AVS2001 Session TR+SS-WeM: Fundamentals of Tribology & Adhesion

Wednesday, October 31, 2001 8:20 AM in Room 132

Wednesday Morning

Start	Invited?	Item
8:20 AM		TR+SS-WeM-1 Bonding and Debonding in Nanometer-Scale Viscoelastic Contacts M. Giri, D.B. Bousfield, W.N. Unertl (University of Maine) Contact to viscoelastic materials, unlike elastic or elastomeric materials, are poorly understood, primarily because of the hysteretic effects caused by the time dependent mechanical properties. We present a quantitative analysis of contacts to viscoelastic materials, specifically crosslinked styrene-butadiene and uncrosslinked styrene-acrylate copolymers. Contacts were made with ultra-low load indentation using diamond probes of various axisymmetric shapes. Both creep and loading-unloading (bonding-debonding) measurements were made for penetration depths ranging from a few nanometers up to a few micrometers. This data was analyzed using the cohesive zone model recently developed by Hui and coworkers.¹ This model demonstrates that the most information that can be extracted from a contact experiment is the Mode I Stress Intensity Functional K_I. We show that no knowledge of the interfacial bonding mechanism is required to determine K_I from the displacement versus load data. In effect, K_I is analogous to the rate constant of a chemical reaction. Its measurement does not require knowledge of the bonding mechanism, but once measured, it can be used to test models of the mechanism. We illustrate this by testing the widely used bonding-debonding theory of Schapery² for propagation of cracks at viscoelastic interfaces. We show that this model is inadequate to explain our results and attribute this failure to the assumption of a rate-independent interaction potential. ¹Y.Y. Lin, C.K. Hui, J.M. Baney, J. Phys. D: Appl. Phys. 32 (1999) 2250. ²R.A. Schapery, Intl. J. Fracture 39 (1989) 163.
8:40 AM		TR+SS-WeM-2 A Multi-Scale Elasto-hydrodynamic Contact Model of Chemical Mechanical Planarization A. Kim, J. Tichy, T.S. Cale (Rensselaer Polytechnic Institute) We present a physically based multi-scale finite element model to help better understand the CMP process. We extend a model that is presented in Ref. 1. This extended "soft" elasto-hydrodynamic contact model captures the fundamental mechanical and tribological aspects of the CMP process and requires few ad hoc assumptions or adjustable parameters. Recent experimental results show that fluid suction pressures exist,² and the friction coefficient decreases as the Hersey number (i.e., viscosity*velocity/ pressure) increases.³ These results indicate that there exist mixed direct solid-solid contact and partial fluid lubrication, i.e., elasto-hydrodynmic lubrication. The theoretical results presented in this work support elasto-hydrodynamic contact (abrasion) at the pad-wafer interface. The constitutive equation for the soft polymer pad material must be some form of large strain nonlinear elasticity such as hyperelasticity as the strains of a well-deformed asperity are of order one. A physically based asperity-scale hyperelastic model, which includes a frictional effect, is presented to calculate local stresses at asperity tips. These local stresses are directly related to widely accepted material removal models. In most CMP tools, the external downward force is applied to the wafer-carrier head through a ball joint, which in principle cannot transmit a moment. In order to obtain closure of the analysis, the mean depth into the pad and tilt angle of the wafer are determined by the normal global force applied and momentum balances using the Levenberg-Marquardt method. Finally, we summarize our approach to linking the asperity scale contact analysis to the wafer scale model through a statistical method. ¹J. A. Tichy, J. Levert, L. Shan and S. Danyluk, J. Electrochem. Soc. 146(4), 1523 (1999). ²L. Shan, Ph.D. Thesis, Georgia Institute of Technology, 2000. ³Y. Moon, Ph. D. Thesis, University of California, Berkeley, CA, 1999.
9:00 AM		TR+SS-WeM-3 Comparative Energy Dissipation in Nanoscale Shear and Tensile Interactions G. Haugstad (University of Minnesota) A case study of ultrathin polyvinyl alcohol films is presented, comparing energy dissipation in three modes of scanning force microscopy: friction force, pulsed force mode and "tapping mode". Relative energy dissipation is measured on three distinct film components: a continuous first layer (~1 nm thick) strongly adsorbed to mica; a thicker, discontinuous overlayer, autophobically dewetted from the first layer; ordered overlayer islands (1 nm thick) located within the breaks of component #3. These films are chemically homogeneous but structurally heterogeneous. The components differ in amorphous content (free volume) and confinement, giving rise to differences in energy dissipation. Energy dissipation during sliding is quantified as friction force multiplied by a sliding distance of one contact diameter. Dissipation per cycle during vertical cantilever oscillation is quantified from the cantilever phase lag and the ratio of reduced to free resonance amplitude, via the method proposed by Cleveland et al.¹ Dissipation during pull-off in pulsed force mode is quantified with a newly proposed method: by measuring the cantilever deflection with high time resolution (5 MHz) and comparing the (slowly damped) free oscillation amplitude (squared) immediately following pull-off to the quasistatic deflection (squared) immediately prior to pull-off. Corrections arise from (a) the relationship between linear deflection and (measured) angular tilt near the end of the cantilever;² (b) the excitation of higher cantilever eigenmodes² upon pull-off. Our results demonstrate that energy dissipation contrast in pulsed force and "tapping" modes is very similar, though the time scales of interaction are very different, whereas contrast in sliding friction is markedly different from either. ¹J. P. Cleveland, B. Anczykowski, A. E. Schmid and V. B. Elings, Appl. Phys. Lett. 72, 2613-2615 (1998). ²H. J. Butt and M. Jaschke, Nanotechnology 6, 1 (1995).
9:20 AM		TR+SS-WeM-4 A Nanoscale JKR Test for Adhesive Contacts to Polymers S.A.S. Asif (Geo-Centers); K.J. Wahl (Naval Research Laboratory) Contact mechanics measurements at the nanoscale are important for understanding the behavior of ultrathin films developed for adhesives, electronics packaging, microelectromechanical devices, colloidal particles, and lubrication. Determining surface mechanical properties of small devices, thin films or small volumes may be impossible by traditional methods, which lack either high spatial resolution or surface sensitivity. In this paper, we present a dynamic nanoscale Johnson-Kendall-Roberts (JKR) test to examine adhesive contacts to polymers and thin films. The nanoscale JKR test, based on a depth-sensing nanoindenter with AC force modulation capabilities,¹ combines measurements of load and contact or interaction stiffness as a function of tip-surface separation and indenter penetration depth. With this method, and appropriate contact mechanics, it is possible to make localized mechanical property measurements (e.g. loss and storage moduli, adhesion energy, cohesive stress, and strain energy release rate) for contacts with diameters smaller than the optical limit. We present results of studies using probes with tip radii between 1 and 10 microns against polydimethyl siloxane surfaces with varying cross-link densities. Smaller probe diameters and increased cross-link density shifted the measured response away from a pure JKR model into the Maugis-Dugdale transition regime. The storage modulus and surface energy measured from nanoscale JKR results are compared to both calculated values and those measured with conventional nanoindentation. ¹ S.A. Syed Asif, K.J. Wahl, and R.J. Colton, Rev. Sci. Instrum. 70 2408-2413 (1999).
9:40 AM		TR+SS-WeM-5 Combined Nanoindenter and Quartz Crystal Microbalance Studies of Realistic Tribological Contact B. Borovsky, J. Krim (North Carolina State University); S.A.S. Asif, K.J. Wahl (Naval Research Laboratory) There has recently been increased interest in studying friction at nanometer and micron length scales at much higher speeds than are obtainable with instruments such as atomic force microscope and surface forces apparatus. Sliding contacts in computer hard drives, micromachines, and many macroscopic applications move at speeds on the order of 1 m/s. This speed regime is routinely accessed by the vibrating surface of a quartz crystal microbalance (QCM). We have therefore constructed a device capable of studying both high-speed sliding friction and contact mechanics by combining a nanoindenting probe and QCM. By measuring normal load, contact stiffness, and QCM response simultaneously, this combination is well-suited to developing the theoretical understanding of probe-QCM systems. In order to establish the relationship between the QCM response and the properties of the interface, we have carried out detailed studies of glass-metal and metal-metal contacts in air. The interfaces are characterized by a contact area (derived from the square of the contact stiffness) proportional to the normal load, consistent with multi-asperity contact and elastoplastic deformation.¹ We observe that the frequency shift of the QCM is proportional to the true area of contact as inferred from the contact stiffness. Following an earlier suggestion, we model the interaction in the near-field acoustic regime.² We find that our results are explained by accounting for the roughness of the opposing surfaces. The magnitude of QCM frequency shift is found to reflect the elasticity of the interface, the number and size of contact regions, and the degree of slippage. Research supported by NSF, AFOSR, and ONR. ¹ J.A. Greenwood, in Fundamentals of Friction: Macroscopic and Microscopic Processes, NATO ASI Series, I.L. Singer and H.M. Pollock, eds., (Kluwer, Boston, 1992) p. 37. ² A. Laschitsch and D. Johannsmann, J. Appl. Phys. 85, 3759 (1999).
10:00 AM		TR+SS-WeM-6 Molecular Layer Effects on Friction Between Single Crystalline Metal Surfaces A.J. Gellman (Carnegie Mellon University); J.S. Ko (Merck & Co.) The combined use of an ultrahigh vacuum tribometer and a variety of surface science techniques has enabled us to explore the tribological properties of interfaces between Ni(100) surfaces and to observe phenomena attributable to molecular layering. Friction measurements have been made between a pair of clean Ni(100) surfaces, modified by the presence of adsorbed atomic sulfur with and without adsorbed ethanol. Friction measurements made with ethanol coverages ranging from 0 â€“ 10 monolayers on each Ni(100) surface reveal that the friction coefficient is discontinuous in coverage and can be correlated to the coverage dependence of the ethanol desorption energy. During shearing sliding never commences between clean Ni(100) surfaces or sulfided Ni(100) surfaces without adsorbed ethanol. In the submonolayer coverage regime of either atomic sulfur or adsorbed ethanol the behavior is characterized by a high friction coefficient (µ_s > 5.5) accompanied by high adhesive forces (µ_ad = 1.5 Â± 0.7). An abrupt decrease in both the friction coefficient and adhesion coefficient occurs at a coverage of one monolayer of ethanol on each surface. The friction coefficient drops to (µ_s = 3.1 Â± 1. while the adhesion coefficient is lowered to µ_ad ~ 0.25. At coverages between 1.0 and 2.5 monolayers of ethanol on each Ni(100) surface the static friction coefficient decreases in a step-wise manner that is correlated with discontinuities in the ethanol desorption energy. This step-wise decrease in both the friction coefficient and the desorption energy may be due to molecular layering of the ethanol.
10:20 AM	Invited	TR+SS-WeM-7 The Effect of Packing Density on the Friction of Alkane Monolayers J.A. Harrison, P.T. Mikulski (United States Naval Academy) Hydrocarbon materials have traditionally been used to prevent the friction and wear of mechanical components in sliding contact. One important example of this is the use of oil in conventional combustion engines. The advent of chemical vapor deposition technology has piqued interest in the use of solid hydrocarbons as lubricants in systems such as microelectromechanical devices. A detailed knowledge of the molecular-scale mechanisms responsible for lubrication would be invaluable in the design of novel solid lubricants. We are using molecular dynamics to examine the atomic-scale phenomena governing the tribology of hydrocarbon-containing systems. Because liquid hydrocarbons and boundary layer lubricants, such as self-assembled monolayers, are to be studied, the potential energy function must include intermolecular interactions. The new adaptive intermolecular reactive empirical bond-order potential (AIREBO)¹ can simulate reactive and non-reactive processes in a wide range of environments, including graphite, liquid hydrocarbons, and self-assembled monolayers. We have conducted extensive simulations that have examined the friction of alkane monolayers attached to diamond surfaces or model self-assembled monolayer systems. We have examined friction as a function of packing density, chain length,² and sliding direction. Recent AFM results of Perry and coworkers³ unambiguously demonstrate that decreasing the packing density, or the disorder of the film, increases the friction. Simulations reproduce this trend and provide an atomic-scale explanation for this observation. *Supported by ONR and AFOSR. . ¹ S. J. Stuart, A. B. Tutein, and J. A. Harrison, J. Chem. Phys. 112, 6472-6486 (2000). ² A. B. Tutein, S. J. Stuart, and J. A. Harrison, Langmuir 16, 291-296 (2000); Ibid. , J. Phys. Chem. B 103, 11357-11365 (1999). ³ Lee et al., Langmuir 16 2220 (2000).
11:20 AM		TR+SS-WeM-10 A Study of Triboelectricity on Dielectric Surfaces by Measuring Electric-discharge Luminescence during Friction T. Miura, I. Arakawa (Gakushuin University, Japan) Triboluminescence during friction between a spherical surface and a flat surface of dielectrics, i.e., a typical pin-on-disk technique, was observed by a spectrometer and an optical microscopy. Spectrum measurement of the luminescence in ambient air made it clear that electric discharge occurs around a contact point. To investigate a relation between the electric discharge and triboelectricity the luminescent intensity distribution along the sliding direction was measured using the microscope with only UV-translucent filter. In short, the breakdown characteristics depending on the gap distance was aimed. When it is assumed that the UV distribution indicates electric discharge currents at each gap distance, the distribution is accounted for a well-known equation of semi-empirical discharge-current. As a result, voltage between the both sides was evaluated. Then the surface charge density at immediately after the friction in several ms was shown. We believe this is innovative technique to evaluate electric potential difference of triboelectricity between dielectric surfaces. In this conference we will present our experimental results, methods, analysis, and discussions in detail.