Polymer Tribology

The following sections are included:

• EDITORIAL POLYMER TRIBOLOGY — A PREFACE

• ACKNOWLEDGEMENTS

• CONTENTS

Surface phenomena and the structure of polymers affect essentially their tribological characteristics, the adhesion of polymers to solids being one of the dominating factors. The latter is explained, in particular, by the adsorption (chemisorption) of the functional groups, variations in the polymer crystallinity and morphology, constrained molecular mobility, and catalytic effects of the additives on the reactions occurring in the contact zone. It is shown how load, sliding velocity, and temperature affect friction. Different modes of wear of polymers and friction transfer are considered.

This chapter gives an overview of micro-structural changes at polymer surfaces induced by friction, and the way they influence the sliding behaviour. Raman spectroscopy is presently applied on worn polymer surfaces giving quantitative and qualitative information about the chemical nature (structural units or additives), conformational order (trans-gauche molecular isomerism), state of order (amorphous, crystalline) and molecular orientation (alignment of polymer chain and side groups) after sliding. Depositions of internal lubricants and formation of a sliding film on the polymer surface are characterized. Degradation mechanisms due to chemical reactions, thermal effects or radical formation are illustrated for polyacetals, polyamides, polyesters and thermosetting or thermoplastic polyimides. Crystallization deteriorates the sliding stability while the formation of a rigid amorphous phase results in low friction and controllable wear rates. Other information is obtained from thermo-analytical analysis (DTA/TGA) of wear debris particles. Through the joint action of repeated loading and high temperature, the wear debris undergoes physical and chemical reactions since its generation. As a result, the properties of those small particles would not be identical to those of the bulk material and are characteristic for the wear process. For polyimides, hydrolysis is the origin of high friction at 100 to 180°C and lacking transfer film while imidization at 180 to 260°C promotes transfer film formation with low friction and stable wear rates.

Correlations between meso-, small-, large- and full-scale tribotesting of engineering polymers are discussed in this chapter. A common parameter characterizing tribological data is the “pv-value” (product of contact pressure and sliding velocity), but its use is restricted to a single test scale. The influence of visco-elasticity and thermal effects should be taken into consideration. Three experimental extrapolation models are presented with up to two mechanical parameters. A macroscopic geometry model considers thermal effects, sample geometry and contact conditions. Coefficients of friction for bulk polymers can be extrapolated from small-scale to large-scale tests. Extrapolation of wear rates is impossible due to transitions between mild wear, softening and melting. Tribological data for solid-lubricated or internally oil-lubricated polymers are more difficult to extrapolate due to interactions of lubricant supply mechanisms with softening, melting and deformation. Different tribological behaviour obtained at different test scales is due to stress concentrations and debris mobility, which is confined within large contact areas and promotes film formation on the counterfaceor polymer s urface. Small-scale tests at high-temperature are irrepresentative for large-scale tests at high load due to variations in polymer structures: orientation concentrates in either a crystalline phase (high temperature) or a rigid amorphous phase (high load). Debris polymerization mainly improves the homogeneity of the transfer film at long resident time. Specifically designed large-scale tests agree with full-scale performance of bearing elements, as verified by finite element modeling.

The scratch behaviour of an amorphous polymer was investigated to determine how the characteristics of the material affect the contact geometry during scratch experiments with spherical indenters. A thermoplastic polymer (Polymethylmethmcrylate) was studied using both an experimental device allowing in situ observation of the contact area during scratching with spherical tips and finite element modelling (FEM). The rheological properties of the PMMA surface were approximated by a simplified bilinear law, while the friction at the interface between the indenter and the material was modelled with Coulomb's friction coefficient varying between 0 and 1, for each computed ratio a/R from 0.3 to 0.6 (where a is the contact radius and R the radius of the tip). FEM results for elastoplastic contacts indicate that the contact geometry is directly related to the plastic strain field in the deformation beneath the indenter during scratching. We show that the dimensions of the plastically deformed volume and the gradient of plastic strain both depend on the ratio a/R and also on the friction coefficient. An equivalent average plastic strain is calculated by FEM over a representative plastically deformed volume. The average plastic strain increases with a/R, as predicted by Tabor's empirical rule, and with the local friction coefficient for a given ratio a / R. Clear correlations are demonstrated between the average plastic strain and the geometrical parameters classically used to describe the shape of the contact area.

Indentation, in general, is a well-known experimental technique to determine hardness of materials. Nanoindentation is a later development of the technique which can be used to determine elastic modulus and hardness at submicron to nanometer levels. However, application of the nanoindentation to polymeric materials is still a challenging issue as for polymeric materials, the rate- and time-dependent properties of the materials are not included in the conventional nanoindentation analysis, and this usually results in over estimation of the elastic modulus and hardness from the nanoindentation tests for the polymeric materials. In addition, the indentation unloading curve of polymers appears to be dependent on both the holding time at the maximum load and the unloading rate; this further leads to certain uncertainty and errors during the nanoindentation experiments and analysis.

This paper will review the current understanding of the experimental and analysis of nanoindentation of polymeric materials. First, traditional analysis of nanoindentation load-displacement data will be presented, followed by discussions of the uncertainties and problems when applying the traditional analysis to the nanoindentation of polymeric materials. The paper will then focus on some of the central issues with applying nanoindentation techniques to the polymeric materials including: (i) how to modify the commonly-used nanoindentation unloading analysis; (ii) how to characterize the timedependent properties through indentation creep experiments; (iii) how to analysis the rate-dependent properties during nanoindentation experiments; and (iv) the use of the atomic-force microscopy or scanning probe microscope to the polymeric materials as well as the comparison with the conventional nanoindentation experiments where appropriate. When appropriate, the discussions are aided with the results from several research projects in author's group.

Wear of Ultra-high molecular weight polyethylene (UHMWPE) has been the most important tribological issue of artificial joint prostheses since fine wear particles released from UHMWPE components exert serious adverse reactions on living tissues. Those tissue reactions finally induce the aseptic loosening of prosthetic joint components. Therefore, a large number of studies have been conducted to understand the in vivo wear characteristics of UHMWPE and to extend the longevity of artificial joints by improving wear resistance of polyethylene components. However, the polyethylene wear mechanism in human body is very unique and complex because it is inevitably affected by the complicated joint kinematics and various constituents contained in the joint environment. This chapter focuses on physiological factors involved in the in vivo wear mechanism of UHMWPE. We first review representative studies about effects of kinematical and environmental factors on the in vivo wear mechanism of UHMWPE components of joint prostheses. Subsequently, some experimental results are presented to discuss about the considerable effects of multidirectional nature of the sliding motion in prosthetic joints and biological macromolecules contained in the periprosthetic fluid.

This chapter focuses on the tribology of biopolymers used in total joint replacements. These biopolymers include ultra high molecular weight polyethylene, various types of cross-linked polyethylene and, for historical completeness, poly tetra fluoro ethylene and polyacetal. The main applications of these polymers, and where they have shown the greatest success, are in replacement hip and knee joints, though these same polymers have also been used in other prostheses implanted around the human body such as the shoulder, elbow, wrist, finger, ankle and toe. The key tribological concern regarding these biopolymers is their wear and this area is examined in detail including consideration of why wear is so critical, details of polymeric wear debris, the important types of wear, and laboratory testing which aims to replicate the wear rates which are found in the body. The study of wear necessarily leads on to a consideration of friction and lubrication. While most replacement joints generally have one component of a polymeric material, consideration is given to the potential of all-polymer prostheses. The chapter ends with a look at future possible developments in biopolymers and current challenges for tribologists concerned with biopolymers.

The frictional behavior of miniature polymer-on-polymer bearings is discussed. The transition from static to kinetic friction was simulated based on material parameters, surface topography and loading conditions. Therefore the effect of rheological behavior of contacting polymeric materials on friction was investigated. The elaborated model was used for prediction of friction coefficient (torque) in the analyzed bearings. The friction force was considered to be a sum of adhesive and mechanical (deformation) components. The extended experiments then have been carried out to compare the results from computer simulation with the results obtained in the experimental part of the studies. Very good agreement between simulation and experimental results was found. The results of the studies can be used in design process of miniature components to predict their frictional behavior and to select optimum combination of rubbing polymeric materials to obtain a required coefficient of friction.

Along with the developments and applications for tribological purposes of rubber and rubber-like materials, remarkable progress has been made in rubber tribology over the years. This chapter is mainly aimed at reviewing the recent advances in rubber tribology in the last five years from the following five aspects: rubber friction, rubber lubrication, rubber wear, wear of metal by rubber, and tribology of rubber assemblies. This comprehensive material could served as a valuable source of references for the future tribologists. Up to now, rubber tribology seems still in its infancy in the strict sense. However, increasing the understanding of rubber tribology will put the tribological application of elastomers forward. Conversely, spreading the tribological application of rubbers will certainly bring about a great advance in rubber tribology. It is expected that a new developing prospect of the rubber tribology would be opened up in the near future.

Polytetrafluoroethylene (PTFE) was discovered by chance in 1938 by Roy Plunkett at the DuPont company, and within a decade establishment of initial commercial production facilities brought PTFE's widespread availability. In addition to this polymer's high melting point and chemical resistance, its unusually low friction behavior was noted early on,¹ leading to a great deal of activity over the half-century since developing a basic understanding of its tribological capabilities as well as its useful application as a self-lubricating bearing material. This chapter surveys the molecular and crystalline structure of PTFE and its related transfer film and friction behavior, the severe wear mechanism often accompanying such transfer behavior and its prevention via hard micro-fillers within a PTFE composite, as well as alternate approaches towards wear resistance which avoid the abrasive countersurface damage such hard filler particles also typically present. Finally, the recent development of extremely wear-resistant PTFE nano-composites is presented.

This study deals with the development of PEEK (polyetheretherketone) and PTFE (polytetrafluoroethylene) based composites, optimized for low friction and low wear performance under extreme environments. It is demonstrated that the incorporation of a harder polymer component into PTFE (such as PEEK particles), a short fibre reinforcement (e.g. carbon fibres CF), and internal lubricants (e.g. PTFE particles), helps to reduce the friction and to improve the wear resistance over a very wide temperature range.

Polymers are filled with particulate materials to enhance the tribological properties and often to change the physical and mechanical properties as well. The composites made with such fillers are amenable to common molding processes unlike fiber-reinforced composites which makes them very attractive economically. As such a large number of studies on the tribological behavior of filled polymers have been made. The representative results from such studies are presented in this chapter. In this context, the friction and wear behavior of these composites is examined. The results show that some particulate fillers improve the wear resistance of polymers while others affect it adversely. The mechanism of the improvement in wear behavior is discussed by referring to the transfer film characteristics such as texture, uniformity and thickness as well as bonding to the counterface surface. In order to support these observations, the micrographs of transfer films along with the experimental results from transfer film-counterface bonding are presented. In addition the results from energy dispersive X-ray analysis are presented to support the hypothesis of bonding. The similarities and differences with respect to the tribological behavior of the composites made with micro and nano fillers are also discussed.

Thermoplastic polymers are light in weight and easily process-able materials hence they are extensively used in auto parts and in several other industries. The materials with the unique combination of good friction and wear properties with the easy processing-ability are in demand. Polypropylene and its blends exhibit excellent mechanical and thermal properties coupled with excellent processability. The sliding wear performance of polypropylene or its blends have been improved significantly by introducing UHMWPE. This chapter gives a review on the sliding wear properties of PP and its blends with polyethylene terephthalate (PET) and recycled PET. Mechanism of wear under the sliding wear condition has been explained with the help of wear rates, mechanical properties and worn surfaces. The improved wear performance on loading a small amount of UHMWPE is an interesting feature of these materials. The sliding wear of UHMWPE modified polypropylene and its blends depend primarily upon the composition, sliding conditions, wear mechanism, the coefficient of friction and rise in temperature of interface.

This chapter reviews the use of engineering polymers and short-fibre reinforced thermoplastics for machine elements such as gears, cams and pulleys where the polymers are in non-conformal contact. Both twin-disc testing and gear tests are examined for a variety of engineering polymers as well as the effects of temperature, fibre-reinforcement and geometry on performance. Gear efficiency, failure mechanisms and wear of polymer gears are also covered in detail. Finally polymer gears for high-performance applications are considered together with the effects of external lubrication.

Tribological properties of brake friction materials are presented considering compositional effects, processing parameters, and microstructural aspects of sliding interfaces during brake applications. In the first part, the role of ingredients on the brake performance is summarized by grouping the constituents into binders, reinforcements, and property modifiers. The manufacturing parameters that affect compressibility, hardness, and porosity are also examined for optimization of brake performance. Secondly, the sliding interfaces between brake pads and a disk are studied since the friction films on the disk surface and the plateaus on the pad surface are closely related to the critical brake performance including noise propensity, fade phenomena, etc. In the last section, the environmental issues of the brake friction material and the current development efforts toward next generation friction materials are discussed.

Molded polypropylene is increasingly used as the interior and exterior materials in the automobile industry to replace metals and other materials, due to their much desired properties. To optimize and further expand the applications, correct characterization of their tribological and mechanical properties is important. A single-probe test technique is applied to test the tribological properties of four polypropylene samples, products of Advanced Composites Inc., in the present study, using a Nano-Indenter for controlled damaging and using a scanning probe microscope (SPM) to examine the damages, as well as test their mechanical properties. The study shows the technique is capable of distinguishing the samples' performances. A new method to quantitatively and comprehensively characterize the surface damage is also introduced in the study, i.e. the measurement of surface area increase.

This chapter addresses the problem of the mechanical properties of thin polymer films geometrically confined within contacts between elastic substrates. Analytical contact mechanics solutions for coated substrates are used to derive, within the limits of confined contacts situations, an approximate oedometric solution for the indentation of a thin film lying on a more rigid substrate. From a discussion of this approximate model, it is shown that lateral contact methods are an interesting alternative to indentation experiments for the measurement of the visco-elastic properties of polymer films in their glass transition range or rubbery state. As an example, the hydrostatic pressure dependence of the visco-elastic properties of confined polymer films is analyzed in the light of lateral contact stiffness measurements. The effects of hydrostatic pressure on the onset of plastic flow within confined polymer coatings are also discussed.

This article describes the tribological properties of well-defined highdensity polymer brushes prepared by the “grafting-from” method based on surface-initiated living radical polymerization. The authors have demonstrated here a water-lubrication system using hydrophilic polymer brushes consisting of 2,3-dihydroxypropyl methacrylate and 2-methacryloyloxyethyl phosphorylcholine (MPC). Macro- and microscopic frictional properties were carried out by a ball-on-plate type tribotester and an AFM cantilever attached with a colloidal sphere, respectively. Friction coefficients and adhesion forces of brush surfaces depend on the solvent quality. For example, the friction coefficient of poly(MPC) brush was reduced to be 0.02 under the highly humid atmosphere because water molecules adsorbed into the brush surfaces acted as a lubricant. Furthermore, high-density polymer brushes showed the low friction coefficient even after 100 reciprocating friction test, which clearly indicates an excellent wear resistance of brush film.

This paper reviews the current materials selection criteria for polymeric resists used in Nanoimprint Lithography (NIL).The tribological and adhesive behavior of some selected polymeric NIL resists is presented. The paper concludes with an offer of several solutions to overcoming tribological problems and the problem of adhesion experienced in NIL.

In the present chapter, we first review the literature on the tribological properties of polymer thin films and then present the results on novel ultra-high molecular weight polyethylene (UHMWPE) films formed on Si surface using a simple dip-coating technique. The presently developed UHMWPE films have reduced the coefficient of friction of Si by at least 6 times to 0.09 and increased the wear resistance of the Si by many orders of magnitude with no sign of wear debris formation. Further enhancement of tribological properties of UHMWPE film is described where perfluoropolyether (PFPE) overcoating onto UHMWPE film has increased the wear-life of pristine UHMWPE film by several orders. Finally, the associated wear mechanisms are explained. Extremely wear durable polymer films can find applications in bearing and gear surfaces of machines that are made of Si.

This chapter presents the strategies to improve the friction and wear properties of UHMWPE thin film on Si substrate by suitable interfacial modifications. Firstly, hard DLC was used as an intermediate layer in order to increase the load carrying capacity of the polymer film. As a result, the contact area decreased and the wear life of the composite film, Si/DLC/UHMWPE increased significantly. After investigating the effect of UHMWPE thickness on the tribological properties of Si/DLC/UHMWPE, it is found that there is an optimum range of thickness of the polymer film that shows the best friction and wear performance of this composite film. Secondly, Si substrates were modified with different surface energies that could determine the adhesion strength of UHMWPE film to the substrates. The two extreme conditions, the most hydrophilic, 21º and the most hydrophobic, 104º showed much lower wear resistance when compared with moderate surface wettability. It is concluded that the wear performance of UHMWPE thin films on Si substrate could be extended by several orders of magnitude by suitably using a hard interface layer and by modifying the interfaces for different surface energies of the Si substrate.

The following sections are included:

• INDEX