ICMCTF2009 Session E5: Nano- and Microtribology Instrumentation and Characterization

At most solid-solid interfaces of technological relevance, contact occurs at numerous asperities. A sharp atomic/friction force microscope (AFM/FFM) tip sliding on a surface simulates just one such contact. However, asperities come in all shapes and sizes which can be simulated using tips of different shapes and sizes. AFM/FFM techniques are commonly used for tribological studies of engineering surfaces at scales ranging from atomic- to microscales. Studies include surface characterization, adhesion, friction, scratching/wear, boundary lubrication, and surface potential and capacitance mapping^1-5. AFMs and their modifications are also used for nanomechanical characterization, which includes measurement and analysis of hardness, elastic modulus and viscoelastic properties, and in-situ localized deformation studies. State-of-the-art contact mechanics models have been developed and are used to analyze dry and wet contacting interfaces. Experimental data exhibit scale effects in adhesion, friction, wear, and mechanical properties, and a comprehensive model for scale effects due to adhesion/deformation and meniscus effects has been developed. Generally, coefficients of friction and wear rates on micro- and nanoscales are smaller, whereas hardness is greater. Therefore, micro/nanotribological studies may help define the regimes for ultra-low friction and near-zero wear. New lubrication strategies such as the use of self-assembled monolayers promise to be very versatile and effective at these scales. To improve adhesion between biomolecules and silicon based surfaces, chemical conjugation as well as surface patterning have been used⁶. Friction and wear studies of biomolecules show that these act as a lubricant but wear away in AFM experiments, even when attached to the underlying substrate⁷. In the area of biomimetics, surface roughness present on lotus and other leaves has been measured and the surface films are characterized to understand the mechanisms responsible for superhydrophobicity (high contact angle). A model for surface-roughness-dependent contact angle has been developed and optimum distributions have been developed for superhydrophobic surfaces^8-10. These fundamental nanotribological studies provide insight to molecular origins of interfacial phenomena including adhesion, friction, wear and lubrication. Friction and wear of lightly loaded micro/nano components are highly dependent on the surface interactions (few atomic layers). Nanotribological and nanomechanics studies are also valuable in the fundamental understanding of interfacial phenomena in macrostructures to provide a bridge between science and engineering. This talk will present an overview of nanotribological and nanomechanics studies and their applications.

¹Bhushan, B., Israelachvili, J.N., and Landman, U., “Nanotribology: Friction, Wear and Lubrication at the Atomic Scale,” Nature, Vol. 374, 1995, pp. 607-616.

²Bhushan, B., Handbook of Micro/Nanotribology, second ed., CRC Press, Boca Raton, Florida, 1999.

³Bhushan, B. et al.., Applied Scanning Probe Methods, Vol. 1-10, Springer-Verlag, Heidelberg, Germany, 2004, ‘06, ‘07, ‘08.

⁴Bhushan, B., Nanotribology and Nanomechanics – An Introduction, Springer-Verlag, Heidelberg, Germany, 2005.

⁵Bhushan, B., Springer Handbook of Nanotechnology, second ed., Springer-Verlag, Heidelberg, Germany, 2007.

⁶Bhushan, B., Tokachichu, D. R., Keener, M. T., and Lee, S. C., “Morphology and Adhesion of Biomolecules on Silicon Based Surfaces,” Acta Biomat. 1, 327-341 (2005).

⁷Bhushan, B., Tokachichu, D. R., Keener, M. T., and Lee, S. C., “Nanoscale Adhesion, Friction and Wear Studies of Biomolecules on Silicon Based Surfaces,” Acta Biomaterialia 2, 39-49 (2006).

⁸Burton Z. and Bhushan, B., “Hydrophobicity, Adhesion, and Friction Properties of Nanopatterned Polymers and Scale Dependence for Micro- and Nanoelectromechanical Systems,” Nano Letters 5, 1607-1613 (2005).

⁹Bhushan, B., Nosonovsky, M., and Jung, Y. C., “Towards Optimization of Patterned Superhydrophobic Surfaces,” J. Roy. Soc. Interf. 4, 643-648 (2007).

¹⁰Nosonovsky, M. and Bhushan, B., “Multiscale Friction Mechanisms and Hierarchical Surfaces in Nano- and Bio-Tribology,” Mat. Sci. Eng.R 58, 162-193 (2007).

A variety of fabrication methods are available to build Microsystems: bulk micromachining, surface micromachining (SMM), LIGA (German acronym for Lithography, Galvanoformung und Abformung), and meso-scale machining. Microsystems fabricated by LIGA and meso-scale machining are more amenable to applying thin films and coatings to mitigate friction and wear. The major focus of this presentation will be the on tribological issues associated with structures created in the LIGA process, specifically electroformed metal structures with dimensions of 0.01 to several mm, and thickness up to 500 µm. Diamond-like nanocomposite (DLN) coatings processed from siloxane precursors by plasma enhanced chemical vapor deposition are well known for their low friction and wear behavior.

In this study, we have examined the feasibility of applying these coatings on the sidewalls of LIGA-fabricated microsystems parts. Novel tribological tests were designed to evaluate the frictio n behavior of coated parts in planar-sidewall and sidewall-sidewall contacts, in regimes relevant to MEMS. Friction coefficients of DLN coated LIGA parts ranged from an extremely low value of 0.02 in dry nitrogen to 0.2 in air with 50% RH, with minimal amounts of wear in both environments.

Like many solid lubricating materials, DLN coatings exhibited non-Amontonian friction behavior; with coefficient of friction (COF) decreasing with an increase in Hertzian contact stress. The main mechanism responsible for low friction and wear is governed by the interfacial sliding between the DLN coating and the friction-induced transfer film on the counterface. This interfacial shear strength, computed from COF-inverse Hertzian contact stress plots, was found to be 9 Mpa in dry nitrogen and 78 Mpa in humid air. Time-of-Flight Secondary Ion Mass Spectroscopy (ToF-SIMS) analysis of the interfaces (wear tracks and transfer films) was used to gain a fundamental understanding of tr ibochemical phenomena. The role of finite element analysis and novel characterization techniques (e.g., FIB, EBSD, Micro-impact testing) to evaluate the surface coatings for microsystems technology will be discussed.

¹Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under Contract DE-AC04-94AL85000.

Surface roughness controlled ultra thin nano crystalline diamond film was fabricated on Si(100) surface¹. To simulate MEMS and NEMS patterned surface, 800 nanometer and 1 micrometer wide lines with 200 nanometer wide space pattern were prepared on Si(100) substrate by E-Beam lithography and RIE process. ESAND (Electrostatic Self-assembly of Nano Diamond) seeding process was employed to initiate nano crystalline diamond growth. For seeding procedure, negatively charged substrate surface was covered with cationic polymer (PMMA) monolayer, and anionic polymer (PSS) coated cationic nano diamond particles were spontaneously attached on the substrate. Through this process, undulated nanocrystalline diamond pattern was successfully formed by conventional hot filament CVD system. The roughness of deposited surface was controlled by regulating the size of seeding nano diamond particles. Crushing nanodiamond aggregates and dispersion of nano diamond solution was performed in attrition milling system. AFM probe was used for the wear test and surface profiling of nanocrystalline diamond coatings. 2D frictional coefficient mapping by LFM (Lateral Force Microscopy) scanning showed low friction coefficient (< 0.1) on line patterned diamond surface, and higher friction coefficient (> 0.3) on narrow area adjacent to undulated pattern edges. With prolonged LFM scanning, the high coefficient of friction was decreased down to below 0.1. The morphology of nanocrystalline diamond was analyzed with Raman spectroscopy.

¹J.H.Kim, S.K.Lee, O.M.Kwon, and D.S.Lim, “Ultra Thin CVD Diamond Film Deposition by Electrostatic Self-Assembly Seeding Process with Nano Diamond Particles”, Accepted for Publication in Journal of Nanoscience and Nanotechnology (2007).