Post on 15-Apr-2017
Friction Force and its Relationship to the Electrostatic Charges at Interfaces
Dr. Thiago A L Burgo
Outline
Unicamp • Friction coefficient induced
by electrostatic charges – Coefficient of rolling
resistance x Electrostatic potential (EP)
– Friction angle x EP – Friction at nanoscale
• Lateral Force Microscopy (LFM)
Argonne Na/onal Laboratory • Tribocurrent and
macroscopic friction force – Dependence on the
atmosphere – Friction force fluctuations – Electrostatic adhesion
• Nanomechanical Mapping on tested PTFE
– Triboemission
• Concluding Remarks
• Electrostatic Discharge (ESD) and Technological Challenges
Explosions triggered by electrostatic discharge
https://www.youtube.com/watch?v=6lKUsUycBNA&spfreload=10
www.firesciencetools .com
In October 2013, a fire of great proportions hit Copersucar facilities.
• For nanotechnology – Electrostatic force is even
larger than the inertial force, for micromachine parts made of insulators.
– The electrification of the
insulator is not well understood, especially at the micro-scale.
• Safety and technologies
– Dust explosions – Fires – Pharmaceuticals – Polymer recycling – Electrospinning – Solid paint – Electrocopying – Toner
Consequences
Challenges Faced By Solar Energy Use
Calle et al. Active dust control and mitigation technology for lunar and Martian exploration. Acta Astronautica 69, 1082-1088 (2011).
Electrodynamic Repulsion
Courtesy of Carlos Calle, Electrostatics and Surface Physics Laboratory - NASA
Surfaces under rela<ve mo<on: Triboplasma
Heinicke G. Tribochemistry. (1984) Matta et al. J. Phys. D: Appl. Phys. (2009) Camara, et al. Nature (2008) Burgo, et al. Polym. Degrad. Stabil. (2014)
is altered, and loose wear particles are generated. All theseseemingly random and complex phenomena follow a certainorder and satisfy the laws of nature, as explained by Suh [3].Not only such variables as sliding velocity, normal load,material hardness, but also the environment of hydrocarbonsunder boundary lubrication conditions [4,5], are correlatedwith triboemission charge intensities. According toNakayama and Hashimoto [5], triboemission of bothnegatively and positively charged particles was measuredduring the scratching of various metals, ceramics, andpolymers under boundary lubrication conditions withsaturated hydrocarbons.
Presently, the triboemission process should be consideredalong with micro-triboplasma, recently discovered byNakayama and Nevshupa [6], because it is mostly composedof UV photons and exoelectrons. According to the mostrecent Nevshupa’s paper [7], mechanoemission is anensemble of emission phenomena resulting frommechanicalaction between solids. Study of mechanoemission is anintricate problem since it includes a multitude of mechanicalactions and a multitude of emission phenomena of variousphysical natures, which may vary depending on the ambientconditions and type of materials. Mechanoemission strictlyrelates tomechanochemistry defined as a branch of chemistrydealing with the chemical and physicochemical changes ofsubstances of all states of aggregation due to the influence ofmechanical energy [8]. Historically, the termmechanochem-istry was coined by Ostwald for this field of research and themechanoemission has been a subject of physics andchemistry, while studies were focused especially on theelectromagnetic aspects, i.e. emission of electrons, ions andelectromagnetic radiation. Many experimental and theore-tical studies of the mechanoemission have been done withdisperse systems using milling of materials, peeling out ofadhesive films as well as with plastically deformed andfractured materials like polymers, ionic and covalentcrystals, etc. According to [7], the following main types oftriboemission phenomena can be distinguished:
(i) emission of gas atoms and molecules includingemission of radicals and molecular clusters,
(ii) emission of electromagnetic radiation,(iii) emission of electrons,(iv) emission of ions,(v) emission of magnetic field,(vi) emission of electric field including emission of electric
charges and generation of tribocurrents,(vii) emission of noise, vibration and acoustic emission
(AE),(viii) emission of heat.
These emission types are shown in Fig. 1. Emission ofgas atoms and molecules at friction results from thecompetition gas release and gas adsorption processes.When at certain sliding conditions, the rate of gasadsorption exceeds the rate of gas release, total emission
rate becomes negative. Such emission of negative rate hasbeen called ‘anti-emission’.
By physical nature, triboemission phenomena areclassified into two classes: (1) emission of particles(‘corpuscular’), and (2) emission of energy (Fig. 2). Theparticle emission includes neutral particles (atoms, mol-ecules, radicals and clusters) and charged particles (elec-trons, negative ions and positive ions). Three main types ofenergy emission encompass: electromagnetic energy,mechanical energy and heat. Mechanical energy includesmechanical oscillations of various frequency ranges, i.e.vibration, noise, ultrasonic emission, acoustic emission, etc.Electromagnetic emission can be classified into static anddynamic. Static emission includes static electric andmagnetic fields, while dynamic emission includes electro-magnetic waves of various wavelengths, i.e. radio waves;IR, visible, UV light; and X-rays.
For many years, mechanoemission has been consideredalmost exclusively originating from high energetic ‘inter-intermediate excited states’ [7], which are formed in thematerial of mechanically affected solids due to breaking ofchemical bonds, i.e. due to plastic deformation, fracture,peeling of adhesive films, etc. [8–10]. The excited states areformed because the rate of the energy release exceeds therate of the energy dissipation. These excited states give riseto emission of electrons, ions, molecules, etc. Thementioned triboplasma [6] introduced a new activationmechanism that does not require initial breaking ofchemical bonds in the mechanically affected solids, asfrictional electrification can occur without breaking bonds,e.g. due to contact potential difference and thermoelectriceffect [11]. On the other hand, from the standpoint ofmechanoemission, friction is considered simplistically asplastic deformation and breaking of chemical and adhesivebonds.
Looking at the frictional contact ‘black box’, it is toassume that ‘high energy intermediate excited states’ ofmechanoemission should also include ‘flash temperature’.
Fig. 1. General scheme of triboemission [7].
C.K. Kajdas / Tribology International 38 (2005) 337–353338
How powerful is the triboplasma?
!
Camara et al. Correlation between nanosecond X-ray flashes and stick–slip friction in peeling tape. Nature, 455, 1089-1093 (2008).
X-Rays!
X-ray Fluorescence (XRF) technology
Tribogenics
What creates sta/c electricity?
Williams, M.W. What creates static electricity? American Scientist 100, 316-326 (2012)
• Complex and unexpected charge distribution in every material scanned by Kelvin electrodes (EFM, KFM/SEPM, Macro-Kelvin):
– Cardoso et al., Langmuir 1998, 1999 – Galembeck et al., Polymer 2001 – Gouveia et al., J. Phys. Chem B, 2005, 2008 – Soares et al., J. Braz. Chem. Soc. 2008 – Gouveia et al., J. Am. Chem. Soc. 2009 – Ducati et al., Langmuir 2010 – Bernardes et al., J. Phys. Chem. C 2010 – Burgo et al., J. Electrostatics 2011
Polymer Ethanol
2 hours
Clean Polymer
drying
Electrical poten<al scanning with Kelvin electrode
OR
EXPERIMENTAL
Balance
Polymer
P: 1,5 kPa RPM: 5000 Time: 3 s PE foam
Shaking table
Polymer
Glass spheres or pellets de PTFE Amount: 4 g
Time: 60 min
1-‐ Cleaning
2-‐ Charging
3-‐ Charge mapping
X
Y
Polymer
1 cm
Bipolar Segregated Domain
Formation free-radicals is followed by electron transfer: from the hydrocarbon free-radicals to the more electronegative fluorocarbon radicals. Ions are segregated due to the chain size, following Flory−Huggins theory and superseding weak electrostatic interactions between highly spaced charges.
!
Burgo et al. Triboelectricity: macroscopic charge patterns formed by self-arraying ions on polymer surfaces, Langmuir, 28(19), 7407-7416 (2012).
Francisco, K. R., Burgo, T. A. L., Galembeck, F. Tribocharged Polymer Surfaces: Solvent Effect on Pattern Formation and Modification. Chem. Lett. 41, 1256-1258 (2012).
• Procedure used to transfer charges from PTFE to LDPE using paraffin oil as transfer agent;
• Also, ethanol can be used to suppress charges on a previous tribocharged PTFE surface.
Charge transfer and electrostatic lithograph
Mo<va<on and hypothesis
• Triboeletrifica<on: glass beads over PE or PTFE;
• Fric<on generates surfaces with both posi<ve and nega<ve paWerns;
• Since, coulombian forces (long range) describe interac<ons between electrical charges, how these charges affect fric<on on electrified interfaces?
Tribology: science of friction • “The science and technology of interac<ng surfaces in rela<ve
mo<on and of associated subjects and prac<ces.” (Peter Jost, 1966);
• Amontons’ laws: – Fric,on is propor,onal to normal load – Independent of apparent contact area
Mate, C. M. Tribology on the small scale. Oxford University Press, 2008. Fall, et al. Sliding Friction on wet and dry sand. Phys. Rev. Lett. (2014).
Real contact area and adhesion • Connec<ons from micro to macro
scale is very difficult;
• Real x apparent contact area;
• Elasto-‐plas<c deforma<ons;
• Adhesion: van der Waals forces (only???): “…The primary obstacle to inclusion of sliding triboelectrifica<on into our model is the mysterious and complex nature of the process…”
Bowden, F. P. & Tabor, D. Friction and Lubrication. 2nd ed., Oxford (1954). Nakayama, K. Wear 194, 185-189 (1996). Ireland, P. M. J. Electrostat. 70, 524-531 (2012).
Mechanical contact: JKR, DMT and Maugis
• JKR: Adhesion forces change contact area
• DMT: Contact area remains the same, but with addi<onal aWrac<ve interac<ons
• Maugis: contact area is in between!!!
Johnson, K. L.; Kendall, K. & Roberts, A. D. Proc. R. Soc. London A (1971). Derjaguin, B. V.; Muller, V. M. & Toporov, Y. P. J. Colloid Interface Sci. (1975). Maugis, D. J. Colloid Interface Sci. (1992).
with adhesion aH aH
a a
without adhesion (Hertz)
van der Waals forces
…and the Coulombic contribu<on?
“…The primary obstacle to inclusion of sliding triboelectrification into our model is the mysterious and complex nature of the process…”
Ireland, P. M. J. Electrostat. (2012).
Coefficient of Rolling Resistance (CoRR) x Electrosta<c Poten<al
A"
B" C"h
d CoRR = h/d
glass beads
tribocharged PTFE
CoRR: silanized glass beads
a b
θ=25˚ θ=93˚• Rolling coefficients are strongly
modified by the surface silanitazion of glass.
θ = 15º θ = 93º
Movement restric<on
Movement is restricted on electrified interfaces!!!
Beads on top Beads withdrawn
Fric,on angle: PTFE x PE
• Triboelectrification between PTFE and PE increases friction angles
• Some PE pellets does not slide even at 90º. PTFE + PE pellets
after shaking
PTFE
PTFE + PE pellets before shaking
Polyethylene pellets PTFE
Fric,on angle: PTFE x PE PTFE + PE pellets
after shaking PTFE PTFE + PE pellets
before shaking
Fric<on at a microscopic level: Lateral Force Microscopy
• First verified by Mate et al.;
• AFM plahorm;
• Deflec<on signal is a qualita<ve measurement of fric<on.
http://www.doitpoms.ac.uk/tlplib/afm/lfm.php
α∆´ = (1+µ2)sinθcosθ cosθ2 − µ2 sin2θ
αW´ = µ cosθ2 − µ2 sin2θ
µ + = 2∆´
W´sin2θ 1 µ
LFM calibra<on: volts (V) to units of
force (N)
Ogletree, D. F., Carpick, R. W. & Salmeron M. Calibration of frictional forces in atomic force microscopy. Rev. Sci. Instrum. 67(9), 3298-3306 (1996).
Lateral force microscopy (LFM)
• Friction is largely affected by surface charges at the nanometer scale;
• Fractal dimension D of
friction signal is bigger than topography signal!!!
Force-‐distance curves (Fd) on tribocharged PTFE
In Geckos, each hair produces 100 nN (due to van der Waals and/or capillary interactions)!!! Geim, et al. Nat. Materials (2003). Burgo et al. Friction coefficient dependence on electrostatic tribocharging. Nature Sci. Rep. (2013).
Parcial Conclusions • Tribocharges produced by friction have a large effect
on the friction coefficients of dielectrics: – They may exceed all other factors for mechanical energy
dissipation;
• Controlling surface electrostatics should thus open the way to new approaches for controlling friction in many important systems and equipment.
• Since tribocharge paWerns are fractal, their contribu<on to fric<on coefficients is also fractal.
Bipolar Tribocharging Signal During
Fric3on Force Fluctua3ons at
Metal–Insulator Interfaces
Escobar, JV, Chakravarty, A, Putterman SJ, Diamond Relat. Mater. 2013
Akbulut M, Godfrey Alig AR, Israelachvili J. J. Phys. Chem. B. 2006.
Tribocurrent
Setup
Tribocurrent x friction force
vacuum nitrogen
hydrogen open air
1N normal load
50 cm/s speed
PTFE x Steel
Trib
ocur
rent
(nA
)
Fric
tion
forc
e (N
)
1N 2N 5N
5 cm/s 5 cm/s 5 cm/s
25 cm/s 25 cm/s 25 cm/s
50 cm/s 50 cm/s 50 cm/s
Results Friction force fluctuations: ü Fluctuations of friction force
occur with certain regularity in
tribological tests and Singer has
shown that this effect is generally
caused by the presence of third
bodies (material transfer).
Singer, et al., J. Vac. Sci. Technol. A 2003.
Tribocurrent and transient friction force fluctuation
Although tribocurrent signal depends on speed and load, charged species per force ratio is constant around 10 µC/N.
TRACK: negatively charged
Xerox® Cyan Developer powder (iGen3, 5R706). When placed on the sample, the track promptly repeals the negatively charged toner particles.
Quantitative NanomechanicsTM (QNM)
Quantitative material properties obtained by force curve analysis at every pixel!!!
Z Position (~2KHz)
Peak Force Setpoint
Force distance curves
Clean PTFE
Charged PTFE
Adhesion mapping on PTFE track
Strongly adhered PTFE flakes
5 mm
Pull-off force: 150 nN for most of the pixels!
PTFE
track 5x
Triboemission (triboluminescence) under Neon atmosphere
Burgo, T. A. L. & Erdemir, A. Ang. Chem. Int. Ed., 53, 2014.
Concluding Remarks
n Friction force fluctuations are always accompanied by two tribocharging mechanisms at metal-insulator interfaces: – injection of electrons from the
metal to PTFE subsequently followed by material/charge transfer from PTFE to the metal surface;
n Friction and triboelectrification have a common origin: – which must be associated with the
formation of strong electrostatic interactions at the interface.
Prospects: Controlling friction?
n The nature of the fundamental processes that give rise to friction between sliding bodies in close proximity is a long standing question in tribology, both theoretically and experimentally!!!
Park, J. Y., Ogletree, D. F., Thiel, P. A. & Salmeron, M. Electronic control of friction in silicon pn junctions. Science 313, 186–186 (2006).
Electrostatically stimulated additives?
Electrostatic potential naturally built up at interfaces under relative motion should attract charged molecules. Moreover, an external dc source must increase ionic migration.
Tribocurrent at the nanoscale? In progress…
AFM contact modes could be combined with techniques for monitoring the triboelectrification of surfaces, for example by measuring the tribocurrent, which would result in a powerful complementary method to AFM.
Electrometer ~
Computer
Tribocurrent image
Acknowledgements
n Department of Energy (DOE)
n Argonne National Laboratory – Tribology Section – Center for Nanoscale Materials
Unicamp Instituto de Química
“God made the bulk; surfaces were invented by the devil.”
Wolfgang Pauli
Thank you…