Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session L24: Focus Session: Friction, Fracture, and Deformation III |
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Sponsoring Units: GSNP DMP Chair: Roland Bennewitz, McGill University Room: LACC 411 |
Tuesday, March 22, 2005 2:30PM - 2:42PM |
L24.00001: Microparticle Manipulation Using Inertial Forces Michael Eglin, Mark A. Eriksson, Robert W. Carpick Manipulation (transport, positioning, separation, or removal) of micro- and nanoparticles has become an increasingly vibrant field of research. We present a simple method to transport a large number of microparticles in parallel. Piezoelectric shear plates are used to excite asymmetric shear waves which are coupled into a substrate. At the surface of the substrate, linear motion of particles is induced due to inertial forces on the particles and the stick-slip effect. While the approach is very versatile and can be used for a wide range of particle and substrate combinations, it is selective to the particle mass and the surface chemistry. In addition to the study of particle transport, the tribological behavior of the particles on the surface can be investigated by applying a symmetric waveform to the piezo, which allows the probing of the static friction force between the particle and substrate. A simple dynamic model to describe the behavior will be discussed. The frictional behavior of particles on chemically functionalized surfaces will be presented. [Preview Abstract] |
Tuesday, March 22, 2005 2:42PM - 2:54PM |
L24.00002: Contact area dependence of frictional forces: Moving adsorbed antimony nanoparticles Udo Schwarz, Claudia Ritter, Markus Heyde, Klaus Rademann Despite its daily-life importance, the fundamentals of friction are still insufficiently understood. In particular, the interplay between friction, adhesion, ``true'' contact area, and crystalline structure at the interface is an issue of current debate. In this work, antimony nanoparticles grown on highly oriented pyrolytic graphite and molybdenum disulfide were used as a model system to investigate the contact area dependence of frictional forces. This system allows to accurately determine both the interface structure and the effective contact area. Controlled translation of the antimony nanoparticles was induced by the action of the oscillating tip in a dynamic force microscope. During manipulation, the power dissipated due to tip-sample interactions was recorded. We found that the threshold value of the power dissipation needed for translation depends linearly on the contact area between the antimony particles and the substrate. Assuming a linear relationship between dissipated power and frictional forces implies a direct proportionality between friction and contact area. Particles smaller than 10000~nm$^{2}$ in size, however, were found to show lower dissipation than expected, which might be explained by structural lubricity. [Preview Abstract] |
Tuesday, March 22, 2005 2:54PM - 3:06PM |
L24.00003: Intrinsic Friction Analysis of the Glass Forming Process of Polymer Films Rene Overney, Scott Sills, Tomoko Gray Many modern and future technological applications involve ultrathin polymer films with a thickness below the 100-nanometer scale, where statistical bulk averaging is jeopardized and interfacial constraints dictate transport properties. In such confined polymeric systems, transport properties strongly depend on molecular relaxation and structural phases that deviate from the bulk. This is particularly relevant in thermally assisted nanoindentation processes near the glass transition temperature, Tg. In this paper, an elaborate isothermal friction-velocity analysis is introduced, as a material distinctive characterization tool that provides fundamental insight into the glass forming process. It is the glass forming process in constrained thin films that leads to a non-monotonous Tg-profile, which is responsible for a strongly film thickness dependent effective modulus during nanoindentation. The presented study involves ultrathin polystyrene films that serve as model systems in a thermomechanical NEMS storage application designed to circumvent the superparamagnetic limit associated with magnetic data storage. [Preview Abstract] |
Tuesday, March 22, 2005 3:06PM - 3:18PM |
L24.00004: Molecular dynamics simulation of crack propagation in highly cross-linked polymers under uniaxial deformation Mesfin Tsige, Mark J. Stevens The strength of the interface between a structural adhesive and a solid surface is a fundamental issue. We study fracture in highly cross-linked polymer networks (e.g. epoxy) bonded to a solid surface using large-scale molecular dynamics simulations. An initial crack is created by forbidding bonds to occur on a fraction of the solid surface up to a crack tip. The time and length scales involved in this process dictate the use of a coarse grained bead-spring model of the epoxy network. In order to avoid unwanted boundary effects, large systems of up to a million particles are used. Stress-strain curves are determined for each system from tensile pull molecular dynamics simulations. Comparison with standard fracture mechanics will be presented. The dependence of the interfacial fracture energy on film thickness and on the ratio of the width of the unbonded solid surface to film thickness will be described. [Preview Abstract] |
Tuesday, March 22, 2005 3:18PM - 3:30PM |
L24.00005: Reactive MD simulations of deformation and failure in cross-linking polymers. Chandrashekar Shankar, John Kieffer We have developed a computational framework for studying the behavior of polymer systems simultaneously undergoing strain deformations and polymerization reactions, such as epoxy resins and other reacting polymeric systems. Using our simulations we predict system properties, such as yield strength, toughness and ultimate failure of the system at various stress regimes, and examine how these properties are affected by variations in reaction rates and curing conditions. Accordingly, we use this framework to optimize the design of autonomously healing polymer matrix composites, containing di-cyclo-penta-diene (DCPD) as the healing agent. Polymerization reactions occur on time scales, currently inaccessible by conventional MD. We bridge this chasm by coarse graining real monomers as soft interacting beads. Timescale mapping between the coarse-grained simulations and a reactive MD model of atomically detailed systems with reacting monomers such as DCPD, is achieved using diffusion timescale matching. Reaction processes in the coarse-grained simulations are accounted by a MC scheme, in which we can adjust the probability of reaction and the geometric aspects reflecting the reaction mechanisms in a particular polymer system. [Preview Abstract] |
Tuesday, March 22, 2005 3:30PM - 3:42PM |
L24.00006: Short-Range Exponential Repulsive Force Between Randomly Rough Surfaces Kenneth Rosenberg, Marcel Benz, Jacob Israelachvili Using a Surface Forces Apparatus we have studied the effects of surface roughness on the interaction forces and deformations of polymeric surfaces. We measured the force-distance functions on approach and separation of two rough surfaces which, on approach, exhibited an almost perfect exponentially repulsive force-distance regime, and a weak adhesion on separation. Random roughness may be a prerequisite for the exponential force regime, which appears to be due to the local compressions (micro- or fine- grained deformations) of the surface asperities. The resulting characteristic decay lengths were fitted to common surface roughness parameters obtained by Atomic Force Microscopy to draw possible correlations. The coarse-grained (global) deformations of the initially curved surfaces appear to be Hertzian. [Preview Abstract] |
Tuesday, March 22, 2005 3:42PM - 4:18PM |
L24.00007: Oscillating viscoelastic JKR contacts Invited Speaker: Adhesion of micron-scale probes with model elastomers was studied with a depth-sensing nanoindenter under oscillatory loading conditions. Force-displacement curves were highly reversible, consistent with Johnson-Kendall-Roberts (JKR) behavior. However, experiments revealed striking differences between the measured tip-sample interaction stiffness and the theoretical prediction from the JKR relationship. Measured stiffness was always greater than zero, and varying probe radius or polymer modulus resulted in stiffness curve shapes remarkably similar to Maugis’ JKR/DMT transition curves. These apparent paradoxes are resolved by considering viscoelasticity of an oscillating crack tip. Under well described conditions determined by oscillation frequency, sample viscoelasticity, and Tabor’s parameter, an oscillating crack tip will neither advance nor recede. Thus, contact size is fixed at any given instant, and experimentally measured stiffness is equal to the punch stiffness. For fixed oscillation frequency, transition between JKR and punch stiffness can be brought about by increasing probe radius, decreasing sample modulus, or varying frequency. Comparisons of experiments and theory will be presented. Storage modulus and surface energy measured from nanoscale JKR results were compared to calculated and measured with conventional nanoindentation and JKR force-displacement analyses. With this method it is possible to make localized mechanical property measurements for contacts with diameters smaller than the optical limit. \newline \newline Collaborators: S.A. Syed Asif (Hysitron, Inc.); K.L., Johnson, J.A. Greenwood (Cambridge Univ., UK) [Preview Abstract] |
Tuesday, March 22, 2005 4:18PM - 4:30PM |
L24.00008: Cantilever tilt compensation for variable-load atomic-force microscopy R.J. Cannara, M.J. Brukman, R.W. Carpick In a typical atomic-force microscope (AFM), the cantilever forms an angle with respect to the sample surface. This tilt is important for contact mode experiments, because the free end of the cantilever (constrained to move along the surface) displaces laterally as applied load varies. As a result, the AFM tip makes contact with a different point on the surface at each load. These positions lie along the surface projection of the lever's long-axis. The amount of relative tip-sample displacement is proportional to load and is shown to be substantial. Thus, care should be taken when performing load-dependent contact mode experiments, such as friction versus load or force versus separation measurements, when it is required that the tip scan the same line or remain on the same position at each load. We present a method that compensates for in-plane tip-displacement versus load, based on a simple geometric calculation that depends only on the range of vertical motion. We demonstrate the successful use of this method on surfaces with nano-scale inhomogeneities and show that the tip can remain localized over a large load range. [Preview Abstract] |
Tuesday, March 22, 2005 4:30PM - 4:42PM |
L24.00009: Carbon, Hydrogen, and Silicon-containing Solid Lubricants Judith Harrison, Paul Mikulski, Ginger Chateauneuf, J. David Schall, Guangtu Gao The development of micro-sized devices has prompted the need for protection of the surfaces of these devices. Amorphous carbon films (a-C and a-CH), doped carbon films, and self- assembled monolayers (SAMs) are all possible candidates for the passivation and lubrication of these devices. The fundamental problem associated with controlling friction and wear is a lack of understanding of the underlying atomic-scale chemical and physical processes that govern them. Extensive molecular dynamics (MD) simulations have been done that have examined the compression and friction of model hydrocarbon SAMs, amorphous carbon- and silicon-containing films both with and without hydrogen. We have examined the contact forces present at the interface between a tip and pure, or mixed-length, SAMs during sliding. Compression and shear-induced polymerization have also been modeled in unsaturated hydrocarbon films. In addition, we have also done simulations that analyze the mechanical and tribological properties of a-C, a-CH, a-C-Si, and a-C-Si-H films. Some of our recent results will be discussed. [Preview Abstract] |
Tuesday, March 22, 2005 4:42PM - 4:54PM |
L24.00010: Nanotribology of high performance amorphous carbon films D.S. Grierson, A.V. Sumant, K. Sridharan, E.E. Flater, T.A. Friedmann, J.P. Sullivan, R.W. Carpick High performance carbon films are attracting a great deal of interest as candidate materials to improve the tribological characteristics of mechanical parts ranging from the macroscale to the nanoscale. We have investigated the nanotribology and the surface chemistry of two types of diamond-like carbon (DLC) films. One, known as tetrahedral amorphous carbon (ta-C), is grown with Pulsed Laser Deposition (PLD). It is essentially hydrogen-free and contains a high (up to $\sim $80{\%}) fraction of sp3-bonded carbon. The other is grown with Plasma Immersion Ion Implantation and Deposition (PIIID). This film is a hydrogenated DLC with a lower fraction of sp3-bonded carbon (30-50{\%}). The nanotribology is characterized quantitatively with atomic force microscopy (AFM) utilizing silicon AFM tips coated with diamond, DLC, or ta-C. The surface chemistry is characterized via near edge x-ray absorption fine structure (NEXAFS) spectroscopy. We will discuss how doping the DLC and annealing the ta-C affects the nanotribology and surface chemistry of these films. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Tuesday, March 22, 2005 4:54PM - 5:06PM |
L24.00011: Metallic Adhesion in Atomic-Size Junctions Nicolas Agrait, Gabino Rubio-Bollinger, Philippe Joyez We report high resolution simultaneous measurements of electrical conductance and force gradient between two sharp gold tips as their separation is varied from the tunneling distance to atomic-size contact. The use of atomically sharp tips minimizes van der Waals interaction, making it possible to identify the short-range metallic adhesion contribution to the total force. [Phys. Rev. Lett. 93, 116803] [Preview Abstract] |
Tuesday, March 22, 2005 5:06PM - 5:18PM |
L24.00012: Incipient Plasticity During Indentation of a Well Characterized 3 Nanometer Radius Tip Graham Cross, Andre Schirmeisen, Peter Gruetter, Urs Duerig We present the results of nanoindentation testing of a well characterized tip geometry with a spatial scale easily matched by existing atomistic simulation. The atomically defined tungsten asperity of 3 nm radius was fabricated and imaged by field ion microscopy and brought into contact with a Au(111) terrace in ultra-high vacuum conditions. The mechanical evolution of the asperity contact under cyclic indentation testing was monitored by a simultaneous load-displacement and electrical current-displacement measurement. Load displacement curves of the pristine surface showed multiple discrete plastic (``pop-in'') events during loading, with energies consistent with the nucleation of individual defects. During unloading, we observe reverse plasticity and complete self-healing of the induced defect. Both the qualitatative behaviour and measured energy dissipation values are in agreement with recent molecular dynamics simulations of incipient plasticity in metallic asperity contacts. [Preview Abstract] |
Tuesday, March 22, 2005 5:18PM - 5:30PM |
L24.00013: Molecular Dynamics Simulations of Nanoindentation and Nanoscratching of $\beta$-SiC A. Noreyan, J.G. Amar, I. Marinescu We present the results of molecular dynamics simulations of nanoindentation of the Si-terminated (001) surface of $\beta$-SiC by a diamond tip. In particular we investigate the dependence of the critical depth and pressure for the elastic-to-plastic transition as a function of indentation velocity, tip size, and workpiece temperature. Our simulations were carried out using the Tersoff potential, which accurately reproduces the lattice and elastic constants of $\beta$-SiC. Over the range of indenter sizes used in our simulations, both the critical pressure and indentation depth decrease with increasing indenter size. In contrast, the critical indentation depth for the elastic-to-plastic transition does not depend on the indenter velocity. For indentation depths beyond the critical depth, the pressure saturates at 100 GPa, which corresponds to the experimental pressure at which $\beta$-SiC transforms to the rocksalt structure. An analysis of the pair-correlation function and bond-angle distribution as a function of indenter depth supports our conjecture that the observed plastic behavior is related to the onset of a phase transition from the zinc-blende structure to the rocksalt structure. Results for nanoscratching of the (100) surface of $\beta$-SiC are also presented. [Preview Abstract] |
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