ICMCTF2008 Session F3/E5: Nanotribology Instrumentation and Diagnostics

Tuesday, April 29, 2008 8:00 AM in Room Royal Palm 4-6

Tuesday Morning

Start	Invited?	Item
8:00 AM		F3/E5-1 Investigation of Creep Behaviour With a New Innovative Nanoindentation Tester N. Conte, R. Consiglio, N.X. Randall (CSM Instruments, Switzerland) Nanoindentation testing is particularly appropriate for creep and stress relaxation tests because it can measure materials whose properties are highly viscoelastic. However, the main drawback of nanoindentation tests is linked to the low thermal stability of most instruments. These instabilities introduce an uncontrollable penetration drift superimposed to the viscoelastic deformation of the sample. For some polymers thermal expansion of the instrument frame can be quite significant. The recent development of a new innovative instrument (the Ultra Nanoindentation Tester) has allowed such drawbacks to be avoided, and has allowed the precise investigate of the creep behaviour of samples using very long duration tests. These results were made possible thanks to a quasi elimination of the thermal drift by the use of specific dedicated materials with very low thermal expansion coefficients and a special design of the measurement head. Furthermore, the influence of the deformation of the frame has been eliminated owing to an active top referencing which continuously monitors the position of the surface of the sample through a reference applying a very small and controlled pressure. The indenter penetration depth is then measured relative to that reference. A series of long time quasi static tests on PMMA will be used to demonstrate the efficiency of this novel instrument design and its ability to almost totally eliminate the thermal drift. In this study, it is thus demonstrated that nanoindentation testing, when performed in good conditions with appropriate apparatus, constitutes a reliable tool to study the time dependent mechanical properties of materials.
8:20 AM		F3/E5-2 Bio-Derived Bimetallic Hybrid Nanostructures for Surface Modification in MEMS Switch Contacts S.T. Patton (University of Dayton Research Institute); J. Slocik, R. Naik, A.A. Voevodin (Air Force Research Laboratory) RF MEMS switches hold great promise in a myriad of commercial, aerospace, and military applications including cellular phones and phased array antennas. However, there has been little development of lubricants/surface modifiers to improve switch performance and reliability. Self-assembled monolayers of diphenyl disulfide were found to decompose in metallic MEMS switch contacts, which led to growth in contact resistance. In this study, bimetallic nanostructures are used to lubricate metallic MEMS switch contacts. For these studies, bimetallic nanoparticles Au/Pd with 11 nm (Au) and 3 nm (Pd) sizes were used. The role of nanoparticle addition was studied from the two perspectives: i) increasing electrical conductivity between lubricated gold contact interfaces without organic component degradation and contact melting; ii) preventing adhesive failure of the contact by introduction of nanosized asperities. Experiments were conducted in dry nitrogen at MEMS-scale forces using a micro/nanoadhesion apparatus as a switch simulator. For high current (1 mA) switching, the material prevented shorting and extended lifetime by over five orders of magnitude over that of self-assembled monolayers. At low current (0.01 mA), no degradation of contact resistance was observed through 106 cycles. Detailed physical and chemical analysis of fresh and worn contact surfaces was conducted and results are summarized along with bimetallic nanoparticle surface modifier degradation mechanisms.
8:40 AM		F3/E5-3 Lubrication Mechanism of Pulsed Laser Deposited MoS₂-Te Composite Films at Moderate High Temperatures J.J. Hu, J.E. Bultman, J.S. Zabinski, A.A. Voevodin (Air Force Research Laboratory) MoS₂-Te composite films were prepared by pulsed laser deposition on steel substrates. The tribological properties of the films were measured in dry and humid conditions using a ball-on-disc tribometer, which showed a low friction coefficient and much longer wear lifetime than pure MoS2 films in humid air. A high-temperature tribometer was also used to measure friction coefficients at moderate high temperatures in air. The films showed a low friction coefficient at a temperature of up to 450°C. The wear scar surfaces and cross-sectional microstructures were studied using a micro Raman spectroscope, scanning electron microscope, transmission electron microscope and focused ion beam microscope, which provided the information on chemical compositions and microstructure features on the surface and in the depth of the films. Lubricious MoS₂-Te tribofilms were built up on wear scar surfaces, and produced the low friction. In addition, high-resolution electron microscope and X-ray energy dispersive spectrometer measurements showed that the MoS₂–Te composite films consisted of core-like Mo-rich nanoparticles encapsulated with shell-like Te-rich materials. The layer structures of hexagonal MoTe₂ were formed on the nanoparticle surface that was linked to the diffusion-controlled migration and crystallization in laser deposition. The Te-rich shell can play an important role to protect the lubricant films from thermal oxidation so as to improve the film properties at high temperatures.
9:00 AM		F3/E5-4 The Unusually Low Hardness and Modulus of Detonation and Cold Sprayed Coatings: Underlying Mechanisms G. Sundarajan (International Advanced Research Centre for Powder Metallurgy and New Materials, India) Thermal sprayed thick coatings obtained using detonation or cold spray technique are characterized by their splat microstructure. These thick coatings usually have low porosity in the range 0.1 to 0.5%. However, these coatings exhibit unusually low elastic modulus, hardness and electrical conductivity values as compared to the bulk material of identical composition. Such a behaviour cannot be explained only on the basis of the low porosity levels obtained in these coatings and is very likely linked to the splat microstructure characterized by numerous weak inter-splat boundaries. To understand how the intersplat boundaries are responsible for low hardness, modulus and electrical conductivity of the coating, two model coating materials, namely copper and 316 stainless steel, were chosen. Cu and SS coatings were obtained using both detonation spray and cold spray techniques over a range of process parameters so that the interrelationship between splat morphology (extent of flattening), inter-splat boundary area and porosity with the coating properties like hardness, modulus and electrical conductivity could be established. In addition, the bonding between the intersplat boundaries of the coating were improved by heat treatment. Porosity of the coating alone was reduced by CIP and the intersplat bonding with the coating was increased with a simultaneous decrease in porosity using HIP. After the above treatments, the resulting effect on microstructure, hardness, modulus and electrical conductivity of the coatings were evaluated. The results from the above experimental study will be presented and a qualitative model for the influence of splat and porosity related features on the hardness, modulus and electrical conductivity will be presented.
9:20 AM	Invited	F3/E5-5 Fundamental Aspects of Nanotribology: How is Energy Dissipated in Friction? M. Salmeron (Lawrence Berkeley National Laboratory) I will review some of the fundamental mechanisms of energy dissipation in friction. The review summarizes and analyzes many research results obtained over the last decade in my group using modern techniques, particularly Atomic Force Microscopy, Pin-on-disc Tribometry, Surface Forces Apparatus and several imaging and spectroscopic techniques that can be used for in situ characterization. I will focus on the effect of lubricants and their response to load and shear. I will also describe our latest findings on the effect of surface structure in quasicrystals because these exotic materials can reveal fundamental aspects of the origin of friction and the connection between nanoscale and macroscale friction.