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Nanotribology LabNC State Micro-Electro-Mechanical Systems: These Squeaky Wheels will get no Grease Jacqueline Krim, Department of Physics.

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Presentation on theme: "Nanotribology LabNC State Micro-Electro-Mechanical Systems: These Squeaky Wheels will get no Grease Jacqueline Krim, Department of Physics."— Presentation transcript:

1 Nanotribology LabNC State Micro-Electro-Mechanical Systems: These Squeaky Wheels will get no Grease Jacqueline Krim, Department of Physics

2 Iron on copper Temp. = 4 K M.F. Crommie, C.P. Lutz, D.M. Eigler, E.J. Heller., Surf. Rev. and Lett. 2 (1), (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 SiO 2 film on Si, etched in HF. 50 nm line widths Flourocarbon / Hydrocarbon mixtures. 70 nm line widths

3 Molecular Machines: Future or Fancy?

4 Atomic Scale Engines: Cars and Wheels M. Porto et al., PRL 84, 1608 (2000)

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6 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

7 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

8 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… Maxwell Owner’s Manual

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10 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

11 Photo Courtesy M. Adrian Michalicek, University of Colorado at Boulder MEMS Application

12 Science News, July 22, 2000

13 Surface effects that dominate bulk effects High temperature processing conditions Lubricant delivery Lubricant 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

14 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

15 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

16 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

17 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

18 Nanotribology LabNC State Nanotribology and the Atomic-Scale Origins of Friction: What Once Was Old Is New Again. m mgFNFN v F f =  F N m v Amontons, 1699 F = ma Newton, 1686 F FfFf

19 m mgFNFN v 1, 2 Guillaume Amontons, Charles-Augustin de Coulomb, 1785 FfFf Classical Laws of Friction: 1) F f =  F N 2)  independent of apparent contact area 3)  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,

20 Leonardo da Vinci Codex Atlanticus Codex Arundel ca  k = 0.25 Charles-Augustin de Coulomb Théorie des Machines Simple 1785

21 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

22 SFA Measurement Compared toluene on mica to C 60 /toluene solution on mica. Found that C 60 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 C 60 /toluene solution as compared to the toluene alone. The C 60 toluene solution exhibited full- slip boundary conditions. Does this imply it will be a good additive to lubricants? Toluene aloneC 60 /Toluene Solution S.E. Campbell, G. Luengo, V.I. Srdanov, F. Wudl, and J.N. Israelachvili, Nature, 382, (1996).

23 AFM Measurements Scan speed: 500 nm/sec with silicon nitride cantilever 10% C 60 /Toluene solution

24 AFM Measurements mica under toluene, force error image mica under toluene, lateral force image mica under ~20% C 60 /toluene solution, force error image 60 angstroms 50 angstroms

25 Ag(111) Cu(111) Ag(111) Control 0 Hz When C 60 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) Hz For Ag(111), the C 60 molecules in the second layer rotate at frequencies matching that of bulk C 60. E.I. Altman and R.J. Colton, Surface Science 295 (1993) < 1 Hz For Ag(111), the C 60 molecules in the first monolayer do rotate, but slowly. E.I. Altman and R.J. Colton, Surface Science 295 (1993) C 60 Rotation

26 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.

27 Quartz Crystal Microbalance Single crystal quartz Metal film electrode

28 A. B. Figure 8

29 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 10 3 N/m 2, it will “slip” enough to be detected by the QCM: The slip time is deduced from Q and f: (Krim and Widom, PRB, v. 38, n.17, 1988)

30 QCM Results Toluene on Ag Toluene on C 60 /Ag

31 QCM Results Toluene on Ag Toluene on Ag/C 60 Here, we find that C 60 is sticky, while toluene is slippery.

32 Pressure, MPa Velocity, mm/s AFM/IFM SFA micromachines atomistic simulation microengine speed record STM-QCM P = GPa V = mm/s Contact radius = 10 nm - 1  m Proceedings of NIST Nanotribology Workshop Gaithersburg, 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 operating P-V space of macroscopic machines….

33 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 Pressure, MPa Velocity, mm/s AFM/IFM SFA micromachines atomistic simulation microengine speed record STM-QCM P = GPa V = mm/s Contact radius = 10 nm - 1  m Proceedings of NIST Nanotribology Workshop Gaithersburg, MD March 13-15, 2000

34 Surface micromachined device to investigate friction & wear Photo Courtesy University of California at Berkley

35 Comb-drive Photo courtesy Sandia National Laboratories

36 MEMS Friction tester Diagram

37 Vacuum system used to release the vapor-phase lubricants

38 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

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40 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

41 Quartz Crystal Microbalance Scanning Tunneling Microscope STM-QCM quartz disk metal electrodes film metal tip

42 500 × 500 nm 2 STM tip Tunneling current Metal electrode Quartz stationary vibrating

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44 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

45 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

46 TRICRESYL PHOSPHATE (TCP)/Fe M. Abdelmaksoud, J. Bender and J. Krim, Trib. Lett. submitted OCPHOCPH

47 TCP slip times are comparable to those of physisorbed monolayers! OCPHOCPH

48 TCP at high temperature: Polymeric material Formation observed in combination with oxygen gas uptake, but only for iron substrates. How does this film respond to tribological contact?

49 STM-QCM of 10 Å TBPP film 200 × 200 nm 2 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.

50 STM-QCM of 10 Å TBPP film 40 × 40 nm 2 After Heating  F = Hz After annealing, the polymeric surface is more conducting, and ‘nonrigid’.

51 STM-QCM of 10 Å TBPP film 40 × 40 nm 2 QCM OFF QCM ON After annealing Images from article by Peter Weiss in Science News, July 22, 2000  F = 0.0 Hz  F = Hz After rubbing, the rubbed region exhibits lower friction, and evidence for a tribochemical reaction.

52 Frequency shift data for 10 Å film of TBPP  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

53 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

54 Nanotribology LabNC State Micro-Electro-Mechanical Systems: These Squeaky Wheels will get no Grease Jacqueline Krim, Department of Physics

55 The end !! * *On the QCM-STM test setup

56 Bucky Ball Background C 60 Trivia an aromatic molecule molecular diameter: 0.71 nm forms an FCC lattice with nearest neighbor distance of nm a blackish powder, dissolves readily in toluene and benzene sublimes readily at 450 C Some Previous Work B. Bhushan et al., Appl. Phys. Lett., Experiment They coated silicon with C 60 in vacuum. They performed friction and wear tests with a ball on flat tribometer under reciprocating motion. The friction force was measured with strain gauges. Results The C 60 did reduce the coefficient of friction, but the films scratched off at high load. T. Thundat et al., Appl. Phys. Lett., Experiment They sublimed C 60 onto silicon wafers and cleaved mica. They acquired AFM images and LFM. Results They found a higher coefficient of friction for C 60 on mica (0.9) than for bare mica (0.1). They found that for high forces the tip pushed through the fullerene layers.


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