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Wear and scratch damage in polymer nanocomposites
CHAPTER
Wear and scratch damage in polymer nanocomposites
16
Aravind Dasari*, Zhong-Zhen Yu†, Yiu-Wing Mai‡ *School of Materials Science & Engineering, Nanyang Technological University, Singapore, Key Laboratory of Organic-Inorganic Composites, Department of Polymer Engineering, College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing, China, ‡Centre for Advanced Materials Technology (CAMT), School of Aerospace, Mechanical and Mechatronic Engineering, The University of Sydney, Sydney, NSW, Australia
†State
CHAPTER OUTLINE HEAD 16.1 Background���������������������������������������������������������������������������������������������������� 551 16.2 Wear/Scratch Damage in Polymer Nanocomposites������������������������������������������� 552 16.2.1 Effect of coefficient of friction��������������������������������������������������������� 552 16.2.2 Evaluation of scratch/wear resistance����������������������������������������������� 557 16.2.3 Surface roughness�������������������������������������������������������������������������� 561 16.2.4 Anisotropic response����������������������������������������������������������������������� 562 16.3 Coatings��������������������������������������������������������������������������������������������������������� 565 16.4 Concluding Remarks��������������������������������������������������������������������������������������� 567 Acknowledgments..................................................................................................... 568 References�������������������������������������������������������������������������������������������������������������� 568
16.1 BACKGROUND In our previous chapter on “Wear and Scratch Damage in Polymer Nanocomposites” (Chapter 16 of first edition), a review of wear/scratch damage processes occurring in various polymer nanocomposite systems was presented and the parameters responsible for controlling the surface integrity and material removal were deduced and described [1]. In short, we have pointed to the inherent tribological complexities with polymer nanocomposites and the qualitative nature of specified/identified mechanisms. Results varied widely from study to study with only subtle changes of testing conditions, material/filler or even characterization techniques. Considering the complexities involved in quantitatively evaluating the tribological response of polymer nanocomposites, limited efforts have been made to model stress fields induced by different slider geometries, and no explicit correlations between material parameters and wear/scratch damage, particularly for polymer nanocomposites at nanoscale, were available. Tribology of Polymeric Nanocomposites. http://dx.doi.org/10.1016/B978-0-444-59455-6.00016-7 Copyright © 2013 Elsevier B.V. All rights reserved.
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Further, it was suggested that the presence of nanoparticles by themselves or the improved material mechanical properties do not always result in improvements in wear performance [1,2]. However, the formation of uniform and stable transferred materials adhered to the counterface as a result of tribochemical reactions or by some other means between the fillers and the slider counterface was a governing wear response factor. Other reasons were also given including the generation of free radicals during the sliding process, polishing of the counterface slider by fillers, and wear debris acting like a lubricant or third-body in reducing the wear rate by a rolling action. Specifically, during scratching of polymeric materials, the yield stress was considered as the main parameter influencing the scratch depth [3–5]. Moreover, the coefficient of adhesive friction and strain at stress recovery were found to affect the scratch depth and shoulder height. Besides, all these reasons were directly or indirectly influenced by size, volume fraction, geometry, orientation and dispersion of nanoparticles in the polymer matrix. In this chapter, we will update the status of the advancements/knowledge in the field in the past 5 years. For prior knowledge and background on this subject area, interested readers are encouraged to consult our previous contributions [1,2].
16.2 WEAR/SCRATCH DAMAGE IN POLYMER NANOCOMPOSITES As mentioned in our earlier review, added to the coupling of inherent tribological complexities, poor characterization of nanocomposites and their properties often made interpretations of experimental results more difficult. This was even the case with some of the recent studies. However, there are many others that revealed some interesting results with novel approaches. Below, we review both types of studies and delineate the issues of importance.
16.2.1 Effect of coefficient of friction Analogous to the approach of self-healing of epoxy-based composites by reinforcing with resin-filled capsules/fibers, Zhang and coworkers [6] adopted an approach by adding lubricant oil-loaded microcapsules (8 phr) into hybrid epoxy composites (containing 1 phr short carbon fibers and 5 phr silica (SiO2) nanoparticles). It was expected that during wear/scratch conditions, the capsules would break, releasing the oil in the contact area, and thereby reducing the frictional coefficient and material loss. As shown in Fig. 16.1, the presence of oil-loaded microcapsules alone has a significant effect on wear resistance (increased by 60 times) and coefficient of friction (reduced by ~75%) compared to neat epoxy; additional presence of short carbon fiber and/or silica nanoparticles has little effect on tribological properties. However, flexural strength and modulus of these hybrid composites improved considerably compared to epoxy with oil-loaded microcapsules. But, these improvements are negligible when compared to neat epoxy. Although this approach seems to yield positive results on tribological properties, the probability of fracture of oil-loaded
16.2 Wear/Scratch Damage in Polymer Nanocomposites
FIGURE 16.1 Specific wear rate and friction coefficient of epoxy and its composites. Sliding wear test conditions: block-on-ring apparatus, carbon-steel ring, constant velocity ~0.42 m s−1 and a constant pressure ~3 MPa. Recipe #1, 8 phr oil-loaded capsules; recipe #2, 8 phr oil-loaded capsules and 5 phr SiO2; recipe #3, 8 phr oil-loaded capsules and 1 phr short carbon fiber; and recipe #4, 8 phr oil-loaded capsules along with 5 phr SiO2 and 1 phr short carbon fiber.
microcapsules during the manufacturing process of the composites is large. This in turn results in localized pockets of oil in the matrix, which might affect not only the processability of these materials but also some other properties. Similarly, some studies have also noted that to improve wear/scratch resistance, addition of slip agents (mainly derived from fatty acids) in polymers is the best approach. These compounds generally “bloom” or migrate to the surface of the polymer (during the processing step) [7,8] forming a waxy layer that lowers the coefficient of adhesive friction. However, migration will be hindered in the additional presence of other fillers, as slip agents tend to adsorb on the surface of the former. Browning et al. [9] noted that the incorporation of slip agents like erucamide (Fig. 16.2) to thermoplastic olefin (TPO) resulted in a significant decrease in the sliding friction coefficient during scratching, thereby improving the scratch resistance. We have also used stearic acid (SA) (octadecanoic acid, CH3(CH2)16COOH, Fig. 16.2), a saturated fatty acid with a long hydrophobic (aliphatic) tail (CH3(CH2)16) and a hydrophilic head (–COOH), as a slip agent in polypropylene (PP) and PP/(calcium carbonate) CaCO3 systems [10]. In neat PP, the coefficient of friction was similar at all the wear testing conditions studied. This was attributed to similar wear mechanisms operating under all testing conditions (owing to frictional heating). When 1.5 phr of SA was incorporated, friction was reduced (and also specific wear rate) irrespective of the presence of CaCO3 or its content. For example, at a sliding speed of 0.1 m s−1 and a load of 7 N, the friction coefficient for neat PP was 0.296 ± 0.01, while for PP with unmodified 15 wt%
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OH
Carboxylic group
Stearic Acid O
O
H3C
H2N Amine functional group
-CH3 non-polar group
Erucamide
FIGURE 16.2 Chemical structures of stearic acid and erucamide. For color version of this figure, the reader is referred to the online version of this book.
CaCO3, PP/SA blend (1.5 phr SA), and PP with 1.5 phr SA-treated CaCO3 are 0.329 ± 0.01, 0.237 ± 0.007, and 0.241 ± 0.005, respectively. Ha et al. [11] noted that friction coefficients and wear volume losses of epoxy/ clay nanocomposites at low loadings of 2–6 wt% were higher than that of neat epoxy. When the loading increased to 10 wt%, significant reductions in both frictional coefficient and wear loss were noted (Fig. 16.3). They believed that the clay layers acted as a lubricating material with increase in clay concentration. This result cannot be clarified as even the clay layers were not (organically) modified in this study and actually negates the idea of a surfactant acting as a lubricating/slip agent. Besides, why this phenomenon occurs only at 10 wt% is unclear. In another study, the concept of (debonded) nanoparticles acting as rolling balls in retarding the wear and coefficient of friction was extended to even scratching of a polymer nanocomposite system [12]. A polycarbonate (PC)/SiO2 system was scratched with loads of 500, 1000, 2000, and 3000 μN to a distance of 10 μm. As frictional coefficients and scratch penetration depths of the composites are lower than those of neat PC under all testing conditions investigated (Fig. 16.4), the authors suggested that spherical-like SiO2 nanoparticles detached from the matrix during the scratching process and acted as rolling balls between the counterface slider and surface. It is hard to imagine that this process can occur even during a one-step scratching process, as generally, this was suggested to occur during the wearing process after the surface material disintegrates over a period of time. It is also important to note that although several studies on wearing of polymer nanocomposites have used this concept to explain their results, no clear evidence was provided to date to substantiate this. For instance, in a study on the tribological properties of epoxy nanocomposites, Chang et al. [13] proposed a positive rolling effect of the nanoparticles to interpret the remarkable reduction in the frictional coefficient after the addition of nano-TiO2. Pendleton et al. [14] used fullerene as an additive to biofluids and investigated the lubrication response of total joint replacements (TJRs). This is to develop an alternative
16.2 Wear/Scratch Damage in Polymer Nanocomposites
FIGURE 16.3 Wear volume loss of epoxy and its nanocomposites at different clay loadings. Wear testing conditions: counterface slider, carbon-steel ball (∼12.7 mm in diameter); sliding distance, 3251 m; applied load, 10 N; and sliding speed, 0.12 m s−1.
FIGURE 16.4 (a) Average scratch depth and (b) coefficient of friction of neat PC and PC/SiO2 nanocomposite at different applied loads.
to the commonly used in vivo lubrication with periprosthetic synovial fluid, which has to be done for many years after the surgery to restrict TJRs deterioration via wear. In the study, the authors also compared the wear performance of fullerene with a ringlike molecule, crown ether. These additives were simply added to deionized water to make ~37.6 wt%, or 27.2 vol% and 24.9 vol% of crown ether and fullerene, respectively. Tribological testing was done in a reciprocal motion using a ball-on-disc tribometer,
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with a Ti-6Al-4V ball against a pure titanium disc. During the tribological experiment, 1 ml of fullerene or crown ether-containing fluid was applied to the substrate using a pipette. Results indicated that fullerene provided the lowest coefficient of friction under the testing conditions (sliding speeds varying from 0.1 to 0.5 cm s−1, track length 1 cm and applied loads 1, 4, and 5 N) (see Fig. 16.5(a)). They further performed transmission electron microscopic (TEM) investigations of crown ether and fullerene particles before and after wear tests to understand the reasons behind the results and found that crown
FIGURE 16.5 (a) Coefficient of friction for the three fluids tested over a period of time. (b–e) TEM images of additives before (b, d) and after (c, e) wear testing; (b, c) crown ether and (d, e) fullerene. For color version of this figure, the reader is referred to the online version of this book.
16.2 Wear/Scratch Damage in Polymer Nanocomposites
ether molecules were adhered together during the wearing process, plausibly due to friction (Fig. 16.5(b), (c)), while fullerene particles were separated (Fig. 16.5(d), (e)). However, it is not clear how crown ether particles aggregated and why this affects the tribological properties as they are still in the size range of 100–400 nm compared to 50–200 nm sizes of fullerene. Moreover, the slider diameter was 6 mm.
16.2.2 Evaluation of scratch/wear resistance Liu et al. [15] correlated the dielectric properties of high-density polyethylene (HDPE)/carbon nanofiber (CNF) nanocomposites to the magnitude of wearing, that is, using dielectric response as a reflection of dipole movements (in insulating materials), which is very sensitive to the polar groups in polymers. Evidently, this is system dependent and only for qualitative comparison; any attempt to quantitatively evaluate the performance will result in misleading conclusions. For instance, it is hard to envelop the effects of frictional heat, ups/downs in the wear coefficient with time, and lubrication among other factors. Nevertheless, to enhance the interaction of CNF with HDPE, silanization was used, which also provides a nonpolar hydrocarbon layer on top of the nanofiber surface. During the wear process, it was believed that cutting some polymer chains and separating nanofibers from the polymer matrix, as well as causing damage to the surface of the nanofiber, would create polar groups. Therefore, with longer wear time, the dielectric response could be stronger since more wear would result in more polar groups on or near the surface. To clarify this, the authors also varied the thickness of silane coating on nanofibers and obtained reasonable correlation with permittivity (see Table 16.1). Figure 16.6 also indicates that the permittivity increases linearly with increasing wear coefficient. In another approach, plasticity index obtained via indentation was used as an indicator of scratch penetration depth. This index is defined as ψ = Wir/(Wir + Wr), where Wir and Wr represent the irreversible work done during the indentation and the reversible work recovered by viscoelastic processes during the unloading, respectively (see Fig. 16.7(a)). Therefore, the lower the plasticity index, the higher the elasticity recovery. To illustrate this, epoxy/silica nanocomposite was used as an example, for which the plasticity index values at different loadings are shown in Fig. 16.7(b) along
Table 16.1 Effect of Silane Coating Thickness on CNFs versus Permittivity of the Nanocomposites Before and After Wear Testing
Samples
Thickness of Silane Coating on CNF Surface (nm)
Permittivity Before Wearing
Permittivity After 120 h of Wear Process
Nanocomp-ox Nanocomp-A Nanocomp-B Nanocomp-C
0 1.2 2.8 46
5.2 4.8 4.6 4.1
6.3 5.3 4.9 4.4
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FIGURE 16.6 Permittivity trends at 1 × 103 Hz versus corresponding wear coefficient of HDPE/CNF nanocomposites with untreated and treated CNFs. Compositions and designations of materials are identified in Table 16.1. For color version of this figure, the reader is referred to the online version of this book.
with the scratch penetration depths (Fig. 16.7(c)). About 8% of SiO2/epoxy samples exhibited the lowest plasticity index and correspondingly better scratch resistance. Clearly, this approach cannot accommodate the frictional heat (tangential force) and distinctive changes in scratch damage mechanisms. Devaprakasam et al. [16] compared the tribological properties (i.e. friction and wear) at the nanoscale for a nanocomposite and a microcomposite based on a resin matrix consisting of various monomeric dimethacrylates, bisphenol-A-glycidyldimethacrylate, triethylene glycoldimethacrylate, and urethane dimethacrylate. Both nano- and microscale SiO2 particles at a volume fraction of 56% were used. They noted that the average friction force was always higher for the microcomposite than the nanocomposite for a given applied load (for a small scan area 1 × 1 μm2 using the lateral force microscopy mode of atomic force microscopy, AFM). The wear depths of both composites increased with increasing number of scans (Fig. 16.8(a)); a significant increase was noted with the microcomposite compared to the nanocomposite (wear depths were measured after scanning 10 × 10 μm2 area with a load of 15 μN). In addition, based on the AFM phase images, they noted that energy dissipation of the microcomposite during the wear process was higher due to its high surface heterogeneity (Fig. 16.8(b) and (c)); that is, inelastic collisions of an AFM tip with sharp edges on the surface resulted in higher phase differences than the nanocomposite. However, correlating the phase differences to the energy dissipation is not as direct
16.2 Wear/Scratch Damage in Polymer Nanocomposites
FIGURE 16.7 (a) Schematic showing the calculation process of plasticity index, (b) plasticity index and (c) residual depth of epoxy/silica nanocomposites at different loadings of silica. For color version of this figure, the reader is referred to the online version of this book.
as claimed by the authors. Phase differences are always expected even for a simple polymer blend whether it is ductile or brittle. Even the protrusions of the microparticles (as they are not uniformly covered with resin) from the composite surface could result in significant phase differences. For the nanocomposite, considering the size, it is expected that the nanoparticles (even on the surface) were well covered by the resin and might not result in such large phase differences. Chang et al. [17] used poly(2-hydroxyethyl methacrylate) as a matrix and silica nanoparticles modified by a silane, 3-trimethoxysilyl propyl methacrylate, to prepare hard hybrid coatings. To reduce particle–particle aggregation, before incorporation of silane-modified nanoparticles in the matrix, they were further modified by a capping agent, trimethyethoxylsilane (TMES) to consume the Si-OH groups on their surface. They used polyethylene terepthalate as a substrate for these ultra violet-curable coatings.
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FIGURE 16.8 (a) Maximum wear depth versus the number of scanning laps for micro- and nanocomposites; (b and c) phase images of micro- and nanocomposites, respectively. For color version of this figure, the reader is referred to the online version of this book.
However, it is surprising to note that the pencil hardness for all hybrid coatings is the same irrespective of the loadings of silica and TMES (a value of 4H). Although the authors suggested increased cross-linking as a possible reason for this result, the substrate effect could not be excluded. As the coating thickness is only 15 μm, the application of pencil hardness for characterization is questionable. They have also conducted the Taber abrasion test, but only tested the transmittance after the test and revealed that similar transmission results under all conditions (varied from 66 to 70%) including the neat matrix. Similarly, Yao et al. [18] prepared water-soluble polyurethanebased CaCO3 coatings on glass slides, and based on their pencil hardness tests (all the values are essentially between 2H and 3H), they concluded that reinforced coatings are scratch resistant. This again reiterates the need for appropriate application of testing techniques to delineate the differences among the materials and/or rank candidate
16.2 Wear/Scratch Damage in Polymer Nanocomposites
materials. Seo and Han [19] also reported that the pencil hardness of acrylate coatings increased with the percentage loading of silane coupling agent-modified silica. They attributed this to the chemical bonding between the particles and the matrix. However, their resistance to penetration decreased with more than 30 phr of silica nanoparticles. Similarly, Lin and Kim [20] noted that the friction coefficient of 10-μm-thick polymethyl methacrylate (PMMA) and polystyrene (PS)-based silica composite coatings on silicon substrate increased by more than 60% with silica loading (up to 0.3 wt%) in all composites independent of the matrix. It was also noted that the depth of the worn groove reduced in PMMA/silica composites and increased in PS/silica composites compared to their corresponding neat polymers. However, no specific details on the depth or width of the worn groove were listed, which makes it even harder to compare between samples. Besides, these differences were attributed to the compatibility and interfacial bonding variation of silica with PMMA and silica with PS; however, no supporting information was presented to validate the proposition.
16.2.3 Surface roughness Surface roughness is another parameter that affects the surface stresses and frictional properties and thereby the scratch/wear response of polymeric materials. When two surfaces slide against each other, the actual contact is between the asperities of the two surfaces. Junctions will be formed at these contact regions due to physical or chemical interactions possibly caused by heating at the interface [21]. Depending on the contact surface area of the two interacting surfaces, the stress state and the resulting deformation can change substantially during testing. Several attempts were made to model the effect of surface roughness on the contact stresses during sliding. Comprehensive theories and models of surface roughness effects on friction and wear, such as Greenwood–Williamson (G–W) model [22], Majumdar–Bhushan (M–B) model [23], and Cantor set contact models [24,25], have been proposed. The G–W model is based on the assumption that all the asperities have the same radius of curvature, while the M–B model is fractal based on elastic and plastic contact between rough surfaces. Contact models (based on Cantor sets) are also developed on fractal characterization of surfaces aiming to quantify. In general, the deformation undergoes a transition from an elastic regime to a plastic regime as the applied load increases [26]. To account for this transition, a parameter scratch coefficient of friction (SCOF) was introduced. It is the ratio of the tangential force experienced by the scratch tip during scratching to the applied normal force similar to the conventional coefficient of sliding friction. SCOF is divided into two parts [9,27,28] such that SCOF = (μs + μr), where μs is the surface sliding coefficient of friction (dependent on surface adhesion and contacts) and μr accounts for material deformation mechanisms, such as ploughing, cracking and crazing. As expected, at lower applied loads, the magnitude of scratch-induced deformation will be subtle and μs dominates. At higher loads, an additional contribution from μr should be accounted for due to the inherent material resistance to deformation (Fig. 16.9). Sue and his coworkers [26] correlated surface roughness, applied loads and scratch visibility for a model TPO. They noted that surface roughness was less important at higher
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FIGURE 16.9 SCOF versus applied load for a TPO system with differing surface roughness. For color version of this figure, the reader is referred to the online version of this book.
loads where μr dominates. More importantly, the onset of scratch visibility became convoluted as the background average roughness increased. For a given surface background, scratch visibility is an indication of the first position at which the contrast is sufficient to be observed. Therefore, if the surface scatters more visible light and introduces noise, it will mask the onset point. This was also confirmed by studying a textured surface, like the “random animal skin”, which hid the true onset of visible surface damage.
16.2.4 Anisotropic response Considering the asymmetric response of some of the polymers in compression and tension, many of the tribological properties were found to vary with the direction of sliding. Sue and coworkers [3] reported that this behavior will significantly influence the scratching response of polymers as the stress field evolves with the indenter movement during a progressive load scratch test. For this purpose, different model systems were considered with variations in tensile and compressive behaviors (see Table 16.2). Figure 16.10 shows comparisons of shoulder height and scratch depth as a function of the scratch normal load. In the first case, three systems were compared with similar compressive properties, but differ in their tensile properties. Irrespective of these differences, as shown in Fig. 16.10(a) and (b), the shoulder height and scratch depth of the materials remain essentially the same. In the second scenario, two systems were compared with similar tensile strength values and different compressive yield strengths. Clearly, the lower compressive yield strength of PC compared to styrene acrylonitrile (SAN) 19 makes it more susceptible to
16.2 Wear/Scratch Damage in Polymer Nanocomposites
Table 16.2 Tensile and Compressive Properties of Selected Model Systems Property Tensile modulus, GPa Tensile strength, MPa Compressive modulus, GPa Compressive yield strength, MPa
SAN 19
SAN 27B
SAN 27C
Neat PC
3.4 ± 0.0 68.9 ± 1.5 3.5 ± 0.1
3.7 ± 0.1 75.1 ± 3.0 3.6 ± 0.3
3.7 ± 0.1 79.0 ± 1.0 3.5 ± 0.2
2.3 ± 0.0 65.2 ± 0.0 2.1 ± 0.1
117.6 ± 0.8
117.2 ± 0.4
117.2 ± 0.2
75.3 ± 0.7
The numbers after SAN represent the weight percentage of AN; 27B and 27C differ in molecular weight, 119 and 134 kg mol, respectively.
FIGURE 16.10 Comparisons of (a, c) shoulder height and (b, d) scratch penetration depth of neat PC and other model systems as a function of applied normal load illustrating the importance of compressive properties. For color version of this figure, the reader is referred to the online version of this book.
scratch damage resulting in higher shoulder height and deeper scratch depth. This suggests that shoulder height and scratch depth are predominately affected by compression loading properties. However, it should also be noted that SAN 19 exhibited brittle behavior and neat PC yielded a ductile behavior under uniaxial tensile testing conditions.
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In the case of nanocomposites, recently, we have shown that even the orientation and extent of intercalation of clay layers are important parameters influencing the magnitude of scratch damage [29]. Residual depths were lower for scratches performed on the cross-sections (normal to the flow direction) of the nanocomposites compared to those on the surface (parallel to the flow direction), and greater scratch penetration resistance was noted for the nanocomposite with higher intercalation extent of organoclay (Fig. 16.11). This clearly suggests that the orientation and dispersion
FIGURE 16.11 (a) Schematic of two different locations on an injection-molded sample (parallel and normal to flow direction) where scratch tests are performed showing the orientation of clay platelets at both locations and (b) average scratch residual penetration depth for neat polyamide (PA) 6 (A0), exfoliated PA 6/organoclay (90/10) nanocomposite (A1) and intercalated PA 6/organoclay (90/10) composite (A2) at different loads parallel and normal to the injection-molding direction using a Berkovich indenter. For color version of this figure, the reader is referred to the online version of this book.
16.3 Coatings
Table 16.3 Instantaneous and Residual Penetration Depths and Viscoelastic Recovery of PC and PC/ZnO Composite Along Longitudinal and Transverse (Sliding) Directions Sliding Direction: Longitudinal
Sliding Direction: Transverse
Material
Penetra tion Depth (μm)
Residual Depth (μm)
Visco elastic Recovery (%)
Penetra tion Depth (μm)
Residual Depth (μm)
Visco elastic Recovery (%)
PC PC/ZnO
292.8 157.3
37.5 45.9
87.7 70.8
130.2 143.1
48.8 59.4
62.5 58.5
Viscoelastic recovery = [1−(Rh/Rp)] × 100; where Rp instantaneous penetration depth and Rh is the residual depth (measured after 2 min of testing).
of clay layers must be simultaneously considered in determining the effective structural reinforcement in polymer/clay nanocomposites. Otherwise, the results can be misleading. In another similar study, Bermudez et al. [30] also showed that neat PC exhibited an anisotropic behavior during scratching; that is, the scratch response was dependent on the direction of scratching. It was observed that the instantaneous penetration depth for neat PC increased by almost 55% in the longitudinal direction compared to the transverse direction under progressive loading (0.03–29 N) scratch test at the highest applied load (Table 16.3). However, in contrast to the abovediscussed study [29], with the incorporation of ZnO nanoparticles, the anisotropic behavior was minimal, and a mere 9.9% increase was noted under similar conditions. Even the residual penetration depth varied with the sliding direction, but in this case, both PC and PC/ZnO materials showed a similar variation of ~23% increase in the transverse direction compared to the longitudinal direction. These differences were attributed to the variations in viscoelastic recovery of PC and PC/ZnO composite in the longitudinal and transverse directions. However, this reasoning cannot answer the differences in the instantaneous penetration depths (Rp) between longitudinal and transverse directions, which are significant.
16.3 COATINGS Generally, in a metal–metal contact (such as drilling and cutting tools) and in dry sliding systems, to prevent the abrasive and adhesive wear of the softer material by the harder material, application of functional coatings is one of the common techniques utilized. With much advancement in coatings technology, deposition of films (or multilayered films) with tailored properties is achievable. Many such coatings were developed depending on the specific purpose; soft coatings were chosen particularly for lubricating purposes and hard (and superhard) coatings for load bearing as well as sacrificial layers [31–34]. Examples of hard coating materials
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include TiN, MoS2, TiC, Al2O3, diamond, and diamond-like carbon (DLC) as single or multilayer combinations. Particularly, DLC coatings have found tremendous applications as they decreased the coefficient of friction and the wear rate by more than an order of magnitude. The presence of hydrogen up to 40 at% in these materials strongly influences their mechanical and tribological behaviors [35]. Depending on the requirement of these coatings, other elements like nitrogen, silver, silicon, and tungsten were also incorporated. In some specific cases, very low coefficients of friction 0.003–0.008 in dry sliding were also achieved [36]. For some applications like magnetic recording discs, a combination of hard DLC and soft polymeric coatings were used. Cavallin et al. [37] used cold plasma enhanced chemical vapor deposition to treat poly(vinyl chloride) (PVC) and PC surfaces with TiO2 coatings with an objective to increase surface hardness and induce superhydrophilic characteristics. Although hardness was increased (an average value of 0.57 GPa for PVC with TiO2 compared to 0.15 GPa for neat PVC and 0.78 GPa for PC with TiO2 compared to 0.23 GPa for neat PC) and contact angles were reduced to <10° from 65 to 80° for neat materials, there were huge cracks in the coatings (see Fig. 16.12 for TiO2 on PC). The island-like structures clearly suggest the differences in the thermal expansion coefficients of the polymeric materials and TiO2 coating. Besides, the interaction (adhesion) between the substrate and coating is not considered in the study. In another investigation, polyetherimide (PEI), polyether-ether-ketone (PEEK) and PA 12 materials were deposited on low-carbon steel by low-velocity flame spray technology with an optimized spray distance of 85 mm and a preheating temperature of 230 °C [32]. The
FIGURE 16.12 SEM micrograph of an 8-μm-thick TiO2 coating on PC substrate demonstrating the islandlike structures.
16.4 Concluding Remarks
average coating thickness was ~300 μm. Three-body abrasion tests (abrasive ~250-μm silica dry sand, applied load ~45 N, 6000 revolutions and a sand flow ~400 g min−1) were performed on the coatings, which revealed that the performance of PA 12 coating was far better than the other two. The mass loss value of PA 12 was ~50% of that of PEEK and ~20% less than PEI values. This is despite minor differences in the (tensile) adhesion values and microhardness of the coatings. However, the authors neither provided any reasons for the differences nor studied the mechanisms of wear, which makes it tough to deduce any specific reason for this behavior. Despite the many advancements in this field, particularly deposition technologies, there are many issues to be addressed. Some studies have already been directed to understand the significance of properties of coating materials such as elastic modulus, hardness or shear strength and fracture toughness as well as other parameters like critical load in scratching [38]. However, there are many other parameters that should be completely evaluated. For inorganic hard coatings (on inorganic substrates), intrinsic stresses are critical in many studies resulting in failure. These are a combination of stresses induced during the deposition/growth process of coatings and due to the mismatch of coating–substrate thermomechanical properties [39]. For polymeric substrates, parameters like chemical inertness, thermal resistance and structural weakness to ultraviolet radiation and ion bombardment were also important. As they change with the inherent properties of the polymer, the effects of these parameters vary from system to system. Another important facet is the adhesion of coatings and substrates. For polymer–polymer systems, many approaches were already established like the chemical functionalization through the use of reactive groups or the creation of a broad interface region that enables the realization of covalent bonds. However, for polymer-inorganic systems, as mentioned before, there are still many questions to be answered.
16.4 CONCLUDING REMARKS As discussed in this chapter, in some of the contributions, novel approaches were adopted to tackle the issues of friction and wear, while some dealt with enhancing our understanding of the existing tribological knowledge of polymer nanocomposites. Unfortunately these contributions are limited in number, and the majority of the studies, although considered issues of serious concern, failed either at the approach level or in providing a deeper insight into the wear/scratch/friction mechanisms. This in turn has indirect impacts, particularly when dealing with biomedical applications, as more research is diverted into bioceramics and biocompatible polymer nanocomposites that experience tribological contacts at different length scales. Another new area of promise is the usage of polymer-based nanocomposites as shoe soles, particularly in the sports field. The potential of nanoparticles to enhance adhesion and friction between the materials and surface would be critical in these applications. Obviously the usage of polymer nanocomposites in these different fields under different contact conditions requires the overcoming of several hurdles discussed earlier.
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Acknowledgments We thank Singapore's Ministry of Education through AcRF Tier 1 grant (RG45/11), the Start-up Grant from Nanyang Technological University, Australian Research Council and the National Natural Science Foundation of China (50873006 and 51125010) for financially supporting our research on various issues of polymer nanocomposites.
References [1] A. Dasari, Z.Z. Yu, Y.-W. Mai, Wear and scratch damage in polymer nanocomposites, . (Chapter 16), in: K. Friedrich, A.K. Schlarb (Eds.), Tribology of Polymeric Nanocomposites, vol. 55, Elsevier, Oxford, 2008, pp. 374–399. [2] A. Dasari, Z.Z. Yu, Y.-W. Mai, Fundamental aspects and recent progress on wear/scratch damage in polymer nanocomposites, Materials Science and Engineering R: Reports 63 (2009) 31–80. [3] M.M. Hossain, R. Browning, R. Minkwitz, H.-J. Sue, Effect of asymmetric constitutive behavior on scratch-induced deformation of polymers, Tribology Letters 47 (2012) 113–122. [4] M.M. Hossain, H. Jiang, H.-J. Sue, Effect of constitutive behavior on scratch visibility resistance of polymers: a finite element method parametric study, Wear 270 (2011) 751–759. [5] H. Jiang, G.T. Lim, J.N. Reddy, J.D. Whitcomb, H.J. Sue, Finite element method parametric study on scratch behavior of polymers, Journal of Polymer Science Part B: Polymer Physics 45 (2007) 1435–1447. [6] Q.B. Guo, K.T. Lau, M.Z. Rong, M.Q. Zhang, Optimization of tribological and mechanical properties of epoxy through hybrid filling, Wear 269 (2010) 13–20. [7] A.S. Rawls, D.E. Hirt, M.R. Havens, W.P. Roberts, Evaluation of surface concentration of erucamide in LLDPE films, Journal of Vinyl and Additive Technology 8 (2002) 130–138. [8] J. Edenbaum, Plastics Additives and Modifiers Handbook, vol. 107, Chapman & Hall, London, 1996. [9] R. Browning, G.T. Lim, A. Moyse, L. Sun, H.-J. Sue, Effects of slip agent and talc surface-treatment on the scratch behavior of thermoplastic olefins, Polymer Engineering & Science 46 (2006) 601–608. [10] A. Dasari, On Toughening and Wear/scratch Damage in Polymer Nanocomposites, PhD School of Aerospace, Mechanical and Mechatronic Engineering, The University of Sydney, Sydney, 2007, http://hdl.handle.net/2123/1911. [11] S.-R. Ha, K.-Y. Rhee, H. Shin, Effect of MMT concentration on tribological behavior of MMT/Epoxy nanocomposite, Journal of Nanoscience and Nanotechnology 8 (2008) 4869–4872. [12] Z.Z. Wang, P. Gu, Z. Zhang, Indentation and scratch behavior of nano-SiO2/polycarbonate composite coating at the micro/nano-scale, Wear 269 (2010) 21–25. [13] L. Chang, Z. Zhang, C. Breidt, K. Friedrich, Tribological properties of epoxy nanocomposites I. Enhancement of the wear resistance by nano-TiO2 particles, Wear 258 (2005) 141–148. [14] A. Pendleton, P. Kar, S. Kundu, S. Houssamy, H. Liang, Effects of nanostructured additives on boundary lubrication for potential artificial joint applications, Journal of Tribology 132 (2010) 031201.
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[15] T. Liu, W. Wood, W.-H. Zhong, Sensitivity of dielectric properties to wear process on carbon nanofiber/high-density polyethylene composites, Nanoscale Research Letters, 2010. [16] D. Devaprakasam, P.V. Hatton, G. Möbus, B.J. Inkson, Nanoscale tribology, energy dissipation and failure mechanisms of nano- and micro-silica particle-filled polymer composites, Tribology Letters 34 (2008) 11–19. [17] C.-C. Chang, T.-Y. Oyang, F.-H. Hwang, C.-C. Chen, L.-P. Cheng, Preparation of polymer/silica hybrid hard coatings with enhanced hydrophobicity on plastic substrates, Journal of Non-crystalline Solids 358 (2012) 72–76. [18] L. Yao, J. Yang, J. Sun, L. Cai, L. He, H. Huang, R. Song, Y. Hao, Hard and transparent hybrid polyurethane coatings using in situ incorporation of calcium carbonate nanoparticles, Materials Chemistry and Physics 129 (2011) 523–528. [19] J.Y. Seo, M. Han, Multi-functional hybrid coatings containing silica nanoparticles and anti-corrosive acrylate monomer for scratch and corrosion resistance, Nanotechnology 22 (Jan 14 2011) 025601. [20] L.-Y. Lin, D.-E. Kim, Tribological properties of polymer/silica composite coatings for microsystems applications, Tribology International 44 (2011) 1926–1931. [21] B.J. Briscoe, B.H. Stuart, S. Sebastian, P.J. Tweedale, The failure of poly (ether ether ketone) in high-speed contacts, Wear 162 (Apr 1993) 407–417. [22] J.A. Greenwood, J.B.P. Williamson, Contact of nominally flat surfaces, Proceedings of the Royal Society of London: Series A – Mathematical Physical and Engineering Sciences 295 (1966) 300–319. [23] A. Majumdar, B. Bhushan, Fractal model of elastic-plastic contact between rough surfaces, Journal of Tribology-transactions of the ASME 113 (1991) 1–11. [24] T.L. Warren, D. Krajcinovic, Random Cantor set models for the elastic-perfectly plastic contact of rough surfaces, Wear 196 (1996) 1–15. [25] G. Liu, Q.J. Wang, C. Lin, A survey of current models for simulating the contact between rough surfaces, Tribology Transactions 42 (Jul 1999) 581–591. [26] H. Jiang, R. Browning, J. Fincher, A. Gasbarro, S. Jones, H.-J. Sue, Influence of surface roughness and contact load on friction coefficient and scratch behavior of thermoplastic olefins, Applied Surface Science 254 (2008) 4494–4499. [27] H. Jiang, R.L. Browning, M.M. Hossain, H.-J. Sue, M. Fujiwara, Quantitative evaluation of scratch visibility resistance of polymers, Applied Surface Science 256 (2010) 6324–6329. [28] P. Liu, R. Lee Browning, H.-J. Sue, J. Li, S. Jones, Quantitative scratch visibility assessment of polymers based on Erichsen and ASTM/ISO scratch testing methodologies, Polymer Testing 30 (2011) 633–640. [29] A. Dasari, Z.-Z. Yu, Y.-W. Mai, J.-K. Kim, Orientation and the extent of exfoliation of clay on scratch damage in polyamide 6 nanocomposites, Nanotechnology 19 (2008) 055708. [30] M.D. Bermúdez, W. Brostow, F.J. Carrión-Vilches, J. Sanes, Scratch resistance of polycarbonate containing ZnO nanoparticles: effects of sliding direction, Journal of Nanoscience and Nanotechnology 10 (2010) 6683–6689. [31] A. Rempp, A. Killinger, R. Gadow, New approach to ceramic/metal-polymer multilayered coatings for high performance dry sliding applications, Journal of Thermal Spray Technology 21 (2012) 659–667. [32] C.R.C. Lima, N.F.C. de Souza, F. Camargo, Study of wear and corrosion performance of thermal sprayed engineering polymers, Surface and Coatings Technology, 2012.
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[33] X. Guan, L. Wang, The tribological performances of multilayer graphite-like carbon (GLC) coatings sliding against polymers for mechanical seals in water environments, Tribology Letters 47 (2012/07/01) 67–78. [34] S.K. Field, M. Jarratt, D.G. Teer, Tribological properties of graphite-like and diamondlike carbon coatings, Tribology International 37 (2004) 949–956. [35] G. Dearnaley, J.H. Arps, Biomedical applications of diamond-like carbon (DLC) coatings: a review, Surface and Coatings Technology 200 (2005) 2518–2524. [36] J.A. Heimberg, K.J. Wahl, I.L. Singer, A. Erdemir, Superlow friction behavior of diamond-like carbon coatings: time and speed effects, Applied Physics Letters 78 (2001) 2449. [37] T. Cavallin, N. El Habra, M. Casarin, F. Bordin, A. Sartori, M. Favaro, R. Gerbasi, G. Rossetto, Superhydrophilic and tribological improvements of polymeric surfaces via plasma enhanced chemical vapor deposition Ceramic coatings, Journal of Nanoscience and Nanotechnology 11 (2011) 8079–8082. [38] D.L. Burris, B. Boesl, G.R. Bourne, W.G. Sawyer, Polymeric nanocomposites for tribological applications, Macromolecular Materials and Engineering 292 (2007) 387–402. [39] K.H. Lau, K.Y. Li, Y.-W. Mai, Influence of hardness ratio on scratch failure of coatings, International Journal of Surface Science and Engineering 1 (2007) 3–21.
Wear and scratch damage in polymer nanocomposites
16
Aravind Dasari*, Zhong-Zhen Yu†, Yiu-Wing Mai‡ *School of Materials Science & Engineering, Nanyang Technological University, Singapore, Key Laboratory of Organic-Inorganic Composites, Department of Polymer Engineering, College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing, China, ‡Centre for Advanced Materials Technology (CAMT), School of Aerospace, Mechanical and Mechatronic Engineering, The University of Sydney, Sydney, NSW, Australia
†State
CHAPTER OUTLINE HEAD 16.1 Background���������������������������������������������������������������������������������������������������� 551 16.2 Wear/Scratch Damage in Polymer Nanocomposites������������������������������������������� 552 16.2.1 Effect of coefficient of friction��������������������������������������������������������� 552 16.2.2 Evaluation of scratch/wear resistance����������������������������������������������� 557 16.2.3 Surface roughness�������������������������������������������������������������������������� 561 16.2.4 Anisotropic response����������������������������������������������������������������������� 562 16.3 Coatings��������������������������������������������������������������������������������������������������������� 565 16.4 Concluding Remarks��������������������������������������������������������������������������������������� 567 Acknowledgments..................................................................................................... 568 References�������������������������������������������������������������������������������������������������������������� 568
16.1 BACKGROUND In our previous chapter on “Wear and Scratch Damage in Polymer Nanocomposites” (Chapter 16 of first edition), a review of wear/scratch damage processes occurring in various polymer nanocomposite systems was presented and the parameters responsible for controlling the surface integrity and material removal were deduced and described [1]. In short, we have pointed to the inherent tribological complexities with polymer nanocomposites and the qualitative nature of specified/identified mechanisms. Results varied widely from study to study with only subtle changes of testing conditions, material/filler or even characterization techniques. Considering the complexities involved in quantitatively evaluating the tribological response of polymer nanocomposites, limited efforts have been made to model stress fields induced by different slider geometries, and no explicit correlations between material parameters and wear/scratch damage, particularly for polymer nanocomposites at nanoscale, were available. Tribology of Polymeric Nanocomposites. http://dx.doi.org/10.1016/B978-0-444-59455-6.00016-7 Copyright © 2013 Elsevier B.V. All rights reserved.
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Further, it was suggested that the presence of nanoparticles by themselves or the improved material mechanical properties do not always result in improvements in wear performance [1,2]. However, the formation of uniform and stable transferred materials adhered to the counterface as a result of tribochemical reactions or by some other means between the fillers and the slider counterface was a governing wear response factor. Other reasons were also given including the generation of free radicals during the sliding process, polishing of the counterface slider by fillers, and wear debris acting like a lubricant or third-body in reducing the wear rate by a rolling action. Specifically, during scratching of polymeric materials, the yield stress was considered as the main parameter influencing the scratch depth [3–5]. Moreover, the coefficient of adhesive friction and strain at stress recovery were found to affect the scratch depth and shoulder height. Besides, all these reasons were directly or indirectly influenced by size, volume fraction, geometry, orientation and dispersion of nanoparticles in the polymer matrix. In this chapter, we will update the status of the advancements/knowledge in the field in the past 5 years. For prior knowledge and background on this subject area, interested readers are encouraged to consult our previous contributions [1,2].
16.2 WEAR/SCRATCH DAMAGE IN POLYMER NANOCOMPOSITES As mentioned in our earlier review, added to the coupling of inherent tribological complexities, poor characterization of nanocomposites and their properties often made interpretations of experimental results more difficult. This was even the case with some of the recent studies. However, there are many others that revealed some interesting results with novel approaches. Below, we review both types of studies and delineate the issues of importance.
16.2.1 Effect of coefficient of friction Analogous to the approach of self-healing of epoxy-based composites by reinforcing with resin-filled capsules/fibers, Zhang and coworkers [6] adopted an approach by adding lubricant oil-loaded microcapsules (8 phr) into hybrid epoxy composites (containing 1 phr short carbon fibers and 5 phr silica (SiO2) nanoparticles). It was expected that during wear/scratch conditions, the capsules would break, releasing the oil in the contact area, and thereby reducing the frictional coefficient and material loss. As shown in Fig. 16.1, the presence of oil-loaded microcapsules alone has a significant effect on wear resistance (increased by 60 times) and coefficient of friction (reduced by ~75%) compared to neat epoxy; additional presence of short carbon fiber and/or silica nanoparticles has little effect on tribological properties. However, flexural strength and modulus of these hybrid composites improved considerably compared to epoxy with oil-loaded microcapsules. But, these improvements are negligible when compared to neat epoxy. Although this approach seems to yield positive results on tribological properties, the probability of fracture of oil-loaded
16.2 Wear/Scratch Damage in Polymer Nanocomposites
FIGURE 16.1 Specific wear rate and friction coefficient of epoxy and its composites. Sliding wear test conditions: block-on-ring apparatus, carbon-steel ring, constant velocity ~0.42 m s−1 and a constant pressure ~3 MPa. Recipe #1, 8 phr oil-loaded capsules; recipe #2, 8 phr oil-loaded capsules and 5 phr SiO2; recipe #3, 8 phr oil-loaded capsules and 1 phr short carbon fiber; and recipe #4, 8 phr oil-loaded capsules along with 5 phr SiO2 and 1 phr short carbon fiber.
microcapsules during the manufacturing process of the composites is large. This in turn results in localized pockets of oil in the matrix, which might affect not only the processability of these materials but also some other properties. Similarly, some studies have also noted that to improve wear/scratch resistance, addition of slip agents (mainly derived from fatty acids) in polymers is the best approach. These compounds generally “bloom” or migrate to the surface of the polymer (during the processing step) [7,8] forming a waxy layer that lowers the coefficient of adhesive friction. However, migration will be hindered in the additional presence of other fillers, as slip agents tend to adsorb on the surface of the former. Browning et al. [9] noted that the incorporation of slip agents like erucamide (Fig. 16.2) to thermoplastic olefin (TPO) resulted in a significant decrease in the sliding friction coefficient during scratching, thereby improving the scratch resistance. We have also used stearic acid (SA) (octadecanoic acid, CH3(CH2)16COOH, Fig. 16.2), a saturated fatty acid with a long hydrophobic (aliphatic) tail (CH3(CH2)16) and a hydrophilic head (–COOH), as a slip agent in polypropylene (PP) and PP/(calcium carbonate) CaCO3 systems [10]. In neat PP, the coefficient of friction was similar at all the wear testing conditions studied. This was attributed to similar wear mechanisms operating under all testing conditions (owing to frictional heating). When 1.5 phr of SA was incorporated, friction was reduced (and also specific wear rate) irrespective of the presence of CaCO3 or its content. For example, at a sliding speed of 0.1 m s−1 and a load of 7 N, the friction coefficient for neat PP was 0.296 ± 0.01, while for PP with unmodified 15 wt%
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OH
Carboxylic group
Stearic Acid O
O
H3C
H2N Amine functional group
-CH3 non-polar group
Erucamide
FIGURE 16.2 Chemical structures of stearic acid and erucamide. For color version of this figure, the reader is referred to the online version of this book.
CaCO3, PP/SA blend (1.5 phr SA), and PP with 1.5 phr SA-treated CaCO3 are 0.329 ± 0.01, 0.237 ± 0.007, and 0.241 ± 0.005, respectively. Ha et al. [11] noted that friction coefficients and wear volume losses of epoxy/ clay nanocomposites at low loadings of 2–6 wt% were higher than that of neat epoxy. When the loading increased to 10 wt%, significant reductions in both frictional coefficient and wear loss were noted (Fig. 16.3). They believed that the clay layers acted as a lubricating material with increase in clay concentration. This result cannot be clarified as even the clay layers were not (organically) modified in this study and actually negates the idea of a surfactant acting as a lubricating/slip agent. Besides, why this phenomenon occurs only at 10 wt% is unclear. In another study, the concept of (debonded) nanoparticles acting as rolling balls in retarding the wear and coefficient of friction was extended to even scratching of a polymer nanocomposite system [12]. A polycarbonate (PC)/SiO2 system was scratched with loads of 500, 1000, 2000, and 3000 μN to a distance of 10 μm. As frictional coefficients and scratch penetration depths of the composites are lower than those of neat PC under all testing conditions investigated (Fig. 16.4), the authors suggested that spherical-like SiO2 nanoparticles detached from the matrix during the scratching process and acted as rolling balls between the counterface slider and surface. It is hard to imagine that this process can occur even during a one-step scratching process, as generally, this was suggested to occur during the wearing process after the surface material disintegrates over a period of time. It is also important to note that although several studies on wearing of polymer nanocomposites have used this concept to explain their results, no clear evidence was provided to date to substantiate this. For instance, in a study on the tribological properties of epoxy nanocomposites, Chang et al. [13] proposed a positive rolling effect of the nanoparticles to interpret the remarkable reduction in the frictional coefficient after the addition of nano-TiO2. Pendleton et al. [14] used fullerene as an additive to biofluids and investigated the lubrication response of total joint replacements (TJRs). This is to develop an alternative
16.2 Wear/Scratch Damage in Polymer Nanocomposites
FIGURE 16.3 Wear volume loss of epoxy and its nanocomposites at different clay loadings. Wear testing conditions: counterface slider, carbon-steel ball (∼12.7 mm in diameter); sliding distance, 3251 m; applied load, 10 N; and sliding speed, 0.12 m s−1.
FIGURE 16.4 (a) Average scratch depth and (b) coefficient of friction of neat PC and PC/SiO2 nanocomposite at different applied loads.
to the commonly used in vivo lubrication with periprosthetic synovial fluid, which has to be done for many years after the surgery to restrict TJRs deterioration via wear. In the study, the authors also compared the wear performance of fullerene with a ringlike molecule, crown ether. These additives were simply added to deionized water to make ~37.6 wt%, or 27.2 vol% and 24.9 vol% of crown ether and fullerene, respectively. Tribological testing was done in a reciprocal motion using a ball-on-disc tribometer,
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with a Ti-6Al-4V ball against a pure titanium disc. During the tribological experiment, 1 ml of fullerene or crown ether-containing fluid was applied to the substrate using a pipette. Results indicated that fullerene provided the lowest coefficient of friction under the testing conditions (sliding speeds varying from 0.1 to 0.5 cm s−1, track length 1 cm and applied loads 1, 4, and 5 N) (see Fig. 16.5(a)). They further performed transmission electron microscopic (TEM) investigations of crown ether and fullerene particles before and after wear tests to understand the reasons behind the results and found that crown
FIGURE 16.5 (a) Coefficient of friction for the three fluids tested over a period of time. (b–e) TEM images of additives before (b, d) and after (c, e) wear testing; (b, c) crown ether and (d, e) fullerene. For color version of this figure, the reader is referred to the online version of this book.
16.2 Wear/Scratch Damage in Polymer Nanocomposites
ether molecules were adhered together during the wearing process, plausibly due to friction (Fig. 16.5(b), (c)), while fullerene particles were separated (Fig. 16.5(d), (e)). However, it is not clear how crown ether particles aggregated and why this affects the tribological properties as they are still in the size range of 100–400 nm compared to 50–200 nm sizes of fullerene. Moreover, the slider diameter was 6 mm.
16.2.2 Evaluation of scratch/wear resistance Liu et al. [15] correlated the dielectric properties of high-density polyethylene (HDPE)/carbon nanofiber (CNF) nanocomposites to the magnitude of wearing, that is, using dielectric response as a reflection of dipole movements (in insulating materials), which is very sensitive to the polar groups in polymers. Evidently, this is system dependent and only for qualitative comparison; any attempt to quantitatively evaluate the performance will result in misleading conclusions. For instance, it is hard to envelop the effects of frictional heat, ups/downs in the wear coefficient with time, and lubrication among other factors. Nevertheless, to enhance the interaction of CNF with HDPE, silanization was used, which also provides a nonpolar hydrocarbon layer on top of the nanofiber surface. During the wear process, it was believed that cutting some polymer chains and separating nanofibers from the polymer matrix, as well as causing damage to the surface of the nanofiber, would create polar groups. Therefore, with longer wear time, the dielectric response could be stronger since more wear would result in more polar groups on or near the surface. To clarify this, the authors also varied the thickness of silane coating on nanofibers and obtained reasonable correlation with permittivity (see Table 16.1). Figure 16.6 also indicates that the permittivity increases linearly with increasing wear coefficient. In another approach, plasticity index obtained via indentation was used as an indicator of scratch penetration depth. This index is defined as ψ = Wir/(Wir + Wr), where Wir and Wr represent the irreversible work done during the indentation and the reversible work recovered by viscoelastic processes during the unloading, respectively (see Fig. 16.7(a)). Therefore, the lower the plasticity index, the higher the elasticity recovery. To illustrate this, epoxy/silica nanocomposite was used as an example, for which the plasticity index values at different loadings are shown in Fig. 16.7(b) along
Table 16.1 Effect of Silane Coating Thickness on CNFs versus Permittivity of the Nanocomposites Before and After Wear Testing
Samples
Thickness of Silane Coating on CNF Surface (nm)
Permittivity Before Wearing
Permittivity After 120 h of Wear Process
Nanocomp-ox Nanocomp-A Nanocomp-B Nanocomp-C
0 1.2 2.8 46
5.2 4.8 4.6 4.1
6.3 5.3 4.9 4.4
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FIGURE 16.6 Permittivity trends at 1 × 103 Hz versus corresponding wear coefficient of HDPE/CNF nanocomposites with untreated and treated CNFs. Compositions and designations of materials are identified in Table 16.1. For color version of this figure, the reader is referred to the online version of this book.
with the scratch penetration depths (Fig. 16.7(c)). About 8% of SiO2/epoxy samples exhibited the lowest plasticity index and correspondingly better scratch resistance. Clearly, this approach cannot accommodate the frictional heat (tangential force) and distinctive changes in scratch damage mechanisms. Devaprakasam et al. [16] compared the tribological properties (i.e. friction and wear) at the nanoscale for a nanocomposite and a microcomposite based on a resin matrix consisting of various monomeric dimethacrylates, bisphenol-A-glycidyldimethacrylate, triethylene glycoldimethacrylate, and urethane dimethacrylate. Both nano- and microscale SiO2 particles at a volume fraction of 56% were used. They noted that the average friction force was always higher for the microcomposite than the nanocomposite for a given applied load (for a small scan area 1 × 1 μm2 using the lateral force microscopy mode of atomic force microscopy, AFM). The wear depths of both composites increased with increasing number of scans (Fig. 16.8(a)); a significant increase was noted with the microcomposite compared to the nanocomposite (wear depths were measured after scanning 10 × 10 μm2 area with a load of 15 μN). In addition, based on the AFM phase images, they noted that energy dissipation of the microcomposite during the wear process was higher due to its high surface heterogeneity (Fig. 16.8(b) and (c)); that is, inelastic collisions of an AFM tip with sharp edges on the surface resulted in higher phase differences than the nanocomposite. However, correlating the phase differences to the energy dissipation is not as direct
16.2 Wear/Scratch Damage in Polymer Nanocomposites
FIGURE 16.7 (a) Schematic showing the calculation process of plasticity index, (b) plasticity index and (c) residual depth of epoxy/silica nanocomposites at different loadings of silica. For color version of this figure, the reader is referred to the online version of this book.
as claimed by the authors. Phase differences are always expected even for a simple polymer blend whether it is ductile or brittle. Even the protrusions of the microparticles (as they are not uniformly covered with resin) from the composite surface could result in significant phase differences. For the nanocomposite, considering the size, it is expected that the nanoparticles (even on the surface) were well covered by the resin and might not result in such large phase differences. Chang et al. [17] used poly(2-hydroxyethyl methacrylate) as a matrix and silica nanoparticles modified by a silane, 3-trimethoxysilyl propyl methacrylate, to prepare hard hybrid coatings. To reduce particle–particle aggregation, before incorporation of silane-modified nanoparticles in the matrix, they were further modified by a capping agent, trimethyethoxylsilane (TMES) to consume the Si-OH groups on their surface. They used polyethylene terepthalate as a substrate for these ultra violet-curable coatings.
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FIGURE 16.8 (a) Maximum wear depth versus the number of scanning laps for micro- and nanocomposites; (b and c) phase images of micro- and nanocomposites, respectively. For color version of this figure, the reader is referred to the online version of this book.
However, it is surprising to note that the pencil hardness for all hybrid coatings is the same irrespective of the loadings of silica and TMES (a value of 4H). Although the authors suggested increased cross-linking as a possible reason for this result, the substrate effect could not be excluded. As the coating thickness is only 15 μm, the application of pencil hardness for characterization is questionable. They have also conducted the Taber abrasion test, but only tested the transmittance after the test and revealed that similar transmission results under all conditions (varied from 66 to 70%) including the neat matrix. Similarly, Yao et al. [18] prepared water-soluble polyurethanebased CaCO3 coatings on glass slides, and based on their pencil hardness tests (all the values are essentially between 2H and 3H), they concluded that reinforced coatings are scratch resistant. This again reiterates the need for appropriate application of testing techniques to delineate the differences among the materials and/or rank candidate
16.2 Wear/Scratch Damage in Polymer Nanocomposites
materials. Seo and Han [19] also reported that the pencil hardness of acrylate coatings increased with the percentage loading of silane coupling agent-modified silica. They attributed this to the chemical bonding between the particles and the matrix. However, their resistance to penetration decreased with more than 30 phr of silica nanoparticles. Similarly, Lin and Kim [20] noted that the friction coefficient of 10-μm-thick polymethyl methacrylate (PMMA) and polystyrene (PS)-based silica composite coatings on silicon substrate increased by more than 60% with silica loading (up to 0.3 wt%) in all composites independent of the matrix. It was also noted that the depth of the worn groove reduced in PMMA/silica composites and increased in PS/silica composites compared to their corresponding neat polymers. However, no specific details on the depth or width of the worn groove were listed, which makes it even harder to compare between samples. Besides, these differences were attributed to the compatibility and interfacial bonding variation of silica with PMMA and silica with PS; however, no supporting information was presented to validate the proposition.
16.2.3 Surface roughness Surface roughness is another parameter that affects the surface stresses and frictional properties and thereby the scratch/wear response of polymeric materials. When two surfaces slide against each other, the actual contact is between the asperities of the two surfaces. Junctions will be formed at these contact regions due to physical or chemical interactions possibly caused by heating at the interface [21]. Depending on the contact surface area of the two interacting surfaces, the stress state and the resulting deformation can change substantially during testing. Several attempts were made to model the effect of surface roughness on the contact stresses during sliding. Comprehensive theories and models of surface roughness effects on friction and wear, such as Greenwood–Williamson (G–W) model [22], Majumdar–Bhushan (M–B) model [23], and Cantor set contact models [24,25], have been proposed. The G–W model is based on the assumption that all the asperities have the same radius of curvature, while the M–B model is fractal based on elastic and plastic contact between rough surfaces. Contact models (based on Cantor sets) are also developed on fractal characterization of surfaces aiming to quantify. In general, the deformation undergoes a transition from an elastic regime to a plastic regime as the applied load increases [26]. To account for this transition, a parameter scratch coefficient of friction (SCOF) was introduced. It is the ratio of the tangential force experienced by the scratch tip during scratching to the applied normal force similar to the conventional coefficient of sliding friction. SCOF is divided into two parts [9,27,28] such that SCOF = (μs + μr), where μs is the surface sliding coefficient of friction (dependent on surface adhesion and contacts) and μr accounts for material deformation mechanisms, such as ploughing, cracking and crazing. As expected, at lower applied loads, the magnitude of scratch-induced deformation will be subtle and μs dominates. At higher loads, an additional contribution from μr should be accounted for due to the inherent material resistance to deformation (Fig. 16.9). Sue and his coworkers [26] correlated surface roughness, applied loads and scratch visibility for a model TPO. They noted that surface roughness was less important at higher
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FIGURE 16.9 SCOF versus applied load for a TPO system with differing surface roughness. For color version of this figure, the reader is referred to the online version of this book.
loads where μr dominates. More importantly, the onset of scratch visibility became convoluted as the background average roughness increased. For a given surface background, scratch visibility is an indication of the first position at which the contrast is sufficient to be observed. Therefore, if the surface scatters more visible light and introduces noise, it will mask the onset point. This was also confirmed by studying a textured surface, like the “random animal skin”, which hid the true onset of visible surface damage.
16.2.4 Anisotropic response Considering the asymmetric response of some of the polymers in compression and tension, many of the tribological properties were found to vary with the direction of sliding. Sue and coworkers [3] reported that this behavior will significantly influence the scratching response of polymers as the stress field evolves with the indenter movement during a progressive load scratch test. For this purpose, different model systems were considered with variations in tensile and compressive behaviors (see Table 16.2). Figure 16.10 shows comparisons of shoulder height and scratch depth as a function of the scratch normal load. In the first case, three systems were compared with similar compressive properties, but differ in their tensile properties. Irrespective of these differences, as shown in Fig. 16.10(a) and (b), the shoulder height and scratch depth of the materials remain essentially the same. In the second scenario, two systems were compared with similar tensile strength values and different compressive yield strengths. Clearly, the lower compressive yield strength of PC compared to styrene acrylonitrile (SAN) 19 makes it more susceptible to
16.2 Wear/Scratch Damage in Polymer Nanocomposites
Table 16.2 Tensile and Compressive Properties of Selected Model Systems Property Tensile modulus, GPa Tensile strength, MPa Compressive modulus, GPa Compressive yield strength, MPa
SAN 19
SAN 27B
SAN 27C
Neat PC
3.4 ± 0.0 68.9 ± 1.5 3.5 ± 0.1
3.7 ± 0.1 75.1 ± 3.0 3.6 ± 0.3
3.7 ± 0.1 79.0 ± 1.0 3.5 ± 0.2
2.3 ± 0.0 65.2 ± 0.0 2.1 ± 0.1
117.6 ± 0.8
117.2 ± 0.4
117.2 ± 0.2
75.3 ± 0.7
The numbers after SAN represent the weight percentage of AN; 27B and 27C differ in molecular weight, 119 and 134 kg mol, respectively.
FIGURE 16.10 Comparisons of (a, c) shoulder height and (b, d) scratch penetration depth of neat PC and other model systems as a function of applied normal load illustrating the importance of compressive properties. For color version of this figure, the reader is referred to the online version of this book.
scratch damage resulting in higher shoulder height and deeper scratch depth. This suggests that shoulder height and scratch depth are predominately affected by compression loading properties. However, it should also be noted that SAN 19 exhibited brittle behavior and neat PC yielded a ductile behavior under uniaxial tensile testing conditions.
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In the case of nanocomposites, recently, we have shown that even the orientation and extent of intercalation of clay layers are important parameters influencing the magnitude of scratch damage [29]. Residual depths were lower for scratches performed on the cross-sections (normal to the flow direction) of the nanocomposites compared to those on the surface (parallel to the flow direction), and greater scratch penetration resistance was noted for the nanocomposite with higher intercalation extent of organoclay (Fig. 16.11). This clearly suggests that the orientation and dispersion
FIGURE 16.11 (a) Schematic of two different locations on an injection-molded sample (parallel and normal to flow direction) where scratch tests are performed showing the orientation of clay platelets at both locations and (b) average scratch residual penetration depth for neat polyamide (PA) 6 (A0), exfoliated PA 6/organoclay (90/10) nanocomposite (A1) and intercalated PA 6/organoclay (90/10) composite (A2) at different loads parallel and normal to the injection-molding direction using a Berkovich indenter. For color version of this figure, the reader is referred to the online version of this book.
16.3 Coatings
Table 16.3 Instantaneous and Residual Penetration Depths and Viscoelastic Recovery of PC and PC/ZnO Composite Along Longitudinal and Transverse (Sliding) Directions Sliding Direction: Longitudinal
Sliding Direction: Transverse
Material
Penetra tion Depth (μm)
Residual Depth (μm)
Visco elastic Recovery (%)
Penetra tion Depth (μm)
Residual Depth (μm)
Visco elastic Recovery (%)
PC PC/ZnO
292.8 157.3
37.5 45.9
87.7 70.8
130.2 143.1
48.8 59.4
62.5 58.5
Viscoelastic recovery = [1−(Rh/Rp)] × 100; where Rp instantaneous penetration depth and Rh is the residual depth (measured after 2 min of testing).
of clay layers must be simultaneously considered in determining the effective structural reinforcement in polymer/clay nanocomposites. Otherwise, the results can be misleading. In another similar study, Bermudez et al. [30] also showed that neat PC exhibited an anisotropic behavior during scratching; that is, the scratch response was dependent on the direction of scratching. It was observed that the instantaneous penetration depth for neat PC increased by almost 55% in the longitudinal direction compared to the transverse direction under progressive loading (0.03–29 N) scratch test at the highest applied load (Table 16.3). However, in contrast to the abovediscussed study [29], with the incorporation of ZnO nanoparticles, the anisotropic behavior was minimal, and a mere 9.9% increase was noted under similar conditions. Even the residual penetration depth varied with the sliding direction, but in this case, both PC and PC/ZnO materials showed a similar variation of ~23% increase in the transverse direction compared to the longitudinal direction. These differences were attributed to the variations in viscoelastic recovery of PC and PC/ZnO composite in the longitudinal and transverse directions. However, this reasoning cannot answer the differences in the instantaneous penetration depths (Rp) between longitudinal and transverse directions, which are significant.
16.3 COATINGS Generally, in a metal–metal contact (such as drilling and cutting tools) and in dry sliding systems, to prevent the abrasive and adhesive wear of the softer material by the harder material, application of functional coatings is one of the common techniques utilized. With much advancement in coatings technology, deposition of films (or multilayered films) with tailored properties is achievable. Many such coatings were developed depending on the specific purpose; soft coatings were chosen particularly for lubricating purposes and hard (and superhard) coatings for load bearing as well as sacrificial layers [31–34]. Examples of hard coating materials
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include TiN, MoS2, TiC, Al2O3, diamond, and diamond-like carbon (DLC) as single or multilayer combinations. Particularly, DLC coatings have found tremendous applications as they decreased the coefficient of friction and the wear rate by more than an order of magnitude. The presence of hydrogen up to 40 at% in these materials strongly influences their mechanical and tribological behaviors [35]. Depending on the requirement of these coatings, other elements like nitrogen, silver, silicon, and tungsten were also incorporated. In some specific cases, very low coefficients of friction 0.003–0.008 in dry sliding were also achieved [36]. For some applications like magnetic recording discs, a combination of hard DLC and soft polymeric coatings were used. Cavallin et al. [37] used cold plasma enhanced chemical vapor deposition to treat poly(vinyl chloride) (PVC) and PC surfaces with TiO2 coatings with an objective to increase surface hardness and induce superhydrophilic characteristics. Although hardness was increased (an average value of 0.57 GPa for PVC with TiO2 compared to 0.15 GPa for neat PVC and 0.78 GPa for PC with TiO2 compared to 0.23 GPa for neat PC) and contact angles were reduced to <10° from 65 to 80° for neat materials, there were huge cracks in the coatings (see Fig. 16.12 for TiO2 on PC). The island-like structures clearly suggest the differences in the thermal expansion coefficients of the polymeric materials and TiO2 coating. Besides, the interaction (adhesion) between the substrate and coating is not considered in the study. In another investigation, polyetherimide (PEI), polyether-ether-ketone (PEEK) and PA 12 materials were deposited on low-carbon steel by low-velocity flame spray technology with an optimized spray distance of 85 mm and a preheating temperature of 230 °C [32]. The
FIGURE 16.12 SEM micrograph of an 8-μm-thick TiO2 coating on PC substrate demonstrating the islandlike structures.
16.4 Concluding Remarks
average coating thickness was ~300 μm. Three-body abrasion tests (abrasive ~250-μm silica dry sand, applied load ~45 N, 6000 revolutions and a sand flow ~400 g min−1) were performed on the coatings, which revealed that the performance of PA 12 coating was far better than the other two. The mass loss value of PA 12 was ~50% of that of PEEK and ~20% less than PEI values. This is despite minor differences in the (tensile) adhesion values and microhardness of the coatings. However, the authors neither provided any reasons for the differences nor studied the mechanisms of wear, which makes it tough to deduce any specific reason for this behavior. Despite the many advancements in this field, particularly deposition technologies, there are many issues to be addressed. Some studies have already been directed to understand the significance of properties of coating materials such as elastic modulus, hardness or shear strength and fracture toughness as well as other parameters like critical load in scratching [38]. However, there are many other parameters that should be completely evaluated. For inorganic hard coatings (on inorganic substrates), intrinsic stresses are critical in many studies resulting in failure. These are a combination of stresses induced during the deposition/growth process of coatings and due to the mismatch of coating–substrate thermomechanical properties [39]. For polymeric substrates, parameters like chemical inertness, thermal resistance and structural weakness to ultraviolet radiation and ion bombardment were also important. As they change with the inherent properties of the polymer, the effects of these parameters vary from system to system. Another important facet is the adhesion of coatings and substrates. For polymer–polymer systems, many approaches were already established like the chemical functionalization through the use of reactive groups or the creation of a broad interface region that enables the realization of covalent bonds. However, for polymer-inorganic systems, as mentioned before, there are still many questions to be answered.
16.4 CONCLUDING REMARKS As discussed in this chapter, in some of the contributions, novel approaches were adopted to tackle the issues of friction and wear, while some dealt with enhancing our understanding of the existing tribological knowledge of polymer nanocomposites. Unfortunately these contributions are limited in number, and the majority of the studies, although considered issues of serious concern, failed either at the approach level or in providing a deeper insight into the wear/scratch/friction mechanisms. This in turn has indirect impacts, particularly when dealing with biomedical applications, as more research is diverted into bioceramics and biocompatible polymer nanocomposites that experience tribological contacts at different length scales. Another new area of promise is the usage of polymer-based nanocomposites as shoe soles, particularly in the sports field. The potential of nanoparticles to enhance adhesion and friction between the materials and surface would be critical in these applications. Obviously the usage of polymer nanocomposites in these different fields under different contact conditions requires the overcoming of several hurdles discussed earlier.
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Acknowledgments We thank Singapore's Ministry of Education through AcRF Tier 1 grant (RG45/11), the Start-up Grant from Nanyang Technological University, Australian Research Council and the National Natural Science Foundation of China (50873006 and 51125010) for financially supporting our research on various issues of polymer nanocomposites.
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