AVS1997 Session NS+SS-MoM: Friction and Nanoscale Tribology

Monday, October 20, 1997 8:20 AM in Room K

Monday Morning

Start	Invited?	Item
8:20 AM		NS+SS-MoM-1 Molecular Aspects of Friction: Systematic Studies of the Frictional Properties of Selectively Fluorinated Self-Assembled Monolayers S.S. Perry, H.I. Kim, T. Koini, R. Colorado, T.R. Lee (University of Houston) The frictional properties of hydrocarbon and fluorocarbon based self-assembled films have been investigated on a molecular scale with atomic force microscopy. Control of the molecular composition of the films has been achieved by organic synthesis of alkyl thiols and their subsequent molecular self assembly on gold substrates. Frictional properties of the films have been measured as a function of applied load and as a function of film composition. Systematic increases in frictional properties have been observed for films terminated in CF₃ species as compared to fully hydrogenated films. By studying films of equivalent chain length, packing density, and packing energy, we are able to conclude that the difference in size of the methyl and trifluoromethyl terminating groups is responsible for the increase in friction. We propose that the larger size of the trifluoromethyl species, arising from the larger van der Waals radii of fluorine, leads to increased steric interactions. In turn, such intermolecular interactions provide additional pathways for the dissipation of energy at the sliding interface and result in a greater frictional response.
8:40 AM		NS+SS-MoM-2 Mechanical Properties of Alkylsilane Monolayers with Mixed-Chain-Length N. Li, L. Xu, D.F. Ogletree, M. Salmeron (Lawrence Berkeley National Laboratory) Using an atomic force microscope, the morphology and mechanical properties of mixed self-assembled monolayers were investigated. The monolayers were formed of Octadecyltriethoxysilane(OTE) and shorter-chain alkylsilanes on freshly cleaved mica. We have made comparative experiments on the mixed-chain-length and uniform monolayer. The effects of normal force on the film height difference and the friction behaviour were investigated under controled conditions.
9:00 AM		NS+SS-MoM-3 Applications of Multi-task Scanning Probes to Friction and Adhesion A.R. Burns, J.E. Houston (Sandia National Laboratories) Molecular monolayers such those produced by self-assembly on gold and oxide surfaces are model systems for the study of nanoscale friction and adhesion. In order to better understand some of the fundamental aspects of molecular friction, we need to establish a correlation between lubricant structure and dynamic mechanical response. Thus we have developed a multi-task scanning near-field optical microscope that integrates near-field optical detection through a tapered optical fiber probe with simultaneous probe shear force detection and normal force detection via a balanced-capacitance-based sensor¹. The optical signal consists of fluorescence from probe chromophores embedded in the monolayers. Preliminary results on patterned, self-assembled monolayers will be discussed.² ¹J. E. Houston and T. A. Michalske, Nature 356, p.266 (1992). ²This work was supported by the United States Department of Energy under Contract DE-AC04-94AL85000. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the Unites States Department of Energy.
9:20 AM	Invited	NS+SS-MoM-4 Nanofriction and Nanoimaging at the Insulator-Liquid Interface N.D. Spencer (ETH Zurich) AFM and LFM not only provide a window onto friction on the nanometer scale, but also open up a host of new possibilities for tribology-based chemical imaging of surfaces. Thanks to the suitability of scanning probe methods for in situ experiments, this approach is becoming increasingly important for systems where the liquid-solid interface is of particular interest, such as lubricated contacts, biological interactions, or electrochemical phenomena. We have exploited some inherent physical properties of liquid-solid interfaces in order to gain spatially resolved chemical information about both the solid surface and its specific behavior in the liquid. Systems studied have included pure oxide surfaces, oxidized alloys, and polymers. The spatially resolved chemical information obtained by this approach is virtually inaccessible by any other analytical method.
10:00 AM		NS+SS-MoM-6 Atomic-Scale and Nano-Scale Frictional Behavior of Clean and Hydrogen-Terminated Diamond(111) Surfaces R.J.A. van den Oetelaar, C.F.J. Flipse (Eindhoven University of Technology, The Netherlands) The frictional behavior of surfaces at the atomic-scale and nano-scale can be totally different from friction at macro-scale. The study of friction at such a small scale (nanotribology) is not only of interest from a fundamental point of view, but is also of increasing technological interest due to the ongoing miniaturization of industrial components. We have studied the frictional behavior of a silicon tip on a diamond(111) single-crystal surface, one of the hardest and most common orientations of diamond, using an ultra-high vacuum atomic force microscope and low-energy electron diffraction. Atomic-scale friction is observed on both the clean diamond(111)-(2x1) surface and the hydrogen-terminated diamond(111)-(1x1) surface. Fluctuations in friction at nano-scale on the hydrogen-terminated surface can be attributed to the surface topography. On the clean surface, enormous stick-slip patterns are observed at nano-scale giving rise to a more than two orders of magnitude increase in friction compared to the hydrogen-terminated surface. In contrast, the magnitude of atomic-scale friction on clean diamond(111) is much and much smaller, even compared to atomic-scale friction on the hydrogen-terminated surface. The possible mechanisms responsible for the remarkable difference in frictional behavior between clean and hydrogen-terminated diamond(111) will be discussed, both at the atomic-scale and nano-scale.
10:20 AM		NS+SS-MoM-7 Quantized Friction and Atomic Lattice Resolution in AFM Imaging D.F. Ogletree (Lawrence Berkeley National Laboratory); R.W. Carpick (Uinversity of California, Berkeley); M. Salmeron (Lawrence Berkeley National Laboratory) Topographic AFM images that resolve the atomic lattice of the sample are widely reported for many classes of materials in many environments. Lattice resolution has been observed for layered materials, metals, alkali halide crystals, oxides, molecular crystals, and ordered molecular films produced by self-assembly or Langmuir-Blodgett techniques. Lattice resolution is observed even though the typical AFM tip-sample contact involves 10 to 1000 atoms. We will show that apparent topographic resolution can be explained as a product of AFM instrumental response and quantized or "stick-slip" friction at the atomic level. Numerical simulations of AFM instruments operating in the presence of quantized tip-sample lateral forces will be presented to show the relations between friction and topography.
10:40 AM		NS+SS-MoM-8 Nanotribology of Single Crystal Metals M. Enachescu (Lawrence Berkeley National Laboratory); R.W. Carpick (University of California, Berkeley, and Lawrence Berkeley National Laboratory); D.F. Ogletree, M. Salmeron (Lawrence Berkeley National Laboratory) We present novel results of nanotribological measurements using friction force microscopy (FFM) with a Pt(111) single crystal sample in ultrahigh vacuum. We have examined the friction and adhesion behavior of FFM tips with ordered adsorbate layers on Pt(111) to determine the dependence upon adsorbate species, coverage and structure. The load dependence of friction and its relation to contact area for this structurally and chemically well-defined interface will be discussed.
11:00 AM		NS+SS-MoM-9 Friction at Sub-Micron Lengthscales J. Krim, A. Dayo (Northeastern University) Recent studies of friction's atomic-level origins, or nanotribology, have indicated that the force stems from various unexpected sources, including sound energy (phonons) and electronic excitations. Progress in this field will be discussed, including our use of a quartz microbalance to measure the relative contributions of phonons and electronic excitations to friction in well-characterized contact geometries. The results of such studies are most applicable to friction and adhesion at the interface between liquid and solid materials, where the complicating factors associated with asperity contacts are minimized, and to the MEMS community, where component dimensions are rapidly approaching those routinely probed by the nanotribological community. Work Supported by NSF DMR 9204044
11:20 AM		NS+SS-MoM-10 Growth and Tribological Properties of Thin Ice Films. H. Bluhm, L. Xu, T. Inoue, D.F. Ogletree, M. Salmeron (Lawrence Berkeley National Laboratory) The growth and tribological properties of ice films on mica were studied by a scanning force microscope. The instrument operates in a vacuum chamber at a temperature down to -60 C. Both contact and non-contact imaging were performed. Ice was found to grow in a 2-D manner, often exhibiting dendritic patterns. The surfaces of the ice islands were flat and contained steps several molecular layers in height. The frictional force was found to be smaller on the ice than on the mica surface. Damage to the ice was also observed at high tip loads.
11:40 AM		NS+SS-MoM-11 Scanning Force Microscope Observations of Corrosive Wear in Single Crystal Carbonates and Phosphates L. Scudiero, J.T. Dickinson, S.C. Langford (Washington State University) Mechanical wear in the presence of a corrosive medium provides a new way to manipulate material on a nanometer size scale. We emplosy Scanning force microscopy (SFM) to study nm-scale wear of single crystal calcite (CaCO₃) (100) and brushite (CaHPO₄^.2H₂O) (010) surfaces in aqueous solutions. The rhombohedral structure of calcite allows for two distinct surface steps that display markedly different dissolution rates. When the SFM tip is drawn back and forth in a linear fashion across the edge of a pre-existing single atomic layer etch pit, dissolution is strongly enhanced at the point where the tip crosses the step. The growth of the resulting "wear track" as a function of contact force is consistent with a thermally activated wear process (double kink nucleation), where the activation energy is locally reduced in the strain field of the SFM tip. This strain dependence can be characterized by activation volumes (different for the two distinct steps), which are close to the volume of a single ion in the calcite lattice. Although the dissolution rates depend strongly on solution chemistry, the activation volumes do not. The lower symmetry of monoclinic brushite allows for three distinct, nonparallel surface steps under dissolution conditions. The orientation of these steps on any given crystal display a consistent chiral symmetry, determined by the underlying crystal structure. Step dissolution on brushite is also strongly enhanced by tip induced stress, again yielding activation volumes on the order of typical ion volumes. No threshold stress for stress-enhanced dissolution is observed in either material. These measurements provide fundamental information on dissolution under tribological stimulation and relate to mechanochemical polishing and allows production of large, atomically flat surfaces. This work was supported by the National Science Foundation Surface Engineering and Tribology Program under Grant CMS-9414405.