Tribology Issues in

Electrical Contacts

M.D. BryantMechanical Engineering

The University of Texas at AustinAustin, Texas 78712-1063

Dedicated to the late Dr. Ralph Ashley Burton,who introduced me to Electrical Contacts.

Roadmap• Introduction

Define electrical contactsReview: Contact principlesReview: Electrical concepts

• Electrical contact physics

• Selected problem areasConnectorsSliding contacts/brushesSwitches and relays

Electrical Contacts Review• Definition:

Ragnar Holm, Electric Contacts, Theory andApplication, 4th edition, Springer-Verlag, NewYork, 1981.

electrical contact ….. a releasable junctionbetween two conductors which is apt to carryelectric current.

• Purpose: Transfer charge across a mechanicalinterface between conductors. Electric contactspermit frequent and convenient connection anddisconnection of circuits.

• Common ApplicationsSwitches & RelaysConnectors & PlugsSliding contacts: brushes

• Background = All areas of classical physics:Tribology + Electrical Engineering + Chemistry +Materials + Mechanics + Quantum effects

Electrical Contact MakePress bodies together ⇒ Contact pressures & areaVoltage difference ⇒ currentCharge carriers “jump” interfacial gapContact impedance (resistance) depends on

Real contact areamaterialsfilms

conductor 1

conductor 2

lines of current

Electrical ContactRequirements• Electrical

ConductiveNegligible effect oncircuit: small impedanceStable: no impedancefluctuations

• MechanicalContact stays togetherCompact

• Chemically & thermallystable

• Cost effective

conductor 1

conductor 2

lines of current

1

Contact Principles

HERTZIAN THEORY

2a

P

P

Zf (x, y)x

p(x,y)

• p(x, y) = po a2 - x2 - y2

po = 3P

2π a2

• a = 3πP(k1+k2)R1R24(R1+R2)

13

ki = 1- νi2πEi

• α =

9π2P2(k1+k2)2(R1+R2)16R1R2

13

2

PLASTIC CONTACT THEORY

• Indentation (Meyer) hardness

P

P

Bodies in contact

δA

P

P

p

Load P > elastic limit⇒ plastic deformations

• Contact pressures p(x, y) approximately uniform

• Hardness pressure (indentation hardness)

Η ≡ p ≈ PδA

H ≈ 3 x Yield stress

• Use: estimate contact area, given H and P

3

OVERALL CONTACT MODEL

2a

P

P

p(x,y)

P

P

α2

α1

• Spheres

• Increasing normal load P

0 ≤ P < Pe ; Elastic (Hertzian) contact model

α = α1+α2 = 9π2P2(k1+k2)2(R1+R2)16R1R2

13

P ≥ Pe ; Plastic contact model

α = α1+α2 > 9π2Pe2(k1+k2)2(R1+R2)16R1R2

13

• Similar formulations, tangential loads & deformations

4

Electrical ConceptsElectric field: force per charge [N C–1 = V m-1]Voltage: energy per charge [volt V = J C-1]Current: charges in motion [ampere A = C s-1]• requires charge [coulomb C] carriers

Electrons: e-Holes: p+

• Conduction mechanismsFree electrons e- in metalsBarrier gap/work function potential φgap

e- jumps gap if sufficient kinetic energyV < φgap suggests no current

Tunnel effect: distribution of energies overpopulation of charge carriers, some have enoughkinetic to jump

distance

energypercharge

electrode 1 electrode 2

φgap

V

gap

kinetic energy distribution

V Φgap

kinetic energy: e-

5Current density: [A m-2]

Impedance: Z = V/I, resistance to current flowZ = R + j X

Resistance: R = ρ l/A [ohm Ω = V A-1]Resistivity: ρ [Ω–m]Area: A [m2]

Length: l [m]

• Reactance: X fromInductance L [henry H = Ω s]

ZL = jω L

Capacitance C [farad F = C V-1 = Ω-1 s]ZC = 1/jω C

6

Constriction Resistance

• Lines of current constrict near contact• Water draining from bathtub: constriction impedes

flow• Generates resistance

R = ∫ ρ dr/A(r) = ∫ ρ/(2 π r2 ) dr ≈ ρ/2π (1/a – 1/r∞ )

ρ: bulk resistivity of body; a: contact radius

• Neglects volume r < a• Actual Constriction Resistance: Rc = ρ/2a• Contact radius a via mechanical contact

r + drr2a

7

Surface Films

• Thin films: nm to µm• Thickness dependent

resistivity: ρf = ρf(s)• Film formed via

contaminant diffusion& corrosion

• Mechanical Effects offilm negligible

• Film Resistance

Rf = ∫ ρf/A ds

• Tunnel effect candominate ρf & Rf

body 1

body 2

film

8

Continental Analogy of Contact

• Rough surfaces:earth & electrical contacts

North America

South America

AndesMountains

AppalachianMountains

RockyMountains

BrazilHighlands

• Invert South America,place atop NorthAmerica

• Contact: highest peaksagainst highest peaks

Andes/AppalachiaHighlands/Rockies

• Small, discrete contact areas at highest peaks

14• Analogous to bodies in contact

Rough surfacesSmall, discrete contact areas (a-spots)

• ⇒ parallel circuits & micro-constrictions

• “Pores” in contact allows easy contaminant diffusion

contact area withmicro-contacts

a-spot

global constriction

roughsurfaces

15

Contact Impedance• Zc = Rc1 + jωLc1 + 1/( Σ Rm

–1 ) + 1/ ( 1/Rf + jωC)+ Rc2 + jωLc2 + 1/( Σ Rm

–1 )

≈ Rc = Rc1 + 1/( Σ Rm–1 ) + Rf + Rc2 + 1/( Σ Rm

–1 )

• Inductance & capacitance effects small @ lowfrequency ω

Rc

Rf Cf

globalconstriction

surface filmeffects

rough contactmicro-constrictions

lines of current

16

Joule Heating• Contact resistance dissipates power: P = i2 Rc

Heat sources• Field equations

Electrical: ∇ • J = ∇ • [1

ρe(T) ∇ V] = 0 (Laplace)

Thermal: ∇ • [k(T) ∇ Τ ]+ q = ρCp(T) ∂T∂t (Fourier)

q = qe = J • E = 1ρe

∇∇∇∇ V • ∇∇∇∇ V

2

1

00 1 2

z/a

x/a

2.0

1.6

1.2

0.8

0.4

2

1

00 1 2

x/a

z/a

1500

1750

2000

2000

15001000

Equipotential contours and isotherms for electricalheating of carbon graphite with a cold contactboundary with a = 1 mm. Applied voltage: 2 V.

17

Polarization Effects

DC voltageBodies become anode and cathode

AffectsChemistryMaterial behaviorArcing

ExamplesCathode brush wear > anode brush wearGraphite “sticks” to cathode

Selective film formation on anode/cathodeAffects contact resistance

Anode/cathode arc erosion different

18

Switches & RelaysFunction, design, & purpose: frequent and rapidopening & closing of circuits. Often contains twometallic contactor parts.

Switches / Relays / Reed Switches

Pictures: Eaton website (www.eaton.com)

19

Arcing• Existing contact with current flow• Contact break interrupts circuit• Circuit inductance L opposes current change via

induced voltage VL = L di/dt• Analogy: mass induces inertial force FI = m dv/dt• Charge carriers needed for current to jump gap• Air molecules ionize under voltage VL = VL(t)• Plasma formed: fourth state of matter• Arc = Current through ionized “column”

closed contact open circuit:inductancesinduce air ions

arc currentflows viaions from air

20• Destructive: melts, welds & erodes electrodes

Material transfer: anode - cathodeSegregation & recrystallizationSurface roughening

Photo: C. H. Leung & A. Lee, 1993, “Silver Tin Oxide Contact Erosion inAutomotive Relays,” Proc. 39th IEEE Holm Conference, pp. 61-67

21

Elevated & sporadic contact resistance

Chart: Z. Chen & K. Sawa, 1994, “Polarity Effect of Unsymmetrical MaterialCombination on the Arc Erosion and Contact Resistance Behaviour,” Proc. 40th

IEEE Holm Conference, pp. 79-88.

• Always present in switches, relays, plugs, etc.• Often: arc travels along cathode, stationery on

anode

_

22

Research Issues/Design Problems• Arc suppression• Guide arc path• Control contact bounce: repeated arcing• Materials selection: powdered

Cu-CrNi-CdOAg-CdOAg-SnO2Ag-SnO2In2O3Ag-MeO

23

Connectors• Function, design, & purpose: permanent but quick

connections/disconnects5-40 year stable life

Connector classespower connectors: power lines & junction boxesautomotiveelectronic connectors: low power & compact

Telephoneselectronicscomputers

24

Power connectors: Aluminium• Lightweight• Economic• Conductive• Form passivating films• Problem:

stress relaxation @ higher temperaturesloss of contact area ⇒ higher Rcrisk: fires

Examples: wire nuts, wedge connectors, clamps

Photo: J. J Schindler, R.T. Axon & R.S. Timsit, 1995, “Mechanical andElectrical Contact Properties of Wedge Connectors,” Proc. 41st IEEE HolmConference, pp. 1-9.

25

Electronic Connectors

• Low power: contamination problemsdustpollutantscorrosive agents ⇒ fretting corrosion

• IBM, late 1980s: connector problems cause 50% ofall computer failures

Pictures: Molex website (www.molex.com)

26

Fretting Corrosion

fretting time (cycles)

100 102 104 106

Contact resistance (mΩ)

1

10

100

Surfacefilmsbroken

Stable

Increasewithfluctuations Drastic

Increase

Stage 1 Stage 2 Stage 3

• Elevated & fluctuating contact resistance• Elevated resistance: digital high ⇒ low,

computer failure• Fluctuations ⇒ “noise” to signals

27

Fretting Mechanism

virginasperities

mth cycle: motions expose a-spotscorrosive layers form

h ≈≈≈≈ 1to 100 nm

m

Aex posed(m )

• Micro-motions @ contactStray vibrationsThermal expansions via temperature variation

• Normal & tangential motions• Surfaces exposed: corrode• Surface wear: corrosion & metal• Buildup of fretting debris• Surfaces separated, resistance changes

28

Connector Issues• Connector Insertion Force: Springs ⇒ relaxation• Healing mechanisms: breaking/penetrating films

• Contact force: fractures• Wipe: fractures & displaces• Fritting: Joule heat from small filaments through

film softens/melts metal; plastic deformationsbrings electrodes closer & breaks film.

• Lubricants: reduces wear & seals off corrosiveagents

• Platings: Corrosion ion migration barriers• Au, Pt, Ni, Cu

base metal

platingpores

s

• Micro-connectors• Accelerated testing & aging:

Mixed gases & vibrations simulateBatelle & IBM tests

29

Sliding Electrical ContactsFunction, design, & purpose: Transfer charge (collectcurrent) across moving interface.

• BrushesDC & AC motorsSlip rings

Picture: Reliance website( www.reliance.com)

• Brushes with commutator bars connect acrossinterface

30

Carbon Brushes

Carbon Brush Commutator

Pictures: Morganite website (www.morganite.com)

Operation• Carbon graphite electrically conductive• Sliding: graphite film deposits on metal• Humidity ⇒ water layer on graphite• Self lubricating

Problems• Excessive Wear

Hot-spotting/thermal moundingBrush dusting/low humidity lubricationCarbon graphite: water lubrication mechanism

• Arcing across commutator bars

31

Hot-Spotting/Thermal Moundingslider

countersurface

• contact between rough surfaces ⇒ discrete spots (islands) of contact between surfaces

slider

countersurface

disconnecte d

expanded expanded

• sliding commences, @ higher sliding speeds

friction & electrical heats @ contact spotsspots expand & growsome spots separateloads transferred to still-connected spots

32

slider

countersurface

disconnecte d

expanded expanded

disconnecte d

• loads transferred to still-connected spots

• more intense conditions

• process continues until slider runs on few spots

• spots fragile, break off

• large wear particles ⇒ extreme wear

33

Research Issues• Attain very high current densities (> MA/cm2 )

Low wearLow frictionReasonable temperatures

• Possible GeometriesSolid brushesFiber brushes

• Liquid metalsNo contact resistanceConductive lubricantPoisonous: no leakage!NaKEutectics

• Materials selection: conductive & low frictionmetal graphites: silver, copperother materials?

34

References

Ragnar Holm, Electric Contacts Handbook, 3rd ed.,Berlin, Springer-Verlag, 1958.

International Conference on Electric Contacts, everyother year, 1960-present.

Proceedings of the (IEEE) Holm Conference onElectric Contacts, annually, 1954-present.

IEEE Transactions on Components, Hybrids, &Manufacturing Technology