- Email: [email protected]
Study on lotus-type porous copper electroplated with a Ni coating on inner surface of pores
Applied Surface Science 264 (2013) 772–778
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Study on lotus-type porous copper electroplated with a Ni coating on inner surface of pores Hao Du a,c,∗ , Guihong Song b , Hideo Nakajima c , Yanhui Zhao d , Jinquan Xiao a , Tianying Xiong a a
Division of Surface Engineering of Materials, Institute of Metal Research, 72 Wenhua Road, Shenyang 110016, PR China School of Material Science and Technology, Shenyang University of Technology, 111 Shenliao West Road, Shenyang 110870, PR China The Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka 567-0047, Japan d Division of Specialized Materials and Devices, Institute of Metal Research, 72 Wenhua Road, Shenyang 110016, PR China b c
a r t i c l e
i n f o
Article history: Received 27 July 2012 Received in revised form 18 October 2012 Accepted 20 October 2012 Available online 6 November 2012 Keywords: Lotus-type porous copper Coating Inner surface of pore Mechanical properties
a b s t r a c t Deposition of Ni coating on inner surface of pores was attempted by electroplating for lotus-type porous copper with pore size of 0.6 mm and pore length of 6 mm. The surface morphology, thickness, thickness distribution along the pore length, and phase composition of the coating were characterized. It is proven that the Ni coating with a polycrystalline structure can be deposited on the inner surface of the pores with length/diameter of 10 for lotus-type porous copper by agitating the electroplating solution properly during the process. It is indicated that the coating thickness distributes uniformly along the pore depth and is about 4–5 m. Furthermore, the mechanical properties including vicker hardness, compressive yield strength and absorbed energy ability of the electroplated porous copper were evaluated. It is found that the mechanical properties are improved significantly after depositing the nickel coating inside pores of the lotus-type porous copper. Among them, 0.2% yield stress increases from 22.96 to 30.15 MPa, while absorbed energy per volume from 60.83 to 96.01 MJ/m3 when compressed to strain of 80%, which is attributed mainly to the Ni coating as an obstacle to dislocation slip during deformation and its strengthening effect for the higher strength, and the good adhesion to the pore wall of the porous copper. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Gasar or lotus-type porous metal has received much attention recently for not only a smart structure on porosity, pore morphology and pore aligning direction, but also higher mechanical properties than those formed by other techniques, such as foaming or powder sintering [1,2]. It is not easy to fabricate lotus-type porous alloy with straight and elongated pores by unidirectional solidification method through the mold casting technique for the sake of the dendrite growth during alloy solidification, compared with lotus-type porous pure metal [3]. Furthermore, it is also difficult to control the porosity of lotus-type porous alloy by the traditional unidirectional solidification method for the very small hydrogen solubility difference between solid and liquid of alloy due to addition of alloying elements [4]. To deal with this problem, Nakajima et al. [5] attempted to fabricate lotus-type porous brass by evaporating and depositing zinc onto and into lotus-type porous copper and diffusing into the copper by annealing. This suc-
∗ Corresponding author at: Division of Surface Engineering of Materials, Institute of Metal Research, 72 Wenhua Road, Shenyang 110016, PR China. Tel.: +86 24 8397 8952; fax: +86 24 8397 8952. E-mail address: [email protected] (H. Du). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.10.119
cessful fabrication motivates us an idea if coating can be employed on lotus-type porous metal for not only alloy fabrication but also surface protection, and structure and property modification. In this case, the first question to be answered is if and how a metallic or ceramic coating can be deposited into pores of lotus-type porous metal successfully; the second question is the effect of the metallic or ceramic coating on the surface of pore wall on properties of lotus-type porous metals or alloys. To the best knowledge of the present authors’, no work has been carried out on this point for lotus-type porous metals. Coating deposition on inner surface of pores for lotus-type porous metal may be possibly achieved by several methods such as electroplating, electroless plating, PVD, CVD, etc., in which gas or liquid can pass the pores for coating deposition [6,7]. The key point of these methods on coating deposition inside pore is how to deposit a uniform coating to a depth as long as possible. Among these methods, electroplating is flexible and extremely feasible in industrialization [8]. It is accepted that thickness and properties of the electroplating metal layer can be controlled by several factors including composition and concentration of the electrolyte solution, temperature, electrolyte flow rate, deposition time and the applied potential [9–11]. Furthermore, it has been demonstrated that mechanical properties of foamed metals could potentially be increased with electroplated Ni, Ni alloy coatings [12–14].
H. Du et al. / Applied Surface Science 264 (2013) 772–778
773
Cu(111)
On cross section On surface
Cu(222)
Ni(111) Cu(200) Ni(200)
30
40
50
Cu(220)
60
70
Cu(311)
80
90
100
2θ, degree Fig. 2. XRD pattern for electroplated lotus-type porous copper on both surface and cross section.
average pore length of 9.8 mm was employed as substrate. For electroplating, rectangular specimens of 30 mm × 15 mm × 6 mm were cut from the lotus-type porous copper ingot, and mechanically polished by SiC papers from grit 400# to 1500#, followed by degreasing, etching, and cleaning. The specimens were first degreased with a hot solution of 5% H2 SO4 for 3 min, and etched in a solution of HNO3 for 3 s with vibration. After rinsed with deionized water for a minute, the specimens were cleaned with alcohol for 3 min ultrasonically and then used immediately. Ni electroplating was performed under an optimal condition by our experimental results, using a pure nickel plate as anode and the lotus-type porous copper specimens as cathode with a 2:1 ratio in surface area for obtaining a more uniform coating. The lotus-type porous copper specimens were placed with a distance of 15 mm and pore direction perpendicular to the nickel plate surface. The solution for electroplating contained NiSO4 ·7H2 O 250 g/L, NiCl2 ·6H2 O 40 g/L, H3 BO3 35 g/L, C12 H25 SO4 Na 0.1 g/L and a nickel additive. The pH value of the solution was adjusted to 3.5. A magnetic pellet was used to stir the solution for keeping the solution concentration in the pores as uniform as possible and preventing the formation of hydrogen bubbles on the surface. The electroplating process was carried out in the solution by a proper agitating at a controlled and constant potential (18 V), current density (2.5 A/dm2 ), and temperature (55–60 ◦ C). During the electroplating, the lotus-type porous specimens were turned around once each 5 min. The thickness of the nickel coating was controlled by the electroplating time. Fig. 1. Observation on electroplated lotus-type porous copper specimen. (a) Whole image, (b) on cross section, (c) on inner surface of pores.
In this work, deposition of Ni coating on the inner surface of pores of lotus-type porous copper by electroplating was attempted. The structure of the Ni coating including surface image, thickness, thickness distribution along pore depth, and XRD pattern was characterized. Furthermore, mechanical properties of the electroplated lotus-type porous copper, including vicker hardness, compressive yield strength and absorbed energy ability were evaluated and analyzed. 2. Experimental procedures 2.1. Electroplating of Ni coating for lotus-type porous copper A lotus-type porous copper, fabricated in Nakajima’s laboratory, with porosity of 43.3%, average pore size of 603.5 m and
2.2. Characterization of electroplated lotus-type porous copper After being observed by an optical microscope (OM; VHX200, Keyence Corp., Japan), the coated specimens were divided into 5 mm × 5 mm × 6 mm using an electric discharge machine (LN1W, Sodick Co., Japan), and examined by an X-ray diffractometer (XRD; D8 Discover, Bruker AXS, Germany) operating with Cu K␣ ( = 0.154056 nm) radiation on both surface and cross section. The analyzed range of the diffraction angle 2 was between 30◦ and 100◦ by a step width of 0.04◦ . At the same time, coating surface was observed on pore wall by a scanning electron microscope (SEM; JSM 6360T, JEOL, Japan). Then, they were further cut on transverse sections along the sample height at 0.5, 1.0, 2.0, 3.0, 4.0, 5.0 and 5.6 mm, mounted, grinded and polished. The coating thickness was obtained by observing the cross section using the SEM, which is the average value of 10 points chosen randomly. The element distributions on cross section of coated pore wall were determined by electron probe microanalysis (EPMA; JXA-8800, JEOL, Japan).
774
H. Du et al. / Applied Surface Science 264 (2013) 772–778
Fig. 3. Observation of Ni coating on inner surface of pores and coating thickness. (a) The image of cross section, (b) a magnified image of (a), (c) element distribution on cross section of coated pore wall, (d) coating thickness distribution along pore length, (e) change in coating thickness with deposition time.
2.3. Measurement on mechanical properties The hardness of both uncoated and coated porous copper samples was evaluated by using a microhardness tester with a Vicker indenter (AKASHI MVK-EIII, Japan) on surface under two loads of 10 g and 25 g and a dwelling time of 10 s. The distance between two indentations was more than five times the minor diagonal to prevent stress field effect from nearby indentations. The average hardness values were recorded from five points chosen randomly on the sample surface. Compressive tests were performed on both the uncoated and the coated porous copper samples with size of 5 mm × 5 mm × 6 mm using a universal testing machine (Model 5582, Instron Co. Ltd., USA) in air at room temperature. The cross-head speed was set to be 0.1 mm/min. The strain was measured from the actuator displacement and the initial sample length. The highest strain was
chosen as 80%. The compressive yield strength (0.2% offset strength) was determined from the stress–strain curve and reported by the testing machine automatically. The mean values and standard deviation of the compressive yield strength were obtained from results tested on 5 samples in each case. After the compression measurements, the absorbed energy during the compression was calculated by integrating the area under the stress–strain curve by [15]:
W=
ε
(ε) dε 0
where W is the absorbed energy per unit volume and ε is the strain. The average values and standard deviations of the absorbed energy were reported from five samples in each case.
H. Du et al. / Applied Surface Science 264 (2013) 772–778
775
Fig. 4. Images of pore walls after compressed 20% of uncoated porous copper (a) and coated porous copper (b). In each case, 2 is the amplified part labeled in Image 1.
3. Results and discussion 3.1. Morphology observation, phase composition of the electroplated lotus-type porous copper After electroplating, the lotus-type porous copper specimens changed from copper appearance to nickel’s, as shown in Fig. 1(a). To be sure if the Ni coating was deposited on the inner surface of the penetrable pores, the specimen was cut on cross section and observed, which is shown in Fig. 1(b). It is found that the inner surface of the pores of the sample is also covered by a layer of coating, which is comprised of continuous and compact nodules with size in range of 2–4 m, as shown in Fig. 1(c). The surface characteristic of the coating is traditional of that for electroplating metal coating, as reported by Gao et al. [16]. On the other hand, no coating is found on the inner surface of the close pores in the specimens, as access of electrolyte to the inner surface of the pores is unavailable. XRD results confirm that the coating deposited on not only surface but also inner surface of pores of the lotus-type porous copper is Ni coating, as shown in Fig. 2. On the whole, the coating in this work is only composed of the Ni phase. There are no signals related to Ni alloy or other nickel oxide phases existing in the coating. In the surface case, Ni phase is the main phase for the sample. In the cross section (inner surface) case, Ni phase can be distinguished although Cu phase is the main phase, which stems from the naked copper substrate and the limited thickness of the Ni coating (presented in the following section). The XRD results also indicate that the electroplated Ni coating on inner surface of pores possesses a polycrystalline structure with two diffraction peaks of nickel (1 1 1) and (2 0 0).
3.2. Thickness and its distribution of the electroplated Ni coating on inner surface The experimental results mentioned above demonstrated that Ni coating can be deposited on inner surface of pores of lotustype porous copper by electroplating successfully. It is necessary and useful to know thickness and thickness distribution along pore length of the electroplated Ni coatings. The cross sectional surface of a polished sample is shown in Fig. 3(a). A white line with a thickness of 4–5 m can be found clearly between pores and copper substrate, which is confirmed as Ni coating by EMPA result, as shown in Fig. 3(b) and (c), respectively. Variation in coating thickness along the pore length is shown in Fig. 3(d) on sample electroplated with Ni for 60 min. It is interesting to find that the coating thickness distributes along the pore length uniformly although the ratio of pore length/pore diameter reaches 10, which is attributed to the proper electrolyte circulation [17]. It should be mentioned that the thickness increases a little from 4.3 to 4.5 m near the outer edge of the pores, indicating there is still a coating cumulation at the outer edge of pores, which is generally the key problem for coating deposition on inner surface of long tube especially with a small diameter. The influence of stirring direction and velocity of the electroplating solution on thickness and thickness distribution of the coating on inner surface of pores will be presented and discussed in another paper. At the same time, variation in coating thickness with electroplating time was obtained on both sample surface and inner surface of pores, as shown in Fig. 3(e). The coating thickness on sample surface increases with the deposition time almost linearly with a little decrease for longer deposition time. The coating thickness on the inner surface increases from 3.9 to 4.8 m although that on sample
776
H. Du et al. / Applied Surface Science 264 (2013) 772–778
Table 1 Hardness of the uncoated and the coated lotus-type porous copper.
300
HV0.01
HV0.025
250
Uncoated sample Coated sample
89.3 ± 4.2 217.4 ± 15.6
79.5 ± 5.1 176.1 ± 7.1
200
surface increases from 19.0 to 49.6 m when the deposition time increases from 30 to 80 min. The lower and the relatively stable coating thickness on inner surface, especially when the electroplating time is longer is connected with a shielding effect by pore walls-copper substrate, resulting in a reduced potential in the center of the elongated pores and a limited passage and supply of the electrolyte through the pores, which reduces the coating deposition velocity markedly.
Stress, MPa
Hardness
(a)
Lotus-type porous copper Coated lotus-type porous copper
150
100
50
0 0
10
20
30
40
50
60
70
80
60
70
80
Table 1 shows the vicker hardness of the coated and uncoated porous copper obtained at the surface. The hardness of the louts-type porous copper with the coating is much higher compared with that of the uncoated specimen. In the case of 10 g load, the average hardness reaches 217.4, for the coated specimen, more than 2.4 times for the uncoated specimen. This is obviously related to the higher hardness of the Ni since the coating thickness is between 25 m and 50 m, which is much higher than the indentation depth. From this result, it is predicted that the hardness of pore inner surface is also improved by the Ni coating, which may improve the wear resistance when fluid passes through pores of lotus-type porous metal using as a filter. It is interesting to find that the yield strength increases from 22.96 ± 2.03 to 30.15 ± 3.11 MPa (about 31% on growth rate) with the addition of the Ni coating for the lotus-type porous copper. The improvement is supposedly connected mainly with the Ni coating as a barrier for dislocation slip movement during the compression. Fig. 4 shows the SEM images of the inner surface of pores of the uncoated and the coated samples when compression at strain of 20% was applied along the pore direction. It is indicated that it is slip deformation that occurs during compression of uncoated sample, as conventional fine slip lines are clearly found (Fig. 4(a2)). In the case of the coated samples, dislocation still has to move out from copper matrix through pore walls during the compression. As the Ni coating deposited on the inner surface of pores, the dislocations encounter potential barriers and hardly slip out the substrate as before. Thus, higher stress is necessary to move the dislocations, resulting in the improved yield strength. It is predicted that the yield strength of the coated porous copper will increase further if the Ni coating thickness increases. During the compression, the Ni coating underwent the same load with the lotus-type porous copper. The rigidity and the strength of the electroplated Ni coating increase with its thickness [18], which may play another positive role in the improvement of yield strength of the coated porous copper on the basis of the barrier effect. Unfortunately, the thickness effect cannot be investigated in this work for the coating thickness limitation, which will be dealt with in our following work. 3.4. Ability to absorb energy during compression The compression stress–strain curves of both the uncoated and the coated samples are presented in Fig. 5(a). Significant increase in stress can be found for the porous copper deposited with the Ni coating on inner surface of pores in the whole strain range. Unlike foamed metals, no constant plateau stress
Absorbed energy per volume, MJ m
3.3. Hardness and compressive strength of the electroplated lotus-type porous copper
-3
Strain, % 100
(b)
uncoated porous copper coated porous copper
90 80 70 60 50 40 30 20 10 0 0
10
20
30
40
50
Strain, % Fig. 5. Compression stress–strain curves (a) and absorbed energy per volume–strain curves (b) for both uncoated and coated lotus-type porous copper.
appears on both samples but stress continuously increases with the deformation progress, indicating that stress concentration does not occur around the pores on both samples during compression in the pore direction [19]. According to the compression stress–strain curves, the absorbed energy can be obtained for both the uncoated and the coated lotus-type porous copper, as shown in Fig. 5(b). The absorbed energy per volume of the lotus-type porous copper is almost the same as that reported by Hyun and Nakajima [19]. It should be mentioned that this result gives the total absorbed energy consisting of both elastic and plastic absorbed energy. It is interesting to find that the coating significantly improves the energy absorption of the lotus-type porous copper during the whole compression at each strain, and the improvement increases with the strain, in which the absorbed energy per volume increases from 60.83 to 96.01 MJ m−3 (about 57.8% on growth rate) when the porous copper samples were compressed to 80%. The absorbed energy of the coated lotus-type porous copper is much superior to that of the uncoated porous copper, which can be attributed to not only higher strength including compressive strength and hardness but also the deformation of the Ni coating during the compression. The coated samples were observed by the SEM with interrupting the compression tests at strains of 20, 40 and 80%. After the compressions, it is interesting to find that the Ni coating still adheres to the pore walls even when the compression ratio reaches 80%, as shown in Fig. 6. It is proved that the Ni coating has a good adhesion with the lotus-type porous copper and will deform with the copper during the compression. Thus, the higher strength of Ni coating plays an important role on the increased absorbed energy
H. Du et al. / Applied Surface Science 264 (2013) 772–778
777
4. Conclusion The inner surface of pores could be deposited with a Ni coating by electroplating with a proper agitating for lotus-type porous copper with pore length 6 mm comparable to the 0.6 mm mean pore diameter. The coating thickness uniformity along the pore length is the main advantage by this simple method. The Ni coating deposited on inner surface of pores with thickness of 4.5 m improves significantly both the yield strength and the energy absorption in compression for the lotus-type porous copper, which is attributed to the obstacle effect to dislocation slip during the deformation and the strengthening effect by the Ni coating. As the coating thickness can be ignored comparing to the pore diameter for lotus-type porous copper, the improvement is interesting and useful. It is predicted that in applications where a porous metal is used for a structural role and the weight must be minimized, depositing a coating on the inner surface of pores could be benefit for such application in the future.
Acknowledgments The financial supports of Natural Science of Foundation from Liaoning Province (no. 201104137), Visiting Scientist Program from Nanoscience and Nonotechnology Center in the Institute of Scientific and Industrial Research (ISIR), Osaka University are acknowledged. The first author would like to express his appreciation to Mr. Yefei Gao, Miss Fei Zhao and Mr. Fan Yang in ISIR, Osaka University, for their technical support.
References
Fig. 6. Optical observation of lotus-type porous copper electroplated with Ni layer after compression. (a) Compressive ratio 20%, (b) compressive ratio 40%, (c) compressive ratio 80%.
for lotus-type porous copper. Further analysis and discussion about the improvement will be given in another paper [20]. Not the same as yield strength, it is hard to predict that the absorbed energy per volume of the coated lotus-type porous copper will increase with the Ni coating thickness. Although the strength of the Ni coating increase with its thickness [18], it should be considered that the intrinsic stress between the Ni coating and the substrate will also increase, especially for the thickness between 3 and 10 m [21], which may deteriorate the adhesion between the Ni coating and the pore wall. Further investigation in this case on the electroplated porous copper is necessary when the Ni coating thickness can be arranged in a wider range in our work in the future.
[1] Hideo Nakajima, Fabrication, properties and application of porous metals with directional pores, Progress in Materials Science 52 (2007) 1091–1173. [2] V. Shapovalov, Development of gas–solid structures at various orientations of the solidification front with respect to the earth gravity field, Materials Science Forum 215–216 (1996) 485–488. [3] Soong-Keun Hyun, Teruyuki Ikeda, Hideo Nakajima, Fabrication of lotus-type porous Ni3 Al intermetallics, Journal of the Japan Institute of Metals 68 (2) (2004) 39–42. [4] Hideo Hoshiyama, Teruyuki Ikeda, Kenji Murakami, Hideo Nakajima, Pore morphology and microstructure of porous magnesium alloys fabricated by unidirectional solidification under hydrogen atmosphere, Journal of the Japan Institute of Metals 67 (12) (2003) 714–720. [5] Takeshi Aoki, Teruyuki Ikeda, Hideo Nakajima, Fabrication of lotus-type porous brass by zinc diffusion into porous copper, Materials Transactions 44 (1) (2003) 89–93. [6] K.L. Choy, J.E. Durodola, B. Derby, C. Ruiz, Effect of TiB2 , TiC and TiN protective coatings on tensile strength and fracture behaviour of SiC monofilament fibres, Composites 26 (1995) 531–539. [7] T. Abdulla, A. Yerokhin, R. Goodall, Effect of plasma electrolytic oxidation coating on the specific strength of open-cell aluminium foams, Materials and Design 32 (2011) 3742–3749. [8] M.P. Nascimento, R.C. Souza, I.M. Miguel, W.L. Pigatin, H.J.C. Voorwald, Effects of tungsten carbide thermal spray coating by HP/HVOF and hard chromium electroplating on AISI 4340 high strength steel, Surface and Coatings Technology 138 (2001) 113–124. [9] A.R. Jones, Microcracks in Hard Chromium Electrodeposits, Plating and Surface Finishing, April 1989, pp. 62–66. [10] G. Dubpernell, F.A. Lowenheim, Modern Electroplating, 1968, p. 80. [11] H.S. Kuo, J.K. Wu, Passivation treatment for inhibition of hydrogen absorption in chromium-plated steel, Journal of Materials Science 31 (22) (1996) 6095–6098. [12] Y. Boonyongmaneerat, C.A. Schuhb, D.C. Dunand, Mechanical properties of reticulated aluminum foams with electrodeposited Ni–W coatings, Scripta Materialia 59 (2008) 336–339. [13] M. Suralvo, B. Bouwhuis, J.L. McCrea, G. Palumbo, G.D. Hibbard, Hybrid nanocrystalline periodic cellular materials, Scripta Materialia 58 (2008) 247–250. [14] B.A. Bouwhuis, J.L. McCrea, G. Palumbo, G.D. Hibbard, Mechanical properties of hybrid nanocrystalline metal foams, Acta Materialia 57 (2009) 4046–4053. [15] L.J. Gibson, M.F. Ashby, Cellular Solids, 2nd ed., Cambridge University Press, UK, 1997. [16] Z. Ning, Y. He, W. Gao, Mechanical attrition enhanced Ni electroplating, Surface & Coatings Technology 202 (2008) 2139–2146. [17] Dj. Djozan, M. Amir-Zehni, Anodizing of inner surface of long and small-bore aluminum tube, Surface and Coatings Technology 173 (2003) 185–191.
778
H. Du et al. / Applied Surface Science 264 (2013) 772–778
[18] Zhang, Kewei Xu, On the evaluation of the adhesion of electroplated Ni coatings upon steel substrate with extended microbridge technique, Materials Science and Engineering A 494 (2008) 122–126. [19] S.K. Hyun, H. Nakajima, Anisotropic compressive properties of porous copper produced by unidirectional solidification, Materials Science and Engineering A 340 (2003) 258–264.
[20] Hao Du, Hideo Nakajima, Guihong Song, Improvement of compressive properties of lotus-type porous copper by Ni coating on inner surface of pores, Materials Science and Engineering A, submitted for publication. [21] S. Basrour, L. Robert, X-ray characterization of residual stresses in electroplated nickel used in LIGA technique, Materials Science and Engineering A288 (2000) 270–274.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Study on lotus-type porous copper electroplated with a Ni coating on inner surface of pores Hao Du a,c,∗ , Guihong Song b , Hideo Nakajima c , Yanhui Zhao d , Jinquan Xiao a , Tianying Xiong a a
Division of Surface Engineering of Materials, Institute of Metal Research, 72 Wenhua Road, Shenyang 110016, PR China School of Material Science and Technology, Shenyang University of Technology, 111 Shenliao West Road, Shenyang 110870, PR China The Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka 567-0047, Japan d Division of Specialized Materials and Devices, Institute of Metal Research, 72 Wenhua Road, Shenyang 110016, PR China b c
a r t i c l e
i n f o
Article history: Received 27 July 2012 Received in revised form 18 October 2012 Accepted 20 October 2012 Available online 6 November 2012 Keywords: Lotus-type porous copper Coating Inner surface of pore Mechanical properties
a b s t r a c t Deposition of Ni coating on inner surface of pores was attempted by electroplating for lotus-type porous copper with pore size of 0.6 mm and pore length of 6 mm. The surface morphology, thickness, thickness distribution along the pore length, and phase composition of the coating were characterized. It is proven that the Ni coating with a polycrystalline structure can be deposited on the inner surface of the pores with length/diameter of 10 for lotus-type porous copper by agitating the electroplating solution properly during the process. It is indicated that the coating thickness distributes uniformly along the pore depth and is about 4–5 m. Furthermore, the mechanical properties including vicker hardness, compressive yield strength and absorbed energy ability of the electroplated porous copper were evaluated. It is found that the mechanical properties are improved significantly after depositing the nickel coating inside pores of the lotus-type porous copper. Among them, 0.2% yield stress increases from 22.96 to 30.15 MPa, while absorbed energy per volume from 60.83 to 96.01 MJ/m3 when compressed to strain of 80%, which is attributed mainly to the Ni coating as an obstacle to dislocation slip during deformation and its strengthening effect for the higher strength, and the good adhesion to the pore wall of the porous copper. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Gasar or lotus-type porous metal has received much attention recently for not only a smart structure on porosity, pore morphology and pore aligning direction, but also higher mechanical properties than those formed by other techniques, such as foaming or powder sintering [1,2]. It is not easy to fabricate lotus-type porous alloy with straight and elongated pores by unidirectional solidification method through the mold casting technique for the sake of the dendrite growth during alloy solidification, compared with lotus-type porous pure metal [3]. Furthermore, it is also difficult to control the porosity of lotus-type porous alloy by the traditional unidirectional solidification method for the very small hydrogen solubility difference between solid and liquid of alloy due to addition of alloying elements [4]. To deal with this problem, Nakajima et al. [5] attempted to fabricate lotus-type porous brass by evaporating and depositing zinc onto and into lotus-type porous copper and diffusing into the copper by annealing. This suc-
∗ Corresponding author at: Division of Surface Engineering of Materials, Institute of Metal Research, 72 Wenhua Road, Shenyang 110016, PR China. Tel.: +86 24 8397 8952; fax: +86 24 8397 8952. E-mail address: [email protected] (H. Du). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.10.119
cessful fabrication motivates us an idea if coating can be employed on lotus-type porous metal for not only alloy fabrication but also surface protection, and structure and property modification. In this case, the first question to be answered is if and how a metallic or ceramic coating can be deposited into pores of lotus-type porous metal successfully; the second question is the effect of the metallic or ceramic coating on the surface of pore wall on properties of lotus-type porous metals or alloys. To the best knowledge of the present authors’, no work has been carried out on this point for lotus-type porous metals. Coating deposition on inner surface of pores for lotus-type porous metal may be possibly achieved by several methods such as electroplating, electroless plating, PVD, CVD, etc., in which gas or liquid can pass the pores for coating deposition [6,7]. The key point of these methods on coating deposition inside pore is how to deposit a uniform coating to a depth as long as possible. Among these methods, electroplating is flexible and extremely feasible in industrialization [8]. It is accepted that thickness and properties of the electroplating metal layer can be controlled by several factors including composition and concentration of the electrolyte solution, temperature, electrolyte flow rate, deposition time and the applied potential [9–11]. Furthermore, it has been demonstrated that mechanical properties of foamed metals could potentially be increased with electroplated Ni, Ni alloy coatings [12–14].
H. Du et al. / Applied Surface Science 264 (2013) 772–778
773
Cu(111)
On cross section On surface
Cu(222)
Ni(111) Cu(200) Ni(200)
30
40
50
Cu(220)
60
70
Cu(311)
80
90
100
2θ, degree Fig. 2. XRD pattern for electroplated lotus-type porous copper on both surface and cross section.
average pore length of 9.8 mm was employed as substrate. For electroplating, rectangular specimens of 30 mm × 15 mm × 6 mm were cut from the lotus-type porous copper ingot, and mechanically polished by SiC papers from grit 400# to 1500#, followed by degreasing, etching, and cleaning. The specimens were first degreased with a hot solution of 5% H2 SO4 for 3 min, and etched in a solution of HNO3 for 3 s with vibration. After rinsed with deionized water for a minute, the specimens were cleaned with alcohol for 3 min ultrasonically and then used immediately. Ni electroplating was performed under an optimal condition by our experimental results, using a pure nickel plate as anode and the lotus-type porous copper specimens as cathode with a 2:1 ratio in surface area for obtaining a more uniform coating. The lotus-type porous copper specimens were placed with a distance of 15 mm and pore direction perpendicular to the nickel plate surface. The solution for electroplating contained NiSO4 ·7H2 O 250 g/L, NiCl2 ·6H2 O 40 g/L, H3 BO3 35 g/L, C12 H25 SO4 Na 0.1 g/L and a nickel additive. The pH value of the solution was adjusted to 3.5. A magnetic pellet was used to stir the solution for keeping the solution concentration in the pores as uniform as possible and preventing the formation of hydrogen bubbles on the surface. The electroplating process was carried out in the solution by a proper agitating at a controlled and constant potential (18 V), current density (2.5 A/dm2 ), and temperature (55–60 ◦ C). During the electroplating, the lotus-type porous specimens were turned around once each 5 min. The thickness of the nickel coating was controlled by the electroplating time. Fig. 1. Observation on electroplated lotus-type porous copper specimen. (a) Whole image, (b) on cross section, (c) on inner surface of pores.
In this work, deposition of Ni coating on the inner surface of pores of lotus-type porous copper by electroplating was attempted. The structure of the Ni coating including surface image, thickness, thickness distribution along pore depth, and XRD pattern was characterized. Furthermore, mechanical properties of the electroplated lotus-type porous copper, including vicker hardness, compressive yield strength and absorbed energy ability were evaluated and analyzed. 2. Experimental procedures 2.1. Electroplating of Ni coating for lotus-type porous copper A lotus-type porous copper, fabricated in Nakajima’s laboratory, with porosity of 43.3%, average pore size of 603.5 m and
2.2. Characterization of electroplated lotus-type porous copper After being observed by an optical microscope (OM; VHX200, Keyence Corp., Japan), the coated specimens were divided into 5 mm × 5 mm × 6 mm using an electric discharge machine (LN1W, Sodick Co., Japan), and examined by an X-ray diffractometer (XRD; D8 Discover, Bruker AXS, Germany) operating with Cu K␣ ( = 0.154056 nm) radiation on both surface and cross section. The analyzed range of the diffraction angle 2 was between 30◦ and 100◦ by a step width of 0.04◦ . At the same time, coating surface was observed on pore wall by a scanning electron microscope (SEM; JSM 6360T, JEOL, Japan). Then, they were further cut on transverse sections along the sample height at 0.5, 1.0, 2.0, 3.0, 4.0, 5.0 and 5.6 mm, mounted, grinded and polished. The coating thickness was obtained by observing the cross section using the SEM, which is the average value of 10 points chosen randomly. The element distributions on cross section of coated pore wall were determined by electron probe microanalysis (EPMA; JXA-8800, JEOL, Japan).
774
H. Du et al. / Applied Surface Science 264 (2013) 772–778
Fig. 3. Observation of Ni coating on inner surface of pores and coating thickness. (a) The image of cross section, (b) a magnified image of (a), (c) element distribution on cross section of coated pore wall, (d) coating thickness distribution along pore length, (e) change in coating thickness with deposition time.
2.3. Measurement on mechanical properties The hardness of both uncoated and coated porous copper samples was evaluated by using a microhardness tester with a Vicker indenter (AKASHI MVK-EIII, Japan) on surface under two loads of 10 g and 25 g and a dwelling time of 10 s. The distance between two indentations was more than five times the minor diagonal to prevent stress field effect from nearby indentations. The average hardness values were recorded from five points chosen randomly on the sample surface. Compressive tests were performed on both the uncoated and the coated porous copper samples with size of 5 mm × 5 mm × 6 mm using a universal testing machine (Model 5582, Instron Co. Ltd., USA) in air at room temperature. The cross-head speed was set to be 0.1 mm/min. The strain was measured from the actuator displacement and the initial sample length. The highest strain was
chosen as 80%. The compressive yield strength (0.2% offset strength) was determined from the stress–strain curve and reported by the testing machine automatically. The mean values and standard deviation of the compressive yield strength were obtained from results tested on 5 samples in each case. After the compression measurements, the absorbed energy during the compression was calculated by integrating the area under the stress–strain curve by [15]:
W=
ε
(ε) dε 0
where W is the absorbed energy per unit volume and ε is the strain. The average values and standard deviations of the absorbed energy were reported from five samples in each case.
H. Du et al. / Applied Surface Science 264 (2013) 772–778
775
Fig. 4. Images of pore walls after compressed 20% of uncoated porous copper (a) and coated porous copper (b). In each case, 2 is the amplified part labeled in Image 1.
3. Results and discussion 3.1. Morphology observation, phase composition of the electroplated lotus-type porous copper After electroplating, the lotus-type porous copper specimens changed from copper appearance to nickel’s, as shown in Fig. 1(a). To be sure if the Ni coating was deposited on the inner surface of the penetrable pores, the specimen was cut on cross section and observed, which is shown in Fig. 1(b). It is found that the inner surface of the pores of the sample is also covered by a layer of coating, which is comprised of continuous and compact nodules with size in range of 2–4 m, as shown in Fig. 1(c). The surface characteristic of the coating is traditional of that for electroplating metal coating, as reported by Gao et al. [16]. On the other hand, no coating is found on the inner surface of the close pores in the specimens, as access of electrolyte to the inner surface of the pores is unavailable. XRD results confirm that the coating deposited on not only surface but also inner surface of pores of the lotus-type porous copper is Ni coating, as shown in Fig. 2. On the whole, the coating in this work is only composed of the Ni phase. There are no signals related to Ni alloy or other nickel oxide phases existing in the coating. In the surface case, Ni phase is the main phase for the sample. In the cross section (inner surface) case, Ni phase can be distinguished although Cu phase is the main phase, which stems from the naked copper substrate and the limited thickness of the Ni coating (presented in the following section). The XRD results also indicate that the electroplated Ni coating on inner surface of pores possesses a polycrystalline structure with two diffraction peaks of nickel (1 1 1) and (2 0 0).
3.2. Thickness and its distribution of the electroplated Ni coating on inner surface The experimental results mentioned above demonstrated that Ni coating can be deposited on inner surface of pores of lotustype porous copper by electroplating successfully. It is necessary and useful to know thickness and thickness distribution along pore length of the electroplated Ni coatings. The cross sectional surface of a polished sample is shown in Fig. 3(a). A white line with a thickness of 4–5 m can be found clearly between pores and copper substrate, which is confirmed as Ni coating by EMPA result, as shown in Fig. 3(b) and (c), respectively. Variation in coating thickness along the pore length is shown in Fig. 3(d) on sample electroplated with Ni for 60 min. It is interesting to find that the coating thickness distributes along the pore length uniformly although the ratio of pore length/pore diameter reaches 10, which is attributed to the proper electrolyte circulation [17]. It should be mentioned that the thickness increases a little from 4.3 to 4.5 m near the outer edge of the pores, indicating there is still a coating cumulation at the outer edge of pores, which is generally the key problem for coating deposition on inner surface of long tube especially with a small diameter. The influence of stirring direction and velocity of the electroplating solution on thickness and thickness distribution of the coating on inner surface of pores will be presented and discussed in another paper. At the same time, variation in coating thickness with electroplating time was obtained on both sample surface and inner surface of pores, as shown in Fig. 3(e). The coating thickness on sample surface increases with the deposition time almost linearly with a little decrease for longer deposition time. The coating thickness on the inner surface increases from 3.9 to 4.8 m although that on sample
776
H. Du et al. / Applied Surface Science 264 (2013) 772–778
Table 1 Hardness of the uncoated and the coated lotus-type porous copper.
300
HV0.01
HV0.025
250
Uncoated sample Coated sample
89.3 ± 4.2 217.4 ± 15.6
79.5 ± 5.1 176.1 ± 7.1
200
surface increases from 19.0 to 49.6 m when the deposition time increases from 30 to 80 min. The lower and the relatively stable coating thickness on inner surface, especially when the electroplating time is longer is connected with a shielding effect by pore walls-copper substrate, resulting in a reduced potential in the center of the elongated pores and a limited passage and supply of the electrolyte through the pores, which reduces the coating deposition velocity markedly.
Stress, MPa
Hardness
(a)
Lotus-type porous copper Coated lotus-type porous copper
150
100
50
0 0
10
20
30
40
50
60
70
80
60
70
80
Table 1 shows the vicker hardness of the coated and uncoated porous copper obtained at the surface. The hardness of the louts-type porous copper with the coating is much higher compared with that of the uncoated specimen. In the case of 10 g load, the average hardness reaches 217.4, for the coated specimen, more than 2.4 times for the uncoated specimen. This is obviously related to the higher hardness of the Ni since the coating thickness is between 25 m and 50 m, which is much higher than the indentation depth. From this result, it is predicted that the hardness of pore inner surface is also improved by the Ni coating, which may improve the wear resistance when fluid passes through pores of lotus-type porous metal using as a filter. It is interesting to find that the yield strength increases from 22.96 ± 2.03 to 30.15 ± 3.11 MPa (about 31% on growth rate) with the addition of the Ni coating for the lotus-type porous copper. The improvement is supposedly connected mainly with the Ni coating as a barrier for dislocation slip movement during the compression. Fig. 4 shows the SEM images of the inner surface of pores of the uncoated and the coated samples when compression at strain of 20% was applied along the pore direction. It is indicated that it is slip deformation that occurs during compression of uncoated sample, as conventional fine slip lines are clearly found (Fig. 4(a2)). In the case of the coated samples, dislocation still has to move out from copper matrix through pore walls during the compression. As the Ni coating deposited on the inner surface of pores, the dislocations encounter potential barriers and hardly slip out the substrate as before. Thus, higher stress is necessary to move the dislocations, resulting in the improved yield strength. It is predicted that the yield strength of the coated porous copper will increase further if the Ni coating thickness increases. During the compression, the Ni coating underwent the same load with the lotus-type porous copper. The rigidity and the strength of the electroplated Ni coating increase with its thickness [18], which may play another positive role in the improvement of yield strength of the coated porous copper on the basis of the barrier effect. Unfortunately, the thickness effect cannot be investigated in this work for the coating thickness limitation, which will be dealt with in our following work. 3.4. Ability to absorb energy during compression The compression stress–strain curves of both the uncoated and the coated samples are presented in Fig. 5(a). Significant increase in stress can be found for the porous copper deposited with the Ni coating on inner surface of pores in the whole strain range. Unlike foamed metals, no constant plateau stress
Absorbed energy per volume, MJ m
3.3. Hardness and compressive strength of the electroplated lotus-type porous copper
-3
Strain, % 100
(b)
uncoated porous copper coated porous copper
90 80 70 60 50 40 30 20 10 0 0
10
20
30
40
50
Strain, % Fig. 5. Compression stress–strain curves (a) and absorbed energy per volume–strain curves (b) for both uncoated and coated lotus-type porous copper.
appears on both samples but stress continuously increases with the deformation progress, indicating that stress concentration does not occur around the pores on both samples during compression in the pore direction [19]. According to the compression stress–strain curves, the absorbed energy can be obtained for both the uncoated and the coated lotus-type porous copper, as shown in Fig. 5(b). The absorbed energy per volume of the lotus-type porous copper is almost the same as that reported by Hyun and Nakajima [19]. It should be mentioned that this result gives the total absorbed energy consisting of both elastic and plastic absorbed energy. It is interesting to find that the coating significantly improves the energy absorption of the lotus-type porous copper during the whole compression at each strain, and the improvement increases with the strain, in which the absorbed energy per volume increases from 60.83 to 96.01 MJ m−3 (about 57.8% on growth rate) when the porous copper samples were compressed to 80%. The absorbed energy of the coated lotus-type porous copper is much superior to that of the uncoated porous copper, which can be attributed to not only higher strength including compressive strength and hardness but also the deformation of the Ni coating during the compression. The coated samples were observed by the SEM with interrupting the compression tests at strains of 20, 40 and 80%. After the compressions, it is interesting to find that the Ni coating still adheres to the pore walls even when the compression ratio reaches 80%, as shown in Fig. 6. It is proved that the Ni coating has a good adhesion with the lotus-type porous copper and will deform with the copper during the compression. Thus, the higher strength of Ni coating plays an important role on the increased absorbed energy
H. Du et al. / Applied Surface Science 264 (2013) 772–778
777
4. Conclusion The inner surface of pores could be deposited with a Ni coating by electroplating with a proper agitating for lotus-type porous copper with pore length 6 mm comparable to the 0.6 mm mean pore diameter. The coating thickness uniformity along the pore length is the main advantage by this simple method. The Ni coating deposited on inner surface of pores with thickness of 4.5 m improves significantly both the yield strength and the energy absorption in compression for the lotus-type porous copper, which is attributed to the obstacle effect to dislocation slip during the deformation and the strengthening effect by the Ni coating. As the coating thickness can be ignored comparing to the pore diameter for lotus-type porous copper, the improvement is interesting and useful. It is predicted that in applications where a porous metal is used for a structural role and the weight must be minimized, depositing a coating on the inner surface of pores could be benefit for such application in the future.
Acknowledgments The financial supports of Natural Science of Foundation from Liaoning Province (no. 201104137), Visiting Scientist Program from Nanoscience and Nonotechnology Center in the Institute of Scientific and Industrial Research (ISIR), Osaka University are acknowledged. The first author would like to express his appreciation to Mr. Yefei Gao, Miss Fei Zhao and Mr. Fan Yang in ISIR, Osaka University, for their technical support.
References
Fig. 6. Optical observation of lotus-type porous copper electroplated with Ni layer after compression. (a) Compressive ratio 20%, (b) compressive ratio 40%, (c) compressive ratio 80%.
for lotus-type porous copper. Further analysis and discussion about the improvement will be given in another paper [20]. Not the same as yield strength, it is hard to predict that the absorbed energy per volume of the coated lotus-type porous copper will increase with the Ni coating thickness. Although the strength of the Ni coating increase with its thickness [18], it should be considered that the intrinsic stress between the Ni coating and the substrate will also increase, especially for the thickness between 3 and 10 m [21], which may deteriorate the adhesion between the Ni coating and the pore wall. Further investigation in this case on the electroplated porous copper is necessary when the Ni coating thickness can be arranged in a wider range in our work in the future.
[1] Hideo Nakajima, Fabrication, properties and application of porous metals with directional pores, Progress in Materials Science 52 (2007) 1091–1173. [2] V. Shapovalov, Development of gas–solid structures at various orientations of the solidification front with respect to the earth gravity field, Materials Science Forum 215–216 (1996) 485–488. [3] Soong-Keun Hyun, Teruyuki Ikeda, Hideo Nakajima, Fabrication of lotus-type porous Ni3 Al intermetallics, Journal of the Japan Institute of Metals 68 (2) (2004) 39–42. [4] Hideo Hoshiyama, Teruyuki Ikeda, Kenji Murakami, Hideo Nakajima, Pore morphology and microstructure of porous magnesium alloys fabricated by unidirectional solidification under hydrogen atmosphere, Journal of the Japan Institute of Metals 67 (12) (2003) 714–720. [5] Takeshi Aoki, Teruyuki Ikeda, Hideo Nakajima, Fabrication of lotus-type porous brass by zinc diffusion into porous copper, Materials Transactions 44 (1) (2003) 89–93. [6] K.L. Choy, J.E. Durodola, B. Derby, C. Ruiz, Effect of TiB2 , TiC and TiN protective coatings on tensile strength and fracture behaviour of SiC monofilament fibres, Composites 26 (1995) 531–539. [7] T. Abdulla, A. Yerokhin, R. Goodall, Effect of plasma electrolytic oxidation coating on the specific strength of open-cell aluminium foams, Materials and Design 32 (2011) 3742–3749. [8] M.P. Nascimento, R.C. Souza, I.M. Miguel, W.L. Pigatin, H.J.C. Voorwald, Effects of tungsten carbide thermal spray coating by HP/HVOF and hard chromium electroplating on AISI 4340 high strength steel, Surface and Coatings Technology 138 (2001) 113–124. [9] A.R. Jones, Microcracks in Hard Chromium Electrodeposits, Plating and Surface Finishing, April 1989, pp. 62–66. [10] G. Dubpernell, F.A. Lowenheim, Modern Electroplating, 1968, p. 80. [11] H.S. Kuo, J.K. Wu, Passivation treatment for inhibition of hydrogen absorption in chromium-plated steel, Journal of Materials Science 31 (22) (1996) 6095–6098. [12] Y. Boonyongmaneerat, C.A. Schuhb, D.C. Dunand, Mechanical properties of reticulated aluminum foams with electrodeposited Ni–W coatings, Scripta Materialia 59 (2008) 336–339. [13] M. Suralvo, B. Bouwhuis, J.L. McCrea, G. Palumbo, G.D. Hibbard, Hybrid nanocrystalline periodic cellular materials, Scripta Materialia 58 (2008) 247–250. [14] B.A. Bouwhuis, J.L. McCrea, G. Palumbo, G.D. Hibbard, Mechanical properties of hybrid nanocrystalline metal foams, Acta Materialia 57 (2009) 4046–4053. [15] L.J. Gibson, M.F. Ashby, Cellular Solids, 2nd ed., Cambridge University Press, UK, 1997. [16] Z. Ning, Y. He, W. Gao, Mechanical attrition enhanced Ni electroplating, Surface & Coatings Technology 202 (2008) 2139–2146. [17] Dj. Djozan, M. Amir-Zehni, Anodizing of inner surface of long and small-bore aluminum tube, Surface and Coatings Technology 173 (2003) 185–191.
778
H. Du et al. / Applied Surface Science 264 (2013) 772–778
[18] Zhang, Kewei Xu, On the evaluation of the adhesion of electroplated Ni coatings upon steel substrate with extended microbridge technique, Materials Science and Engineering A 494 (2008) 122–126. [19] S.K. Hyun, H. Nakajima, Anisotropic compressive properties of porous copper produced by unidirectional solidification, Materials Science and Engineering A 340 (2003) 258–264.
[20] Hao Du, Hideo Nakajima, Guihong Song, Improvement of compressive properties of lotus-type porous copper by Ni coating on inner surface of pores, Materials Science and Engineering A, submitted for publication. [21] S. Basrour, L. Robert, X-ray characterization of residual stresses in electroplated nickel used in LIGA technique, Materials Science and Engineering A288 (2000) 270–274.