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Influence of FeSO4 concentration on thermal emissivity of coatings formed on titanium alloy by micro-arc oxidation
Applied Surface Science 257 (2011) 10839–10844
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Influence of FeSO4 concentration on thermal emissivity of coatings formed on titanium alloy by micro-arc oxidation Hui Tang a , Tiezhu Xin b , Qiu Sun a , Chuangui Yi a , Zhaohua Jiang a , Fuping Wang a,∗ a b
School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin 150001, China School of Material Science and Engineering, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 24 January 2011 Received in revised form 8 June 2011 Accepted 25 July 2011 Available online 2 August 2011 Keywords: Microarc oxidation Titanium alloy FeSO4 High emissivity Bonding strength Thermal shock resistance
a b s t r a c t Ceramic coatings with high emission were fabricated on Ti6Al4V alloy by microarc oxidation (MAO) with additive FeSO4 into the electrolyte. The microstructure, chemical composition and chemical state of the coatings were determined by SEM, XRD, EDS and XPS, respectively. The bonding strength between the coating and substrate was studied by tensile strength test, together with the thermal shock resistance of the coating. The results showed that Fe content in the coating layer significantly affect its thermal emissivity. The relative content of Fe in the coatings surface increased at first and then decreased with increasing the concentration of FeSO4 in electrolytes, so does the emissivity of the coatings. The bonding strength became weaker with increasing the concentration of FeSO4 . In addition, the coating remains stable over 40 cycles of thermal shocking. The coating formed at 3 g/L FeSO4 demonstrates the highest an average spectral emissivity value around of 0.87, and bonding strength higher than 33 MPa. © 2011 Elsevier B.V. All rights reserved.
1. Introduction High friction heat coming from the acute friction between vehicle’s surface and atmosphere causes severe increase of surface temperature during hypervelocity flights, which leads to reduce the lifetime and performance of the space vehicles material. A thermal protection system (TPS) is demanded to avoid aerodynamic heating, which can decrease the surface temperature, prevent the heat transferring inward and protect the electron apparatus [1–3]. TPS usually bases on materials with high temperature stability and excellent thermal insulation [4]. Moreover, the materials must possess high thermal emission. It can effectively radiate the heat on the surface of vehicle back into space. The existing of traditional thermal protection system on metal surface, contains a ceramic coating filled with highly emissive materials. Silicone adhesives are adopted to bond the coating and metal. The bond strength between the ceramic coating and the metal substrate is weak, so it is high instability. And the coating may be cracked and destroyed under thermal stress, due to its different expansion/contraction coefficients from metals. Ceramic coating formed by micro-arc oxidation technology can alleviate this problem, and result in formation of coating layer
∗ Corresponding author. Tel.: +86 451 86418409; fax: +86 451 86418409. E-mail address: [email protected] (F. Wang). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.07.118
with super bonding strength, excellent mechanical and thermal properties. Recently, micro-arc oxidation (MAO), which is also commonly called “plasma electrolytic oxidation (PEO)”, has attracted more attention in virtue of its convenience and effectiveness to prepare oxide ceramic coatings with porous structure on the surface of Ti, Al, Mg and their alloys, In addition, MAO is a room-temperature electrochemical process that is suitable for the formation of uniform coatings on the substrate with complex geometries and the MAO coatings offer a good combination of wear resistance, corrosion resistance and bonding strength. In MAO process, the elements in substrate and electrolyte were incorporated into coatings, so functional coatings can be prepared by changing electrolytes and the power parameter in MAO processing [5–7]. Due to high thermal stability, oxidation resistance and low thermal conductivity, Fe2 O3 is used as infrared radiation materials in industrial furnace for energy saving [8,9]. Fe2 O3 also possesses high emission in near and middle infrared [10]. For these reasons, Fe2 O3 also can be used as thermal protective materials in vehicle. In this paper, ceramic coatings were prepared by microarc oxidation on the surface of Ti6Al4V alloy. FeSO4 was added into the electrolyte in order improve the thermal emission of the coating. The influence of FeSO4 concentration on the morphology, bonding strength and thermal shock resistance of the coatings was studied in detail also.
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Fig. 2. Current–times curve obtained during the evolution of coating under different concentrations of FeSO4 .
Fig. 1. Schematic diagram of the tensile strength test.
2. Experiment A 20-kW AC microarc oxidation device was used to fabricate ceramic coatings on the Ti6Al4V surface. Ti6Al4V discs with a diameter of 30 mm and thickness of 6 mm were used as the substrate. The surface of the discs was polished with waterproof abrasive paper up to 1200 grits, make the roughness of the surface Ra = 0.2 m, and then ultrasonically cleaned in distilled water followed in acetone. The discs were used as anode, a watercooled electrolyser made of stainless steel, which also served as cathode. The reaction temperature was controlled to below 30 ◦ C by adjusting stirring and the cooling water flow. The electrolyte was prepared from the solution of Na3 PO4 (7 g/L), with FeSO4 0 g/L, 2 g/L, 3 g/L, 5 g/L, respectively. Noted as MAO0, MAO2, MAO3, and MAO5. During the MAO treatment, a constant voltage mode was applied. The anodic voltage was kept constantly at 350 V and the cathodic voltage was controlled at 0 V, the frequency was 100 Hz; the duration time for the MAO treatment was 10 min. Phase composition of the coatings was examined with a RICOH D/max-rB automatic X-ray diffractometer (XRD, D/max-rB, Japan) using a Cu Ka source. Surface morphologies of the produced coatings were studied by scanning electron microscopy (SEM, S-4800, Hitachi Co., Japan). The element distribution on the surface of the coating was investigated by energy dispersive spectroscopy (EDS, Oxford Model 7537, England). X-ray photoelectron spectroscopy (XPS; PHI 5700, America Physical Electronics, USA) was performed on a PHI 5700 system at a base pressure of less than 10−7 Pa, using Mg Ka radiation 12.5 kV and 20 mA. The roughness of the coatings was measured by current-based roughness gauge (TR-200, Time Company, China). The average roughness of each sample was obtained from 8 measurements at different positions. The tensile strength of coatings was investigated by the direct pull-off method and impact tests [11]. In pull-off tests, the side of a MAO treated sample was bonded to the side of an untreated sample using epoxy resin and then a force applied to coating–substrate interface to determinate the force for detachment, as shown in Fig. 1. The pull-off test was carried out on Instron-4467 device. During the thermal shock tests, coated samples are heated up to 500 ◦ C at which temperature they are held for 2 min to make body temperature uniform. Then the specimens were quickly taken out and immersed in the cooling water. The tests were repeated 40
cycles, observe carefully the coatings surface and identify whether visual cracks or large area flakes are produced. Fourier transform infrared (FTIR) spectrometer was used to measure the emission of the coatings in vertical directions at 700 ◦ C. The spectrometer uses a simple Michelson interferometer that consists of a corner cube mirror and a KBr beam splitter. The spectral range between 0.6 m and 25 m is covered by a photovoltaic HgCdTe and a silicon photodiode detector. A chamber with circulating water was designed for supporting the heater. Detailed information about the equipment can be found in other references [12]. 3. Results and discussions 3.1. Current density–time response Fig. 2 shows the plot of the current–time response during the MAO treatment process on Ti6Al4V at 350 V. For all the samples, the trend of the current–time curves was the same. As is known, there are three main steps underlying the formation of MAO coatings [13]. Firstly, an oxide film develops on the surface by conventional anodizing. This stage is characterized by a current increased rapidly, and accompanied by gas evolution. Secondly, the current decreased quickly and a few white sparks can be seen on the surface of sample. The final stage is characterized by the current decreased slowly and the yellow sparks covers the whole surface. The current remains stable eventually. Furthermore, it can be noted that the maximum current and final current were found to be significantly influenced by the concentration of FeSO4 in the electrolyte, with increasing the concentration of FeSO4 , the largest current and the final current became larger. The distinct deviations in the above observed behavior may be attributed to different conductivity of the four electrolytes, which is increased with the increasing concentration of FeSO4 . It is similar to the previous research results [14]. And this implies that the addition of FeSO4 into the electrolyte, could cause the change of MAO process and subsequently lead to the differences in the formation and the characteristics of the oxidation coating as well. 3.2. Surface microstructure characterization Fig. 3 shows the morphologies of the oxide coatings formed in four electrolytes. All the coatings have a porous surface microstructure and some volcano top-like pores. The molten oxide particles
H. Tang et al. / Applied Surface Science 257 (2011) 10839–10844
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Fig. 3. Surface morphologies of MAO coatings formed under different concentrations of FeSO4 : (a) 0 g/L, (b) 2 g/L, (c) 3 g/L, and (d) 5 g/L.
with different sized around the volcano. As shown by the previous studies, the porosity of the coating was formed by the oxygen bubbles in the coating growth process. During microarc oxidation, the temperature and pressure in the discharge channels can reach about 2 × 104 K and 102 MPa [15], which make the bubbles and molten particles erupt from the channels. The bubbles escaped from the electrolyte and the molten particles were cooled by the electrolyte. In addition, with increasing the concentration of FeSO4 , the number of micropores on the surface of the coatings decrease and the average of micropores size increase, and whole the porosity of the coatings decrease. However, noticeable micropores cannot be seen in MAO5 coating. The difference of the surface morphology may be due to the higher energy in formation MAO5 coating. It is clear that the current in formation of MAO5 processing is much higher than that in other processing. The effect of increasing current make the power of the discharge increased, so more production erupted from the discharge channel, deposited on the surface and cooled by the electrolyte. The melted product mass can deposited in the existed discharge channels and sealed the channels rapidly. The relative contents of elements in the coating are shown in Table 1. Ti is found to be incorporated into the coatings from metal substrate, and some electrolyte components, such as Fe and P, were
also incorporated into the coating layer. With increasing the concentration of FeSO4 in electrolytes, the content of Fe on the surface of the coating became larger at first, and then it became smaller. When the concentration of FeSO4 is 3 g/L, the content of the Fe on the coating surface is largest. However, EDS results show that the S element cannot be detected in the surface of the coatings. Maybe the sulfur was removed as SO2 gases from electrolyte, and the mechanisms responsible need further detailed investigation. Fig. 4 shows the surface roughness of the MAO coatings formed in different electrolytes. It is obvious that the introduction of FeSO4
Table 1 The element contents of the MAO coatings formed different FeSO4 concentrations.
Fe Ti P O
0 g/L
2 g/L
3 g/L
5 g/L
– 29.51 15.40 55.09
10.10 13.87 20.18 55.86
14.27 10.96 20.35 54.48
10.64 11.84 20.04 57.13
Fig. 4. Surface roughness of MAO coatings under different concentrations of FeSO4 .
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Fig. 5. Cross-section morphologies of MAO coatings formed under different concentrations of FeSO4 : (a) 0 g/L, (b) 2 g/L, (c) 3 g/L, and (d) 5 g/L.
into the electrolyte increases the surface roughness. This change is in good agreement with the change of surface morphologies shown in Fig. 3. 3.3. Cross-section microstructure characterization The cross-section morphologies of samples MAO0, MAO2, MAO3, and MAO5 are shown in Fig. 5a–d, respectively. Two layers are observed on the coatings, which are the outer layer and the inner layer. The outer layer is loose and the inner layer is compact. There are also some pores and micro-cracks in the cross-section morphologies, but these pores and micro-cracks were not connected each other or perforated through the whole oxide film. It indicates that the adhesion between the coating and substrate will be good. Furthermore, it can be noted that the thickness of the coating increased with increasing the concentration of FeSO4 .
obtain a clean material surface. It is noted that all of the binding energies were referenced to the C(1s) peak at 284.3 eV. The study of the Fe(2p1/2) and Fe(2p3/2) peak were shown in Fig. 7a, the core level of Fe(2p3/2) was at 711.0 eV, and that for Fe(2p1/2) peak was at 723.8 eV. The binding energy is comparable to that of the energy of corresponding photoelectrons of Fe3+ in Fe2 O3 [16,17]. Fig. 7b depicts the Ti(2p3/2) core level at binding energy of 458.9 eV, and Ti(2p1/2) located at 464.8 eV, asserting the existence of titanium in the form of Ti4+ state [18,19]. From Fig. 7c and d, The O(1s) peak
3.4. Phase composition of the coatings Fig. 6 shows the XRD patterns of the coatings on MAO, MAO2, MAO3 and MAO5. The coatings are composed of rutile and anatase TiO2 , but their ratio varies with different addition of FeSO4 in the electrolyte. More rutile and less anatase is formed in the presence of higher concentration of FeSO4 . Peaks conformed to titanium substrate also appear in the patterns. No peaks corresponding to Fe and P elements are detected, possibly due to their existence in amorphous phase in the coatings. 3.5. Chemical state of the coatings The XPS measurement results of the MAO3 coating were shown in Fig. 7. The specimens were sputtered with Ar ions for 2 min to
Fig. 6. XRD patterns of the coatings under different FeSO4 addition.
H. Tang et al. / Applied Surface Science 257 (2011) 10839–10844
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Fig. 7. XPS spectra of the MAO3 coating (a) Fe(2p) core level, (b) Ti(2p) core level, (c) O(1s) core level, and (d) P(2p) core level binding energies.
located at 531.6 eV, corresponds to the typical binding energy of Ti–O bonding [20]. The peak location of P(2p) at 133.3 eV, represents the state of P5+ [21]. There are the evident peak of Fe(2p3/2) and Fe(2p1/2), indicating that Fe2+ in the electrolyte was incorporated into the coatings during the MAO process, and formed Fe2 O3 .
spreading speed of heat fluxes and the temperature change rate of the substrate. Furthermore, they could also decrease the heat stresses of the interface area between the coatings and the substrate. 3.8. Emissivity characterization
3.6. Bonding strength Besides persuiting of high emissivity, the other properties such as adhesion strength and thermal shock resistance, should be also taken into account when the high emissivity coating are used in space vehicle surface. The tensile test is a effective method to evaluate the bonding strength of a coating–substrate system. In order to minimize the errors and exclude accidental factors, several tests are used for the coating prepared in the same electrolyte, and the final result is an average. The results of the tensile strength tests are plotted in Fig. 8. It indicates that the bonding strength of the coatings decrease a little with increasing the concentration of FeSO4 . But all the coatings show a bonding strength higher than 30 MPa.
The FTIR spectral emissivity values of the as-prepared coatings at wavelength of 3–20 m are shown in Fig. 9. The spectral emissivity–wavelength curve for a freshly polished sample of
3.7. Thermal shock resistance tests After subjected to severe thermal shocking with temperature difference 500 ◦ C for 40 cycles, no peeling off of the coating occurred, which indicates that the coating possesses a good thermal shock resistance. The good thermal shock resistance may attribute to the special coating structure formed on titanium metal by microarc oxidation method. The coatings contain a lot of pores which can lead micro-cracks spreading in more directions thereby inhibiting very big crack formation, meanwhile, can decrease the
Fig. 8. Bonding strength of MAO coatings between substrate under different concentrations of FeSO4 .
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the intermediate region [28,26,29]. Studies show that microcavities can enhance the thermal emission of the coatings [30,31]. The previous achievements provide valuable insight into the emissivity principles, concerning the influence of the composition, the surface roughness, the microcavities and the thickness of the coating on the emissivity principle. However, the impact factors of emission are much complicated, and a great deal of research is still required. 4. Conclusions High emissivity coatings on titanium alloy were prepared by microarc oxidation. The structure, phase composition, chemical state, bonding strength and thermal shock resistance of the coatings were investigated and the following conclusions can be drawn:
Fig. 9. Spectral emissivity of substrate and MAO coatings under different.
Ti6Al4V titanium alloy with no coating was included as a comparison. The substrate has lowest spectral emissivity value in the wavelength of 3–20 m, whose average is 0.33. Comparing with the coating formed in the electrolyte without FeSO4 , the coatings prepared in electrolytes containing FeSO4 have possessed higher spectral emissivity value in the wavelength of 3–8 m range. The spectral emissivity of the coatings reaches maximum at a wavelength of about 7.6 m, and the peak position of dominant peaks shows very little change dependence with the concentration of FeSO4 in the electrolytes. However, the coating prepared in the electrolyte without FeSO4 do not exist clearly dominant peak. The dominant peaks may be caused by the presence of Fe2 O3 in the structure of the coating. When the wavelength is about 3–8 m, the spectral emissivity value of MAO3 is located at the range from 0.8 to 1.0, the average value is about 0.91. The average of the spectral emissivity value of MAO3 is 0.87 with wavelength region at 3–20 m, which is the highest among all the coatings. Past studies have shown that oxide has higher emission than metal in the wavelength range from 3 to 20 m [1]. That maybe the reason the substrate has lowest spectral emissivity value across this range. Campo investigated the oxidation kinetics of iron and showed that Fe2 O3 possessed high emission in the wavelength of 3–20 m range [22]. Others studies demonstrated that the spectral emission increasing with increased the content of the Fe2 O3 [23]. The peaks position of dominant peaks seemed to be changed by changing the temperature, but there is no doubt that dominant peak is existence. [24]. In microarc oxidation process, Fe2+ in electrolyte was incorporated into microarc oxidation coating and formed Fe2 O3 . And the content of Fe2 O3 in the surface of the coatings can be controlled by changing the concentration of FeSO4 in the electrolytes and adjusting the parameters in microarc oxidation process. In this investigation, the content of Fe2 O3 in the surface of MAO3 is highest among all the coatings, and so is the spectral emissivity value. But the impact factors of emission are much complicated, and the composition is one of those. The roughness, the microcavities and the thickness of the coating may be involved in this study [25–27]. The correlation between emissivity and surface roughness has been explored. For a gray body, the surfaces can be divided into three regions based on optical roughness (ı/, the ratio between surface roughness to wavelength): the specular region (0 < ı/ < 0.2), intermediate region ((0.2 < ı/ < 1) and the geometric region (ı/ > 1). Theoretical and experimental confirmed that the emission increased with increasing the roughness of the surface in
(1) The ceramic coatings were composed of rutile and anatase TiO2 . Fe, P elements in electrolyte were incorporated into the coatings. The surface roughness and the coating thickness increased with increasing the concentration of FeSO4 . (2) All the coatings showed a bonding strength higher than 30 MPa. After subjected a severe thermal shocking for 40 cycles, no peeling off of the coating occurred, the coatings showed a good thermal shock resistance. (3) The coating which is prepared in an aqueous solution of 7 g/L Na3 PO4 with 3 g/L FeSO4 , has a highest spectral emissivity. With wavelength increased from 3 to 8 m, the spectral emissivity average value is about 0.91, the average of the spectral emissivity value is 0.87 with wavelength region at 3–20 m. References [1] L. Campo, R.B. Pérez-Sáez, L. González-Fernández, X. Esquisabel, I. Fernández, P. González-Martín, M.J. Tello, J. Alloys Compd. 489 (2010) 482–487. [2] R. Savino, M.D.S. Fumo, D. Paterna, A.D. Maso, F. Monteverde, Aerosol Sci. Technol. 14 (2010) 178–187. [3] M. Schüßler, M. Auweter-Kurtz, G. Herdrich, S. Lein, Acta Aeronaut. 65 (2009) 676–686. [4] J. Yi, X.D. He, Y. Sun, Y. Li, M.W. Li, Appl. Surf. Sci. 253 (2007) 7100–7103. [5] A.L. Yerokhin, A. Leyland, A. Matthews, Appl. Surf. Sci. 200 (2002) 172–184. [6] Y.J. Xu, Z.P. Yao, F.Z. Jia, Y.L. Wang, Z.H. Jiang, H.T. Bu, Curr. Appl. Phys. 10 (2010) 698–702. [7] X.T. Sun, Z.H. Jiang, S.G. Xin, Z.P. Yao, Thin Solid Films 471 (2005) 194–199. [8] G. Neuer, G. Jaroma-Weiland, Int. J. Thermophys. 19 (1998) 917–929. [9] B. Stanley, Ind. Heat 8 (1982) 49–53. [10] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, S.J. Dowey, Surf. Coat. Technol. 122 (1999) 73–93. [11] J.M. Dai, X.B. Wang, G.B. Yuan, J. Phys.: Conf. Ser. 13 (2005) 63–66. [12] J. Liang, B.G. Guo, J. Tian, H.W. Liu, J.F. Zhou, T. Xu, Appl. Surf. Sci. 252 (2005) 345–351. [13] Y.M. Wang, T.Q. Lei, B.L. Jiang, L.X. Gu, Appl. Surf. Sci. 233 (2004) 258–267. [14] G. Wu, X.Y. Tan, G.Y. Li, C.W. Hu, J. Alloys Compd. 504 (2010) 371–376. [15] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, Surf. Coat. Technol. 130 (2000) 195–206. [16] J.D. Desai, H.M. Pathan, S.K. Min, K.D. Jung, O.S. Joo, Appl. Surf. Sci. 252 (2005) 1870–1875. [17] M.R. Bayati, A.Z. Moshfegh, F. Golestani-Fard, Electrochim. Acta 55 (2010) 2760–2766. [18] Y.M. Wang, B.L. Jiang, T.Q. Lei, L.X. Guo, Surf. Coat. Technol. 201 (2006) 82–89. [19] J. Liang, L.T. Hu, J.C. Hao, Electrochim. Acta 52 (2007) 4836–4840. [20] J. Baszkiewicz, D. Krupa, J. Mizera, J.W. Sobczak, A. Bilinski, Vacuum 78 (2005) 143–147. [21] D.Q. Wei, Y. Zhou, D.C. Jia, Y.M. Wang, Mater. Chem. Phys. 104 (2007) 177–182. [22] L. Campo, R.B. Pérez-Sáez, M.J. Tello, Corros. Sci. 50 (2008) 194–199. [23] Z.J. Ye, C.F. Ma, S.Y. Huang, J. Therm. Sci. 5 (1995) 128–131. [24] G.B. Smith, A. Gentle, P.D. Swift, A. Earp, N. Mronga, Sol. Energy Mater. Sol. Cells 79 (2003) 179–197. [25] F. Cernuschia, S. Ahmaniemib, P. Vuoristoc, T. Mantyla, J. Eur. Ceram. Soc. 24 (2004) 2657–2667. [26] C.D. Wen, I. Mudawar, Int. J. Heat Mass Transf. 49 (2006) 4279–4289. [27] C.Y. Zhao, T.J. Lu, H.P. Hodson, Int. J. Heat Mass Transf. 47 (2004) 2927–2939. [28] C.D. Wen, I. Mudawar, Int. J. Heat Mass Transf. 48 (2005) 1316–1329. [29] F. Ghmari, T. Ghbara, J. Appl. Phys. 96 (2004) 2656–2664. [30] M. Ziegler, J.W. Tomm, T. Elsaesser, C. Monte, J. Hollandt, H. Kissel, J. Biesenbach, J. Appl. Phys. 103 (2008) 104508–104519. [31] O. Rozenbaum, D.D.S. Meneses, P. Echegut, Int. J. Thermophys. 30 (2009) 580–590.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Influence of FeSO4 concentration on thermal emissivity of coatings formed on titanium alloy by micro-arc oxidation Hui Tang a , Tiezhu Xin b , Qiu Sun a , Chuangui Yi a , Zhaohua Jiang a , Fuping Wang a,∗ a b
School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin 150001, China School of Material Science and Engineering, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 24 January 2011 Received in revised form 8 June 2011 Accepted 25 July 2011 Available online 2 August 2011 Keywords: Microarc oxidation Titanium alloy FeSO4 High emissivity Bonding strength Thermal shock resistance
a b s t r a c t Ceramic coatings with high emission were fabricated on Ti6Al4V alloy by microarc oxidation (MAO) with additive FeSO4 into the electrolyte. The microstructure, chemical composition and chemical state of the coatings were determined by SEM, XRD, EDS and XPS, respectively. The bonding strength between the coating and substrate was studied by tensile strength test, together with the thermal shock resistance of the coating. The results showed that Fe content in the coating layer significantly affect its thermal emissivity. The relative content of Fe in the coatings surface increased at first and then decreased with increasing the concentration of FeSO4 in electrolytes, so does the emissivity of the coatings. The bonding strength became weaker with increasing the concentration of FeSO4 . In addition, the coating remains stable over 40 cycles of thermal shocking. The coating formed at 3 g/L FeSO4 demonstrates the highest an average spectral emissivity value around of 0.87, and bonding strength higher than 33 MPa. © 2011 Elsevier B.V. All rights reserved.
1. Introduction High friction heat coming from the acute friction between vehicle’s surface and atmosphere causes severe increase of surface temperature during hypervelocity flights, which leads to reduce the lifetime and performance of the space vehicles material. A thermal protection system (TPS) is demanded to avoid aerodynamic heating, which can decrease the surface temperature, prevent the heat transferring inward and protect the electron apparatus [1–3]. TPS usually bases on materials with high temperature stability and excellent thermal insulation [4]. Moreover, the materials must possess high thermal emission. It can effectively radiate the heat on the surface of vehicle back into space. The existing of traditional thermal protection system on metal surface, contains a ceramic coating filled with highly emissive materials. Silicone adhesives are adopted to bond the coating and metal. The bond strength between the ceramic coating and the metal substrate is weak, so it is high instability. And the coating may be cracked and destroyed under thermal stress, due to its different expansion/contraction coefficients from metals. Ceramic coating formed by micro-arc oxidation technology can alleviate this problem, and result in formation of coating layer
∗ Corresponding author. Tel.: +86 451 86418409; fax: +86 451 86418409. E-mail address: [email protected] (F. Wang). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.07.118
with super bonding strength, excellent mechanical and thermal properties. Recently, micro-arc oxidation (MAO), which is also commonly called “plasma electrolytic oxidation (PEO)”, has attracted more attention in virtue of its convenience and effectiveness to prepare oxide ceramic coatings with porous structure on the surface of Ti, Al, Mg and their alloys, In addition, MAO is a room-temperature electrochemical process that is suitable for the formation of uniform coatings on the substrate with complex geometries and the MAO coatings offer a good combination of wear resistance, corrosion resistance and bonding strength. In MAO process, the elements in substrate and electrolyte were incorporated into coatings, so functional coatings can be prepared by changing electrolytes and the power parameter in MAO processing [5–7]. Due to high thermal stability, oxidation resistance and low thermal conductivity, Fe2 O3 is used as infrared radiation materials in industrial furnace for energy saving [8,9]. Fe2 O3 also possesses high emission in near and middle infrared [10]. For these reasons, Fe2 O3 also can be used as thermal protective materials in vehicle. In this paper, ceramic coatings were prepared by microarc oxidation on the surface of Ti6Al4V alloy. FeSO4 was added into the electrolyte in order improve the thermal emission of the coating. The influence of FeSO4 concentration on the morphology, bonding strength and thermal shock resistance of the coatings was studied in detail also.
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Fig. 2. Current–times curve obtained during the evolution of coating under different concentrations of FeSO4 .
Fig. 1. Schematic diagram of the tensile strength test.
2. Experiment A 20-kW AC microarc oxidation device was used to fabricate ceramic coatings on the Ti6Al4V surface. Ti6Al4V discs with a diameter of 30 mm and thickness of 6 mm were used as the substrate. The surface of the discs was polished with waterproof abrasive paper up to 1200 grits, make the roughness of the surface Ra = 0.2 m, and then ultrasonically cleaned in distilled water followed in acetone. The discs were used as anode, a watercooled electrolyser made of stainless steel, which also served as cathode. The reaction temperature was controlled to below 30 ◦ C by adjusting stirring and the cooling water flow. The electrolyte was prepared from the solution of Na3 PO4 (7 g/L), with FeSO4 0 g/L, 2 g/L, 3 g/L, 5 g/L, respectively. Noted as MAO0, MAO2, MAO3, and MAO5. During the MAO treatment, a constant voltage mode was applied. The anodic voltage was kept constantly at 350 V and the cathodic voltage was controlled at 0 V, the frequency was 100 Hz; the duration time for the MAO treatment was 10 min. Phase composition of the coatings was examined with a RICOH D/max-rB automatic X-ray diffractometer (XRD, D/max-rB, Japan) using a Cu Ka source. Surface morphologies of the produced coatings were studied by scanning electron microscopy (SEM, S-4800, Hitachi Co., Japan). The element distribution on the surface of the coating was investigated by energy dispersive spectroscopy (EDS, Oxford Model 7537, England). X-ray photoelectron spectroscopy (XPS; PHI 5700, America Physical Electronics, USA) was performed on a PHI 5700 system at a base pressure of less than 10−7 Pa, using Mg Ka radiation 12.5 kV and 20 mA. The roughness of the coatings was measured by current-based roughness gauge (TR-200, Time Company, China). The average roughness of each sample was obtained from 8 measurements at different positions. The tensile strength of coatings was investigated by the direct pull-off method and impact tests [11]. In pull-off tests, the side of a MAO treated sample was bonded to the side of an untreated sample using epoxy resin and then a force applied to coating–substrate interface to determinate the force for detachment, as shown in Fig. 1. The pull-off test was carried out on Instron-4467 device. During the thermal shock tests, coated samples are heated up to 500 ◦ C at which temperature they are held for 2 min to make body temperature uniform. Then the specimens were quickly taken out and immersed in the cooling water. The tests were repeated 40
cycles, observe carefully the coatings surface and identify whether visual cracks or large area flakes are produced. Fourier transform infrared (FTIR) spectrometer was used to measure the emission of the coatings in vertical directions at 700 ◦ C. The spectrometer uses a simple Michelson interferometer that consists of a corner cube mirror and a KBr beam splitter. The spectral range between 0.6 m and 25 m is covered by a photovoltaic HgCdTe and a silicon photodiode detector. A chamber with circulating water was designed for supporting the heater. Detailed information about the equipment can be found in other references [12]. 3. Results and discussions 3.1. Current density–time response Fig. 2 shows the plot of the current–time response during the MAO treatment process on Ti6Al4V at 350 V. For all the samples, the trend of the current–time curves was the same. As is known, there are three main steps underlying the formation of MAO coatings [13]. Firstly, an oxide film develops on the surface by conventional anodizing. This stage is characterized by a current increased rapidly, and accompanied by gas evolution. Secondly, the current decreased quickly and a few white sparks can be seen on the surface of sample. The final stage is characterized by the current decreased slowly and the yellow sparks covers the whole surface. The current remains stable eventually. Furthermore, it can be noted that the maximum current and final current were found to be significantly influenced by the concentration of FeSO4 in the electrolyte, with increasing the concentration of FeSO4 , the largest current and the final current became larger. The distinct deviations in the above observed behavior may be attributed to different conductivity of the four electrolytes, which is increased with the increasing concentration of FeSO4 . It is similar to the previous research results [14]. And this implies that the addition of FeSO4 into the electrolyte, could cause the change of MAO process and subsequently lead to the differences in the formation and the characteristics of the oxidation coating as well. 3.2. Surface microstructure characterization Fig. 3 shows the morphologies of the oxide coatings formed in four electrolytes. All the coatings have a porous surface microstructure and some volcano top-like pores. The molten oxide particles
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Fig. 3. Surface morphologies of MAO coatings formed under different concentrations of FeSO4 : (a) 0 g/L, (b) 2 g/L, (c) 3 g/L, and (d) 5 g/L.
with different sized around the volcano. As shown by the previous studies, the porosity of the coating was formed by the oxygen bubbles in the coating growth process. During microarc oxidation, the temperature and pressure in the discharge channels can reach about 2 × 104 K and 102 MPa [15], which make the bubbles and molten particles erupt from the channels. The bubbles escaped from the electrolyte and the molten particles were cooled by the electrolyte. In addition, with increasing the concentration of FeSO4 , the number of micropores on the surface of the coatings decrease and the average of micropores size increase, and whole the porosity of the coatings decrease. However, noticeable micropores cannot be seen in MAO5 coating. The difference of the surface morphology may be due to the higher energy in formation MAO5 coating. It is clear that the current in formation of MAO5 processing is much higher than that in other processing. The effect of increasing current make the power of the discharge increased, so more production erupted from the discharge channel, deposited on the surface and cooled by the electrolyte. The melted product mass can deposited in the existed discharge channels and sealed the channels rapidly. The relative contents of elements in the coating are shown in Table 1. Ti is found to be incorporated into the coatings from metal substrate, and some electrolyte components, such as Fe and P, were
also incorporated into the coating layer. With increasing the concentration of FeSO4 in electrolytes, the content of Fe on the surface of the coating became larger at first, and then it became smaller. When the concentration of FeSO4 is 3 g/L, the content of the Fe on the coating surface is largest. However, EDS results show that the S element cannot be detected in the surface of the coatings. Maybe the sulfur was removed as SO2 gases from electrolyte, and the mechanisms responsible need further detailed investigation. Fig. 4 shows the surface roughness of the MAO coatings formed in different electrolytes. It is obvious that the introduction of FeSO4
Table 1 The element contents of the MAO coatings formed different FeSO4 concentrations.
Fe Ti P O
0 g/L
2 g/L
3 g/L
5 g/L
– 29.51 15.40 55.09
10.10 13.87 20.18 55.86
14.27 10.96 20.35 54.48
10.64 11.84 20.04 57.13
Fig. 4. Surface roughness of MAO coatings under different concentrations of FeSO4 .
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Fig. 5. Cross-section morphologies of MAO coatings formed under different concentrations of FeSO4 : (a) 0 g/L, (b) 2 g/L, (c) 3 g/L, and (d) 5 g/L.
into the electrolyte increases the surface roughness. This change is in good agreement with the change of surface morphologies shown in Fig. 3. 3.3. Cross-section microstructure characterization The cross-section morphologies of samples MAO0, MAO2, MAO3, and MAO5 are shown in Fig. 5a–d, respectively. Two layers are observed on the coatings, which are the outer layer and the inner layer. The outer layer is loose and the inner layer is compact. There are also some pores and micro-cracks in the cross-section morphologies, but these pores and micro-cracks were not connected each other or perforated through the whole oxide film. It indicates that the adhesion between the coating and substrate will be good. Furthermore, it can be noted that the thickness of the coating increased with increasing the concentration of FeSO4 .
obtain a clean material surface. It is noted that all of the binding energies were referenced to the C(1s) peak at 284.3 eV. The study of the Fe(2p1/2) and Fe(2p3/2) peak were shown in Fig. 7a, the core level of Fe(2p3/2) was at 711.0 eV, and that for Fe(2p1/2) peak was at 723.8 eV. The binding energy is comparable to that of the energy of corresponding photoelectrons of Fe3+ in Fe2 O3 [16,17]. Fig. 7b depicts the Ti(2p3/2) core level at binding energy of 458.9 eV, and Ti(2p1/2) located at 464.8 eV, asserting the existence of titanium in the form of Ti4+ state [18,19]. From Fig. 7c and d, The O(1s) peak
3.4. Phase composition of the coatings Fig. 6 shows the XRD patterns of the coatings on MAO, MAO2, MAO3 and MAO5. The coatings are composed of rutile and anatase TiO2 , but their ratio varies with different addition of FeSO4 in the electrolyte. More rutile and less anatase is formed in the presence of higher concentration of FeSO4 . Peaks conformed to titanium substrate also appear in the patterns. No peaks corresponding to Fe and P elements are detected, possibly due to their existence in amorphous phase in the coatings. 3.5. Chemical state of the coatings The XPS measurement results of the MAO3 coating were shown in Fig. 7. The specimens were sputtered with Ar ions for 2 min to
Fig. 6. XRD patterns of the coatings under different FeSO4 addition.
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Fig. 7. XPS spectra of the MAO3 coating (a) Fe(2p) core level, (b) Ti(2p) core level, (c) O(1s) core level, and (d) P(2p) core level binding energies.
located at 531.6 eV, corresponds to the typical binding energy of Ti–O bonding [20]. The peak location of P(2p) at 133.3 eV, represents the state of P5+ [21]. There are the evident peak of Fe(2p3/2) and Fe(2p1/2), indicating that Fe2+ in the electrolyte was incorporated into the coatings during the MAO process, and formed Fe2 O3 .
spreading speed of heat fluxes and the temperature change rate of the substrate. Furthermore, they could also decrease the heat stresses of the interface area between the coatings and the substrate. 3.8. Emissivity characterization
3.6. Bonding strength Besides persuiting of high emissivity, the other properties such as adhesion strength and thermal shock resistance, should be also taken into account when the high emissivity coating are used in space vehicle surface. The tensile test is a effective method to evaluate the bonding strength of a coating–substrate system. In order to minimize the errors and exclude accidental factors, several tests are used for the coating prepared in the same electrolyte, and the final result is an average. The results of the tensile strength tests are plotted in Fig. 8. It indicates that the bonding strength of the coatings decrease a little with increasing the concentration of FeSO4 . But all the coatings show a bonding strength higher than 30 MPa.
The FTIR spectral emissivity values of the as-prepared coatings at wavelength of 3–20 m are shown in Fig. 9. The spectral emissivity–wavelength curve for a freshly polished sample of
3.7. Thermal shock resistance tests After subjected to severe thermal shocking with temperature difference 500 ◦ C for 40 cycles, no peeling off of the coating occurred, which indicates that the coating possesses a good thermal shock resistance. The good thermal shock resistance may attribute to the special coating structure formed on titanium metal by microarc oxidation method. The coatings contain a lot of pores which can lead micro-cracks spreading in more directions thereby inhibiting very big crack formation, meanwhile, can decrease the
Fig. 8. Bonding strength of MAO coatings between substrate under different concentrations of FeSO4 .
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the intermediate region [28,26,29]. Studies show that microcavities can enhance the thermal emission of the coatings [30,31]. The previous achievements provide valuable insight into the emissivity principles, concerning the influence of the composition, the surface roughness, the microcavities and the thickness of the coating on the emissivity principle. However, the impact factors of emission are much complicated, and a great deal of research is still required. 4. Conclusions High emissivity coatings on titanium alloy were prepared by microarc oxidation. The structure, phase composition, chemical state, bonding strength and thermal shock resistance of the coatings were investigated and the following conclusions can be drawn:
Fig. 9. Spectral emissivity of substrate and MAO coatings under different.
Ti6Al4V titanium alloy with no coating was included as a comparison. The substrate has lowest spectral emissivity value in the wavelength of 3–20 m, whose average is 0.33. Comparing with the coating formed in the electrolyte without FeSO4 , the coatings prepared in electrolytes containing FeSO4 have possessed higher spectral emissivity value in the wavelength of 3–8 m range. The spectral emissivity of the coatings reaches maximum at a wavelength of about 7.6 m, and the peak position of dominant peaks shows very little change dependence with the concentration of FeSO4 in the electrolytes. However, the coating prepared in the electrolyte without FeSO4 do not exist clearly dominant peak. The dominant peaks may be caused by the presence of Fe2 O3 in the structure of the coating. When the wavelength is about 3–8 m, the spectral emissivity value of MAO3 is located at the range from 0.8 to 1.0, the average value is about 0.91. The average of the spectral emissivity value of MAO3 is 0.87 with wavelength region at 3–20 m, which is the highest among all the coatings. Past studies have shown that oxide has higher emission than metal in the wavelength range from 3 to 20 m [1]. That maybe the reason the substrate has lowest spectral emissivity value across this range. Campo investigated the oxidation kinetics of iron and showed that Fe2 O3 possessed high emission in the wavelength of 3–20 m range [22]. Others studies demonstrated that the spectral emission increasing with increased the content of the Fe2 O3 [23]. The peaks position of dominant peaks seemed to be changed by changing the temperature, but there is no doubt that dominant peak is existence. [24]. In microarc oxidation process, Fe2+ in electrolyte was incorporated into microarc oxidation coating and formed Fe2 O3 . And the content of Fe2 O3 in the surface of the coatings can be controlled by changing the concentration of FeSO4 in the electrolytes and adjusting the parameters in microarc oxidation process. In this investigation, the content of Fe2 O3 in the surface of MAO3 is highest among all the coatings, and so is the spectral emissivity value. But the impact factors of emission are much complicated, and the composition is one of those. The roughness, the microcavities and the thickness of the coating may be involved in this study [25–27]. The correlation between emissivity and surface roughness has been explored. For a gray body, the surfaces can be divided into three regions based on optical roughness (ı/, the ratio between surface roughness to wavelength): the specular region (0 < ı/ < 0.2), intermediate region ((0.2 < ı/ < 1) and the geometric region (ı/ > 1). Theoretical and experimental confirmed that the emission increased with increasing the roughness of the surface in
(1) The ceramic coatings were composed of rutile and anatase TiO2 . Fe, P elements in electrolyte were incorporated into the coatings. The surface roughness and the coating thickness increased with increasing the concentration of FeSO4 . (2) All the coatings showed a bonding strength higher than 30 MPa. After subjected a severe thermal shocking for 40 cycles, no peeling off of the coating occurred, the coatings showed a good thermal shock resistance. (3) The coating which is prepared in an aqueous solution of 7 g/L Na3 PO4 with 3 g/L FeSO4 , has a highest spectral emissivity. With wavelength increased from 3 to 8 m, the spectral emissivity average value is about 0.91, the average of the spectral emissivity value is 0.87 with wavelength region at 3–20 m. References [1] L. Campo, R.B. Pérez-Sáez, L. González-Fernández, X. Esquisabel, I. Fernández, P. González-Martín, M.J. Tello, J. Alloys Compd. 489 (2010) 482–487. [2] R. Savino, M.D.S. Fumo, D. Paterna, A.D. Maso, F. Monteverde, Aerosol Sci. Technol. 14 (2010) 178–187. [3] M. Schüßler, M. Auweter-Kurtz, G. Herdrich, S. Lein, Acta Aeronaut. 65 (2009) 676–686. [4] J. Yi, X.D. He, Y. Sun, Y. Li, M.W. Li, Appl. Surf. Sci. 253 (2007) 7100–7103. [5] A.L. Yerokhin, A. Leyland, A. Matthews, Appl. Surf. Sci. 200 (2002) 172–184. [6] Y.J. Xu, Z.P. Yao, F.Z. Jia, Y.L. Wang, Z.H. Jiang, H.T. Bu, Curr. Appl. Phys. 10 (2010) 698–702. [7] X.T. Sun, Z.H. Jiang, S.G. Xin, Z.P. Yao, Thin Solid Films 471 (2005) 194–199. [8] G. Neuer, G. Jaroma-Weiland, Int. J. Thermophys. 19 (1998) 917–929. [9] B. Stanley, Ind. Heat 8 (1982) 49–53. [10] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, S.J. Dowey, Surf. Coat. Technol. 122 (1999) 73–93. [11] J.M. Dai, X.B. Wang, G.B. Yuan, J. Phys.: Conf. Ser. 13 (2005) 63–66. [12] J. Liang, B.G. Guo, J. Tian, H.W. Liu, J.F. Zhou, T. Xu, Appl. Surf. Sci. 252 (2005) 345–351. [13] Y.M. Wang, T.Q. Lei, B.L. Jiang, L.X. Gu, Appl. Surf. Sci. 233 (2004) 258–267. [14] G. Wu, X.Y. Tan, G.Y. Li, C.W. Hu, J. Alloys Compd. 504 (2010) 371–376. [15] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, Surf. Coat. Technol. 130 (2000) 195–206. [16] J.D. Desai, H.M. Pathan, S.K. Min, K.D. Jung, O.S. Joo, Appl. Surf. Sci. 252 (2005) 1870–1875. [17] M.R. Bayati, A.Z. Moshfegh, F. Golestani-Fard, Electrochim. Acta 55 (2010) 2760–2766. [18] Y.M. Wang, B.L. Jiang, T.Q. Lei, L.X. Guo, Surf. Coat. Technol. 201 (2006) 82–89. [19] J. Liang, L.T. Hu, J.C. Hao, Electrochim. Acta 52 (2007) 4836–4840. [20] J. Baszkiewicz, D. Krupa, J. Mizera, J.W. Sobczak, A. Bilinski, Vacuum 78 (2005) 143–147. [21] D.Q. Wei, Y. Zhou, D.C. Jia, Y.M. Wang, Mater. Chem. Phys. 104 (2007) 177–182. [22] L. Campo, R.B. Pérez-Sáez, M.J. Tello, Corros. Sci. 50 (2008) 194–199. [23] Z.J. Ye, C.F. Ma, S.Y. Huang, J. Therm. Sci. 5 (1995) 128–131. [24] G.B. Smith, A. Gentle, P.D. Swift, A. Earp, N. Mronga, Sol. Energy Mater. Sol. Cells 79 (2003) 179–197. [25] F. Cernuschia, S. Ahmaniemib, P. Vuoristoc, T. Mantyla, J. Eur. Ceram. Soc. 24 (2004) 2657–2667. [26] C.D. Wen, I. Mudawar, Int. J. Heat Mass Transf. 49 (2006) 4279–4289. [27] C.Y. Zhao, T.J. Lu, H.P. Hodson, Int. J. Heat Mass Transf. 47 (2004) 2927–2939. [28] C.D. Wen, I. Mudawar, Int. J. Heat Mass Transf. 48 (2005) 1316–1329. [29] F. Ghmari, T. Ghbara, J. Appl. Phys. 96 (2004) 2656–2664. [30] M. Ziegler, J.W. Tomm, T. Elsaesser, C. Monte, J. Hollandt, H. Kissel, J. Biesenbach, J. Appl. Phys. 103 (2008) 104508–104519. [31] O. Rozenbaum, D.D.S. Meneses, P. Echegut, Int. J. Thermophys. 30 (2009) 580–590.