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Investigation of a self-lubricating coating for diesel engine pistons, as produced by combined microarc oxidation and electrophoresis
Wear 394–395 (2018) 109–112
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Investigation of a self-lubricating coating for diesel engine pistons, as produced by combined microarc oxidation and electrophoresis
T
⁎
Chunsheng Ma , Dong Cheng, Xinhe Zhu, Zhijun Yan, Jingguo Fu, Jing Yu, Zeze Liu, Guangyu Yu, Shibin Zheng School of Marine Engineering, Dalian Maritime University, Dalian 116026, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Ceramic matrix composites Sliding wear Microarc oxidation Electrophoresis deposition Self-lubricating
The purpose of this work was to improve reliability and durability of high-silicon aluminum alloy piston skirts for diesel engines. Novel ceramic matrix composites were fabricated on ZL109 aluminum alloy substrates by two steps combining microarc oxidation (MAO) with electrophoresis deposition (EPD). MoS2 is incorporated into an aluminum oxide matrix during processing. The effects of the ceramic matrix composites on anti-wear and selflubricating were investigated using a reciprocating test method and cylinder liner samples (boron copper cast iron) as the sliding partner. Compared to the high-silicon aluminum alloys substrate, the friction coefficient of ceramic matrix composites against the liner material was reduced by 35% under dry sliding, the wear loss was decreased by 95%, the worn surfaces were flat and smooth, and friction coefficient was relatively stable. The mechanisms by which the observed advantages were produced are discussed.
1. Introduction In the design of diesel engines, it is important to ensure the reliability and durability of the cylinder-piston group [1,2]. The piston, which is the component of the cylinder-piston group, bears the complex thermal stress, mechanical erosion, friction and wear with the cylinder liner and so on. It is largely responsible for the life and running costs of the engine. High-silicon aluminum alloys are widely used in the piston manufacturing. However, high-silicon aluminum alloys are characteristics of low surface hardness, poor abrasion and corrosion resistance. Improving the piston reliability can decrease the malfunction rate of the engine, especially, using effective methods of surface hardening [1,2]. Composite coatings are widely applied to mechanical components due to their high mechanical, chemical and tribological properties, as well as excellent corrosion resistance [3–6]. For examples, Sun et al. prepared Ni-Al2O3/graphite composite coatings on LY12 aluminum alloys using a three-step process that involved electrophoresis and electrodeposition. They found that the new Ni-Al2O3/graphite composite coatings presented excellent lubricating properties and wear resistance due to the effects of graphite and Al2O3 particles [7]. Liu et al. prepared a Cu2O–CoO/Al2O3 composite coating on an aluminum substrate by MAO in a phosphate electrolyte modified with Cu(Ac)2 and Co (Ac)2 solutions. The catalyst exhibited an excellent chemical stability with negligible leaching ions [8]. Furthermore, composite coatings containing fine particles of ⁎
graphite, SiC, Al2O3, Si3N4 etc. can remarkably reduce the friction coefficient as well as the wear rate of the mechanical components. However, the binding modes of the substrate and composite coatings via EPD, electrodeposition, and spraying etc. include epitaxial growth, chemical bond combination, molecular bond, and mechanical bond etc., which are difficult to satisfy the requirement of pistons working conditions. MAO is a kind of surface treatment technology including electrochemical and plasma-chemical process etc. It occurs on the surface of valve metals, such as Al, Mg, Ti, Zr, Ta, and Nb and their alloys, and forms a porous ceramic coating. The binding mode of ceramic coating and the substrate is a metallic bond combination with the highest bonding strength [9]. In our previous study, MAO has been demonstrated to significantly improve wear-resisting, and the average microhardness is more than 1200 HV. A hardness gradient exists across the coating thickness from the dense layer to the loose layer of ceramic coating [10]. However, the hardness of cylinder liner surface is far less than the dense layer, and the loose layer is easy to wear off due to the pores inside it. Although by MAO alone, we may increase the wear resistance at the surface of a piston made from high-silicon aluminum alloys, some measures must be taken to increase the wear resistance of cylinder liner surface [2], which will observably raise the costs of technology. This work was devoted to fabricating the anti-wear and self-lubricating ceramic matrix composites on high-silicon aluminum alloys
Corresponding author. E-mail address: [email protected] (C. Ma).
http://dx.doi.org/10.1016/j.wear.2017.10.012 Received 28 February 2017; Received in revised form 19 October 2017; Accepted 23 October 2017 Available online 25 October 2017 0043-1648/ © 2017 Elsevier B.V. All rights reserved.
Wear 394–395 (2018) 109–112
C. Ma et al.
coatings, then forms a uniform coating on the ceramic coating, with a thickness of approximately 10 µm (Fig. 1C). The morphology of ups and downs also improves the bonding strength between ceramic coating and EPD coating by inlaying. As is illustrated in Fig. S1A, B, the white flake materials are MoS2. So the regions of pores and thinner coatings contain much more MoS2 wrapped by electrophoretic paint (Fig. 2B, C and Fig. 1C). In the process of friction and wear, although the EPD coating may be worn, the ceramic coating has much better wear-resisting property. MoS2 retained in the regions of pores and thinner coatings, would continue to play a role of anti-wear and self-lubricating. Therefore, the preparation of ceramic matrix composites requires the porous structure and appropriate thickness of the ceramic coating. The larger ceramic coating thickness is, the greater power is required. Fig. 3A shows the XRD spectra of the ceramic coating and the ceramic matrix composites. Because of the porous structure, there are remarkable Al and γ-Al2O3 peaks for the ceramic coating (Fig. 3A (a)). It is seen that there are not only typical Al and γ-Al2O3 peaks for the ceramic matrix composites, but also the (002) where 2θ is 14.331° (Fig. 3A (b)) when compared with the XRD spectrum of the ceramic coating. This observation demonstrates incorporation of MoS2 particles into the ceramic matrix composites. This result further indicates that the combination method belongs to a mechanical combination.
with MoS2 particles in two steps by a combination of MAO and EPD, due to the good lubricating property of MoS2 and the adaptive hardness, porous structure and excellent wear-resisting property of the MAO ceramic coating. In addition, the bonding strength of EPD coating and ceramic coating is much larger than traditional paddings, such as boiling and brush–painting methods [11]. Specifically, it can be described that the two steps are padding in micro-texture on the highsilicon aluminum alloy substrate. The padding is operated by EPD [11,12], the micro-texture is treated by MAO. 2. Experimental details ZL109 aluminum alloy sample (wt%: 11–13% Si, 0.5–1.5% Cu, 0.8–1.3% Mg, 0.8–1.5% Ni, Residue Al. 40 mm*10 mm*10 mm) was selected to fabricate the ceramic matrix composites. The MAO electrolyte was prepared by dissolving of Na2SiO3, Na2WO3, KOH, and EDTA-2Na in deionized water. EPD electrolyte was prepared by homodisperse of acrylic anodic electrophoretic paint (10% Solid points), MoS2 particles (10 g/l, average size: 40 nm), and polyethylene glycol (mass ratio to MoS2 1:2) in deionized water. Meanwhile, the bath was stirred by a magnetic stirrer at a speed of 150 rpm for 30 min and then ultrasonic oscillations for 1 h by using an ultrasonic cleaner before the MAO and EPD. MAO and EPD were operated by self-developing power. First, the sample was operated by MAO for 15 min, next, cleaned by ultrasonic for 30 min. Then, the sample was operated by EPD for 1 min. Whereafter, the sample was baked for 30 min at 170 °C. Natural cooling in the end. The whole process is shown in Fig. 1. The surface and cross-section micrographs were examined by scanning electron microscope (SEM, VEGA 3, TESCAN). The composition in ceramic matrix composites was determined by X-ray diffraction (XRD, EMPYREAN) and energy dispersive x-ray spectroscopy (EDXS) coupled to the SEM. The wear resistance of the ceramic matrix composites was tested under dry sliding conditions in the atmosphere at room temperature by the reciprocating friction and wear tester (selfdeveloping, DALIAN MARITIME UNIVERSITY).
3.2. Friction and wear properties In the present research, the anti-friction and wear resistance of the ceramic matrix composites were examined by the reciprocating friction and wear tester driven by a servo motor. The sliding counterbody material is boron copper cast iron which is used as a predominant material of cylinder liner in diesel engine design for the higher hardness and wear resistance. The friction pairs were the ZL109 aluminum alloy (40 mm*10 mm*10 mm) and cylinder liner samples (boron copper cast iron, 110 mm*10 mm*2 mm), the ceramic matrix composites (40 mm*10 mm*10 mm) and cylinder liner samples (110 mm*10 mm*2 mm). The test was carried out with 20 N loading at 0.2 m/s sliding speed for 5 min. Each sample was cleaned by ultrasonic washing in absolute ethyl alcohol before and after the test, so as to reduce the impurities on the surface of samples which may affect the whole wear test, and thus to improve the calculation accuracy of the weight loss and reduce the fluctuation of the friction coefficients. The friction coefficient and sliding time were obtained automatically during the test by acquisition cards, transducers and software for collecting information. The weight loss of samples was measured by weighing the samples before and after each wear test, using an electrical balance with a precision of 0.1 mg. Five samples of each set of conditions were tested for each data, so as to avoid the fluctuations in the data, and the reported values are the average resulted from these measurements. The purpose of the test is to prove the self-lubrication property of the ceramic matrix composites itself. If the tests had run with
3. Results and discussion 3.1. composition and morphology Surface morphologies of the ceramic coating and the ceramic matrix composites are shown in Fig. 2. The SEM micrograph of the ceramic coating surface, as illustrated in Fig. 2A, shows that the ceramic coating surface is characteristic of a porous structure of ups and downs, with a thickness of approximately 10 µm (Fig. 1B). The surface structure of ceramic coating is the important basic condition of EPD and self-lubricating ceramic matrix composites, because EPD requires having an electric field, meanwhile the regions of pores and thinner coatings provide stronger electric field intensity. So EPD prefers forming in the regions of pores and thinner
Fig. 1. Scheme showing the two-step method of ceramic matrix composites: (A) cross-section morphology of ZL109 aluminum alloy substrate. (B) cross-section morphology of the ZL109 aluminum alloy substrate after MAO. (C) cross-section morphology of the ZL109 aluminum alloy substrate after MAO and EPD. 1. ZL109 aluminum alloy substrate. 2. MAO ceramic coating. 3. EPD coating. 4. MAO for 15 min 5. EPD for 1 min.
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Fig. 2. SEM micrographs of surfaces: (A) surface of the ceramic coating. (B) surface of the ceramic matrix composites (1.43kx). (C) surface of the ceramic matrix composites (21.5kx).
Fig. 3. XRD patterns and friction coefficients curves: (A) XRD patterns of (a) the ceramic coating and (b) the ceramic matrix composites. (B) friction coefficients of (a) the ceramic matrix composites and (b) the ZL109 aluminum alloy substrate.
stabilizes at around 0.45 (Fig. 3B (a)). In the testing process the friction coefficient curve was more smooth, and the tester vibrated slightly. The ceramic matrix composites and cylinder liner samples worn surfaces are very smooth (Fig. 4C, D). The worn surface of ceramic matrix composites was in black, which is due to the diffusion of MoS2 molecules and the substrate surface. It can be seen that MoS2 particles formed a thin uneven lubricating film on the contact regions during dry sliding (Fig. 4C). The lubricating film could strongly adhere to the contact zone by self-characteristics of MoS2 [13,14], inlaying of ceramic coating and EPD coating, the bonding strength of EPD coating and ceramic coating. It also can be seen that the ceramic coating exposed in some regions (Fig. 4C), which improve the wear resistance of the ceramic matrix composites. So the average wear loss of cylinder liner samples cooperated with the ceramic matrix composites is almost 0.5 mg. Because of the pores of ceramic coating, the microhardness of ceramic coating surface is difficult to be measured directly. However, we can speculate indirectly that the ceramic coating possesses adaptive hardness by the result of friction and wear properties testing.
lubricants, MoS2 particles would have dispersed in lubricants, and then MoS2 particles would have become a lubricant additive, the friction coefficient and wear loss might have also been reduced. In addition, during the reciprocating movement of a piston in a cylinder liner, the lubricating film is hardly established between piston skirt and cylinder liner surface at Top Dead Center and Bottom Dead Center, the particular micro-regions may approach dry condition. As well known, after long-time operation of a diesel engine the moving parts of the diesel engine may appear location deviations, for example, the deviation of side axial clearance of connecting rod big end. The location deviations may lead to clearance decrease between piston skirt and cylinder line in some area, then the lubrication of piston skirt and cylinder liner will become very bad. So, we carried out our tests in dry condition. The applied variables are friction coefficient and wear loss. The test conditions were also selected based on lots of previous experiments, and relevant literatures, including literature [7]. But there are many conditions which were hardly simulated in a university labs, such as the vibration of diesel engines and dynamic thermal and mechanical load of pistons. Fig. 3B shows the friction coefficients of the two friction pairs. Fig. 4 shows the worn surfaces morphologies of the two friction pairs. Fig. S1C, D show the composition of ceramic matrix composites worn surface. The average friction coefficient of ZL109 aluminum alloy substrate and cylinder liner samples initially slowly decreases and then stabilizes at around 0.7 (Fig. 3B (b)). In the testing process the friction coefficient fluctuated remarkably, and the tester vibrated acutely. ZL109 aluminum alloy substrate and cylinder liner samples surfaces were seriously worn, distributed many scratches and abraded badly unevenly (Fig. 4A, B). The average wear loss of cylinder liner samples is almost 10 mg. However, the average friction coefficient of the ceramic matrix composites and cylinder liner samples initially slowly increases and
4. Conclusions For improving reliability and durability of high-silicon aluminum alloy piston skirts for diesel engines, microarc oxidation and electrophoresis deposition were employed on ZL109 aluminum alloy substrates obtaining novel ceramic matrix composites. The conclusions as follows: (1) A compact and uniform ceramic matrix composites with a high volume content of MoS2 particles can be prepared by EPD on the basis of MAO, the whole thickness is approximately 20 µm. (2) The ceramic matrix composites exhibited excellent anti-wear and 111
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Fig. 4. SEM images of worn surfaces: (A) ZL109 aluminum alloy substrate. (B) cylinder liner sample cooperated with the ZL109 aluminum alloy substrate. (C) ceramic matrix composites. (D) cylinder liner sample cooperated with the ceramic matrix composites.
Appendix A. Supporting information
self-lubricating properties, which could be attributed to the ceramic coating with excellent anti-wear property and MoS2 particles which were uniformly dispersed into the porous structure of the ceramic coating.
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.wear.2017.10.012. References
The results of this paper are valuable for the research on the preparation piston skirt surface strengthening coating with good tribological properties, and expands the application areas of MAO and EPD in the wear science and technology. The vibration of diesel engines and dynamic thermal and mechanical load of pistons are not included in the present experiment. Thus, for further validating excellent anti-wear and self-lubricating properties of the ceramic matrix composites, the diesel bench test would be carried to completely simulate the practical working conditions.
[1] C. Bing-Quan, C. Cang-Xiu, P. Jun-Bo, J. Wuhan Univ. Technol. Mater. Sci. Ed. 18 (1) (2003) 25–28. [2] I.A. Butusov, N.Y. Dudareva, Russ. Eng. Res. 36 (2) (2016) 116–118. [3] J.J. Sobczak, L. Drenchev, J. Mater. Sci. Technol. 29 (4) (2013) 297–316. [4] Q. Feng, T. Li, H. Teng, X. Zhang, Y. Zhang, C. Liu, J. Jin, Surf. Coat. Technol. 202 (17) (2008) 4137–4144. [5] W.X. Chen, J.P. Tu, L.Y. Wang, H.Y. Gan, Z.D. Xu, X.B. Zhang, Carbon 41 (2) (2003) 215–222. [6] K.I. Triantou, D.I. Pantelis, V. Guipont, M. Jeandin, Wear 336–337 (2015) 96–107. [7] W.-C. Sun, P. Zhang, K. Zhao, M.-M. Tian, Y. Wang, Wear 342–343 (2015) 172–180. [8] D. Liu, B. Jiang, Z. Liu, Y. Ge, Y. Wang, Ceram. Int. 40 (7) (2014) 9981–9987. [9] X. Jiang, C. Pan, 2015, pp. 257–276. [10] L. Rama Krishna, K.R.C. Somaraju, G. Sundararajan, Surf. Coat. Technol. 163–164 (2003) 484–490. [11] V. Meille, Appl. Catal. A: Gen. 315 (2006) 1–17. [12] R. Nedyalkova, A. Casanovas, J. Llorca, D. Montané, Int. J. Hydrog. Energy 34 (6) (2009) 2591–2599. [13] J.-W. Jiang, Front. Phys. 10 (3) (2015) 287–302. [14] Y.-C. Lin, D.O. Dumcenco, Y.-S. Huang, K. Suenaga, Nat. Nano 9 (5) (2014) 391–396.
Acknowledgement We are grateful to Dr. Lina Wang for scientific editing of the manuscript. This work was supported by Ministry of Transport of the People's Republic of China (2015329225230); the National Natural Science Foundation of China (51605066);
112
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Investigation of a self-lubricating coating for diesel engine pistons, as produced by combined microarc oxidation and electrophoresis
T
⁎
Chunsheng Ma , Dong Cheng, Xinhe Zhu, Zhijun Yan, Jingguo Fu, Jing Yu, Zeze Liu, Guangyu Yu, Shibin Zheng School of Marine Engineering, Dalian Maritime University, Dalian 116026, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Ceramic matrix composites Sliding wear Microarc oxidation Electrophoresis deposition Self-lubricating
The purpose of this work was to improve reliability and durability of high-silicon aluminum alloy piston skirts for diesel engines. Novel ceramic matrix composites were fabricated on ZL109 aluminum alloy substrates by two steps combining microarc oxidation (MAO) with electrophoresis deposition (EPD). MoS2 is incorporated into an aluminum oxide matrix during processing. The effects of the ceramic matrix composites on anti-wear and selflubricating were investigated using a reciprocating test method and cylinder liner samples (boron copper cast iron) as the sliding partner. Compared to the high-silicon aluminum alloys substrate, the friction coefficient of ceramic matrix composites against the liner material was reduced by 35% under dry sliding, the wear loss was decreased by 95%, the worn surfaces were flat and smooth, and friction coefficient was relatively stable. The mechanisms by which the observed advantages were produced are discussed.
1. Introduction In the design of diesel engines, it is important to ensure the reliability and durability of the cylinder-piston group [1,2]. The piston, which is the component of the cylinder-piston group, bears the complex thermal stress, mechanical erosion, friction and wear with the cylinder liner and so on. It is largely responsible for the life and running costs of the engine. High-silicon aluminum alloys are widely used in the piston manufacturing. However, high-silicon aluminum alloys are characteristics of low surface hardness, poor abrasion and corrosion resistance. Improving the piston reliability can decrease the malfunction rate of the engine, especially, using effective methods of surface hardening [1,2]. Composite coatings are widely applied to mechanical components due to their high mechanical, chemical and tribological properties, as well as excellent corrosion resistance [3–6]. For examples, Sun et al. prepared Ni-Al2O3/graphite composite coatings on LY12 aluminum alloys using a three-step process that involved electrophoresis and electrodeposition. They found that the new Ni-Al2O3/graphite composite coatings presented excellent lubricating properties and wear resistance due to the effects of graphite and Al2O3 particles [7]. Liu et al. prepared a Cu2O–CoO/Al2O3 composite coating on an aluminum substrate by MAO in a phosphate electrolyte modified with Cu(Ac)2 and Co (Ac)2 solutions. The catalyst exhibited an excellent chemical stability with negligible leaching ions [8]. Furthermore, composite coatings containing fine particles of ⁎
graphite, SiC, Al2O3, Si3N4 etc. can remarkably reduce the friction coefficient as well as the wear rate of the mechanical components. However, the binding modes of the substrate and composite coatings via EPD, electrodeposition, and spraying etc. include epitaxial growth, chemical bond combination, molecular bond, and mechanical bond etc., which are difficult to satisfy the requirement of pistons working conditions. MAO is a kind of surface treatment technology including electrochemical and plasma-chemical process etc. It occurs on the surface of valve metals, such as Al, Mg, Ti, Zr, Ta, and Nb and their alloys, and forms a porous ceramic coating. The binding mode of ceramic coating and the substrate is a metallic bond combination with the highest bonding strength [9]. In our previous study, MAO has been demonstrated to significantly improve wear-resisting, and the average microhardness is more than 1200 HV. A hardness gradient exists across the coating thickness from the dense layer to the loose layer of ceramic coating [10]. However, the hardness of cylinder liner surface is far less than the dense layer, and the loose layer is easy to wear off due to the pores inside it. Although by MAO alone, we may increase the wear resistance at the surface of a piston made from high-silicon aluminum alloys, some measures must be taken to increase the wear resistance of cylinder liner surface [2], which will observably raise the costs of technology. This work was devoted to fabricating the anti-wear and self-lubricating ceramic matrix composites on high-silicon aluminum alloys
Corresponding author. E-mail address: [email protected] (C. Ma).
http://dx.doi.org/10.1016/j.wear.2017.10.012 Received 28 February 2017; Received in revised form 19 October 2017; Accepted 23 October 2017 Available online 25 October 2017 0043-1648/ © 2017 Elsevier B.V. All rights reserved.
Wear 394–395 (2018) 109–112
C. Ma et al.
coatings, then forms a uniform coating on the ceramic coating, with a thickness of approximately 10 µm (Fig. 1C). The morphology of ups and downs also improves the bonding strength between ceramic coating and EPD coating by inlaying. As is illustrated in Fig. S1A, B, the white flake materials are MoS2. So the regions of pores and thinner coatings contain much more MoS2 wrapped by electrophoretic paint (Fig. 2B, C and Fig. 1C). In the process of friction and wear, although the EPD coating may be worn, the ceramic coating has much better wear-resisting property. MoS2 retained in the regions of pores and thinner coatings, would continue to play a role of anti-wear and self-lubricating. Therefore, the preparation of ceramic matrix composites requires the porous structure and appropriate thickness of the ceramic coating. The larger ceramic coating thickness is, the greater power is required. Fig. 3A shows the XRD spectra of the ceramic coating and the ceramic matrix composites. Because of the porous structure, there are remarkable Al and γ-Al2O3 peaks for the ceramic coating (Fig. 3A (a)). It is seen that there are not only typical Al and γ-Al2O3 peaks for the ceramic matrix composites, but also the (002) where 2θ is 14.331° (Fig. 3A (b)) when compared with the XRD spectrum of the ceramic coating. This observation demonstrates incorporation of MoS2 particles into the ceramic matrix composites. This result further indicates that the combination method belongs to a mechanical combination.
with MoS2 particles in two steps by a combination of MAO and EPD, due to the good lubricating property of MoS2 and the adaptive hardness, porous structure and excellent wear-resisting property of the MAO ceramic coating. In addition, the bonding strength of EPD coating and ceramic coating is much larger than traditional paddings, such as boiling and brush–painting methods [11]. Specifically, it can be described that the two steps are padding in micro-texture on the highsilicon aluminum alloy substrate. The padding is operated by EPD [11,12], the micro-texture is treated by MAO. 2. Experimental details ZL109 aluminum alloy sample (wt%: 11–13% Si, 0.5–1.5% Cu, 0.8–1.3% Mg, 0.8–1.5% Ni, Residue Al. 40 mm*10 mm*10 mm) was selected to fabricate the ceramic matrix composites. The MAO electrolyte was prepared by dissolving of Na2SiO3, Na2WO3, KOH, and EDTA-2Na in deionized water. EPD electrolyte was prepared by homodisperse of acrylic anodic electrophoretic paint (10% Solid points), MoS2 particles (10 g/l, average size: 40 nm), and polyethylene glycol (mass ratio to MoS2 1:2) in deionized water. Meanwhile, the bath was stirred by a magnetic stirrer at a speed of 150 rpm for 30 min and then ultrasonic oscillations for 1 h by using an ultrasonic cleaner before the MAO and EPD. MAO and EPD were operated by self-developing power. First, the sample was operated by MAO for 15 min, next, cleaned by ultrasonic for 30 min. Then, the sample was operated by EPD for 1 min. Whereafter, the sample was baked for 30 min at 170 °C. Natural cooling in the end. The whole process is shown in Fig. 1. The surface and cross-section micrographs were examined by scanning electron microscope (SEM, VEGA 3, TESCAN). The composition in ceramic matrix composites was determined by X-ray diffraction (XRD, EMPYREAN) and energy dispersive x-ray spectroscopy (EDXS) coupled to the SEM. The wear resistance of the ceramic matrix composites was tested under dry sliding conditions in the atmosphere at room temperature by the reciprocating friction and wear tester (selfdeveloping, DALIAN MARITIME UNIVERSITY).
3.2. Friction and wear properties In the present research, the anti-friction and wear resistance of the ceramic matrix composites were examined by the reciprocating friction and wear tester driven by a servo motor. The sliding counterbody material is boron copper cast iron which is used as a predominant material of cylinder liner in diesel engine design for the higher hardness and wear resistance. The friction pairs were the ZL109 aluminum alloy (40 mm*10 mm*10 mm) and cylinder liner samples (boron copper cast iron, 110 mm*10 mm*2 mm), the ceramic matrix composites (40 mm*10 mm*10 mm) and cylinder liner samples (110 mm*10 mm*2 mm). The test was carried out with 20 N loading at 0.2 m/s sliding speed for 5 min. Each sample was cleaned by ultrasonic washing in absolute ethyl alcohol before and after the test, so as to reduce the impurities on the surface of samples which may affect the whole wear test, and thus to improve the calculation accuracy of the weight loss and reduce the fluctuation of the friction coefficients. The friction coefficient and sliding time were obtained automatically during the test by acquisition cards, transducers and software for collecting information. The weight loss of samples was measured by weighing the samples before and after each wear test, using an electrical balance with a precision of 0.1 mg. Five samples of each set of conditions were tested for each data, so as to avoid the fluctuations in the data, and the reported values are the average resulted from these measurements. The purpose of the test is to prove the self-lubrication property of the ceramic matrix composites itself. If the tests had run with
3. Results and discussion 3.1. composition and morphology Surface morphologies of the ceramic coating and the ceramic matrix composites are shown in Fig. 2. The SEM micrograph of the ceramic coating surface, as illustrated in Fig. 2A, shows that the ceramic coating surface is characteristic of a porous structure of ups and downs, with a thickness of approximately 10 µm (Fig. 1B). The surface structure of ceramic coating is the important basic condition of EPD and self-lubricating ceramic matrix composites, because EPD requires having an electric field, meanwhile the regions of pores and thinner coatings provide stronger electric field intensity. So EPD prefers forming in the regions of pores and thinner
Fig. 1. Scheme showing the two-step method of ceramic matrix composites: (A) cross-section morphology of ZL109 aluminum alloy substrate. (B) cross-section morphology of the ZL109 aluminum alloy substrate after MAO. (C) cross-section morphology of the ZL109 aluminum alloy substrate after MAO and EPD. 1. ZL109 aluminum alloy substrate. 2. MAO ceramic coating. 3. EPD coating. 4. MAO for 15 min 5. EPD for 1 min.
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Fig. 2. SEM micrographs of surfaces: (A) surface of the ceramic coating. (B) surface of the ceramic matrix composites (1.43kx). (C) surface of the ceramic matrix composites (21.5kx).
Fig. 3. XRD patterns and friction coefficients curves: (A) XRD patterns of (a) the ceramic coating and (b) the ceramic matrix composites. (B) friction coefficients of (a) the ceramic matrix composites and (b) the ZL109 aluminum alloy substrate.
stabilizes at around 0.45 (Fig. 3B (a)). In the testing process the friction coefficient curve was more smooth, and the tester vibrated slightly. The ceramic matrix composites and cylinder liner samples worn surfaces are very smooth (Fig. 4C, D). The worn surface of ceramic matrix composites was in black, which is due to the diffusion of MoS2 molecules and the substrate surface. It can be seen that MoS2 particles formed a thin uneven lubricating film on the contact regions during dry sliding (Fig. 4C). The lubricating film could strongly adhere to the contact zone by self-characteristics of MoS2 [13,14], inlaying of ceramic coating and EPD coating, the bonding strength of EPD coating and ceramic coating. It also can be seen that the ceramic coating exposed in some regions (Fig. 4C), which improve the wear resistance of the ceramic matrix composites. So the average wear loss of cylinder liner samples cooperated with the ceramic matrix composites is almost 0.5 mg. Because of the pores of ceramic coating, the microhardness of ceramic coating surface is difficult to be measured directly. However, we can speculate indirectly that the ceramic coating possesses adaptive hardness by the result of friction and wear properties testing.
lubricants, MoS2 particles would have dispersed in lubricants, and then MoS2 particles would have become a lubricant additive, the friction coefficient and wear loss might have also been reduced. In addition, during the reciprocating movement of a piston in a cylinder liner, the lubricating film is hardly established between piston skirt and cylinder liner surface at Top Dead Center and Bottom Dead Center, the particular micro-regions may approach dry condition. As well known, after long-time operation of a diesel engine the moving parts of the diesel engine may appear location deviations, for example, the deviation of side axial clearance of connecting rod big end. The location deviations may lead to clearance decrease between piston skirt and cylinder line in some area, then the lubrication of piston skirt and cylinder liner will become very bad. So, we carried out our tests in dry condition. The applied variables are friction coefficient and wear loss. The test conditions were also selected based on lots of previous experiments, and relevant literatures, including literature [7]. But there are many conditions which were hardly simulated in a university labs, such as the vibration of diesel engines and dynamic thermal and mechanical load of pistons. Fig. 3B shows the friction coefficients of the two friction pairs. Fig. 4 shows the worn surfaces morphologies of the two friction pairs. Fig. S1C, D show the composition of ceramic matrix composites worn surface. The average friction coefficient of ZL109 aluminum alloy substrate and cylinder liner samples initially slowly decreases and then stabilizes at around 0.7 (Fig. 3B (b)). In the testing process the friction coefficient fluctuated remarkably, and the tester vibrated acutely. ZL109 aluminum alloy substrate and cylinder liner samples surfaces were seriously worn, distributed many scratches and abraded badly unevenly (Fig. 4A, B). The average wear loss of cylinder liner samples is almost 10 mg. However, the average friction coefficient of the ceramic matrix composites and cylinder liner samples initially slowly increases and
4. Conclusions For improving reliability and durability of high-silicon aluminum alloy piston skirts for diesel engines, microarc oxidation and electrophoresis deposition were employed on ZL109 aluminum alloy substrates obtaining novel ceramic matrix composites. The conclusions as follows: (1) A compact and uniform ceramic matrix composites with a high volume content of MoS2 particles can be prepared by EPD on the basis of MAO, the whole thickness is approximately 20 µm. (2) The ceramic matrix composites exhibited excellent anti-wear and 111
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Fig. 4. SEM images of worn surfaces: (A) ZL109 aluminum alloy substrate. (B) cylinder liner sample cooperated with the ZL109 aluminum alloy substrate. (C) ceramic matrix composites. (D) cylinder liner sample cooperated with the ceramic matrix composites.
Appendix A. Supporting information
self-lubricating properties, which could be attributed to the ceramic coating with excellent anti-wear property and MoS2 particles which were uniformly dispersed into the porous structure of the ceramic coating.
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.wear.2017.10.012. References
The results of this paper are valuable for the research on the preparation piston skirt surface strengthening coating with good tribological properties, and expands the application areas of MAO and EPD in the wear science and technology. The vibration of diesel engines and dynamic thermal and mechanical load of pistons are not included in the present experiment. Thus, for further validating excellent anti-wear and self-lubricating properties of the ceramic matrix composites, the diesel bench test would be carried to completely simulate the practical working conditions.
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Acknowledgement We are grateful to Dr. Lina Wang for scientific editing of the manuscript. This work was supported by Ministry of Transport of the People's Republic of China (2015329225230); the National Natural Science Foundation of China (51605066);
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