Ando, Yasuhisa Effects of condensed water on friction Yasuhisa Ando Tribology Group, National Institute of Advanced Industrial Science and Technology (AIST), 1-2-1 Namiki, Tsukuba 305-8564, JAPAN E-mail: yas.ando@aist.go.jp
The friction coefficient is constant stated in Coulomb’s low is not always valid especially when adhesion force is not negligible, which is mainly caused by surface tension of condensed water. The adhesion force acts as an additional normal load and increases the friction force. Then, the friction force is proportional to the sum of external applied load and adhesion force.
The condensed water could affect the friction force in other the ways. The friction force could be increased by the viscous resistance of the condensed water film between the solid surface. There is also a possibility that the water film might prevent the direct contact between the solid surfaces and act as a liquid lubricating film. The effect of the resistance and lubrication would be conspicuous under a low normal load or friction force caused by direct contact between the solid surfaces is extremely low.
The friction force and pull-off force were measured in a high vacuum (HV) at various substrate temperatures in this study in order to determine the effects of condensed water on friction. Asperities with spherical peaks were fabricated on a silicon substrate using a focused ion beam. Pull-off and friction forces were measured on each asperity using atomic force microscopy (AFM)in a high vacuum (HV) of 2 x 10-5 Pa. The probe of the AFM cantilever had a flat square tip, approximately 1 x 1 µm2 in area. The results showed that the pull-off force increased with increasing contact time at a substrate temperature of 100°C or lower, but was independent of contact time at 190°C or higher. The friction force increased with lower sliding velocities at 100°C or lower, suggesting the condensed water has a lubricating effect that prevents direct solid contact.
Bain, Colin In Situ Study of Lubricating Films under Pressure and Shear by Vibrational Spectroscopy Colin D. Bain Department of Chemistry University of Oxford, UKRecent developments in the sensitivity of vibrational spectroscopy now make it possible to obtain infrared, Raman or sum-frequency spectra from nanometre-thick films of lubricants in a solid–solid contact. The structure of both fluid and boundary lubricants under pressure and/or shear offers insights into the mode of action of lubricants and possible mechanisms for the dissipation of energy in a sliding or rolling contact. This talk will report results in our laboratory for boundary lubricants between two dielectric surfaces obtained by Raman scattering and sum-frequency generation. Attempts to detect ‘frozen’ layers of confined liquids under zero shear conditions will be described. Preliminary results from a new project to study the contact between a rotating steel ball and a dielectric flat by Raman spectroscopy will be presented.
Bobji, M.S In situ TEM for contact measurements
M.S Bobji, Department of Mechanical Engineering Indian Institute of Science, INDIA
Abstract Physical processes that take place, at nano-scale, as materials come into contact are very important in tribology. Nanoindentation has been extensively used to determine the local mechanical properties as it is an excellent tool in creating local disturbance and measuring the mechanical force response. Transmission Electron Microscope uses the smaller wavelength of the accelerated electron to see things beyond the optical resolution. It is an important tool used for characterization of very small volume of materials and to understand the deformation mechanisms. In-situ TEM nanoindenter is a new instrument that can do indentation inside a TEM. The deformation process can be observed as the material is being indented leading to simultaneous measurement of force and displacement and the associated deformation events. This has been made possible mainly by the improvement in the TEM specimen preparation methods by Focused Ion Beam microscope.
Gellman, Andrew
Dynamics of Oligomeric Lubricants on Surfaces
Andrew J. Gellman
Carnegie Mellon University
The interface between the read-write head and the magnetic data storage disk of a hard disk drive presents one of the most ‘high tech’ of tribological problems. The head flies over the disk surface at a height of only 100 Å but at a speed of about 10 m/sec. The surface of the disk is protected from contact with the head by a film of perfluoropoly-alkylether lubricant that is only 10 Å in thickness. This ultra-thin film of lubricant is expected to survive on the disk surface for periods up to 5 years without evaporating or failing. The magnetic data storage medium of the disk is protected by a thin film of sputtered amorphous carbon that is in turn coated with the lubricant film. We have studied a number of aspects of the surface chemistry of the perfluorpolyalkylether lubricants on the surfaces of the amorphous carbon. These include the mechanism of interaction between the lubricant and the carbon film, the properties of the carbon film and the dynamics of these oligomeric lubricants on their surfaces.
Granick, Steve From Nanofluidics to Microfluidics Steve Granick Professor of Materials Science and Engineering, Chemistry, and Physics University of Illinois at Urbana-ChampaignViscous flow is familiar and useful, yet the underlying physics is surprisingly subtle and complex. Recent experiments and simulations show that the textbook assumption of "no slip at the boundary" can fail massively when walls are sufficiently smooth. The reasons appear to involve materials chemistry interactions that can be controlled. To know what boundary condition is appropriate in solving continuum equations requires inquiry into microscopic particulars. Attention is drawn to unresolved topics of investigation, to the potential to purposefully capitalize on "slip at the wall" for purposes of materials engineering, and to the use of laser-based techniques (especially fluorescence correlation spectroscopy) to probe nanoenvironments that are fundamental to boundary, elastohydrodyanamic, and hydrodynamic lubrication.
Grunze, Michael
Heuberger, Manfred Understanding friction as a non equilibrium surface force M. Heuberger Laboratory for Surface Science and Technology, Materials Department, ETH Zürich, SwitzerlandDissipative processes such as friction are dictated by the dynamics of the system constituents. This simple statement has profound consequences for the fundamental study of nanotribology - making it an exigent field of research. Apart from the inherent challenge of realizing reproducible experimental conditions at the nm-scale, an exceptionally large experimental window is required to probe the dynamic response to allow for any fundamental conclusions. Using the extended surface forces apparatus we are probing molecular re-arrangements and conformational transitions - in some cases at sub-Ångstrom resolution. The phenomenon of stick-slip is revisited in the case of a simple linear alkane system, which acts as model lubricant. Film-thickness changes are monitored during intermittent slip events and the film relaxation during stop-start experiments is recorded. The effects of molecular ordering and the presence of nano-particles on the dynamics and statistics of friction are discussed.
Kato, Koji Title : The Effect of Gases on Friction of Ceramics and Hard Coatings. Author : Koji Kato, Tribology Laboratory, Tohoku University, Sendai, Japan
The effect of atmospheric gas such as Ar, He, CO2, O2 or N2 on the friction between Si3N4/a-C, and CNx/CNx are tested experimentally by changing the gas in the test chamber or by supplying each gas to the contact interface in air.
It was found that O2 increases friction by these material combinations and N2 decreases friction by these all combinations. Ar, He and CO2 do not change friction at these material combinations by a big amount like O2 and N2.
The friction coefficient observed with Cnx/CNx combination in the stream of O2 gas in air is about 0.16, and the friction coefficient observed with CNx/CNx in the stream of N2 gas in air is about 0.006.
This result suggests the possibility of N2-lubrication of CNx/CNx in air.Krim, Jacqueline QCM Studies of the Slippage of Adsorbed Monolayers in Open and Confined Geometries J. Krim, Physics Department, North Carolina State University Raleigh, North Carolina 27695Studies of the fundamental origins of friction have undergone rapid progress in recent years with the development of new experimental and computational techniques for measuring and simulating friction at atomic length and time scales. The increased interest has sparked a variety of discussions and debates concerning the nature of the atomic-scale mechanisms that dominate the dissipative process by which mechanical energy is transformed into heat. Measurements of the sliding friction of physisorbed monolayers and bilayers provides information on the relative contributions of electronic to phononic dissipative mechanisms, since phonon dissipation is present at all film coverages, while electronic dissipation primarily impacts the monolayer. We will be reporting our measurements for physisorbed layers adsorbed on lead, C60 and copper surfaces in open geometries and also in confined geometries. Lead is of particular interest on account the phenomenon of superconductivity-dependent sliding friction. The system Xe/Cu is interesting because the interaction potential of Xe/Cu is known to a high degree of accuracy, allowing highly reliable comparisons of theory to experiment. C60 is of interest as a model system for examining the impact of rotational motion on sliding friction.
Landman, Uzi Friction Forces, Amontons’ Law, and Diffusion: From the molecular to the macroscopic scale Uzi Landman, J. Gao, W.D. Luedtke School of Physics, Georgia Institute of Technology, Atlanta, GA 30332-0430 D. Gourdon, M. Ruths, J.N. Israelachvili Department of Chemical Engineering, UC Santa Barbara, CA 93106Amontons’ law, which was already known to Leonardo da Vinci, states that the friction force is directly proportional to the (normal) applied load, with a constant of proportionality - the friction coefficient - that is constant and independent of the contact area, the surface roughness and the sliding velocity. No theory has yet satisfacorily explained this surprisingly general law, all attempts being model or system dependent. On the basis of large-scale molecular dynamics simulations pertaining to lubricated adhesive and non-adhesive junctions, with morphologically rough (as well as crystallographically flat) confining solid surfaces, and in conjunction with recent experiments, we show that the local energy-dissipation mechanisms are not 'mechanical', as assumed in most models, but “thermodynamic” in nature. We show that a local analysis of the simulation results, based on division of the system into small cells, leads to a natural description in terms of the Weibull distribution. For the dynamic. non-equilibrium, energy-dissipating process that we study, this long-tail distribution serves a similar purpose as the Boltzmann distribution for classical systems at equilibrium. While Amontons law does not hold on the local scale, it is recovered on the global scale, with the spatio-temporal averaging utilizing the Weibull distribtion of the local friction forces. Interestingly, the concept of "area of contact", often used in frictional studies, does not enter into our analysis. J. Gao et. al., J. Phys. Chem. B 108, 3480 (2004).
Luckham, Paul Determining the adhesion and the Elastic properties of living cells using Atomic Force Microscopy Paul Luckham and I-Ming Chung Dept. Chem. Eng., Imperial College, London, UK
For the last decade people have been using the AFM to measure colloidal interactions. This is achieved by attaching a particle onto an AFM cantilever and measuring that particles interaction, either with a second particle, or more usually a flat surface made of a similar material to that of particle. Initially DLVO forces were measured, quickly followed by steric and hydrophobic interactions. The AFM has been used also in the field of biology, largely as an imaging technique, but also to measure the strength of specific interactions such as antibody-antigen interactions.
In this talk I shall describe a method whereby single living biological cells, namely yeast, erythrocytes and fibroblasts can be attached to AFM cantilevers, and their interactions with various surfaces have been determined. The technique enables the adhesion of cells to surfaces to be studies and results for a range of surfaces will be presented. One problem encountered during this work, which has proven to yield interesting mechanical details about the cells was that during an experiment the cells deform. By comparing data of the interactions of the cells with a surface and the bare cantilever with the surface, it is possible to estimate the deformation of the cell. It is found that the deformation is well described by Hertz theory of elasticity, enabling us to estimate the Young’s modulus of the cells: yeast 0.65+/- 0.15 MPa; erythrocytes 0.25+/-0.1 MPa; and fibroblasts 20+/-10 kPa.
Persson, Bo
Friction: Perspective & Status Bo Persson Institut für Festkörperforschung, Deutschland
Pethica, John Dissipation Mechanisms in Nano Contact
Ralston, John STRUCTURED SURFACES AND THEIR IMPACT UPON STATIC AND DYNAMIC WETTING John Ralston Ian Wark Research Institute University of South Australia Mawson Lakes Campus Mawson Lakes Adelaide South Australia 5095 AUSTRALIAABSTRACT
Our research on the static and dynamic wetting of structured surfaces deals with surfaces that respond to different external stimuli; ultra-smooth surfaces where the influence of liquid structure is explored; the wetting of single particles, fibres and packed beds; the influence of physical and chemical heterogeneity on wetting; and thin film stability and surface charge effects on wettability.
In the case of external stimuli, a tethered DNA base and its derivatives undergo reversible photodimerization when subjected to UV irradiation of specific wavelengths. This leads to large changes in contact angle that are linked to a concerted molecular reorganization taking place when a dimer is formed. Electrowetting enables the wettability of hydrophobic materials to be enhanced in a rapid and reproducible fashion, but electrical double layer contributions lead to a divergence from theory as does contact angle saturation. Heat and radiation can alter the wettability of metal oxide surfaces, caused by a delicate interplay between dispersion energy and hydrogen bonding contributions. A combination of hydrodynamic and molecular kinetic approaches enable the wetting and dewetting of very smooth amorphous fluoropolymer surfaces to be described over a very wide range of velocities and dynamic contact angles, however large challenges remain for heterogeneous surfaces. For example surface defects, depending upon their size, may lead to restricted spreading and considerable divergence from theory. Physically rough, but chemically homogeneous surfaces have been developed that permit roughness of different magnitudes to be explored and the links between advancing, receding and equilibrium behaviour to be established.
Rieker, Claude How does tribology influence the lifespan of total joint prostheses?
Even if the first total joint prostheses were developed more than 50 years ago, their global propagation started only in the mid seventies. Today, more than 1.5 million total joint prostheses are implanted annually world-wide and they offer a real improvement in the quality of life for the patients having arthritis or similar disorders. Most of these total joint prostheses have a lifespan of about 10 to 20 years. Even if this lifespan may be long enough for elderly patients, this lifespan is not long enough for more demanding active and young patients.
The tribology of total joint prostheses was recognised in the mid eighties as being a fundamental parameter controlling the lifespan of total joint prostheses. The wear is practically low enough to assure a satisfactory mechanical integrity of the total joint prostheses, but these prostheses failed mainly due to the body reaction induced by the wear products. These products (wear particles) have typically a size of 0.5 to 2.0 µm and the human body recognises these particles as foreign bodies, which trigger a body reaction against these particles. Macrophages and giant cells start to release cytokines, which try to dissolve the wear particles. As most of the wear particles have a good bio-compatibility, the cytokines are unable to dissolve these particles and as “collateral” consequences, these cytokines active osteoclasts and induce therefore the loosening mechanisms of the total joint prostheses.
This lecture will present these body reactions producing the wear particles and will also present the today’s solutions used to prolong the lifespan of total joint prostheses. The lifespan of the ideal joint prosthesis should be long enough to “survive” the patient, even if the patient has a life expectancy of 20 years or more.
Claude Rieker, Ph.D. 23 August 2004Robbins, Mark
From atomic simulations to macroscopic models of contact mechanics and friction
The Johns Hopkins University, USA
Friction is determined by both atomic interactions in contacts, and the macroscopic elastic and plastic deformations that determine the contact area and
geometry. The talk will begin with recent continuum results for contact between
elastic and plastic surfaces with self-affine surface roughness. One conclusion is that
the typical contact size is always comparable to the small scale cutoff in the
roughness. The talk will next use atomistic simulations to test continuum
contact mechanics in contacts whose width is a few to a few tens of
molecular diameters. The results imply that continuum analysis of AFM measurements
can give accurate elastic moduli, but that contact areas and yield stresses may
be off by a factor of two. The talk will finish with atomistic and multiscale
simulations of friction between self-affine surfaces, and contrast the results to
continuum calculations of elastic and plastic solids.
Singer, Irwin "Role of Third Bodies in Low Friction Sliding Contacts." I.L. Singer US Naval Research Laboratory, Code 6176 Washington, D.C. 20375 singer@nrl.navy.mil
Investigations of sliding friction at the macro- and microscale are often discussed in terms of bulk and/or surface properties of the two bodies placed in sliding contact, even when third body debris is prominent in and out of the wear track. However, recent investigations of low friction materials provide direct and conclusive evidence that third bodies control friction and wear1. Using a tribometer with in situ optical and Raman microscopy, we were able to obtain videos, Raman intensities and spectra that revealed the third body processes involved. Tribotests were performed in reciprocating motion at low speed (1 - 4 mm/s) in both dry and humid air with transparent glass and sapphire hemispheres.
In this talk, I will present evidence for third body control of friction at various stages of sliding: run-in, steady state, friction instabilities, changes in steady state friction between two values and rapidly rising friction coefficients. Coatings and surface treatments studied include: metal-doped MoSx; diamond-like carbon, DLC; boron carbide and boric-acid coated surfaces. For all three surface conditions, interfacial sliding between transfer film and wear track was the dominant velocity accommodation mode at low (<0.1) and high (0.2 to 0.6) friction coefficients. I will also address such questions as: do third body effects scale with contact size, e.g., might there be a size scale below which third bodies cannot control friction?
1 I.L. Singer, S.D. Dvorak, K.J. Wahl and T.W. Scharf, “Role of Third Bodies in Friction and Wear of Protective Coatings,” J Vac Sci Technol A 21 (2003) S232-S240 (Suppl. S). Invited talk, Nano Sikkim II, 8th-12th November 2004 7th - 13th November, 2004 Pelling, Sikkim, INDIASokoloff, Jeffrey
Models for Static, Dry Friction and possible
mechanisms for lubrication
J. B. Sokoloff, Physics Department and Center
for Interdisciplinary Research
in Complex Systems,
Northeastern University, Boston, MA 02115.
Spencer, Nic Biomimetic Approaches to Lubrication
aLaboratory for Surface Science and
Technology, Department of Materials,
ETH-Hönggerberg, CH-8093 Zürich, Switzerland
Houston, TX 77204-5003, USA
Nature lubricates with water, but man typically has not. The main reason for this discrepancy lies in corrosion issues when lubricating steel and, possibly more seriously, the unsuitability of water as an EHL lubricant, due to its low pressure-coefficient of viscosity. Nature’s approach to lubrication, which it generally does at room temperature, involves both complex lubricants and smart surfaces. The latter are frequently covered with heavily hydrated brushes, which apparently lead to a very low friction coefficient. Water would have numerous advantages as a lubricant if a suitable biomimetic approach were to be developed, since it has clear environmental and safety advantages over oil, as well as being a significantly better heat-transfer medium.
In our laboratories, we have been investigating biomimetic lubrication systems that involve immobilized, hydrated block copolymers. These molecules appear to be capable of lubricating a variety of different surfaces in an aqueous environment. Both macrotribological and nanotribological measurements have been carried out, and the results suggest that certain architectural features of the polymers play a critical role in their effectiveness as lubricants. The presence of polymer in solution also seems to be essential in maintaining a low friction coefficient.
The results of our experimental investigations will be
presented, and possible models for the low friction coefficients encountered
will be discussed.
Spikes, Hugh
Mechanism by which lubricant additives influence friction
Tung, Simon AN INVESTIGATION OF SURFACE INTERACTION AND BREAK-IN FILM DEVELOPMENT DURING PISTON RING/CYLINDER BORE LUBRICATED CONTACT USING ELECTRICAL CONTACT RESISTANCE AND SURFACE SPECTROSCOPIC MEASUREMENTS Simon C. Tung Chemical and Environmental Sciences laboratory, General Motors R & D Center 30500 Mound Road, Warren, MI 48090-9055 E-mail: simon.c.tung@gm.com, Fax: (586) 986-2094 This paper will be presented at the 2004 NanoSikkim II Friction and Tribology Conference, Pelling, India, on November 8-12, 2004Abstract:
The surface film formation including hydrodynamic oil film and chemical boundary films was studied experimentally by using an electrical contact resistance method during the surface break-in period of the coated piston rings rubbing against a cylinder bore segment. A simplified surface film model was developed to describe the film formation and correlated to the electrical contact resistance measurements. Three major break-in factors were determined from the electrical resistance measurement during the break-in film development. The modeling results agreed well with the experimental measurements when these major factors were selected appropriately in the model. Parameters such as applied normal load, surface roughness, lubricant viscosity, and Young’s modulus of the ring coatings were considered in film modeling. The experimental results showed that the behavior of break-in film development was affected by the surface interaction with oil additives. The numerical simulation results showed that the film developed more quickly with decreasing normal load and lubricant viscosity and with increasing surface roughness and Young’s modulus.
In addition, based on energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD) analyses of wear tracks on the tested tribo-components, it indicated that the wear-resistant tribofilms formed on the piston ring and cast iron bores. The EDX and XRD analyses also indicated that surface plateaus after surface break-in process have rich concentrations of the main additive functional elements, i.e. Mo, S, Zn, P, and Ca, suggesting intensive tribochemical reactions in the contact.