nanotribology labnc state micro-electro-mechanical systems: these squeaky wheels will get no grease...
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Nanotribology Lab NC State
Micro-Electro-Mechanical Systems:
These Squeaky Wheels will get no Grease
Jacqueline Krim, Department of Physics
Iron on copperTemp. = 4 K
M.F. Crommie, C.P. Lutz, D.M. Eigler, E.J. Heller.,Surf. Rev. and Lett. 2 (1), 127-137 (1995)
U. Kunze and B. Klehn,Adv. Mat. 11, 1473 (1999)
R. Overney and E. Meyer,MRS Bulletin, May 1993, p. 26.
Polymer mask over SiO2 film on Si,etched in HF.
50 nm line widths
Flourocarbon / Hydrocarbon mixtures.
70 nm line widths
Molecular Machines: Future or Fancy?
Atomic Scale Engines:Cars and Wheels
M. Porto et al., PRL 84, 1608 (2000)
Every week
• Apply a few drops of engine oil to the spark and throttle cross-shaft brackets
• Apply sufficient amounts of engine oil to all brake clevised, oiler, and cross-shaft brackets, at least 12 locations
• Force a “grease gun full”(half cup) of grease into the universal joint
• Pack the ball joints of the steering mechanism with grease more…..
Every Day
• Check Oil in Engine, oil lubricated clutch, tranmission, and differential gear housing
• Turn grease cup caps on the 8 spring bolts, one turn
• Apply a few drops of engine oil to the tie rod clevises
• Turn the grease cup on the fan support
• …….more
1916 Maxwell Owner’s Manual
2000 miles
• Drain rear axle, flush with kerosene and refill.
• Drain crank case, flush with kerosene, and refill (several quarts)
• Jack up car by the frame, pry spring leaves apart, and insert graphite grease between the leaves.
Monthly
• Force a “grease gun full” of grease into the engine timing gear.
• Force a “grease gun full” of grease into the steering gear case.
• Apply a few drops of 3-in-1 oil to the magneto bearing.
• Turn the grease cup on the generator drive shaft, one turn.
• Turn the grease cup on the drive shaft bearing, one turn.
• ……more
1916 Maxwell Owner’s Manual
Regularly • Check engine valve action• Inspect ignition wiring.• Check battery fluid level and
color.• Inspect cooling system for
leaks.• Check fan belt tension.• Inspect steering parts.• Tighten body and fender
bolts.• Check effectiveness of
brakes.
• more…..
Biweekly
• Check engine compression.
• Listen for crankshaft bearing noises.
• Clean and regap spark plugs.
• Adjust carburetor mixture.
• Clean gasoline strainer.
• Drain water from carburetor bowl .
• Inspect springs.• More….
1916 Maxwell Owner’s Manual
MicroElectro Mechanical Systems
Advantages: Mass-fabrication, low-cost and IC integration
Application : Whole new line of applications, limited only by imagination
• MEMS Microsurgery devices
• Miniature valves, pumps
• MEMS accelerometer used in Airbags
An emerging cutting-edge technology which relies on microfabrication of small scale IC compatible mechanical components
Photo Courtesy M. Adrian Michalicek, University of Colorado at Boulder
MEMS Application
Science News, July 22, 2000
Surface effects that dominate bulk effectsHigh temperature processing conditionsLubricant deliveryLubricant replenishment
Tribological issues related:
• Stiction (Release and/or In-use)
• Friction and wear
Stiction: Unintentional adhesion of microstructure surfaces where the restoring forces are unable to overcome interfacial forces
MEMS Tribology Issues
Release related stiction
• Caused mainly by liquid capillary forces
Adhesion of micromachined structures to the underlying substrate after the final sacrificial layer etch
Photo Courtesy University of California at Berkley
Approaches to solving release related stiction
• Self-assembled monolayer (SAMs) OTS, FDTS, DDMS
- Capillary pull can be made into a push if the contact angle is made larger than 90 °
- Due to hydrophobicity of these coatings, capillary forces responsible for release-related stiction are eliminated
Photo Courtesy University of California at Berkley
• Supercritical carbon dioxide drying of Microstructures
• Freeze sublimation drying
- Avoidance of liquid-vapor interfaces through supercritical fluid
- Allows samples to be dried without any surface tension, thus reducing the likelihood of stiction
Alternate release methods
• developing stiction reducing chemical additives for final rinse stages
• development of vapor-phase lubricants for use in extreme MEMS operating environments
• A knowledge of nanotribology is required, as contact areas may include only tens of atoms!
Permanent adhesion through acceleration or electrostatic forces and/or adhesive forces between surfaces causing permanent device failure
Approaches towards solving Stiction/Friction
In-use stiction and/or Friction
Nanotribology Lab NC State
Nanotribology and the Atomic-Scale Origins of Friction:
What Once Was Old Is New Again.
m
mg FN
v
Ff = FN
m
v
Amontons, 1699
F = ma
Newton, 1686
F
Ff
m
mg FN
v
1, 2 Guillaume Amontons, 1699 3 Charles-Augustin de Coulomb, 1785
Ff
Classical Laws of Friction:
1) Ff = FN
2) independent of apparent contact area3) independent of sliding speed
depends on whether object is at rest or moving - “static friction” vs. “kinetic friction”.
s k
Meanwhile, for solid-liquid interfaces, “viscous friction” applies,
where, vm
F
Leonardo da VinciCodex AtlanticusCodex Arundelca. 1500k = 0.25
Charles-Augustin de CoulombThéorie des Machines Simple1785
• QCM: unconfined geometry, “viscous friction”, no static friction
• SFA: Confined “planar” geometry, higher friction levels, “barrier to induce motion” always observed
• LFM: Confined “point” geometry, highest friction levels, static friction always observed.
J. Krim, Scientific American, Oct. 1996.
vm
F
SFA Measurement
Compared toluene on mica to C60/toluene solution on mica.Found that C60 formed 1-2 monolayers on the mica--and these adsorbed layers “possess unusually high fluidity and are easily disrupted.”Found that the viscous response of the fluid near the mica surface was completely different for the C60/toluene solution as compared to the toluene alone. The C60 toluene solution exhibited full-slip boundary conditions. Does this imply it will be a good additive to lubricants?
Toluene alone C60/Toluene Solution
S.E. Campbell, G. Luengo, V.I. Srdanov, F. Wudl, and J.N. Israelachvili, Nature, 382, 520-522 (1996).
-1.0 -0.5 0.0 0.5 1.0-1.0
-0.8
-0.6
-0.4
-0.2
0.0
Fri
ctio
na
l Fo
rce
(vo
lts)
Normal Force (volts)
Toluene on Mica C
60/Toluene Solution on Mica
AFM Measurements
Scan speed: 500 nm/sec with silicon nitride cantilever10% C60/Toluene solution
AFM Measurements
mica under toluene, force error image
mica under toluene, lateral force image
mica under ~20% C60/toluene solution, force error image
60 angstroms
50 angstroms
Ag(111)
Ag(111)
Cu(111)
Ag(111) Control
0 HzWhen C60 molecules form a monolayer on Cu(111), the molecules lock in to a specific direction on the terraces and the free rotation is suppressed.T. Sakurai et al, Applied Surface Science 87/88 (1995) 405-413.
109 HzFor Ag(111), the C60 molecules in the second layer rotate at frequencies matching that of bulk C60. E.I. Altman
and R.J. Colton, Surface Science 295 (1993) 13-33
< 1 HzFor Ag(111), the C60 molecules in the first monolayer do rotate, but slowly. E.I. Altman and R.J. Colton, Surface Science 295 (1993) 13-33
C60 Rotation
AFM Results
The films were evaporated in UHV conditions onto freshly cleaved mica surfaces. They were then transferred to a liquid cell and completely submerged in methanol for the AFM measurements. The measurements were acquired under methanol in order to avoid capillary effects.
Quartz Crystal Microbalance
Single crystal quartz
Metal film electrode
A.
B.
Figure 8
Measuring friction with a quartz crystal microbalance (QCM)
• Thin crystal disk oscillates in a shear mode
• Adsorbed material lowers the resonant frequency
• If the shear stress is below about 103 N/m2, it will “slip” enough to be detected by the QCM:
• The slip time is deduced from Q and f:fQ 4)( 1
)()( 11 AQ
(Krim and Widom, PRB, v. 38, n.17, 1988)
QCM Results
Toluene on Ag
Toluene on C60/Ag
QCM Results
Toluene on Ag
Toluene on Ag/C60
Here, we find that C60 is sticky, while toluene is slippery.
Pre
ssur
e, M
Pa
Velocity, mm/s0 50 100 150 200 250 800 1000
0
200
400
600
800
1000
AFM/IFM
SFA
micromachines
atomisticsimulation
microenginespeed record
STM-QCMP = 0 - 1000 GPaV = 100 - 3000 mm/sContact radius = 10 nm - 1 m
Proceedings of NIST Nanotribology WorkshopGaithersburg, MD March 13-15, 2000
“Existing molecular scale test methods do not duplicate the operating P-V space of micromachines” -- M.T. Dugger, Sandia Labs
They also do not duplicate the operatingP-V space of macroscopic machines….
Alternate approaches are required to study MEMS
lubricants.“Existing molecular scale test methods do not duplicate the operating P-V space of micromachines” -- M.T. Dugger, Sandia Labs
Pre
ssur
e, M
Pa
Velocity, mm/s0 50 100 150 200 250 800 1000
0
200
400
600
800
1000
AFM/IFM
SFA
micromachines
atomisticsimulation
microenginespeed record
STM-QCMP = 0 - 1000 GPaV = 100 - 3000 mm/sContact radius = 10 nm - 1 m
Proceedings of NIST Nanotribology WorkshopGaithersburg, MD March 13-15, 2000
Surface micromachined device to investigate friction & wear
Photo Courtesy University of California at Berkley
Comb-drive
Photo courtesy Sandia National Laboratories
MEMS Friction tester Diagram
05
1015202530354045
0 1 2 3 4 5 6
Time
Am
plit
ude
(um
)
Vacuum system used to release the vapor-phase lubricants
Why vapor-phase lubricants?
• semiconductor-like fabrication of MEMS devices
• small size
• monolithic nature of micromachines
Difficulties in lubricating MEMS devices because of
Vapor phase may ultimately prove to be an effective and perhaps exclusive means to deliver and/or replenish lubricants
Current focus on:
• Development of realistic laboratory test set-ups which are both well controlled and relevant to operating machinery
• Understanding the chemical and tribochemical reaction which occur in sliding contact
• Characterization of the microstructural and mechanical properties of the micromachined contact region
Quartz Crystal Microbalance
Scanning Tunneling Microscope
STM-QCM
quartz disk
metal electrodesfilm
metal tip
500 × 500 nm2
STM tip
TunnelingcurrentMetal electrode
Quartz
stationary vibrating
Desirable Properties of a MEMS lubricant
Low friction Low wear Effective as very thin film Uniform adhesion to substrate Durable and Replenishable Specificity Usable in extreme environments (temperature,
pressure)
TCP is known to exhibit many of these properties in macroscopic tests. (downside: possible corrosion)
QUESTION: Does it exhibit these same favorable properties in nanometer-scale tests using STM-QCM?
P
O
C
H
TCP in purified form
3D-TRICRESYL PHOSPHATE (TCP)
P
O
C
H
Atomic-scale studies of an anti-wear additive proven effective in extreme
environments of high temperature and pressure.
Experimental System
Quartz Crystal Microbalance
(QCM)
TCP vapor
Metal substrate
TCP film
0 10 20 30 40 50 60 700
10
20
30
40
50
60
-F
(Hz)
0 10 20 30 40 50 60 700.000.020.040.060.080.100.120.14
(1/
A)
(v-1)
10-7 *
(1
/Q) fi
lm
Time (min)
0
50
100
150
200
Mas
sD
epo
site
d (
ng
)
02468101214
TRICRESYL PHOSPHATE (TCP)/Fe
M. Abdelmaksoud, J. Bender and J. Krim, Trib. Lett. submitted
O
C
P
H
0.0 0.5 1.0 1.5 2.05E-10
1E-9
2E-9
3E-94E-95E-9
1E-8
2E-8
3E-84E-85E-8
S
lip T
ime
(s)
Coverage (monolayers)
TCP slip times are comparable to those of physisorbed monolayers!
O
C
P
H
0 2 4 6 8 10 12 14
0
200
400
600
TCP/Fe at 3000C
TCP/Cr at 3000C
O2 uptake
on TCP exposed surfaces
-F
(Hz)
0 2 4 6 8 10 12 14
0
20
40
60
80
10-7 *
(1
/Q) fil
m
Time (min)
TCP at high temperature: Polymeric materialFormation observed in combination with
oxygen gas uptake, but only for iron substrates.
How does this film respond to tribological contact?
STM-QCM of 10 Å TBPP film
200 × 200 nm2
QCM OFF
QCM ON
Room Temperature
Liquid TBPP cannot diffuse back into the rubbed region faster than the QCM vibrational speed. Therefore, an image can be obtained when the QCM is vibrating.
STM-QCM of 10 Å TBPP film
40 × 40 nm2
After Heating
F = + 0.6 Hz
After annealing, the polymeric surface is more conducting , and ‘nonrigid’.
STM-QCM of 10 Å TBPP film
40 × 40 nm2
QCM OFF
QCM ON
After annealing
Images from article by Peter Weiss in Science News, July 22, 2000
F = 0.0 Hz
F = + 2.3 Hz
After rubbing, the rubbed region exhibits lower friction, and evidence for a tribochemical reaction.
Frequency shift data for 10 Å film of TBPP
0 10 20 30 40 50 60 70 80 90100-80-60-40-20
020406080
100120140160
F (
Hz)
Normal Load (a.u.)
Lubricated contact Lubricated and annealed contact Unlubricated contact
Molecularly thin films dramatically change interfacial properties.
Films are tenacious, durable, yet very thin.
Best performance in extreme environments - high temperature and pressure
~100 GPa
Conclusions• A good MEMS lubricant exhibits high
flexibility, very low friction and high adhesion.
• Static friction (and the closely related phenomenon of stick-slip) is largely associated with confined (like a sandwich) geometries
• Static friction can be totally absent in an unconfined geometry at both microscopic and macroscopic scales, and for both solid-solid and solid-liquid interfaces.
• Sliding friction has been observed to increase by five orders of magnitude in going from an unconfined to confined geometry
Nanotribology Lab NC State
Micro-Electro-Mechanical Systems:
These Squeaky Wheels will get no Grease
Jacqueline Krim, Department of Physics
The end !! *
*On the QCM-STM test setup
Bucky Ball Background
C60 Trivia• an aromatic molecule
• molecular diameter: 0.71 nm• forms an FCC lattice with• nearest neighbor distance
of 1.003 nm• a blackish powder, dissolvesreadily in toluene and benzene
• sublimes readily at 450 C
Some Previous Work
B. Bhushan et al., Appl. Phys. Lett., 1993.ExperimentThey coated silicon with C60 in vacuum. They performed friction andwear tests with a ball on flat tribometer under reciprocating motion. Thefriction force was measured with strain gauges.ResultsThe C60 did reduce the coefficient of friction, but the films scratched offat high load.
T. Thundat et al., Appl. Phys. Lett., 1993.ExperimentThey sublimed C60 onto silicon wafers and cleaved mica. They acquired AFM images and LFM. ResultsThey found a higher coefficient of friction for C60 on mica (0.9) than for bare mica (0.1). They found that for high forces the tip pushed through the fullerene layers.