

• Excellent electrical conductivity but higher erosion rate.

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Powder

Mixing

Pressing

Sintering

Contact tips

Drawing

Wire

Contact Rivets

Extruding

Solder Backing

Repressing Infiltrating

Solder Backing

Contact tips

SUMMERY

REFERENCES

ELECTRICAL CONTACTS PRODUCED BY POWDER METALLURGY

METHODS, I. N. Frantsevich

SILVER – BASED INFILTRATED COMPOSITES, M. MADEJ

Electrical contact materials based on silver, NADEZDA M. TALIJAN

Contact technologies, Inc.

http://www.plansee.com/en/products/components/electrical-

contacts/

http://www.electrical-contacts-

wiki.com/index.php/Contact_Materials_for_Electrical_Engineering

https://en.wikipedia.org/wiki/Electrical_contacts

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Thank You

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