Tribology level

Molecular dynamics (MD) metliods can be used to simulate tribological phenomena at a molecular level. These have been used primarily to simulate behaviour observed in AFM and SFA measurements. Such simulations are limited to short-timescale events, but provide a weaitli of infonnation and insight into tribological phenomena at a level of detail tliat cannot be realized by any experimental metliod. One of tire most interesting contributions of molecular dynamics... 

A typical model system used in tribological simulations is shown in Figure 8. In this system, two walls are separated by a fluid and shear is applied by pulling the top wall with an external device, whereas the bottom wall is held fixed. In atomic-level simulations, the two walls correspond to atomically discrete surfaces and the fluid is composed of atoms or molecules, which represent lubricants or contaminants. 

In this section, we give a brief overview of theoretical methods used to perform tribological simulations. We restrict the discussion to methods that are based on an atomic-level description of the system. We begin by discussing generic models, such as the Prandtl-Tomlinson model. Below we explore the use of force fields in MD simulations. Then we discuss the use of quantum chemical methods in tribological simulations. Finally, we briefly discuss multiscale methods that incorporate multiple levels of theory into a single calculation. 

The last few years have seen an explosion in the number of atomic-level tribological simulations aimed not just at understanding fundamental aspects of friction, but also at determining the frictional properties of systems used in real-world applications. In this section, we will discuss selected studies in an effort to demonstrate how the principles discussed earlier in this chapter are used in practice. Unfortunately, it is not possible to consider all of the important, high-caliber research within the space of this chapter. Instead, we focus on a few key areas that encompass both fundamental and applied research in computational tribology. 

Somorjai, G. A., Introduction to Surf ace Chemistry and Catalysis, Wiley, New York, 1994. (Undergraduate level. This in-depth treatment of surface chemistry and catalysis brings the experience and perspectives of a pioneer in the field to the general audience. The book is meant to be an introductory-level description of modern developments in the area for students at the junior level. However, it is also an excellent source of the current literature and contains numerous, extensive tables of data on kinetic parameters, surface structure of catalysts, and so on. Chapter 3, Thermodynamics of Surfaces, and Chapter 7, Catalysis by Surfaces, cover information relevant to the present chapter. Chapter 8 discusses applications in tribology and lubrication (not discussed in this chapter).)... 

Tribological behavior has its origin at the molecular level. Current theories suggest that adhesive forces have an important effect on die magnitude of frictional forces measured between two surfaces. At the molecular level, the two... 

Yang et al. 2005 (59) MWCNT CVD Purified In situ polymerization CNT Loading levels 0.05 to 2.5wt% Block Specimens Microhardness increased, friction coefficient decreased and wear rate decreased with increasing CNT content SEM analyses of worn surfaces of PMMA showed the positive effect of CNTs on tribological behavior of composites ... 

Potassium titanate whiskers were found to improve the tribological properties of NAO friction materials. These soft, synthetic refractory materials stabilize friction level, reduce pad wear, noise, fade, thermal conductivity, control porosity, and provide thermal reinforcement. Fibrous potassium titanates are being replaced by platelet and powder versions because of health concerns associated with respirable fibers. 

The kinetics of tribochemical reactions depends on the conditions of the tribological process and, as a result, it is necessary to find more than one function of the tribochemical reaction mechanism. For example, one function can be related to the mechanics of the tribological process, the second function to its chemical reactions. It was assumed that the relationship between those functions makes it possible to analyse the mechanism of tribological processes on a basic level . Two functions have been taken into account /(y) and (piy). It was assumed that in both functions y is the only state variable. 

The HH, HL, HM, HR, HV and HG hydraulic lubricants are the most important and widely used. As viscosity is one of their most important properties, they are produced in a wide range of viscosities, ISO VG 10-100. For a given application, viscosity should be as low as possible to give a rapid response for the hydraulic system, meaning that the oil should be sufficiently fluid for efficient power transmission. A minimum viscosity level is necessary to eliminate or minimize leakage losses and ensure lubrication of all tribological elements of the hydraulic system. 

When we glue a broken chair, light a match, walk on a street, or ski on snow, we make use of the mechanical properties of surfaces. These include (a) static properties such as hardness or adhesion and (b) dynamic properties such as slide, friction, lubrication, or fracture. The study of the mechanical properties of surfaces in relative motion is often called tribology. It is the purpose of surface scientists to describe and explore many of the macroscopic mechanical properties on the molecular level in order to provide fundamental answers to some simple questions Why are materials hard or soft How do the mechanical properties of surfaces enable us to walk What occurs when we repeatedly move surfaces relative to each other at variable speeds (such as the piston rod against the piston wall of the internal combustion engine) ... 

A monolayer of adsorbates sandwiched between interfaces of solids can markedly modify adhesive and tribological properties. These monolayers are being investigated increasingly on the molecular level. 

Avoidable losses because of friction and wear occur because many who are involved in the design, specification, operation, and maintenance of industrial equipment are unaware of current technology in tribology. There is also an inadequate level of research and development in tribology, through which the reliability and economy of industrial processes and products can be improved. 

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