Surfaces in relative motion

Friction can now be probed at the atomic scale by means of atomic force microscopy (AFM) (see Section VIII-2) and the surface forces apparatus (see Section VI-4) these approaches are leading to new interpretations of friction [1,1 a,lb]. The subject of friction and its related aspects are known as tribology, the study of surfaces in relative motion, from the Greek root tribos meaning mbbing. 

The surface force apparatus (SFA) is a device that detects the variations of normal and tangential forces resulting from the molecule interactions, as a function of normal distance between two curved surfaces in relative motion. SFA has been successfully used over the past years for investigating various surface phenomena, such as adhesion, rheology of confined liquid and polymers, colloid stability, and boundary friction. The first SFA was invented in 1969 by Tabor and Winterton [23] and was further developed in 1972 by Israela-chivili and Tabor [24]. The device was employed for direct measurement of the van der Waals forces in the air or vacuum between molecularly smooth mica surfaces in the distance range of 1.5-130 nm. The results confirmed the prediction of the Lifshitz theory on van der Waals interactions down to the separations as small as 1.5 nm. 

The failure of TFL only means a loss of mobility here, but monolayers can stay on solid surfaces to separate the solid surfaces in relative motion, and subsequently sustain a feasible boundary lubrication state [10]. Because the film thickness of TFL is of the nano scale or molecular order, from a mechanical point of view, TFL is the last one of the lubrication regimes where the Reynolds equation can be applied. 

Finally we will consider the two flow fields that yield extensional deformation. The first is axial elongational in which the fluid is contained between two planar surfaces in relative motion along their planar normals. With the axial extension direction taken as z, one has nzz = e = —2nxx = —2xyy and... 

Lubricity is a term used to describe the ability of a fluid to minimize friction between, and damage to, surfaces in relative motion under load. Fuel helps to lubricate and prevent wear of high-pressure fuel injection pump components, especially under boundary lubrication conditions. Boundary lubrication is defined as a condition whereby friction and wear between two surfaces in relative motion are determined by the properties of the surfaces and the bulk properties of the contacting fluid. The bulk viscosity of the fuel or oil is not a critical factor under boundary lubrication conditions. 

Tribology is the branch of science and engineering of surfaces in relative motion. Included are issues of friction, wear, and lubrication of surfaces. Modem technology has enabled the study of these characteristics in a number of different ways. These studies have given rise to a new branch atomic-scale tribology. This branch deals with issues and processes from atomic/molecular scale to microscale. These... 

Also called tribosystem and tribomechanical system, whose functional behavior is system connected with interacting surfaces in relative motion. 

Attrition is the mechanical removal of hard tissue by direct contacts between teeth (either natural or restored) with no foreign substance intervening [5]. This mechanism causes wear by tooth-tooth contacts as well as by tooth-restoration, and indeed restoration-restoration contacts. The action of mastication and bruxism are known causes of attrition. In the field of tribology, the term abrasion refers to the loss of material from a surface by sliding, rubbing or scratching. Two-body abrasion refers to abrasion caused by two contacting surfaces in relative motion, i.e. the mechanism in dentistry that is described as attrition. Three-body abrasion refers to abrasion caused by surfaces in... 

The development of instrumentation capable of studying and manipulating matter on a molecular scale has ushered in new opportunities to explore surfaces in relative motion (Fig. 1). The field of nanotribology has emerged from the belief that a more... 

Lubrication, a part of tribology created by the UK scientist H. Peter lost in his famous lost Report submitted to the government in 1966, refers to the science and technology of interacting surfaces in relative motion and their associated subjects and practice [1]. Tribology is regarded as an aspect of mechanical engineering and includes lubrication, friction, wear, erosion, and corrosion [2]. Friction and lubrication have attracted attention all over the world, especially in the industrial and... 

Boundary lubrication describes processes which can occur at surfaces in relative motion to reduce friction and also control wear under conditions where fluid film formation is not possible. Boundary lubrication functions by several mechanisms... 

Dependent upon load, speed, viscosity, temperature and the nature of the lubricated surfaces in relative motion, differing degrees of separation of these surfaces occur. When significant oil film penetration occurs, frictional heating begins to cause wear, and then both friction and wear can be controlled by chemical/physical surface reactions. Within an engine a range of the above conditions operate. The influence of these conditions on friction can be seen from the Stribeck curve. Fig. 3.6. 

Virtually in all dry sliding contacts we observe that the frictional force required to initiate motion is more than that needed to maintain the surfaces in the subsequent relative sliding thus there are two values reported for the coefficient of friction. The static coefficient of friction is used in reference to the initial movement of the object from the rest position. In this case, the F ml. The kinetic coefficient of friction is used for two surfaces in relative motion. This feature, together with the inevitable natural elasticity of any mechanical system, can often lead to the troublesome phenomenon of stick-slip motion (the displacement of surface materials with time). Displacement increases linearly with time during periods of sticking when slipping occurs, the deformed surface materials are released. Representative of dry static and kinetic coefficients of friction for various material pairs are found in tribology and physics references 11 see Table 3.1. 

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) ... 

Surfaces in contact and moving parallel to each other exhibit phenomena that are encountered incessantly in our everyday life. These phenomena include friction, slide, wear, and lubrication (the process used to modify friction). We could not walk without controlled friction, nor could the internal combustion engine operate without the lubrication of its moving parts. The name tribology (from the Greek tribein—to rub) is given to phenomena that involve surfaces in relative motion. 

Friction, slide, and lubrication are the properties of surfaces in relative motion (tribological properties). 

Abrasive Wear. Abrasive wear(18) is common for brittle, ductile and elastomeric polymers. Abrasion is the wear by displacement of materials from surfaces in relative motion caused by the presence of hard protruberanees or by the presence of hard particles either between the surfaces or embedded in one of them. As a result, microploughing, microshearing or microcutting can occur. Thus, fracture energetics and contact mechanics are involved in analyzing the wear results. We shall discuss briefly the wear rate with respect to different types of polymers. 

LUBRICATION, BOUNDARY - A condition of lubrication in which the friction and wear between two surfaces in relative motion are determined by the properties of the surfaces and by the properties of the lubricant other than bulk viscosity. 

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