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…