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Tribological behavior and energy dissipation of hybrid nanoparticle-reinforced HPMC composites during sliding wear
Surface & Coatings Technology 389 (2020) 125617
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Tribological behavior and energy dissipation of hybrid nanoparticlereinforced HPMC composites during sliding wear
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Shih-Chen Shi , Xiao-Ning Tsai, Shia-Seng Pek Department of Mechanical Engineering, National Cheng Kung University (NCKU), Tainan, Taiwan
A R T I C LE I N FO
A B S T R A C T
Keywords: MoS2 Friction-reducing coating Wear mechanism Solid lubricant additive Self-lubricating composite Tribology
In recent years, nanoparticle additives have been frequently used to enhance the tribological properties of polymers, especially in the biopolymer system. In this study, we analyzed the tribological behavior and energy dissipation properties of MoS2 and nano-copper-reinforced biopolymer hydroxypropyl methylcellulose (HPMC) composites. The tribological behavior of the composite was evaluated at low load using a ball-on-disk cycle tester. The addition of MoS2 and nano-copper additives improved the wear resistance and reduced the friction coefficient of the HPMC composites. The addition of both MoS2 and copper additives had a synergistic effect on enhancing the tribology behavior. The dissipation energy results indicated a linear correlation between wear and energy consumed. This study suggests that the wear mechanism varies with the type and quantity of additive, regardless of the normal load applied.
1. Introduction Nanoparticle-polymer composites have been widely used since 1900 because nanoparticles significantly improve the strength, compression resistance, and wear resistance of composite materials due to their high coordination deficiency of surface atoms and strong van der Waals forces [1]. Most previous research has ascribed the enhanced mechanical properties of polymer-inorganic additive composites to the high stiffness of particles [2]. However, the stiffness, strength, and Young's modulus of composites depend on the stress transferred between the matrix materials and additives. If the stress can be effectively transferred from matrix to additives, the mechanical properties of the composites will be greatly improved [3]. Therefore, bonding between the additives and the matrix is critical. Aggregation leads to the growth of nanoparticles, which results in failure of the small size and surface effects of nanoparticles and weak bonding between the matrix and large particles [4]. The mechanical properties of the composites are then affected by these weak bonds, which exist as defects in the composites. Therefore, it is important to establish an effective procedure for the dispersion and application of particles [5]. Polymer composites are commonly utilized in lightweight components. Among these, polyimide (PI) exhibits the advantages of high strength, a wide range of operational temperatures, and chemical corrosion resistance. Therefore, the National Aeronautics and Space Administration (NASA) often applies PI-based composites in the
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structural components of unmanned drones [6]. In order to promote the development of composites, the relationship between the matrix and additives should be established to predict the mechanical properties of bulk composites before synthesis. Many models have previously been built, such as the model of carbon nanotube-polymer composites [7,8]. However, despite the excellent mechanical properties of these composites, they are restricted by the high manufacturing costs of carbon nanotubes. Therefore, some researchers have focused on establishing models for composites using polymers and low-cost nanoparticles [9]. In these models, molecular dynamics were adopted to simulate the motion relationship between spherical silicon dioxide (SiO2) nanoparticles and PI molecules. The simulations showed that the molecular density of PI around the SiO2 nanoparticles increased by 33–40%. Moreover, the shear modulus and Young's modulus of the composites also increased. These results were consistent with those of the MoriTanaka model for large-scale particles [10]. The effect of nanoparticles on wear has recently been attracting increasing attention. In contrast to bulk materials, nanoparticles possess a high specific surface area. As such, they form strong bonds with the polymer matrix in composites. Consequently, they demonstrate an excellent load-carrying capacity and prevent direct contact between wear parts [11]. Spherical and near-spherical particles exhibit a rolling effect instead of sliding friction, which effectively reduces the friction coefficient and wear rate. Hard and polyhedral particles such as alumina (Al2O3), silicon carbide (SiC), and artificial diamond demonstrate
Corresponding author. E-mail address: [email protected] (S.-C. Shi).
https://doi.org/10.1016/j.surfcoat.2020.125617 Received 21 November 2019; Received in revised form 4 March 2020; Accepted 10 March 2020 Available online 27 March 2020 0257-8972/ © 2020 Elsevier B.V. All rights reserved.
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2. Materials and methods
grooving or rolling effects at different concentrations and loads [12]. At low concentration and high load, hard particles pierce the soft surface of wear parts, causing two-body abrasive wear because of a high stress per unit area on the particles. As a result, the frictional resistance and wear rate increase. At high concentration and low load, the effects are the same as for spherical particles; the wear changes from two-body to three-body rolling effects [13]. Additives with sharp corners become embedded into the wear parts. Assuming that micro particles and nanoparticles bear the same small load, the depth to which nanoparticles are embedded into the material is much smaller than that of microparticles. Consequently, the abrasive wear and shape effects of nanoparticles are low [14]. Most tribology research into polymer composites evaluates the surface roughness of nanoscale additives and corresponding wear parts. The particles can easily refine the wear debris and enter asperity valleys during the process of abrasion. Moreover, a stable transfer film forms by mechanical interlocking to achieve the effect of abrasion resistance [15,16]. Regarding their use in composites, the specific surface area of nanoparticles is relatively large, and the bonding strength of nanoparticles and polymers is stronger than that of microscale additives and polymers; therefore, the mechanical properties and tribological properties of nanoparticle-composites are relatively good. If the additives are soft materials such as polymers with low surface energy, they can adhere to the parts due to a surface energy difference or a frictional chemical reaction with the wear parts, thereby reducing abrasion by the formation of a transfer layer [17,18]. Furthermore, if the additives are solid lubricants with self-lubricating characteristics, such as molybdenum disulfide, graphite, and PTFE, the friction coefficient and wear rate can be reduced simultaneously [19–21]. Shi et al. coated a silicon substrate with hydroxypropyl methylcellulose (HPMC) solution at an appropriate ratio, which acted as a sacrificial layer during the wear process. They then revealed that HPMC debris resided in the contact region and adhered onto the wear parts to form a transfer layer, which effectively reduced both the friction coefficient and wear rates [22]. Shi and Huang proved that HPMC exhibits self-healing and refilling properties by injecting organic solvents into the wear scar, or by controlling the temperature and humidity to generate condensed water on the HPMC film surface, which dissolved HPMC to fill the wear scar [23,24]. Other researchers added molybdenum disulfide (MoS2) micro particles into HPMC to exploit the MoS2 structural characteristics and effectively reduce the friction coefficient under high load and high sliding speed [25–28]. A transfer layer of HPMC also formed, achieving an abrasion resistance effect. Shi and Peng added stearic acid by mixing or coating to improve the hydrophobicity and tribological properties of HPMC [29]. Their results showed that the stearic acid in HPMC can effectively improve the surface hydrophobicity of HPMC. The stearic acid is in crystalline form in the HPMC film and, as the third body in the process of abrasion, the stearic acid on the bi-layer film adheres to the surface of chromium steel balls to prevent direct contact between the film and the wear parts; both can improve the macro tribological properties of HPMC. The purpose of this study is to explore the influence of additives on the load-carrying capacity and tribological properties of composite films made using biopolymer HPMC as a green coating to improve tribological properties. The concept of dissipation energy and the thirdbody issues are introduced to illustrate the velocity accommodation mode provided by different additives and the wear mechanism of the dominant wear behavior [30]. In this study, the tribological behavior of nano-MoS2 and nano-copper-reinforced biopolymer HPMC composites was evaluated at low load using a ball-on-disk cycle tester. The morphology and energy dissipation characteristics of the material were then evaluated.
2.1. Preparation of HPMC, MoS2/HPMC, and cu/MoS2/HPMC composite coatings HPMC (606, Shin Etsu, Osaka, Japan) was used as the base material. Molybdenum disulfide powder (average particle size: 2.0 ± 0.1 μm) was commercially obtained from Sigma-Aldrich Corporation (USA). The nano-Cu (average particle size 137.6 ± 39.6 nm) was obtained from Yi-Mei Company (New Taipei City, Taiwan). Sorbitan monoester (Span 80), is a long-chain nonionic surfactant with 18 carbons in the molecule and use as dispersant for nanoparticles. Borosilicate glass (Paul Marienfeld, Lauda-Königshofen, Germany) was used as the substrate for the following analysis and wear test. First, the HPMC solution was prepared. 10 g of HPMC powder was added into 90 cc of a mixed ethanol and deionized water solution (8:2). The solution was then heated to 60 °C on a hot plate then cooled. Next, a mixed solution of MoS2/HPMC was prepared; i.e., 5 g of MoS2 particles was added to 100 cc of HPMC solution and subjected to ultrasonic oscillating for 20 min. Subsequently, a solution of Cu/MoS2/HPMC was prepared. Different contents of Cu nanoparticles (0.2–0.4 g) were dissolved in a mixture of Span 80 and alcohol at a w/w ratio of 1, then subjected to ultrasonic oscillation for 5 min. Finally, a composite solution with a uniform distribution of nanoparticles was obtained by pouring the solution into previously prepared MoS2/HPMC solution and performing ultrasonic oscillation for 20 min. Then, 486 μl of mixed Cu/MoS2/ HPMC solution was directly dripped onto a glass substrate with a micropipette, which was placed in a device with a constant temperature and humidity of 25 ± 3 °C and RH40 ± 5% for 6 h. 2.2. Analysis and measurement setup Scanning electron microscopy (SEM, XL-40FEG, Philips, Amsterdam, Netherlands & SU-5000, Hitachi, Tokyo, Japan) was used to observe the morphology of nanoparticles and the surface of the wear scar. The crystal structure of the HPMC-based composite coating was analyzed using X-ray diffraction (XRD, Bruker AXS Gmbh, D8 DISCOVER). Quality of the composite coating and distribution of the additive particles were analyzed by Raman spectroscopy (MRID, ProTrusTech Co., Ltd., Taiwan). Surface roughness (Ra) and wear volume were measured by a 3D laser scanner (VK9700, Keyence, Osaka, Japan). 2.3. Tribological performance analysis AISI 52100 chrome steel balls with a diameter of 6.35 mm were used as counter bodies. Wear tests were conducted on a rotary ball-ondisk tribometer (POD-FM406-10NT, Fu Li Fong Precision Machine, Kaohsiung, Taiwan). The wear behavior under an applied normal force of 2 N and a sliding speed of 3 mm/s with various wear distances (10, 30, 70 m) was measured under dry sliding conditions. The tribometer was equipped with a controlled environment; i.e., a temperature of 25 °C ± 5 °C and a relative humidity of 70 ± 10%. 2.4. Third-body theory Third-body theory is used to describe the interaction and different motion states of the abrasive parts. Third body theory is used to describe the interaction and different motion states of the abrasive parts. Velocity accommodation mechanism (VAM) is applied to describe behavioral patterns of materials during abrasion. Velocity accommodation mode refers to the wear position and motion state as “Sites” and “Modes”. The velocity accommodation site S is the location where the velocity is accommodated. S1/S5 is defined as first bodies, and refer to chrome steel ball and coating in this experiment. S3 is defined as third bodies and refers to natural wear debris or added lubricants; S2/S4 is defined as third-body screens. It is interface layer between S1/S5 and 2
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Fig. 1. SEM images of (a) Cu 1%/HPMC; (b) Cu 2%/HPMC; (c) Cu 1%/MoS2/HPMC; and (d) Cu 2%/MoS2/HPMC composite coating surfaces; Raman mapping of (e) Cu; and (f) MoS2 signal in Cu 1%/MoS2/HPMC composite coating.
material. Fig. 1e–f represents the results of Raman mapping, showing the positions of Cu and MoS2 particles, respectively. This study is based on the results of Raman mapping as the basis for particle dispersion. The locked characteristic peaks of Cu and MoS2 are 280 and 408 cm−1, respectively [37,38]. The additives in this study are well dispersed in the HPMC matrix, which will have the positive effect to the tribology performance. Tribology behavior, such as coefficient of friction and anti-wear property, is a function of film thickness [39]. Fig. 2a shows film thickness of composite coatings with different additive contents. Thickness of the composite coatings can be controlled by the amount of composite solution added; thus, film thickness can be accurately manipulated with an average thickness of 75 ± 5 μm. Therefore, influence of coating thickness on the abrasive behavior can be eliminated. Similarly, surface roughness of coating is an important factor of wear performance. Fig. 2b exhibits surface roughness of composite coatings. Surface roughness increases with the quantity of additives. It is speculated that these surface macro-scale changes are caused by the gradual evaporation of solvents during film formation; thus, a rough peak forms due to the partially exposed nanoparticles. When the volume ratio of nanoparticles dispersed in the solution is high, the nanoparticles are in contact with each other due to a reduction of the solvent. The roughness of the composite film is also enhanced by the aggregation of surface particles. Therefore, as shown by the SEM images, nanoparticles protrude from the film surface. Moreover, particle aggregation on the film with 2 wt% nanoparticles is larger than that on the film with 1 wt% of the same nanoparticles. However,
S3. Velocity accommodation mode M specifies the manner in which the velocity is accommodated, which is the elastic M1, normal breaking/ rupture M2, shearing/sliding M3, and rolling M4. VAM is made out of one site Si, and one mode Mj will be identified by a code SiMj. Wear mechanism caused by additives can be clearly described through clear definition of VAM [31,32]. 3. Results and discussion 3.1. Analysis of composite coating Hybrid materials strengthening is a method often used to improve characteristics of polymers, especially in mechanical property. Adding additives to the polymer substrate (matrix), especially in the nanometer size, are likely to achieve the desired purpose [33]. The most critical factors are the size and dispersion of the additives [34,35]. When the additive aggregates, it will greatly reduce its ability of anti-wear and tribology behavior [36]. Fig. 1a–d shows the surface morphology of composite coatings Cu 1%/HPMC, Cu 2%/HPMC, Cu 1%/MoS2/HPMC, and Cu 2%/MoS2/HPMC, respectively. As the concentration of additives increases, the coverage ratio of nanoparticles on the surface of the composite film also increases. The presence of Cu and MoS2 particles in the HPMC film is supposed having higher loading capacity. Raman mapping is a very effective method for determining the position of material. The method is to lock the characteristic peaks of the material, high-intensity location indicate the existence of the 3
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Fig. 2. (a) Film thickness and (b) surface roughness of the composite coating.
relative to the thickness of the coating and the size of the wear counter ball, the change in surface roughness between coatings is not obvious. Therefore, in the subsequent analysis, the influence of surface roughness is also excluded. The characteristics of the additives play an important role on the properties of hybrid composite coating [40]. In the process of preparing composite coating, additives interact with various chemicals, it is important to monitor structural properties of the additives. Fig. 3a reveals the XRD results of the composite coating, in which the signals of HPMC, Cu, and MoS2 can be observed. This indicates that Cu and MoS2 nanoparticles maintain their original characteristics without damage from surface contamination during the mixing process. Raman analysis of the composite coatings is shown in Fig. 3b. Characteristic peaks of HPMC and MoS2 can be observed, which demonstrates that the additive particles are not contaminated. 3.2. Tribological properties of the composite coating Fig. 4. Comparison of (a) wear volume and (b) average coefficient of friction of the composite coatings (2 N, 0.03 m/s, 30 m).
Fig. 4 presents the wear volume and average friction coefficient of HPMC films and composite films after 30 m off wear at a load of 2 N. The average friction coefficient of pure HPMC films is 0.58 with a wear volume of approximately 39.5 ∗ 10−3 mm3, whereas the wear volume of all composite films is less than 15 ∗ 10−3 mm3. It is worth noting that the average friction coefficient of Cu 2%/HPMC and Cu 2%/MoS2/ HPMC composite coatings is approximately 0.1. Therefore, it can be
concluded that the addition of Cu and MoS2 effectively improves the tribological properties of HPMC. It is proposed that the hard Cu additives provide the load capacity to the composite coating and separate the contact material. MoS2 exhibits self-lubricating behavior due to its
Fig. 3. (a) XRD analysis and (b) Raman spectra of the composite coating. 4
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layered structure. The above statement is consistent with the results obtained by Harris et al. [17] and Shi et al. [38]. Surface morphology of the wear scar, dissipation energy and third-body observation are applied for further understanding about the wear mechanism of additives. The wear scar of HPMC films is shown in Fig. 5a. The apparent scratches at the scar are attributed to severe adhesion wear. In comparison to Fig. 5b–c, the scars on the composite films are narrow and typically in the removal stage of rough peaks; thus, the wear is not obvious. Cu nanoparticles were embedded into soft HPMC matrix and suggested having ploughing behavior, resulting in variation of scar profiles. 3.3. Wear behavior and wear mechanism of the composite coating Fig. 6a and c illustrates the surface morphology of the wear scars on Cu 1%/HPMC and Cu 2%/HPMC composite coatings, respectively. Enlarged parts are shown in Fig. 6b and d, respectively. Matrix material HPMC deforms elastically under stress; thus, its VAM is classified as S5M1 (S5: composite coating; M1: elastic deformation of HPMC). Cu nanoparticles on the surface of composite coatings deform under friction force during the wear process. VAM is classified as S5M2 (M2: flattening or damage of Cu, as pointed by arrow). Two VAMs (S5M1 and S5M2) can be observed at the surface of Cu/HPMC composite
Fig. 5. Observations of wear scar profiles in (a) HPMC; (b) Cu 2%/HPMC; and (c) Cu 2%/MoS2/HPMC composite coatings (2 N, 0.03 m/s, 30 m).
Fig. 6. Wear marks on: (a) Cu 1%/HPMC, 100× magnification; (b) Cu 1%/HPMC, 5000× magnification; (c) Cu 2%/HPMC, 100× magnification; and (d) Cu 2%/ HPMC, 5000× magnification. (e) Wear mechanism of the Cu/HPMC composite coating. 5
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Fig. 7. Wear marks on: (a) Cu 1%/MoS2/HPMC, 100× magnification; (b) Cu 1%/ MoS2/HPMC, 5000× magnification; (c) Cu 2%/ MoS2/HPMC, 100× magnification; and (d) Cu 2%/ MoS2/HPMC, 5000× magnification. (e) Wear mechanism of the Cu/ MoS2/HPMC composite coating.
the Cu 2%/MoS2/HPMC composite coating is the narrowest and faintest, as shown in Fig. 7c–d. MoS2 particles on the coating surface slip and delaminate under stress; thus, VAM is defined as S5M3 (M3: shearing/delamination of MoS2, as pointed by arrow). Cu particle deforms and represents VAM of S5M2. Matrix material HPMC exhibits S5M1 (M1: elastic deformation). In the Cu/MoS2/HPMC composite coating, three VAMs can be observed in the surface of composite coating, i.e. S5M1, S5M2 and S5M3. 3.4. Dissipation energy analysis of the Cu/MoS2/HPMC composite coating Surface friction is directly related to energy dissipation, and the form of wear changes the material structure. Sliding wear models can be used to illustrate the dissipated energy in the system. The volume of material removed by sliding wear is directly proportional to the work done due to friction [40]. The slope of the graph showing the volume of material removed vs the friction work represents the dominant wear mechanism. The Archard wear model, which is a model used to describe sliding wear, is given below:
Fig. 8. Wear volume and dissipation energy curves of HPMC-based composites.
∆E = F × V × ∆t
coatings. Fig. 7a and c shows slight scratches on the Cu 1%/MoS2/HPMC composite coating. High magnification images reveal that some copper nanoparticles deform and others retain their original shape. The scar on
E=
∑ ∆E
F = μN 6
(1) (2) (3)
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Acknowledgement
where F is the average value of the friction force, V is the constant sliding speed, and Δt is the time interval. Two sets of material systems with different slopes are observed in Fig. 8. According to the third-body theory, materials in tribological pairs are subject to load while accommodating the velocity difference with the first bodies. The role of VAM is to convert the energy generated by friction. Therefore, dissipation energy in the system can be reduced by efficient VAM [41]. In Cu/HPMC material system, bulk behavior is still dominated by HPMC due to small amount of Cu additive. Therefore, in HPMC and Cu/ HPMC, S5M1 dominates the tribological behavior of the entire system. Previous studies have reported that the addition of MoS2 is highly beneficial for the formation of transfer layers, especially in the case of MoS2 found on the surface of wear parts, suggesting that MoS2 delaminates and is transferred under stress [33]. In Cu/MoS2/HPMC, three velocity accommodation mechanisms coexist in the interface of the composite coating. Delamination of MoS2 in the composite can provide a more efficient VAM (S5M3), resulting in a lower friction coefficient and wear rate.
The authors would also like to thank the Center for Micro/Nano Science and Technology and Instrument center, National Cheng Kung University (NCKU) for the technical support. Funding The authors gratefully acknowledge the financial support for this project from the Ministry of Science and Technology, Taiwan (MOST 106-2221-E-006-092-MY3). References [1] B. Boonstra, Role of particulate fillers in elastomer reinforcement: a review, Polymer 20 (1979) 691–704. [2] N. Kida, M. Ito, F. Yatsuyanagi, H. Kaido, Studies on the structure and formation mechanism of carbon gel in the carbon black filled polyisoprene rubber composite, J. Appl. Polym. Sci. 61 (1996) 1345–1350. [3] S.-Y. Fu, X.-Q. Feng, B. Lauke, Y.-W. Mai, Effects of particle size, particle/matrix interface adhesion and particle loading on mechanical properties of particulate–polymer composites, Compos. Part B 39 (2008) 933–961. [4] T. Jiang, T. Kuila, N.H. Kim, B.-C. Ku, J.H. Lee, Enhanced mechanical properties of silanized silica nanoparticle attached graphene oxide/epoxy composites, Compos. Sci. Technol. 79 (2013) 115–125. [5] A.C. Balazs, T. Emrick, T.P. Russell, Nanoparticle polymer composites: where two small worlds meet, Science 314 (2006) 1107–1110. [6] D.R. Tenney, J.G. Davis Jr., R.B. Pipes, N. Johnston, NASA Composite Materials Development: Lessons Learned and Future Challenges, DOI (2009). [7] R. Bradshaw, F. Fisher, L. Brinson, Fiber waviness in nanotube-reinforced polymer composites—II: modeling via numerical approximation of the dilute strain concentration tensor, Compos. Sci. Technol. 63 (2003) 1705–1722. [8] J.N. Coleman, U. Khan, W.J. Blau, Y.K. Gun’ko, Small but strong: a review of the mechanical properties of carbon nanotube–polymer composites, Carbon 44 (2006) 1624–1652. [9] D. Barbier, D. Brown, A.-C. Grillet, S. Neyertz, Interface between end-functionalized PEO oligomers and a silica nanoparticle studied by molecular dynamics simulations, Macromolecules 37 (2004) 4695–4710. [10] G. Odegard, T. Clancy, T. Gates, Modeling of the mechanical properties of nanoparticle/polymer composites, Polymer 46 (2005) 553–562. [11] L. Pena-Paras, J. Taha-Tijerina, L. Garza, D. Maldonado-Cortés, R. Michalczewski, C. Lapray, Effect of CuO and Al2O3 nanoparticle additives on the tribological behavior of fully formulated oils, Wear 332 (2015) 1256–1261. [12] N. Chand, A. Naik, S. Neogi, Three-body abrasive wear of short glass fibre polyester composite, Wear 242 (2000) 38–46. [13] R. Trezona, D. Allsopp, I. Hutchings, Transitions between two-body and three-body abrasive wear: influence of test conditions in the microscale abrasive wear test, Wear 225 (1999) 205–214. [14] E. Rabinowicz, A. Mutis, Effect of abrasive particle size on wear, Wear 8 (1965) 381–390. [15] D.-X. Peng, C.-H. Chen, Y. Kang, Y.-P. Chang, S.-Y. Chang, Size effects of SiO2 nanoparticles as oil additives on tribology of lubricant, Industrial Lubrication and Tribology 62 (2010) 111–120. [16] C.J. Reeves, P.L. Menezes, M.R. Lovell, T.-C. Jen, The influence of surface roughness and particulate size on the tribological performance of bio-based multi-functional hybrid lubricants, Tribol. Int. 88 (2015) 40–55. [17] K.L. Harris, A.A. Pitenis, W.G. Sawyer, B.A. Krick, G.S. Blackman, D.J. Kasprzak, C.P. Junk, PTFE tribology and the role of mechanochemistry in the development of protective surface films, Macromolecules 48 (2015) 3739–3745. [18] S. Bahadur, The development of transfer layers and their role in polymer tribology, Wear 245 (2000) 92–99. [19] S.-C. Shi, Tribological performance of green lubricant enhanced by sulfidation IFMoS2, Materials 9 (2016) 856. [20] M. Conte, A. Igartua, Study of PTFE composites tribological behavior, Wear 296 (2012) 568–574. [21] M.N. Gardos, The synergistic effects of graphite on the friction and wear of MoS2 films in air, Tribol. Trans. 31 (1988) 214–227. [22] S.-C. Shi, T.-F. Huang, J.-Y. Wu, Preparation and tribological study of biodegradable lubrication films on Si substrate, Materials 8 (2015) 1738–1751. [23] S.-C. Shi, T.-F. Huang, Self-healing materials for ecotribology, Materials 10 (2017) 91. [24] S.-C. Shi, T.-F. Huang, Effects of temperature and humidity on self-healing behaviour of biopolymer hydroxylpropyl methylcellulose for ecotribology, Surf. Coat. Technol. 350 (2018) 997–1002. [25] S.-C. Shi, J.-Y. Wu, T.-F. Huang, Raman, FTIR, and XRD study of MoS2 enhanced hydroxypropyl methylcellulose green lubricant, Opt. Quant. Electron. 48 (2016) 474. [26] S.-C. Shi, J.-Y. Wu, T.-F. Huang, Y.-Q. Peng, Improving the tribological performance of biopolymer coating with MoS2 additive, Surf. Coat. Technol. 303 (2016) 250–255. [27] W.T. Hay, G.F. Fanta, S.C. Peterson, A. Thomas, K.D. Utt, K.A. Walsh, V.M. Boddu,
4. Conclusion In this study, MoS2 and Cu nanoparticles were added to biopolymer HPMC to form functional composite coatings. We then evaluated the tribological properties of the composite coatings, the third-body behaviors of the nanoscale additives during abrasive wear, and the correlation between wear volume and energy dissipation. The following key conclusions were drawn. 1. The dispersion effect of the matrix HPMC enabled the MoS2 and Cu nano-additives to disperse in the solution with good compatibility and good film formation. A composite coating was generated, demonstrating abrasion resistance effects, in which the crystal structure of the additives remained intact. 2. In the wear tests, the tribological properties of Cu/MoS2/HPMC films were superior to those of Cu/HPMC films. This indicates that the addition of MoS2 effectively improved the macro-scale tribological properties of HPMC. When MoS2 was subjected to a tangential force, it was easily sheared and stratified. The layered MoS2 exhibited fluidity and good wettability. Debris aggregated to form a good protective layer and transfer layer. When the transfer layer formed, the transfer layer and the third-body layer on the composite coating surface exhibited self-lubricating properties, which could reduce friction resistance and generate good tribological effects. 3. Based on the surface morphology and dissipation energy results, it can be concluded that the Cu/MoS2/HPMC composite systems exhibit multiple tribological mechanisms (S5M3, S5M2, and S5M1). Therefore, it can effectively reduce the wear rate and friction coefficient of wear parts.
CRediT authorship contribution statement Shih-Chen Shi:Conceptualization, Methodology, Writing - review & editing, Supervision, Project administration, Funding acquisition.Xiao-Ning Tsai:Investigation.Shia-Seng Pek:Writing original draft.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 7
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Tribological behavior and energy dissipation of hybrid nanoparticlereinforced HPMC composites during sliding wear
T
⁎
Shih-Chen Shi , Xiao-Ning Tsai, Shia-Seng Pek Department of Mechanical Engineering, National Cheng Kung University (NCKU), Tainan, Taiwan
A R T I C LE I N FO
A B S T R A C T
Keywords: MoS2 Friction-reducing coating Wear mechanism Solid lubricant additive Self-lubricating composite Tribology
In recent years, nanoparticle additives have been frequently used to enhance the tribological properties of polymers, especially in the biopolymer system. In this study, we analyzed the tribological behavior and energy dissipation properties of MoS2 and nano-copper-reinforced biopolymer hydroxypropyl methylcellulose (HPMC) composites. The tribological behavior of the composite was evaluated at low load using a ball-on-disk cycle tester. The addition of MoS2 and nano-copper additives improved the wear resistance and reduced the friction coefficient of the HPMC composites. The addition of both MoS2 and copper additives had a synergistic effect on enhancing the tribology behavior. The dissipation energy results indicated a linear correlation between wear and energy consumed. This study suggests that the wear mechanism varies with the type and quantity of additive, regardless of the normal load applied.
1. Introduction Nanoparticle-polymer composites have been widely used since 1900 because nanoparticles significantly improve the strength, compression resistance, and wear resistance of composite materials due to their high coordination deficiency of surface atoms and strong van der Waals forces [1]. Most previous research has ascribed the enhanced mechanical properties of polymer-inorganic additive composites to the high stiffness of particles [2]. However, the stiffness, strength, and Young's modulus of composites depend on the stress transferred between the matrix materials and additives. If the stress can be effectively transferred from matrix to additives, the mechanical properties of the composites will be greatly improved [3]. Therefore, bonding between the additives and the matrix is critical. Aggregation leads to the growth of nanoparticles, which results in failure of the small size and surface effects of nanoparticles and weak bonding between the matrix and large particles [4]. The mechanical properties of the composites are then affected by these weak bonds, which exist as defects in the composites. Therefore, it is important to establish an effective procedure for the dispersion and application of particles [5]. Polymer composites are commonly utilized in lightweight components. Among these, polyimide (PI) exhibits the advantages of high strength, a wide range of operational temperatures, and chemical corrosion resistance. Therefore, the National Aeronautics and Space Administration (NASA) often applies PI-based composites in the
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structural components of unmanned drones [6]. In order to promote the development of composites, the relationship between the matrix and additives should be established to predict the mechanical properties of bulk composites before synthesis. Many models have previously been built, such as the model of carbon nanotube-polymer composites [7,8]. However, despite the excellent mechanical properties of these composites, they are restricted by the high manufacturing costs of carbon nanotubes. Therefore, some researchers have focused on establishing models for composites using polymers and low-cost nanoparticles [9]. In these models, molecular dynamics were adopted to simulate the motion relationship between spherical silicon dioxide (SiO2) nanoparticles and PI molecules. The simulations showed that the molecular density of PI around the SiO2 nanoparticles increased by 33–40%. Moreover, the shear modulus and Young's modulus of the composites also increased. These results were consistent with those of the MoriTanaka model for large-scale particles [10]. The effect of nanoparticles on wear has recently been attracting increasing attention. In contrast to bulk materials, nanoparticles possess a high specific surface area. As such, they form strong bonds with the polymer matrix in composites. Consequently, they demonstrate an excellent load-carrying capacity and prevent direct contact between wear parts [11]. Spherical and near-spherical particles exhibit a rolling effect instead of sliding friction, which effectively reduces the friction coefficient and wear rate. Hard and polyhedral particles such as alumina (Al2O3), silicon carbide (SiC), and artificial diamond demonstrate
Corresponding author. E-mail address: [email protected] (S.-C. Shi).
https://doi.org/10.1016/j.surfcoat.2020.125617 Received 21 November 2019; Received in revised form 4 March 2020; Accepted 10 March 2020 Available online 27 March 2020 0257-8972/ © 2020 Elsevier B.V. All rights reserved.
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2. Materials and methods
grooving or rolling effects at different concentrations and loads [12]. At low concentration and high load, hard particles pierce the soft surface of wear parts, causing two-body abrasive wear because of a high stress per unit area on the particles. As a result, the frictional resistance and wear rate increase. At high concentration and low load, the effects are the same as for spherical particles; the wear changes from two-body to three-body rolling effects [13]. Additives with sharp corners become embedded into the wear parts. Assuming that micro particles and nanoparticles bear the same small load, the depth to which nanoparticles are embedded into the material is much smaller than that of microparticles. Consequently, the abrasive wear and shape effects of nanoparticles are low [14]. Most tribology research into polymer composites evaluates the surface roughness of nanoscale additives and corresponding wear parts. The particles can easily refine the wear debris and enter asperity valleys during the process of abrasion. Moreover, a stable transfer film forms by mechanical interlocking to achieve the effect of abrasion resistance [15,16]. Regarding their use in composites, the specific surface area of nanoparticles is relatively large, and the bonding strength of nanoparticles and polymers is stronger than that of microscale additives and polymers; therefore, the mechanical properties and tribological properties of nanoparticle-composites are relatively good. If the additives are soft materials such as polymers with low surface energy, they can adhere to the parts due to a surface energy difference or a frictional chemical reaction with the wear parts, thereby reducing abrasion by the formation of a transfer layer [17,18]. Furthermore, if the additives are solid lubricants with self-lubricating characteristics, such as molybdenum disulfide, graphite, and PTFE, the friction coefficient and wear rate can be reduced simultaneously [19–21]. Shi et al. coated a silicon substrate with hydroxypropyl methylcellulose (HPMC) solution at an appropriate ratio, which acted as a sacrificial layer during the wear process. They then revealed that HPMC debris resided in the contact region and adhered onto the wear parts to form a transfer layer, which effectively reduced both the friction coefficient and wear rates [22]. Shi and Huang proved that HPMC exhibits self-healing and refilling properties by injecting organic solvents into the wear scar, or by controlling the temperature and humidity to generate condensed water on the HPMC film surface, which dissolved HPMC to fill the wear scar [23,24]. Other researchers added molybdenum disulfide (MoS2) micro particles into HPMC to exploit the MoS2 structural characteristics and effectively reduce the friction coefficient under high load and high sliding speed [25–28]. A transfer layer of HPMC also formed, achieving an abrasion resistance effect. Shi and Peng added stearic acid by mixing or coating to improve the hydrophobicity and tribological properties of HPMC [29]. Their results showed that the stearic acid in HPMC can effectively improve the surface hydrophobicity of HPMC. The stearic acid is in crystalline form in the HPMC film and, as the third body in the process of abrasion, the stearic acid on the bi-layer film adheres to the surface of chromium steel balls to prevent direct contact between the film and the wear parts; both can improve the macro tribological properties of HPMC. The purpose of this study is to explore the influence of additives on the load-carrying capacity and tribological properties of composite films made using biopolymer HPMC as a green coating to improve tribological properties. The concept of dissipation energy and the thirdbody issues are introduced to illustrate the velocity accommodation mode provided by different additives and the wear mechanism of the dominant wear behavior [30]. In this study, the tribological behavior of nano-MoS2 and nano-copper-reinforced biopolymer HPMC composites was evaluated at low load using a ball-on-disk cycle tester. The morphology and energy dissipation characteristics of the material were then evaluated.
2.1. Preparation of HPMC, MoS2/HPMC, and cu/MoS2/HPMC composite coatings HPMC (606, Shin Etsu, Osaka, Japan) was used as the base material. Molybdenum disulfide powder (average particle size: 2.0 ± 0.1 μm) was commercially obtained from Sigma-Aldrich Corporation (USA). The nano-Cu (average particle size 137.6 ± 39.6 nm) was obtained from Yi-Mei Company (New Taipei City, Taiwan). Sorbitan monoester (Span 80), is a long-chain nonionic surfactant with 18 carbons in the molecule and use as dispersant for nanoparticles. Borosilicate glass (Paul Marienfeld, Lauda-Königshofen, Germany) was used as the substrate for the following analysis and wear test. First, the HPMC solution was prepared. 10 g of HPMC powder was added into 90 cc of a mixed ethanol and deionized water solution (8:2). The solution was then heated to 60 °C on a hot plate then cooled. Next, a mixed solution of MoS2/HPMC was prepared; i.e., 5 g of MoS2 particles was added to 100 cc of HPMC solution and subjected to ultrasonic oscillating for 20 min. Subsequently, a solution of Cu/MoS2/HPMC was prepared. Different contents of Cu nanoparticles (0.2–0.4 g) were dissolved in a mixture of Span 80 and alcohol at a w/w ratio of 1, then subjected to ultrasonic oscillation for 5 min. Finally, a composite solution with a uniform distribution of nanoparticles was obtained by pouring the solution into previously prepared MoS2/HPMC solution and performing ultrasonic oscillation for 20 min. Then, 486 μl of mixed Cu/MoS2/ HPMC solution was directly dripped onto a glass substrate with a micropipette, which was placed in a device with a constant temperature and humidity of 25 ± 3 °C and RH40 ± 5% for 6 h. 2.2. Analysis and measurement setup Scanning electron microscopy (SEM, XL-40FEG, Philips, Amsterdam, Netherlands & SU-5000, Hitachi, Tokyo, Japan) was used to observe the morphology of nanoparticles and the surface of the wear scar. The crystal structure of the HPMC-based composite coating was analyzed using X-ray diffraction (XRD, Bruker AXS Gmbh, D8 DISCOVER). Quality of the composite coating and distribution of the additive particles were analyzed by Raman spectroscopy (MRID, ProTrusTech Co., Ltd., Taiwan). Surface roughness (Ra) and wear volume were measured by a 3D laser scanner (VK9700, Keyence, Osaka, Japan). 2.3. Tribological performance analysis AISI 52100 chrome steel balls with a diameter of 6.35 mm were used as counter bodies. Wear tests were conducted on a rotary ball-ondisk tribometer (POD-FM406-10NT, Fu Li Fong Precision Machine, Kaohsiung, Taiwan). The wear behavior under an applied normal force of 2 N and a sliding speed of 3 mm/s with various wear distances (10, 30, 70 m) was measured under dry sliding conditions. The tribometer was equipped with a controlled environment; i.e., a temperature of 25 °C ± 5 °C and a relative humidity of 70 ± 10%. 2.4. Third-body theory Third-body theory is used to describe the interaction and different motion states of the abrasive parts. Third body theory is used to describe the interaction and different motion states of the abrasive parts. Velocity accommodation mechanism (VAM) is applied to describe behavioral patterns of materials during abrasion. Velocity accommodation mode refers to the wear position and motion state as “Sites” and “Modes”. The velocity accommodation site S is the location where the velocity is accommodated. S1/S5 is defined as first bodies, and refer to chrome steel ball and coating in this experiment. S3 is defined as third bodies and refers to natural wear debris or added lubricants; S2/S4 is defined as third-body screens. It is interface layer between S1/S5 and 2
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Fig. 1. SEM images of (a) Cu 1%/HPMC; (b) Cu 2%/HPMC; (c) Cu 1%/MoS2/HPMC; and (d) Cu 2%/MoS2/HPMC composite coating surfaces; Raman mapping of (e) Cu; and (f) MoS2 signal in Cu 1%/MoS2/HPMC composite coating.
material. Fig. 1e–f represents the results of Raman mapping, showing the positions of Cu and MoS2 particles, respectively. This study is based on the results of Raman mapping as the basis for particle dispersion. The locked characteristic peaks of Cu and MoS2 are 280 and 408 cm−1, respectively [37,38]. The additives in this study are well dispersed in the HPMC matrix, which will have the positive effect to the tribology performance. Tribology behavior, such as coefficient of friction and anti-wear property, is a function of film thickness [39]. Fig. 2a shows film thickness of composite coatings with different additive contents. Thickness of the composite coatings can be controlled by the amount of composite solution added; thus, film thickness can be accurately manipulated with an average thickness of 75 ± 5 μm. Therefore, influence of coating thickness on the abrasive behavior can be eliminated. Similarly, surface roughness of coating is an important factor of wear performance. Fig. 2b exhibits surface roughness of composite coatings. Surface roughness increases with the quantity of additives. It is speculated that these surface macro-scale changes are caused by the gradual evaporation of solvents during film formation; thus, a rough peak forms due to the partially exposed nanoparticles. When the volume ratio of nanoparticles dispersed in the solution is high, the nanoparticles are in contact with each other due to a reduction of the solvent. The roughness of the composite film is also enhanced by the aggregation of surface particles. Therefore, as shown by the SEM images, nanoparticles protrude from the film surface. Moreover, particle aggregation on the film with 2 wt% nanoparticles is larger than that on the film with 1 wt% of the same nanoparticles. However,
S3. Velocity accommodation mode M specifies the manner in which the velocity is accommodated, which is the elastic M1, normal breaking/ rupture M2, shearing/sliding M3, and rolling M4. VAM is made out of one site Si, and one mode Mj will be identified by a code SiMj. Wear mechanism caused by additives can be clearly described through clear definition of VAM [31,32]. 3. Results and discussion 3.1. Analysis of composite coating Hybrid materials strengthening is a method often used to improve characteristics of polymers, especially in mechanical property. Adding additives to the polymer substrate (matrix), especially in the nanometer size, are likely to achieve the desired purpose [33]. The most critical factors are the size and dispersion of the additives [34,35]. When the additive aggregates, it will greatly reduce its ability of anti-wear and tribology behavior [36]. Fig. 1a–d shows the surface morphology of composite coatings Cu 1%/HPMC, Cu 2%/HPMC, Cu 1%/MoS2/HPMC, and Cu 2%/MoS2/HPMC, respectively. As the concentration of additives increases, the coverage ratio of nanoparticles on the surface of the composite film also increases. The presence of Cu and MoS2 particles in the HPMC film is supposed having higher loading capacity. Raman mapping is a very effective method for determining the position of material. The method is to lock the characteristic peaks of the material, high-intensity location indicate the existence of the 3
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Fig. 2. (a) Film thickness and (b) surface roughness of the composite coating.
relative to the thickness of the coating and the size of the wear counter ball, the change in surface roughness between coatings is not obvious. Therefore, in the subsequent analysis, the influence of surface roughness is also excluded. The characteristics of the additives play an important role on the properties of hybrid composite coating [40]. In the process of preparing composite coating, additives interact with various chemicals, it is important to monitor structural properties of the additives. Fig. 3a reveals the XRD results of the composite coating, in which the signals of HPMC, Cu, and MoS2 can be observed. This indicates that Cu and MoS2 nanoparticles maintain their original characteristics without damage from surface contamination during the mixing process. Raman analysis of the composite coatings is shown in Fig. 3b. Characteristic peaks of HPMC and MoS2 can be observed, which demonstrates that the additive particles are not contaminated. 3.2. Tribological properties of the composite coating Fig. 4. Comparison of (a) wear volume and (b) average coefficient of friction of the composite coatings (2 N, 0.03 m/s, 30 m).
Fig. 4 presents the wear volume and average friction coefficient of HPMC films and composite films after 30 m off wear at a load of 2 N. The average friction coefficient of pure HPMC films is 0.58 with a wear volume of approximately 39.5 ∗ 10−3 mm3, whereas the wear volume of all composite films is less than 15 ∗ 10−3 mm3. It is worth noting that the average friction coefficient of Cu 2%/HPMC and Cu 2%/MoS2/ HPMC composite coatings is approximately 0.1. Therefore, it can be
concluded that the addition of Cu and MoS2 effectively improves the tribological properties of HPMC. It is proposed that the hard Cu additives provide the load capacity to the composite coating and separate the contact material. MoS2 exhibits self-lubricating behavior due to its
Fig. 3. (a) XRD analysis and (b) Raman spectra of the composite coating. 4
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layered structure. The above statement is consistent with the results obtained by Harris et al. [17] and Shi et al. [38]. Surface morphology of the wear scar, dissipation energy and third-body observation are applied for further understanding about the wear mechanism of additives. The wear scar of HPMC films is shown in Fig. 5a. The apparent scratches at the scar are attributed to severe adhesion wear. In comparison to Fig. 5b–c, the scars on the composite films are narrow and typically in the removal stage of rough peaks; thus, the wear is not obvious. Cu nanoparticles were embedded into soft HPMC matrix and suggested having ploughing behavior, resulting in variation of scar profiles. 3.3. Wear behavior and wear mechanism of the composite coating Fig. 6a and c illustrates the surface morphology of the wear scars on Cu 1%/HPMC and Cu 2%/HPMC composite coatings, respectively. Enlarged parts are shown in Fig. 6b and d, respectively. Matrix material HPMC deforms elastically under stress; thus, its VAM is classified as S5M1 (S5: composite coating; M1: elastic deformation of HPMC). Cu nanoparticles on the surface of composite coatings deform under friction force during the wear process. VAM is classified as S5M2 (M2: flattening or damage of Cu, as pointed by arrow). Two VAMs (S5M1 and S5M2) can be observed at the surface of Cu/HPMC composite
Fig. 5. Observations of wear scar profiles in (a) HPMC; (b) Cu 2%/HPMC; and (c) Cu 2%/MoS2/HPMC composite coatings (2 N, 0.03 m/s, 30 m).
Fig. 6. Wear marks on: (a) Cu 1%/HPMC, 100× magnification; (b) Cu 1%/HPMC, 5000× magnification; (c) Cu 2%/HPMC, 100× magnification; and (d) Cu 2%/ HPMC, 5000× magnification. (e) Wear mechanism of the Cu/HPMC composite coating. 5
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Fig. 7. Wear marks on: (a) Cu 1%/MoS2/HPMC, 100× magnification; (b) Cu 1%/ MoS2/HPMC, 5000× magnification; (c) Cu 2%/ MoS2/HPMC, 100× magnification; and (d) Cu 2%/ MoS2/HPMC, 5000× magnification. (e) Wear mechanism of the Cu/ MoS2/HPMC composite coating.
the Cu 2%/MoS2/HPMC composite coating is the narrowest and faintest, as shown in Fig. 7c–d. MoS2 particles on the coating surface slip and delaminate under stress; thus, VAM is defined as S5M3 (M3: shearing/delamination of MoS2, as pointed by arrow). Cu particle deforms and represents VAM of S5M2. Matrix material HPMC exhibits S5M1 (M1: elastic deformation). In the Cu/MoS2/HPMC composite coating, three VAMs can be observed in the surface of composite coating, i.e. S5M1, S5M2 and S5M3. 3.4. Dissipation energy analysis of the Cu/MoS2/HPMC composite coating Surface friction is directly related to energy dissipation, and the form of wear changes the material structure. Sliding wear models can be used to illustrate the dissipated energy in the system. The volume of material removed by sliding wear is directly proportional to the work done due to friction [40]. The slope of the graph showing the volume of material removed vs the friction work represents the dominant wear mechanism. The Archard wear model, which is a model used to describe sliding wear, is given below:
Fig. 8. Wear volume and dissipation energy curves of HPMC-based composites.
∆E = F × V × ∆t
coatings. Fig. 7a and c shows slight scratches on the Cu 1%/MoS2/HPMC composite coating. High magnification images reveal that some copper nanoparticles deform and others retain their original shape. The scar on
E=
∑ ∆E
F = μN 6
(1) (2) (3)
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Acknowledgement
where F is the average value of the friction force, V is the constant sliding speed, and Δt is the time interval. Two sets of material systems with different slopes are observed in Fig. 8. According to the third-body theory, materials in tribological pairs are subject to load while accommodating the velocity difference with the first bodies. The role of VAM is to convert the energy generated by friction. Therefore, dissipation energy in the system can be reduced by efficient VAM [41]. In Cu/HPMC material system, bulk behavior is still dominated by HPMC due to small amount of Cu additive. Therefore, in HPMC and Cu/ HPMC, S5M1 dominates the tribological behavior of the entire system. Previous studies have reported that the addition of MoS2 is highly beneficial for the formation of transfer layers, especially in the case of MoS2 found on the surface of wear parts, suggesting that MoS2 delaminates and is transferred under stress [33]. In Cu/MoS2/HPMC, three velocity accommodation mechanisms coexist in the interface of the composite coating. Delamination of MoS2 in the composite can provide a more efficient VAM (S5M3), resulting in a lower friction coefficient and wear rate.
The authors would also like to thank the Center for Micro/Nano Science and Technology and Instrument center, National Cheng Kung University (NCKU) for the technical support. Funding The authors gratefully acknowledge the financial support for this project from the Ministry of Science and Technology, Taiwan (MOST 106-2221-E-006-092-MY3). References [1] B. Boonstra, Role of particulate fillers in elastomer reinforcement: a review, Polymer 20 (1979) 691–704. [2] N. Kida, M. Ito, F. Yatsuyanagi, H. Kaido, Studies on the structure and formation mechanism of carbon gel in the carbon black filled polyisoprene rubber composite, J. Appl. Polym. Sci. 61 (1996) 1345–1350. [3] S.-Y. Fu, X.-Q. Feng, B. Lauke, Y.-W. Mai, Effects of particle size, particle/matrix interface adhesion and particle loading on mechanical properties of particulate–polymer composites, Compos. Part B 39 (2008) 933–961. [4] T. Jiang, T. Kuila, N.H. Kim, B.-C. Ku, J.H. Lee, Enhanced mechanical properties of silanized silica nanoparticle attached graphene oxide/epoxy composites, Compos. Sci. Technol. 79 (2013) 115–125. [5] A.C. Balazs, T. Emrick, T.P. Russell, Nanoparticle polymer composites: where two small worlds meet, Science 314 (2006) 1107–1110. [6] D.R. Tenney, J.G. Davis Jr., R.B. Pipes, N. Johnston, NASA Composite Materials Development: Lessons Learned and Future Challenges, DOI (2009). [7] R. Bradshaw, F. Fisher, L. Brinson, Fiber waviness in nanotube-reinforced polymer composites—II: modeling via numerical approximation of the dilute strain concentration tensor, Compos. Sci. Technol. 63 (2003) 1705–1722. [8] J.N. Coleman, U. Khan, W.J. Blau, Y.K. Gun’ko, Small but strong: a review of the mechanical properties of carbon nanotube–polymer composites, Carbon 44 (2006) 1624–1652. [9] D. Barbier, D. Brown, A.-C. Grillet, S. Neyertz, Interface between end-functionalized PEO oligomers and a silica nanoparticle studied by molecular dynamics simulations, Macromolecules 37 (2004) 4695–4710. [10] G. Odegard, T. Clancy, T. Gates, Modeling of the mechanical properties of nanoparticle/polymer composites, Polymer 46 (2005) 553–562. [11] L. Pena-Paras, J. Taha-Tijerina, L. Garza, D. Maldonado-Cortés, R. Michalczewski, C. Lapray, Effect of CuO and Al2O3 nanoparticle additives on the tribological behavior of fully formulated oils, Wear 332 (2015) 1256–1261. [12] N. Chand, A. Naik, S. Neogi, Three-body abrasive wear of short glass fibre polyester composite, Wear 242 (2000) 38–46. [13] R. Trezona, D. Allsopp, I. Hutchings, Transitions between two-body and three-body abrasive wear: influence of test conditions in the microscale abrasive wear test, Wear 225 (1999) 205–214. [14] E. Rabinowicz, A. Mutis, Effect of abrasive particle size on wear, Wear 8 (1965) 381–390. [15] D.-X. Peng, C.-H. Chen, Y. Kang, Y.-P. Chang, S.-Y. Chang, Size effects of SiO2 nanoparticles as oil additives on tribology of lubricant, Industrial Lubrication and Tribology 62 (2010) 111–120. [16] C.J. Reeves, P.L. Menezes, M.R. Lovell, T.-C. Jen, The influence of surface roughness and particulate size on the tribological performance of bio-based multi-functional hybrid lubricants, Tribol. Int. 88 (2015) 40–55. [17] K.L. Harris, A.A. Pitenis, W.G. Sawyer, B.A. Krick, G.S. Blackman, D.J. Kasprzak, C.P. Junk, PTFE tribology and the role of mechanochemistry in the development of protective surface films, Macromolecules 48 (2015) 3739–3745. [18] S. Bahadur, The development of transfer layers and their role in polymer tribology, Wear 245 (2000) 92–99. [19] S.-C. Shi, Tribological performance of green lubricant enhanced by sulfidation IFMoS2, Materials 9 (2016) 856. [20] M. Conte, A. Igartua, Study of PTFE composites tribological behavior, Wear 296 (2012) 568–574. [21] M.N. Gardos, The synergistic effects of graphite on the friction and wear of MoS2 films in air, Tribol. Trans. 31 (1988) 214–227. [22] S.-C. Shi, T.-F. Huang, J.-Y. Wu, Preparation and tribological study of biodegradable lubrication films on Si substrate, Materials 8 (2015) 1738–1751. [23] S.-C. Shi, T.-F. Huang, Self-healing materials for ecotribology, Materials 10 (2017) 91. [24] S.-C. Shi, T.-F. Huang, Effects of temperature and humidity on self-healing behaviour of biopolymer hydroxylpropyl methylcellulose for ecotribology, Surf. Coat. Technol. 350 (2018) 997–1002. [25] S.-C. Shi, J.-Y. Wu, T.-F. Huang, Raman, FTIR, and XRD study of MoS2 enhanced hydroxypropyl methylcellulose green lubricant, Opt. Quant. Electron. 48 (2016) 474. [26] S.-C. Shi, J.-Y. Wu, T.-F. Huang, Y.-Q. Peng, Improving the tribological performance of biopolymer coating with MoS2 additive, Surf. Coat. Technol. 303 (2016) 250–255. [27] W.T. Hay, G.F. Fanta, S.C. Peterson, A. Thomas, K.D. Utt, K.A. Walsh, V.M. Boddu,
4. Conclusion In this study, MoS2 and Cu nanoparticles were added to biopolymer HPMC to form functional composite coatings. We then evaluated the tribological properties of the composite coatings, the third-body behaviors of the nanoscale additives during abrasive wear, and the correlation between wear volume and energy dissipation. The following key conclusions were drawn. 1. The dispersion effect of the matrix HPMC enabled the MoS2 and Cu nano-additives to disperse in the solution with good compatibility and good film formation. A composite coating was generated, demonstrating abrasion resistance effects, in which the crystal structure of the additives remained intact. 2. In the wear tests, the tribological properties of Cu/MoS2/HPMC films were superior to those of Cu/HPMC films. This indicates that the addition of MoS2 effectively improved the macro-scale tribological properties of HPMC. When MoS2 was subjected to a tangential force, it was easily sheared and stratified. The layered MoS2 exhibited fluidity and good wettability. Debris aggregated to form a good protective layer and transfer layer. When the transfer layer formed, the transfer layer and the third-body layer on the composite coating surface exhibited self-lubricating properties, which could reduce friction resistance and generate good tribological effects. 3. Based on the surface morphology and dissipation energy results, it can be concluded that the Cu/MoS2/HPMC composite systems exhibit multiple tribological mechanisms (S5M3, S5M2, and S5M1). Therefore, it can effectively reduce the wear rate and friction coefficient of wear parts.
CRediT authorship contribution statement Shih-Chen Shi:Conceptualization, Methodology, Writing - review & editing, Supervision, Project administration, Funding acquisition.Xiao-Ning Tsai:Investigation.Shia-Seng Pek:Writing original draft.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 7
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