Session K9 - Tribology II: Energy Dissipation & Sliding Mechanisms.
MIXED session, Wednesday morning, March 22
102D, MCC

[K9.001] Sliding Friction of Adsorbed Monolayers and Bilayers on Pb and Cu Substrates

Studies 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 scales [1]. The increased interest has sparked a variety of discussions about the nature of atomic-scale mechanisms by which mechanical energy is dissipated at sliding interfaces. We report here our measurements of sliding friction of monolayers and bilayers on Cu and Pb surfaces. Such studies provide information about the relative contribution of electronic and phononic dissipation at the interface. The Cu surface has a well known interaction potential, which allows highly reliable comparisons of theory to experiment, and Pb exhibits a superconducting phase transition at low temperature, allowing a direct means of probing the electronic contribution to friction. This work is funded by NSF DMR#9896280.

[1] J. Krim, Scientific American, vol. 275, pp 74 (1996)

[K9.002] Strongly Temperature Dependent Sliding Friction for a Superconducting Interface

A sudden drop in mechanical friction, between an adsorbed nitrogen monolayer and a lead substrate, occurs when the lead passes through the superconducting transition temperature. We attribute this effect to a sudden drop at the superconducting transition temperature of the substrate Ohmic heating. The Ohmic heating is due to the electronic screening current that results from the sliding adsorbed film. We also consider the question of whether the film is completely pinned in place by defects, which has recently been raised.

[K9.003] Electrostatic damping of center-of-mass oscillations of a molecular monolayer.

The quadrupole moments of the molecules in a monolayer herringbone lattice are the source of an electrostatic field with spatial variation determined by the reciprocal lattice vectors of the monolayer solid. For lattice oscillations parallel to a metal substrate, as in recent quartz crystal microbalance experiments(A. Dayo, W. Alnasrallah, and J. Krim, Phys. Rev. Lett. 80), 1690 (1998)., there is an oscillating electric field applied to the metal. The contribution of the resulting dissipation to the damping of the oscillation is evaluated with parameters appropriate to N_2/Pb. This is an unusual regime with strong similarities to the anomalous skin effect, but here the anomalous response arises because the spatial length scale of the driving field is small compared to the mean free path of electrons in the normal metal.

[K9.004] Static Friction due to Damping of Low Frequency Phonons

It is well known to every student in an elementary physics class that there exists static friction between sliding bodies and the kinetic friction has very little velocity dependence (``dry friction"). Molecular dynamics simulations and analytic calculations based on perturbation theory show that a monolayer film with underdamped phonons sliding on a substrate, incommensurate with it and possessing some disorder, gives nearly velocity independent sliding friction at slow speeds and exhibits static friction. Perturbation theory calculations done for a three dimensional solid with underdamped phonons sliding on a substrate, however, give viscous friction and no divergence in the mean square atomic displacement at zero sliding velocity, consistent with the film not being pinned (i.e., no static friction). When one takes into account the fact that those low frequency modes which contribute to the friction are highly damped, one finds that the critical dimension, below which there is static friction and at and above which there is not, switches from two to four, implying static friction for the three dimensional solid. The problem is also examined from the point of view of scaling and Larkin domain arguments.

[K9.005] The Effect of C60 on Interfacial Friction and Wetting of Toluene

Experimental investigations of friction, lubrication and adhesion at nanometer length scales have traditionally been performed by employing force microscopy, surface forces apparatus (SFA), or quartz crystal microbalance (QCM) techniques. While collectively these techniques have yielded much useful information, their results have never to date been mutually cross-referenced. In order to achieve such a cross referencing, we have performed a QCM study of the system toluene/Ag(111) with and without C60 deposited on the surface of the Ag(111). This system has been previously studied by means of SFA [1], whereby it was reported that C60 at a toluene/mica interface resulted in a significant reduction in friction levels. Our QCM studies of toluene/C60 reveal a distinct change in the wetting behavior of the toluene on the C60-coated surface, and also indicate that the decrease in friction reported in [1] is most likely due to the manner in which the C60 is bound to the surface. The investigation is currently being continued with AFM lateral force measurements of toluene/mica and Ag(111) surfaces, as well as macroscopic contact angle measurements of toluene on the various substrates of interest.

[1] S.E. Campbell, G. Luengo, V.I. Srdanov, F. Wudl, and J.I. Israelachvili, Nature, vol. 82, p. 520 (1996)

[K9.006] Joint QCM-STM Studies of Vapor-Phase Lubricants

The concept of lubricating high temperature bearing surfaces with organic vapors which react with a surface to form a solid lubricating film has existed for at least forty years, with substantial efforts beginning in the 1980's and continuing to the present day. While vapor phase lubricants have primarily been studied within the context of macroscopic system performance, they may well prove to be of critical importance to tribological performance in sub micron mechanical systems as well: The vapor phase may ultimately prove to be the most effective, if not only, means to deliver and/or replenish a lubricant on account of the submicron scale of the device. In order to investigate the viability of vapor-phase lubrication for MEMS applications, we have studied the molecular scale properties of a number of known or proposed vapor-phase lubricants in controlled environments and well-defined contact geometries. In particular, we have also constructed a simple nanomechanical systems consisting of a Scanning Tunneling Microscope tip dragging on the surface of a Quartz Crystal Microbalance electrode. The buried contact is directly imaged throughout the course of the measurements, which are performed in realistic sliding conditions. Work is also underway to compare the STM-QCM results to the effect of vapor-phase lubricants on actual MEMS devices.

[K9.007] Advanges and limitations of hollow nanoparticles of WS2 under friction and wear

It has been shown that, nanoparticles of WS2 with the structure related to nested carbon fullerenes (IF) possess superior lubricating properties over solid lubricants with lamellar structures, like graphite and MoS2 (2H platelets). The goal of the present work was to analyze the advantages and limitations of IF-WS2 in comparison to 2H platelets. The friction experiment was performed on ring-block tester. TEM analysis of the solid lubricant particles after wear testing showed that substantial part of the 2H platelets, was destroyed. Contrarily, most of the IF nanoparticles were found to suffer only a minor damage. XPS analysis indicated substantial oxidation of the block surface and wear particles in the case of the 2H platelets. One of the prime advantages of IF over 2H particles is shown to be in the absence of dangling bonds in the former. The main limitations are the plastic deformation of nanoparticles in definite load range and the compaction of IF in friction contact.

[K9.008] Triboemission and Tribochemistry

Results of an experimental study of triboemission - the emission of charged particles from rubbing surfaces - are presented with an emphasis on tribochemical effects, in particular tribopolymerization as a mechanism of boundary lubrication. In this concept, proposed by Furey and Kajdas [1], triboemitted charged-particles (e.g., low-energy electrons) initiate the surface polymerization of addition-type monomers.

A new instrument for measuring the burst-type triboemission is described [2]. Experimental results are presented for the sliding contact of diamond on the three related materials - alumina, sapphire and aluminum - and for the sliding contact of alumina-on-alumina. Features of the triboemission intensity for negatively and positively-charged emission as a function of retarding potential are discussed. A stochastic model is proposed to characterize the triboemission data in the frequency domain.

[1] Furey,M.J., Kajdas,C., Models of Tribopolymerization as an Anti-Wear Mechanism, Proc.Jap.Intl.Trib.Conf., 1990, 1089-1094. [2] Furey,M.J., Ritter,A.L., Vick,B., Molina,G.J., Kajdas,C., Triboemission and Surface Temperatures Generated in a Ball-in-Cone Tribocontact, Proc.5th Intl.Trib.Conf. AUSTRIB'98, Dec.6-9, 1998, Brisbane, Australia, 257-262.

[K9.009] Mechanochromism and Friction Anisotropy in Functional Organic Films

Organic films that are composed of ordered, densely-packed molecules have been used extensively as a means to study the structural, mechanical, and chemical aspects of adhesion and tribology. In most cases, these films are monolayers of simple alkanes, of variable chain length and tail group chemistry, that are created either by self-assembly or Langmuir-Blodgett techniques. We have been exploring the tribological aspects of organic films that not only have a high degree of alkane chain ordering, but also incorporate a cross-linking, conjugated backbone that exhibits a chromophoric response to stress and molecular rearrangement ("mechanochromism"). These "functional" materials (polydiacetylenes) have many unique optical properties that lend themselves to sensing applications, and many unique structural properties for scanning probe studies of friction anisotropy, sheer-induced molecular rearrangement, and mechanochromism. Our experimental approach includes atomic force microscopy, fluorescence microscopy, and a unique "multi-tasking" scanning probe that combines shear force, interfacial force, and near-field optical detection. Sandia is a multiprogram laboratory operated by Sandia Corp., a Lockheed Martin Co., for the U.S. Dept. of Energy under Contract DE-AC04-94AL85000.

[K9.010] Ultrasensitive Measurements of Vacuum-Mediated Friction and Force Fluctuations

We report measurements of non-contact friction/force fluctuations between a sample and an AFM cantilever tip. Vacuum-mediated force fluctuations as small as an attonewton/\sqrtHz were directly observed by the real-time thermal (Brownian) motion of the tip. Measurements were made for tip motion parallel to the sample surface with tip-sample separations of 10 to 1000~Åtemperatures of 3 to 300~K, and for various tip and sample materials and bias voltages. For the case of a Au(111) sample and gold-coated tip with zero potential difference, we find friction/force fluctuations approximately 1000 times smaller than previously reported. We interpret our force fluctuations in terms of thermal charge fluctuations interacting with local static inhomogeneous electric fields. The relevance of this work to the detection of a single spin by magnetic resonance force microscopy (MRFM) will be discussed.

[K9.011] Gear-like rolling motion at the nanometer scale

We will present results of AFM/LFM manipulation studies of carbon nanotubes on graphite (HOPG). In this system, the contact zone consists of two facing graphene lattices. We find the friction is highly anisotropic depending on the degree of commensurability, and that there are certain orientations where the CNT locks-in to atomic registry with the substrate. The lateral force required to move the tube in this locked-in (commensurate) state is up to 10 times larger than the out-of-registry state. We also observe that rolling motion occurs only in this locked-in state. We have never observed rolling on any other substrate. We have also performed atomistic computer simulations identifying the energy barriers for sliding and rolling, elucidating atomic-scale features of slip-roll motion and explaining the details of the lateral force data in terms of the intrinsic faceting of multiwall carbon nanotubes. Our experiments and simulations show that interlocking of the atomic lattices in the contact region of two bodies can determine whether the body slides or rolls. In essence, the atomic lattice can act like a gear mechanism. This work is supported by the NIH (NCRR), NSF, and ONR (MURI).

[K9.012] Tribochemical Deposition--A New Way of Generating Atomically Flat Surfaces

When one puts a dent in the wall one does not wear away square meters of wall to restore it’s planarity --instead, one fills in the dent with putty. In terms of energy and time, considerable savings can be achieved on nearly flat surfaces by such an approach. We show for the first time that atomic layer by layer crystal growth can be controlled mechanically in such a fashion that “fills in” micron-sized features a few atomic layers deep, thereby creating atomically flat surfaces. The experiments are done under supersaturated solution conditions. Step edges are stimulated with low contact force scanning using a scanning force microscope which is also used to image the changes in surface topography. We present recent results on single crystal brushite (CaHPO_4.2H_2O) where we find that the ability to "fill" depends strongly on solution chemistry, mechanical parameters (e.g., normal force), and step crystallography. A model for the mechanism involving relevant tip-solution-step interactions will be presented.

[K9.013] Investigation of the nano-scale friction and wear mechanisms at the boron carbide film surface-atomic force microscope tip interface

The investigation of the modifications of the nano-scale tribological properties of boron carbide films induced by the energy dissipation at the interface between the atomic force microscope tip and the film surface is presented. It is shown that the microscope tip induces a modification at the surface that results in a decrease on the friction forces between the tip and the film surface. The influences of the friction coefficient, the scanning speed and the applied normal force on the film wear are investigated. Using a microscopic model, the dissipated energy at the tip-surface interface during scanning was estimated. The influence of the dissipated energy on the nano-scale wear is presented and a strong correlation between friction and wear, in nano-scale, is shown. The influence of capillary condensation of water (on the atomic force microscopy tip and boron carbide films surface) on the friction forces and wear of the films was also studied. It is shown that friction and wear are strongly dependent on the humidity. At low humidity no wear at the films surface is observed while at high humidities a constant wear rate is measured. The presented results show that the wear mechanisms of boron carbide films is highly dependent on the environment humidity. A wear model based on a film-surface interaction with the moisture in the air, induced by the dissipated energy at the microscope tip-surface interface while in relative motion, is presented.

Part K of program listing