AVS2001 Session TR+MM-WeA: Nanotribology

Wednesday, October 31, 2001 2:00 PM in Room 132

Wednesday Afternoon

Start	Invited?	Item
2:00 PM	Invited	TR+MM-WeA-1 Tribological Issues in MEMS R. Maboudian (University of California, Berkeley) Given the dimension of most microelectromechanical systems, gravity and other body forces are negligible. In contrast, interfacial forces dominate due to their large surface area-to-volume ratios. As a consequence, adhesion, friction, and wear are prevalent problems in many MEMS devices. Additionally, MEMS technology provides us with the opportunity to study tribology on a length scale not easily accessible by other techniques, namely the mesoscopic length scale. This presentation will discuss the use of several tribological microinstruments in conjunction with other surface characterization techniques, such as atomic force microscopy, to measure the surface forces present between polycrystalline silicon surfaces and to manipulate them by utilizing various surface treatments. The successes and the limitations of current surface coating technologies as well as areas for improvement will be discussed.
2:40 PM	Invited	TR+MM-WeA-3 Friction Properties of Self-assembled Monolayers: Influence of Adsorbate Molecular Structure and Molecular Organisation N.J. Brewer, G.J. Leggett (UMIST, UK) The frictional properties of self-assembled monolayers (SAMs) of alkanethiols on gold are influenced by both the chemistry of the adsorbate tail groups and the organisation of adsorbate molecules within the monolayer. Studies of mixed self-assembled monolayers composed of adsorbates with contrasting terminal group chemistries and equal chain lengths have revealed a linear relationship between SAM composition and the coefficient of friction. However, for mixed SAMs formed from molecules with differing alkyl chain lengths, molecular organisation may be quite different from that observed for single component SAMs, leading to substantial changes in the coefficient of friction. Moreover, while methyl terminated SAMs exhibit markedly different frictional properties for adsorbates of different chain length, this is not the case for SAMs formed from hydroxyl and carboxylic acid terminated adsorbates. For these polar SAMs the coefficient of friction varies little between short and long chain adsorbates. This is attributed to hydrogen bonding between adsorbate terminal groups, which has a dominating effect on SAM stability and organisation. The friction-velocity behaviour of SAMs with polar terminal groups in contact with polar tips is quite different from the behaviour observed for the same materials in contact with non-polar tips, and the expected linear relationship between the friction force and the log of the velocity is not observed; instead the friction force rises rapidly and then reaches a plateau. Finally, friction properties of specific thiols adsorbed onto gold and silver surfaces are markedly different, reflecting changes in molecular organisation and providing valuable insights into the structures of these materials.
3:20 PM		TR+MM-WeA-5 Interfacial Friction of Methanol Sliding on Rigid vs. Rotating Fullerenes T.S. Coffey, M. Abdelmaksoud, J. Krim (North Carolina State University) Due to the ability of C60 molecules to rapidly rotate within their lattice position, tribologists had hopes that C60 molecules would work like nano-sized ball bearings. C60 has not been proven to be an effective lubricant. We measured the interfacial friction of toluene on Ag(111) in both the presence and absence of interfacial C60 layers employing quartz crystal microbalance (QCM) and atomic force microscopy (AFM) techniques.¹ We see the friction double when C60 is present. However, the difference in interfacial friction between a rigid vs. a rotating molecule in a substrate remains an interesting topic. C60 is known to form close packed hexagonal films on both Ag(111) and Cu(111) substrates. However, on Ag(111) surfaces, C60 is known to spin freely in its lattice position, while it is not free to rotate on a Cu(111) surface.² In order to determine whether the spinning of the C60 molecules affects the interfacial friction, we are employing QCM techniques to compare the friction of methanol on C60/Ag(111) vs. methanol on C60/Cu(111). We examine here whether the rolling nature of the C60 layer impacts the sliding friction as probed by QCM, AFM, and QCM/STM measurements. ¹ ¹ T. Coffey, M. Abdelmaksoud, and J. Krim, "A Scanning Probe and Quartz Crystal Microbalance Study of the Impact of C60 on Friction at Solid-Liquid Interfaces", Journal of Physics Condensed Matter, in press. ² Applied Surface Science 87/88 (1995) 405.
4:00 PM		TR+MM-WeA-7 Superconductivity Dependent Friction of Adsorbed Monolayers on Pb(111) J. Krim, A. Mayer, L. Wagner (North Carolina State University) In order to gain a fundamental understanding of friction, one must understand, at the molecular level, how the energy associated with the work to overcome friction is converted to heat. Such knowledge is key to understanding the rate at which an interface will heat, and in addition how chemical reactions and other physical processes triggered by heat will be affected by friciton. One of the simplest possible geometries in which friction can occur, and thus be studied, is that of a fluid or crystalline monolayer adsorbed on an atomically flat surface. This geometry is experimentally accessible to experiments with a Quartz Crystal Microbalance (QCM), to numerical simulation techniques, and to analytic theory. A recent QCM experiment¹ sought to explore the nature of electronic contributions to friction by measuring the friction associated with nitrogen monolayers sliding on Pb substrates as the temperature passed through the superconducting transition at 7.2K. The work inspired a number of subsequent theoretical and experimental efforts, which yielded contradictory results. A major complication associated with the Ref. 1 result arose from the fact that the data were reported for Pb substrates which had been exposed to air. We have thus repeated the measurements on Pb substrates which have been prepared in situ for both nitrogen and rare gas monolayers. The latter have been predicted by some theories to exhibit no dependence on the superconducting state of the sample. We present these results and compare them to the various conflicting theories. ¹ A. Dayo, W. Alnasrallah and J. Krim, Phys. Rev. Lett. vol 80, 1690 (1998) Work funded by NSF.
4:20 PM		TR+MM-WeA-8 Nanotribology of Total Joint Replacement Prosthesis (TJR) S.P. Ho (Clemson University); R.W. Carpick (University of Wisconsin - Madison); T. Boland, P.F. Joseph, M. LaBerge (Clemson University) Atomic force microscope (AFM) was used to represent nanotribological asperity-to-asperity nanocontact between a cast CoCr alloy and an unsterilized surgical-grade direct compression molded ultra high molecular weight polyethylene (UHMWPE) of TJR prosthesis. UHMWPE is a semicrystalline material consisting of crystalline domains within an amorphous matrix. The primary dependence of friction upon normal load during an abrasive nanocontact may involve gross plastic deformation leading to plowing of UHMWPE surface. We have studied the nanotribological properties and evolution of plastic deformation of UHMWPE, to determine the precursors to wear particle formation in TJR prosthesis. It was hypothesized that variability in friction due to the individual nano-constituents in the semicrystalline polymer could cause a stress concentration leading to the generation of nanoscale wear debris particle; the originator for osteolysis in a total joint arthroplasty. Results from AFM analyses will be presented in this work to elucidate the average coefficient of friction and wear mechanisms and to reveal friction coefficients of the individual nano-constituents in the UHMWPE. Results showed that loading the nanocontact within the elastic regime resulted in an average coefficient of friction of 0.25+/-0.04. Higher normal load ranges revealed a sudden increase in lateral force indicating ploughing of the nanocontact, eventually leading to generation of nanoscale wear debris. The sudden increase in lateral force indicated that the UHMWPE plastically deforms causing an increase in contact area leading to an increase in lateral forces. We will present data supporting the individual contribution of crystalline and amorphous regions to the coefficient of friction for the purpose of understanding the observed plastic deformation at the modeled nanocontact of total joint replacement prosthesis (TJR).
4:40 PM		TR+MM-WeA-9 Surface Force Dynamics and Nanotribology of Self-healing Monolayers Applied to MEMS Lubrication: A Study Using Dip Pen Nanolithography J.J. Nainaparampil (Systran Federal/MLBT); K.C. Eapen (UDRI/MLBT); J.S. Zabinski (AFRL/MLBT) Self-assembled monolayers (SAMS) have good potential for lubrication of microelectromechanical systems (MEMS). However, monolayers tend to wear away relatively quickly, which limits their usefulness. Mechanisms for monolayer replenishment have the potential to expand the role of SAMS in MEMS lubrication schemes. The dynamics of adhesion and friction forces of self-healing monolayer coatings is studied using techniques derived from dip pen nanolithography (DPN). Typically, DPN makes use of an Atomic Force Microscope (AFM) to write patterns with 'ink' that is deposited on the cantilever tip or delivered through a nanometric aperture.^1,2 An AFM is used here to write monolayer patterns of various thiols, nonane dione and aminoalkylsilane on gold, copper, aluminum and silicon surfaces. Nanotribological measurements of adhesion and lubricity are measured using a combination of topography and lateral force scans. Special patterns that constrain molecular surface diffusion were used to permit studies of wear and replenishment dynamics. Measurements made on thiol and nonane dione monolayers on gold surfaces showed that stronger adhesion occurs for thiol coatings compared to nonane dione. The effects of relative humidity, temperature and aging on surface forces and monolayer dynamics will be presented. ¹Richard D. Piner, Jin Zhu, Feng Xu, Seunghun Hong and Chad A. Mirkin, Science 283, (1999) 661. ²Mun-Heon Hong, Ki Hyun Kim, Joono Bae and Wonho Jhe, Appl. Phys. Lett. 77(16) (2000) 2604.
5:00 PM		TR+MM-WeA-10 Observation and Simulation of Dislocation Emission after Nanoindentation of an FCC (100) Surface O. Rodríguez de la Fuente (Universidad Complutense de Madrid, Spain); J.A. Zimmerman, J. de la Figuera (Sandia National Laboratories); M.A. González (Universidad Complutense de Madrid, Spain); J.C. Hamilton (Sandia National Laboratories); J.M. Rojo (Universidad Complutense de Madrid, Spain) The traditional use of indentation of materials to characterize mechanical hardness has benefited both from recent experimental and theoretical advances. The routine use of Scanning Probe Microscopes allows the detailed examination of the surface of materials after nanoindentation, using the same instrument for both deformation and analysis. Techniques used to model the defects generated during indentation have also advanced by the development of fast, parallel computations of million-atom systems governed by semi-empirical potential energy functions. Together, these advances are starting to bridge the gap between theory and experiment. We present a combined study of the emission of dislocation loops by indentation on the surface of Au(100) using Scanning Tunneling Microscope (STM) experiments and atomistic simulations employing the Embedded Atom Method (EAM). Our experiments show dislocation loops emitted in <110> directions that extend out to distances hundreds of nanometers away from the indentation region. These loops consist of dissociated edge loops that intersect the surface. The locations of these dissociated loops are identified by the sub-angstrom height hillocks observed on the crystal surface. Atomistic simulation of nanoindentation reveals that these dislocation loops are generated close to the indentation region and glide away along <110> directions. The sub-surface structure of the dislocation loops verifies the stacking faults beneath the hillocks, which intersect at a stair-rod dislocation. Our simulation permits an estimate of the Peierls barrier for loop glide, revealing why the dislocations can glide so far from the indentation region. We have also observed the same dislocation loops by annealing an ion-irradiated Au(100) surface. The atomistic simulations help to understand how the dislocation loops can withstand the annealing.