Tribology of Natural Fiber Polymer Composites (eBook)

eBook Download: PDF | EPUB

2008 | 1. Auflage
220 Seiten
Elsevier Science (Verlag)
978-1-84569-505-7 (ISBN)

Infos

Environmental concerns are driving demand for bio-degradable materials such as plant-based natural fiber reinforced polymer composites. These composites are fast replacing conventional materials in many applications, especially in automobiles, where tribology (friction, lubrication and wear) is important. This book covers the availability and processing of natural fiber polymer composites and their structural, thermal, mechanical and, in particular, tribological properties.
Chapter 1 discusses sources of natural fibers, their extraction and surface modification. It also reviews the thermal, structural, mechanical, spectroscopic and morphological properties of unmodified and chemically modified natural fibers such as sisal, jute, wood, bamboo and cotton together with their potential applications. Chapter 2 gives a brief introduction to the tribology of polymer composites and the role of fiber reinforcement and fillers in modifying their tribological properties. Further chapters discuss the chemical composition, physical structure, mechanical properties and tribological behaviour of polymer composites reinforced with sisal, jute, cotton and bamboo fibers. The tribological behaviour of wood polymer composites (WPCs) is also discussed.
Tribology of natural fibre polymer composites is a useful reference guide for engineers, scientific and technical personnel involved in the development of natural fiber composites. In particular it will give an insight into mechanical properties and failure mechanisms in situations where wear, lubrication and friction are a problem.

• Examines the availability and processing of natural fiber composites and their structural, thermal, mechanical and tribological properties
• Explores sources of natural fibers, their extraction and surface modification as well as properties of chemically modified natural fibers
• Provides an overview of the tribology of polymer composites and the role of fiber reinforcement and filters in modifying tribological composites

Dr Navin Chand is Head of the Polymer and Fiber Group at the Advanced Materials and Processes Research Institute, Bhopal, India (Formerly RRL Bhopal).

Textauszug

2

Introduction to tribology of polymer Composites

Abstract

This chapter gives a brief introduction to the interdisciplinary subject of tribology which is a relatively new subject dealing with the interface between two or more bodies in relative motion such as in gears, bearings, piston–cylinder assembly, gyroscopes, etc. In broader terms tribology, derived from the Greek word tribos which means rubbing, deals with the subject of friction, wear, lubrication and related phenomena. Tribological characterization of a material, determining its wear and friction coefficient and lubrication properties at different operating parameters such as load, sliding distance, sliding velocity, temperature, etc., assumes paramount significance with regard to developing economically viable and energy efficient tribo-materials.

Key words

tribology

friction coefficient

wear coefficient

lubrication

operating parameters

2.1 What is tribology?

We come across tribology in everyday life. Since our childhood we have been taught that during the evolution of man, fire was discovered when two stones were rubbed against each other. Since then the ‘rubbing’ between two surfaces has caused a sort of revolution, and today this has become a full-fledged branch of science, now known as tribology (derived from the Greek word tribos which means rubbing).1 The friction between the stones which led to the discovery of fire has had serious implications and bearing on man’s life. Life cannot exist without friction. If there is no friction between ground and feet we cannot walk; vehicles cannot move on the roads if there is no friction between the road and the tyres; we cannot write on paper without friction between the tip of the pen and the paper. An eraser will not rub out the pencil marks on the paper. In fact we cannot hold a pen if there is no friction between pen and fingers. As we grow and learn to drive a vehicle, we realize that almost no friction is required between the machine parts like the piston and cylinder in the engine, the gears and bearings and numerous other moving parts. Lubricants in the form of engine oil and grease are used to increase the life of a component. Likewise, the face cream we use should be smooth enough to be applied to the face without causing harm to the skin. The tanning cream used for sunbathing consists of nanoparticles of a solid lubricant so that it can be spread and applied smoothly. The friction between hair, shaving, wear due to cutting by teeth and wear and friction caused at joints in the human body sand-blasting, polishing and grinding are all examples of tribology. In fact, tribology exists everywhere right from the human body to micro-electro mechanical systems (MEMS), to aircraft components and the Earth’s tectonic plates.

Studies of all the three factors mentioned above, namely friction, wear and lubrication of materials, comprise the science of tribology. On a broader perspective, it deals with studies of the interface between two or more bodies in relative motion, such as in gears, bearings, piston–cylinder assembly, gyroscopes, etc. (Fig. 2.1). The interactions at the interface between two surfaces cause friction and wear of the materials involved. These interactions lead to the transmission of forces and dissipation of mass (wear) and energy (friction). Friction and wear are interrelated in the sense that frictionless processes will not result in wear. On the other hand, increasing friction forces does not always result in increased wear loss. The consumption of energy alters the physical and chemical behavior of the materials, changes the surface topography and causes generation of loose wear particles (wear debris). Since the consumption of mass as well as energy is involved, the subject of tribology assumes paramount significance. A significant amount of energy is consumed ultimately in friction processes and a similar amount of energy is lost as a result of wear processes. Thus, a scientific effort is required to develop materials and technologies that can control the wear and friction of components and increase their life.

2.1 Tribological components; (a) ball bearings; (b) roller bearings; (c) gears.

2.2 Origin of friction

One of the most important aspects of tribology, and one that is still being investigated today, is how friction is generated at the atomic level. The genesis of friction between two surfaces in relative motion is the key to finding an answer to the ever-increasing problem of huge financial losses due to wear and friction. Friction is closely related to the energy dissipation at the surface. Whenever work is done by a thermodynamic system, heat is generated and this heat is equal to a change in the internal energy of the system plus the work done by the system. This is known as the first law of thermodynamics. In the case of friction, when external work is done by a frictional force it should be equal to the energy dissipated plus the change in the internal energy. Since the change in internal energy is very small compared to energy dissipated, friction is mainly due to the latter. Friction is the measure of energy dissipated, which is caused by chemical and mechanical damage. These damage processes take place in the bulk of the material (ploughing) and at the interface (adhesion).2 Consequently, theories of friction are based on adhesion and ploughing. According to an earlier theory of friction based on adhesion alone, it was assumed that asperities of the surfaces in contact form welded junctions that shear during sliding causing friction. Thus, friction was thought to be dependent on the actual or real area of contact* which, in turn, depends on the applied and tangential load. However, this theory could not explain satisfactorily the large discrepancy between the theoretical and experimental values of the friction coefficient. Moreover, the reasoning that metals with greater solubility form junctions readily and, thus, have higher wear and friction coefficients did not hold much water because phenomena such as chemisorption and physisorption easily contaminate the surface and the compositions of surface and bulk are quite different. Hence, the adhesion theory was modified using the combined effects of surface (adhesion, asperity deformation) and bulk (ploughing) properties. According to the modified theory, the coefficient of friction μ between the sliding surfaces is a combination of asperity deformation μd,ploughing by wear particles and hard surface asperities μp and the adhesion between the flat surfaces μa. The individual contributions of the three mechanisms depend on the contact surface topography, operating conditions and the type of material.

A typical curve between coefficient of friction and sliding distance is shown in Fig. 2.2. From this figure it is clear that initially adhesion does not play any significant role at this stage due to the contaminated nature of the surface. Similarly, asperity deformation is not of much significance because the asperities in contact deform as soon as sliding commences and the surface is easily polished. As sliding progresses, the frictional force increases due to enhanced adhesion and hence a small increase in μ is observed. The slope of this curve would have increased had there been entrapped wear particles, which would have ploughed the surfaces. With further increase in sliding duration, μ increased linearly due to ploughing by the entrapped wear particles. If the entrapped wear particles are of equal hardness then they can plough both of the surfaces and the ploughing will be greater. With further increase in sliding time, μ becomes steady as the number of entrapped particles between the interface becomes constant. The number of entrapped particles leaving the surface becomes equal to the particles leaving the interface. However, these mechanisms depend on the experimental conditions and the nature of the materials. The initial increase in the value of μ is known as the static friction coefficient and the steady state is known as the dynamic friction coefficient. The contribution of asperity deformation is substantial for the static coefficient of friction and minimal for the dynamic friction coefficient. Similarly, the contribution of adhesion and ploughing is substantial for the dynamic friction coefficient and minimal for the static friction coefficient.

2.2 Typical friction curve.

Friction force increases due to adhesion of surfaces in contact as a result of welding and formation of junctions. Due to contamination of surfaces, adhesion is not good initially (static friction coefficient) but, as sliding progresses, the deformation of asperities exposes new surfaces and adhesion increases. The dynamic friction coefficient is attributed more to the ploughing component of the frictional force. Either the entrapped wear particles or the hard asperities of the surface or both can penetrate the surface. This results in the formation of grooves. However, when either of the two surfaces is very hard and smooth, the wear particles slide along the hard surface and no ploughing occurs. When the hard surface is very rough, wear particles plough the softer surface and create debris which causes wear of sliding surfaces. This becomes evident when ridges are formed along the sides of ploughed...

Produkt-Details

• Erscheint lt. Verlag: 14.10.2008
• Sprache: englisch
• Themenwelt: Naturwissenschaften ► Chemie ► Technische Chemie
• Themenwelt: Technik ► Maschinenbau
• ISBN-10: 1-84569-505-4 / 1845695054
• ISBN-13: 978-1-84569-505-7 / 9781845695057