Friction and Wear Research, Volume 4, 2016 www.seipub.org/fwr doi: 10.14355/fwr.2016.04.001
Influence of Aluminum Oxide Nanofibers Reinforcing Polyethylene Coating on the Abrasive Wear Ezzat A. A.*, Mousa M. O.**, Ali W. Y.** *El‐Minia High Institute of Technology, El‐Minia, EGYPT **Faculty of Engineering, Minia University, P. N. 61111, El‐Minia, EGYPT Abstract Experiments have been carried out to investigate the abrasive wear of steel specimens coated by polyethylene (PE) coating in a sandy soil. Aluminum oxide (Al2O3) nanofibers were employed as fillers in PE for fabricating the tested composites. The reinforced polyethylene coating has been impregnated by oil. An abrasive wear tester was developed to simulate the abrasion caused by a sandy soil against surfaces subjected to abrasive contaminants. Motion was transmitted to the disc via the drill chuck. The test time was 15 min. Experiments were carried out at 25 °C. Wear was measured by digital balance with an accuracy of ± 0.001 g. Wear and embedment of sand particles were analyzed using optical microscopy after the test. Based on the experimental results, it was found that, the addition of aluminum oxide fibers showed a considerable mitigation in the wear. When oil content increased, the wear decreased. Embedment of the sand particles for the smaller size particles is higher than the larger particles. Increasing of oil content enhanced the embedment of sand particles. The compatibility of Al2O3 nanofibers with PE leads to the inseparability of aluminum oxide due to good adhesion and decreased the wear. Keywords Wear, Polyethylene, Aluminum Oxide, Oil, Sand, Embedment
Introduction In a study investigated the abrasive wear of surfaces subjected to abrasive contaminants. It was found that, the addition of aluminum oxide particles to PE enhanced the wear resistance and the hardness of the matrix. The increase of oil content decreased the wear. PE coating filled by aluminum oxide particles content and 10% oil content showed zero wear, [1]. Where, nanoparticles of aluminum oxide were used as filling material in PVC coating. Steel specimens were used as substrate coated by the tested composites and oil. It was concluded that, where the aluminum oxide nanoparticles increased the wear decreased. Minimum wear illustrated in PVC with 9% aluminum oxide particles content and 10 % oil content. Increasing of oil content enhanced the embedment of the sand particles, [2]. One way to improve the wear properties of polyethylene (PE) is the use of inorganic fillers, such as kaolin, alumina, zirconium oxide and various carbon materials, [3]. The search for advanced tribological materials and concepts that will dramatically reduce size, weight and power consumption in modern aviation, space, and propulsion applications, while also satisfying extreme mechanical, environmental and endurance challenges, has dramatically increased over the past decade, [4]. There has been an increased use of aluminum and its alloys in engineering due to their excellent resistance to corrosion, good thermal conductivity, low density, and moderate cost, [5]. Alumina is one of the most widely used ceramic material finds applications as structural component for high temperature applications, heat engine and aerospace applications, as electronic substrate, [6]. Oxide ceramics such as Al2O3 ceramic coatings having superior hardness, chemical stability and refractory character, are commonly utilized to resist wear by friction and solid particle erosion, [7]. Among variety of ceramic fibers available on market, alumina fibers have received a particular attention due to unique high temperature properties and chemical stability, [8]. Latest investigations have shown that inorganic fullerene like materials (IFLM) can be used as nanomaterial for reduction of friction, [9]. The reinforcement of the epoxy matrix by nanoparticles and aluminum oxide thin fibers is especially attractive because these fillers increase the strength and thermal stability of the material and impart resistance to corrosive media, except for strongly alkaline media. It was demonstrated that aluminum oxide nanoparticles and aluminum oxide nanofibers have a reinforcing effect on
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epoxy matrices. The filling of the reinforcing effect on epoxy matrices. The filling of the reinforcing effect on epoxy matrices. The filling of the strength of the composite to a larger extent, whereas the filling of the matrices with nanofibers favors an increase in the Young modulus. Possibly, a combination of filling with aluminum oxide nanoparticles and aluminum oxide nanofibers will appear to be the most optimum variant of the reinforcement of epoxy matrices, [10]. The addition of alumina nanoparticles into epoxy resin demonstrates their ability to simultaneously improve stiffness, impact energy and failure strain at low filler contents (1–2 vol. %), [11]. Many studies on polymer composites have been done using inorganic compounds as a single filler material, [12]. Polymers and polymer–matrix composites have been finding great potentials in industry as a class of important tribo‐engineering materials, not just for their ease in manufacturing and low unit cost, but also for their potentially excellent tribological performance in engineered forms. Polymer composites filled with fibers and/or solid lubricants have been widely accepted as tribo‐materials and used on the components supposed to run without any external lubricants The former mainly improves the mechanical strength and wear resistance of polymers, while the latter improves friction characteristics and contributes to the control of wear. Tribological properties of polymer composites can also be greatly enhanced with the addition of nanoparticles, such as nano‐Al2O3, [13]. Polymeric materials having ultra‐high molecular weight (UHMW), such as polyethylene (PE), have superior friction and wear characteristics; thus, they are widely used as bearings and artificial joints. Various modifications of the chemical structure of the chains, including cross‐linking by irradiation ion implantation and blending with other components are made for the further improvement of the tribological properties, [14]. In desert areas, fine sand particles are attracted into machine lubricating systems through the air or fuel filters This condition causes serious wear problems for the sliding components in lubrication and, under extreme circumstances, a total failure of the equipment, [15].The wear of specimens coated with PE containing fillers of different hardness showed a good correlation between wear and fillers hardness. PE coatings gave a relatively soft matrix of plastic materials, which enhanced the embedment of sand particles. The embedment, [16] increased the abrasion resistance. In the present work, the wear of polyethylene (PE) filled with nanofibers of aluminum oxide and oil is discussed. Experimental An abrasive wear tester was constructed to simulate the abrasion caused by sand against surfaces subjected to abrasive contaminants. The tester was composed of a circular steel disc holder 180 mm in diameter capable of holding eight specimens of carbon steel (St 60). The specimens had the form of a pin with 40 mm length, 8 mm outer diameter and 4 mm inner diameter. Motion was transmitted to the disc via the drill chuck, Fig. 1. A speed of 280 rev/min was chosen. The test specimens were immersed at a fixed depth in a pan full of sand. The test time was 15 min. Experiments were carried out at 25 °C. Wear was measured by digital balance with an accuracy of ± 0.01 g. The specimens were coated with polyethylene filled with 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 % nanofibers of aluminum oxide content, Fig. 2 and 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 % oil content. Two‐ theta scale for aluminum oxide nanofibers is shown in Fig. 3. The sequence of the operations followed in the experimental work is shown in Fig. 4.
FIG. 1 LAYOUT OF SAND TEST RIG, [16].
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FIG. 2 SEM MICROGRAPH OF AL2O3 NANOFIBERS, [17].
FIG. 3 2‐THETA SCALE FOR ALUMINUM OXIDE NANOFIBERS, [17].
FIG. 4 THE SEQUENCE OF THE OPERATIONS FOLLOWED IN THE EXPERIMENTAL WORK.
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Results and Discussion
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FIG. 5 EFFECT OF ALUMINUM OXIDE NANOFIBERS ON WEAR FOR PE.
Figure 5 shows the effect of Al2O3 nanofibers on wear for PE. Where, increasing of Al2O3 nanofibers caused a decrease in wear. This can be attributed to the compatibility of Al2O3 nanofibers with PE which leads to the inseparability of Al2O3 due to its good adhesion. Moreover, the higher surface area for nanofibers increases the adherence of the Al2O3 fibers into the PE matrix and increases the wear resistance and the hardness of the matrix trend owing to the high wear resistance and hardness of aluminum oxide. The best results have been observed for PE with 7, 9 and 10 wt. % aluminum oxide content.
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Aluminum Oxide (Al2 O3) Content, % FIG. 6 EFFECT OF ALUMINUM OXIDE NANOFIBERS ON WEAR OF PE WITH 1% OIL CONTENT.
Figure 6 indicates the relation between wear and Al2O3 in PE with 1 wt. % oil content. The specimens covered by PE with Al2O3 nanofibers and impregnated by 1 wt. % oil gave good wear results as comparing to PE with Al2O3 nanofibers without oil. This behavior means that when oil content increases, wear decreases. This can be attributed to the improvement of the oil over the running surface by the addition of the oil content in the matrix. As the oil gets into the contact surfaces, it forms film of oil and reduces wear. Moreover, where Al2O3 increases wear decreases. The minimum wear was 1 mg at 8 wt. % Al2O3 content. The high adhesion for Al2O3 nanofibers
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increased the accordance between the PE and the nanofibers and showed a considerable mitigation in the wear due to the high hardness of Al2O3.
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Aluminum Oxide (Al2 O3) Content, % FIG. 7 EFFECT OF ALUMINUM OXIDE NANOFIBERS ON WEAR OF PE WITH 2% OIL CONTENT.
Figure 7 observes that, Al2O3 nanofibers enhanced the strength and the hardness of the coating. This enhancement increased with increasing Al2O3 fibers concentration. The higher surface area for nanofibers plays a noticeable role in enhancement of the adherence between the fibers and PE, as shown in Fig. 8. Also, the relatively high adhesion between the matrix and fibers tends to make the fibers to be inherent with the coating and increases the hardness of the coating as result of the high hardness of Al2O3. On the other hand, the effect of oil on the wear of PE with Al2O3 also is revealed, as shown in Fig. 7. For PE with Al2O3 and 2 wt. % oil content, wear is remarkably lowered as compared to the value of the wear of PE with Al2O3 content with 1 wt. % oil content. Wear was 1 mg at 10 wt. % Al2O3 content and 2 wt. % oil content, where wear was 3 mg at 10 wt. % Al2O3 content with 1 wt. % oil content. This can be interpreted on the better presence of the oil over the contact area due to the higher percentage of the oil. It seems that oil plays the role of the lubricant and tends to reduce the wear because it covers the surfaces and protects it from wear.
FIG. 8 THE SURFACE AREA FOR NANOFIBERS ALUMINUM OXIDE AND NANOPARTICLES ALUMINUM OXIDE.
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8 6 Wear, mg
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FIG. 9 EFFECT OF ALUMINUM OXIDE NANOFIBERS ON WEAR OF PE WITH 3% OIL CONTENT.
Figure 9 clearly demonstrates the effect of Al2O3 on wear of impregnating PE by 3 wt. % oil content. It is evident that the wear of PE with Al2O3 and 3 wt. % oil content is lower than wear of PE with Al2O3 and 2 wt. % oil content. The minimum wear was zero at PE with Al2O3 and 3 wt. % oil content. That improvement may be attributed to the higher percentage of the oil which allows the oil to cover the contact surface and decrease the wear. Moreover, it seems that increasing Al2O3 decreased the wear. The high hardness of Al2O3 causes a significant improvement in the surface of the coating and increases the hardness of the matrix. 5 4
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2Aluminum 3 4Oxide5 (Al 6O ) Content, 7 8% 9 2
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FIG. 10 EFFECT OF ALUMINUM OXIDE NANOFIBERS ON WEAR OF PE WITH 4% OIL CONTENT.
Wear, mg
Figure 10 illustrates the same trend for PE with Al2O3 and impregnated by 4 wt. % oil content. Where, the minimum wear was zero mg at 7 and 10 wt. % Al2O3 content. Moreover, impregnating PE by3 wt. % oil content improved the wear resistance. This can be related to the good lubricating properties of oil, which builds a film at the contact zone and reduces wear. Moreover, Al2O3 as filling material in PE coating enhanced the strength and the hardness of the polymeric matrix. 5 4 3 2 1 0 -1 -2 -3 -4 0
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Aluminum Oxide (Al2 O3) Content, % FIG. 11 EFFECT OF ALUMINUM OXIDE NANOFIBERS ON WEAR OF PE WITH 5% OIL CONTENT.
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Figure 11 concludes that, the embedment of the abrasive sand particles in the surface of the coating offers an explanation for the observed initial increase of weight recorded for both results of PE with 7, and 10 wt. % Al2O3 content with 5 wt. % oil content. Where, wear was ‐1 mg at 7 and 10 wt. % Al2O3 content and the negative sign indicated that the weight increased after test. This means that the sand particles embedded in the surface of the coating. The microscopic examination of the specimen surfaces confirmed the presence of the sand particles embedded in the surface, Fig. 12. Here, embedment of the sand particles in the surface of the test specimens can be expected due to the relatively higher hardness of the sand particles. On the other hand, the effect of oil content on wear of PE with aluminum oxide content also is revealed, as shown in Fig. 11. For PE with aluminum oxide content and 5 wt. % oil content, wear is remarkably lowered as comparing to the value of the wear of PE with Al2O3 content and 4 wt. % oil content. This can be related to the low hardness of the matrix because increasing oil content, which enables the hard particles of sand to be embedded in the surface of specimen, as shown in Fig. 13.
FIG. 12 SAND EMBEDMENT.
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FIG. 13 SCHEMATIC ILLUSTRATION THE EMBEDMENT OF SAND PARTICLES.
6 5 4 3 Wear, mg
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Aluminum Oxide (Al2 O3) Content, % FIG. 14 EFFECT OF ALUMINUM OXIDE NANOFIBERS ON WEAR OF PE WITH 6% OIL CONTENT.
Figure 14 presents the effect of Al2O3 on wear of impregnating PE by 6 wt. % oil content. The specimens covered by PE with Al2O3 nanofibers content and 6 wt. % oil content gave good wear results as comparing with the specimens covered by PE with Al2O3 nanofibers and 5 wt. % oil content. Wear was ‐2 mg at 5 and 10 wt. % Al2O3 content. This behavior means that the embedment is higher than the embedment in PE with 5 wt. % oil content, where minimum wear was ‐1 mg, as shown in Fig. 11. Where, the increasing of oil content decreased the hardness of the surface and enhanced the embedment of sand particles. Moreover, embedment is attributed to the fact of the higher hardness of the sand particles. To have more information about embedment of sand particles on the surface of the specimens, wear and embedment of sand particles was analyzed using optical microscopy after the test. A careful survey of Fig. 15 indicates that embedment of the sand particles for the smaller size particles are higher than the larger particles. Difference in embedment is probably due to their different angularity and the smaller particles are more angular, Fig. 16.
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FIG. 15 EFFECT OF SAND PARTICLES SIZE ON THE EMBEDMENT.
FIG. 16 SCHEMATIC ILLUSTRATION THE EFFECT OF THE SAND PARTICLE SIZE ON THE EMBEDMENT.
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Aluminum Oxide (Al2 O3) Content, % FIG. 17 EFFECT OF ALUMINUM OXIDE NANOFIBERS ON WEAR OF PE WITH 7% OIL CONTENT.
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Figure 17 shows that, increasing the oil content causes a significant improvement in the surface of the coating as the wear decreases with increasing oil content. It is well known that factors such as particle shape, size and hardness affect on the embedment of abrasive particles. In most cases, embedment of sand particles is revealed. The best of results was ‐3 mg for wear because the high percentage of oil content increases the plasticity of the coating surface and decreases the hardness of the coating matrix. Where, the hardness of the matrix decreases, the sand particles embedment increases. On the other hand, embedment of sharp particles is higher than the spherical particles as the sharp edge of these particles facilitates the embedment in the surface. 3 2 1 0
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FIG. 18 EFFECT OF ALUMINUM OXIDE NANOFIBERS ON WEAR OF PE WITH 8% OIL CONTENT.
Figure 18 observes that, embedment displayed in all results expect only at 0 and 7 wt. % Al2O3 content. This behavior confirms that, increasing the oil content causes a main role in the wear resistance of the coating surface. The percentage of the sand particles embedment is higher due to the high percentage of oil content which decreases the hardness of the matrix and enables the hard particles of sand to be easily embedded in the surface, allowing for growth the protective wear layer of sand particles. According to Fig. 18, wear was ‐5 mg at 3 and 8 wt. % Al2O3 particles content. The negative sign indicated the increase of the weight caused by embedment of the sand particles. 3 2 1 Wear, mg
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Aluminum Oxide (Al2 O3) Content, % FIG. 19 EFFECT OF ALUMINUM OXIDE NANOFIBERS ON WEAR OF PE WITH 9% OIL CONTENT.
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Figure 19 concludes that, the weight increased after test in all results expect only at 1 and 2 wt. % Al2O3 content. PE with Al2O3 content and 9 wt. % oil content showed promising results. It is clearly seen that the embedment is higher than the embedment in PE with Al2O3 content and 8 wt. % oil content. The wear was ‐6 mg at 9 wt. % aluminum oxide content. The negative sign indicated the increase of weight causes by embedment of sand particles. This phenomenon can be attributed to the increasing of oil which enhanced the embedment of sand particles. Where, the oil decreased the hardness of the coating by increasing the plasticity of it. The high abrasive action of sand particles facilitates the embedment. So the sand particles embedded in the surface of the specimen and increased the abrasion resistance of the coating appreciably, by forming a protective wear layer of hard sand particles on the surface of coating. The microscope examination illustrated the embedment of sand particles in the surface of the coating, as shown in Fig. 20.
FIG. 20 SAND EMBEDMENT.
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Aluminum Oxide (Al2 O3) Content, % FIG. 21 EFFECT OF ALUMINUM OXIDE NANOFIBERS ON WEAR OF PE WITH 10% OIL CONTENT.
Figure 21 clearly demonstrates that, minimum wear illustrated in PE with 10 wt. % oil content and Al2O3 content, Fig. 21. Wear was ‐7 mg at 8 and 9 wt. % Al2O3 content. The percentage of the sand particles embedment is higher due to the high percentage of oil content which decreased the hardness. So the sand particles embedded in the surface of the specimen. The decrease in hardness increased embedment of the sand particles in the surface of the coating and forming a protective wear layer of hard and particles on the surface of the coating leading to a significant reduction wear.
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Conclusions 1‐Minimum wear was observed in PE with 10% oil with 8 and 9% aluminum oxide content. 2‐Aluminum oxide fibers enhanced the wear resistance and the hardness of the coating. Where, the high adhesion for Al2O3 nanofibers increased the accordance between the PE and the nanofibers and showed a considerable mitigation in the wear due to the high hardness of Al2O3. 3‐Increase of oil impregnating PE enhanced the embedment of sand particles. Where, the oil decreased the hardness of the coating by increasing the plasticity of it. The high abrasive action of sand particles facilitates the embedment. So the sand particles embedded in the surface of the specimen and increased the abrasion resistance of the coating appreciably, by forming a protective wear layer of hard sand particles on the surface of coating. 4‐As oil content increased, the wear decreased. This can be attributed to the improvement of the oil over the running surface by the addition of the oil content in the matrix. As the oil gets into the contact surfaces and forms film of oil and reduces wear. 5‐Embedment of the sand particles for the smaller size particles are higher than the larger particles. Difference in embedment is probably due to their different angularity and the smaller particles are more angular. REFERENCE
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[14] T. Aoike, D. Yokoyama, H. Uehara, T. Yamanobe, T. Komoto, ʺTribology of ultra‐high molecular weight polyethylene disks molded at different temperaturesʺ, Wear 262, pp. 742–748, (2007). [15] L. Du, B. Xu, S. Dong, H. Yang, Y. Wu, ʺPreparation, microstructure and tribological properties of nano‐Al2O3/Ni brush plated composite coatingsʺ, Surface & Coatings Technology 192, pp. 311–316, (2005). [16] W. Y. Ali, F. M. H. Ezzat, ʺWear of tillage tools coated tools coated by thermoplastic coatingsʺ, Wear 173, pp. 115‐119, (1994). [17] Inorganic nanofiber_Sample Data, Pardam s.r.o., Jindřišská 2025, 530 01 Pardubice, Czech Republic (Production Facility in Nové Město na Moravě), ID: 26077914, (2013).
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