Papers

ICMCTF2006 Session F3-1: Nanotribology Instrumentation and Diagnostics

Monday, May 1, 2006 10:30 AM in Room Royal Palm 1-3

Monday Morning

Start	Invited?	Item
10:30 AM		F3-1-1 Developing a Nanoscale Understanding of Friction and Wear C.M. Mate (Hitachi San Jose Research Center) Thin film coatings are commonly used to improve the tribology performance, for example, to increase surface hardness and toughness and to reduce friction and wear. Tremendous progress has been made during the past few decades in determining how friction and wear occur at the atomic and molecular level, leading to greatly enhanced tribological performance of thin films. For example, improved understanding of tribology issues in the disk drive industry has lead to the reduction of the disk overcoat thicknesses from tens of nanometers to just a few nanometers over the last fifteen years, enabling several orders of magnitude increase in areal density of data stored on the disk with no sacrifice of tribological reliability. Much of this improved tribological understanding has been driven by the development of new instrumentation for determining material properties at the nanometer scale. In this talk, I will discuss some of the instrumentation developed in our lab for probing the nanoscale tribology associated with disk drive overcoats and lubricants. These include Atomic Force Microscope (AFM) techniques for determining surface roughness and surface forces, ellipsometer techniques for determining overcoat thickness and lubricant distribution, and new methods for precisely determining nanoscale friction forces and wear rates at high sliding speeds (>1m/s).
11:10 AM		F3-1-3 Microtribology of DLC Coatings R.R. Chromik, K.J. Wahl (US Naval Research Laboratory) We have studied the microtribology of diamond-like carbon (DLC) nanocomposite coatings using a commercially available nanoindentation system. Reciprocating sliding tests were conducted at 4 µm/s over track lengths of 8 µm with diamond and sapphire counterfaces with nominal diameters of 20 and 300 microns, respectively. The friction behavior as a function of the contact stress (between 0.2 and 2.0 GPa) was explored in the elastic regime. Measured interfacial shear stress for these sliding contacts were 6-7 MPa for sapphire and 21-26 MPa for diamond against DLC coatings. For the same coating, microscale friction coefficients were higher than for macroscale sliding at comparable contact stresses. Friction was highest for the sapphire counterface, which exhibited multi-asperity contact and no evidence of transfer film formation via atomic force microscopy and micro-Raman spectroscopy. In contrast, the diamond counterface showed the lowest friction with shear strengths similar to macroscopic contacts against the same DLC coating. Micro-Raman spectroscopy showed increased G-band signal (sp²-bonded carbon) from the tip after sliding. These results suggest that what we know from macroscopic friction studies, that the presence or absence of transfer films controls friction performance, may also be true for microscale contacts.
11:30 AM		F3-1-4 Investigation of Film Failure and Wear Behavior of Aluminum Coated Glass by Cyclic Wear and Friction Tests in the Micro Scale V. Linss, T. Chudoba (ASMEC GmbH, Germany) In order to optimize coatings for mechanical protection they have to be characterized under conditions that are similar to the conditions in a real application. However, conventional wear measurements are often not sensitive enough to detect the first failure and to understand the local conditions which were responsible for the loss of the protecting effect of the film. A Universal Nanomechanical Tester (UNAT) was used to investigate wear and friction properties in the micro scale by a combination of highly resolved normal and lateral force-displacement measurements. In one example a constant normal load was applied to a spherical diamond tip and then the sample was repeatedly moved in lateral direction with amplitudes of some µm up to 150µm. The normal displacement was measured simultaneously with nanometer resolution. In other experiments the application of normal and lateral loads and displacements was combined in different manner. Every single lateral load-displacement cycle was analyzed afterwards. In this way several coatings were investigated in dependence on normal load, lateral speed, lateral displacement and cycle number. It will be shown how the failure of the coating can be noted in the normal and lateral force-displacement cycles and how several critical material parameters can be identified.
11:50 AM		F3-1-5 Lubrication of DC RF MEMS Switches Using Self-Assembled Monolayers S.T. Patton, K.C. Eapen (University of Dayton Research Institute); J.S. Zabinski, J.H. Sanders (Air Force Research Laboratory) MEMS switches hold great promise in a myriad of commercial, aerospace, and military applications including cellular phones and phased array antennas. However, there is limited understanding of the factors that determine the performance and reliability of these devices. In previous studies using hot-switched DC (gold versus gold) MEMS radio frequency (RF) switch contacts, it was found that adhesion growth with cycling was the failure mechanism at low current (1-10 µA), and switch shorting was the failure mechanism at high current (1-10 mA). In this study, an attempt is made to lubricate the switch contacts using a self-assembled monolayer (SAM) of diphenyl disulfide. This material was used due to its thermal stability, good conductivity due to delocalization of its electrons, and its ability to form a SAM on the gold electrode surface. Fundamental studies of lubricated switch contacts were conducted in air at MEMS-scale forces using a micro/nanoadhesion apparatus as a switch simulator. This presentation will focus on contact physics, chemistry, and failure mechanisms. During switch cycling experiments at low current, lubricated contacts failed by growth in adhesion and contact resistance (R). Contact resistance degradation was linked to the formation and growth of a non-conducting film on the switch electrodes that prevents metallic contact. Under some conditions, the film can trap parasitic charge leading to other failure mechanisms such as self-actuation, failure to actuate, and self-release. During cycling at high current, switch contacts failed immediately by growth in R. This was also linked to the formation of a non-conducting deposit on switch electrodes. More heat generation at points of contact at high current is thought to accelerate failure. Detailed physical and chemical analysis of virgin and worn contacts was conducted and will be summarized in this paper. Lubricant degradation mechanisms will also be put forward.
12:10 PM		F3-1-6 Ionic Liquid Lubricity Under Low Load; An AFM Based Study J.J. Nainaparampil (UES Inc.); K.C. Eapen (University of Dayton Research Institute); A.A. Voevodin, J.S. Zabinski, J.H. Sanders (Air Force Research Laboratory) Ionic liquids (IL's) have received attention due to their myriad of potential uses as low vapor pressure solvents, catalysts and conducting liquids. Quite recently, certain ionic liquids were found to be lubricious when applied in appropriate combinations (1,2). Phase changes that lead to micro structural entropy variations can lead to lubrication under low load conditions (3). In this report, the potential of ionic liquid lubricants for low load conditions is probed at both the micro-nanoscopic, and macroscopic level. Solutions in acetonitrile or other suitable ketones of 0.2% to 15% of 1- ethyl-3-methylimidazolium ethyl sulfate and 1- ethyl-3-methyl-pyridinium ethyl sulfate, which are low melting (~ -60 º C) and stable under humidity, are characterized under varied loads in interfaces ranging from nano to macro scale in cross section. Certain load conditions have produced nanostructures possessing low friction when observed under contact mode AFM. Results from in-situ observations under a liquid cell will be presented along with characterization of nano structures from Dip Pen Nanolithography of these fluids. An atomic force microscope operated in non-contact mode was capable of phase imaging and provided data on the energetics of contact interactions. Development of a basic model for the formation of a lubricious phase generated by rubbing is also underway. (1)J. J. Nainaparampil, K. C. Eapen B. S. Phillips and J. S. Zabinski, Nanotechnology 16 (2005) 2474-2481. (2)oIonic Liquid Thin Films: Potential Solution To Lubricate Miniaturized Devices, Qunji Xue Bo Yu Feng Zhou Ping Gao Yongmin Liang Weimin Liu, Proceedings of WTC2005World Tribology Congress III September 12- 16, 2005, Washington, D.C., USA. (3) Kinetics and Energetics in Nanolubrication, Ren© M. Overney, George Tyndall and Jane Frommer, Chapter 29 in Nanotechnology Handbook, B. Bhushan (ed.), Springer-Verlag, Heidelberg, Germany, 2004 .