AVS1996 Session SS+TF+NS-MoM: Tribological Properties of Coatings and Lubricants

Monday, October 14, 1996 8:20 AM in Room 203B

Monday Morning

Start	Invited?	Item
8:20 AM		SS+TF+NS-MoM-1 Real-Time Photoelectron Emission Microscopy of Lubrication on a Real Surface E. Montei, M. Kordesch (Ohio University) Thin films of fluorinated lubricants have been imaged with photoelectron emission microscopy on a rotating cylindrical polycrystalline molybdenum sample. In situ stylus contact is imaged in situ and in real-time. Areas of lubrication are clearly resolved in the image, as are wear tracks formed by contact with the lubricated and clean areas of the surface. Changes in the lubricant coverage in the form of "dragging" the lubricant into formerly clean areas of the surface provide direct observation of lubrication on the 100 micron scale. Initial observations indicate that in areas of sufficient lubricant coverage, no wear tracks are produced (as indicated by the absence of material transfer in the lubricated regions). Tentatively, contrast in the photoelectron emission microscope images are attributed to electron attenuation by the thicker lubricant layers. Wear studies used Fomblin Y diluted in Freon applied to a Mo surface and rubbed with a pure aluminum stylus. Fluorinated compounds that serve as models for the lubricant degradation process were also studied on polycrystalline Mo using vibrational spectroscopy. A major component of the adsorbed Fomblin Y oil vibrational spectrum is a mode associated with CF\sub 3\ at 156 meV. Comparison with smaller fluorocarbon compounds under tribological stress will be discussed. Future work aimed at understanding the reaction of lubricants under mechanically stressful stimuli may provide insight into the nature of tribochemical degradation processes which evolve over time. Acknowledgement is made to the donors of The Petroleum Research Fund, administered by the ACS, for support of this research
8:40 AM		SS+TF+NS-MoM-2 Fluid Structure, Dynamics, and Solvation Forces in AFM D. Patrick (Cambridge University, United Kingdom); R. Lynden-Bell (The Queen's University of Belfast, United Kingdom) When atomic force microscopy (AFM) is performed under a liquid, fluid molecules in the region of the tip-sample junction act as a perturbation, resulting in a tip-sample interaction different from that occurring under vacuum conditions. Understanding how tip-sample interactions are affected by a liquid, as well as the structure and dynamics of liquid molecules in the neighborhood of the tip is important because the majority of AFM experiments are conducted on samples possessing an adsorbed liquid film. To investigate these issues, we have performed molecular dynamics simulations on a system consisting of an AFM tip positioned above an fcc (100) surface, immersed under a simple fluid. The atomistic nature of the model gives rise to a number of interesting phenomena associated with the discreet size of the tip, surface, and fluid particles. Key questions addressed in this study include (i) What affect does a fluid have on an AFM image? (ii) To what extent can a fluid act as a "lubricant", spreading out the tip-sample interaction forces over a larger effective contact area? (iii) What are the structural and diffusional characteristics of the fluid near the tip-sample junction? and (iv) How do the answers to these questions depend upon the position and sharpness of the tip, the tip and sample lyophilicity, and the size of the fluid particles?
9:00 AM		SS+TF+NS-MoM-3 Adhesion Force Analysis of Interactions between AFM Tips and Substrates Modified with Organosilanes L. Wenzler, G. Moyes, T. Beebe, Jr. (University of Utah) Recent atomic force microscopy (AFM) studies have probed specific chemical interactions between the tip and the surface. Some have investigated interactions of hydrogen bonding and van der Waals bonding between functionalized self-assembled thiols, ligand receptor interactions, ligand-protein interactions, and interactions between strands of DNA. We have developed a statistical method which produces information about individual chemical bond force magnitudes. We will develop the method and present evidence for detection of individual bond force changes between hydrophobic and hydrophilic forces. We will also discuss the characterization of modified tips and surfaces by a variety of organosilanes with imaging x-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry. This technique has been expanded to study self-assembled films, specifically organosilane compounds (silane coupling agents) covalently attached to hydroxyl-bearing surfaces. Since both the tip and the surface can be modified by functionalized organosilanes, a variety of tip-surface interactions have been studied.
9:20 AM		SS+TF+NS-MoM-4 Tribological Atomic Force Microscopy Studies of Grafted Organo-Silanes Monolayers J. Servat (Universitat de Barcelona, Spain); M. Garcia-Parajo (University of Twente); C. Longo (Universidade de Sao Paulo, Brazil); P. Gorostiza, F. Sanz (Universitat de Barcelona, Spain) There is a continous going on interest on the understanding of the microscopic mechanical properties of thin organic layers, which are known to work exceedingly well as boundary lubricants. We have focused our attention on the study of Octadecyl-TrichloroSilane CH\sub n\3-(CH\subn\2)\sub n\17-SiCl\sub n\3,(OTS) thin layers, grafted on silicon oxide surfaces. This material has been used recently in the Microelectronic Industry to reduce friction and increase the lifetime of devices. We have tested the mechanical response of the OTS layers using the Atomic Force Microscopy (AFM) working in contact and tapping modes. To assess the adhesion hysteresis and the anelasticity of the films, we have performed loading and unloading experiments in contact mode. The effect of compression (slope less than 1) is observed while the load is being applied to the sample. Once the load is fully removed, the film remains compressed showing a clear deviation from perfect elasticity. In the tapping regime, the loading/unloading experiments show again the anelastic behavior of the films. Upon loading three different regions are encountered. These can be associated with an initial disorder of the headgroups of the chains, followed by lateral folding and a final hardening effect due to full compression of the folded chains. The tapping regime is also used to provide strong evidence of a cushioning effect: the OTS film absorbs the pressure imposed by the tip (up to 20GPa in our experiments) protecting the underlying silicon substrate from plastic deformation. Results of these will be presented and a preliminary model based on the molecular properties of OTS will be advanced.
9:40 AM		SS+TF+NS-MoM-5 Friction and Adhesion at an Incommensurate Solid-Solid Interface C. Daly, J. Krim (Northeastern University) We have employed a quartz crystal microbalance (QCM) technique to measure the friction required to slide one- and two-atom thick solid films of xenon along a Ag(111) surface, and observe that the friction associated with the bilayer is approximately 25\% greater than that associated with the monolayer. We further observe that both submonolayer coverages of incommensurate Xe islands and complete, compressed monolayers exhibit lower friction levels than a complete, uncompressed monolayer, despite the fact that all three are characterized by equal adhesive forces. The results are well explained however in terms of current atomic-scale theories of frictional energy dissipation. Work Supported by NSF DMR\#9204022.
10:00 AM		SS+TF+NS-MoM-6 External and Internal Vibrations of Large Hydrocarbons Adsorbed on Metal Substrates D. Fuhrmann, C. W\um o\ll (Max-Planck-Institut f\um u\r Str\um o\mungsforschung, Germany) We have investigated the low energy (\<\ 15 meV) vibrational excitation spectrum of several large hydrocarbons, n-octane (C\sub 8\H\sub 18\), n-nonane (C\sub 9\H\sub 20\), n-decane (C\sub 10\H\sub 22\), n-hexatriacontane (C\sub 36\H\sub 74\) and perfluorohexane (C\sub6 \F\sub 14\), adsorbed on different (Pb, Cu) metal surfaces by means of high-resolution inelastic scattering of thermal energy He-atoms. In all cases an external vibration, the frustrated translation normal to the surface, could be detected with energies between 6.6 meV and 7.2 meV. The corresponding line-widths were analyzed and allow for the conclusion that the damping of the molecular motion normal to the surface occurs via emission of phonons into the substrate. In contrast the excitation of electron-hole pairs has been proposed to dominate the damping of the motion parallel to the surface and thus the energy dissipation in sliding friction for alkane-lubricants [1]. In addition to these external vibrations we were also able to observe low energy internal vibrations. By carrying out a normal-mode analysis these modes, which have not yet been resolved with other spectroscopic methods, could be identified as out-of-plane bending modes. Experiments with deuterated hydrocarbons support this assignment. [1]B.N.J. Persson and A.I. Volokitin, J. Chem. Phys. 103, 8679 (1995)
10:20 AM		SS+TF+NS-MoM-7 Surface Reactivity and Frictional Properties of Titanium Carbide (100) S. Perry, P. Merrill (University of Houston) Titanium carbide has found widespread use as a coating material due to its extreme hardness and wear resistant characteristics. An understanding of the fundamental reactivity of titanium carbide with atmospheric species is of great importance in predicting the friction and wear behavior of this material in different environments as well as in designing suitable lubricant systems. We have prepared a single crystal TiC (100) in vacuum by means of sputter/anneal cycles and studied the resulting surface structure with low energy electron diffraction (LEED) and ultrahigh vacuum (UHV) scanning tunneling microscopy (STM). The reactivity of D\sub 2\O and O\sub 2\ with this surface has been studied by temperature programmed reaction spectroscopy (TPRS) and illustrates the complex reactivity of oxygen containing adsorbates with this compound surface. UHV atomic force microscopy (AFM) has also been used to probe the resulting changes in frictional properties of the native surface as a function of adsorbate exposure and reactivity. This has been accomplished by measuring the force of friction as a function of applied load between silicon nitride probe tips and the prepared TiC surfaces Correlations between chemical modifications of the titanium carbide surface and friction properties will be presented.
10:40 AM		SS+TF+NS-MoM-8 Use of Sputtering and Negative Carbon Ion Sources to Prepare Carbon Nitride Films I. Murzin, G. Tompa, E. Forsythe (Structured Materials Industries, Inc.); J. Wei, T. Fischer (Stevens Institute of Technology) There have been quite a few efforts to prepare carbon nitride material over the last few years. Carbon nitride may have many different applications as wear and corrosion resistant coatings, electrical insulators and optical coatings. In this work we compare the tribological properties of the carbon films enriched with nitrogen that were prepared by two separate ion-beam assisted techniques. The first approach used an ONYX-2 gun to sputter deposit carbon in a nitrogen atmosphere under simulteneous bombardment of the growing film with low energy (~50 to 200 eV) nitrogen ions. The second method utilizes a beam of negatively charged carbon ions of 1 to 5 mA/cm2 current density impinging the substrate at the same time with either radical or positive nitrogen ions produced by the ion gun. We were able to produce smooth continious coatings that are harder than silicon. These films possess wear rates lower than 2x10-7 mm3/Nm and friction coefficients in the range of 0.2 to 0.6. Further work is under progress to better determine the differences in the properties of the films formed with each method. This work is supported in part by the Ballistic Missile Defense Organization under the contract No. DAAH04-95-C-0042.
11:00 AM		SS+TF+NS-MoM-9 Reduction of Friction through Finite-Size Effects C. Mak, J. Krim (Northeastern University) We have employed a quartz crystal microbalance (QCM) technique to measure the friction levels associated with sliding of solid and liquid krypton films on crystalline and disordered gold substrates, and have observed reduced friction levels for certain disordered substrates. We also observe reduced friction levels in cases where small islands, rather than complete layers, of krypton are sliding. Both results are potentially explained by limited correlation sizes in the system being studied.Work Supported by NSF DMR\#9204022.
11:20 AM		SS+TF+NS-MoM-10 Tapping Mode/Phase Imaging of Composite Surfaces D. Chernoff, J. Lohr (Advanced Surface Microscopy, Inc.) Composite surfaces of industrial interest may be created either deliberately (microfabrication of thin film recording heads, polymer processing) or accidentally (contaminants and defects). Tapping Mode/Phase images can map the material domains with spatial resolution down to 10 nm. Such images can be a powerful aid in process control. Phase images show the mechanical phase of the tapping tip relative to the drive signal which oscillates the cantilever. The phase image supplements the ordinary height image and often provides unique contrast related to material differences in stiffness and adhesion. We present several examples for both inorganic and organic systems, including: - detection of photoresist residue on silicon - identification of contaminant/wear particles on a magnetic recording head - growth of corrosion films - domains on a copolymer surface - thin coatings of lignin on cellulose - mixed Langmuir-Blodgett films By scanning the same spot with different probes and operating modes, we showed that phase contrast correlated well with friction contrast in LFM. We discuss correlations with adhesion images. We compare phase images captured under ordinary low force conditions in air ("soft tapping") with those captured during "hard tapping" in air (as described by S. Magonov) and soft tapping in liquid.
11:40 AM		SS+TF+NS-MoM-11 UHV Friction Force Microscopy Studies of Ionic Materials R. Carpick (Lawrence Berkeley National Laboratory and University of California, Berkeley); Q. Dai (Storemedia, Inc.); D. Ogletree, M. Salmeron (Lawrence Berkeley National Laboratory) We present recent results from friction force microscopy studies of ionic single crystal surfaces cleaved and measured in ultrahigh vacuum, including KBr(001), KCl(001), KF(001) and KMnF3(001). We have investigated the load dependence of friction in the low load regime and compared the results with contact mechanical models. We discuss measurements of adhesion energies and shear strengths, the onset of wear, stick-slip behavior, and tip-induced modifications of the surface.