Self-assembled monolayers for nanotribology and surface protection

Abstract

In many applications, hydrophobic and lubricating films are needed to minimize adhesion, stiction, friction, and wear. In various applications, surfaces need to be protected from exposure to the operating environment, and hydrophobic films are of interest. Self-assembled monolayers are molecularly thick, well organized, and chemically bonded to the substrate. Some can be insensitive to the environment and can provide lower adhesion, friction, and wear. Ordered molecular assemblies with high hydrophobicity and desirable nanotribological properties can be engineered using chemical grafting of various polymer molecules with suitable functional head groups, spacer chains, and nonpolar surface terminal groups. In this chapter, we present an overview of self-assembled monolayers (SAMself-assembled monolayer (SAM)s) with hydrophobicity and desirable nanotribological properties. SAMs are produced by various organic precursors. We first present a primer to organic chemistry, followed by an overview of selected SAMs with various substrates, spacer chains, and terminal groups in the molecular chains, and an overview of nanotribological properties of SAMs. The contact angle, adhesion, friction, and wear properties of SAMs having various spacer chains with different surface terminal and head groups (hexadecane thiol, biphenyl thiol, perfluoroalkylsilane, alkylsilane, perfluoroalkylphosphonate, and alkylphosphonate) and on various substrates (Au, Si, and Al) are surveyed. Chemical degradation mechanisms and environmental effects are presented. Based on the contact angle and nanotribological properties of various SAM films discovered using an atomic force microscope (AFMatomic force microscopy (AFM)), it is found that perfluoroalkylsilane and perfluorophosphonate SAMs exhibit attractive hydrophobic and nanotribological properties.