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Heat dissipation properties of oxide layers formed on 7075 Al alloy via plasma electrolytic oxidation
Surface & Coatings Technology 269 (2015) 114–118
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Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Heat dissipation properties of oxide layers formed on 7075 Al alloy via plasma electrolytic oxidation Yeon Sung Kim a, Hae Woong Yang b, Ki Ryong Shin a, Young Gun Ko b,⁎, Dong Hyuk Shin a,⁎ a b
Department of Metallurgy and Materials Engineering, Hanyang University, Ansan 426-791, South Korea School of Materials Science and Engineering, Yeungnam University, Gyeongsan 712-749, South Korea
a r t i c l e
i n f o
Available online 4 February 2015 Keywords: Al alloy Plasma electrolytic oxidation Carbon nanotube Metallic oxide Heat emissivity
a b s t r a c t The paper demonstrated the microstructure and radiation characteristics of the oxide layers on 7075 Al alloy samples which were subjected to plasma electrolytic oxidation (PEO) process in the couples of the electrolytes with containing particles of either carbon nanotube (CNT) or yttria-stabilized zirconia (YSZ), or titania (TiO2). For this purpose, a series of PEO coatings were conducted at a current density of 100 mA/cm2. Surface observation revealed that a number of CNT, YSZ, and TiO2 were incorporated successfully in the oxide layer. Moreover, the size and distribution of the micro-pores caused by plasma discharge were changed markedly as compared to Al alloy coated by PEO process using a particle-free electrolyte. The oxide layer formed in the electrolyte with CNT showed the highest heat emission properties among all conditions tested in this study. © 2015 Elsevier B.V. All rights reserved.
1. Introduction In recent years, plasma electrolytic oxidation (PEO) process, which was also known as micro-arc oxidation, has been one of the surface modification coatings used in the various fields of electronic and automobile industries [1–4]. This method has been known as an ecofriendly process because the electrolyte employed various solutions with high pH, which differed mainly from the conventional anodizing. This process could also produce a well-developed oxide layer with amorphous and/or nanocrystalline structures on valve materials by means of the anodic oxidation and concurrent electrochemical reactions between the substrate and electrolyte [5–7]. Under either DC or AC conditions, the plasma discharges began to appear at the onset of dielectric breakdown where both gas and plasma bubbles formed at the anode surface and, thereby, the hard coating layer formed due to the growth of the metal-oxide which was originated from the electrolyte, substrate metal ions, and oxygen gas produced by water decomposition [8–10]. The oxide layer fabricated in the case of Al alloys was composed of an inner barrier layer next to the substrate and an outer porous layer containing the numerous micro-pores which were attributed to the severe discharging and gas/plasma bubble on the surface of the substrate. This structure might be controlled by handling the electrical parameters such as current density, frequency, and wave form and the chemical composition of the electrolyte. Furthermore, the development of the oxide layers and their properties would be altered by incorporating the various functional oxide compounds.
⁎ Corresponding authors. E-mail addresses: [email protected] (Y.G. Ko), [email protected] (D.H. Shin).
http://dx.doi.org/10.1016/j.surfcoat.2015.01.059 0257-8972/© 2015 Elsevier B.V. All rights reserved.
In general, the emissivity characteristics of the coating were affected by structural factors, such as coating thickness and surface roughness, leading to the fact that the increase in the thickness and roughness could contribute to the enhancement of the solar emissivity of the coating [11–14]. On the other hand, in the current field of PEO technologies, the successful incorporation of secondary chemical compounds, which would be carbon nanotube (CNT), YSZ, and TiO2 with excellent emissivity properties in nature, would be expected to improve the heat dissipation of the overall coating layer [15–17]. According to the previous results reported by the present authors [18,19], the incorporations of CNT with multi walls would give rise to the appreciable change in structural roughness. Thus, it is necessary to investigate the individual role of these functional compounds after PEO coating on the emissivity response by controlling the structural parameters of the coating layer. In this study, three coating layers incorporating CNT, YSZ, and TiO2, respectively, were fabricated while the thickness and roughness of the coating layers remained nearly identical after PEO coatings in the alkaline electrolytes. The heat emissivity properties as a way of a radiation were examined. 2. Experimental procedures The chemical composition of 7075 Al alloy used in this study was Al-5.1Zn-2.1Mg-1.2Cu in weight percent. The samples whose diameter and length were 20 and 10 mm, respectively, were polished with #1000 SiC paper, rinsed with distilled water, and ultrasonically cleaned in ethanol. A schematic illustration of PEO setup is shown in Fig. 1. The PEO process was conducted utilizing a 20 kW power supply in conjunction with stirring and cooling systems. The electrolytic cell consisted of a 2-liter acrylic-vessel with a sample holder, and stainless steel with a
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the oxide layers was recorded from 5 to 20 μm at 353 K, 200 scans being obtained at 4 cm−1 resolution via HgCdTe detector.
3. Results and discussion
Fig. 1. Schematic diagram of the present PEO process.
dimension of 15 × 25 cm was used as cathode. The distance between cathode and anode was 50 mm in the electrolyte. To verify the influence of adding CNT into the oxide layer on the heat emissivity in the range of from 5 to 20 μm of the wavelength, two different electrolytes with and without CNT were used. Additionally, YSZ and TiO2 particles with 150–200 nm in diameter listed in Table 1 were selected. The sodium silicate was mainly employed in this study whereas, in the cases of Cells C and D, the sodium hexametaphosphate was added in the electrolyte in order to improve their incorporation kinetics of the metal oxide particles. The temperature of the electrolyte was maintained at 293 K during PEO process. The applied current density and coating time were 100 mA/cm2 and 900 s, respectively, and the thicknesses of the oxide layers was controlled uniformly to be about 10 μm in thickness since the difference in the thickness of the oxide layers would result in the interference of the heat emissivity. The diameter and length of the carboxylated multi-walled CNT dispersed in distilled water acquired from Cluster Instruments Co., Ltd were ~35 nm and ~10 μm, respectively. The surface morphologies and chemical compositions of each sample were observed by using a field-emission scanning electron microscope (SEM; S-4700, HITACHI) equipped with an energy dispersive X-ray spectrometer (EDS; Genesis APEX2, AMETEK). A non-contact 3D surface measurement system (U-surf, NANOFOCUS) was used to measure the surface roughness of the PEO-treated samples. To measure the thermal emissivity of the oxide layers, Fourier transform infrared spectrometer (FTIR; M4500, MIDAC) having Michelson interferometer comprised a couple of mirrors, and a KBr beam splitter was employed. The FTIR spectrum of
Table 1 Chemical compositions of the electrolyte used for the present PEO coatings.
Cell A Cell B Cell C Cell D
CNT (1 wt.%) (ml/l)
Na2SiO3 (mol/L)
(NaPO3)6 (mol/L)
YSZ (mol/L)
TiO2 (mol/L)
– 50 – –
0.08 0.08 0.03 0.03
– – 0.05 0.05
– – 0.02 –
– – – 0.02
Fig. 2 shows SEM micrographs revealing the surfaces of the oxide layers formed within the electrolytes with three different particles and without these particles under the same PEO conditions. It is evident from Fig. 2(a) that the surface morphology of the sample coated in the electrolyte without addition of any particles (Cell A) exhibited a number of the micro-pores and irregularly-formed molten oxide, which resulted from the electrochemical oxidation accompanying the micro-discharge randomly generated between the aluminum substrate and the electrolyte. The high temperature of the plasma discharge formed in a time period of less than 10−6 s dropped rapidly with the cooling rate of 108 K/s due to the electrolyte which was maintained at room temperature. Thus, unique oxide compounds such as Al2O3 and SiO2 were fabricated by reacting the cation of the aluminum substrate and the anions of the electrolyte [20]. This was in line with the result of EDS analysis from Table 2, showing the presence of Al, O, and Si. On the other hand, in the case of the sample coated in Cell B, few numbers of the micro-pores were observed throughout the surface of the oxide layer as shown in Fig. 2(b). This fact would be attributed to the fact that the incorporation of CNT played a role in clogging and blocking the micro-pores, which was reported by the present authors [18,19]. EDS analysis of the sample represented Al, O and Si, in addition to ~15 at.% C (Table 2). The amount of Al in the oxide layer was reduced relatively by introducing the appreciable amount of C originating from CNT incorporated. We thought that most of C detected in this study came from not the electrolyte, substrate, and carbon tape for EDS observations but CNT consisting of carbon atoms. Thus, the inset in Fig. 3(b) indicated that the molten oxides caused by the micro-discharges with high temperature might be electrochemically mixed with the numerous CNTs in the oxide layer. In the cases of the oxide layers coated in Cells C and D as shown in Fig. 2(c) and (d), the micro-pores having average diameter of ~ 1 μm which were caused by high temperature of the micro-discharges as well as the particles in the oxide layer were revealed. The incorporation of the metal oxide particles in the oxide layer might be attributed to the migration of those particles via electrophoresis where the metal oxide particles dispersed initially in the electrolyte moved toward the anode surface during PEO process [20,21]. This result was supported by the present EDS analysis in which the oxide layers coated in Cells C and D are shown in Table 2, ~ 11 at.% Zr and ~14 at.% Ti, respectively. Fig. 3 shows the cross sectional images of the oxide layers coated in four different electrolytes. The thickness and surface roughness (Ra) values of the four different oxide layers were also measured, and the results were listed in Table 3. Regardless of the addition of particles, the PEO-treated oxide layers revealed an average thickness of ~10 μm and some micro-pores of a few micrometers in size. The oxide layer coated without any particles as shown in Fig. 3(a) revealed that the outer part of the oxide layer was severely rough, and the middle part contained numerous micro-pores. When CNT was introduced in the oxide layer, the number and size of the network pores were drastically decreased (Fig. 3(b)). It was found, however, that the huge horizontal channel near to the substrate might be inevitably formed due to the influence of the thermal residual stress which was related to the incorporation of CNT [18]. In the cases of the addition of the metal oxide particles in the oxide layers (Cells C and D), Fig. 3(c) and (d) displayed that the inner part of the oxide layer was composed of comparatively large micro-pores. In terms of the Ra in the PEO-treated Al alloys, the oxide layer without particles was measured to be ~ 3 μm, which was the highest value among four different samples. By incorporating particles into the oxide layer, all Ra values were one-third lower than that of the oxide layer in Cell A. These were attributed to the fact that
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Fig. 2. SEM images showing the surface structures of the oxide layers fabricated by PEO process using four different electrolytes: (a) without CNT, (b) with CNT, (c) with YSZ, and (d) with TiO2. The inset of Fig. 2(c) indicates magnified SEM image.
particles could be considered as an important structural factor affecting the surface roughness of the oxide layer. It is apparent that, in all samples with particles, thickness and roughness were successfully controlled to be nearly identical, which would be in line with the main motivation of this study. The particles having the negative value in zeta potential within the present electrolytes would move to an anode surface due to the electrophoresis during PEO coating [22,23]. The local fluctuation of the molten oxide caused by the intense plasma discharges with high energy and temperature would promote the incorporation of these particles into the newly formed oxide layer. As shown in Fig. 2, these particles were incorporated in the coating, resulting in the formation of the different coating structures as compared to those without particles (Cell A). And some particles which were diffused through the discharge channels and/or micro-pores in the oxide layer were likely to be attached to the oxide surface [24]. Then, they worked as a kind of filler in reducing the fraction and size of the micro-pores, lowering the surface roughness as shown in Table 3. The thermal emissivity was an ability which could emit energy by radiation as compared to a blackbody that absorbed most of the incident energy at all wavelength. If all light energy was assumed to be absorbed, the value of thermal emissivity would be a unity. It is of particular interest that the chemical compositions of the oxide layers influenced the emissivity value. The emissivity spectra of the present samples measured in a wavelength from 5 to 20 μm at 353 K are shown in Table 2 EDS analysis of the oxide layers formed by PEO coatings in four different electrolytes.
Cell A Cell B Cell C Cell D
C
O
Al
Si
P
Ti
Zr
– 14.7 – –
36.2 48.9 57.6 55.3
53.2 23.8 22.5 22.4
10.6 12.6 1.9 0.9
– – 6.7 7.9
– – – 13.5
– – 11.3 –
Fig. 4. For a comparison, the thermal emissivity of bare Al alloy was added. The normal total emissivities ε' n of samples were calculated using following equation [25]: Zλ2
0
0
ε λ;n ðλ; T Þ e λb;n ðλ; T Þdλ 0
εn¼
λ1
Zλ2
ð1Þ 0
e λb;n ðλ; T Þdλ λ1
where λ1 and λ2 are measured initial and final wavelengths, e'λb,n is the blackbody emission power in vacuum, and ε' λ,n(λ, T) is the measured spectral emissivity. The emissivity spectra of the whole samples increased with increasing wavelength. The results are tabulated in Table 4. The bare Al alloys used as a reference showed the low emissivity value of ~ 0.35 in the present wavelength due to the poor efficiency of photon emission from the surface [26]. As shown in Fig. 4, the measured emissivity spectra of Al alloy samples coated in the electrolytes with particles such as Cells B, C, and D were higher than Cell A in the wavelength range of 5–8 μm. In terms of the mean emissivity value in this region, Al alloy samples coated in Cells A, C, and D were ~0.68, ~0.74, and ~0.75, respectively. The addition of CNT in the oxide layer led to a significant increase in the emissivity value approaching to be ~ 0.89, which seemed to be the highest among these different results. In general, the general emissivity values of Al alloys after commercial anodizing coating were known to be 0.77–0.83 [27–29]. Wang et al. [30] reported that the highest infrared emissivity properties of the some oxide compounds containing Si and P after PEO coating of 2024 Al alloy were reported to be ~ 0.85 in the wavelength range of 8–20 μm. But, they tested the thermal properties at 773 K, which was higher than the present condition. Wang et al.
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Fig. 3. SEM images showing the cross sectional structure of the oxide layer fabricated by PEO process using four different electrolytes: (a) without CNT, (b) with CNT, (c) with YSZ, and (d) with TiO2.
[25] investigated based on the PEO coating of AZ91D Mg alloy that, regardless of the chemical compositions, such as Si, Cu, W, and V, which were inserted as metal salts in the electrolyte, the four different samples showed a small variation in emissivity values from 0.79 to 0.85. In the case of Ti-6Al-4 V alloy, when PEO coating for 25 min was performed in the electrolyte containing Na2WO4 of 7.5 g/L, resulting in the thickness and roughness of ~ 40 and ~ 2.5 μm, respectively, the highest value of the emissivity was ~ 0.88 [31]. In this study, emissivity values of the oxide layers incorporating particles at 353 K were higher than those reported in the literature. Moreover, the emissivity value of the oxide layer with CNT exhibited ~0.89, which was similar to that of CNT-sprayed on Al alloy (~0.90). The main motivation of this study was to investigate the effect of particle incorporation on the emissivity properties of the oxide layer by reducing the structural differences among the samples fabricated by PEO coating. As seen from Table 3, we thought that the structural difference of the samples after incorporation was insignificant more or less in terms of thickness and roughness. Hence, the present results of heat emissivity were primarily attributed to the incorporation of particles, such as CNT, YSZ, and TiO2. Therefore, the incorporation of the particles in the oxide layer via PEO process could improve the heat emission properties, which was attributed to the inherent high emissivity in mid-infrared region of the particles. The incorporation of CNT resulted in the good emission properties of the PEO-coated Al alloy sample in this study.
4. Summary The influence of the different particles such as CNT, YSZ, and TiO2 in the oxide layer on 7075 Al alloy on the heat emission properties was investigated. The particles incorporated via electrophoresis had an influence on the surface structure, blocking the micro-pores. From the results of the emissivity spectra, the PEO-coated Al alloy samples with particles showed the superior heat emissivity to that without these particles. In the case of the PEO-coated Al alloy with incorporating CNT, the emissivity spectrum was close to being ~ 0.89 in the wavelength. It would be concluded in this study that the incorporation
Table 3 Thickness and surface roughness (Ra) values of the PEO-treated Al alloys.
Thickness (μm) Roughness (Ra, μm)
Cell A
Cell B
Cell C
Cell D
10.1 ± 0.59 3.2 ± 2.25
10.0 ± 0.93 1.1 ± 0.18
10.0 ± 0.28 1.0 ± 0.15
10.1 ± 0.17 1.2 ± 0.27
Fig. 4. Heat emissivity spectra measured in the wavelength of 5–20 μm at 353 K for four different PEO-treated Al alloys. The bare Al alloy was used as a reference.
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Table 4 Average values of heat emissivity of the PEO-coated samples. For a comparison, heat emissivity of bare Al alloy is added. Bare Al alloy
Cell A
Cell B
Cell C
Cell D
0.35
0.82
0.89
0.87
0.87
of CNT would result in the good heat dissipation properties of the PEOcoated 7075 Al alloy sample. Acknowledgment This work was supported by funding from the Yeungnam University Research Project (#214A367008). References [1] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, S.J. Dowey, Surf. Coat. Technol. 122 (1999) 73–93. [2] G. Sundararajan, L.R. Krishna, Surf. Coat. Technol. 167 (2003) 269–277. [3] X. Nie, A. Wilson, A. Leyland, A. Matthews, Surf. Coat. Technol. 131 (2000) 506–513. [4] J.I. Yu, J.S. Yoon, J.G. Yoon, J.H. Kim, S.D. Choi, H.K. Jang, J.Y. Yu, I.H. Bae, Korean J. Met. Mater. 51 (2013) 405–411. [5] L.O. Snizhko, A.L. Yerokhin, A. Pilkington, N.L. Gurevina, D.O. Misnyankin, A. Leyland, A. Matthews, Electrochim. Acta 49 (2004) 2085–2095. [6] S. Verdier, M. Boinet, S. Maximovitch, F. Dalard, Corros. Sci. 47 (2005) 1429–1444. [7] K. Wang, G. Kim, K. Park, S. Byeon, F. Ahmed, B. Koo, Met. Mater. Int. 19 (2013) 77–80. [8] D.Y. Hwang, Y.M. Kim, D.-Y. Park, B. Yoo, D.H. Shin, Electrochim. Acta 54 (2009) 5479–5485.
[9] J. Liang, L. Hu, J. Hao, Electrochim. Acta 52 (2007) 4836–4840. [10] H. Duan, C. Yan, F. Wang, Electrochim. Acta 52 (2007) 5002–5009. [11] H. Tang, T. Xin, Q. Sun, C. Yi, Z. Jiang, F. Wang, Appl. Surf. Sci. 257 (2011) 10839–10844. [12] F. Cernuschi, S. Ahmaniemi, P. Vuoristo, T. Mäntylä, J. Eur. Ceram. Soc. 24 (2004) 2657–2667. [13] C.-D. Wen, I. Mudawar, Int. J. Heat Mass Transfer 49 (2006) 4279–4289. [14] C.Y. Zhao, T.J. Lu, H.P. Hodson, Int. J. Heat Mass Transfer 47 (2004) 2927–2939. [15] X. Zhao, X.D. He, Y. Sun, L.D. Wang, Mater. Lett. 65 (2011) 2592–2594. [16] B.V. Cockeram, J.L. Hollenbeck, Surf. Coat. Technol. 157 (2002) 274–281. [17] Z.W. Wang, Y.M. Wang, Y. Liu, J.L. Xu, L.X. Guo, Y. Zhou, J.H. Ouyang, J.M. Dai, Curr. Appl. Phys. 11 (2011) 1405–1409. [18] D.H. Jeong, S.C. Kil, S.W. Nam, Korean J. Met. Mater. 52 (2014) 873–880. [19] E.Y. Yoon, D.J. Lee, B.H. Park, M.R. Akbarpour, M. Farvizi, H.S. Kim, Met. Mater. Int. 19 (2013) 927–932. [20] E. Matykina, R. Arrabal, P. Skeldon, G. Thompson, J. Appl. Electrochem. 38 (2008) 1375–1383. [21] P. Skeldon, R. Arrabal, E. Matykina, G.E. Thompson, J. Mater. Sci. 43 (2008) 1532–1538. [22] K.M. Lee, B.U. Lee, S.I. Yoon, E.S. Lee, B. Yoo, D.H. Shin, Electrochim. Acta 67 (2012) 6–11. [23] C. Schwer, E. Kenndler, Anal. Chem. 63 (1991) 1801–1807. [24] K.R. Shin, Y.G. Ko, D.H. Shin, J. Alloys Compd. 536 (S1) (2012) S226–S230. [25] L. Wang, J. Zhou, J. Liang, J. Chen, Appl. Surf. Sci. 280 (2013) 151–155. [26] R. Raman, A. Thakur, Appl. Energy 12 (1982) 205–220. [27] A. Gustavsen, P. Berdahl, Nord. J. Build. Phys. 3 (2003) 1–12. [28] Y.S. Touloukian, D.P. DeWitt, Thermophysical Properties of Matter, 3rd ed. Plenum, New York, 1972. [29] C. Siva Kumar, A.K. Sharma, K.N. Mahendra, S.M. Mayanna, Sol. Energy Mater. Sol. Cells 60 (2000) 51–57. [30] Y.M. Wang, H. Tian, X.E. Shen, L. Wen, J.H. Ouyang, Y. Zhou, D.C. Jia, L.X. Guo, Ceram. Int. 39 (2013) 2869–2875. [31] Z. Yao, B. Hu, Q. Shen, A. Niu, Z. Jiang, P. Su, P. Ju, Surf. Coat. Technol. 253 (2014) 166–170.
Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Heat dissipation properties of oxide layers formed on 7075 Al alloy via plasma electrolytic oxidation Yeon Sung Kim a, Hae Woong Yang b, Ki Ryong Shin a, Young Gun Ko b,⁎, Dong Hyuk Shin a,⁎ a b
Department of Metallurgy and Materials Engineering, Hanyang University, Ansan 426-791, South Korea School of Materials Science and Engineering, Yeungnam University, Gyeongsan 712-749, South Korea
a r t i c l e
i n f o
Available online 4 February 2015 Keywords: Al alloy Plasma electrolytic oxidation Carbon nanotube Metallic oxide Heat emissivity
a b s t r a c t The paper demonstrated the microstructure and radiation characteristics of the oxide layers on 7075 Al alloy samples which were subjected to plasma electrolytic oxidation (PEO) process in the couples of the electrolytes with containing particles of either carbon nanotube (CNT) or yttria-stabilized zirconia (YSZ), or titania (TiO2). For this purpose, a series of PEO coatings were conducted at a current density of 100 mA/cm2. Surface observation revealed that a number of CNT, YSZ, and TiO2 were incorporated successfully in the oxide layer. Moreover, the size and distribution of the micro-pores caused by plasma discharge were changed markedly as compared to Al alloy coated by PEO process using a particle-free electrolyte. The oxide layer formed in the electrolyte with CNT showed the highest heat emission properties among all conditions tested in this study. © 2015 Elsevier B.V. All rights reserved.
1. Introduction In recent years, plasma electrolytic oxidation (PEO) process, which was also known as micro-arc oxidation, has been one of the surface modification coatings used in the various fields of electronic and automobile industries [1–4]. This method has been known as an ecofriendly process because the electrolyte employed various solutions with high pH, which differed mainly from the conventional anodizing. This process could also produce a well-developed oxide layer with amorphous and/or nanocrystalline structures on valve materials by means of the anodic oxidation and concurrent electrochemical reactions between the substrate and electrolyte [5–7]. Under either DC or AC conditions, the plasma discharges began to appear at the onset of dielectric breakdown where both gas and plasma bubbles formed at the anode surface and, thereby, the hard coating layer formed due to the growth of the metal-oxide which was originated from the electrolyte, substrate metal ions, and oxygen gas produced by water decomposition [8–10]. The oxide layer fabricated in the case of Al alloys was composed of an inner barrier layer next to the substrate and an outer porous layer containing the numerous micro-pores which were attributed to the severe discharging and gas/plasma bubble on the surface of the substrate. This structure might be controlled by handling the electrical parameters such as current density, frequency, and wave form and the chemical composition of the electrolyte. Furthermore, the development of the oxide layers and their properties would be altered by incorporating the various functional oxide compounds.
⁎ Corresponding authors. E-mail addresses: [email protected] (Y.G. Ko), [email protected] (D.H. Shin).
http://dx.doi.org/10.1016/j.surfcoat.2015.01.059 0257-8972/© 2015 Elsevier B.V. All rights reserved.
In general, the emissivity characteristics of the coating were affected by structural factors, such as coating thickness and surface roughness, leading to the fact that the increase in the thickness and roughness could contribute to the enhancement of the solar emissivity of the coating [11–14]. On the other hand, in the current field of PEO technologies, the successful incorporation of secondary chemical compounds, which would be carbon nanotube (CNT), YSZ, and TiO2 with excellent emissivity properties in nature, would be expected to improve the heat dissipation of the overall coating layer [15–17]. According to the previous results reported by the present authors [18,19], the incorporations of CNT with multi walls would give rise to the appreciable change in structural roughness. Thus, it is necessary to investigate the individual role of these functional compounds after PEO coating on the emissivity response by controlling the structural parameters of the coating layer. In this study, three coating layers incorporating CNT, YSZ, and TiO2, respectively, were fabricated while the thickness and roughness of the coating layers remained nearly identical after PEO coatings in the alkaline electrolytes. The heat emissivity properties as a way of a radiation were examined. 2. Experimental procedures The chemical composition of 7075 Al alloy used in this study was Al-5.1Zn-2.1Mg-1.2Cu in weight percent. The samples whose diameter and length were 20 and 10 mm, respectively, were polished with #1000 SiC paper, rinsed with distilled water, and ultrasonically cleaned in ethanol. A schematic illustration of PEO setup is shown in Fig. 1. The PEO process was conducted utilizing a 20 kW power supply in conjunction with stirring and cooling systems. The electrolytic cell consisted of a 2-liter acrylic-vessel with a sample holder, and stainless steel with a
Y.S. Kim et al. / Surface & Coatings Technology 269 (2015) 114–118
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the oxide layers was recorded from 5 to 20 μm at 353 K, 200 scans being obtained at 4 cm−1 resolution via HgCdTe detector.
3. Results and discussion
Fig. 1. Schematic diagram of the present PEO process.
dimension of 15 × 25 cm was used as cathode. The distance between cathode and anode was 50 mm in the electrolyte. To verify the influence of adding CNT into the oxide layer on the heat emissivity in the range of from 5 to 20 μm of the wavelength, two different electrolytes with and without CNT were used. Additionally, YSZ and TiO2 particles with 150–200 nm in diameter listed in Table 1 were selected. The sodium silicate was mainly employed in this study whereas, in the cases of Cells C and D, the sodium hexametaphosphate was added in the electrolyte in order to improve their incorporation kinetics of the metal oxide particles. The temperature of the electrolyte was maintained at 293 K during PEO process. The applied current density and coating time were 100 mA/cm2 and 900 s, respectively, and the thicknesses of the oxide layers was controlled uniformly to be about 10 μm in thickness since the difference in the thickness of the oxide layers would result in the interference of the heat emissivity. The diameter and length of the carboxylated multi-walled CNT dispersed in distilled water acquired from Cluster Instruments Co., Ltd were ~35 nm and ~10 μm, respectively. The surface morphologies and chemical compositions of each sample were observed by using a field-emission scanning electron microscope (SEM; S-4700, HITACHI) equipped with an energy dispersive X-ray spectrometer (EDS; Genesis APEX2, AMETEK). A non-contact 3D surface measurement system (U-surf, NANOFOCUS) was used to measure the surface roughness of the PEO-treated samples. To measure the thermal emissivity of the oxide layers, Fourier transform infrared spectrometer (FTIR; M4500, MIDAC) having Michelson interferometer comprised a couple of mirrors, and a KBr beam splitter was employed. The FTIR spectrum of
Table 1 Chemical compositions of the electrolyte used for the present PEO coatings.
Cell A Cell B Cell C Cell D
CNT (1 wt.%) (ml/l)
Na2SiO3 (mol/L)
(NaPO3)6 (mol/L)
YSZ (mol/L)
TiO2 (mol/L)
– 50 – –
0.08 0.08 0.03 0.03
– – 0.05 0.05
– – 0.02 –
– – – 0.02
Fig. 2 shows SEM micrographs revealing the surfaces of the oxide layers formed within the electrolytes with three different particles and without these particles under the same PEO conditions. It is evident from Fig. 2(a) that the surface morphology of the sample coated in the electrolyte without addition of any particles (Cell A) exhibited a number of the micro-pores and irregularly-formed molten oxide, which resulted from the electrochemical oxidation accompanying the micro-discharge randomly generated between the aluminum substrate and the electrolyte. The high temperature of the plasma discharge formed in a time period of less than 10−6 s dropped rapidly with the cooling rate of 108 K/s due to the electrolyte which was maintained at room temperature. Thus, unique oxide compounds such as Al2O3 and SiO2 were fabricated by reacting the cation of the aluminum substrate and the anions of the electrolyte [20]. This was in line with the result of EDS analysis from Table 2, showing the presence of Al, O, and Si. On the other hand, in the case of the sample coated in Cell B, few numbers of the micro-pores were observed throughout the surface of the oxide layer as shown in Fig. 2(b). This fact would be attributed to the fact that the incorporation of CNT played a role in clogging and blocking the micro-pores, which was reported by the present authors [18,19]. EDS analysis of the sample represented Al, O and Si, in addition to ~15 at.% C (Table 2). The amount of Al in the oxide layer was reduced relatively by introducing the appreciable amount of C originating from CNT incorporated. We thought that most of C detected in this study came from not the electrolyte, substrate, and carbon tape for EDS observations but CNT consisting of carbon atoms. Thus, the inset in Fig. 3(b) indicated that the molten oxides caused by the micro-discharges with high temperature might be electrochemically mixed with the numerous CNTs in the oxide layer. In the cases of the oxide layers coated in Cells C and D as shown in Fig. 2(c) and (d), the micro-pores having average diameter of ~ 1 μm which were caused by high temperature of the micro-discharges as well as the particles in the oxide layer were revealed. The incorporation of the metal oxide particles in the oxide layer might be attributed to the migration of those particles via electrophoresis where the metal oxide particles dispersed initially in the electrolyte moved toward the anode surface during PEO process [20,21]. This result was supported by the present EDS analysis in which the oxide layers coated in Cells C and D are shown in Table 2, ~ 11 at.% Zr and ~14 at.% Ti, respectively. Fig. 3 shows the cross sectional images of the oxide layers coated in four different electrolytes. The thickness and surface roughness (Ra) values of the four different oxide layers were also measured, and the results were listed in Table 3. Regardless of the addition of particles, the PEO-treated oxide layers revealed an average thickness of ~10 μm and some micro-pores of a few micrometers in size. The oxide layer coated without any particles as shown in Fig. 3(a) revealed that the outer part of the oxide layer was severely rough, and the middle part contained numerous micro-pores. When CNT was introduced in the oxide layer, the number and size of the network pores were drastically decreased (Fig. 3(b)). It was found, however, that the huge horizontal channel near to the substrate might be inevitably formed due to the influence of the thermal residual stress which was related to the incorporation of CNT [18]. In the cases of the addition of the metal oxide particles in the oxide layers (Cells C and D), Fig. 3(c) and (d) displayed that the inner part of the oxide layer was composed of comparatively large micro-pores. In terms of the Ra in the PEO-treated Al alloys, the oxide layer without particles was measured to be ~ 3 μm, which was the highest value among four different samples. By incorporating particles into the oxide layer, all Ra values were one-third lower than that of the oxide layer in Cell A. These were attributed to the fact that
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Fig. 2. SEM images showing the surface structures of the oxide layers fabricated by PEO process using four different electrolytes: (a) without CNT, (b) with CNT, (c) with YSZ, and (d) with TiO2. The inset of Fig. 2(c) indicates magnified SEM image.
particles could be considered as an important structural factor affecting the surface roughness of the oxide layer. It is apparent that, in all samples with particles, thickness and roughness were successfully controlled to be nearly identical, which would be in line with the main motivation of this study. The particles having the negative value in zeta potential within the present electrolytes would move to an anode surface due to the electrophoresis during PEO coating [22,23]. The local fluctuation of the molten oxide caused by the intense plasma discharges with high energy and temperature would promote the incorporation of these particles into the newly formed oxide layer. As shown in Fig. 2, these particles were incorporated in the coating, resulting in the formation of the different coating structures as compared to those without particles (Cell A). And some particles which were diffused through the discharge channels and/or micro-pores in the oxide layer were likely to be attached to the oxide surface [24]. Then, they worked as a kind of filler in reducing the fraction and size of the micro-pores, lowering the surface roughness as shown in Table 3. The thermal emissivity was an ability which could emit energy by radiation as compared to a blackbody that absorbed most of the incident energy at all wavelength. If all light energy was assumed to be absorbed, the value of thermal emissivity would be a unity. It is of particular interest that the chemical compositions of the oxide layers influenced the emissivity value. The emissivity spectra of the present samples measured in a wavelength from 5 to 20 μm at 353 K are shown in Table 2 EDS analysis of the oxide layers formed by PEO coatings in four different electrolytes.
Cell A Cell B Cell C Cell D
C
O
Al
Si
P
Ti
Zr
– 14.7 – –
36.2 48.9 57.6 55.3
53.2 23.8 22.5 22.4
10.6 12.6 1.9 0.9
– – 6.7 7.9
– – – 13.5
– – 11.3 –
Fig. 4. For a comparison, the thermal emissivity of bare Al alloy was added. The normal total emissivities ε' n of samples were calculated using following equation [25]: Zλ2
0
0
ε λ;n ðλ; T Þ e λb;n ðλ; T Þdλ 0
εn¼
λ1
Zλ2
ð1Þ 0
e λb;n ðλ; T Þdλ λ1
where λ1 and λ2 are measured initial and final wavelengths, e'λb,n is the blackbody emission power in vacuum, and ε' λ,n(λ, T) is the measured spectral emissivity. The emissivity spectra of the whole samples increased with increasing wavelength. The results are tabulated in Table 4. The bare Al alloys used as a reference showed the low emissivity value of ~ 0.35 in the present wavelength due to the poor efficiency of photon emission from the surface [26]. As shown in Fig. 4, the measured emissivity spectra of Al alloy samples coated in the electrolytes with particles such as Cells B, C, and D were higher than Cell A in the wavelength range of 5–8 μm. In terms of the mean emissivity value in this region, Al alloy samples coated in Cells A, C, and D were ~0.68, ~0.74, and ~0.75, respectively. The addition of CNT in the oxide layer led to a significant increase in the emissivity value approaching to be ~ 0.89, which seemed to be the highest among these different results. In general, the general emissivity values of Al alloys after commercial anodizing coating were known to be 0.77–0.83 [27–29]. Wang et al. [30] reported that the highest infrared emissivity properties of the some oxide compounds containing Si and P after PEO coating of 2024 Al alloy were reported to be ~ 0.85 in the wavelength range of 8–20 μm. But, they tested the thermal properties at 773 K, which was higher than the present condition. Wang et al.
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Fig. 3. SEM images showing the cross sectional structure of the oxide layer fabricated by PEO process using four different electrolytes: (a) without CNT, (b) with CNT, (c) with YSZ, and (d) with TiO2.
[25] investigated based on the PEO coating of AZ91D Mg alloy that, regardless of the chemical compositions, such as Si, Cu, W, and V, which were inserted as metal salts in the electrolyte, the four different samples showed a small variation in emissivity values from 0.79 to 0.85. In the case of Ti-6Al-4 V alloy, when PEO coating for 25 min was performed in the electrolyte containing Na2WO4 of 7.5 g/L, resulting in the thickness and roughness of ~ 40 and ~ 2.5 μm, respectively, the highest value of the emissivity was ~ 0.88 [31]. In this study, emissivity values of the oxide layers incorporating particles at 353 K were higher than those reported in the literature. Moreover, the emissivity value of the oxide layer with CNT exhibited ~0.89, which was similar to that of CNT-sprayed on Al alloy (~0.90). The main motivation of this study was to investigate the effect of particle incorporation on the emissivity properties of the oxide layer by reducing the structural differences among the samples fabricated by PEO coating. As seen from Table 3, we thought that the structural difference of the samples after incorporation was insignificant more or less in terms of thickness and roughness. Hence, the present results of heat emissivity were primarily attributed to the incorporation of particles, such as CNT, YSZ, and TiO2. Therefore, the incorporation of the particles in the oxide layer via PEO process could improve the heat emission properties, which was attributed to the inherent high emissivity in mid-infrared region of the particles. The incorporation of CNT resulted in the good emission properties of the PEO-coated Al alloy sample in this study.
4. Summary The influence of the different particles such as CNT, YSZ, and TiO2 in the oxide layer on 7075 Al alloy on the heat emission properties was investigated. The particles incorporated via electrophoresis had an influence on the surface structure, blocking the micro-pores. From the results of the emissivity spectra, the PEO-coated Al alloy samples with particles showed the superior heat emissivity to that without these particles. In the case of the PEO-coated Al alloy with incorporating CNT, the emissivity spectrum was close to being ~ 0.89 in the wavelength. It would be concluded in this study that the incorporation
Table 3 Thickness and surface roughness (Ra) values of the PEO-treated Al alloys.
Thickness (μm) Roughness (Ra, μm)
Cell A
Cell B
Cell C
Cell D
10.1 ± 0.59 3.2 ± 2.25
10.0 ± 0.93 1.1 ± 0.18
10.0 ± 0.28 1.0 ± 0.15
10.1 ± 0.17 1.2 ± 0.27
Fig. 4. Heat emissivity spectra measured in the wavelength of 5–20 μm at 353 K for four different PEO-treated Al alloys. The bare Al alloy was used as a reference.
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Table 4 Average values of heat emissivity of the PEO-coated samples. For a comparison, heat emissivity of bare Al alloy is added. Bare Al alloy
Cell A
Cell B
Cell C
Cell D
0.35
0.82
0.89
0.87
0.87
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