Denis MAZUYER - Institut d'études scientifiques de Cargèse (IESC)

2010
22 Mars
26 Mars
THEOROTICAL MODELING &
EXPERIMENTAL SIMULATION
IN TRIBIOLOGY
Denis MAZUYER
Ecole Centrale de Lyon – 69134 Ecully
Cedex
04 72 18 62 88
denis.mazuyer@ec-lyon.fr
Direction scientifique :
Giovanna Chimini
Contact :
Dominique Donzella
tél : 04 95 26 80 40
www.iesc.univ-corse.fr

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Elementary friction processes still remain an open problem in physics. Friction is usually considered as
the energy dissipative mechanism between two solid surfaces moving relative to each other and as
work done in generating wear particles. All these phenomena occur at very different length and time
scales. There have been few attempts to model these processes in the form of a general theory using
continuum mechanics. This is complicated by the fact that friction is a result of many interacting
processes such as geometric locking of two surfaces, elastic and plastic deformations, wear,
lubrication and adhesion. Assuming no-slip boundary conditions, the Reynolds equation has been
solved to equate frictional work done in a journal bearing to the energy dissipated in a viscous
Newtonian fluid. This has provided a powerful tool in the design of the bearings and other machine
parts. However, the condition that exists at the liquid-solid interface has to be considered. Additionally,
some investigations have found that the properties of the confined fluid of a few atomic layers are very
different from those of the bulk. With the recent advances in the instrumentation there have been
many novel experiments in tribology covering the range from nanometres to centimetres. There has
also been associated progress in the simulation of dynamic phenomena at these scales.
Nevertheless, it is still not clear what the dissipative mechanisms are at the atomic/molecular level
especially when there is a third medium (liquid, colloidal state or adsorbed molecules) present at the
interface.
The main goal of this school is to give an overview of the current trends in the theoretical description
and experimental simulation of dissipative processes in rubbing interfaces over a continuum spectrum
of scales, from single to multi-asperity contacts, from molecular relaxation times to machine lifetimes.
By bringing together experimentalists and theoreticians as well as different fields (physics, chemistry,
mechanics, surface and material science), this school will help to develop a fundamental
understanding of energy dissipation in tribology. The school also aims to provide quality and synthetic
education to PhD students and young researchers on the subject of theoretical modeling and
experimental simulation in tribology. There is no doubt that this period of renewal coming with Spring
in the Corsica sunshine will favour the emergence of ideas and fruitful scientific discussions between
all the delegates. This workshop would not have been possible without E. Rigaud, our Webmaster, F.
Brémond and S. Mambingo-Doumbe who helped with the practical organization, S. Moro and B.
Barchasz who were in charge of some administrative tasks. CNRS, Saint-Gobain, L'Oréal and Ecole
Centrale de Lyon must be acknowledged for financial support. Finally, I would like to thank all the
invited speakers for their scientific contribution to this workshop and the chairmen who have kindly
accepted the responsibility for leading the discussions.
On behalf of the scientific committee
Denis Mazuyer
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Dr. J. Cayer-Barrioz, LTDS – ECLyon, France
Prof. E. Charlaix, LPMCN – UCBLyon, France
Dr. C. Frétigny, PPMD – ESPCI Paris, France
Prof. M. Kubo, Tohoku University Sendai, Japan
Prof. JM. Martin, LTDS – ECLyon, France
Prof. D. Mazuyer, LTDS – ECLyon, France
Prof. BNJ. Persson, Forschungzentrum Jülich, Germany
Prof. S. Sinnott, University of Florida, Gainesville, USA
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Molecular organization, dynamics, and dissipation in
nanojunctions, nanofluids and membranes
U. Landman
School of Physics
Georgia Institute of Technology
Atlanta, GA 30332, USA
uzi.landman@physics.gatech.edu
The arrangements of molecules in thin films confined between solid boundaries, layering transitions,
solvation forces versus gap width, and segregation effects in mixtures of long and short molecules, will
be discussed. The effects of surface morphology, that is smooth versus rough confining surfaces, on
the structure, dynamics, and tribological properties of the confined films, with and without shearing
motion, will be highlighted. In the second part of the lecture we discuss trans-membrane transport
processes investigated via molecular dynamics simulations of nanojet injection through lipid bilayer
membranes, illustrating membrane self-healing processes and their dependence on the dimensions of
the injecting jet.
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Influence of molecular structure and alignment on nanometer-scale
tribology
P. Barry, P. Chiu, S. R. Phillpot and S. B. Sinnott
Department of Materials Science and Engineering
University of Florida, Gainesville, Florida, 32611-6400, USA
ssinn@mse.ufl.edu
We report on the effect of small, fluorocarbon molecules on self-mated, aligned
PolyTetraFluoroEthylene (PTFE)-PTFE tribology using atomistic molecular dynamics simulations.
Three fluorocarbon molecular classes were considered: C 2 F 6 , C 4 F 10 and C 8 F 18 with the amount of
lubricant between the classes kept constant. Further, the effects of a relatively thin lubricating layer
and a relatively thick lubricating layer were compared. The simulations predicted that the systems with
thicker lubricating layers exhibited a friction coefficient that was significantly lower than those a thinner
lubricating layer. Correspondingly, substantially more molecular wear of the PTFE surfaces were
predicted for the latter systems. Interestingly, unlubricated PTFE-PTFE self-mated systems
demonstrated low friction coefficients and molecular wear when the chains were slid in a direction
parallel to the chain alignment, and unlubricated, aligned polyethylene (PE)-PE systems exhibited
comparable or lower friction coefficients. The simulations further predicted that unlubricated, aligned
PE-PTFE systems had friction coefficient values in between those of the PE-PE systems and PTFE-
PTFE systems in which the chains slid in directions that were perpendicular to the alignment of the
chains. Surprisingly, the highest friction coefficients in the PE-PTFE system occurred when the chains
were slid in a direction parallel to the direction in which the chains were aligned. This result was
attributed to the incommensurate nature of the sliding interface between the two different polymers.
This work was carried out under the support of an AFOSR MURI.
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Multi-scale modeling of tribological phenomena: bridging from
electronic to mesoscopic density functional theory
M. Mueser
University of Western Ontario, Canada
martin.mueser@gmail.com
In this talk, it will be shown how to parameterize constitutive equations for tribological simulations at
the mesoscopic scale from first-principle calculations. Examples, illustrating the methodology, will
consist of aluminum surfaces and lubricating olefins. The first step in the sequential multi-scale
modeling approach is the design of classical force fields for the interaction between lubricating
molecules and solid surfaces. A self-consistent approach, similar to those used for equilibrium
simulations, will be presented. The second step consists of large-scale molecular dynamics
simulations, in which the flow boundary conditions as well as bulk properties such as wavelengthdependent
compressibility (square gradient corrections), shear thinning, and pressure thickening are
parameterized. In our studied example, it turns out that the slip length is particularly sensitive to small
chemical changes, e.g., the precise location of the double bond in hexene. Lastly, it will be discussed
how to use Green's function molecular dynamics, an efficient method to simulate elastic boundaries,
can be combined with various mesoscopic fluid mechanical treatments.
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Slipups in friction: nanotribology experiments reconsidered
J. W. M. Frenken (1) and S. Yu. Krylov (2) ,
(1) Kamerlingh Onnes Laboratory, Leiden University, P.O. Box 9504, 2300 RA, Leiden, The
Netherlands
(2) Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences,
Leninsky prospect 31, 119991 Moscow, Russia
frenken@physics.leidenuniv.nl
We present the first fully quantitative and self-consistent analysis of atomic scale friction, explicitly
taking into account the flexibility and low effective mass of the mechanical nanocontact. In a
procedure, which is free of the traditional assumptions with respect to the corrugation of the interaction
potential of the contact, the basic but experimentally inaccessible system parameter, we arrive at an
excellent description of recent nanotribology experiments, including the transition from stick-slip to
nearly frictionless sliding. We show that, contrary to original interpretation, the ultra-low friction
observed in some experiments has been largely due to thermal ("thermolubricity") rather than
mechanistic effects (superlubricity). Furthermore, we observe the manifestations of two different forms
of thermally induced sliding dynamics, namely true thermolubricity (slipperiness based on thermal
excitations) and a specific, low-dissipation type of stick-slip motion.
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Friction and wetting behavior of hydrophobic polymer films under
water
I. Siretanu, J. P. Chapel and C. Drummond
Université de Bordeaux, Centre de Recherche Paul Pascal, UPR8641 CNRS
Avenue Schweitzer, 33600 Pessac Cedex, France
isiretanu@crpp-bordeaux.cnrs.fr
When hydrophobic surfaces are in contact with water in ambient conditions a layer of reduced density
is present in the water-solid boundary, preventing the intimate contact between the two phases. In this
work we show how avoiding the formation of this layer can have extraordinary implications for the
interaction between the condensed phases. The enhanced proximity between a hydrophobic glassy
polymer film and an aqueous solution may induce long lasting modifications on the solid surface, with
significant implications on the adhesion, friction and wetting behavior of hydrophobic surfaces. These
results show that the external layer of glassy spin coated polymer films displays an enhanced mobility
compared with the bulk polymer molecules.
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Nanotribology of polymer films in aqueous systems
M. Rutland, N. Nordgren
Chemistry : Surface and Corrosion Science
Royal Institute of Technology, KTH
Stockholm, Sweden
mark@kth.se
Friction measurements in aqueous systems measured using AFM in colloid probe mode will be
presented and discussed. Primary focus will be on the frictional properties of responsive, endgrafted
charged polymers (pDEMAEMA).
The polymers are produced in house via RAFT and the RAFT agent is converted to a thiol moiety to
allow grafting to a gold surface which is performed in situ in a Quartz Crystal Microbablance (QCM)
The advantage of this system is that the number of molecules at the surface is fixed, but their
conformation and hydration can be controlled using both pH and temperature. Considerably lower
friction was observed when the polymers were charged and extended and contained significant water.
The friction properties are strongly dependent on the conformation but some anomolous behaviour
occurs in highly collapsed states where the friction coefficient was somewhat lower for a fully
collapsed film than for partially collapsed. The results are compared with QCM studies of the film
behaviours and dissipation. Comparisons are made with other measurements, in polysaccharide
systems where once again friction is related to polymer conformations with the added contribution of
dynamic adhesion effects.
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Static and dynamic surface properties of polymer-bearing
substrates
B. Liberelle (1) , X. Banquy (2) , B. Lego (1) , S. Giasson (1,2)
(1) Department of Chemistry
(2) Faculty of Pharmacy
Université de Montréal, C.P. 6128, succursale Centre-Ville, Montréal, QC, Canada, H3C 3J7
suzanne.giasson@umontreal.ca
Experimental surface forces studies of different classes of solvated polymer- and nanoparticle- bearing
surfaces were carried out using the surface forces apparatus and similar molecular techniques in order
to elucidate the role of different parameters, such as polymer conformation, surface roughness and
elasticity, solvent quality, ionic strength, in controlling friction and adhesion between polymer-bearing
surfaces.
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Friction dynamics of adsorbed confined polymer layers
J. Cayer-Barrioz (1) , D. Mazuyer (1) , A. Tonck (1) , E. Yamaguchi (2)
(1) Laboratoire de Tribologie et Dynamique des Systèmes – UMR 5513 CNRS
Ecole Centrale de Lyon, 69134 Ecully Cedex, France
(2) Chevron – Oronite Company LLC, P.O. Box 1627, Richmond, CA 94802-0627, USA
juliette.cayer-barrioz@ec-lyon.fr
Polymers are commonly used as a dispersant and to control the temperature dependence of the
lubricant viscosity. It has been shown that some of them can act as friction modifier both in mixed and
boundary regime at moderate contact pressure. In that case, their ability to organize themselves at the
neighbourhood of the sliding surfaces, as a nanometre thick monolayer is a key point in the friction
mechanisms. A molecular tribometre derived from a Surface Force Apparatus has been used to
investigate the sliding dynamics of a confined adsorbed polymer layer. The latter is made from a semidilute
solution of OCPD. The molecular structure of the surface layer is described by the self-similar
adsorption model of de Gennes and is correlated to its rheology that has been experimentally
characterized. The frictional behaviour of the layer is clearly governed by the confinement at short
distances and does not follow the Amontons-Coulomb law. The velocity dependence is supposed to
be governed the dynamics of rupture of bonds that link molecular junctions. From a model that
determines the lifetime of such bonds and their probability of formation, it is possible to deduce an
activation volume that can be related to the size of an elementary junction. This shows the presence of
an overlapping layer whose thickness and stiffness depends on both the load and the sliding velocity.
The role of this interpenetration zone on the friction response of the confined polymer layer is
discussed.
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History and mechanisms of boundary lubrication
H. Spikes
Tribology Group, Department of Mechanical Engineering
Imperial College London, United Kingdom
h.spikes@imperial.ac.uk
Ever since the regime known as “boundary lubrication” was first recognized at the beginning of the
twentieth century there have been continuous efforts to understand its mechanism and to develop
additives able to enhance the boundary properties of liquid lubricants. In recent years these efforts
have become particularly intense. The quest for energy saving is leading to the use of lower viscosity
lubricants, a trend which, since it results in thinner hydrodynamic films, places stronger emphasis on
the boundary lubrication regime. The same quest is also leading to the need for improved friction
modifier additives to reduce boundary friction. Also, concern for the environment is placing constraints
on lubricant composition, which necessitate changes in design of boundary lubricant additives. All of
these trends have led to an increase in interest in boundary lubrication.
It is now clear that boundary lubrication is not one single phenomenon but that it involves a number of
different mechanisms, associated with different types of boundary lubricating film ranging from solidlike
surface deposits to enhancements of local viscosity at the oil-solid interface
This lecture will outline the history of boundary lubrication and describe our current understanding of
its mechanisms. It is apparent that very rapid advances are taking place, driven largely by the
development of increasingly sophisticated experimental and modelling techniques. Finally the lecture
will examine the challenges that still constrain our knowledge and thus full utilisation of boundary
lubrication.
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Surfactant/polymer mixtures as boundary lubricants
C. Drummond
Université de Bordeaux, Centre de Recherche Paul Pascal, UPR8641 CNRS
Avenue Schweitzer, 33600 Pessac Cedex, France
drummond@crpp-bordeaux.cnrs.fr
Fewer studies at a molecular level have addressed the behaviour of surfactant/polymer mixtures as
boundary lubricants. By combining the capabilities of the Surface Forces Apparatus and the Atomic
Force Microscopy, we have studied the normal interaction and the behavior under shear of mica
surfaces covered by several surfactant, copolymers or surfactant-copolymer mixtures. Copolymers
anchored or co-adsorbed onto preadsorbed surfactant bilayers may either reinforced or weakened the
cohesion of surfactant films with important implications on the behaviour of the surfaces under shear.
A review of these studies will be presented.
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Polymer brushes in tribology
N. Spencer
ETH Zürich, Switzerland
nicholas.spencer@mat.ethz.ch
Polymer brushes form when polymer chains are end-grafted close to each other (spacing Rg) on a
surface in the presence of a good solvent. They have been studied extensively, both experimentally
and theoretically by de Gennes among many others. Polymer brushes display a range of interesting
properties that are relevant to tribology. They function as a soft protective coating on surfaces, they
lead to repulsive forces between surfaces, and they bring with them an intrinsic solvent-rich layer
when two brush-coated surfaces are brought together in a good solvent. All these properties can help
lead to low friction between brush-covered surfaces. Polymer brushes can be readily formed on
surfaces of tribological interest by a variety of methods, some spontaneous (e.g. simple adsorption
from solution, even during tribological use) and some requiring a polymerization step on the surface
itself. Both of these approaches have been explored in our laboratories and tested in tribosystems
ranging from atomic force microscopy and surface forces apparatus experiments to testing at GPa
loads in a pin-on-disk setup. In all cases, significant friction reduction is observed, often to values that
are below the measuring capabilities of normal tribological instruments. Of particular interest has been
the possibility of using polymer brushes to facilitate lubrication of a variety of materials by water.
Nature, of course, also lubricates by means of water, and utilizes structures that resemble polymer
brushes, but display certain key differences. In Nature, the "polymers" are sugar chains, which are
highly hydrated and help form water-rich layers on surfaces that display similar behavior to that
observed in the case of water-compatible polymer brushes.
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Active and passive friction of molecular motor assemblies
T. Guerin
Physicochimie 'Curie' - UMR 168 CNRS/Institut Curie
11, rue Pierre et Marie Curie, 75231 Paris Cedex 05, France
thomas.guerin@curie.fr
The behaviour of an assembly of molecular motors acting on a filament shares common features with
tribology, among which the continuous formation and breaking of bonds. Molecular motors may
produce high passive friction, due to the fact that elastic energy is released at each binding-unbinding
cycle. It has also been predicted that the force produced by a motor collection at small velocities can
formally amount to a “negative friction”, resulting in dynamical instabilities, which could be the origin of
many biological oscillations.
We propose a simple model for the collective behaviour of molecular motors. The model includes both
the motor internal flexibility and a periodic interaction potential between the motors and their filament.
The motors are allowed to switch between two states (which mimic a strongly and weakly bound
state), with transition rates that break detailed balance in order to reflect energy consumption.
Our model can be seen as a 2-state non-equilibrium version of Tomlinson's model, which displays the
property of solid friction below a critical value of the stiffness: the friction force does not vanish at small
velocities, but its sign depends on the direction of motion. We show that the property of solid friction
disappears in our model, because the motors can detach instead of remaining “pinned” in a potential
well. However, signatures of the transition to static friction remain: the slope of the force velocity
relation at small velocities can be discontinuous, and the effective friction coefficient increases sharply
above a critical stiffness, which can be interpreted as a transition to "protein friction".
We give an analytical expression for the effective friction coefficient of the motor assembly, and show
that it can become negative in two limiting cases. These limiting cases can be related to two classical
models of molecular motors (the ‘cross-bridge’ and the ‘ratchet’ type models), and thus our model
enables to make an explicit connection between them. In the first limiting case, the motors have a very
low stiffness: they can be strongly pinned in the potential wells, and the model is equivalent to 'crossbridge'
models, originally introduced in the context of muscles. Instability occurs if the detachment rate
depends on the load. In the second limiting case, the motors are 'stiff' (‘weak pinning’), and the model
is equivalent to 'ratchet' type models. In this regime, contributions to positive friction come from the
molecular friction between the motor head and the filament, whereas the contribution from ‘protein
friction’ is negligible.
The negative friction property can result in dynamical instabilities. We compare our results obtained in
the limit of stiff motors to a recent in vitro experiment, where a collection of molecular motors works
against an elastic load and displays oscillations [P.Y Plaçais et al, Phys. Rev. Lett. 103, 158102
(2009)]. We show that high friction coefficient is needed to explain the experiments, and that a formal
analogy with stick slip oscillation can be made to understand the effect of stabilisation by a high
external stiffness (memory effect).
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Onset of frictional slip by domain nucleation in adsorbed
monolayers
M. Reguzzoni, M. Ferrario, S. Zapperi, M. C. Righi
INFM-CNR S3 National Research Center, Universita di Modena e Reggio, Italy
marco.reguzzoni@gmail.com
It has been known for centuries that a body in contact with a substrate will start to slide when the
lateral force exceeds the static friction force. Yet the microscopic mechanisms ruling the crossover
from static to dynamic friction are still the object of active research. Here, I analyze the onset of slip of
a xenon (Xe) monolayer sliding on a copper (Cu) substrate. We consider thermal activated creep
under a small external lateral force and observe that slip proceeds by the nucleation and growth of
domains in the commensurate interface between the film and the substrate. We measure the
activation energy for the nucleation process considering its dependence on the external force, the
substrate corrugation and particle interactions in the film. To understand the results, we use the
classical theory of nucleation and compute analytically the energy of a domain wall which turns out to
be in excellent agreement with numericalresults. I discuss the relevance of our results to understand
experiments on the sliding of adsorbed monolayers.
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Effect of particle agglomeration on the deformation mechanism and
tribological behavior of layered MoS 2 nanoparticles
R. R. Sahoo and S. K. Biswas
Department of Mechanical Engineering
Indian Institute of Science, Bangalore 560012 India
sahoo@mecheng.iisc.ernet.in
Inorganic nanoparticles in suspension in a liquid lubricant often agglomerate and give rise to poor
tribology. In this study we use lateral force microscopy to study the deformation mechanism and
dissipation under traction of two extreme configurations (1) a monolithic MoS 2 particle (~ 20µm width
and 1µm height) and (2) an agglomerate (~ 20µm width), which consists of loosely bound 50nm MoS 2
crystallites (1µm height). The agglomerate accounts for a friction coefficient, which is about 5 to 7
times that of monolithic particle. The present study examines the mechanisms of material removal
from both the particles using continuum modeling and microscopy. It is inferred that while the
agglomerate response to traction can be accounted for by the bulk mechanical properties of the
material, intra-layer and inter-layer basal planar slip determines the friction and wear of the monolithic
particles.
Frictional performance of molybdenum disulfide (MoS 2 ) particles sprayed on a substrate is
investigated in a ball on disc tribometer. The ability, of the monolithic and the agglomerated MoS 2
particles, to generate low friction transfer film is investigated with a view to elucidate requirements for
a transfer film formation. Particle migration, particle stability in the contact region are investigated
within a span of operating parameters; normal load and sliding velocity. It is found that the monolithic
particles are able to migrate to the contact to raise a homogeneous but non-uniform low friction
transfer film which flows plastically to yield large contact areas which aid in wear protection. Within the
present load and speed range, the inability of the agglomerates to reside in the contact region and
undergo basal slip militates against the formation of a low friction transfer film.
The results present a basis for selection of layered particles to be used as suspensions in liquid
lubricants.
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Granular walkers: a ratchet effect for wet clusters of beads
A. Steinberger (1,2) , Z. S. Khan (1) , R. Seemann (1,3) and S. Herminghaus (1)
(1) MPI for Dynamics and Self-Organization, Bunsenstr. 10, 37073 Goettingen, Germany
(2) Laboratoire de Physique, CNRS UMR 5672, Ecole Normale Supérieure de Lyon, 46 allée
d'Italie, 69364 Lyon, France
(3) Experimental Physics, Saarland University, 66041 Saarbruecken, Germany
audrey.steinberger@ens-lyon.fr
We have observed that when a bidisperse mixture of glass beads is moistened by a fluid and shaken
sinusoidally in a vertical container, small clusters of beads take off from the surface of the pile and
rapidly climb up the container walls against gravity. These self-assembled clusters are held together
and against the wall by liquid capillary bridges, and are led by one large grain with one or more small
grains trailing behind. When similar clusters are artificially assembled on a horizontally vibrating
substrate, they travel horizontally along the axis of vibration.
In order to understand this phenomenon, we study the motion of the simplest structure, a dimer
composed of one large bead and one small bead, in the horizontal configuration. we report on the
influence of the driving parameters and the degree of asymmetry of the dimer on the velocity of the
structure, and we discuss a basic model describing this system.
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Testing the local dynamics of polymer chains at interfaces through
mechanical experiments
F. Restagno, C. Cohen and L. Léger
Laboratoire de Physique des Solides, UMR 8502 CNRS Université Paris Sud XI,
Bâtiment 510, 91405 Orsay Cedex, France
restagno@lps.u-psud.fr
We shall present and discuss several series of experiments in which the aptitude of surface anchored
polymer chains to transmit stress through the interface is directly characterized and used to gain
information on both chain conformation and dynamics at polymer interfaces.
In a first series, the friction at polymer melt / solid interfaces has been investigated through direct
measurement of the velocity of the bulk polymer chains in the immediate vicinity of the solid wall (over
distances from the wall smaller or comparable to the size of the chains).
Covering the solid surface with end grafted polymer chains having the same chemical nature as that of
the bulk chains, and varying both the length and the surface density of the grafted brush, we show that
the friction at such interfaces is driven first by the degree of entanglements between surface and bulk
chains, and second by the ability of surface chains to stretch and disentangle from the bulk for high
enough shear rates.
Similarly, in a second series of experiments, the friction force between a crosslinked elastomer and a
layer of end grafted chains having the same chemical nature as that of the elastomer has been
measured for various sliding velocities and various surface densities and chain length of the grafted
chains. These experiments allow one to extract the friction force exerted by the elastomer on one
grafted chain. We show that quantitative agreement can be obtained with molecular models. At high
grafting densities, the chains attached to the surface are expelled from the elastomer, and the
experiment gives access to the shear stress that this confined grafted polymer layer can transmit. We
also show that “start stop” experiments, in which the elastomer is first slide at high velocity in order to
extract all connector chains from the elastomer, then stopped for a certain duration, then slide again,
can be used to test the internal dynamics of surface anchored chains.
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Hydration lubrication: exploring a new paradigm
J. Klein
Weizmann Institute of Science, Rehovot 76100, Israel
jacob.klein@weizmann.ac.il
Hydration layers about ions or charges in aqueous media form as a result of the strong dipole of the
H 2 O molecule. They can be very tenaciously attached and at the same time very labile, with relaxation
times ranging from 10 -9 – 10 5 sec, some 14 orders of magnitude, affording them remarkable
properties. In recent years a new paradigm has emerged [1-6]: This reveals the remarkable - and
unsuspected - role of hydration layers in modulating frictional forces between sliding surfaces or
molecular layers in aqueous media, termed hydration lubrication, in which the lubricating mode is
completely different from the classic one of oils or surfactants. The talk will focus on very recent
studies of these effects, ranging from the behaviour of water and hydrated polymers in nanometricallyconfined
films to the molecular origins of biological lubrication.
!
!
References
[1] Raviv, U.; Laurat, P.; Klein, J. Nature 413, 51-54 (2001).
[2] Raviv, U.; Klein, J. Science 297, 1540-1543 (2002).
[3] Raviv, U.; Giasson, S.; Kampf, N.; Gohy, J.-F.; Jerome, R.; Klein, J. Nature 425, 163-165 (2003).
[4] Briscoe, W. H.; Titmuss, S.; Tiberg, F.; Thomas, R. K.; McGillivray, D. J.; Klein, J. Nature 444, 191-194 (2006).
[5] Klein, J. Science, 323, 47-48 (2009)
[6] Chen, M.; Briscoe, W. H.; Armes, S. P.; Klein, J. Science, 323, 1698-1702 (2009).
! $#!

Nanohydrodynamics: investigating boundary flow with surface
force experiments
E. Charlaix
Laboratoire de Physique de la Matière Condensée et Nanostructures - UMR 1356
Université Lyon 1, France
elisabeth.charlaix@lpmcn.univ-lyon1.fr
The investigation of fluid flows at a small scale is a particularly active field. Many new applications,
beyond the classical area of tribology, have emerged recently. Manipulation of small quantities of
liquid in microfluidic devices, flow around biological objects, lubrication in joints, transport of colloids by
electrokinetic effects, are crucially dependent on the interfacial friction between solid and liquid.
A number of experimental techniques have been used to study interfacial flows. However, while it is
now admitted that simple liquids may undergo substantial slip on solid surfaces, experimental studies
of boundary flows have not provided a consensual picture. Slip effects reported vary quantitatively as
well as qualitatively regarding their variation with the shear rate, with no clear cut relation to relevant
surface parameters such as interactions or roughness.
In this talk I will show that the use of surface force apparatus in the dynamic mode can bring new
insights in this domain. I will first discuss the intrinsic boundary condition for water on smooth surfaces
using various hydrophilic and hydrophobic surfaces, and show that intrinsic slip occurs on hydrophobic
surfaces but remains of moderate amplitude under ambient conditions. Then I will study the influence
of the liquid viscosity, using water-glycerol mixtures, with respect to the model of the "gas layer"
trapped at the solid-liquid interface. Finally I will present first results on textured superhydrophobic
surfaces. On this structured surfaces I will show that the presence of tailored trapped bubbles allows
one to adjust both the boundary flow and the surface elasticity in a controlled way.
! $$!

Observation of dynamic and structural transitions in molecular
confined films
M. Heuberger
Empa, Swiss Federal Laboratories for Materials Testing and Research, Switzerland
Manfred.Heuberger@empa.ch
The presentation includes a short introduction into the experimental technique of the surface forces
apparatus (SFA) - an instrument that builds on the model of a macroscopic single-asperity contact of
atomically perfect geometry. It will be demonstrated how white light can be used to measure the
density and the thickness of confined films that are only a few molecules thick. Some important quasiequilibrium
results obtained with this instrument are reviewed with the basic underlying theory of
intermolecular forces. It is demonstrated that intermolecular forces are most relevant in nano-tribology.
A range of different possibilities for dynamic measurements in the SFA will also be explained in detail.
These include one- or multi-dimensional friction experiments, thin-film rheological experiments as well
as fluid film drainage experiments. Illustrated with experimental data from different research groups,
we will discuss the observation of transitions of dynamic or structural nature at various sliding or
flowing interfaces that have a nanometer thick film of confined fluid between them. We will discuss
some of these non-equilibrium observations in terms of lengths- and time-scales involved. This will
lead us to the subject of controlling friction and energy dissipation at sliding interfaces.
! $%!

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Mechano-chemistry of carbon tribo materials
M. Moseler
Fraunhofer Institute for Mechanics of Materials IWM, Wöhlerstr. 11, 79108 Freiburg,
Germany
Physics Department, University of Freiburg, Hermann-Herder-Str. 3, 79104 Freiburg,
Germany
mos@iwm.fhg.de
Carbon is an ubiquitous element in technical tribology occurring e.g. in lubricants, tribological coatings
and tribo-mutation layers. Despite the fact that diamond and diamond-like carbon coatings (DLC) [1]
are used in an increasing number of applications, not much is known about the atomic scale
processes that occur during sliding these films. For instance, the mechanical and chemical processes
that occur during the running-in of DLC films [2] or the polishing of diamond are still poorly understood
[3]. Molecular dynamics is ideally suited to gain a deeper understanding of the underlying processes.
Here simulations with a novel Brenner bond order potential [4] are reported.
The running-in of hydrogenated DLC is explained by a combination of smoothing and chemical
passivation of a DLC/DLC interface. Fluctuations in the friction coefficient of the DLC coatings can be
explained by welding of the DLC/DLC tribosurfaces, combined with the formation of a transfer film
(transferred from one DLC partner to the other) and the establishment of a new tribointerface with low
friction coeffcient.
For diamond polishing, the occurrence of soft polishing direction can be related to the generation of
thick amorphous layers that are not stable with respect to oxidation or mechanical wear. The wear rate
(i.e. the velocity of the diamond/amorphous-carbon interface) depends crucially on the diamond
surface orientation with the highest speed found for (110) surfaces that are rubbed in the (001)
direction, while the lowest interface speed was observed for the diamond (111) surface. These finding
are in perfect agreement with a 600 years old experimental knowledge of diamond polishers. The
observed anisotropy of the wear rates is rationalized within a simple model that describes the
amorphisation process as a mechanically guided chemical reaction.
References
[1] M. Moseler, P. Gumbsch, C. Casiraghi, A. Ferrari, J. Robertson, The ultrasmoothness of diamond-like carbon
surfaces, Science 309, 1545 (2005)
[2] L.Pastewka, S. Moser, M. Moseler, B. Blug, S. Meier, T. Hollstein, P. Gumbsch, The running-in of amorphous
hydrocarbon tribocoatings: a comparison between experiment and molecular dynamics simulations, Int. J. Mat.
Res., 10, 1136 (2008)
[3] J.Hird and J.Fields, Diamond polishing, Proc. R. Soc. Lond. 460, 3547 (2004)
[4] L. Pastewka, P. Pou, R. Perez, P. Gumbsch, M. Moseler, Describing bond-breaking processes by reactive
potentials: the importance of an environment-dependent interaction range, Phys. Rev. B (R) 78, 161402 (2008)
! $+!

Novel approaches to the design of superhard and low-friction nanocomposite
coatings
A. Erdemir
Argonne National Laboratory
Energy Systems Division
Argonne-IL, USA
erdemir@anl.gov
Super-hard and low-friction coatings have been attracting overwhelming interest in recent years mainly
because of their superior property and performance characteristics under severe tribological
conditions. These novel coatings are truly multi-functional and consist of very unique chemical and
structural designs. The production of such multi-functional coatings can only be achieved by the use of
advanced deposition systems consisting of arc and sputtering processes equipped with a variety of
power sources like HiPIMS, pulsed DC, and cathodic arc, etc. The primary focus of this talk will be on
the chemical and structural design of such coatings for super-low friction and wear properties under
severe tribological conditions. For the structural and chemical design of such coatings, we will
introduce a crystal-chemical model that can be very useful in the selection of specific coating
ingredients which appear to be essential for their much superior performance and property
characteristics. In laboratory tests, such designer coatings can provide friction coefficients of 0.02 to
0.05 under severe boundary lubricated sliding conditions and they could not be scuffed under the
heaviest loadings that are available in reciprocating and block-on-ring test machines. Fundamental
friction and wear mechanisms of these coatings have been elucidated using a variety of surface
analytical tools including TOF-SIMS and XPS.
! %,!

Tribological properties of fluorinated carbon nano/micro particles
P. Thomas (1) , K. Delbe (1) , D. Himmel (1) , J.L. Mansot (1) , W. Zhang (2) , M. Dubois (2) ,
K. Guerin (2) , A. Hamwi (2)
(1) Groupe de Technologie des Surfaces et Interfaces (GTSI EA2432), Université des
Antilles et de la Guyane, Faculté des Sciences Exactes et Naturelles, Campus Fouillole,
Pointe a Pitre Cedex, Guadeloupe, France
(2) Laboratoire des Matériaux Inorganiques, Université Blaise Pascal de Clermont-Ferrand,
UMR-CNRS 6002, 63177 Aubiere, France
jlmansot@univ-ag.fr
Graphite fluorides are well known to present good lubricating properties and are commonly used as
friction reducers [1]. In this paper, the friction behaviour of various nano fluorinated carbons are
investigated under air as a function of fluorination temperature.
The tested materials are high purity carbon nanofibres, amorphous and graphitized carbon nanodics
and nanocones fluorinated at temperatures ranging between 280°C and 520°C in F2 atmosphere[2].
The friction properties of the compounds are investigated using a ball-on-plane tribometer under a
normal load of 10N and a sliding speed of about 2 mm.s-1. The pristine and fluorinated carbon nano
particles are deposited on the 52100 AISI steel planes previously polished and ultrasonically cleaned
in ethanol and acetone. The tribofilm structure at the end of the test is investigated using Raman
spectroscopy and compared to the initial nano particles structure.
In all cases, fluorinated nano compounds present better friction properties than pristine ones.
During the friction process, the fluorinated nanoparticles undergo chemical and physical
transformations leading to the build up of a stable tribofilm.
The Raman investigations of the films structure allowed us to point out that the friction process
induces a defluorination of fluorinated carbon phases and an increase of disorder as already observed
with graphite fluorides. However, the tribologic properties of nanofibres and graphitized nanodiscs and
nanocones remain better than fluorinated graphite ones. An action mechanism is proposed.
References
[1].P. Thomas, K. Delbe, D. Himmel, J.L. Mansot, F. Cadore, K. Guerin, M. Dubois, C. Delabarre, A. Hamwi,
Journal of Phys. Chem. of Solids, 67, (2006), 1095
[2].R. Yazami, A. Hamwi, K. Guerin, Y. Ozawa, M. Dubois, J. Giraudet, F. Masin, Electrochemistry
Communications, 9, (2007), 1850
Acknowledgments
The authors acknowledge the Conseil Régional de la Guadeloupe, the European Regional Development Fund
and the European Social Fund for their financial supports.
! %@!

Modeling tribochemical reactions in the Environmentally Controlled
Analytical Tribometer (ECAT)
M. I. De Barros Bouchet, T. Le Mogne, J. M. Martin
Laboratoire de Tribologie et Dynamique des Systèmes – UMR 5513 CNRS
Ecole Centrale de Lyon, 69134 Ecully Cedex, France
jean-michel.martin@ec-lyon.fr
We have developed a model experiment to study the reactivity of lubricant additives and environments
towards nascent and tribo-stressed metal surfaces. Reactive surfaces are created and friction tests
are conducted using an Environmentally Controlled Analytical Tribometer (ECAT) equipped with
XPS/AES analysis while a reactive gas partial pressure simulates the lubricant additive chemistry or
the atmosphere. First of all, the metal surface is cleaned by ion etching to remove the passivation
layer (usually oxide/hydroxide layers). The chemical purity of the metal surface is checked by XPS and
AES. A pin-on-flat friction test is then performed in UHV conditions (100 nPa residual gas pressure) on
this pure metallic surface previously obtained. Afterwards, a selected reactive gas is introduced in the
chamber and can react with the different activated surfaces which are exposed: the as-received, the
ion-teched and the tribo-stressed, respectively. At the end, the chamber is pumped down to UHV and
XPS analyses are carried out on the different locations, inside and outside the wear scars. We have
performed such model experiments on two metal surfaces (steel and a titanium alloy) and with
different partial pressures of gases (air, nitrogen, oxygen, tri-methyl phosphite, tri-methyl phosphate,
organic polysulfide) and at different temperatures of the substrates from ambient to 400 C. In the
absence (or depletion) of oxygen in the gas molecule, results show that typical species are
preferentially produced on the tribo-stressed surface, such as phophide [1], sulfide [2], nitride and
carbide [3]. The origins of the tribochemical reactions and chemical pathways are discussed in the
light of the chemical hardness model (HSAB principle). The chemical reactivity of the tribo-stressed
surface is explained in terms of exo-electronic emission, presence of defects and the possible
existence of triboplasma. Results are compared with practical surfaces obtained in fretting and
boundary lubrication laboratory experiments in presence of different environments.
References
[1] D. Philippon, M.I. De Barros Bouchet, Th. Le Mogne, E. Gresser, J.M. Martin, Trib.-Mat., Surf. and Interf.,
VOL1 N°3 (2007) 113-123.
[2] J. Tannous, M.I. De Barros Bouchet, Th. Le Mogne, P. Charles, J.M. Martin, Trib.-Mater., Surf. and Interface,
vol.1, N°2 (2007) 98-104.
[3] C. Mary et al, Tribology Letters, 2009 on line.
! %"!

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! %$!

Analysis of the tribological operation of fluid phase phospholipid
biomimetic surfaces
M.-C Corneci (1) , A.-M.Trunfio-Sfarghiu (1) , F. Dekkiche (2)(3) , Y. Berthier (1) , M.-H.
Meurisse (1) , J.-P. Rieu (2)
(1) Laboratoire de Mécanique des Contacts et des Structures, INSA-Lyon, CNRS UMR5259,
F69621 Villeurbanne Cedex, France
(2) Laboratoire de Physique de la Matière Condensée et Nanostructures, Université Claude
Bernard Lyon 1, CNRS UMR5586, F69622 Villeurbanne Cedex, France
(3) Département de Chimie, Faculté de Sciences exactes. Université Mentouri Constantine
(25000) Algeria
magdalena-carla.corneci@insa-lyon.fr
The difficulties associated with the rheological characterization of the \"biological lubricants\" to
understand the remarkable tribological performances of biological rubbing contacts, have led to study
their molecular composition especially that of molecular interfaces created with biological tissues.
Recent studies [Trunfio 2008] have shown the importance of surfactant-type molecules (phospholipid-
SAPL) on obtaining molecular surface layers with high mechanical strength and capable of locating
the velocity accommodation, leading to a very low friction coefficient. These molecular layers are
made of stacks of 3 to 7 SAPL bilayers [Hills 1989], with a thickness of about 5nm each and separated
by aqueous layers (a few nm thick, about 150mM ion and pH around 7 in the healthy case). Low
friction coefficients are due to the location of the velocity accommodation in the aqueous layers [Klein
2002] and it might be influenced by changes in ion concentrations or pH.
This work aims to study the influence of these parameters on the evolution of friction and the
degradation of SAPL bilayers during tests carried on an experimental realistic tribological model. This
model uses HEMA hydrogel to reproduce ex vivo the mechanical and physicochemical properties of
biological tissues. Nanostructural physical techniques such as lipid deposition by vesicle fusion
method [Bayerl 1990] or by co-adsorption of lipid-detergent micelles [Tiberg 2005] and atomic force
microscopy are used to reproduce and characterize the nano-mechanical resistance of SAPL bilayers
forming biological rubbing surfaces. The evolution of SAPL bilayers is visualized in situ, during friction
tests (velocity: 0.5mm/s; contact pressure 0.3MPa), by fluorescence optical microscopy using specific
molecular markers. To study the influence of pH, two TRIS buffer solutions at pH 3 and 7.2 are used.
The influence of ion concentration was tested using TRIS pH 7.2 with ionic concentrations of 0 or 150
mM NaCl.
The main experimental results are summarized below:
- Using non-buffered solution gives to SAPL bilayers low mechanical strength to the nanoindentation
(100% of the bilayers do not resist to normal pressures between a few tenths of MPa and few MPa.),
which promotes their degradation and the increase in the friction coefficient by about 80% after 1 hour
of friction.
- Using buffered solutions (pH 7.2) increases the mechanical strength of the SAPL bilayers (in most
cases they resist to pressures over 20MPa) which prevents their degradation and stabilizes the friction
coefficient to a value depending on the ionic concentration and pH. The ionic solution decreases by
about 1.2 times the friction coefficient compared to a non-ionic solution. The low pH promotes friction
coefficients up to 2 times smaller than the higher pH.
This study therefore shows that good nano-mechanical resistance of the SAPL bilayers is essential to
obtain low friction. It also suggests that low friction is ensured by ion hydration layers between
adjacent SAPL bilayers and that a buffered medium is essential to maintain stable bilayers.
! %%!

Tribological properties of graphite intercalation compounds:
correlation to their electronic structure.
K. Delbé (1) , D. Himmel (1) , J.L. Mansot (1) , P. Thomas (1) , Y. Bercion (1) , F. Boucher (2) , D.
Billaud (3)
(1) Groupe de Technologie des Surfaces et Interfaces (GTSI, EA 2432), Université des
Antilles et de la Guyane, 97159 Pointe-à-Pitre Cedex, France
(2) Institut des Matériaux de Nantes Jean Rouxel, UMR CNRS 110, 2 rue de la Houssinière,
44072 Nantes cedex 03, France
(3) Laboratoire de Chimie du Solide Minéral, UMR 7555, Université Henri Poincaré Nancy I,
B.P. 239, 54506 Vandœuvre-lès-Nancy Cedex, France
jlmansot@univ-ag.fr
The good tribological properties of lamellar compounds (MoS2, Graphite…) are classically associated
to the presence, in their structures, of van der Waals Gaps through which interlayer interactions are
weak leading to low critical shear rate along directions parallel to the layers.
Always due to the presence of the van der Waals Gap, most of the lamellar compounds are subjected
to intercalate various chemical species such as atoms, ions, molecules…[1].
In the present work the intercalation of selected nucleophilic (alkaline atoms) and electrophilic
(Transition metal chlorides) species in graphite is used in order to modulate in a controlled manner the
intergraphene layer distances and the electronic band structure of the host compound.
Ab initio band structure calculations (pseudo-potentials and FLAPW methods) carried on the various
studied intercalated graphite allowed us to obtain band structure, Density of State diagrams, valence
electron density maps and then to access to the bonding interactions between the intercalants and the
graphene planes.
Tribologic investigations are carried out under high purity argon atmosphere using a sphere (AISI
52100 steel) on plane (AISI 52100 steel) tribometer. Thin films (few !m thicknesses) of intercalated
compounds are deposited onto the plane surface by burnishing. The drastic reduction of the friction
coefficient from 0.20 in the case of graphite down to 0.10 when graphite is intercalated is correlated to
the graphene /graphene planes interactions reduction resulting from the presence of intercalated
species or intercalated species mobility. The Raman spectra recorded on the tribologic films at the end
of the tests are compared to the spectra recorded on initial intercalated compounds. They point out
that during friction partial de intercalation processes occur leading to higher stages intercalation
compounds.
Reference
[1] E. Stumpp, “Intercalation of metal chloride and bromide into graphite”, Materials Sciences and Engineering,
31, (1977), 53-59.
Acknowledgements
The authors would like to thank the C3I for its technical support in collecting computational data and the Conseil
Régional de la Guadeloupe, the European Regional Development Fund and the European Social Fund for their
financial supports.
! %&!

Identification and analysis of friction regimes due to the failure of
thin lubricant films
S. Eder (1) , G. Vorlaufer (1) , A. Vernes (1) (2) (1) (2)
and G. Betz
(1) Austrian Center of Competence for Tribology, Viktor-Kaplan-Str. 2,
2700 Wiener Neustadt, Austria
(2) Institute of Applied Physics, Vienna University of Technology,
Wiedner Hauptstr. 8 - 10 / 134, 1040 Vienna, Austria
eder@ac2t.at
The frictional behavior of nano-rough Fe-surfaces lubricated with adsorbed fatty acid molecules is
studied using classical molecular dynamics simulations (MD). The main interest of this contribution lies
in the load-dependent failure of the lubricant film at the critical molecular surface coverage.
Here, the load-friction-dependence exhibits discontinuities which allow the identification of friction
regimes differing in type and intensity of the interaction between the opposing asperities. For a better
understanding of these findings, they are also analysed in terms of the momentary solid-solid contact
area.
! %'!

Simultaneous measurements of pressure, lubricant film thickness
and temperature distributions in lubricated rolling sliding contacts
using High Spatial Resolution in situ Raman Spectrometry
D. Himmel (1) , Y. Bercion (1) , A. Sauldubois (1) , T. Lubrecht (2) , J.L. Mansot (1)
(1) Laboratoire de Technologie des Surfaces et Interfaces (GTSI EA 2432)
Université des Antilles et de la Guyane, Faculté des Sciences Exactes et Naturelles campus
de Fouillole, 97159 Pointe a Pitre Cedex Guadeloupe, France
(2) LaMCos UMR CNRS 5259, Insa de Lyon, 2 Avenue Albert Einstein,Villeurbanne, France
jlmansot@univ-ag.fr
In order to progress in the understanding of energy dissipation in dynamic lubricated contacts,
knowledge of in situ parameters of the tribologic interface are needed. A dedicated experimental set
up, coupling a Raman microprobe to an elasto-hydrodynamic ball on disc tribometer, has been
developped in order to experimentally investigate lubricant film thickness, pressure and temperature
distributions in the running contact.
In situ quantitative Raman microscopy [1] is used to acquire high spatial resolution (10!m)
spectroscopic images in the elasto-hydrodynamic lubricated (EHL) sphere/plane contacts.
The lubricant used, 5P4E, presents a strong Raman band near 1000 cm-1 (trigonal breathing vibration
mode of the aromatic rings ) which allowed us to analyse film thicknesses down to 0.1 !m.[2,3].
The quantitative analyses of the intensity, energy shift and Raman Stoke/anti Stoke band ratio of the
1000 cm-1 Raman band of 5P4E allowed us to deduce the local lubricant film thickness, pressure and
temperature and then to establish simultaneous distribution of these parameters in the rolling sliding
contact with a high spatial resolution [1-4]. The experimentally measured distributions of the three
contacts parameters in the case of pure rolling contacts are in good agreement with theoretical ones.
The significant increase of the temperature in a rolling sliding contact attributed to the shearing of the
lubricant film in the contact area explains the variation of the lubricant film thickness distribution
experimentally observed.
References
[1] J.L. Mansot, "Etude des pressions dans un interface sphère/plan en présence d'une couche mince organique",
PhD Thesis, University of Lyon, France , n° 86.50 (1986)
[2] I. Jubault, J.L. Mansot, P. Vergne and D. Mazuyer, ASME Trans., J.of Tribology, vol.124, (2002) 114.
[3] Jubault, J. Molimard, A.A. Lubrecht, J.L. Mansot and Ph. Vergne, Tribology lettres, vol. 15, n° 4, (2003) 421.
[4] Himmel D., PhD Thesis, (2005) Université des Antilles et de la Guyane
!
Acknowledgements
The authors acknowledge the CNRS, the Conseil Régional de la Guadeloupe, the E Social Européen (FSE) and
Fonds Européens de Développement Régional (FEDER) for their financial supports. !
! %(!

MicroFEM modelling and stress simulation of thick composite
coating structures
K. Holmberg (1) , A.Laukkanen (1) , A. Ghabchi (1) , A.Helle (1) , K. Rissa (2)
(1) VTT Technical Research Centre of Finland, Espoo, Finland
(2) Tampere University of Technology, Tampere, Finland
kenneth.holmberg@vtt.fi
The problem of wear modelling of thick composite coatings has been approached by finite element
method (FEM) modelling on microlevel and stress and strain simulation. The contact between a sliding
spherical diamond tip with increasing load on a thick thermal sprayed WC-CoCr coating has been
modelled by a complex multigrid microFEM mesh. About 5000 carbide particles of various size and
shapes extracted from high resolution SEM images are included in the model. The model includes real
geometrical and mechanical characteristics of microstructural features such as carbide particles,
decarburised regions, pores and cracks. The densest part of the FEM mesh around the particle
contour details has a node distance of only some 10-20 nanometres. Stress peaks and patterns as
well as strain and deformation are shown in simulated loading conditions. The results are compared to
SEM micrograph observations from various loaded regions of WC-CoCr samples that have been
loaded in scratch test under similar experimental conditions. The crack initiation and crack growth
mechanisms are discussed.
!
! %+!

A simpler model for friction between rubber and rough hard
surfaces
S. Jacobson, M. Åstrand and P. Hollman
Tribomaterials Group, Dept of Engineering Sciences, Uppsala University, Sweden
staffan.jacobson@Angstrom.uu.se
A new simplified model for the friction between rubber and hard rough surfaces is presented. The
model primarily treats the bulk term of the friction, but does this without including damping or
hysteresis effects. This simplification compared to previous models obviously introduces limitations to
the predictive abilities, but on the other hand its very simple structure and set of assumptions facilitate
understanding and makes the new model more readily applicable to anlayse real life situations and
real materials.
The model assumes the seemingly naive idea that friction is primarily due to stronger contact (higher
pressure) against the up-hill side of surface irregularities, which results in a backward pointing
component of the normal force. The corresponding adhesive term due to uphill sliding adds to this
normal force component, as can be described by a simple geometrical expression.
Experimentally the model has proven to work very well and giving interesting insights into the
influence from the shape of the rough counter surface on rubber friction. The experiments involve
various devices for sliding shoes or smaller rubber samples against specially designed surfaces that
each effectively gives only one single contact slope. The adhesive component of the friction is reduced
to a minimum by using a lubricant. Friction data is presented from a number of simplified surface
designs, various rubber types, sliding speeds, etc. It is shown that the bulk term of rubber friction is
very precisely predicted by the model for the model surfaces.
For these surfaces, where the contact angle is well defined and the adhesive term made negligible, it
is shown that the coefficient of friction very closely follows the simple geometrical expression ! = tan
ø, where ø is the (uphill) slope angle of the protrusions on the contact surface.
Deviations from the model predictions can be utilised to understand other effects, estimate the effect
of damping, the effect of sliding speed on the damping, the effect of micro roughness, lubrication, etc.
The ability to estimate a minimum friction level, i.e. the minimum friction left in the case of greasy
substances contaminating the surfaces, could potentially become useful in safety work regarding
slipping on floors or skidding of cars.
! &,!

Lubricant properties of mixed cationic surfactant and hydrophilic
diblock copolymer films
J. M. Lagleize, C. Drummond, Ph. Richetti
CRPP, University of Bordeaux, France
drummond@crpp-bordeaux.cnrs.fr,
Biomedical and environmentally friendly applications require specific lubricants that can be use in
aqueous media. As we have shown in the past, water soluble charged surfactants that self assemble
as smooth bilayers are effective in this context. They provoke very low friction forces as long as the
bilayers remain undamaged, by trapping a very thin film of water. Unfortunately, the films are often not
cohesive enough and are readily disrupted under pressure, leading thus to high friction forces and
wear of the surfaces.
We will discuss the effect of coadsorbing a copolymer with bilayer forming surfactants on the cohesion
and lubricant properties of the mix layer. Three different cationic surfactants with increasing
oligomerisation degree that adsorb spontaneously on negatively charged mica surfaces were studied:
a monomer (CTAC), a dimer 12-3-12 and a trimer 12-3-12-3-12. The coadsorbing copolymer is a
diblock hydrophile-hydrophile polyacrilic acid-polyacrilamide. We used two different techniques,
Atomic Force Microscopy (AFM) and the Surface Force Apparatus (SFA) to investigate the self
assembled layers.
We have found that the mix systems are more cohesive than the simple surfactant bilayers. However,
under specific conditions of pressure and composition, the lubricants show an original behavior: a
dynamic transition from a low friction force state to a high friction force state associated with a
thickness decreasing of the boundary layer is observed. We can understand this phenomenon as the
delamination of the boundary lubricant in several steps: first, wear of the films is observed, evidenced
by the growth of friction and film thickness; then matter is expulsed from the contact and the friction
force and the thickness decrease; finally, when a critical low thickness is achieved, a dramatic
increase of friction force is observed. The effect of the oligomerization degree of the cationic surfactant
on the cohesion of the layer and the different dynamic transition observed will be discussed.
! &@!

Rubber friction on wet road track
Y. Le Chenadec, D. Charleux, M. Portigliatti
Michelin Technology Center, Ladoux, France
yohan.le-chenadec@fr.michelin.com
It is well accepted that rubber sliding friction has two main causes: hysteresis losses and adhesion
effects which occur only under dry condition. Under wet condition, it is supposed that a very thin layer
of water remains between the rubber and the track preventing adhesion effects from occurring. When
rubber is sliding over a rigid indenter, deformation occurs in the bulk rubber. Due to the viscoelastic
properties of the material, there is energy dissipation (or hysteresis losses) which creates a force
opposite to the sliding direction.
In this study, we propose to analyse the experimental behaviour of rubber sliding on a wet road track
with special emphasis on the effect of sliding speed, temperature and track. The guiding principle of
our analysis is the following: studying the friction behaviour of rubber compound under realistic tire
operating conditions.
The nominal pressure of contact is directly connected to the tire inflation pressure and the contact
surface ratio. Experiments are thus conducted at 2 and 3 bars, representative of passenger car tire.
The sliding speed during ABS wet braking ranges typically from 0.1 m/s to 5 m/s, so that we focus on
speeds around 1 m/s. The rubber temperature in the contact patch is mainly conditioned by external
temperature of air and track and is thus significantly variable. Last but not least, the road tracks can be
very different and it is obviously necessary to take into account this diversity in experiments.
The experimental set-up is the following: an annular rubber specimen is pressed against a track, then
it is put in rotation. The stationary coefficient of friction (cof) is measured during the rotation.
It is observed that the coefficient of friction describes a bell shape curve in function of both sliding
speed and temperature. This well-known fact reminds the viscoelastic origins of rubber friction. We
show that with these friction data, it is not possible to obtain a master curve by applying time
temperature equivalence: the coefficient of friction at the peak of a temperature curve is for example
decreasing with the sliding speed. The originality of our work is that we pay great attention to analyse
the cof as a function of the temperature and not only as a function of the sliding speed.
Compounds with various glass transition temperatures (Tg) are tested. A shift in temperature curves
and then intersection is observed for these compounds. Moreover, the occurrence of the maximum of
!(T) depends on Tg.
Two types of friction tracks are tested. The first one is road pavement track coming from road coring or
road plates. The second one is artificial tracks: road stones are maintained on a support with resin.
Road pavement friction tracks are realistic for tire condition, but we show that artificial tracks give
similar results, so that is it possible to study rubber friction on these tracks. The main advantage of
these tracks is their robustness with temperature and stone failure. Two facts are enlightened: the
road friction track has a great influence both on the cof level and on the position of the cof maximum
with temperature for the same sliding speed. This can been read as differences between tracks on
their roughness at various scales.
! &"!

Improving the scratch resistance of PMMA surface by using
plasticizers
M. Mansha, C. Gauthier, R. Schirrer
University of Strasbourg, Institute Charles Sadron, CNRS-UPR 22,
23 rue du Loess, BP 84047, F-67034, Strasbourg, cedex 2, France
muhammad.mansha@ics-cnrs.unistra.fr
Industrial use of polymers ranges across a broad field of structural, mechanical, electrical and optical
applications. Scratch durability of polymer surfaces and coatings is becoming critical for the increasing
use of these materials in new applications, replacing other materials with more resistant surfaces. An
understanding of abrasion resistance and the associated surface deformation mechanisms is of
primary importance in the materials engineering and design of many important industrial components
undergoing wear and abrasion. The scratching technique has gained interest in recent times due to its
varied applications to a number of engineering materials, especially for the evaluation of surface
scratch resistance of plastics. The various aspects of friction at appropriate scales had been
investigated in detail. A lot of work is also available on scratch and nano-indentation of various
polymer surfaces. Various physico-chemical processes such as annealing and various ion plantation
techniques have been proved very useful for the modification of polymeric surfaces but their
applications are limited to some certain polymers due to some disadvantages. Especially, some of
them can not be applied to transparent polymers like PMMA due to their darkening effect.
Improvement of the scratch resistance must be investigated primarily as an effect due to decreasing
the friction coefficient. The recovery increases if the tip is smooth or if the local friction coefficient is
low. Investigation of the reduction of friction through use of different plasticizers is an area that has a
great potential of research. Discovery of appropriate plasticizers for different polymers to reduce their
surface friction is yet to be made in order to improve their scratch resistance. This issue is addressed
in our work.
A widely used polymer PMMA was selected for this study. The effect of varius plasticizers like PEG,
stearamide, behenamide, erucamide and two types of crodamide (Crodamide-212 and Crodamide-Er)
on both surface properties and bulk mechanical properties were studied at a wide range of
temperature (ranging from -40 to +85°C). The surface properties included like over all and local friction
coefficient, contact area and pressure, contact angle, groove size etc where as bulk mechanical
properties studied were like Young modulus , storage modulus , loss modulus, loss factor tand,
Poisson ratio, yielding stress and strain, shear stress, elongation at break etc. Our experimental
results show that a decrease in friction coefficient is possible by the introduction of appropriate
plasticizer without having a significant effect on its bulk behaviour and this decrease in friction
depends upon the nature and the content of plasticizer. Moreover, fatty acid amides have been proved
more efficient in decreasing friction than PEG. Among different fatty acid amides studied, erucamide
and crodamide were proved more efficient than others. The friction decreases directly with the
percentage of crodamide whereas erucamide shows minimum friction at 0.05%. Further increase in
the erucamide content gives anti-plasticization effect and hence, again increases friction coefficient.
PMMA samples with 0.10 % Stearamide and 0.10% Behenamide also gave a significant decrease in
friction coefficient.
The values of contact angle and in-situ photographs during scratching show that the decrease in
friction is associated with the nature of the contact between the tip and the polymer surface.
! &#!

Experimental and theoretical approaches of energy dissipation in
boundary lubricated contacts
J.L. Mansot (1) , J.M. Martin (2) , Y. Bercion (1)
(1) Groupe de Technologie des Surfaces et Interfaces (GTSI EA 2432)
UFR Sciences Exactes et Naturelles , Université des Antilles et de la Guyane
Campus de Fouillole, 97159 Pointe à Pitre, Guadeloupe, France
(2) Laboratoire de Tribologie et Dynamique des Systèmes, UMR CNRS 5513, Ecole Centrale
de Lyon, 36 Avenue Guy de Collongue, BP 163, 69631 Ecully Cedex France.
jlmansot@univ-ag.fr
Lots of experiments carried out with monomolecularly lubricated contacts by means of organic acids or
hydrocarbons demonstrated in the past the influence of organic chain length on the friction coefficient
associated to energy dissipation in contacts [1]. Complementary experimental and theoretical works
allowed to propose that friction dissipation can be understood as combination between molecular
interactions (depending on molecules shape) and “dynamic” electrostatic interaction between
substrate surfaces during sliding [2,3]. The present paper is concerned with an experimental approach
of molecularly lubricated sphere plane contacts. The relationship between average adsorbed film
thicknesses in the contact and friction coefficient associated to the careful interpretation of Electrical
Contact Resistance/Friction coefficient dependency tend to confirm the existence of the electrostatic
interaction between rubbing surfaces which contribution, in the energy dissipation process, is
modulated by the separation distance (related to molecular film thickness) and dielectric properties of
the tribologic interface (dielectric properties of molecules in the interface).
An expression of the friction coefficient can be deduced from the experiments as:
!=!(M)+Kh-n
where !(M) is the friction contribution due to molecular interactions
K an experimental constant, h the distance between substrate surfaces and n an
exponent experimentally deduced and close to 4.
References
[1] Bowden, F. P., and Tabor, D., “The Friction and Lubrication of Solids“, Clarendon Press Oxford, (1950).
[2] W.A.ZISMAN, “durability and wettability properties of monomolecular films on solids“, friction and wear, R.
DAVIES (Ed), Elesevier, Amsterdam,p110-148,(1959).
[3] S.N. POSTNIKOV, Electrophysical and electrochemical phenomena in friction cutting and lubrication, Litton
Educational Publishing, Inc. , New York (1978).
[4] J.L. MANSOT, “ Aspects microscopiques de l’action des réducteurs de frottement en lubrification limite”, these
Docteur Ingénieur, Ecole Centrale de Lyon, (1982).
! &$!

An improved description of bond-breaking processes in empirical
interatomic potentials
L. Pastewka, P. Pou, R. Perez, P. Gumbsch, M. Moseler
Fraunhofer Institute for Mechanis of Materials IWM
Wöhlerstraße 1,1 79108 Freiburg, Germany
lars.pastewka@iwm.fraunhofer.de
mos@iwm.fhg.de
First nearest-neighbor models are routinely used for atomistic modeling of covalent materials.
Neighbors are usually determined by looking for atoms within a fixed interaction range. While these
models provide a faithful description of material properties near equilibrium, the limited interaction
range introduces problems in heterogeneous environments and when bond-breaking processes are of
concern. This becomes of special importance when modeling tribology, where high shear stress and
pressure pushes the system into non-equilibrium states and induces mechano-chemical breaking of
bonds. We demonstrate that the reliability of reactive bond-order potentials is substantially improved
by using an environment-dependent first nearest-neighbor definition.
In particular, we extend the hydrocarbon interatomic potential of Brenner and co-workers, also known
as the reactive empirical interatomic potential (REBO). The performance of the improved description is
shown for two test cases: Brittle fracture in diamond and the phase stability of quenched amorphous
carbon films. In the first case, brittleness is rediscovered and even lattice trapping is in reasonable
agreement with density functional theory (DFT) calculations. Also, quenched amorphous carbon
shows the correct number of sp3-hybrids as a function of density as opposed to the original
formulation. We are hence faithful that the improved potential can be used for the reliable simulation of
tribological properties of diamond and diamond-like materials.
Reference
L. Pastewka, P. Pou, R. Perez, P. Gumbsch, M. Moseler, Phys. Rev. B 78, 161402(R) (2008)
!
! &%!

Influence of adsorbates on atomic stick-slip resolution and
influence of scan rate on friction forces studied by AFM
N. Podghainiy , M. Nielinger and H. Baltruschat (1)
(1) Institute for Physical and Theoretical Chemistry, University of Bonn,
53117 Bonn, Germany
podghainiy@pc.uni-bonn.de
Friction under ambient conditions often involves wet surfaces, and thus electrochemical interfaces.
Yet, measurements of friction at an atomic scale under electrochemical conditions are scarce. For
HOPG, a change of friction forces at steps edges was observed with a change of potential.[1, 2] We
recently started to measure friction by AFM and the influence of potential, Cu UPD on Au(111) and
other parameters thereupon.[3]
We will present friction measurements performed on Au(111) single crystal electrodes and the effect of
adsorbates on atomic stick-slip resolution. The range of a normal force necessary to observe stick-slip
resolution depends on the type of ionic adsorbates. Atomic resolution is easily observed if sulfate
anions are adsorbed on the gold or copper submonolayer surface. However, with the type of cantilever
we used, it was not possible to observe stick-slip resolution on a monolayer of Cu on Au(111) and on
Cu bulk deposits.
In contrast to Couloumb’s friction law [4] the tip scan rate has an influence on the friction force,
however, only when sulfate is adsorbed on the clean Au surface.. Such effects were found and
explained with conditions when atomic stick-slip occurs [5].
References
[1] E. Weilandt, A. Menck, O. Marti, Surf. Interface Anal. 1995, 23, 428.
[2] B. Schnyder, D. Alliata, R. Kötz, H. Siegenthaler, Appl. Surf. Sci. 2001, 173, 221.
[3] M. Nielinger, H. Baltruschat, Phys. Chem. Chem. Phys., 2007, 9, 3965–3969
[4] E. Meyer, R.M. Overney, K. Dransfeld, and T. Gyalog, Nanoscience: Friction and Rheology on the Nanometer
Scale (World Scientific Publishing, Singapore, 1998)
[5] E. Gnecco, R. Bennewitz, T. Gyalog, and E. Meyer, J. Phys.: Condens. Matter 13, R619 (2001)
Acknowledgements
Authors acknowledge financial support of the DFG
! &&!

Assessment of the surface state behavior of complex metallic
alloys in sliding contacts
P. Ponthiaux (1) , N. Diomidis (2) , J.M. Dubois (2) , J. P. Celis (3)
(1) École Centrale Paris, Laboratoire de Génie des Procédés et Matériaux,
Grande Voie des Vignes, 92295 Châtenay-Malabry, France
(2) Ecole des Mines de Nancy, 54042 Nancy, France
(3) Katholieke Universiteit Leuven, Dept. MTM, Kasteelpark Arenberg 44, B-3001 Leuven,
Belgium
Jean-Pierre.Celis@mtm.kuleuven.be
Electrochemical measurements and friction measurements during continuous and intermittent
unidirectional sliding are used to monitor and to evaluate the surface characteristics of two types of
metallic materials characterized by a huge unit cell, namely Al 71 Cu 10 Fe 9 Cr 10 and Al 3 Mg 2 .
The modification of the surface characteristics results from the periodic mechanical removal of a
surface film during sliding, and the subsequent (electro)chemical re-growth of a surface film inbetween
successive sliding contacts. Al 71 Cu 10 Fe 9 Cr 10 and Al 3 Mg 2 materials were tested in a
phosphate buffer solution pH 7 at 25 °C to compare their depassivation and subsequent repassivation
behaviour. The Al 3 Mg 2 material was also tested in a 0.1 M KOH solution pH 13 and 25 °C to reveal the
role of constituting metallic elements on the surface film formation.
The effect of film formation and removal on the coefficient of friction recorded during unidirectional
sliding is discussed.
Acknowledgments
This work was done within the Network of Excellence CMA funded by CEC-FP6.
! &'!

Quantitative modeling of steady-state EHL problems: what are the
current limits?
P. Vergne (1) , W. Habchi (2) , N. Fillot (1) , S. Bair (3) , G.E. Morales-Espejel (1)(4)
(1) Université de Lyon, CNRS, INSA-Lyon, LaMCoS UMR5259, 69621, France
(2) Lebanese American University, Department of Industrial and Mechanical Engineering,
Byblos, Lebanon, USA
(3) G.W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology,
Atlanta, USA
(4) SKF Engineering and Research Center, Nieuwegein, The Netherlands
nicolas.fillot@insa-lyon.fr
Quantitative modeling of steady-state elastohydrodynamic lubrication (EHL) problems aims to propose
an accurate prediction of tribological parameters, namely film thickness and friction, based on both,
primary laboratory data and appropriate physically-based models that describe the actual lubricant
behavior. The overall objective of this approach is to offer relevant results to improve our
understanding of the highly coupled physical mechanisms that take place within EHL contacts. It
excludes, in its principle, all data or relationship that could come from adjustable fitting with traction or
film thickness measurements, in contrast to many works published during the 70’s-90’s period. Finally,
it is expected that these calculations may be compared for validation with experimental results. This
further step should be conducted thoroughly and rigorously in a thorough and rigorous fashion to avoid
misinterpretation.
Recent years have seen a substantial development of new numerical methodologies in the field of
modeling EHL problems. For instance, film thickness prediction has been greatly improved, especially
in terms of spatial resolution, dynamics, complex kinematics, calculation time, accuracy, etc. However,
these studies have served to advance the understanding of surface feature effects much more than
issues related to the lubricant itself and its two major primary tribological functions, separation of
surfaces in relative motion and friction reduction. Accurate prediction of both film thickness and friction
under steady-state conditions is still an important challenge. It is directly related to engineering,
industrial and society-concerned issues like lifespan optimization of machines and energy saving.
Tackling an EHD problem requires solving at least Reynolds, film thickness and load balance
equations. But real lubricants behave as non-Newtonian fluids and their response is not only governed
by pressure and the applied shear stress but is strongly temperature dependent requiring solution of
the energy equation when friction is considered. The thermo-physical parameters are also influenced
by pressure and temperature and this should be accounted in a friction simulation, especially when the
contact is subjected to severe operating conditions. This finally requires a multi-physics set of coupled
equations and models.
In this context, the presentation will address several points that should be investigated to achieve a full
quantitative modeling of steady-state elastohydrodynamic lubrication (EHL) problems, as for instance:
- The relevance of the limiting shear stress concept often used for highly loaded lubricated contacts,
what is the mechanism, what is the nature of the transition, and what is the temperature dependence?
- How far are we allowed to consider the lubricant’s response as time independent in EHL?
- What is the extent of wall-lubricant slip or apparent slip, and the role of surface topography in it?
- What is the lower limit where continuum mechanics approach is no longer valid? Does the molecular
dynamics approach provide an appropriate alternative in case of such confined liquid films?
! &(!

Ab initio atomic-scale friction of graphene
A. Vernes (1)(2) , G. Vorlaufer (1) , I. Ilincic (1) , S. Eder (1) (1) (3)
and F. Franek
(1)Austrian Center of Competence for Tribology, Viktor-Kaplan-Str. 2,
2700 Wiener Neustadt, Austria
(2) Institute of Applied Physics, Vienna University of Technology, Wiedner Hauptstr. 8 - 10 /
134, 1040 Vienna, Austria
(3) Institute of Sensor and Actuator Systems, Vienna University of Technology, Floragasse 7
/ 366, 1040 Vienna, Austria
vernes@ac2t.at
In the present contribution the quasi-static sliding of graphene on top of a semi-infinite graphite is
theoretically investigated from an ab initio DFT-based point of view.
It is shown that besides of the matching (either commensurate or incommensurate) of atomic surfaces
in dry contact, the calculated friction forces are also strongly dependent on the relative angle of the
straight sliding path. How this could affect the atomic-scale friction measurements in the case of
graphite, i.e., both stick-slip and structural lubricity regimes, it is also analysed, e.g., by introducing the
ab initio atomic forces into the Prandtl-Tomlinson model.
! &+!

Ab initio calculation of adhesion and potential corrugation of
surfaces in contact, insight into the nanotribological properties of
diamond
G. Zilibotti (1) , M. Ferrario (1) (2) , C. M. Bertoni (1) (2) (1) (2)
and M. C. Righi
(1) Dipartimento di Fisica, Universita di Modena e Reggio Emilia, Via Campi 213/A, 41100
Modena, Italy
(2) INFM-CNR National Research Center on nanoStructures and bioSystems at Surfaces
(S3) 41100 Modena, Italy
giovanna.zilibotti@unimore.it
Diamond films, artificially grown by chemical vapor deposition, are receiving a lot of attention due to
their outstanding mechanical and tribological properties. Diamond is used for coating of tools,
automotive components and is considered a very promising material for micro/nanoscale applications
such as MEMS/NEMS. A microscopic understanding of the mechanisms that govern the tribological
behavior of diamond is necessary to control and design its application in particular at the nanoscales.
It is observed, for example, that the tribological performances of diamond are extremely sensitive to
the environmental conditions, and that the surface passivation is the key mechanism that determines
the changes of the friction coefficient [1]. At the atomic level, the behavior of a slider is dictated by the
potential energy surface (PES) it experiences due to the interaction with the substrate. We applied ab
initio calculations to derive the PES and hence the forces acting during the relative displacement of
two diamond surfaces. In particular we analyzed the effects of i) the surface structure and orientation,
ii) the surface chemical composition, iii) the presence of an applied load. To this aim we studied both
dimer-reconstructed (001) surfaces and (111) surfaces, taking into consideration different kind of
adsorbates like hydrogen, oxygen, hydroxyl groups and water molecules. The surface stability in
presence of the different kind of adsorbates is calculated along with the variations of the surface
adhesion and of the potential corrugation. The role played by dangling carbon bonds is discussed in
relation to previous findings. The magnitude of the friction forces per atom is calculated as a function
of load. This latter analysis, which takes into account both the structural and electronic properties of
the surfaces under pressure, may provide a new piece of information to understand the friction laws at
the nanoscale.
Reference
[1] A. R. Konicek, D. S. Grierson, P. U. Gilbert,W. G. Sawyer, A. V. Sumant, and R.W. Carpick, Phys. Rev. Lett.
100, 235502 (2008).
!
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! '"!

Length-scale issues in adhesive contact theories
E. Barthel
CNRS / Saint-Gobain UMR 125, 39 quai Lucien Lefranc, BP 135, 93303 Aubervilliers Cedex,
France
etienne.barthel@saint-gobain.com
We propose an introduction to classical and more advanced adhesive contact theories using a lengthscale
approach.
First, the main features of adhesive elastic contacts at the macroscopic and microscopic scales will be
reviewed under the length-scale viewpoint, and the connection to other adhesion problems and to
fracture mechanics concepts will be highlighted.
One of the major assumptions of the classical adhesive contact theories is the homogeneity of the half
spaces in contact. In a number of cases however heterogeneous half spaces must be considered, with
a definite impact on the adhesive contact response which we will discuss. The important example of
coated substrates will be reviewed and application of the coated substrate models to literature data will
be reviewed.
Another major assumption is the instantaneous response of the materials in contact. In many systems,
such as polymers and gels, this assumption is severely challenged. For that reason, we will discuss
the case of time dependent materials. As an example, we will detail the adhesive contact of
elastomers, which are elastic at low frequency and viscoelastic at high frequency. Comparison of the
predictions with literature data will be presented.
This course will in part be based on our recent review (J. Phys. D: Appl. Phys. 41 (2008) 163001).
! '#!

Novel aspects of atomic-scale lateral force microscopy
P. Steiner, R. Roth, E. Gnecco, T. Glatzel, A. Baratoff and E. Meyer
Department of Physics, University of Basel, Klingelbergstr. 82, 4056 Basel, Switzerland
alexis.baratoff@unibas.ch
Previous work in our group on the transition to continuous sliding with ultralow friction achieved by
reducing the applied normal force or by applying perpendicular electrical or mechanical oscillations [1]
has been extended in several directions over the past 1 " years. Selected topics among the following
ones will be discussed in detail.
The lateral force, static and kinetic friction have been computed for arbitrary scan directions within a
separable 2D Prandtl-Tomlinson like model (lowest Fourier component of the corrugation potential)
and interpreted in terms of the probe tip trajectories and elastic slip instability boundaries [2, 3].
Detailed analytic predictions are confirmed by dynamic simulations assuming overdamped tip motion.
Continuous sliding sets in all scan directions below a treshold corrugation amplitude. Experimental
verification of the predicted angular dependence in the stick-slip regime is, however, complicated by
instrumental effects if only the flexion and torsion of a rectangular cantilever are detected. So far most
measurements have been concerned with scans parallel to symmetry directions (or nearly so).
Nevertheless several novel phenomena could be observed and successfully interpreted.
Comparison with computed lateral force images on cleaved NaCl(001) samples indicate that in the
wearless regime the main deformation is at the tip apex and is essentially isotropic. The computed
effects of thermal activation and perpendicular actuation are then quite similar to previous predictions
based on the 1D Prandtl-Tomlinson model [3].
The observed more frequent occurrence of slips over several lattice spacings with increased normal
force can be explained by a gradual transition from overdamped to slightly underdamped tip motion. A
comparison of experimental and computed slip-length histograms provides a rough estimate of the
lateral damping in contact with respect to a critical damping value largely unaffected by the corrugation
amplitude [4].
The flexural and torsional resonance frequencies of the cantilever in contact measured with electronics
developed for non-contact atomic force microscopy exhibit atomic-scale contrast which correlates with
the lateral force variations [5]. Moreover, higher resolution of point defects can thus be achieved. The
normal and lateral contact stiffnesses of a few N/m, determined via appropriate dynamic calibration
procedures [1, 5], indicate that the contacts are of atomic size for normal forces of a few nN.
References
[1] E. Gnecco et al., Nanotechnology 20, 025501 (2009)
[2] P. Steiner et al., Phys. Rev. B79, 045414 (2009)
[3] P. Steiner et al., R. Roth et al., unpublished
[4] R. Roth et al., Trib. Lett., in press
[5] P. Steiner et al., Nanotechnology 20, 495701 (2009)
! '$!

Large-scale quantum chemical molecular dynamics simulation on
tribochemical reaction dynamics
M. Kubo
University of Tohoku, Japan
momoji@rift.mech.tohoku.ac.jp
Tribological phenomena are generally caused by the microscopic behaviors of lubricant molecules at
the contacting interface under high pressure. In addition to the experimental researches, recently
computational simulations are strongly expected to clarify such phenomena on atomic-scale. Classical
molecular dynamics is very powerful tool to investigate the atomistic behaviors of the friction dynamics
and to evaluate the friction coefficient. However, more recently in addition to the atomistic
understanding of the tribological dynamics, the electronic-level understanding of the tribochemical
reactions is strongly demanded. The simulations on the multi-physics phenomena including friction,
chemical reaction, fluid, and heat have not been performed because of its complexity. Therefore, we
developed a quantum chemical molecular dynamics simulator “Tribo-Colors” for the elucidation of the
tribochemical reaction dynamics. This simulator is based on our original SCF-tight-binding theory and
realized over 5,000 times acceleration compared to the conventional first-principles molecular
dynamics method. Moreover, we developed a hybrid quantum chemical / classical molecular dynamics
simulator in order to realize the large-scale simulation on the tribochemical reaction dynamics. In the
above simulator, the chemical reaction site is calculated by the quantum chemical molecular dynamics
method and the other parts are calculated by the classical molecular dynamics method. The above
quantum chemical molecular dynamics simulator was successfully applied to various tribochemical
reaction dynamics.
For example, we succeeded in simulating the formation dynamics of the Zinc dialkyldithiophosphate
(Zn-DTP) tribofilm under friction condition. Furthermore, in order to clarify the wear prevention
mechanism of the Zn-DTP tribofilm, we simulated the dissolution dynamics of Fe 2 O 3 wear particle in
the Zn-DTP tribofilm. Our results show that the Fe 2 O 3 wear particle is digested by the Zn(PO 3 ) 2
tribofilm under friction condition, but it is not digested under no friction condition. This result is in good
agreement with the experimental results of Prof. J.-M. Martin group. Moreover, our simulation results
indicate that Al 2 O 3 wear particle is not digested by the Zn(PO 3 ) 2 tribofilm. This result is also in good
agreement with their experimental results. The reason for the above difference is successfully clarified
by the electronic states analysis. Furthermore, the formation dynamics of MoS 2 tribofilm from the Mo-
DTC additives and the low friction coefficient of the MoS 2 tribofilm were well reproduced. The effect of
Fe surface states on the friction coefficient was also clarified. The friction behavior of diamond like
carbon was also investigated and the effect of the sp2/sp3 ratio and the hydrogen contents on its
friction behavior was well elucidated. These successful applications of our quantum chemical
molecular dynamics simulator prove the importance of the electronic-states dynamics investigation on
the tribochemical reactions.
! '%!

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Nano-tribological evaluation, a driving force in cosmetic science
G. Luengo, R. Santoprete
L’Oreal Recherche Avancée, France
rsantoprete@rd.loreal.com
Contact dynamics between surfaces and interfaces are at the base of many cosmetic problems and
treatments. These surfaces are good examples of practical multiasperity contacts. In cosmetics, these
substrates are hair, skin and nail. These are complex but organized structures that can be considered
as good examples of biomaterials; in where both, chemistry and surface morphology, act together to
produce the adhesion and friction observed at the macroscopic level.
We well review the complex structure of these surfaces, the challenge they represent, the observed
tribological properties, and the efforts we have done until now to try explain these properties as a
function of their structure and, in some cases, the chemistry at the boundary interface. As we have
mentioned, these properties play an important role on sensory perception (i.e. touch), but they can
also, as it is the case with hair, control the overall macroscopic shape (straight, curly hair) and
dynamics.
But understanding the substrate Tribology is only one first step towards understanding the way a
products active ingredient or material can protect it or, even better, improve their properties. There are
many challenges that skin, hair and nail have to overcome to give rise to the great variety of cosmetic
effects that today’s products can and will offer to the consumer (from long-lasting lipsticks to
conditioners for example). In these cases polymers are very versatile materials that, if well chosen,
can help reaching these goals. We will show a few examples that will show both the complexity of the
cosmetic challenges and the way in which polymers can modify the dynamics of the contact
mechanics to the consumer’s final interest.
! '+!

Surface corrugation: a relevant concept to assess lubricated
friction at the nanometer scale
H. Berro, N. Fillot and P. Vergne
Université de Lyon, INSA-Lyon, LaMCoS, UMR 5259 CNRS, France
hassan.berro@insa-lyon.fr
Development in lubrication technology confronts many issues owed to the downsizing of modern
tribological applications. An essential issue in this context is related to the understanding of lubricant
performance under severe confinement, down to film thicknesses of the nanometer scale [1].
Experimental [2, 3] and numerical [4, 5, 6] studies have shown that a particular flow behavior develops
in confined nano-films and depends essentially on molecular structures and interactions with the
bounding surfaces.
Most molecular simulations of confined lubricant flow consider atomically flat crystalline bounding
surfaces models. The atomic structure of these surfaces results in interaction potentials that vary with
respect to the normal as well as lateral directions. The normal variation of these potentials defines
surface adsorption whereas the lateral one defines its corrugation [7]. In confined flow, adsorption
potential contributes to the lubricant layering in the direction normal to the surfaces whereas
corrugation potential induces in-plane density modulations in the adjacent lubricant layer parallel to the
surfaces. A direct correlation is found between the level of induced in-plane order (due to surface
corrugation) and the friction in such confined systems [4, 7].
Nevertheless, to the best of our knowledge, the effects of surface corrugation have only been
analyzed qualitatively. Factors such as surface density (atomic spacing), Lennard-Jones interaction
energies for surface atoms, and surface lattice type and orientation have all been studied and their
effects on flow boundary conditions and friction were analyzed in correlation with the lubricant induced
in-plane order, a consequence of surface corrugation potential. However, other factors also contribute
to the corrugation potential but are often harder to quantify than the preceding ones for instance the
surface chemical composition (atoms of different nature) and the actual topographical roughness [8].
Since so many factors can influence the surface corrugation potential in a hardly controllable manner,
we propose to quantify this level of corrugation with a parameter that can characterize any surface
type. By varying this key parameter, the influence of surface corrugation on boundary flow and friction
in confined nano-films can be directly concluded.
References
[1] Dowson. Thin Films in Tribology. Proceedings of the 19th Leeds-Lyon Symposium on Tribology Elsevier,
Amsterdam, 1992.
[2] Christenson, Gruen, Horn, Israelachvili. J. Chem. Phys., 87(3):1834–1841, 1987.
[3] Israelachvili, McGuiggan, Homola. Science, 240(4849):189–191, 1988.
[4] Thompson, Robbins. Phys. Rev. A, 41(12):6830–6837, 1990.
[5] Ribarsky, Landman. J. Chem. Phys., 97(3):1937–1949, 1992.
[6] Jabbarzadeh, Atkinson, Tanner. J. Non-Newtonian Fluid Mech., 77(1-2):53 – 78, 1998.
[7] Robbins, Müser. Computer Simulations of Friction, Lubrication, and Wear. in: Modern Tribology Handbook by
Bhushan (ed.), vol. 1. CRC Press LLC, 2001.
[8] Gao, Luedtke, Landman. Tribol. Lett., 9:3-13, 2000.
! (,!

Molecular dynamics simulations of normal and tangential contact:
a better comprehension of local physics of contact mechanics
M. Solar, C. Gauthier, H. Meyer, C. Fond, O. Benzerara, H. Pelletier, J. Baschnagel,
R. Schirrer
Institut Charles Sadron (ICS)
23 rue du Loess, 67034 Strasbourg Cedex 2 BP 84047, France
solar@ics.u-strasbg.fr
Scratch resistance is an important mechanical property for the surfaces of bulk materials and for thin
layer materials (e.g. glasses, varnishes, nail varnish, paint…). In the case of polymer surfaces, this
resistance can be improved by decreasing the local friction, minimizing the plastic strain or obtaining
better recovery after scratching. Moreover, it has proved to be relevant to differentiate between the
surface behavior and that of the bulk, by investigating the confined domain under the contact, at the
interface between indenter and substrate. Polymers present viscoelastic and viscoplastic properties
and display complex behavior in the presence of confined shearing (friction, i.e. interfacial shear
stress). The improvement of their surface behavior requires a better understanding of the local physics
of their contact mechanics during indentation and scratching. Some previous works assumed the
existence of a small sheared layer under the tip during the scratch [Briscoe (1980) / Charrault (2007)].
This small sheared layer could explain the observed evolution of local friction during a scratch
process.
The molecular dynamics (MD) simulations are more relevant in such a situation because they consider
molecular details and have a microscopic/statistic thermodynamic formulation. As an example, MD
simulations are able to predict changes in the behavior of a material from variations in thermodynamic
parameters (e.g. pressure, temperature, volume…). On theother hand, MD simulations currently
require a lot of CPU time and the simulated time and space scales are still small in comparison with
Continuum mechanics(CM).
Continuum mechanics (CM) has enabled a better identification and comprehension of mechanical
stresses and strains to which the matter is subjected during micro- or nanoindentation / scratch tests.
It uses mainly a macroscopic thermodynamic formulation. Finite element simulations are commonly
used to predict the mechanical behaviour, but the results depend on the phenomenological models
chosen and these models rely on experimental observations. Furthermore, the matter is seen as a
continuous medium without gaps or empty spaces, disregarding its molecular structure, so as to be
able to use differential formalism. This approach is nevertheless limited when the local physics
contributes to the global behaviour, at scale where surface mechanisms take place. Some
phenomena, like for instance the local gradient of a mechanical property, cannot be predicted with CM
if the chosen material behavior model contains no explicit law to model such phenomena.
Molecular dynamics simulations of nano-indentation and nano-scratch tests are studied on linear
amorphous polymer surfaces. The tested volume element (VE) are supposed to be close to the
Representativ Volume Element (RVE) of an amorphous polymer. First results of MD simulations
exhibit good correlation with experimental indentation data. A first analyse of bond orientation of
polymer chains under the tip display the existence of the small sheared layer during indentation and
scratch. At last we sketch out a first link between DM and CM by investigating uniaxial mechanical
behavior of the numerical model of polymer.
! (@!

Modelling the contact of real rough surfaces subjected to normal
and tangential loading
S. Medina, D. Dini, A.V. Olver
Tribology Group, Department of Mechanical Engineering
Imperial College London, United Kingdom
s.medina@imperial.ac.uk
The multi-scale nature of real surface contacts has been recognised for many years but recently more
research has been directed at attempting to model at all levels. With this in mind, we have developed
an adhesive contact solver using continuum analysis to improve scalability, and shown it capable of
simulating local contact conditions down to atomistic scales for normal loading. However, tangential
loading poses a greater challenge. The contact information of most interest is friction and tangential
stiffness. Here, the tangential stiffness of a rough contact is investigated. The stiffness is taken to
consist of a series of contributing factors which combine to a net value. Models of varying complexity
are used to predict the stiffness of each factor, and the importance of each is examined. The influence
of friction for cases of partial slip is also investigated.
Firstly, a precise definition of tangential stiffness is required since different aspects play a role, and
different features will be of interest to those considering different scales of contact. Remote from the
contact (where the load is applied), elastic deflection of the bulk may be included. Zooming-in closer to
the surface, the presence of the roughness reduces the contact stiffness. Taking the contact as a
whole, the approach of two rough surfaces generates equivalent material “voids” (the separation of the
surfaces due to roughness low points not in contact). Such voids, which would not be present in case
the surfaces were perfectly smooth, produce a reduced effective modulus at the contact interface,
therefore introducing extra compliance. The extent of partial slip and how this varies will also influence
the tangential stiffness at that load. Additionally, a thin layer of contamination, oxide or transformed
material may exist, itself having a sufficiently low stiffness to impact on the remote displacement.
Bulk stiffness can be accounted for simply by finite element analysis or analytical solutions for some
configurations. For large scale contacts, it is possible to make a reasonable attempt at simulating the
effect of friction coefficient (and thus partial slip) on tangential stiffness by assuming Coulomb friction
applies at the individual real contact patches. A partial slip contact analysis shows how this affects the
tangent modulus at different loads, with the extent depending upon amount and form of roughness.
Finite element and analytical modelling of material voids has been used to assess the contribution of
roughness itself in reducing the tangential stiffness. Again the amount and form of roughness has
been investigated. Simplified phenomenological models are used to account for contamination or
transformed layers at the contact interface. An examination of how the various stiffness values from
each model combine has been carried out to show the expected importance of each factor. The
models have also been incorporated within the adhesive contact solver previously developed and
used to predict the stiffness of real rough surfaces. Results are compared to various experimental
measurements of contact stiffness.
! ("!

FEM-BEM contact mechanics and multiresolution analysis of rough
surfaces
S. Ilincic (1) , D. Bianchi (1) , A. Vernes (1,2) and G. Vorlaufer (1)
(1) Austrian Center of Competence for Tribology, Viktor-Kaplan-Str. 2, 2700 Wiener
Neustadt, Austria
(2) Institute of Applied Physics, Vienna University of Technology, Wiedner Hauptstr. 8 - 10 /
134, 1040 Vienna, Austria
vernes@ac2t.at
Multiresolution analysis (MRA) is a powerful mathematical tool based on wavelets leading to a
quantitative and hence unambiguous model of tribological / engineering surfaces in terms of their
roughness, waviness and profile (shape).
Exactly this decorrelation of data is addressed by the present contribution, where changes in the
pressure distribution induced separately by the roughness, waviness and profile of a given surface are
also computed and analyzed using a recently developed FEM-BEM scheme.
In this FEM-BEM technique, the elastic multi-asperity contacts problem is basically solved applying the
boundary element method (BEM), for which the influence coefficients are calculated in the framework
of the finite element method (FEM).
! (#!

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Contact mechanics with applications
B.N.J. Persson
IFF, FZ-Juelich, 52425 Juelich, Germany
b.persson@fz-juelich.de
I have developed a theory of contact mechanics and adhesion between elastic solids with randomly
rough surfaces. The theory takes into account that partial contact may occur between the solids on all
length scales. The theory predicts how the area of real contact, the stress distribution in the contact
regions and the distribution of interfacial separations depends on the load and magnification. The
theory is very flexible and can be applied to both elastic and viscoelastic solids with or without
adhesion. Plasticity effects can also be easily included in the theory. As an illustration, I discuss the
leak-rate of seals, adhesion, rubber friction and the contact resistance between elastic solids with
randomly rough surfaces.
! ('!

Friction and adhesion of soft materials
A. Chateauminois
Laboratoire P.P.M.D
Ecole Supérieure de Physique et Chimie industrielles (E.S.P.C.I), Paris, France
antoine.chateauminois@espci.fr
This course will be dealing with the interplay between adhesion and friction at contact interfaces
involving soft materials such as rubbers.
First, evidences for the interactions between adhesion and frictional energy dissipation will be
reviewed in various experimental situations such as the peeling of adhesive tapes or the contact
stiction (or static friction) processes involved during the incipient stages of sliding friction. We will
especially focus on recent contact imaging approaches which provide new insights into adhesive
failure mechanisms in the presence of friction from spatially resolved measurements of the
displacement and stress fields at the mesoscopic scale.
From a theoretical point of view, continuum mechanics descriptions of adhesive failure rely on fracture
mechanics concepts. Issues related to the definition of an adhesion energy under shear and to the
incorporation of frictional energy dissipation into fracture mechanics models will be highlighted.
Comparison of the predictions with experimental data on the adhesive failure of torsional contacts will
be presented.
! ((!

Dynamics of multicontact interfaces
A. Le Bot, D. Mazuyer
Laboratoire de Tribologie et Dynamique des Systèmes – UMR 5513 CNRS
Ecole Centrale de Lyon, 69134 Ecully Cedex, France
denis.mazuyer@ec-lyon.fr
The relatively recent discovery that a contact between two solids is never full but rather composed of a
vast number of spots, has deeply modified our understanding of contact problems. In this
presentation, the so-called multicontact interfaces are introduced from the historical point of view. The
adhesive friction model of Bowden and Tabor, the self-similar contact of Archard and the statistical
theory of Greenwood and Williamson are successively described.
In other respects, the vibration of surfaces appears to be important in an increasing number of contact
problems. Contact instabilities such as stick-slip, Schallamach's wave propagation are some examples
of dynamics of contact. Interferometry technique and high speed camera allow to directly visualize
interfaces at small spacial scale (micrometer) and short time scale (millisecond).
The conjunction of dynamics of surfaces and multicontact interfaces appears in some non-standard
problems. Among them is the roughness noise, that is the friction noise created by the sliding of two
rough surfaces. It is shown that the acoustical description of this friction noise is quite simple: A white
noise filtered by the frequency response function of vibrating solids. But, the description of contact is a
problem of statistical mechanics where the underlying events are the shocks between antagonist
asperities. The rate of shocks is so high that a deterministic description is not conceivable. Some
experimental results are shown: Noise level versus roughness of surfaces, sliding speed and contact
area. In particular, it is shown that the acoustical power is not proportional to the contact area.
! (+!

! +,!

O4:28.4'40$04L4:&.48$
!
!
Marie-Charlotte Audry-Deschamps
Hassan Berro
Florian Brémond
Juliette Cayer-Barrioz
Jean-Pierre Celis
Magdalena-Carla Corneci
Viet-Hung Dang
Stefan Eder
Ali Erdemir
Hélène Fay
Nicolas Fillot
Giacomo Fontani
Joost Frenken
Christian Frétigny
Anthony Galliano
Christian Gauthier
Sabine Geldermann
Suzanne Giasson
Thomas Guérin
Kenneth Holmberg
Staffan Jacobson
Yohan Le Chenadec
Vincent Le Houérou
Thierry Le Mogne
Olivier Lerasle
Samuel Mambingo-Doumbe
PPMD, ESPCI, France
LaMCoS, InsaLyon, France
LTDS, ECLyon, France
LTDS, ECLyon, France
KU Leuven, Belgium
LaMCoS, InsaLyon, France
LTDS, ECLyon, France
AC2T, Austria
Argonne National Laboratory, USA
LTDS, ECLyon, France / CRPP, France
LaMCoS, InsaLyon, France
EMPA, Switzerland
Leiden Institute of Physics, The Netherlands
PPMD, ESPCI, France
L’Oréal Recherche Avancée, France
ICS Strasbourg, France
Bonn University, Germany
Université de Montréal, Canada
Institut Curie, France
VTT, Finland
Uppsala University, Sweden
MFP Michelin
ICS Strasbourg, France
LTDS, ECLyon, France
Total, France
LTDS, ECLyon, France
Jean-Louis Mansot
! +@!
Université des Antilles et de Guyane, France

Jean-Michel Martin
Denis Mazuyer
Simon Medina
Alain Molinari
Michael Moseler
Mansha Muhammad
Danh Toan Nguyen
Nicolas Obrecht
Alejandro Pachon-Rodriguez
Piero Paolino
Nikolay Podghainiy
Shivaprakash Narve Ramakrishna
Marco Reguzzoni
Frédéric Restagno
Bruno Reynard
Philippe Richetti
Anne Rubin
Mark Rutland
Rashmi Sahoo
Roberto Santoprete
Igor Siretanu
Mathieu Solar
Audrey Steinberger
Andras Vernes
Giovanna Zilibotti
LTDS, ECLyon, France
LTDS, ECLyon, France
Imperial College London, UK
Université de Metz, France
University of Freiburg, Germany
ICS Strasbourg, France
PPMD, ESPCI, France
Total, France
LPMCN, Université de Lyon, France
PPMD, ESPCI, France
Bonn University, Germany
ETHZürich, Switzerland
Universita di Modena, Italy
LPS, Orsay, France
ENSLyon, France
CRPP, France
ICS Strasbourg, France
KTH, Sweden
Indian Institute of Science, India
L’Oréal Recherche Avancée, France
CRPP, France
ICS Strasbourg, France
ENSLyon, France
AC2T, Austria
Universita di Modena, Italy
! +"!

,-M2.40$374&H4'8$
Prof. Alexis Baratoff
Dr. Etienne Barthel
Prof. Elisabeth Charlaix
Dr. Antoine Chateauminois
Dr. Carlos Drummond
Dr. Manfred Heuberger
Prof. Jacob Klein
Prof. Momoji Kubo
Prof. Uzi Landman
Dr. Alain Le Bot
Prof. Martin Müser
Prof. Bo Persson
Prof. Susan Sinnott
Prof. Nicholas Spencer
Prof. Hugh Spikes
Basel University, Switzerland
Saint-Gobain Recherches, France
LPMCN, Université de Lyon, France
PPMD, ESPCI, France
CRPP, France
EMPA, Switzerland
Weizmann Institute, Israel
Tohoku University, Japan
Georgia Tech, USA
LTDS, ECLyon, France
University of Saarbrücken, Germany
Forschungzentrum Jülich
University of Florida, USA
ETHZürich, Switzerland
Imperial College London, UK
! +#!