Self-Assembled Monolayers (SAMs) for Controlling Adhesion, Friction, and Wear
Making micro- and nanodevices as well as magnetic storage devices reliable necessitates the use of protective hydrophobic lubricating films that can minimize the adhesion, friction, and wear of sliding surfaces. Because of the small clearances associated
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Summary. Making micro- and nanodevices as well as magnetic storage devices reliable necessitates the use of protective hydrophobic lubricating films that can minimize the adhesion, friction, and wear of sliding surfaces. Because of the small clearances associated with these devices, such films need to be very thin (on the order of a few molecules thick). Chemicallybonded low surface tension liquid films are suitable for this purpose, as are a select number of hydrophobic solid films. Highly hydrophobic ordered molecular assemblies can also be used; these are engineered by chemically grafting various polymer molecules with suitable functional head groups, spacer chains and nonpolar surface terminal groups to the surface involved. In this chapter, we focus on the use of self-assembled monolayers (SAMs) for high hydrophobicity and/or low adhesion, friction and wear applications. SAMs are produced by various organic precursors, so the chapter starts with a primer for the organic chemistry associated with this field. This is followed by an overview of selected SAMs with various spacer chains and terminal groups in their molecular chains on a variety of substrates, and a summary of the tribological properties of SAMs. The adhesion, friction and wear properties of SAMs with various spacer chains and surface terminal and head groups (hexadecane thiol, biphenyl thiol, alkylsilane, perfluoroalkylsilane and alkylphosphonate) on various substrates (Au, Si, and Al) are then surveyed. Degradation mechanisms and environmental effects are studied. Nanotribological studies of various SAM films by atomic force microscopy (AFM), show that perfluoroalkylsilane SAMs in particular exhibit attractive hydrophobic and tribological properties.
17.1 Introduction Maximizing reliability is an important issue in the field of micro/nanodevices, commonly referred to as micro/nanoelectromechanical systems (MEMS/NEMS) and BioMEMS/BioNEMS, as well as for magnetic storage devices (which include magnetic rigid disk drives, flexible disk drives and tape drives). It often necessitates the application of molecular films to sliding surfaces in order to lubricate and protect them [1–8]. A solid or liquid film is generally required in order to obtain acceptable tribological properties for sliding interfaces. However, the presence of a small
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Bharat Bhushan
quantity of liquid between smooth surfaces can substantially increase the adhesion, friction and wear due to the formation of menisci or adhesive bridges [9, 10]. This becomes a major concern in micro/nanodevices operating at ultralow loads, as the magnitude of the liquid-mediated adhesive force may be similar to that of the external load. The liquid film may be pre-existing and/or it can be a capillary condensate of water vapor from the environment. If the liquid wets the surface (0 ≤ θ < 90◦ , where θ is the contact angle between the liquid–vapor interface and the liquid–solid interface for a liquid droplet sitting on a solid surface, Fig. 17.1a), the surface of the liquid is constrained to lie p
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