AVS1997 Session NS+SS-TuM: Nanomechanics and Adhesion

Tuesday, October 21, 1997 8:20 AM in Room K

Tuesday Morning

Start	Invited?	Item
8:20 AM		NS+SS-TuM-1 Viscoelastic Effects in Nanoscale Contacts Under Shear W.N. Unertl (Naval Research Laboratory); S.V. Stepnowski (USNR); K.J. Wahl (Naval Research Laboratory) We have measured viscoelastic response and its time variation for nanometer scale contacts on polyvinylethelene (1,2-polybutadiene) surfaces. The polyvinylethelene films (bulk glass transition temperature 273 K) were cast onto glass from about 10 percent solution in toluene and dried in vacuum. Contacts between the polyvinylethelene and silicon nitride AFM tips were made using a scanned probe microscope. Contacts were sheared by modulating the substrate laterally at frequencies between 50 and 1200 Hz. The modulation amplitude was about 1-2 Å; this amplitude was more than two orders of magnitude below the amplitude required to initiate slip in contacts at constant loads of several nanonewtons. The amplitude and phase of the torsional response of the AFM cantilever force sensor were measured simultaneously with a lock-in detector. Variation of both response amplitude and phase with driving frequency demonstrated that the contact was viscoelastic. A Voigt model was used to extract the frequency dependent contact stiffness κ and relaxation time τ. Unlike κ, τ is independent of sensor stiffness. Measured relaxation times were in the range 0.1 to 1 ms. The transient response of the force sensor torsion to a step change in shear displacement of the sample was also measured. These values are about two-to-three orders of magnitude slower than in the bulk. τ slowly decreased toward the bulk value over a period of about one week and κ simultaneously increased. Possible mechanisms responsible for this decrease include chain reconfiguration, oxidation, and removal of residual solvent.
8:40 AM		NS+SS-TuM-2 Shear Modulation Spectroscopy in Nanometer Scale Contacts K.J. Wahl (Naval Research Laboratory); S.V. Stepnowski (USNR); W.N. Unertl (Naval Research Laboratory) Mechanical properties of contacts with nanometer dimensions are important in microelectronics, microelectromechanics, recording media, and other microscale tribological contacts. We describe application of a scanned probe microscope (SPM) under shear modulation. This technique provides new insights into dynamic processes that occur during the formation and breaking of nanometer scale contacts under shear. In these experiments, the sample position is modulated laterally with amplitudes as small as 1 Å and modulation frequencies in the range 50 to 1200 Hz. The amplitude X and phase α of the torsional response of the SPM force sensor are measured simultaneously with a lock-in detector. Measurements can be made with the tip in contact at constant load or during force-distance curves. We illustrate the technique with examples of materials with a wide range of mechanical properties: diamond, mica, silicone, polyvinylethelene, and Langmuir-Blodgett films. The basic characteristics of the shear response are similar for all these materials. In constant load contacts in air, X increases linearly with increasing modulation amplitude and α is constant until the shear strength of the contact is reached and sliding begins. During force-distance curve measurements, both X and α change considerably between jump-to-contact and pull-off. These changes are largest just after jump-to-contact and just prior to pull-off where the load is tensile. Often, X and α change continuously over the entire range of contact forces. This behavior appears to be inconsistent with purely elastic deformation of the contacts. Possible contributions due to microslip, slip, surface contamination, humidity and tip deformation will be discussed. Viscoelastic response was demonstrated for polyvinylethelene by the frequency dependence of α. Quantification and sensitivity of the method will be discussed.
9:00 AM	Invited	NS+SS-TuM-3 Adhesion and Mechanical Properties at the Nanometer Level. J.E. Houston (Sandia National Laboratories) At the nanometer level, material properties and processes can be remarkably different from those observed in everyday experience. Mechanically, solids can approach the behavior of perfect single-crystals and interparticle interactions can become dominated by surface energetics and dynamics. The study of these properties at the nanometer level is presently enjoying broad attention due in large part to the rapid development of scanning probe techniques. In the present paper, I will illustrate some of these unique nanoscale effects in applications of the Interfacial Force Microscope (IFM) to studies of adhesion and mechanical properties. The IFM is a scanning force-probe microscopy similar to the Atomic Force Microscope but distinguished by its use of a mechanically stable, zero compliance force sensor. I will illustrate the IFM's unique capabilities with examples of adhesion experiments ranging from subtle hydrogen-bonding interactions between self-assembling monolayer end groups to the violent interactions between clean-metal surfaces; and nanomechanical properties in studies ranging from the quantitative measurement of initial dislocation nucleation to the effect of morphological defects, such as steps, on the nanoscale "hardness" of single-crystal surfaces. Finally, I will discuss future directions for this kind of research and suggest its potential implications for impacting advanced materials development. This work was supported by the U. S. Department of Energy under Contract DE-AC04-94AL85000. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed-Martin Company, for the U. S. Department of Energy.
9:40 AM		NS+SS-TuM-5 Nanomechanical Properties of Au (100), (110), and (111) J.D. Kiely, R.Q. Hwang, J.E. Houston (Sandia National Laboratories) Nanometer-scale indentation has been employed to investigate the mechanical properties of Au (100), (110), and (111) surfaces. Single crystal samples were prepared via UHV sputter/annealing and were passivated by a self-assembling alkanethiol monolayer, which eliminates probe-sample adhesion and allows a classical Hertzian analysis of the nanomechanical properties. Using the interfacial force microscope (IFM), which combines zero-compliance sub-micro-Newton force resolution with scanning force imaging, comparisons of elastic modulus and yield point were performed on the three surfaces. Typical indentations were to a depth of 10-15 Å and to forces of 5-8 microNewtons. Au has a relatively high degree of anisotropy and is ideal for investigating elastic and plastic indentation phenomena on the nanometer scale. Measured elastic moduli of the three surfaces indicate of the stress state during indentation through the combination of elastic constants they sample. Indentation moduli for the three surfaces are modeled as combinations of lattice elastic constants, in agreement with previous results [Vlassak and Nix, J. Mech. Phys. Solids 42, 1223 (1994)]. Anisotropy in yield points has also been quantified and must be considered in terms of the orientation of the <110>{111} slip system. Additionally, the effect of stress fields at surface steps on local moduli and yield points has been quantified. Lower moduli, lower initial yield points, and larger deformation were found at the high side of surface steps relative to those found far from the step and at the low side of the step. The results indicate that mechanical properties vary across a single-crystallographic surface by as much as 20% and appear to scale with step height. This work was supported by the U.S. Department of Energy under contract DE-AC04-94AL85000. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed-Martin Company, for the U.S. Department of Energy.
10:00 AM		NS+SS-TuM-6 Calculations of Plastic Deformation and Recovery during Surface Indentation C.L. Kelchner, J.C. Hamilton (Sandia National Laboratories) Experimental techniques such as IFM, AFM, and nanoindentation allow measurements of force as a function of depth when a probe tip is pressed into a surface. Important thin film mechanical properties including elastic modulus and hardness can be derived from these force curves. The strain produced by elastic compression of the surface under a load can be partially relieved by the formation of dislocations and defects in the near-surface region as the tip is pressed further into the surface. We have modeled this behavior using the Embedded Atom Method to calculate the forces on a spherical tip with a radius of 80 Angstroms. In order to prevent avalanche bonding of the tip and the surface, the spherical tip has been replaced by a sphere with a nearly hard wall repulsive interaction with the surface atoms. Force vs. distance profiles for the (111) and (001) Au surfaces were calculated during indentation and retraction of the indenter, via minimum energy calculations. Detailed images of the dislocation structures were obtained by a new method which calculates the deviation from a centrosymmetric environment around each atom. The first yield point, corresponding to the onset of plastic deformation, produced multiple stacking faults and dislocation loops in the thin film for both surfaces. These dislocations completely healed upon retraction of the indenter, meaning that the observed plastic deformation was not permanent. We will also discuss the plastic deformation of the two surfaces during indentation past the first yield point as well as during retraction of the tip from various stages of the indentation process.
10:20 AM		NS+SS-TuM-7 Nanoindentation of Atomically Modified Surfaces S.G. Corcoran (Hysitron, Inc.); S.R. Brankovic, N. Dimitrov, K. Sieradzki (Arizona State University) Nanoindentation studies on metal and semiconducting surfaces often display instabilities in the load-displacement curves which have been attributed to phase transitions, oxide breakthrough, surface contamination effects, and dislocation nucleation under the indenter tip. We have shown¹ recently that displacement excursions were present for nanoindentation on single crystal Au (111), (110) and (100), and were attributed to dislocation nucleation since all other phenomena were ruled out. We present our recent results which have been aimed at understanding the effects of surface modification at the nanoscale on dislocation nucleation. The effects of modifying the Au surface with electrochemically deposited metal monolayers (Pb and Ag), with an electrochemically deposited oxide monolayer and an electrochemically reconstructed surface will be presented. Hardness differences as great as a factor of 3 have been observed for these surfaces. These experiments are unique in that they were carried out under electrochemical control where strict control of the surface cleanliness can be maintained. ¹S.G. Corcoran, R.J. Colton, E.T. Lilleodden, and W.W. Gerberich, Phys. Rev. B Rapid Communication, accepted for publication.
10:40 AM		NS+SS-TuM-8 Adhesion at an Atomically Defined Junction. A. Schirmeisen, G. Cross, A. Stalder, P. Grütter (McGill University, Canada); U. Dürig (IBM Research Division, Switzerland) We perform direct measurement of short range adhesive forces in a tip-sample configuration in UHV at 150 K. An atomically flat Au(111) terrace is approached by W(111) and W(110) tips. The atomic structure of these tips is manipulated and imaged [\it in situ] by field ion microscopy (FIM) immediately before and after force distance spectroscopy. We observe attractive, short-range metallic adhesive forces of about 7 nN. We report on the effect of tip geometry with respect to attraction, short range repulsion, and jump-to-contact behaviour.
11:00 AM		NS+SS-TuM-9 Investigation of Ice-Solid Interfaces by Force Microscopy B. Pittenger, D.J. Cook, C.R. Slaughterbeck, S.C. Fain, Jr. (University of Washington) As part of a program to understand the dynamics of ice surfaces interacting with other solids in controlled environments, we are studying the interaction between oxidized silicon tips and vapor-deposited ice in a pure water vapor environment using force curve measurements. Modifications of an apparatus described previously¹ include direct measurement and control of the temperature of the cantilever holder, a gold-plated substrate for the vapor-deposited ice, electrical connections to cantilever and substrate which avoid silver paint, and larger scan ranges. The measurements described here are for zero external electrical bias applied to the tip. For maximum externally applied load force on the ice exceeding 100 nN, we often observe a correlation between the maximum load and the adhesive pull-off force. Simple elastic contact models predict that the pull-off force from a flat surface can depend on the interfacial energies, tip geometry, and elastic moduli, but it will be independent of the maximum load. Some inelastic flow of ice under the tip must be taking place which is dependent on the maximum load, creating a change in the contact area as the external load is removed. The rapid dynamics of ice surfaces above -20 C allows the indentation to be partially filled in between force curves. Data will be presented taken both in the original and modified apparatus and compared to simple model estimates of the flow of material due to plastic deformation and to heat transport from the tip to the ice. In addition, preliminary measurements of frictional effects between various tips and ice surfaces will be described. Supported by NSF DMR 96-23590. ¹C. R. Slaughterbeck et. al., J. Vac. Sci. Technol. A 14, 1213 (1996)
11:20 AM		NS+SS-TuM-10 Modification of Molecularly Thin Films of Water on Mica Surface by Nanometer Scale Contacts ---- A Scanning Polarization Force Microscopy Study L. Xu, D.F. Ogletree, M. Salmeron (Lawrence Berkeley National Laboratory) The response of a molecularly thick film of water on mica to the perturbation caused by contact with a sharp tip was studied by scanning polarization force microscopy (SPFM). The water layer was formed by exposing freshly cleaved mica to water vapor in a controlled humidity chamber. The perturbation of the film was produced by physical contact with the tip for a short time (ms to s) and imaged afterwards using the non-contact electrostatic force mode. It is found that the modification effects on the water films greatly depend on humidity. It is proposed that the observed modification involves the formation of new layers of water molecules.
11:40 AM		NS+SS-TuM-11 Correlation of Film Stress and the Nano-Mechanical Properties of Au Thin Films K.F. Jarausch (North Carolina State University); J.E. Houston (Sandia National Laboratories); P.E. Russell (North Carolina State University) The interfacial force microscope (IFM) has been used to investigate the nano-mechanical properties of Au thin films as a function of residual and imposed film stress. The technique differs significantly from conventional nanoindentation techniques in the approach to force measurement, in-situ imaging capability, and since the elastic and plastic response of a film is analyzed during the loading portion of an indentation cycle. As was observed in earlier work, the mechanical response of 200 nm thick polycrystalline Au films deposited on various adhesion layers (Ti, Cr, Pt) on Si substrates was consistent for a particular film-adhesion layer combination, however the values of effective elastic modulus and shear-stress threshold were found to vary by as much as a factor of two as a function of adhesion layer. The IFM measurements were found to correlate with the differences in residual stress resulting from the film-adhesion layer combination and were not found to correlate with the films' morphology or adhesion to the substrate. A new four-point bending device is being used to investigate the dependence of IFM nanoindentation measurements on imposed tensile and compressive stress. The observed dependence of the mechanical response on stress will be discussed in terms of possible mechanisms, identifying how the stress alters the measurement process and causes the variation of the films' mechanical response. These experiments suggest that the IFM used in nanoindenter mode has the potential for being able to measure thin film stress on a very local level. This work was supported under DOE under Contract No. DE-AC04-94AL85000.