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PCL coating on Mg–Ca–Bi alloy
Vacuum 119 (2015) 95e98
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Corrosion and mechanical performance of double-layered nano-Al/PCL coating on MgeCaeBi alloy H.R. Bakhsheshi-Rad a, *, E. Hamzah a, M.R. Abdul-Kadir a, M. Daroonparvar a, M. Medraj b, c a
Department of Materials, Manufacturing and Industrial Engineering, Faculty of Mechanical Engineering, Universiti, Teknologi Malaysia, 81310 Johor Bahru, Johor, Malaysia Department of Mechanical Engineering, Concordia University, 1455 De Maisonneuve Blvd.West, Montreal, QC H3G 1M8, Canada c Mechanical and Materials Engineering Department, Masdar Institute, Po Box: 54224, Abu Dhabi, United Arab Emirates b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 April 2015 Received in revised form 27 April 2015 Accepted 28 April 2015 Available online 7 May 2015
To improve corrosion resistance and reduce mechanical properties loss of MgeCaeBi alloy, nanoaluminium (Al) and poly(ε-caprolactone) (PCL) were prepared on a substrate of this alloy through physical vapour deposition (PVD) and dip coating techniques. The coating is composed of a thick outer interconnected pores PCL layer and a thin inner dense Al layer. The results showed that the compressive strength of nano-Al/PCL after immersion in NaCL solution is significantly higher than nano-Al coated samples but nano-Al showed better bonding strength of 25.6 MPa than that of nano-Al/PCL which was 8.1 MPa only. The results of electrochemical tests indicated that the bi-layered nano-Al/PCL coating dramatically increased the corrosion resistance of the MgeCaeBi alloy. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Mg alloy PVD Composite coating Mechanical properties Corrosion behaviour
Mg-based alloys are extensively employed in automobile, aerospace, aircraft, and computer industries [1,2], due to their excellent mechanical and physical properties, including low density, high strength/weight ratio, excellent damping capacity and good electromagnetic shielding [1,3,4]. However, the extensive applications of Mg-based alloys were limited owing partly to their poor corrosion resistance [5]. Surface modification is considered as an effective method to decline the corrosion rate of these Mg alloys [6e8]. Some polymer coatings can improve corrosion resistance of Mg alloys in aggressive solution. PCL (e[(CH2)5COO]ne) is semicrystalline aliphatic polymer and hydrophobic, which is desirable as a coating applied on magnesium alloys because they show high corrosion rate when exposed to solution containing Cl [9]. However, at some stage during service, interaction between the polymer coating and the Mg substrate occurs leading to increase corrosion rate [6]. To address this problem, Al coating beneath organic coatings appear to have excellent potential to obtain both corrosion protection and surface stability without adversely affecting
* Corresponding author. Tel.: þ60 147382258. E-mail addresses: [email protected], (H.R. Bakhsheshi-Rad). http://dx.doi.org/10.1016/j.vacuum.2015.04.039 0042-207X/© 2015 Elsevier Ltd. All rights reserved.
[email protected]
mechanical properties. Al coating was used for protection of AZ31 magnesium alloy in NaCl solution, due to its cost-effectiveness and good corrosion resistance in aggressive media [10]. However, Wu et al. [10] also showed that single-layered Al coating alone on Mg alloy still severely corroded owing to the incidence of the alkalization effect. In this study, a combination of the two methods is used to synthesise nano-Al/PCL coating. PVD technique as a green and environmentally friendly method has been used for different type of coatings, such as TiN, Cr, Al, Al2O3 to improve Mg alloys [10,11]. Dip coating has been also applied because of their simple operation, compact coating, low temperature processing and low cost [12]. In the present study, bi-layered nano-Al/PCL coating was fabricated on a Mg1.2Ca2.5Bi alloy, expecting to avoid the disadvantages of a single nano-Al or PCL coating. Therefore, Mg1.2Ca2.5Bi alloy samples with dimensions of 15 mm 10 mm 10 mm were used as substrates. Al coating with nano size grains was deposited on Mg alloys by a PVD method. A hybrid ion beam deposition system consisting of a linear ion source and a magnetron sputtering source was selected to deposit the coatings on the substrates. The Mg alloys were ultrasonically washed in pure alcohol for 5 min before placement inside a vacuum chamber. An ion source with Ar gas was used to clean the surface of the Mg alloys for 40 min. This pre-treatment was performed when
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the base pressure of the chamber was below 2.55 103 Pa. Physical vapour deposition was performed at room temperature with argon gas as a sputtering gas. The PVD parameters are as follows: a sputtering pressure of 0.24 Pa, a target distance of 13 cm, an RF sputtering power of 200 W, a deposition time of 90 min and a bias voltage of 150 V. Prior to dipping of Al, 2.5 wt.% PCL pellets (Mw ¼ 80000 g/mol, SigmaeAldrich, UK) were dissolved in dichloromethane (DCM; CH2Cl2, SigmaeAldrich, UK) by stirring for 6 h at room temperature. The samples were dipped for 30 s and withdrawn at a constant speed to form a uniform coating and then dried at room temperature. X-ray diffractometry (Siemens-D500) was used for identifying the phases present in the specimens using Cu Ka radiation generated at 40 kV and 35 mA. The crystallite size was determined by the Scherrer equation [13]. Microstructural observation was performed using a scanning electron microscope (SEM; JEOL JSM-6380LA). The surface topography of the coated samples was evaluated by using atomic-force microscopy (AFM, NanoScope IV, Digital Instruments) according to [14]. The compressive strength and bonding strength tests were carried out according to [14,15]. A three-electrode cell was used for potentiodynamic polarization tests (PARSTAT 2263) in 3.5 wt.% NaCl solution according to [14]. Fig. 1a shows that the ternary MgeCaeBi alloy consists of aeMg, Mg2Ca and Mg3Bi2 which is consistent with the MgeCaeBi phase diagram. Phase equilibria of this alloy show that it is composed of 94.5 wt% aeMg, 2.5 wt% Mg2Ca and 3 wt% Mg3Bi2. It can be seen that most of Mg3Bi2 is formed adjacent to the eutectic structure. As can be seen in Fig. 1b, nano-Al coated Mg alloy has fine grains and dense structure accompanied with some pores. The presence of Al in the nano-Al coating besides traces of Mg, Ca and Bi that are picked from the substrate can be observed in Fig. 1d. The nano-Al/PCL coating comprised interconnected pore network where most pores in the PCL layer are sealed by nano-Al as an inner layer, serving as a barrier against corrosive electrolytes (Fig. 1c). This is expected to improve the corrosion resistance of the coated sample. EDS analysis also resulted in the detection of C, O, and Mg in this sample, indicating the formation of PCL as can be seen in Fig. 1d. Fig. 1e shows the cross-section image of nano-Al coating demonstrating that the film is composed of columnar structure
with micro-cracks and pinholes which could establish the galvanic cells between the nano-Al and Mg alloy in the vicinity of the defects. These kinds of defects can be attributed to the columnar structure of the coatings. Some parts of the boundaries between the columns are sources of porosity extending through the coatings [11,16,17]. The formation of pinholes in PVD coatings can be related to the Al coating which tend to initiate and grow in a non-uniform manner. In fact, after the original nucleation stage, the growth process occurs in the isolated islands. These islands grow simultaneously, leaving voids between them [16]. However the nano-Al/ PCL coating formed a thick layer (85.2 mm) that homogeneously covers the surface of the Mg alloy (Fig. 1f). This layer is composed of two sub-layers structure, the PCL layer (80e90 mm) at the top and Al layer with a thickness of around 750e800 nm underneath it (Fig. 1f). Gu et al. [18] showed that the thickness of the coating layer has a significant effect on the corrosion behaviour of the Mg substrate where the thicker coating layer prohibited further corrosion in Mg during tests in chloride solutions. The XRD result of the ternary MgeCaeBi alloy shows the peeks of Mg, Mg2Ca, and Mg3Bi2 (Fig. 2a) confirming the SEM/EDS results shown in Fig. 1a and d. However, Al coated sample presented peaks of Al(200), Al(220), Al(311) and Al(111) planes as preferred orientation due to the close-packed planes (PDF: 01-071-4625). The calculated crystallite size in Al coating sample is 37 nm. Nano-Al/ PCL background spectrum mainly consists of two intense peaks for PCL at 2q ¼ 21.6 and 2q ¼ 23.8 , that account for diffraction on the (1 1 0) and (2 0 0) planes, respectively (PCL has a crystalline structure with polyethylene-like orthorhombic cell disposition, with lattice parameters a ¼ 0.748 nm, b ¼ 0.498 nm and c ¼ 1.727 nm) [19]. AFM images of the nano-Al and nano-Al/PCL coatings show that the average surface roughness of the Al coating is14.7 nm which is significantly lower than that of the Al/ PCL coating which has 364 nm average roughness. The higher roughness value of the nano-Al/PCL coated alloy is due to the large amounts of pores in PCL layer (Fig. 2c, d). Al coating has a granular morphology with nano-structure, as can also be confirmed by the AFM images. The polarization curves of uncoated and coated samples in NaCl solution are shown in Fig. 3a. This figure shows
Fig. 1. SEM images of the surface of (a) uncoated MgeCaeBi alloy; (b) nano-Al coated; (c) nano-Al/PCL coated and (d) EDS analysis of point A and point B and (e) Crosssectional SEM micrograph of nano-Al coated and (f) nano-Al/PCL coated specimens.
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97
Fig. 2. (a) X-ray diffraction patterns of uncoated and coated specimens and (b) AFM topography of nano-Al/PCL coated and (c) nano-Al coated specimens.
Fig. 3. (a) Potentiodynamic polarization curves (b) Electrochemical impedance spectroscopy measurements of uncoated MgeCaeBi alloys, nano-Al coated and nano-Al/PCL coated specimens in NaCl solution.
that the corrosion potential (Ecorr) of the nano-Al coated sample (1376.1 mVSCE) is more positive than that of the uncoated sample (1623.3 mVSCE). This is due to the formation of the more compact and thin layer of nano-Al compared to the oxide film on the uncoated Mg alloy which is not stable enough in the solution and cannot protect the alloy adequately against pitting corrosion [20]. However, coating nano-Al/PCL over Mg alloy shifted the Ecorr to the nobler direction (1361.3 mVSCE). The deposition of a thick layer of the PCL coating over nano-Al with higher densification leads to impede the infiltration of NaCl solution into the coating and hence decline the local galvanic corrosion of the substrate. The corrosion current density (icorr) of nano-Al/PCL (0.04 mA/cm2) was significantly lower than nano-Al coated (2.87 mA/cm2) and uncoated sample (394.3 mA/cm2) which is owing to the existence of large amount of defects in the nano-Al layer. The Nyquist plots of the coated and uncoated samples in Fig. 3b shows only one capacitance loop which is attributed to the protective characteristics of the coatings. In this regard, charge transfer resistance (Rt) of the uncoated sample was around 3.91 kU cm2 which is the lowest corrosion resistance among the three samples. The Rt for nano-Al/ PCL and nano-Al coated Mg alloy was 2215.45 kU cm2 and 9.17 kU cm2, respectively. This indicates that Al coating can cathodically protect the Mg alloy in the aggressive NaCl solution during corrosion. Hence, nano-Al can also be selected as a protective layer [10] for the Mg alloys. However, nano-Al/PCL presented the highest Rt which can be due to the incorporation of nano-grains of Al coating that increased the Rt of the composite coating benefiting from the reduced coating porosity and the improved corrosion barrier performance of the Mg substrate. The compressive strength of uncoated and coated alloys (Fig. 4) showed that the compressive strengths of both nano-Al/PCL and nano-Al coated samples decreased from 287.3 MPa before immersion to 254.6 and 224.2 MPa, respectively, after 10 days of
immersion (Table 1). But, the compressive curves of uncoated Mg alloy dropped to 181 MPa, after 10 days immersion in NaCl solution because of the presence of pits caused by high corrosion rate. This indicates that polymer coating can delay the loss of the mechanical properties of the substrate. It should be noted that Al and Al/PCL coating did not affect the bulk mechanical properties of the Mg alloys, thus the compressive curves of uncoated and coated samples were all similar before immersion. The bonding strength of the nano-Al coating was about 25.6 MPa. However, bonding strength between the PCL and nano-Al layer was about 8.1 MPa. The higher strength of nano-Al is due to the formation of thin films which are
Fig. 4. Compressive stressestrain curves for uncoated MgeCaeBi alloys, nano-Al coated and nano-Al/PCL coated specimens before and after immersion in the NaCl solution.
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Table 1 Compression test results of the uncoated MgeCaeBi alloy, nano-Al and nano-Al/PCL coated Mg alloys before and after immersion in 3.5 wt.% NaCl. Specimen
Uncoated MgeCaeBi alloy before immersion
Nano-Al/PCL coated alloy after 10 days of immersion
Nano-Al coated alloy after 10 days of immersion
Uncoated MgeCaeBi alloy after 10 days of immersion
Compression strength eUCS (MPa)
287.3
254.6
224.2
181.4
more difficult to debond. The poor adhesion bonding between PCL and nano-Al layer leads to more diffusion of corrosive electrolyte through the coating resulting in the protective layers becoming considerably delaminated and peel off from the substrate and eventually accelerating the corrosion rate of substrate. Degner et al. [21] showed that PCL coating has low adhesion strength on the substrate. But coating still can effectively prevent magnesium alloys from corrosion which related to the polymer film thickness. Thus further studies are required to show the relationship between adhesion strength of polymer coating and corrosion resistance of magnesium alloys to meet the different application requirements. To conclude, the present study developed a new surface treatment for the MgeCaeBi alloy, combining PVD and dip coating techniques. The bi-layered coating formed on the MgeCaeBi alloy is composed of nano-aluminium and poly(ε-caprolactone) coating with a total thickness of 86 mm. The bi-layered nano-Al/PCL coating has significantly higher corrosion resistance and compressive strength after immersion than that of the nano-Al coated alloy. The results show that the nano-Al/PCL coating can sufficiently protect the MgeCaeBi alloy and enhance the mechanical properties of the alloy. Acknowledgements The authors would like to acknowledge the Universiti Teknologi Malaysia (UTM) and Nippon Sheet Glass Foundation for providing research facilities and financial support under Grant No. R.J.130000.7324.4B136.
References [1] Kartsonakis IA, Balaskas AC, Koumoulos EP, Charitidis CA, Kordas G. Corros Sci 2012;65:481e93. [2] Xie Z, Luo Z, Yang Q, Chen T, Tan S, Wang Y, et al. Vacuum 2014;101:171e6. [3] Guo X, An M. Corros Sci 2010;52:4017e27. lu S, Demirtas¸ A, Usta M, Üçıs¸ık AH. Vacuum 2013;88: [4] Durdu S, Bayramog 130e3. [5] Liu F, Shan D, Song Y, Han EH, Ke W. Corros Sci 2011;53:3845e52. [6] Wang H, Zhao C, Chen Y, Li J, Zhang X. Mater Lett 2012;68:435e8. [7] Hongxi L, Qian X, Damin X, Bo L, Chunlei M. Vacuum 2013;89:233e7. [8] Gnedenkov SV, Sinebryukhov SL, Mashtalyar DV, Imshinetskiy IM, Gnedenkov AS, Samokhin AV, et al. Vacuum 2015. http://dx.doi.org/10.1016/ j.vacuum.2015.02.004. [9] Lee EJ, Teng SH, Jang TS, Wang P, Yook SW, Kim HE, et al. Acta Biomater 2010;6:3557e65. [10] Wu G, Zeng X, Yuan G. Mater Lett 2008;62:4325e7. [11] Wu G, Wang X, Ding K, Zhou Y, Zeng X. Mater Charact 2009;60:803e7. [12] Zomorodian A, Garcia MP, Silva TM, Fernandes JCS, Fernandes MH, Montemor MF. Acta Biomater 2013;9:8660e70. [13] Cullity BD. Elements of X-Ray diffraction. 2nd ed. Massachusetts: AddisonWesley; 1978. [14] Bakhsheshi-Rad HR, Hamzah E, Daroonparvar M, Saud SN, Abdul-kadir MR. Vacuum 2014;110:127e35. [15] Bakhsheshi-Rad HR, Hamzah E, Daroonparvar M, Yajid MAM, Medraj M. Surf Coat Technol 2014;258:1090e9. [16] Fan X, Gu L, Zeng S, Zhu L, Wang C, Wang Y, et al. Surf Coat Technol 2012;206: 4471e80. [17] Wu G, Zeng X, Yao S, Wang X. Mater Lett 2007;61:4019e22. [18] Gu XN, Zheng W, Cheng Y, Zheng YF. Acta Biomater 2009;5:2790e9. [19] Kim HW, Knowles JC, Kim HE. Biomaterials 2004;25:1279e87. [20] Daroonparvar M, Mat Yajid MA, Bakhsheshi-Rad HR, Yusof NM, Hamzah E, Abdul Kadir MR. Vacuum 2014;108:61e5. [21] Degner J, Singer F, Cordero L, Boccaccini AR, Virtanen S. Appl Surf Sci 2013;282:264e70.
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Rapid communication
Corrosion and mechanical performance of double-layered nano-Al/PCL coating on MgeCaeBi alloy H.R. Bakhsheshi-Rad a, *, E. Hamzah a, M.R. Abdul-Kadir a, M. Daroonparvar a, M. Medraj b, c a
Department of Materials, Manufacturing and Industrial Engineering, Faculty of Mechanical Engineering, Universiti, Teknologi Malaysia, 81310 Johor Bahru, Johor, Malaysia Department of Mechanical Engineering, Concordia University, 1455 De Maisonneuve Blvd.West, Montreal, QC H3G 1M8, Canada c Mechanical and Materials Engineering Department, Masdar Institute, Po Box: 54224, Abu Dhabi, United Arab Emirates b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 April 2015 Received in revised form 27 April 2015 Accepted 28 April 2015 Available online 7 May 2015
To improve corrosion resistance and reduce mechanical properties loss of MgeCaeBi alloy, nanoaluminium (Al) and poly(ε-caprolactone) (PCL) were prepared on a substrate of this alloy through physical vapour deposition (PVD) and dip coating techniques. The coating is composed of a thick outer interconnected pores PCL layer and a thin inner dense Al layer. The results showed that the compressive strength of nano-Al/PCL after immersion in NaCL solution is significantly higher than nano-Al coated samples but nano-Al showed better bonding strength of 25.6 MPa than that of nano-Al/PCL which was 8.1 MPa only. The results of electrochemical tests indicated that the bi-layered nano-Al/PCL coating dramatically increased the corrosion resistance of the MgeCaeBi alloy. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Mg alloy PVD Composite coating Mechanical properties Corrosion behaviour
Mg-based alloys are extensively employed in automobile, aerospace, aircraft, and computer industries [1,2], due to their excellent mechanical and physical properties, including low density, high strength/weight ratio, excellent damping capacity and good electromagnetic shielding [1,3,4]. However, the extensive applications of Mg-based alloys were limited owing partly to their poor corrosion resistance [5]. Surface modification is considered as an effective method to decline the corrosion rate of these Mg alloys [6e8]. Some polymer coatings can improve corrosion resistance of Mg alloys in aggressive solution. PCL (e[(CH2)5COO]ne) is semicrystalline aliphatic polymer and hydrophobic, which is desirable as a coating applied on magnesium alloys because they show high corrosion rate when exposed to solution containing Cl [9]. However, at some stage during service, interaction between the polymer coating and the Mg substrate occurs leading to increase corrosion rate [6]. To address this problem, Al coating beneath organic coatings appear to have excellent potential to obtain both corrosion protection and surface stability without adversely affecting
* Corresponding author. Tel.: þ60 147382258. E-mail addresses: [email protected], (H.R. Bakhsheshi-Rad). http://dx.doi.org/10.1016/j.vacuum.2015.04.039 0042-207X/© 2015 Elsevier Ltd. All rights reserved.
[email protected]
mechanical properties. Al coating was used for protection of AZ31 magnesium alloy in NaCl solution, due to its cost-effectiveness and good corrosion resistance in aggressive media [10]. However, Wu et al. [10] also showed that single-layered Al coating alone on Mg alloy still severely corroded owing to the incidence of the alkalization effect. In this study, a combination of the two methods is used to synthesise nano-Al/PCL coating. PVD technique as a green and environmentally friendly method has been used for different type of coatings, such as TiN, Cr, Al, Al2O3 to improve Mg alloys [10,11]. Dip coating has been also applied because of their simple operation, compact coating, low temperature processing and low cost [12]. In the present study, bi-layered nano-Al/PCL coating was fabricated on a Mg1.2Ca2.5Bi alloy, expecting to avoid the disadvantages of a single nano-Al or PCL coating. Therefore, Mg1.2Ca2.5Bi alloy samples with dimensions of 15 mm 10 mm 10 mm were used as substrates. Al coating with nano size grains was deposited on Mg alloys by a PVD method. A hybrid ion beam deposition system consisting of a linear ion source and a magnetron sputtering source was selected to deposit the coatings on the substrates. The Mg alloys were ultrasonically washed in pure alcohol for 5 min before placement inside a vacuum chamber. An ion source with Ar gas was used to clean the surface of the Mg alloys for 40 min. This pre-treatment was performed when
96
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the base pressure of the chamber was below 2.55 103 Pa. Physical vapour deposition was performed at room temperature with argon gas as a sputtering gas. The PVD parameters are as follows: a sputtering pressure of 0.24 Pa, a target distance of 13 cm, an RF sputtering power of 200 W, a deposition time of 90 min and a bias voltage of 150 V. Prior to dipping of Al, 2.5 wt.% PCL pellets (Mw ¼ 80000 g/mol, SigmaeAldrich, UK) were dissolved in dichloromethane (DCM; CH2Cl2, SigmaeAldrich, UK) by stirring for 6 h at room temperature. The samples were dipped for 30 s and withdrawn at a constant speed to form a uniform coating and then dried at room temperature. X-ray diffractometry (Siemens-D500) was used for identifying the phases present in the specimens using Cu Ka radiation generated at 40 kV and 35 mA. The crystallite size was determined by the Scherrer equation [13]. Microstructural observation was performed using a scanning electron microscope (SEM; JEOL JSM-6380LA). The surface topography of the coated samples was evaluated by using atomic-force microscopy (AFM, NanoScope IV, Digital Instruments) according to [14]. The compressive strength and bonding strength tests were carried out according to [14,15]. A three-electrode cell was used for potentiodynamic polarization tests (PARSTAT 2263) in 3.5 wt.% NaCl solution according to [14]. Fig. 1a shows that the ternary MgeCaeBi alloy consists of aeMg, Mg2Ca and Mg3Bi2 which is consistent with the MgeCaeBi phase diagram. Phase equilibria of this alloy show that it is composed of 94.5 wt% aeMg, 2.5 wt% Mg2Ca and 3 wt% Mg3Bi2. It can be seen that most of Mg3Bi2 is formed adjacent to the eutectic structure. As can be seen in Fig. 1b, nano-Al coated Mg alloy has fine grains and dense structure accompanied with some pores. The presence of Al in the nano-Al coating besides traces of Mg, Ca and Bi that are picked from the substrate can be observed in Fig. 1d. The nano-Al/PCL coating comprised interconnected pore network where most pores in the PCL layer are sealed by nano-Al as an inner layer, serving as a barrier against corrosive electrolytes (Fig. 1c). This is expected to improve the corrosion resistance of the coated sample. EDS analysis also resulted in the detection of C, O, and Mg in this sample, indicating the formation of PCL as can be seen in Fig. 1d. Fig. 1e shows the cross-section image of nano-Al coating demonstrating that the film is composed of columnar structure
with micro-cracks and pinholes which could establish the galvanic cells between the nano-Al and Mg alloy in the vicinity of the defects. These kinds of defects can be attributed to the columnar structure of the coatings. Some parts of the boundaries between the columns are sources of porosity extending through the coatings [11,16,17]. The formation of pinholes in PVD coatings can be related to the Al coating which tend to initiate and grow in a non-uniform manner. In fact, after the original nucleation stage, the growth process occurs in the isolated islands. These islands grow simultaneously, leaving voids between them [16]. However the nano-Al/ PCL coating formed a thick layer (85.2 mm) that homogeneously covers the surface of the Mg alloy (Fig. 1f). This layer is composed of two sub-layers structure, the PCL layer (80e90 mm) at the top and Al layer with a thickness of around 750e800 nm underneath it (Fig. 1f). Gu et al. [18] showed that the thickness of the coating layer has a significant effect on the corrosion behaviour of the Mg substrate where the thicker coating layer prohibited further corrosion in Mg during tests in chloride solutions. The XRD result of the ternary MgeCaeBi alloy shows the peeks of Mg, Mg2Ca, and Mg3Bi2 (Fig. 2a) confirming the SEM/EDS results shown in Fig. 1a and d. However, Al coated sample presented peaks of Al(200), Al(220), Al(311) and Al(111) planes as preferred orientation due to the close-packed planes (PDF: 01-071-4625). The calculated crystallite size in Al coating sample is 37 nm. Nano-Al/ PCL background spectrum mainly consists of two intense peaks for PCL at 2q ¼ 21.6 and 2q ¼ 23.8 , that account for diffraction on the (1 1 0) and (2 0 0) planes, respectively (PCL has a crystalline structure with polyethylene-like orthorhombic cell disposition, with lattice parameters a ¼ 0.748 nm, b ¼ 0.498 nm and c ¼ 1.727 nm) [19]. AFM images of the nano-Al and nano-Al/PCL coatings show that the average surface roughness of the Al coating is14.7 nm which is significantly lower than that of the Al/ PCL coating which has 364 nm average roughness. The higher roughness value of the nano-Al/PCL coated alloy is due to the large amounts of pores in PCL layer (Fig. 2c, d). Al coating has a granular morphology with nano-structure, as can also be confirmed by the AFM images. The polarization curves of uncoated and coated samples in NaCl solution are shown in Fig. 3a. This figure shows
Fig. 1. SEM images of the surface of (a) uncoated MgeCaeBi alloy; (b) nano-Al coated; (c) nano-Al/PCL coated and (d) EDS analysis of point A and point B and (e) Crosssectional SEM micrograph of nano-Al coated and (f) nano-Al/PCL coated specimens.
H.R. Bakhsheshi-Rad et al. / Vacuum 119 (2015) 95e98
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Fig. 2. (a) X-ray diffraction patterns of uncoated and coated specimens and (b) AFM topography of nano-Al/PCL coated and (c) nano-Al coated specimens.
Fig. 3. (a) Potentiodynamic polarization curves (b) Electrochemical impedance spectroscopy measurements of uncoated MgeCaeBi alloys, nano-Al coated and nano-Al/PCL coated specimens in NaCl solution.
that the corrosion potential (Ecorr) of the nano-Al coated sample (1376.1 mVSCE) is more positive than that of the uncoated sample (1623.3 mVSCE). This is due to the formation of the more compact and thin layer of nano-Al compared to the oxide film on the uncoated Mg alloy which is not stable enough in the solution and cannot protect the alloy adequately against pitting corrosion [20]. However, coating nano-Al/PCL over Mg alloy shifted the Ecorr to the nobler direction (1361.3 mVSCE). The deposition of a thick layer of the PCL coating over nano-Al with higher densification leads to impede the infiltration of NaCl solution into the coating and hence decline the local galvanic corrosion of the substrate. The corrosion current density (icorr) of nano-Al/PCL (0.04 mA/cm2) was significantly lower than nano-Al coated (2.87 mA/cm2) and uncoated sample (394.3 mA/cm2) which is owing to the existence of large amount of defects in the nano-Al layer. The Nyquist plots of the coated and uncoated samples in Fig. 3b shows only one capacitance loop which is attributed to the protective characteristics of the coatings. In this regard, charge transfer resistance (Rt) of the uncoated sample was around 3.91 kU cm2 which is the lowest corrosion resistance among the three samples. The Rt for nano-Al/ PCL and nano-Al coated Mg alloy was 2215.45 kU cm2 and 9.17 kU cm2, respectively. This indicates that Al coating can cathodically protect the Mg alloy in the aggressive NaCl solution during corrosion. Hence, nano-Al can also be selected as a protective layer [10] for the Mg alloys. However, nano-Al/PCL presented the highest Rt which can be due to the incorporation of nano-grains of Al coating that increased the Rt of the composite coating benefiting from the reduced coating porosity and the improved corrosion barrier performance of the Mg substrate. The compressive strength of uncoated and coated alloys (Fig. 4) showed that the compressive strengths of both nano-Al/PCL and nano-Al coated samples decreased from 287.3 MPa before immersion to 254.6 and 224.2 MPa, respectively, after 10 days of
immersion (Table 1). But, the compressive curves of uncoated Mg alloy dropped to 181 MPa, after 10 days immersion in NaCl solution because of the presence of pits caused by high corrosion rate. This indicates that polymer coating can delay the loss of the mechanical properties of the substrate. It should be noted that Al and Al/PCL coating did not affect the bulk mechanical properties of the Mg alloys, thus the compressive curves of uncoated and coated samples were all similar before immersion. The bonding strength of the nano-Al coating was about 25.6 MPa. However, bonding strength between the PCL and nano-Al layer was about 8.1 MPa. The higher strength of nano-Al is due to the formation of thin films which are
Fig. 4. Compressive stressestrain curves for uncoated MgeCaeBi alloys, nano-Al coated and nano-Al/PCL coated specimens before and after immersion in the NaCl solution.
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Table 1 Compression test results of the uncoated MgeCaeBi alloy, nano-Al and nano-Al/PCL coated Mg alloys before and after immersion in 3.5 wt.% NaCl. Specimen
Uncoated MgeCaeBi alloy before immersion
Nano-Al/PCL coated alloy after 10 days of immersion
Nano-Al coated alloy after 10 days of immersion
Uncoated MgeCaeBi alloy after 10 days of immersion
Compression strength eUCS (MPa)
287.3
254.6
224.2
181.4
more difficult to debond. The poor adhesion bonding between PCL and nano-Al layer leads to more diffusion of corrosive electrolyte through the coating resulting in the protective layers becoming considerably delaminated and peel off from the substrate and eventually accelerating the corrosion rate of substrate. Degner et al. [21] showed that PCL coating has low adhesion strength on the substrate. But coating still can effectively prevent magnesium alloys from corrosion which related to the polymer film thickness. Thus further studies are required to show the relationship between adhesion strength of polymer coating and corrosion resistance of magnesium alloys to meet the different application requirements. To conclude, the present study developed a new surface treatment for the MgeCaeBi alloy, combining PVD and dip coating techniques. The bi-layered coating formed on the MgeCaeBi alloy is composed of nano-aluminium and poly(ε-caprolactone) coating with a total thickness of 86 mm. The bi-layered nano-Al/PCL coating has significantly higher corrosion resistance and compressive strength after immersion than that of the nano-Al coated alloy. The results show that the nano-Al/PCL coating can sufficiently protect the MgeCaeBi alloy and enhance the mechanical properties of the alloy. Acknowledgements The authors would like to acknowledge the Universiti Teknologi Malaysia (UTM) and Nippon Sheet Glass Foundation for providing research facilities and financial support under Grant No. R.J.130000.7324.4B136.
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