Sol-gel coatings on polymeric substrates have been widely used in optical lenses, automobiles, safety windows, and flexible display panels. The resistance to scratch damage is an important reliability consideration. This article begins with a review of the scratch failure modes, the related mechanisms, and the parametric models related to these failure modes. Generally failure modes can be categorized into coating cracking driven and delamination driven. By analyzing the potential failure modes, the importance of characterizing the coating fracture toughness and interfacial fracture toughness between the coating and the substrate is high-lighted. Controlled buckling test is introduced as an easy and appropriate test method that can be used for the measurement of both properties.Sol-gel coatings with different amount of colloidal silica were prepared on polycarbonate substrates. Pencil scratch test was carried out following the ISO 15184 standard. The indentation hardness, elastic modulus and fracture toughness of the coatings were also determined and correlated to the observed pencil scratch hardness. The scratch failure was found to be tensile trailing coating cracking. Analysis shows that the main factors affecting the scratch resistance are elasticity modulus, thickness, and fracture toughness of the coating. Based on the current and other reported results, ways to improve scratch resistance for brittle coatings on compliant substrates are discussed.
The sliding of polymers against metallic counterfaces is very common due to the light weight of engineering polymers, their low cost and their good tribological performance in many cases. Of utmost importance in polymer-on-metal sliding is the development and maintenance of transfer films deposited on the counterface by the removal of polymeric material from the bulk. This chapter discusses key sliding parameters that affect the transfer film characteristics, as well as the role that transfer films play in the wear rate of polymers in sliding. Typically, the wear resistance of polymers increases when they are filled with materials that enhance the bonding and uniformity of the transfer film. The strong dependence of wear on transfer film quality is of vital importance especially in the wear of polymers filled with nano-sized particles. This chapter discusses the use of nanoparticle filler materials that have shown beneficial results, as well as fillers that have degraded wear resistance of the polymer. Interestingly, nano-sized particles show optimal wear benefits at much lower volume fractions than are required when using micro-sized filler materials. This chapter proposes two primary mechanisms to describe the effects that nanoparticle fillers have on transfer film bonding to the counterface. The first is mechanical interlocking of polymer and nanoparticle debris within the counterface asperities, thus leading to deposition of transfer film and a smoothening of the wear surface. The second mechanism of bond enhancement by nanoparticle fillers is their chemical degradation or chemical reaction with the counterface material. The chapter reports much of the work done in using nanoparticle fillers and how these fillers affect transfer films and thus wear resistance.
Incorporation of micronized powders of poly(ether ether ketone) (PEEK) and poly(tetrafluoro ethylene) (PTFE) did not improve the resistance to wear due to lacking adhesion to the rubber matrix. Carbon nanofibers (CNF) and nanotubes (CNT) proved to be more efficient nanomodifiers than organoclays. "Nanostructuring" by creation of interpenetrating network (IPN) like structures is a promising method to tailor the tribological performance of the related rubber-based hybrids. It was also underlined that the tribological effects of nanoreinforcements and structural modifications strongly depended on the type and testing parameters of the related tribotests.This chapter surveys briefly the dry sliding wear performance of rubbers and their coatings containing novel "micro- and nanofillers." In addition, concepts for their "nanostructuring" were introduced, their feasibility demonstrated and preliminary results on the sliding wear characteristics of such nanostructured systems were reported.Based on the recent research activities some future trends for the development of rubbers with improved resistance to sliding wear were deduced.
In the context of establishing novel polymer nanocomposites for successful industrial use, this chapter is aimed at providing a fundamental overview of comprehensive research regarding carbon nanofiber (CNF)-reinforced poly(ether ether ketone) (PEEK) composites for demanding tribological applications. Following a summary overview describing the potential of nanoscale additives for tribological systems in general, the intrinsic structure-property relationships of the CNFs are discussed To set the frame for the subsequent experimental results regarding their use in PEEK. As demonstrated, successful PEEK-nanofiber composites have been developed, showing a clear enhancement in mechanical properties while minimizing other detrimental effects commonly observed with such nanocomposites. These nanocomposites also reveal a significant beneficial effect on the wear behavior of PEEK under dry sliding conditions. In addition to relating the observed enhancements in the tribological performance to the underlying microstructures and properties of the nanocomposites, evidence for a further optimization of such systems by combining the nanoscale filler with conventional tribological additives and reinforcements such as carbon fibers is provided. Lastly, based on the promising results regarding the wear behavior of such hybrid systems, the performance of advanced nanocomposite hybrid materials for an intended industrial tribological application is introduced and discussed.
An analysis has been made of the literature on friction and wear of polymer nanocomposites prepared by compounding nanofillers with molten thermoplastics. A process is described for preparing the nanocomposites with various binders, e. g., polar and non-polar thermoplastics, aliphatic, aromatic and partially aromatic polymers, as well as amorphous and partially crystalline polymers by reactive extrusion. The most important group of triboengineering nanocomposites includes thermoplastics containing the following fillers, carbon nanomaterials, layered clays, metals and metal-containing compounds. The introduction of nanofillers into thermoplastics influences the kinetics and mechanism of tribochemical transformations in a friction contact with metals or other materials. Nanofillers affect significantly the physicochemical behavior of macromolecules, as well as the macromolecular transformations, at the conditions of frictional interaction. This can be explained by adsorption (chemisorption) of functional groups found on the particle surface, variations in polymer crystallinity and morphology, constrained molecular (segmental) mobility in macrochains, and catalytic (inhibiting) effects of the additives on the course of contact reactions. As a consequence, alterations occur in the formation of transfer layers and wear debris, along with the parameters of friction. Depending on the polymer, the nature and concentration of the fillers, the latter can either improve or impair the performance parameters of friction pairs.
Delamination is the most serious damage of a coating. It is generated even by a single action of contact when the mating material is very hard and its surface is indented into the coating under relatively large contact load.Cracks of ring, radial and/or lateral types are generated in the coating around the contact region, and partial delaminations of the coating are introduced. Critical loads for crack initiation are given by theory.When the load is not large enough to introduce cracks by static contact only, cracks are introduced in the coating after a certain amount of friction cycles.By the repetition of friction, the coating surface is worn microcropically and its thickness is gradually reduced. As a result, the coating is delaminated by the generation and propagation of micro-cracks around the wear scar.The propagation of cracks may take place in the coating, at the bonding interface or in the substrate, depending on both the yield strength ratio between the coating and the substrate as well as the ratio between coating thickness and contact width.Wear maps are given to distinguish the region of crack initiation site in relation to the yield strength ratio and the coating thickness/contact width ratio.
Epoxy, a typical thermosetting polymer, has been used in a wide range of engineering applications because of its outstanding processability, good affinity and considerable thermal and mechanical properties. Incorporation of small amount of inorganic nanoparticles proved to be able to greatly improve the tribological properties of epoxy. The present chapter evaluates the sliding wear behaviors of epoxy and its composites filled with silicon carbide (SiC) nanoparticles. Polyglycidyl methacrylate (PGMA) and a copolymer of glycidyl methacrylate and styrene were grafted onto the nanoparticles as a measure of surface pretreatment, respectively. The grafted polymers were selected because the epoxide groups on PGMA would take part in the curing reaction of epoxy resin and covalently connect the nanoparticles with the matrix, while styrene acted as a copolymerized monomer to adjust the amount of the reactive groups of the grafted macromolecular chains, and hence interfacial compatibility between the grafted polymers and the matrix. In comparison to the composites filled with untreated nano-SiC particles, the composites with the grafted nano-SiC exhibit enhanced sliding wear resistance and reduced frictional coefficient owing to the chemical bonding at the filler-matrix interface. Microstructures of the composites had a critical influence on the composites performance.
In this chapter, two types of hybridized carbon films with different nanocomposite configurations are examined with regard to their tribological properties. The microstructures of the films consisted of graphitic and amorphous regions. The two segments were either arranged in the form of multilayers or as column/inter-column structures aligned normal to the film plane. Both nanocomposite films possessed excellent tribological properties. In particular, the structurally modified column/inter-column films showed a high-wear resistance in spite of a distinctively poor film hardness.
Solid lubricants comprise an important class of materials and find use in applications where the use of more traditional lubrication techniques is undesirable or precluded. Polytetrafluoroethylene (PTFE) is a notable solid lubricant material, being known for its reputably low friction coefficient, high thermal range and chemical resistance, but a high wear rate limits its application in moving mechanical systems. The use of microfillers can reduce the wear rates to more acceptable values, but large particle size, high filler concentration, increased abrasion and increased friction coefficient all contribute to limit the performance of the composite. The use of nanofillers has been shown to provide further improvements in wear rates without introducing detrimental effects on its other beneficial properties. This work outlines recent studies of the wear resistance mechanisms in these novel systems. Several key wear resistance mechanisms have been identified: (1) bonding and strength at the filler/matrix interface, (2) dispersion and mechanical effects of load support and crack deflection, (3) morphological effects of nanoparticles on the matrix, (4) fibrillation and toughening, (5) transfer film coverage, (6) transfer film orientation and (7) chemical degradation. It is found that the ca. 1,000X improvement in wear resistance that trace loadings of nanofillers impart to PTFE is due to a synergism of wear resistance mechanisms that is activated by the small filler size.
To study the wear behavior of polymer-steel sliding pairs an incremental wear simulation technique had been developed, which can consider the temperature and time-dependent behavior of polymer materials and evaluate the contact parameters (location of the contact area and the contact pressure distribution) during the wear process by changing the initial clearance between the pin and disc according to the linear wear equation. The wear simulation technique proposed is applied for a Pin-on-Disk Configuration (PoDC). During the wear simulation, the frictional heat generation was also considered by transient Finite Element (FE) solutions, as well as the thermal expansion in the pin-on-disc environment. The results illustrate the change of the contact parameters and the frictional heat development during the initial part of the wear process, which involves the edge-like contact and the full contact phases. Further results predict the wear process at 150 degrees C, based on temperature-dependent material properties and considering creep behavior of the polyetheretherketone (PEEK) material. Experimental results for the worn surfaces were in good agreement with the numerical ones.
The influence of nanoparticle fillers on the friction and wear behavior of polymer matrices is summarized in this chapter. It is illustrated that the volume content, size and shape of nanoparticle fillers may exert much influence on the friction and wear behavior of polymers. Nanoparticles can change the friction and wear behavior of polymer matrices in different ways and a lower friction coefficient does not necessarily correspond to a decreased wear rate. When nanoparticles are incorporated into polymer matrices in combination with traditional tribofillers, the role of nanoparticles in modifying their friction and wear behavior can be either detrimental or positive depending on the component of the nanofillers and traditional tribofillers. The role of nanoparticles in modifying the friction and wear behavior of polymer matrices is summarized as follows: Firstly, by adding nanoparticles in polymer matrices, the transfer films formed on the counterpart surfaces can be strengthened due to anchoring or tribochemical reaction. Secondly, nanoparticles may act as three body roller bearing in some cases. Thirdly, there may be combinative effect between nanofillers and other traditional fillers. Based on the above roles, influence of nanoparticles on the friction and wear behavior of polymers under different testing conditions is also discussed. It is expected that it is still an open question to study the influence of nanoparticles on the tribological properties of filled polymers, especially in the case of nanoparticles used in combination with traditional tribofillers.
The effect of carbon nanotube (CNT) reinforcement on the sliding wear behavior of epoxy (EP) and ultra-high molecular weight polyethylene (UHMWPE) was studied under uniform sliding against martensitic bearing steel (100Cr6) and austenitic stainless steel (X5CrNi18-10) in a ball-on-prism arrangement.The epoxy-based composites were prepared in various ways to study the effect of the dispersion method on the tribological properties: purification/activation in concentrated nitric acid, treatment in a coupling agent, sonication and a variety of mixing devices were employed. The CNT content was varied systematically to find the optimal filler content. It was found that the wear rate and its scatter are minimal when the CNT content is approximately 1 wt%. The wear rate of this compound could be further reduced by adding dry lubricants. The best wear resistance was found for a compound containing 1% CNTs plus 10% graphite.The CNT-reinforced polyethylene composites were prepared by melt mixing in a kneader. Various amounts of CNTs were dispersed in a mixture of 80 wt% UHMWPE and 20 wt% high density polyethylene. The unreinforced PE matrix had already a relatively good wear resistance. CNT reinforcement could further improve the wear resistance, but only to a minor degree. The optimum filler content was again between 0.2 and 0.5 wt%.The type of counterpart material was found to affect the wear rate of the composite drastically. The wear rates of the compounds were several orders of magnitude lower against austenitic stainless steel than against the hard bearing steel.
This chapter presents a brief account of the current state-of-the-art in the area of the tribology of polymers and their composites. The classical explanation of friction based upon the "two-term model" is presented. Further, important factors affecting friction and wear of polymers from the design and materials selection perspectives are described in detail. Tribological trends for polymer composites, both traditional and nanocomposites, are presented using data currently available in the literature. Finally, based on our current understanding of this field, we have speculated upon some future trends and directions in the area of polymer tribology. Our assessment is naturally very subjective and selective given the vast potential for research growth in this field.This review is not meant to be exhaustive, and hence readers will naturally need to refer other chapters presented in this book for a much more detailed knowledge of the area of polymer tribology in general and tribology of nanocomposites in particular.
Historically, a scratch has been viewed as a sliding indentation of an asperity over the surface of a material. This simplicity is deceiving, as the phenomenon of a scratch has many facets to its nature and requires much care regarding testing and evaluation. This becomes especially important when the scratch behavior of polymers is considered. Viscoelasticity, among other properties, plays a large role in the material response when a polymer is scratched. This creates a special challenge in developing test methods that address scratch behavior from a material science point of view. The challenge is augmented by the need to address the stress state the material experiences during a scratch. This is possible through numerical means (e. g., finite element methods), but the adopted model must be able to encompass the entire material response. This chapter will present a development of the history of scratch testing which led to the development of a new testing and analysis methodology that can address polymer scratch behavior unambiguously. It will also be shown that significant advances have been made in numerical modeling of polymer scratch behavior via the finite element methods which supplements the experimental results to give a comprehensive view of polymer scratch behavior.
Polymer based slide coatings have become indispensable as tribological layers of slide bearings. In modern automobiles they are used as tribological coatings for high-temperature applications in the engine compartment such as anti-friction coatings for piston skirts or in low-lubricant applications like journal bearings of diesel fuel injection pumps which often operate under boundary lubrication. Important structure-property relationships for the achievement of a high operating temperature and a high-compression loading capability are discussed in detail for two of the most important polymeric matrices: epoxy resins (EPs) and polyamide imide (PAI). The beneficial effect of nanoparticulate fillers on the tribology of slide coatings is demonstrated on the basis of three current applications from the automotive industry: slide coatings for piston skirts, polymer/metal-slide bearings for high-temperature bearing applications and for use under boundary lubrication.
The friction and wear behavior of fiber-reinforced composites is dependent on fabrication, operational and environmental parameters. In particular, the type of fibers employed seems to have crucial influence. Carbon nanotubes (CNTs), discovered less than 20 years ago, aroused big enthusiasm around the world because of their excellent properties and potential application in many fields. Compared with carbon fibers, CNTs possess an ultra-high strength, modulus and perfect graphite-layered structure, which imply their promising application as wearable materials in tribological fields. Friction and wear test results suggest that, compared with carbon fiber-reinforced composites, composites with CNTs as reinforcing elements exhibit more stable frictional coefficient and lower wear rates, especially at the high temperature. Based on experimental results conducted on epoxy resin and carbon matrix composites, two possible mechanisms about friction and wear process are proposed. In addition, some potential tribological applications of CNT-reinforced composites are discussed in brief.
Owing to the advantages of self-lubrication and superior cleanliness, polymer composites are increasingly used as sliding elements, which were formerly composed of metallic materials only. The feature that makes polymer composites so promising in industrial applications is the opportunity to tailor their properties with special fillers. In the present overview, the role of various fillers such as short fibers, solid lubricants and nanoparticles, in modifying wear behavior of the polymers is discussed, with particular emphasis on the authors' recent efforts in developing wear-resistant polymeric hybrid composites blending nanoparticles with traditional tribo-fillers.Furthermore, the application of artificial neural networks (ANNs) in the tribology of polymer composites is briefly reviewed. It is stated that a well-trained ANN has the potential for use in composition design and online wear monitoring in practical applications.
This chapter describes a general approach to the friction and wear behavior of poly-ether-ether-ketone (PEEK) coatings under dry sliding conditions. First of all, two efficient coating techniques, thermal spraying and painting, are introduced. Then, the crystalline structure and adhesive property of PEEK coatings are characterized. The correlation between the crystalline structure and the tribological characteristics of PEEK coatings is clarified. In addition, the effects of the sliding conditions, such as sliding velocity, applied load and ambient temperature, on the tribological behavior of an amorphous PEEK coating are studied. Based on 3D morphology analyses of worn tracks and scanning electron microscope (SEM) observations of worn surfaces, the friction and wear mechanisms of the amorphous PEEK coating are discussed. Finally, the tribological behavior of nano-SiC (7wt%)-filled PEEK coating is described. The roles of the nano-SiC particles on the coating tribological behavior are discussed.
