Tribology Issues in Electrical Contacts

M.D. Bryant Mechanical Engineering The University of Texas at Austin Austin, Texas 78712-1063

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

Roadmap
Introduction Define electrical contacts Review: Contact principles Review: Electrical concepts Electrical contact physics Selected problem areas Connectors Sliding contacts/brushes Switches and relays

Electrical Contacts Review

Definition: Ragnar Holm, Electric Contacts, Theory and Application, 4th edition, Springer-Verlag, New York, 1981. electrical contact .. a releasable junction between two conductors which is apt to carry electric current. Purpose: Transfer charge across a mechanical interface between conductors. Electric contacts permit frequent and convenient connection and disconnection of circuits. Common Applications Switches & Relays Connectors & Plugs Sliding contacts: brushes Background = All areas of classical physics: Tribology + Electrical Engineering + Chemistry + Materials + Mechanics + Quantum effects

Electrical Contact Make

Press bodies together Contact pressures & area Voltage difference current Charge carriers jump interfacial gap Contact impedance (resistance) depends on Real contact area materials films

lines of current

conductor 1

conductor 2

Electrical Contact Requirements

Electrical Conductive Negligible effect on circuit: small impedance Stable: no impedance fluctuations Mechanical Contact stays together Compact Chemically & thermally stable Cost effective

lines of current

conductor 1

conductor 2

Contact Principles
HERTZIAN THEORY
p(x, y) = po a2 - x2 - y2
P p(x,y) Zf (x, y) 2a P

3P po = 2 a2 3P(k1+k2)R1R2 a= 4(R1+R2)

1 3

1- i2 ki = E i = 1 3

92P2(k1+k2)2(R1+R2) 16R1R2

PLASTIC CONTACT THEORY

Indentation (Meyer) hardness
P

p
A

P
Load P > elastic limit plastic deformations

Bodies in contact

Contact pressures p(x, y) approximately uniform Hardness pressure (indentation hardness) P p A H 3 x Yield stress

Use: estimate contact area, given H and P

OVERALL CONTACT MODEL

P

P p(x,y)

2
P

2a P

Spheres Increasing normal load P 0 P < Pe ; = 1+2 = P Pe ; = 1+2 > Elastic (Hertzian) contact model 1 92P2(k1+k2)2(R1+R2) 3 16R1R2

Plastic contact model

92Pe2(k1+k2)2(R1+R2) 16R1R2

1 3

Similar formulations, tangential loads & deformations

Electrical Concepts
Electric field: force per charge [N C1 = 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: eHoles: p+ Conduction mechanisms Free electrons e- in metals Barrier gap/work function potential gap e- jumps gap if sufficient kinetic energy V < gap suggests no current Tunnel effect: distribution of energies over population of charge carriers, some have enough kinetic to jump
kinetic energy distribution

energy per charge

gap V distance

electrode 1

gap

electrode 2

gap kinetic energy: eV

Current density: [A m-2] Impedance: Z = V/I, resistance to current flow Z=R+jX Resistance: R = l/A [ohm = V A-1] Resistivity: [m] Area: A [m2] Length:

[m]

Reactance: X from Inductance L [henry H = s] ZL = j L Capacitance C [farad F = C V-1 = -1 s] ZC = 1/j C

Constriction Resistance

r 2a

r + dr

Lines of current constrict near contact Water draining from bathtub: constriction impedes flow Generates resistance R = dr/A(r) = /(2 r2 ) dr /2 (1/a 1/r ) : bulk resistivity of body; a: contact radius Neglects volume r < a Actual Constriction Resistance: Rc = /2a Contact radius a via mechanical contact

Surface Films
body 1 film body 2 Thin films: nm to m Thickness dependent resistivity: f = f(s) Film formed via contaminant diffusion & corrosion Mechanical Effects of film negligible Film Resistance Rf = f/A ds Tunnel effect can dominate f & Rf

Continental Analogy of Contact

Rough surfaces: earth & electrical contacts North America
Rocky Mountains

Invert South America, place atop North America

Appalachian Mountains

South America
Andes Mountains
Brazil Highlands

Contact: highest peaks against highest peaks Andes/Appalachia Highlands/Rockies

Small, discrete contact areas at highest peaks

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Analogous to bodies in contact Rough surfaces Small, discrete contact areas (a-spots) parallel circuits & micro-constrictions Pores in contact allows easy contaminant diffusion

global constriction contact area with micro-contacts rough surfaces

a-spot

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Contact Impedance
Zc = Rc1 + jLc1 + 1/( Rm1 ) + 1/ ( 1/Rf + jC) + Rc2 + jLc2 + 1/( Rm1 ) Rc = Rc1 + 1/( Rm1 ) + Rf + Rc2 + 1/( Rm1 )
lines of current

global Rc constriction rough contact microconstrictions Rf Cf surface film effects

Inductance & capacitance effects small @ low frequency

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Joule Heating
Contact resistance dissipates power: P = i2 Rc Heat sources Field equations 1 Electrical: J = [ (T) V] = 0 (Laplace) e T Thermal: [k(T) ]+ q = Cp(T) t (Fourier) 1 q = qe = J E = e V V
2 2.0 2 1750 1.6 z/a 1 z/a 1.2 0.8 0.4 0 0 1 x/a 2 0 0 1 x/a 2 1500 1000 1 2000 2000 1500

Equipotential contours and isotherms for electrical heating of carbon graphite with a cold contact boundary with a = 1 mm. Applied voltage: 2 V.

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Polarization Effects
DC voltage Bodies become anode and cathode Affects Chemistry Material behavior Arcing Examples Cathode brush wear > anode brush wear Graphite sticks to cathode Selective film formation on anode/cathode Affects contact resistance Anode/cathode arc erosion different

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Switches & Relays

Function, design, & purpose: frequent and rapid opening & closing of circuits. Often contains two metallic contactor parts. Switches / Relays / Reed Switches

Pictures: Eaton website (www.eaton.com)

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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: inductances induce air ions

arc current flows via ions from air

 Destructive: melts, welds & erodes electrodes Material transfer: anode - cathode Segregation & recrystallization Surface roughening

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

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Elevated & sporadic contact resistance

Chart: Z. Chen & K. Sawa, 1994, Polarity Effect of Unsymmetrical Material Combination 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 _

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Research Issues/Design Problems

Arc suppression Guide arc path Control contact bounce: repeated arcing Materials selection: powdered Cu-Cr Ni-CdO Ag-CdO Ag-SnO2 Ag-SnO2In2O3 Ag-MeO

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Connectors
Function, design, & purpose: permanent but quick connections/disconnects 5-40 year stable life Connector classes power connectors: power lines & junction boxes automotive electronic connectors: low power & compact Telephones electronics computers

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Power connectors: Aluminium

Lightweight Economic Conductive Form passivating films Problem: stress relaxation @ higher temperatures loss of contact area higher Rc risk: fires Examples: wire nuts, wedge connectors, clamps

Photo: J. J Schindler, R.T. Axon & R.S. Timsit, 1995, Mechanical and Electrical Contact Properties of Wedge Connectors, Proc. 41st IEEE Holm Conference, pp. 1-9.

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Electronic Connectors
Low power: contamination problems dust pollutants corrosive agents fretting corrosion IBM, late 1980s: connector problems cause 50% of all computer failures
Pictures: Molex website (www.molex.com)

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Fretting Corrosion
100

10 Contact resistance (m) 1

Surface films broken

Stable

Increase with fluctuations

Drastic Increase

Stage 1

Stage 2

Stage 3

100

102

104

106

fretting time (cycles)

Elevated & fluctuating contact resistance Elevated resistance: digital high low, computer failure Fluctuations noise to signals

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Fretting Mechanism
Aex p osed(m )
hm 1 to 100 nm virgin asperities mth cycle: motions expose a-spots corrosive layers form

Micro-motions @ contact Stray vibrations Thermal expansions via temperature variation Normal & tangential motions Surfaces exposed: corrode Surface wear: corrosion & metal Buildup of fretting debris Surfaces separated, resistance changes

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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 deformations brings electrodes closer & breaks film. Lubricants: reduces wear & seals off corrosive agents Platings: Corrosion ion migration barriers Au, Pt, Ni, Cu
pores plating

Connector Issues

base metal
s

Micro-connectors Accelerated testing & aging: Mixed gases & vibrations simulate Batelle & IBM tests

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Sliding Electrical Contacts

Function, design, & purpose: Transfer charge (collect current) across moving interface.

Brushes
DC & AC motors Slip rings

Picture: Reliance website( www.reliance.com)

Brushes with commutator bars connect across interface

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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 mounding Brush dusting/low humidity lubrication Carbon graphite: water lubrication mechanism Arcing across commutator bars

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Hot-Spotting/Thermal Mounding
slider

countersurface

contact between rough surfaces discrete spots (islands) of contact between surfaces
slider

expanded disconnecte d

expanded

countersurface

sliding commences, @ higher sliding speeds friction & electrical heats @ contact spots spots expand & grow some spots separate loads transferred to still-connected spots

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slider

expanded disconnecte d

expanded

disconnecte d

countersurface

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

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Research Issues
Attain very high current densities (> MA/cm2 ) Low wear Low friction Reasonable temperatures Possible Geometries Solid brushes Fiber brushes Liquid metals No contact resistance Conductive lubricant Poisonous: no leakage! NaK Eutectics Materials selection: conductive & low friction metal graphites: silver, copper other materials?

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References
Ragnar Holm, Electric Contacts Handbook, 3rd ed., Berlin, Springer-Verlag, 1958. International Conference on Electric Contacts, every other year, 1960-present. Proceedings of the (IEEE) Holm Conference on Electric Contacts, annually, 1954-present. IEEE Transactions on Components, Hybrids, & Manufacturing Technology