electric contacts
TRANSCRIPT
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