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

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 Friction Friction in in Every-day Every-day Life Life F N v F L 1

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 2

Da Vinci-Amontons Vinci-Amontons‘ ‘ Laws Laws Da 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. 3

Euler Euler Leonhard Euler (1707-1783) Euler experiments with the inclined plane. He discovers the difference between kinetic and static friction Adhesion: Adhesion: Molecular Interaction Molecular Interaction J.T. Desaguliers 1725 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 4

200 years later: Adhesion 200 years later: Adhesion model of Bowden Tabor model of Bowden Tabor Bowden and Tabor discover: The real contact area is much smaller than the geometrical contact area Friction F F is proportional to the material contact area A R F A = " ! F R F.P. Bowden ! is the shear stress and depends molecular 1950 characteristics, similarly as the adhesion The number of contacts increases with the normal force and is independent of the geometrical contact area Real contact Real contact area area A A R 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. 5

Nanotribology: Nanotribology : Examination of individual Examination of individual micro or nano nano-contacts -contacts micro or 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 6

Friction force microscopy (FFM) Friction force microscopy (FFM) Normal forces F N and lateral forces F L are measured Typical forces 1-100nN M. Mate et al. Phys. Rev. Lett. 59 , 1942(1987) microfabricated cantilever cantilever microfabricated l = 450 µ m w = 45 µ m t = 1.5 µ m Tip height: h=12 µ m Tip radius: 10 nm E=1.69 ·10 11 N/m 2 G=0.5 ·10 11 N/m 2 Springconstant k N : Ewt 3 k N 0 . 07 N/m = = 4 3 l Springconstant k T : Gwt 3 k T 390 N/m = = 2 3 h l 7

Spring model model of of experiments experiments Spring F L k con k T vt E 0 a 1 ! 1 1 ' $ k % " = + % " eff k k & # T con k T >> k con k eff ~ k con • the effective stiffness dominated by the contact stiffness 8

Friction contrast on organic films Friction contrast on organic films Topography Mixed Langmuir-Blodgett films (C21H43COO -/C9F19C2H4OCC2H4COO -) Lateral Force C 21 H 43 COO - C 9 F 19 C 2 H 4 OCC 2 H 4 COO - C-terminated F- terminated 2.8x2.8 µ m 2 Friction on Friction on the the atomic atomic scale scale 9

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 F N F L v • In 1D the corresponding potential energies are represented by: x E 1 V cos( 2 tip ) k ( x x ) 2 0 = ! " + ! eff tip 2 a 2 a sinusoid a parabola 10

Atomic Stick-Slip on Stick-Slip on NaCl NaCl(001) (001) Atomic 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) Atomic stik-slip with 2.5Å periodicity adhesion 3-10nN (static) F lat (max)=4.2nN 5x5nm 2 R. Bennewitz et al., Phys. Rev. B 60, R11301 (1999) 11

Friction on Friction on the the Nanometer- Nanometer- scale: : Atomic-Stick Atomic-Stick Slip Slip scale Atomic stick-slip Friction loop F N = 0.44 nN E diss = 1.4 eV KBr(001)-crystal (per slip) Atomic stick-slip stick-slip as a as a function function of normal of normal Atomic force: Observation of „ „Superlubricity Superlubricity“ “ force: Observation of ! > 1 ! < 1 Stick-slip “Superlubricity” A. Socoliuc, R. Bennewitz, E. Gnecco and E. Meyer, Phys. Rev. Lett. 92 , 134301 (2004) 12

Friction of a of a Nano-Asperity Nano-Asperity Friction as a Function Function of Normal of Normal as a Force Force Experiment Theory ! Transition to minimum dissipation state Symmetry Symmetry ? ? 0° 15° 30° 45° 60° We expect quadratic Symmetry ! 60° mirrored Symmetry Group: D4 13

Calibrattion Calibrattion Callibration of Callibration of the the spring spring constant constant 10 nN k= F/s 0.8 nm 14

National Center of Competence in Research Nanoscale Science Thanks Thanks • • Hans Hug Hans Hug • • Martin Guggisberg Martin Guggisberg • • Ernst Meyer Ernst Meyer • Tibor Gyalog Gyalog • Tibor • Mark Lantz Lantz • Mark • Heinz Breitenstein • Heinz Breitenstein • • Regina Hoffmann Regina Hoffmann • • Peter Fornaro Peter Fornaro • • Alexis Baratoff Alexis Baratoff • • Hans - Rudolf Hidber Hans - Rudolf Hidber • Roland Bennewitz Bennewitz • Roland • Stefan Messmer Messmer • Stefan • Christoph Gerber • Christoph Gerber • Peter Reimann • Peter Reimann • Martin Hegner Hegner • Martin • • Thomas Jung Thomas Jung • • Simon Berner Simon Berner • Michael Brunner • Michael Brunner 15