Friction understanding molecular-scale

Understanding molecular-scale adhesion, friction, lubrication, and wear is crucial to modern technologies, such as microelectromechanical systems (MEMS) and hard disk drives.With atomic force/friction force microscopy (AFM/FFM),several studies have shown the correlation of frictional properties of self-assembled monolayers (SAMs) of alkanethiols with their chain lengths and terminal groups. The long chain monolayers (more than 12 carbon units) have lower friction coefficients compared with their short chain counterparts. Recently, Kim et al investigated the effect of the size of chain termination on frictional properties and found that the difference in friction arises predominantly from the difference in the size of the terminal groups. The AFM/FFM has also been applied to study frictional... 

In spite of the strong economic importance of friction and wear and the resulting scientific effort, our understanding of the fundamental processes is still rudimentary. This results from the complexity of these topics. In addition, this complexity demands a multi-disciplinary approach to tribology. In recent years the development of new experimental methods such as the surface forces apparatus, the atomic force microscope, and the quartz microbalance made it possible to study friction and lubrication at the molecular scale. However, this new wealth of information does not alter the fact, that there are no fundamental equations to describe wear or calculate friction coefficients. Engineers still have to rely largely on their empirical knowledge and their extensive experience. 

At the molecular scale, the surface force apparatus is being used to study lubricant behaviour in very close approach of surfaces. Atomic force microscopy can study local frictional variation due to additive deposition on surfaces in addition to giving detailed topographical maps. Nano-indenters can probe the elastic and plastic properties of surface films. These techniques are enhancing the understanding of boundary lubrication and supplement results from the multitude of more widely known boundary friction and wear testers, such as 4-ball, balTon-plate and ring-on-block. 

While the macroscopic concepts of hardness, adhesion, friction, and slide have evolved over the last two centuries, atomic level understanding of the mechanical properties of surfaces eluded researchers. The discovery of the atomic force microscope in recent years promises to change this state of affairs. Being able to measure forces as small as 10 newton or as large as 10 newton [5] over a very small surface area (few atoms) and by simultaneously providing atomic spatial resolution, this technique permits the study of deformation (elastic and plastic), hardness, and friction on the atomic scale. The buried interface between moving solid surfaces can be studied with spectroscopic techniques on the molecular level. Study of the mechanical properties of interfaces is, again, a frontier research area of surface chemistry. 

Nanotribology. Nanotribology is the study of how friction, lubrication, and wear occur at the nanometer and sub-nanometer scale, i.e., at the atomic and molecular level. Since the lubricant films currently in disk drives are only about 1 nm in thickness and, within a few years, the carbon overcoats will only be a few nanometers in thickness, improved understanding of how these materials work at the atomic and molecular level to provide tribological protection is critically needed to develop dramatically more effective materials. 

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