Friction Friction in in Every-day Every-day Life Life F N v F L - - PDF document

1

National Center of Competence in Research Nanoscale Science

Exploring the Nano-World: Friction at the nanometer scale

• L. Zimmerli, F. Müller, H,-R. Hidber,
• T. Gyalog, M. Guggisberg, E. Meyer

http://www.nccr-nano.org http://www.nano-world.org

FN v FL

Friction Friction in in Every-day Every-day Life Life

2

Already the old Egyptians.... Already the old Egyptians....

wood on dry sand: µ ! 0.22-0.5

Leonardo da Vinci Leonardo da Vinci

Leonardo da Vinci (1452-1519)

• 1. Friction is independent on the area of contact
• 2. Friciton is proportional to the loading force

3

Da Da Vinci-Amontons Vinci-Amontons‘ ‘ Laws Laws

Guillaume Amontons (1663-1705)

Amontons discovers the friction laws again. The work from Da Vinci was lost.

Coulomb Coulomb

Charles Augustin Coulomb (1736-1806)

Friction is independent on the velocity Coulomb also suggests that roughness is responsible for the emergence of friction: Tooth from micro contacts.

4

Euler Euler

Euler experiments with the inclined plane. He discovers the difference between kinetic and static friction

Leonhard Euler (1707-1783)

Adhesion: Adhesion: Molecular Interaction Molecular Interaction

Contradiction to the roughness model: High-polished surfaces hold together well and show increased friction Desaguliers invent the concept of the adhesion: Adhesion should be proportional to the contact area

J.T. Desaguliers 1725

5

Bowden and Tabor discover: The real contact area is much smaller than the geometrical contact area Friction FF is proportional to the material contact area AR ! is the shear stress and depends molecular characteristics, similarly as the adhesion The number of contacts increases with the normal force and is independent of the geometrical contact area

200 years later: Adhesion 200 years later: Adhesion model of Bowden Tabor model of Bowden Tabor

R F

A F ! ="

F.P. Bowden 1950

Real Real contact contact area area A AR

R

The material contact area is much smaller than the geometrical surface (typical 10-5) i.e. the macroscopic contact is made by micro contacts. Shearing of the micro contacts is responsible for the macroscopic friction.

6

Nanotribology Nanotribology: : Examination of individual Examination of individual micro or micro or nano nano-contacts

• contacts

1-100nm

AFM on NaF(001) AFM on NaF(001)

• contact mode imaging on NaF(001)
• observation of the atomic periodicity
• steps area distorted in a range of 1 nm

! 1 nm contact radius

1nm

7

Friction force microscopy (FFM) Friction force microscopy (FFM)

Normal forces FN and lateral forces FL are measured Typical forces 1-100nN

• M. Mate et al.
• Phys. Rev. Lett. 59, 1942(1987)

microfabricated microfabricated cantilever cantilever

l = 450 µm w = 45 µm t = 1.5 µm Tip height: h=12 µm Tip radius: 10 nm E=1.69 ·1011N/m2 G=0.5 ·1011N/m2

N/m 07 . 4 3

3

= = l Ewt kN

Springconstant kN: Springconstant kT:

N/m 390 3

2 3

= = l h Gwt kT

8

FL vt a E0 kcon kT

1

1 1

!

" " # $ % % & ' + =

con T eff

k k k

Spring Spring model model of

• f experiments

experiments

kT >> kcon keff ~ kcon

• the effective stiffness dominated by the

contact stiffness

9

Friction contrast on organic films Friction contrast on organic films

Topography Lateral Force 2.8x2.8µm2

C21H43COO - C9F19C2H4OCC2H4COO - C-terminated F-terminated

Mixed Langmuir-Blodgett films (C21H43COO -/C9F19C2H4OCC2H4COO -)

Friction Friction on

• n the

the atomic atomic scale scale

10

Tomlinson mechanism Tomlinson mechanism Explanation of stick slip phenomena Explanation of stick slip phenomena

Tomlinson Tomlinson model model

The tip is subject to: 1) periodic interaction with the underlying surface 2) elastic deformation of the cantilever

2

) ( 2 1 ) 2 cos( 2 x x k a x E V

tip eff tip

! + ! = "

a sinusoid a parabola

• In 1D the corresponding potential

energies are represented by: FL FN v

11

Atomic Atomic Stick-Slip on Stick-Slip on NaCl NaCl(001) (001)

Friction image and Friction loop on NaCl(001) in UHV

Atomic Friction on metals Atomic Friction on metals

Silicon tip on Cu(111) in UHV 5·10-11 mbar) 5x5nm2 Atomic stik-slip with 2.5Å periodicity adhesion 3-10nN (static) Flat(max)=4.2nN

• R. Bennewitz et al., Phys. Rev. B 60, R11301 (1999)

12

Friction Friction on

• n the

the Nanometer- Nanometer- scale scale: : Atomic-Stick Atomic-Stick Slip Slip

FN = 0.44 nN Ediss = 1.4 eV

(per slip) Atomic stick-slip Friction loop KBr(001)-crystal

Atomic Atomic stick-slip stick-slip as a as a function function of normal

• f normal

force: Observation of force: Observation of „ „Superlubricity Superlubricity“ “

! > 1 ! < 1 Stick-slip “Superlubricity”

• A. Socoliuc, R. Bennewitz, E. Gnecco and E. Meyer,
• Phys. Rev. Lett. 92, 134301 (2004)

13

Friction Friction of a

• f a Nano-Asperity

Nano-Asperity as a as a Function Function of Normal

• f Normal

Force Force

!Transition to minimum dissipation state Experiment Theory

Symmetry Symmetry ? ?

0° 15° 30° 45° We expect quadratic Symmetry ! Symmetry Group: D4 60° 60° mirrored

14

Calibrattion Calibrattion Callibration Callibration of

• f the

the spring spring constant constant

k= F/s 10 nN 0.8 nm

15

National Center of Competence in Research Nanoscale Science

• Martin Guggisberg

Martin Guggisberg

• Tibor

Tibor Gyalog Gyalog

• Heinz Breitenstein

Heinz Breitenstein

• Peter

Peter Fornaro Fornaro

• Hans - Rudolf

Hans - Rudolf Hidber Hidber

• Stefan

Stefan Messmer Messmer

• Peter Reimann

Peter Reimann

• Hans

Hans Hug Hug

• Ernst Meyer

Ernst Meyer

• Mark

Mark Lantz Lantz

• Regina Hoffmann

Regina Hoffmann

• Alexis

Alexis Baratoff Baratoff

• Roland

Roland Bennewitz Bennewitz

• Christoph Gerber

Christoph Gerber

• Martin

Martin Hegner Hegner

• Thomas Jung

Thomas Jung

• Simon Berner

Simon Berner

• Michael Brunner

Michael Brunner

Thanks Thanks