AVS1996 Session NS+AS-TuM: Nanomechanics, Friction, and Adhesion

Tuesday, October 15, 1996 8:20 AM in Room 202 A/B

Tuesday Morning

Start	Invited?	Item
8:20 AM		NS+AS-TuM-1 Nonlinearity in Stick - Slip Dynamics F. Family, Y. Braiman, H. Hentschel (Emory University) We study the nonlinear contribution to stick-slip motion in a class of weakly coupled discrete one-dimensional arrays of oscillators subject to a periodic substrate potential. The nonlinear dynamics leads to a fundamentally different friction law, in particular when the driving force is barely larger than the minimal force needed to start the motion. We find a dramatic increase in the friction coefficient of the array compared to that of a single uncoupled oscillator, even though the same constant force f is applied to each oscillator in the array. The sliding friction coefficient \eta\ is found to diverge as \eta\ proportional to (\kappa\-\kappa\\sub c\)\super -1/2\ where \kappa\sub c\ is the critical value of the coupling constant \kappa\. The coefficient \eta\ also grows linearly with the number of elements in the array N and shows dynamical transitions as the external force f applied to each of the oscillators is increased. The addition of thermal noise and quenched spatial disorder significantly alters the dynamics of an array, leading to decrease or increase of the friction. We believe that these results are of particular importance in understanding friction in the regime where the contribution of nonlinear processes on the dynamics of the system can not be ignored.
8:40 AM		NS+AS-TuM-2 Tribological Characteristics of Novel Dichalcogenide Nanostructures and Oriented Films R. Tenne, T. Tsirlina, S. Cohen (Weizmann Institute of Science, Israel); L. Rappaport (Technological Education Center, Israel); C. L\aa e\vy-Cl\aa e\ment, E. Ponomarev (CNRS, France) A new class of materials developed in our laboratories, inorganic fullerenes, consist of nanoclusters of layered metal dichalcogenides, e.g. WS\sub 2\ and MoS\sub 2\. These clusters are unstable against folding due to the large energy associated with dangling covalent bonds and thus fold in against themselves. The closed structures, which range from under ten to over a hundred nm in breadth have been imaged using electron microscopies and scanned probe microscopies. Similar to the powdered dichalcogenides, these materials exhibit excellent lubricating properties. In contrast to the powders, these new materials can be stably bound to a substrate, greatly increasing their efficacy for solid lubrication. In this paper, friction and wear tests made on these films will be presented, comparing macroscopic friction measurements with nano-scale studies using force microscopy. Different models of friction will be considered, including rolling friction of the polyhedral structures. In addition, the role of adhesion will be discussed. Comparison will be made with friction on highly oriented films of layered dichalcogenide compounds formed by van-der Waals rheotaxy.
9:00 AM		NS+AS-TuM-3 AFM Studies of Corrosive Tribological Wear of Single Crystals J. Dickinson, N. Park, M. Kim, S. Langford (Washington State University) In several mechanical wear situations, a surface experiences simultaneous tribological loading and corrosive chemical exposure; the combination can greatly increase wear rates. We examine the exposure of single crystal CaCO3 (calcite) and CaMg(CO3)2 (dolomite) to buffered aqueous solutions and mechanical stimulation with the AFM tip. Si3N4 tips are used with applied normal loads, from 0-300 nN, and tip velocities from 1-200 micrometers/s. We present the influence of normal force, tip velocity, and solution chemistry on the rates of corrosive wear of CaCO3. Images of the wear of atomic steps can be used to examine the wear rates and propagation of dissolution around the stimulated region. Mechanical stimulation includes small area scans, linear reciprocation, and indentation. Our results are interpreted in terms of a mechanically induced double kink nucleation mechanism which dramatically promotes the corrosive attack by the solution. We present a quantitative model which relates applied stress to dissolution rate. In the case of dolomite, we are able to create straight channels several atomic layers deep and less than 20 nm wide. These experiments have important implications in the area of mechano-chemical polishing. * This work supported by NSF under Grant No. CMS-9414405 and NSF Instrumentation Grant No. DMR-9205197.
9:20 AM	Invited	NS+AS-TuM-4 Atomic Scale Studies of Interfacial Shear and Adhesion D. Ogletree (Lawrence Berkeley National Laboratory); R. Carpick (Lawrence Berkeley National Laboratory and University of California, Berkeley); M. Salmeron (Lawrence Berkeley National Laboratory) When solids make contact, the actual contact is made by a few asperities, which make up a very small fraction of the geometric contact area. Mechanical properties such as adhesion and friction which depend on these asperitiy contacts have been difficult to study directly, since the asperities are difficult to locate and characterize. With the Atomic Force Microscope, it is now possible to study a single asperity contact directly and to measure adhesion and shear between well defined surfaces in controlled environments including ultra-high vacuum. In this way the fundamental mechanisms of adhesion and friction can be investigated, and models of contact formation (contract mechanics) can be verified. Recent results of quantitative AFM studies of friction in ultra-high vacuum will be presented. The application of contact mechanics to nanometer size AFM contacts will be discussed, along with some instrumental issues related to friction studies with the AFM. The dominant role of friction in the phemonena of "Atomic Resolution" AFM imaging will be discussed.
10:00 AM		NS+AS-TuM-6 The Influence of pH on Adhesion and Friction of Oxide Surfaces in Electrolytes on Nanometer Scale: A Lateral Force Microscopy Study G. H\um a\hner, A. Marti, N. Spencer (ETH Zurich, Switzerland) The relation between adhesion and friction on a submicroscopic scale is of fundamental importance to the understanding of frictional phenomena in the macroscopic regime [1]. We have investigated the influence of pH in an electrolyte solution on both normal and lateral forces between a Si\sub 3\N\sub 4\ tip and various oxide surfaces. These are usually electrically charged in aqueous electrolyte solutions due to the interaction of surface OH\super -\ groups with H\sub 3\O\super +\ or OH\super -\ ions The influence of pH on surface adhesion can be directly monitored in normal force-distance curves. It was found that beside this well-known normal-force dependence, the lateral force is also highly sensitive to the pH of the surrounding liquid and to the isoelectric point IEP (where the overall net surface charge is zero) of the materials under investigation [2]. Experimental results for SiO\sub 2\ and AlO\sub x\ substrates are presented, and a microscopic model based on entropy is suggested, which explains the observed hysteresis in the normal force curves. The dissipation process during friction, its relation to adhesion phenomena, and potential applications of these phenomena will be discussed.[1] Israelachvili, J. N.; Chen, Y.-L.; Yoshizawa, H. J. Adhesion Sci. Technol. 1994, 8, 1231[2] A. Marti, G. H\um a\hner, and N. D. Spencer, Langmuir 11, (1995) 4632
10:20 AM		NS+AS-TuM-7 Electrical Resistivity and Sliding Friction Measurements of Xe/Ag(111) A. Dayo, J. Krim (Northeastern University) We have combined a quartz crystal microbalance technique with dc-resistivity measurements to study the relation between sliding friction levels and adsorbate-induced resistivity changes in thin metallic films. The resistance of a metal film exhibits a sensitive dependence on conduction electron scattering from its surface, which may be directly related to electronic dissipative mechanisms in sliding friction. Quartz microbalance measurements meanwhile yield information on the total energy dissipation due to both electronic and phononic mechanisms. We have carried out measurements on the system Xe/Ag(111) to investigate the relative contribution of electronic dissipative mechanisms. Work Supported by NSF DMR\#9204022.
10:40 AM		NS+AS-TuM-8 Silicon Coupling Agents as Microlubricants: Reaction with Silicon R. Rye, G. Nelson, M. Dugger (Sandia National Laboratories) The operation and reliability of micromachines is related to the friction and wear between the moving parts, and to their mutual attraction. Because of the size of these machines, the requirements on a lubricant are stringent. We are exploring the use of the silane based coupling agents, octadecyltrichlorosilane (ODTS) and octadecyldimethylchlorosilane (ODMS), as micromachine lubricants. The relative coverages of ODTS and ODMS as a function of reaction time and the interaction with the Si surface layer are being studied by XPS and TOF-SIMS. The reaction of ODTS and ODMS with silicon surfaces is dominated by reaction with the natural thin water layer as well as with surface hydroxyl groups. The XPS and TOF-SIMS data show that the reaction of ODMS with normal silicon wafers and the reaction of ODTS with baked Si (water free) leads to comparable amounts of adsorbed hydrocarbon, while the reaction of ODTS with unbaked Si yields ~5 times the amount of adsorbed hydrocarbon. For ODMS or ODTS with baked Si, only the reaction with surface hydroxyls are possible, while with ODTS on unbaked Si water catalyzed two-dimensional cross-linking of the ODTS molecules occurs. The involvement of water has been confirmed by exposure of ODTS coated baked Si to water and a second ODTS reaction. Friction and wear of these films are being investigated using bulk sliding experiments, as well as scanning probe microscope (SPM) techniques. Conventional pin-on-disk sliding experiments indicate that the coefficient of friction between Si3N4 and silicon surfaces is reduced by an order of magnitude (from 0.5 to 0.05) by treatment with ODTS. Microscale friction measurements made using SPM methods indicate that that the lateral force between a silicon probe and an ODTS treated surface is reduced by rubbing. Preliminary SIMS results suggest that this reduction is not due to film removal. The mechanism for reaction of silicon coupling agents with Si surfaces will be discussed. Supported by the U.S. DOE under contract #DE-AC04-94AL85000. This is the end of the abstract form. Make sure the second bracket goes at the end of the last sentence.*
11:00 AM		NS+AS-TuM-9 Interfacial Force Microscope Studies of the Nanomechanical Properties of Au(111) R. Hwang, J. Houston, A. Schmid, T. Michalske (Sandia National Laboratories) Nanoindentation techniques are now widely employed to measure the mechanical properties of bulk materials and thin films. The sensitivity and resolution of these techniques provide an opportunity to investigate the transition from elastic to plastic behavior on an extremely local scale where it is expected to be dominated by surface properties and dislocation nucleation. An immediate obstacle that arises concerns the difficulty of decoupling the responses of the various inhomogeneities of most samples such as grain boundaries and surface contamination and the effects of tip-surface adhesion. For these reasons, we have systematically investigated the mechanical properties of well characterized single crystal Au(111) surfaces using the interfacial force microscope (IFM). Samples were prepared in an UHV environment and their cleanliness and crystallinity characterized by AES and STM. Large terraces were found that exhibit the herringbone reconstruction which were separated by step bunches. Samples were then passivated by the adsorption of self-assembling alkanethiol molecules that prevent the normally strong tip-surface bonding. This allows the indentation to be analyzed by classic contact-mechanics techniques. By measuring force profiles (loading curves) followed by imaging, we have studied the nanoscale mechanical properties of this surface and the onset of plastic deformation, i.e., the threshold of dislocation nucleation. We contrast the measured properties of large perfect terraces with those in areas of high step density and compare our results with dislocation-nucleation models. This work supported under DOE under Contract No. DE-AC04-94AL85000.
11:20 AM		NS+AS-TuM-10 Mechanics of Self-assembled Monolayers: Measurement and Atomistic Simulations N. Shinn, T. Schneider, T. Michalske (Sandia National Laboratories); U. Landman (Georgia Institute of Technology); T. Kim (Hallym University, Korea); R. Crooks (Texas A&M University) The viscoelastic properties of nanometer-thick molecular films critical to sub-micron lubrication and adhesion technologies are controlled by interfacial effects rather than by bulk characteristics. We have used Acoustic Wave Damping (AWD) experiments and molecular dynamics simulations of self-assembled monolayers (SAMs) to study the atomistic origins of interfacial viscoelasticity at the nanometer scale. The AWD technique measures the elastic energy storage and dissipative loss within a SAM adsorbed onto the gold electrode of a quartz crystal transverse shear resonator. The shear modulus of the monolayer is obtained from the resonator frequency response data using an equivalent circuit model. Because the AWD experiment induces shear deformation of the SAM without a contact probe, the measured shear modulus can be directly related to atomic-scale interfacial structure via molecular dynamics simulations using accurate potentials. For alkane thiol SAMs, we find a chain length dependence in the shear modulus. Photo-polymerization of a novel class of diacetylene-based SAMs incrementally increases the monolayer's mechanical stiffness. Defects and impurities are found to be important in determining the ensemble viscoelasticity of certain films. These results demonstrate the ability to engineer film mechanical properties using chemical composition, molecular structure and ordering. We gratefully acknowledge valuable contributions by additional collaborators C. Daly and J. Krim (Northeastern U), and S. J. Martin (Sandia). Supported by the DOE-BES (DE-AC04-94AL85000) and the NSF.
11:40 AM		NS+AS-TuM-11 Dislocation Nucleation at Nano-scale Contacts T. Michalske, J. Houston, P. Tangyunyong (Sandia National Laboratories); O. Warren (University of Western Ontario, Canada); R. Crooks (Texas A&M University) The plastic deformation of metals in highly localized stress fields is critical to macroscopic phenomena such as fracture, surface wear, and adhesion where crack tips and surface asperities concentrate the applied stress into a very small region of the material. Single asperity contact experiments using Scanning Force Microscopy techniques can be used to measure forces and displacements on the atomic-scale thereby directly exploring elastic and plastic deformation of solids on the nanometer-scale. Previous results from single asperity contact measurements indicate that single crystal metal surfaces can support shear stresses near the theoretical limit (G/10) prior to the onset of permanent deformation, suggesting that the initial plasticity event may involve the homogeneous nucleation of dislocations in the localized contact stress field. In this study we use Interfacial Force Microscopy to examine the nano-scale elastic and plastic response of single crystal gold (111) surfaces using single asperity contacts with radii extending down to 50nm. Our experimental results show that the critical resolved shear stress needed to initiate plastic deformation increases as the probe tip radius decreases. We develop a continuum model based on homogeneous dislocation nucleation that reproduces the scaling and absolute magnitude of our experimental deformation results. This work supported by DOE under Contract No. DE-AC04-94AL85000.