Tribology of Polymeric Nanocomposites: Friction and Wear of Bulk Materials and Coatingsby Klaus Friedrich
Providing engineers and designers with the preparation techniques,/b>/ul>… See more details below
Providing engineers and designers with the preparation techniques, friction and wear mechanisms, property information and evaluation methodology needed to select the right polymeric nanocomposites for the job, this unique book also includes valuable real-world examples of polymeric nanocomposites in action in tribological applications.
- Provides a complete reference to polmer nanocomposite material use in tribology from preparation through to selection and use.
- Explains the theory through examples of real-world applications, keeping this high-level topic practical and accessible.
- Includes contributions from more than 20 international tribology experts to offer broad yet detailed coverage of this fast-moving field.
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Tribology of Polymeric Nanocomposites
Friction and Wear of Bulk Materials and Coatings
By Klaus Friedrich, Alois K. Schlarb
ElsevierCopyright © 2013 Elsevier B.V.
All rights reserved.
Tribological applications of polymers and their composites – past, present and future prospects
CHAPTER OUTLINE HEAD
1.1 Introduction 1
1.2 Classical Works on Polymer Tribology 3
1.2.1 Friction 3
1.2.2 Wear 4
1.3 Tribology of Polymer Composites 7
1.3.1 Bulk modification – "hard and strong" fillers in a "softer" matrix 8
1.3.2 Interface modification – "soft" and "lubricating" fillers in a
"hard and strong" matrix 8
1.4 Tribology of Polymer Nanocomposites 9
1.5 Future Prospects 17
1.6 Final Remarks 18
Notations and Abbreviations 19
Polymers play an important part in materials and mechanical engineering, not just for their ease in manufacturing and low unit cost, but also for their potentially excellent tribological performance in engineered forms. In the pristine or bulk form, only a few of the polymers would satisfy most of the tribological requirements; however, in the composite and hybrid forms, polymers often have an advantage over other materials such as metals and ceramics. Polymer tribology, as a research field, is now well mature given that roughly 50 plus years have seen publication of numerous research articles and reports dealing with a variety of tribological phenomena on a considerably large number of polymers, in bulk, composite and hybrid forms. Tribological applications of polymers include gears, a range of bearings, bearing cages, artificial human joint bearing surfaces, bearing materials for space applications including coatings, tires, shoe soles, automobile brake pads, nonstick frying pans, floorings and various types of surfaces for optimum tactile properties such as fibers. The list is growing. For example, in the new area of microelectromechanical systems (MEMS), polymers (such as poly(methylemethacrylate) (PMMA) and poly(dimethylsiloxane) (PDMS)) are gaining popularity as structural materials over the widely used material, Si. Often, Si is modified by a suitable polymeric film in order to enhance frictional, antiwear or antistiction properties.
Similar to the bulk mechanical responses, the tribological characteristics of polymers are greatly influenced by the effects of temperature, relative speed of the interacting surfaces, normal load and the environment. Therefore, to deal with these effects and for better control of the responses, polymers are modified by adding appropriate fillers to suit a particular application. Thus, they are invariably used in composite or, at best, blended form for an optimum combination of mainly friction and wear performances. In addition, pragmatically fillers may be less expensive than the polymer matrix. The composition of the filler materials, often a closely guarded secret of the manufacturer, is both science and art, for the final performance may depend upon the delicately balanced recipe of the matrix and filler materials. However, the last many years of research in the area of polymer tribology in various laboratories have shed much light into the mechanisms of friction and wear. This has somewhat eased the work of materials selection for any particular tribological application.
This chapter on the tribology of nanocomposite, an area which is still in its infancy, would endeavor to set the background of the research in polymer tribology. We will refer to the term "polymer" for synthetic organic solid in pristine form, with some additives but no fillers aimed at modifying mechanical properties. The word "composite" would be used when one or more than one fillers have been added to a base polymer with the aim of drastically changing mechanical and tribological properties. "Nanocomposite" will mean a composite in which at least one filler material has one of its dimensions in the range of a few to several nanometers.
The chapter would review some of the past, but now classical, works when much of the mechanisms of friction and wear for general polymers were studied and these explanations have stood the test of the time. The early works led to the area of polymer composites where polymers were reinforced with particles and/or short or long fibers. Often, the use of the filler materials has followed two trends that mainly reflect the actual function that the fillers are expected to perform. This type of work on the design of multiphase tribological materials continues mainly aimed at improving an existing formulation, or, using a new polymer matrix or novel filler. The present trend is expected to extend into the future but with much more refinement in materials and process selections. For example, the use of nanosized particles or fibers coupled with chemical enhancement of the interactions between the filler and the matrix seems to produce better tribological performance. Also, there have been some very recent attempts on utilizing some unique properties of polymers, often mimicking the biological systems in one way or the other, which have opened up new possibility of using polymers in tribological applications. One example of this is polymer brush that can be used as a boundary lubricant. This trend will definitely continue into the future with great promises for solving new tribological issues in micro, nano and bio systems.
1.2 CLASSICAL WORKS ON POLYMER TRIBOLOGY
The earliest works on polymer tribology probably started with the sliding friction studies on rubbers and elastomers. Further work on other polymers (thermosets and thermoplastics) led to the development of the two-term model of friction. The two-term model proposes that the frictional force is a consequence of the interfacial and the cohesive works done on the surface of the polymer material. This is assuming that the counterface is sufficiently hard in comparison to the polymer mating surface and undergoes only mild or no elastic deformation. Figure 1.1 shows a schematic diagram of the energy dissipation processes in the two-term model.
The interfacial frictional work is the result of adhesive interactions and the extent of this component obviously depends upon factors such as the hardness of the polymer, molecular structure, glass transition temperature and crystallinity of the polymer, surface roughness of the counterface and chemical–electrostatic interactions between the counterface and the polymer. For example, an elastomeric solid, which has its glass transition temperature below the room temperature and hence is very soft, would have very high adhesive component leading to high friction. Beyond interfacial work is the contribution of the cohesive term, which is a result of the plowing actions of the asperities of the harder counterface into the polymer. The energy required for the plowing action will depend primarily upon the tensile strength and the elongation before fracture (or toughness) of the polymer and, the geometric parameters (height and the cutting angle) of the asperities on the counterface. The elastic hysteresis is another factor generally associated with the cohesive term for polymers that show large viscoelastic strains such as in the case of rubbers and elastomers. Further, both the interfacial and the cohesive works would be dependent upon the prevailing interface and ambient temperatures, and the rate of relative velocity as these factors would in turn modify the polymer's other materials parameters. Pressure has some effect on the interfacial friction as normal contact pressure tends to modify the shear strength of the interface layer by a relation given as
τ = τo + αp (1.1)
The implication of the above relation is that as the contact pressure increases, the shear stress would increase linearly leading to high friction. Equation (1.1), as simple as it may look in the form, hides the very complex nature of polymer. Also, it does not include the temperature and the shear rate effects on the shear stress.
In a normal sliding experiment, it is nontrivial to separate the two terms (interfacial and cohesive) and therefore most of the data available in the literature generally include a combined effect. Often, the practice among experimentalists is to fix all other parameters and vary one parameter to study its effect on the overall friction coefficient for a polymer. Looking at the published data one can easily deduce that depending upon other factors, the friction is greatly influenced by the class of polymers viz. elastomers, thermosets and thermoplastics (semicrystalline and amorphous). Semicrystalline linear thermoplastic would give the lowest coefficient of friction, whereas elastomers and rubbers show large values. This is because of the molecular architecture of the linear polymers that helps molecules stretch easily in the direction of shear giving least frictional resistance. Table 1.1 provides some typical values of the coefficient of friction for pristine or virgin polymers.
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Meet the Author
Klaus Friedrich was formerly a Professor in the Institute for Composite Materials (IVW GmbH) at Technische Universität Kaiserslautern (University of Kaiserslautern), Germany, and is now a part time Professor of Materials Science at the King Saud University, Saudi Arabia. He is an editorial board member of several key publications in the area, including Composites Science and Technology, contributes to committees and conferences internationally relating to composite materials, and has received numerous awards and honours throughout his prolific research career.
Alois K. Schlarb is Professor of Composite Engineering at Technische Universität Kaiserslautern (University of Kaiserslautern), Germany. His research includes work on the structure and properties of plastics, including modeling and simulation of wear behaviour in nanoparticle-reinforced plastics. He is an editorial board member of several key publications in the area and contributes to conferences internationally relating to plastics and composite materials.
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