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Fabrication of friction-reducing texture surface by selective laser melting of ink-printed (SLM-IP) copper (Cu) nanoparticles(NPs)
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Fabrication of friction-reducing texture surface by selective laser melting of ink-printed (SLM-IP) copper (Cu) nanoparticles(NPs) Xinjian Wang a,b , Junyan Liu a,b,∗ , Yang Wang a,b,∗ , Yanan Fu a,b a b
State Key Laboratory of Robotics and System, Harbin Institute of Technology, Harbin, 150001, PR China School of Mechatronics Engineering, Harbin Institute of Technology, Harbin, 150001, PR China
a r t i c l e
i n f o
Article history: Received 28 September 2016 Accepted 1 November 2016 Available online xxx Keywords: Copper Nanoparticles Selective laser melting Texture Friction-reducing
a b s t r a c t This paper reports a process of selective laser melting of ink-printed (SLM-IP) copper (Cu) nanoparticles(NPs) for the fabrication of full dense Cu friction-reducing texture on the metallic surface in ambient condition. This technique synthesizes pure Cu by chemical reduction route using an organic solvent during laser melting in the atmosphere environment, and provides a flexible additive manufacture approach to form complex friction-reduction texture on the metallic surface. Microtextures of ring and disc arrays have been fabricated on the stainless steel surface by SLM-IP Cu NPs. The friction coefficient has been measured under the lubricating condition of the oil. Disc texture surface (DTS) has a relatively low friction coefficient compared with ring texture surface (RTS), Cu film surface (Cu-FS) and the untreated substrate. The study suggests a further research on SLM-IP approach for complex microstructure or texture manufacturing, possibly realizing its advantage of flexibility. © 2016 Elsevier B.V. All rights reserved.
1. Introduction The problem of tribology is always an unavoidable challenge for mechanical systems. As the increasing demand for faster vehicles, lower energy consumption and longer lifetime of use, reducing and controlling of friction and wear has aroused interests of many researchers [1–5]. Advanced materials and surface texture are the two major research branches for friction reduction. Hamilton [6] early proposed the surface irregular texture to improve tribological properties in the 1960s, and then, surface textures were implemented in several manufacturing techniques. So far, various geometrical textures are deeply investigated and many technics, including machining, photo-etching, etching, ion beam texturing, and laser texturing, are carried out for the fabrication of different kinds of textures for friction-reducing application [6–10]. Shallow pores,which act as a fluid reservoir for retention of the lubricating thin film between mating parts, are artificially distributed on the frictional surface. The fluid film could also reduce the temperature rise when abnormal running condition occurred. The shape, size, depth and distribution of the fabricated shallow pores are essential factors for friction reduction [11]. Coating or deposition a specific kind of material (like Al, Cu, MoS2 , C, TiN, etc.) is another way for
∗ Corresponding authors at: School of Mechatronics Engineering, Harbin Institute of Technology, Harbin, 150001, PR China. E-mail addresses: [email protected] (J. Liu), [email protected] (Y. Wang).
friction reducing [12–14]. The soft or hard film of coatings would change the response of a tribo-system [15]. This brings an idea of manufacturing the textures with a specific kind of friction-reducing material. Additive manufacturing (AM), known for its flexibility to manufacture complex parts [16–18], provides a possibility to achieve this goal which will shed some light on improving and optimizing the current friction reducing texture manufacturing situation. Selective laser melting (SLM) is one technique of AM process which uses a laser beam to scan over a deposited metal powder layer according to the CAD model. A complex component could be built layer by layer basing on the laser beam scanning and the deposited metal powder successively. SLM has a high process flexibility, high material utilization and reduced production time for metal component manufacturing. Generally, the size of the textures are commonly around hundreds of micrometers, but the metal particle size of SLM is in the range of 50–200 m [19] which is too large for the fabrication of the friction-reducing texture. To our best knowledge, a laser beam exposes upon a smaller diameter of the particles layer is helpful to fabricate a fine surface with high precision. Thus, nanosized particles are suitable for texture manufacturing. However, due to the high surface energy of the nanoparticle, metal powders tend to be agglomerated which makes it very hard for layering and sintering. Nanoparticles have been found to disperse in organic liquid homogeneously with the help of polyvinylpyrrolidone (PVP). PVP is
http://dx.doi.org/10.1016/j.apsusc.2016.11.003 0169-4332/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: X. Wang, et al., Fabrication of friction-reducing texture surface by selective laser melting of ink-printed (SLM-IP) copper (Cu) nanoparticles(NPs), Appl. Surf. Sci. (2016), http://dx.doi.org/10.1016/j.apsusc.2016.11.003
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a kind of long-chained charged polymer which reduces the surface energy of nanoparticles and strut the particles by electric property. In 2007, Bong Kyun Park et al. [20] synthesized copper nanoparticles in ambient atmosphere by a polyol method of chemisorbed PVP with a thickness around 1.5 nm. In 2011, Bongchul Kang et al. [21] successfully dispersed copper oxide (CuO) nanoparticles into a reduction agent with PVP to stabilize the particles from agglomeration, and used a laser beam to sinter the agent. Recently, Chung-Hyeon Ryu et al. [22] applied two-step flash light to sinter the nanoparticles solution (copper nanoparticles with oxide shells, PVP and diethylene glycol) and copper nanoparticle-ink film was fully sintered and densely agglomerated owing to the splitting sintering process into two steps of chemical reduction and sintering. Thereby, this brings us an idea to combine the SLM process and Cu NPs ink-print to fabricate full dense copper surface reducing texture. In this work, we demonstrate a process of selective laser melting of ink-printed (SLM-IP) copper (Cu) nanoparticles(NPs) for the fabrication of full dense Cu friction-reducing texture on the metallic surface in ambient condition. Firstly, parameters of power and scan speed are the crucial factors, and experimental study of the process SLM-IP Cu NPs is carried out to obtain the process window. Secondly, The composition of the fabricated layer is examined to verify the pure dense Cu layer forming in SLM-IP process through X-ray diffraction (XRD). Finally, two textures of rings and discs are designed and manufactured by SLM-IP Cu NPs. In order to compare and analyze the influence of texturing, a fully covered copper friction test sample by SLM-IP Cu NPs and an untreated stainless steel substrate are also introduced. The tribological behaviors of the four friction samples are carried out on Pin-on-disc tests.
2. SLM-IP Cu NPs and experimental details 2.1. SLM-IP Cu NPs SLM-IP Cu NPs is based on the ideology of additive manufacturing. The solution of copper nanoparticles (NPs) was sintered by a fiber laser (wave length of 1070 nm) to form a small layer of surface texture. The solution consists of 46.2% copper in weight, organic solvent (made from 20.8% polyvinylpyrrolidone (PVP, MW10000) and 10% ethylene glycol in weight to prevent agglomeration and oxidation of Cu of NPs during SLM) and distilled water 23% in weight. The Cu NPs were dispersed in the solution with the ultrasonic wave. The average diameter of the particles lies between 70 nm and 100 nm. Rather than pure copper, the Cu NPs used in this work were covered with copper oxide shells. The Cu NPs could be sintered and formed into pure copper film by the illumination of light combined with photo-reactive polymer coating, i.e. PVP and polyvinyl alcohol, at room temperature in ambient condition. Under the irradiation of laser, the NPs were reordered, melted, premelted and reacted with intact chemical species, due to the temperature increase exciting the ionic degrees of copper lattices. In this case, the SLM-IP Cu NPs process is described in Fig. 1. Firstly, the Cu NPs surface was heated under the condition of laser irradiation (seen in Fig. 1(a)), and then the temperature on the surface was increased. Subsequently, three processes of reaction would simultaneously occur. NPs on the top of the layer would be further oxidized by the interaction with air due to the evaporation of ethylene glycol, but NPs on the bottom layer could be formed into pure Cu by the chemical reduction reaction of ethylene glycol and fully melt at high temperature (seen in Fig. 1(b)). Moreover, the organic solvent in the unheated zone would enter into the laser action region by capillary force, and the Cu oxide would be further deoxidized into pure Cu (seen Fig. 1(c)). Finally, the residual Cu oxide on the top very thin film would be covered by the next Cu
Fig. 1. Schematics of SLM-IP Cu NPs: (a) The heated Cu NPs, (b) The oxidized Cu NPs and full molten Cu, (c) Chemical reduction of oxidation and increasing of molten Cu and (d) The forming of full dense Cu layer.
NPs ink and turned into pure copper by the repeated SLM process (seen in Fig. 1(d)). 2.2. Experimental set-up The experimental setup of SLM-IP Cu NPs (seen in Fig. 2) consists of two parts: a continuous wave laser fast scanning optical system and a Cu NPs ink spreading device. By applying an X-Y galvanometer with a focal length of 255 mm to the fiber laser, a laser beam spot size of 18.2 m diameter is reliably focused on the target position and in a controlled manner on demand for scanning area of 300 mm × 300 mm. A slot die coating method was employed for spreading Cu NPs ink which used a modified spraying head and nozzle with a meniscus structure to enable a good control of Cu NPs ink flow and forming a stable liquid layer on the objective surface. Finally, utilizing the lifted operation stage, the Cu NPs ink would be printed layer by layer on the substrate surface. 2.3. Friction-reducing texture and tribological experiment The textural shapes, orientations and shapes of the cross-section would largely affect the friction performance of surface textures. Different from our work, texture studies were mostly based on the laser surface texturing and other subtractive means. Concerning to avoid the influence of orientation, shapes of circular was adopted as the optimal shape for this work. Disc texture surface (DTS), ring texture surface (RTS) were designated as the objective samples considering the process of SLM-IP, both of which share the same outer diameter. Fig. 3 illustrates the morphography of the textures and the insets are the profiles of each surface. The friction behavior of the samples was characterized using a pin-on-disc tribometer (UMT-3, CETR, USA), unidirectional sliding, in which the coefficient of friction was recorded. A stainless steel ball (AISI 440C, diameter 5 in.) was used as the counterpart. The tribology tests were conducted under the lubricant in an oil (#4050 SINOPEC, velocity 0.516 Pa/s) bath with a temperature around 23 ◦ C. The sliding speed was about 0.89 m/s (500 rpm) with a normal load of 5 N. The normal load was applied on the upper ball part as shown in Fig. 4. 3. Results and discussion The formed film quality of SLM-IP Cu NPs is mainly dependent on the laser power and scanning speed. In order to fabricate the expected texture, the Cu film fabrication experiments were conducted through SLM-IP Cu NPs on ANSI 304 stainless steel sample
Please cite this article in press as: X. Wang, et al., Fabrication of friction-reducing texture surface by selective laser melting of ink-printed (SLM-IP) copper (Cu) nanoparticles(NPs), Appl. Surf. Sci. (2016), http://dx.doi.org/10.1016/j.apsusc.2016.11.003
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Fig. 2. Experimental set-up of SLM-IP metal NPs.
Fig. 3. Shape of the textures: (a) ring texture surface (RTS); (b) disc texture surface (DTS).
Fig. 4. Schematic representation of pin-on-disk arrangement and motion form.
surfaces. The sample surfaces were firstly polished to a surface roughness Ra about 0.1 m. Then, the samples were cleaned with alcohol in an ultrasonic bath. In the experiment, laser scanning space was set to 0.02 mm with a laser beam spot size around 18.2 m. Fig. 5 shows the laser power P versus scan speed process window of the first and second ink-printed layer sintering parameters, respectively. The plane of coordinates of Fig. 5(a) and (b) are divided into 3 regions, insufficient parameters region I, available parameters region II as well as excessive parameters region III. The insufficient parameters region I is on the left, parameters of which could not make the NPs ink be sintered. The excessive parameters region III is on the right, parameters in which would make the NPs ink oxidize, balling or even devastate the substrate. The available parameters region II is in the middle(gray covered), parameters of which would make the result smooth and regular. For the experiment of the second layer, the first layer was sintered with a velocity of 80 mm/s and laser power of 40 W, of which the scan direction is set perpendicular to the first layer. In order to make it obvious, the second sintered layer was smaller than the first layer.
From Fig. 5(a) and (b), the shape and tendency of the available regions are similar. But the process window in Fig. 5(b) is slightly moved to the direction of small value compare with Fig. 5(a). This may be result from the substrates changing that the substrate of the second layer is the last-time sintered copper layer. As the thermal conductivity of copper is much higher than stainless steel, the heat affected zone in copper is larger than in stainless steel with a same amount of thermal energy provided. Therefore, for the purpose of forming high quality Cu layer, the effective power density of the second layer should be lower compared with the first layer sintering. Moreover, the process window of the above layers is similar to the second. Though the process window is similar to SLM of micro metal particles [23], SLM-IP possesses some specific features. The powder and substrate properties of size, the material would tremendously affect result and the operation. Nanoparticles are dispersed in the reduction solvent which can be evaporated by the laser. The evaporation rate should be lower than the rate of melting, reduction and solidification. Due to the nano-sized particle, high laser power causes serious evaporation of the material, and the corresponding laser momentum also impacts on the nanoparticles, which leads to a splash of the powder bed. As a consequence, low power combined with a proper scan speed would be the available parameters resulting in less balling and dense bulk. Finally, the fabrication parameters of processing are experimentally determined with a velocity of 80 mm/s and laser power of 40W for the first layer and the decreased laser power of 23W with the same scan speed for the second layer and above. The Cu films fabricated by SLM-IP are presented in Fig. 6. Fig. 6(a) is the micrograph of the manufactured copper surface, which shows the morphography of Cu film after the leftover nanoparticle solution was washed away. Fig. 6(b) and (c) illustrate the SEMs of Cu
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Fig. 5. Process window for (a) first layer (insets micrograph: I 20 W, 120 mm/s, II 40 W, 80 mm/s, II100 W, 80 mm/s) and (b) second layer and above (insets micrograph: I 20 W, 160 mm/s; II 23 W, 80 mm/s; III 120 W, 60 mm/s).
Fig. 6. (a) Surface micrograph of SLM-IP Cu NPs layers, (b) SEM of the SLM-IP Cu surface, (c) SEM of cross section, and (d) XRD of SLM-IP Cu NPs layers.
film surface and cross section. There is no crack or hole found in Fig. 6(b). It can be found that a dense bulk is formed using SLM-IP Cu NPs. To confirm the purity of Cu formed by SLM-IP Cu NPs, crystal phase analysis was performed using X-ray diffraction (XRD). The XRD pattern of Cu film after rinsed in an ultrasonic bath is shown in Fig. 6(d), and we found that the XRD peaks corresponding to the FCC pure copper phase (43.2◦ , 50.4◦ , 74.1◦ , and 89.9◦ ) according to existing references (JCPDS No. 040836) occurred completely. It is obvious that a pure copper layer is formed by SLM-IP Cu NPs. To compare and analyze the influence of texturing, four types of samples were prepared, these are, disc DTS, RTS, full covered copper friction plate prepared by the process of SLM-IP and an untreated substrate of ANSI 304. Fig. 7 represents the surface micrograph of DTS and RTS. The morphography of surface texture DTS was designed as an array of discs with a diameter of 500 m and distance of 750 m as shown in Fig. 7(a). Texture RTS was designed as an array of rings with an outer diameter of 500 m, inner diameter of 400 m and distance of 750 m as shown in Fig. 7(b). The dimension of the two surface textures was measured using Laser Scanning Confocal Microscopy (LSCM). Fig. 7(a) shows one geometrical situation of the
texture DTS. Height change of the disc along the radial direction was tested, the measurement tack of which is displayed in blue and red arrows as shown in Fig. 7(a). The height of the disc is around 6 m. Fig. 7(b) shows the texture of RTS measured by LSCM. In order to increase the accuracy of the measurement, a quarter part of a ring texture was tested as shown in Fig. 7(b). Measurement tack was set along the radial direction of the ring, as the blue and red arrow illustrated in Fig. 7(b). The height change along the measurement track is shown in Fig. 7(b), from which the height of the ring is around 6 m. Fig. 8 demonstrates the coefficient of the friction (COF) behavior of DTS, RTS, full covered copper friction plate prepared by the process of SLM-IP(Cu-FS) and an untreated substrate of ANSI 304 with a roughness of Ra 0.1 nm under the sliding speed of 0.89 m/s (500 rpm) with a normal load of 5 N. From Fig. 8, each COF is gradually reduced and reaches up to a stable value after about 150s. This may result from the fact that it was arduous for the test samples to establish an effective lubricant film and could not maintain hydrodynamic effects between the friction pairs with insufficient lubrication at the beginning. Therefore, the COFs after 150s are regarded stable and comparable. The COF of test sample Cu-FS is lower than the COF of the untreated sub-
Please cite this article in press as: X. Wang, et al., Fabrication of friction-reducing texture surface by selective laser melting of ink-printed (SLM-IP) copper (Cu) nanoparticles(NPs), Appl. Surf. Sci. (2016), http://dx.doi.org/10.1016/j.apsusc.2016.11.003
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Fig. 7. Micrographs and Laser Scanning Confocal Microscopy (LSCM) graphs of (a) DTS, and (b)RTS.
Fig. 8. Friction coefficients .vs. time.
strate. This may be caused by the material property of soft which contributes the reduction of COF. Owing to the function of the texture, the COFs of DTS and RTS are relatively low. Comparing the COF results of Cu-FS and DTS confirm the positive effect of friction reducing texture. In the aspect of surface morphography, textures of RTS should have a lower COF than DTS, due to the superiority of the micro-dimples to maintain more lubricant than the arrays
of flat discs. However, the test result shows that the COF of RTS is higher than the COF of DTS. This could be caused by the role of soft material property, for the effective volume of DTS is larger than RTS in the aspect of surface topography. This result further the assumption of soft friction mechanism (the material property of soft plays a major role comparing with surface roughness [15]) that the soft property of the material plays a more effective role than surface texture in the condition of oil lubricant. Accordingly, the manufactured copper surface texture on a stainless steel which would possess the two advantages of soft coating and friction-reducing texture as expected. Fig. 9(a) and (b) show the worn morphography of the texture RTS and DTS after friction experiment, respectively. Some white bright spots are observed (illustrated in the blue circle), which resulted from compressing and worn of the “copper ball” during the tribology test. These can be explained by the “balling effect” of selective laser melting [24]. Additionally, the surfaces of the rings and discs are not smooth, the lubricant oil not only could be maintained in textures but also in the rough surface, which could also contribute to friction-reducing effect slightly.
4. Conclusions In summary, the process of SLM-IP Cu NPs was proposed which synthesizes pure Cu by chemical reduction route using an organic solvent during laser melting in ambient conditions, and provides a flexible additive manufacture approach to form complex friction-reduction texture on the metallic surface.
Please cite this article in press as: X. Wang, et al., Fabrication of friction-reducing texture surface by selective laser melting of ink-printed (SLM-IP) copper (Cu) nanoparticles(NPs), Appl. Surf. Sci. (2016), http://dx.doi.org/10.1016/j.apsusc.2016.11.003
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Fig. 9. (a) Worn morphography of RTS; (b) worn morphography of DTS.
Friction-reducing textures were fabricated on the ANSI 304 stainless steel. The texture COFs of RTS and DTS are both lower than the COFs of Cu-FS and untreated substrate confirming the frictionreducing mechanism of surface texture. The COF results of DTS and RTS indicates that the material property of soft plays a more important role than surface texture in the lubricated soft tribology system. The work of SLM-IP Cu NPs shows a great potential for achieving a microstructure fabrication sequence realizing possibility and flexibility of additive manufacturing in the area of manufacturing and studying friction-reducing surface texture. Acknowledgements This work was supported in part by National Natural Science Foundation of China under grant No. 61571153, Self-planned Task of State Key Laboratory of Robotics and System (HIT) and the Program of Introducing Talents of Discipline to Universities (grant No. B07108). References [1] D.Z. Segu, S.G. Choi, J. hyouk Choi, S.S. Kim, The effect of multi-scale laser textured surface on lubrication regime, Appl. Surf. Sci. 270 (2013) 58–63. [2] M. Wakuda, Y. Yamauchi, S. Kanzaki, Y. Yasuda, Effect of surface texturing on friction reduction between ceramic and steel materials under lubricated sliding contact, Wear 254 (2003) 356–363. [3] I. Etsion, Improving tribological performance of mechanical components by laser surface texturing, Tribol. Lett. 17 (2004) 733–737. [4] I. Etsion, State of the art in laser surface texturing, J. Tribol. 127 (2005) 248–253. [5] G. Ryk, I. Etsion, Testing piston rings with partial laser surface texturing for friction reduction, Wear 261 (2006) 792–796. [6] D. Hamilton, J. Walowit, C. Allen, A theory of lubrication by microirregularities, J. Basic Eng. 88 (1966) 177–185. [7] P. Sreejith, B. Ngoi, Dry machining: machining of the future, J. Mater. Process. Technol. 101 (2000) 287–291. [8] H. Costa, I. Hutchings, Hydrodynamic lubrication of textured steel surfaces under reciprocating sliding conditions, Tribol. Int. 40 (2007) 1227–1238.
[9] L. Zhou, K. Kato, N. Umehara, Y. Miyake, Nanometre scale island-type texture with controllable height and area ratio formed by ion-beam etching on hard-disk head sliders, Nanotechnology 10 (1999) 363. [10] A. Kovalchenko, O. Ajayi, A. Erdemir, G. Fenske, I. Etsion, The effect of laser surface texturing on transitions in lubrication regimes during unidirectional sliding contact, Tribol. Int. 38 (2005) 219–225. [11] I. Etsion, E. Sher, Improving fuel efficiency with laser surface textured piston rings, Tribol. Int. 42 (2009) 542–547. [12] S.M. Rowland, R.B. Wright, Catheter and Hydrophilic, Friction-reducing Coating Thereon, in Google Patents, 1991. [13] W. Münz, D. Hoffmann, K. Hartig, A high rate sputtering process for the formation of hard friction-reducing TiN coatings on tools, Thin Solid Films 96 (1982) 79–86. [14] R. Gilmore, M.A. Baker, P.N. Gibson, W. Gissler, Preparation and characterisation of low-friction TiB2-based coatings by incorporation of C or MoS2, Surf. Coat. Technol. 105 (1998) 45–50. [15] K. Kato, Wear in relation to friction—a review, Wear 241 (2000) 151–157. [16] X. Zhou, K. Li, D. Zhang, X. Liu, J. Ma, W. Liu, Z. Shen, Textures formed in a CoCrMo alloy by selective laser melting, J. Alloys Compd. 631 (2015) 153–164. [17] J. Liu, C. Gao, P. Feng, S. Peng, C. Shuai, Selective laser sintering of -TCP/nano-58S composite scaffolds with improved mechanical properties, Mater. Des. 84 (November (5)) (2015) 395–401. [18] Z. Zhu, V. Dhokia, A. Nassehi, S.T. Newman, Investigation of part distortions as a result of hybrid manufacturing, Rob. Comput. Integr. Manuf. 37 (2016) 23–32. [19] I. Yadroitsev, P. Bertrand, I. Smurov, Parametric analysis of the selective laser melting process, Appl. Surf. Sci. 253 (2007) 8064–8069. [20] B.K. Park, S. Jeong, D. Kim, J. Moon, S. Lim, J.S. Kim, Synthesis and size control of monodisperse copper nanoparticles by polyol method, J. Colloid Interface Sci. 311 (2007) 417–424. [21] B. Kang, S. Han, J. Kim, S. Ko, M. Yang, One-step fabrication of copper electrode by laser-induced direct local reduction and agglomeration of copper oxide nanoparticle, J. Phys. Chem. C 115 (2011) 23664–23670. [22] C.-H. Ryu, S.-J. Joo, H.-S. Kim, Two-step flash light sintering of copper nanoparticle ink to remove substrate warping, Appl. Surf. Sci. 384 (2016) 182–191. [23] J.P. Kruth, L. Froyen, J. Van Vaerenbergh, P. Mercelis, M. Rombouts, B. Lauwers, Selective laser melting of iron-based powder, J. Mater. Process. Technol. 149 (2004) 616–622. [24] N.K. Tolochko, S.E. Mozzharov, I.A. Yadroitsev, T. Laoui, L. Froyen, V.I. Titov, M.B. Ignatiev, Balling processes during selective laser treatment of powders, Rapid Prototyp. J. 10 (2004) 78–87.
Please cite this article in press as: X. Wang, et al., Fabrication of friction-reducing texture surface by selective laser melting of ink-printed (SLM-IP) copper (Cu) nanoparticles(NPs), Appl. Surf. Sci. (2016), http://dx.doi.org/10.1016/j.apsusc.2016.11.003
ARTICLE IN PRESS
APSUSC-34305; No. of Pages 6
Applied Surface Science xxx (2016) xxx–xxx
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Full length article
Fabrication of friction-reducing texture surface by selective laser melting of ink-printed (SLM-IP) copper (Cu) nanoparticles(NPs) Xinjian Wang a,b , Junyan Liu a,b,∗ , Yang Wang a,b,∗ , Yanan Fu a,b a b
State Key Laboratory of Robotics and System, Harbin Institute of Technology, Harbin, 150001, PR China School of Mechatronics Engineering, Harbin Institute of Technology, Harbin, 150001, PR China
a r t i c l e
i n f o
Article history: Received 28 September 2016 Accepted 1 November 2016 Available online xxx Keywords: Copper Nanoparticles Selective laser melting Texture Friction-reducing
a b s t r a c t This paper reports a process of selective laser melting of ink-printed (SLM-IP) copper (Cu) nanoparticles(NPs) for the fabrication of full dense Cu friction-reducing texture on the metallic surface in ambient condition. This technique synthesizes pure Cu by chemical reduction route using an organic solvent during laser melting in the atmosphere environment, and provides a flexible additive manufacture approach to form complex friction-reduction texture on the metallic surface. Microtextures of ring and disc arrays have been fabricated on the stainless steel surface by SLM-IP Cu NPs. The friction coefficient has been measured under the lubricating condition of the oil. Disc texture surface (DTS) has a relatively low friction coefficient compared with ring texture surface (RTS), Cu film surface (Cu-FS) and the untreated substrate. The study suggests a further research on SLM-IP approach for complex microstructure or texture manufacturing, possibly realizing its advantage of flexibility. © 2016 Elsevier B.V. All rights reserved.
1. Introduction The problem of tribology is always an unavoidable challenge for mechanical systems. As the increasing demand for faster vehicles, lower energy consumption and longer lifetime of use, reducing and controlling of friction and wear has aroused interests of many researchers [1–5]. Advanced materials and surface texture are the two major research branches for friction reduction. Hamilton [6] early proposed the surface irregular texture to improve tribological properties in the 1960s, and then, surface textures were implemented in several manufacturing techniques. So far, various geometrical textures are deeply investigated and many technics, including machining, photo-etching, etching, ion beam texturing, and laser texturing, are carried out for the fabrication of different kinds of textures for friction-reducing application [6–10]. Shallow pores,which act as a fluid reservoir for retention of the lubricating thin film between mating parts, are artificially distributed on the frictional surface. The fluid film could also reduce the temperature rise when abnormal running condition occurred. The shape, size, depth and distribution of the fabricated shallow pores are essential factors for friction reduction [11]. Coating or deposition a specific kind of material (like Al, Cu, MoS2 , C, TiN, etc.) is another way for
∗ Corresponding authors at: School of Mechatronics Engineering, Harbin Institute of Technology, Harbin, 150001, PR China. E-mail addresses: [email protected] (J. Liu), [email protected] (Y. Wang).
friction reducing [12–14]. The soft or hard film of coatings would change the response of a tribo-system [15]. This brings an idea of manufacturing the textures with a specific kind of friction-reducing material. Additive manufacturing (AM), known for its flexibility to manufacture complex parts [16–18], provides a possibility to achieve this goal which will shed some light on improving and optimizing the current friction reducing texture manufacturing situation. Selective laser melting (SLM) is one technique of AM process which uses a laser beam to scan over a deposited metal powder layer according to the CAD model. A complex component could be built layer by layer basing on the laser beam scanning and the deposited metal powder successively. SLM has a high process flexibility, high material utilization and reduced production time for metal component manufacturing. Generally, the size of the textures are commonly around hundreds of micrometers, but the metal particle size of SLM is in the range of 50–200 m [19] which is too large for the fabrication of the friction-reducing texture. To our best knowledge, a laser beam exposes upon a smaller diameter of the particles layer is helpful to fabricate a fine surface with high precision. Thus, nanosized particles are suitable for texture manufacturing. However, due to the high surface energy of the nanoparticle, metal powders tend to be agglomerated which makes it very hard for layering and sintering. Nanoparticles have been found to disperse in organic liquid homogeneously with the help of polyvinylpyrrolidone (PVP). PVP is
http://dx.doi.org/10.1016/j.apsusc.2016.11.003 0169-4332/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: X. Wang, et al., Fabrication of friction-reducing texture surface by selective laser melting of ink-printed (SLM-IP) copper (Cu) nanoparticles(NPs), Appl. Surf. Sci. (2016), http://dx.doi.org/10.1016/j.apsusc.2016.11.003
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a kind of long-chained charged polymer which reduces the surface energy of nanoparticles and strut the particles by electric property. In 2007, Bong Kyun Park et al. [20] synthesized copper nanoparticles in ambient atmosphere by a polyol method of chemisorbed PVP with a thickness around 1.5 nm. In 2011, Bongchul Kang et al. [21] successfully dispersed copper oxide (CuO) nanoparticles into a reduction agent with PVP to stabilize the particles from agglomeration, and used a laser beam to sinter the agent. Recently, Chung-Hyeon Ryu et al. [22] applied two-step flash light to sinter the nanoparticles solution (copper nanoparticles with oxide shells, PVP and diethylene glycol) and copper nanoparticle-ink film was fully sintered and densely agglomerated owing to the splitting sintering process into two steps of chemical reduction and sintering. Thereby, this brings us an idea to combine the SLM process and Cu NPs ink-print to fabricate full dense copper surface reducing texture. In this work, we demonstrate a process of selective laser melting of ink-printed (SLM-IP) copper (Cu) nanoparticles(NPs) for the fabrication of full dense Cu friction-reducing texture on the metallic surface in ambient condition. Firstly, parameters of power and scan speed are the crucial factors, and experimental study of the process SLM-IP Cu NPs is carried out to obtain the process window. Secondly, The composition of the fabricated layer is examined to verify the pure dense Cu layer forming in SLM-IP process through X-ray diffraction (XRD). Finally, two textures of rings and discs are designed and manufactured by SLM-IP Cu NPs. In order to compare and analyze the influence of texturing, a fully covered copper friction test sample by SLM-IP Cu NPs and an untreated stainless steel substrate are also introduced. The tribological behaviors of the four friction samples are carried out on Pin-on-disc tests.
2. SLM-IP Cu NPs and experimental details 2.1. SLM-IP Cu NPs SLM-IP Cu NPs is based on the ideology of additive manufacturing. The solution of copper nanoparticles (NPs) was sintered by a fiber laser (wave length of 1070 nm) to form a small layer of surface texture. The solution consists of 46.2% copper in weight, organic solvent (made from 20.8% polyvinylpyrrolidone (PVP, MW10000) and 10% ethylene glycol in weight to prevent agglomeration and oxidation of Cu of NPs during SLM) and distilled water 23% in weight. The Cu NPs were dispersed in the solution with the ultrasonic wave. The average diameter of the particles lies between 70 nm and 100 nm. Rather than pure copper, the Cu NPs used in this work were covered with copper oxide shells. The Cu NPs could be sintered and formed into pure copper film by the illumination of light combined with photo-reactive polymer coating, i.e. PVP and polyvinyl alcohol, at room temperature in ambient condition. Under the irradiation of laser, the NPs were reordered, melted, premelted and reacted with intact chemical species, due to the temperature increase exciting the ionic degrees of copper lattices. In this case, the SLM-IP Cu NPs process is described in Fig. 1. Firstly, the Cu NPs surface was heated under the condition of laser irradiation (seen in Fig. 1(a)), and then the temperature on the surface was increased. Subsequently, three processes of reaction would simultaneously occur. NPs on the top of the layer would be further oxidized by the interaction with air due to the evaporation of ethylene glycol, but NPs on the bottom layer could be formed into pure Cu by the chemical reduction reaction of ethylene glycol and fully melt at high temperature (seen in Fig. 1(b)). Moreover, the organic solvent in the unheated zone would enter into the laser action region by capillary force, and the Cu oxide would be further deoxidized into pure Cu (seen Fig. 1(c)). Finally, the residual Cu oxide on the top very thin film would be covered by the next Cu
Fig. 1. Schematics of SLM-IP Cu NPs: (a) The heated Cu NPs, (b) The oxidized Cu NPs and full molten Cu, (c) Chemical reduction of oxidation and increasing of molten Cu and (d) The forming of full dense Cu layer.
NPs ink and turned into pure copper by the repeated SLM process (seen in Fig. 1(d)). 2.2. Experimental set-up The experimental setup of SLM-IP Cu NPs (seen in Fig. 2) consists of two parts: a continuous wave laser fast scanning optical system and a Cu NPs ink spreading device. By applying an X-Y galvanometer with a focal length of 255 mm to the fiber laser, a laser beam spot size of 18.2 m diameter is reliably focused on the target position and in a controlled manner on demand for scanning area of 300 mm × 300 mm. A slot die coating method was employed for spreading Cu NPs ink which used a modified spraying head and nozzle with a meniscus structure to enable a good control of Cu NPs ink flow and forming a stable liquid layer on the objective surface. Finally, utilizing the lifted operation stage, the Cu NPs ink would be printed layer by layer on the substrate surface. 2.3. Friction-reducing texture and tribological experiment The textural shapes, orientations and shapes of the cross-section would largely affect the friction performance of surface textures. Different from our work, texture studies were mostly based on the laser surface texturing and other subtractive means. Concerning to avoid the influence of orientation, shapes of circular was adopted as the optimal shape for this work. Disc texture surface (DTS), ring texture surface (RTS) were designated as the objective samples considering the process of SLM-IP, both of which share the same outer diameter. Fig. 3 illustrates the morphography of the textures and the insets are the profiles of each surface. The friction behavior of the samples was characterized using a pin-on-disc tribometer (UMT-3, CETR, USA), unidirectional sliding, in which the coefficient of friction was recorded. A stainless steel ball (AISI 440C, diameter 5 in.) was used as the counterpart. The tribology tests were conducted under the lubricant in an oil (#4050 SINOPEC, velocity 0.516 Pa/s) bath with a temperature around 23 ◦ C. The sliding speed was about 0.89 m/s (500 rpm) with a normal load of 5 N. The normal load was applied on the upper ball part as shown in Fig. 4. 3. Results and discussion The formed film quality of SLM-IP Cu NPs is mainly dependent on the laser power and scanning speed. In order to fabricate the expected texture, the Cu film fabrication experiments were conducted through SLM-IP Cu NPs on ANSI 304 stainless steel sample
Please cite this article in press as: X. Wang, et al., Fabrication of friction-reducing texture surface by selective laser melting of ink-printed (SLM-IP) copper (Cu) nanoparticles(NPs), Appl. Surf. Sci. (2016), http://dx.doi.org/10.1016/j.apsusc.2016.11.003
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Fig. 2. Experimental set-up of SLM-IP metal NPs.
Fig. 3. Shape of the textures: (a) ring texture surface (RTS); (b) disc texture surface (DTS).
Fig. 4. Schematic representation of pin-on-disk arrangement and motion form.
surfaces. The sample surfaces were firstly polished to a surface roughness Ra about 0.1 m. Then, the samples were cleaned with alcohol in an ultrasonic bath. In the experiment, laser scanning space was set to 0.02 mm with a laser beam spot size around 18.2 m. Fig. 5 shows the laser power P versus scan speed process window of the first and second ink-printed layer sintering parameters, respectively. The plane of coordinates of Fig. 5(a) and (b) are divided into 3 regions, insufficient parameters region I, available parameters region II as well as excessive parameters region III. The insufficient parameters region I is on the left, parameters of which could not make the NPs ink be sintered. The excessive parameters region III is on the right, parameters in which would make the NPs ink oxidize, balling or even devastate the substrate. The available parameters region II is in the middle(gray covered), parameters of which would make the result smooth and regular. For the experiment of the second layer, the first layer was sintered with a velocity of 80 mm/s and laser power of 40 W, of which the scan direction is set perpendicular to the first layer. In order to make it obvious, the second sintered layer was smaller than the first layer.
From Fig. 5(a) and (b), the shape and tendency of the available regions are similar. But the process window in Fig. 5(b) is slightly moved to the direction of small value compare with Fig. 5(a). This may be result from the substrates changing that the substrate of the second layer is the last-time sintered copper layer. As the thermal conductivity of copper is much higher than stainless steel, the heat affected zone in copper is larger than in stainless steel with a same amount of thermal energy provided. Therefore, for the purpose of forming high quality Cu layer, the effective power density of the second layer should be lower compared with the first layer sintering. Moreover, the process window of the above layers is similar to the second. Though the process window is similar to SLM of micro metal particles [23], SLM-IP possesses some specific features. The powder and substrate properties of size, the material would tremendously affect result and the operation. Nanoparticles are dispersed in the reduction solvent which can be evaporated by the laser. The evaporation rate should be lower than the rate of melting, reduction and solidification. Due to the nano-sized particle, high laser power causes serious evaporation of the material, and the corresponding laser momentum also impacts on the nanoparticles, which leads to a splash of the powder bed. As a consequence, low power combined with a proper scan speed would be the available parameters resulting in less balling and dense bulk. Finally, the fabrication parameters of processing are experimentally determined with a velocity of 80 mm/s and laser power of 40W for the first layer and the decreased laser power of 23W with the same scan speed for the second layer and above. The Cu films fabricated by SLM-IP are presented in Fig. 6. Fig. 6(a) is the micrograph of the manufactured copper surface, which shows the morphography of Cu film after the leftover nanoparticle solution was washed away. Fig. 6(b) and (c) illustrate the SEMs of Cu
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Fig. 5. Process window for (a) first layer (insets micrograph: I 20 W, 120 mm/s, II 40 W, 80 mm/s, II100 W, 80 mm/s) and (b) second layer and above (insets micrograph: I 20 W, 160 mm/s; II 23 W, 80 mm/s; III 120 W, 60 mm/s).
Fig. 6. (a) Surface micrograph of SLM-IP Cu NPs layers, (b) SEM of the SLM-IP Cu surface, (c) SEM of cross section, and (d) XRD of SLM-IP Cu NPs layers.
film surface and cross section. There is no crack or hole found in Fig. 6(b). It can be found that a dense bulk is formed using SLM-IP Cu NPs. To confirm the purity of Cu formed by SLM-IP Cu NPs, crystal phase analysis was performed using X-ray diffraction (XRD). The XRD pattern of Cu film after rinsed in an ultrasonic bath is shown in Fig. 6(d), and we found that the XRD peaks corresponding to the FCC pure copper phase (43.2◦ , 50.4◦ , 74.1◦ , and 89.9◦ ) according to existing references (JCPDS No. 040836) occurred completely. It is obvious that a pure copper layer is formed by SLM-IP Cu NPs. To compare and analyze the influence of texturing, four types of samples were prepared, these are, disc DTS, RTS, full covered copper friction plate prepared by the process of SLM-IP and an untreated substrate of ANSI 304. Fig. 7 represents the surface micrograph of DTS and RTS. The morphography of surface texture DTS was designed as an array of discs with a diameter of 500 m and distance of 750 m as shown in Fig. 7(a). Texture RTS was designed as an array of rings with an outer diameter of 500 m, inner diameter of 400 m and distance of 750 m as shown in Fig. 7(b). The dimension of the two surface textures was measured using Laser Scanning Confocal Microscopy (LSCM). Fig. 7(a) shows one geometrical situation of the
texture DTS. Height change of the disc along the radial direction was tested, the measurement tack of which is displayed in blue and red arrows as shown in Fig. 7(a). The height of the disc is around 6 m. Fig. 7(b) shows the texture of RTS measured by LSCM. In order to increase the accuracy of the measurement, a quarter part of a ring texture was tested as shown in Fig. 7(b). Measurement tack was set along the radial direction of the ring, as the blue and red arrow illustrated in Fig. 7(b). The height change along the measurement track is shown in Fig. 7(b), from which the height of the ring is around 6 m. Fig. 8 demonstrates the coefficient of the friction (COF) behavior of DTS, RTS, full covered copper friction plate prepared by the process of SLM-IP(Cu-FS) and an untreated substrate of ANSI 304 with a roughness of Ra 0.1 nm under the sliding speed of 0.89 m/s (500 rpm) with a normal load of 5 N. From Fig. 8, each COF is gradually reduced and reaches up to a stable value after about 150s. This may result from the fact that it was arduous for the test samples to establish an effective lubricant film and could not maintain hydrodynamic effects between the friction pairs with insufficient lubrication at the beginning. Therefore, the COFs after 150s are regarded stable and comparable. The COF of test sample Cu-FS is lower than the COF of the untreated sub-
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Fig. 7. Micrographs and Laser Scanning Confocal Microscopy (LSCM) graphs of (a) DTS, and (b)RTS.
Fig. 8. Friction coefficients .vs. time.
strate. This may be caused by the material property of soft which contributes the reduction of COF. Owing to the function of the texture, the COFs of DTS and RTS are relatively low. Comparing the COF results of Cu-FS and DTS confirm the positive effect of friction reducing texture. In the aspect of surface morphography, textures of RTS should have a lower COF than DTS, due to the superiority of the micro-dimples to maintain more lubricant than the arrays
of flat discs. However, the test result shows that the COF of RTS is higher than the COF of DTS. This could be caused by the role of soft material property, for the effective volume of DTS is larger than RTS in the aspect of surface topography. This result further the assumption of soft friction mechanism (the material property of soft plays a major role comparing with surface roughness [15]) that the soft property of the material plays a more effective role than surface texture in the condition of oil lubricant. Accordingly, the manufactured copper surface texture on a stainless steel which would possess the two advantages of soft coating and friction-reducing texture as expected. Fig. 9(a) and (b) show the worn morphography of the texture RTS and DTS after friction experiment, respectively. Some white bright spots are observed (illustrated in the blue circle), which resulted from compressing and worn of the “copper ball” during the tribology test. These can be explained by the “balling effect” of selective laser melting [24]. Additionally, the surfaces of the rings and discs are not smooth, the lubricant oil not only could be maintained in textures but also in the rough surface, which could also contribute to friction-reducing effect slightly.
4. Conclusions In summary, the process of SLM-IP Cu NPs was proposed which synthesizes pure Cu by chemical reduction route using an organic solvent during laser melting in ambient conditions, and provides a flexible additive manufacture approach to form complex friction-reduction texture on the metallic surface.
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Fig. 9. (a) Worn morphography of RTS; (b) worn morphography of DTS.
Friction-reducing textures were fabricated on the ANSI 304 stainless steel. The texture COFs of RTS and DTS are both lower than the COFs of Cu-FS and untreated substrate confirming the frictionreducing mechanism of surface texture. The COF results of DTS and RTS indicates that the material property of soft plays a more important role than surface texture in the lubricated soft tribology system. The work of SLM-IP Cu NPs shows a great potential for achieving a microstructure fabrication sequence realizing possibility and flexibility of additive manufacturing in the area of manufacturing and studying friction-reducing surface texture. Acknowledgements This work was supported in part by National Natural Science Foundation of China under grant No. 61571153, Self-planned Task of State Key Laboratory of Robotics and System (HIT) and the Program of Introducing Talents of Discipline to Universities (grant No. B07108). References [1] D.Z. Segu, S.G. Choi, J. hyouk Choi, S.S. Kim, The effect of multi-scale laser textured surface on lubrication regime, Appl. Surf. Sci. 270 (2013) 58–63. [2] M. Wakuda, Y. Yamauchi, S. Kanzaki, Y. Yasuda, Effect of surface texturing on friction reduction between ceramic and steel materials under lubricated sliding contact, Wear 254 (2003) 356–363. [3] I. Etsion, Improving tribological performance of mechanical components by laser surface texturing, Tribol. Lett. 17 (2004) 733–737. [4] I. Etsion, State of the art in laser surface texturing, J. Tribol. 127 (2005) 248–253. [5] G. Ryk, I. Etsion, Testing piston rings with partial laser surface texturing for friction reduction, Wear 261 (2006) 792–796. [6] D. Hamilton, J. Walowit, C. Allen, A theory of lubrication by microirregularities, J. Basic Eng. 88 (1966) 177–185. [7] P. Sreejith, B. Ngoi, Dry machining: machining of the future, J. Mater. Process. Technol. 101 (2000) 287–291. [8] H. Costa, I. Hutchings, Hydrodynamic lubrication of textured steel surfaces under reciprocating sliding conditions, Tribol. Int. 40 (2007) 1227–1238.
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Please cite this article in press as: X. Wang, et al., Fabrication of friction-reducing texture surface by selective laser melting of ink-printed (SLM-IP) copper (Cu) nanoparticles(NPs), Appl. Surf. Sci. (2016), http://dx.doi.org/10.1016/j.apsusc.2016.11.003