Abdelbary Wear of Polymers and Composites

1. Auflage 2015
ISBN: 978-1-78242-178-8
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 256 Seiten

In the field of tribology, the wear behaviour of polymers and composite materials is considered a highly non-linear phenomenon. Wear of Polymers and Composites introduces fundamentals of polymers and composites tribology. The book suggests a new approach to explore the effect of applied load and surface defects on the fatigue wear behaviour of polymers, using a new tribometer and thorough experiments. It discusses effects of surface cracks, under different static and cyclic loading parameters on wear, and presents an intelligent algorithm, in the form of a neural network, to map the relationship between wear rate and relevant factors. Using the aforementioned method leads to more accurate and cost effective prediction of surface fatigue wear rates, under different service conditions. The first three chapters of the book introduce polymers and composite materials tribology, followed by three chapters that cover testing in wear, applied load and contact pressure and surface defects. The remaining chapter moves on to predicting wear of polymers, and concludes by discussing questions and problems. - Prepares senior undergraduates as well as postgraduate students - Focuses on the factors influencing wear of polymers and composites - Contains detailed design of Tribometer, wear test procedures and detailed dataset of more than 50 experimental wear tests - Introduces an artificial neural network approach as one of the recently developed wear prediction models

Ahmed Abdelbary is a Chief Engineering and tribology consultant working for the Egyptian Government. He holds a B.Sc. from the Military Technical College MTC in Cairo, and received his MSc and Ph.D. from Alexandria University. He is a full member of the Egyptian Society of Tribology EGTRIB and Egypt Syndicate for Engineers. His research aims to provide a better understanding of the fatigue wear mechanisms in polymers and other materials, and his areas of technical expertise extend to design and manufacture of many mechanical and tribometer systems. He has authored and edited multiple books, chapters, and several research papers with reputable publishers including Elsevier.

Autoren/Hrsg.

Abdelbary, Ahmed

Leseproben

1

Polymer tribology

Abstract

This chapter reviews the basics of polymer tribology. Abrasive, adhesive, surface fatigue and other forms of wear are considered. The concepts of adhesion and deformation components of friction are discussed. The effects of contact and material parameters on friction and wear are established. Also, the influences of external and internal lubrication upon polymer wear are briefly explained.

Key words

wear mechanisms

friction

contact parameters

lubricating wear

1.1 Introduction

Tribology is the study of surfaces in moving contact, having its origin in the Greek word tribos meaning rubbing. It is the science of two contacting solid surfaces in relative motion that produce effects such as friction wear, while the effect of lubrication between them is considered. Furthermore, it is an interfacial phenomenon that is truly interdisciplinary, embodying physics, chemistry, mechanics, thermodynamics and materials science of the two interacting bodies.

Polymers are being used increasingly in tribological applications due to their elasticity, accommodation to shock loading, low friction and wear resistance. However, tribology of polymers is different from tribology of metals for many reasons. In contrast to metals, polymers are visco-elastic and their properties are dependent on time. External liquid lubricants, which work well for other classes of materials, are easily absorbed by polymers. Further complexity arises as polymers are easily influenced by the operating conditions and the prevailing environment. Nevertheless, it is a fascinating area because polymers can be modified, both on the surface and in bulk, by various chemical and physical means to suit a particular application. For this reason, they became attractive candidates and very promising materials for tribologists with the ability to control their friction and wear behaviours. This has resulted in various tribo-systems composed of polymers, metals and ceramic materials in sliding or rolling tribological contacts.

In fact, traditional tribology, as well as traditional experimental methods developed originally for metals, may not be applicable for polymers. Interfacial and operational conditions such as transfer film formation, thermal heat and contact pressure prevail in polymers tribology. The earliest reference that can be found on tribology studies of polymers is by Shooter and Thomas (1949). The success of introducing bulk polymers in bearing applications led to employment of polymers as composites, where their tribological and mechanical properties were modified by using filler additives. To date, thousands of studies introduced by hundreds of researchers have been conducted on understanding the fundamental mechanisms of friction and wear of polymers and composites. This chapter aims to review three basic branches in polymer tribology: wear, friction and lubrication, with a special concern for mechanisms and factors influencing wear behaviour.

1.2 Wear of polymers

The Committee of the Institution of Mechanical Engineers has defined wear as ‘the progressive loss of substance from the surface of a body brought about by mechanical action’ (Pascal, 1970). Wear occurs as a natural phenomenon involving many diverse phenomena, and interacts in a wildly unpredictable manner. During the past decade, the use of polymers in tribological applications, bearings, gears, biomaterials, etc., has been increasing. Studying the wear of polymers is therefore important from both the scientific and technological point of view.

Since polymers are being used more and more in sliding applications, understanding the wear mechanisms of polymers in contact with counterface surfaces becomes important from a practical standpoint. The tribo-system comprises polymer and counterface that interact in the operative environment under given conditions of applied load, speed, temperature, etc., resulting in the polymer wear process. A general classification of wear types in polymers is still an open matter. Earlier research has established that the wear of polymers can be subdivided into three main groups: adhesion, abrasion and surface fatigue. Each wear mechanism is governed by its own laws and, on many occasions, it may act in such a way as to affect the others. It is important to emphasise that it is not always easy to differentiate between these types of wear, as they are inter-related and rarely occur separately. However, other wear forms such as corrosive, erosive or fretting wear are also included by other researchers.

1.2.1 Abrasive wear

Abrasive wear is caused by hard asperities on the counterface, which dig into the rubbing surface of the polymer and remove material, resulting in micro-machining, wear grooves, tearing, ploughing, scratching and surface cracking (Figure 1.1). The wear debris produced usually takes the shape of fine chips or flecks, similar to those produced during machining (Figure 1.2). The abrasive wear is dependent on the shape and apex angle of the abrasive points moving along the polymer surface. Many approaches have been introduced to correlate abrasion to the mechanical properties of polymers. The abrasive wear of polymers is inversely proportional to the product of the nominal tensile breaking stress su and the elongation-to-break eu (Figure 1.3). The abrasion of polymers may also correlate with its cohesive energy, flexure modulus, yield strain or energy-to-rupture (Giltrow, 1970). A wide range of studies on the effect of the counterpart surface on wear of polymers demonstrated that the abrasive wear process involves plastic deformation and shear, and it was found that for abrasion the dominant material property is the energy-to-fracture of the polymer (Lancaster, 1969).

Figure 1.1 Rubbing surface of polyamide 66 after 80 km of sliding against dry steel counterface, at 90 N. Grooves run across the surface of the wear pin parallel to the sliding direction (after Abdelbary, 2011)

Figure 1.2 Wear debris of polyamide 66 after 20 km of sliding against dry steel counterface, at 90 N (after Abdelbary, 2011)

Figure 1.3 Ratner-Lancaster correlation for abrasion (after Lancaster, 1969)

Based on the above, a number of equations have been proposed in order to express the abrasive wear of polymers. Mainly there are three stages involved in the production of wear debris:

1. deformation of the surfaces to an area of contact is determined by the indentation hardness, H;

2. relative motion opposed by the frictional force (f), f = µL, where L is the normal load and µ represents the coefficient of sliding friction; and

3. disruption of material at the contact points involving an amount of work equal to the integral of the stress-strain relationship (Ratner et al., 1964).

An approximate measure of the latter is the product of the breaking stress and the elongation to break. As these three processes occur sequentially, the total wear can be regarded as proportional to the probability of completion of each stage. Thus, for the sliding distance X, the worn volume V is given by the following expression (Lancaster, 1969):

=µLXHsueu

  [1.1]

because the product (su eu) is related to the area under the stress-strain curve, the toughness or impact strength of the material. Therefore, correlations have been sought between abrasive wear and impact strength, or notched impact strength. The particular importance of the parameter sueu is demonstrated by Lancaster (1969), who obtained a linear relation between sueu and the resulting wear during single traverses of different polymers. It should be noted that both elongation and breaking strength are sensitive to strain rate and temperature variation. However, other attempts have been made to relate notched impact strength to the wear of a number of polymers, but the correlation obtained was not convincing.

As a result of these arguments, it is considered that the resistance of a polymer to abrasive wear can be increased by changing its mechanical properties. For example, both the breaking strength and elongation to break tend to increase with increasing molecular weight, to a limiting value. In particular, with polyethylene, the impact strength increased with molecular weight to a maximum at an average molecular weight of about 1.5 × 106 (Margolies, 1971). The abrasion test showed that the abrasion resistance improved greatly with increasing molecular weight, reaching a maximum and constant value at the molecular weight of 1.75 × 106 and greater. Thus abrasion resistance and impact strength show similar correlation with molecular weight.

A quantitative equation for abrasive wear of metals (Rabinowicz and Mutis, 1965), may also be applicable to polymers. A simple model was assumed in which the asperities on the hard counterface were conical (apex angle of...