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.
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