Tribology frictional energy dissipation

Study on mechanisms of ordered molecular films as a model lubricant is of particular importance in the field of micro and nano-tribology, for it would help to understand how molecules contribute to the creation of friction and wear. The understanding has been much improved in recent years through MD simulations, performed by investigators around the world, to detect interactions between the molecular films in relative motion, and to reveal the process and specific mode of energy dissipation. 

Unlike traditional textbooks of tribology, in this book we regard boundary lubrication as a limit state of hydrodynamic lubrication when film thickness is down to molecular dimension and independent of the velocity of relative motion. The discussions are based on the existing results, some from literatures but mostly from the authors own work. The topics are mainly focused on the mechanical properties of boundary films, including rheology transitions, molecular ordering, and shear responses. Ordered molecule films, such as L-B films and SAM, are discussed, with emphasis on the frictional performance, energy dissipation and the effects from structural features. Boundary films can be modeled either as a confined substance, or an adsorbed/reacted layer on the... 

The tribologic solicitation occurs to the contact surfaces by the overlapping of three components, namely - mechanical, thermal, and chemical one - being accompanied by the loosing of friction energy and by concomitant conversion of the kinetic energy (dissipation) in heat, especially in the contact points. 

Polymers generally exhibit complex tribological behaviors due to different energy dissipation mechanisms, notably those induced by internal friction (chain movement), which is dependent on both time and temperature. Polymer friction is then governed by interfacial interactions and viscoelastic dissipation mechanisms that are operative in the interfacial region and also in the bulk, especially in the case of soft materials. Friction of a polymer can be closely linked to its molecular structure. The role of chain mobility has been studied in the case of elastomers, based on dissipation phenomena during adhesion and friction processes of the elastomer in contact with a silicon wafer covered by a grafted layer [1-5]. 

Two approaches have been used to investigate frictional forces in confined systems the tribological, where a constant drive velocity is applied(6), and the rheological, which applies an oscillatory external drive X(t)=Ax sin oi (7,8), When the oscillatory drive is applied, the response of the system is analyzed in terms of complex moduli (P). The rheological analysis of the moduli enables to distinguish between the elastic and viscous components of the response. The viscous part is directly related to the energy dissipation in the system. A few interesting questions arise ... 

Friction between rubbing surfaces dissipates energy in the form of heat and causes material loss by wear. In Table 10.1 we distinguish three types of tribological contacts between solid surfaces sliding, rolling and fi etting. 

The study of aqueous solutions of surface-active agents is interesting from both research and application points of view [1-13]. This chapter attempts to correlate the tribological properties with molecular structure, surface activity, and micelle and LLC formation ability both in the surface phase and in the bulk phase. The physicochemical properties are determined under static conditions, and relating them to dynamic conditions is difficult. Pressures, relative movement, and dissipation of energy in the form of heat aU occur under friction conditions. 

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