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Characterization and mechanical properties of coatings on magnesium by micro arc oxidation
Applied Surface Science 261 (2012) 774–782
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Characterization and mechanical properties of coatings on magnesium by micro arc oxidation Salih Durdu ∗ , Metin Usta Department of Materials Science and Engineering, Gebze Institute of Technology, Gebze, Kocaeli, 41400, Turkey
a r t i c l e
i n f o
Article history: Received 26 June 2012 Received in revised form 23 August 2012 Accepted 24 August 2012 Available online 31 August 2012 Keywords: Micro arc oxidation (MAO) Plasma electrolytic oxidation (PEO) Hardness Adhesion strength Wear resistance
a b s t r a c t The commercial pure magnesium was coated by micro arc oxidation method in different aqueous solution, containing sodium silicate and sodium phosphate. Micro arc oxidation process was carried out at 0.060 A/cm2 , 0.085 A/cm2 and 0.140 A/cm2 current densities for 30 min. The thickness, phase composition, morphology, hardness, adhesion strength and wear resistance of coatings were analyzed by eddy current, X-ray diffraction (XRD), scanning electron microscope (SEM), micro hardness tester, scratch tester and ball-on disk tribometer, respectively. The average thicknesses of the micro arc oxidized coatings ranged from 27 to 48 m for sodium silicate solution and from 45 to 75 m for sodium phosphate solution. The dominant phases formed on the pure magnesium were found to be a mixture of spinel Mg2 SiO4 (Forsterite) and MgO (Periclase) for sodium silicate solution and Mg3 (PO4 )2 (Farringtonite) and MgO (Periclase) for sodium phosphate solution. The average hardnesses of the micro arc oxidized coatings were between 260 HV and 470 HV for sodium silicate solution and between 175 HV and 260 HV for sodium phosphate solution. Adhesion strengths and wear resistances of coatings produced in sodium silicate solution were higher than those of the ones in sodium phosphate solution due to high hardness of coatings produced in sodium silicate solution. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.
1. Introduction Magnesium is the lightest metal in all construction metals [1]. Moreover, it is known that the magnesium and its alloys have good properties such as low density, high strength to weight ratio, excellent dimensional stability, good electromagnetic shielding, superior damping capacity, good machinability and recycling ability [1–5]. Because of these features, the magnesium and its alloys are widely used in many applications such as automotive, aerospace and communication fields [6]. Unfortunately, the magnesium and its alloys have very poor wear resistance and corrosion resistance especially in aggressive environment due to high chemical activity of the magnesium [7–12]. Therefore, the micro arc oxidation (MAO) is used to improve wear and corrosion resistance of magnesium [13–15]. The micro arc oxide coatings on the surface of the pure magnesium and magnesium alloys result in high thickness, high hardness, excellent adhesion strength and good wear resistance [16]. In addition, this method is preferred because of low cost and environmental characteristics in many fields [17].
∗ Corresponding author. Tel.: +90 262 6052689; fax: +90 262 6538490. E-mail address: [email protected] (S. Durdu).
The corrosion resistance of the pure magnesium coated in different electrolytic solutions by MAO method was generally investigated in the literature [18–20]. In our previous study, we investigated the corrosion behavior of the MAO coatings on the commercial pure magnesium produced in the silicate and phosphate based solution for different current densities [20]. The use of the commercial pure magnesium in the tribological applications is not practically available. However, the commercial pure magnesium coated by MAO can be used for tribological applications. The MAO of alloyed magnesium was studied in the literature [4,16,21–28]. In these studies, mainly wear properties, corrosion properties and coating structure of magnesium alloys coated by MAO were investigated. However, the effect of MAO on the mechanical and tribological behavior such as hardness, adhesion strength and wear resistance etc. was not studied for the commercial pure magnesium in the literature. In this study, unlike the literature, the commercial pure magnesium was coated in different solutions by MAO method. The MAO coatings were systematically characterized for different coating parameters such as solution, current density etc. In addition, to observe the effect of the solution and current density on the mechanical and tribological behavior such as hardness, adhesion strength, the wear resistance of the coatings on the commercial pure magnesium was investigated in detail.
0169-4332/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.08.099
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Table 1 Micro arc oxidation coating parameters for the coatings produced in different solutions. Solution code
Electrolyte composition
Time (min)
Current density (A/cm2 )
Final voltage (V) (Vcathodic /Vanodic ) 145/535 155/545 145/580 115/535 112/545 130/580
The silicate solution
5 g/L Na2 SiO3 ·5H2 O, 1 g/L KOH, distilled water
30
0.060 0.085 0.140
The phosphate solution
5 g/L Na3 PO4 , 1 g/L KOH, distilled water
30
0.060 0.085 0.140
2. Experimental 2.1. Materials and preparation of coatings Magnesium blocks with the purity of 99.96% were used as substrates. Before the micro arc oxide coatings, the samples were cut in the sizes of 50 mm × 25 mm × 5 mm, 68 mm × 25 mm × 5 mm and 75 mm × 25 mm × 5 mm. Surfaces of the magnesium samples were ground to grids of 400, 800 and 1200 by SiC papers. Then, the samples were cleaned in distilled water and acetone. The electrolyte solution which consists of 5 g/L sodium silicate (Na2 SiO3 ·5H2 O), 1 g/L potassium hydroxide (KOH) and pure water mixture was prepared and this solution was called as a silicate solution. The phosphate solution was composed of 5 g/L sodium phosphate (Na3 PO4 ), 1 g/L potassium hydroxide (KOH) and pure water. The MAO equipment of 100 kW was composed of an AC power supply, a stainless steel container as well as cooling and stirring systems. The magnesium substrate was used as the anode, while the stainless steel container was used as the cathode. Different current densities, namely 0.060 A/cm2 , 0.085 A/cm2 and 0.140 A/cm2 , were used for the anodizing. The micro arc oxidation was carried out at 0.060, 0.085 and 0.140 A/cm2 current densities in sodium silicate solutions and sodium phosphate solutions for 30 min. Electrolyte temperature did not exceed 30 ◦ C during the MAO. After the MAO, the coated Mg samples were washed with distilled water and dried out under the air. Table 1 shows the electrolyte composition, final voltages and coating parameters such as current density and duration time.
2.2. Characterization of the coatings In this study, the influence of the coating parameters such as the current density and different electrolyte types on the coating thickness, the phase composition, the microstructure, the hardness, the adhesion and the wear of the coating was investigated. For this reason, the following equipment was used in the current study. The coating thicknesses of MAO coatings were measured by using “Fischer Dualscope MP20” device. The average thickness of coating was taken from at least twenty measurements due to nonuniform coating morphology. Surface morphology of coating was analyzed by using Philips XL30 SFEG SEM. In order to analyze the phase structures of the MAO coatings, “Bruker D8 Advance” Xray diffraction (XRD) device was used between 20◦ and 80◦ angles by 2◦ /min rate. Vickers hardness of the coatings was measured by using a microhardness tester (Anton Paar MHT-10) with load of 0.3 N. At least five measurements of Vickers hardness were taken from the different locations of the coatings and the pure magnesium. After that, the average values were given in the results. In order to evaluate adhesion strengths of coatings, “Nanovea Scratch Tester” device was used. The progressive load was applied from 1 N to 175 N along 5 mm on the coatings. Wear test on the coatings was carried out with a ball-on-disk tribometer (CSM Instruments) in dry condition at room temperature. A normal load of 3 N was
applied in all tests. The coated sample and uncoated samples were reciprocated against alumina (Al2 O3 ) ball of 6 mm diameter for the maximum linear speed of 30 cm/s along 300 m. Dektak 8 Profiler was used to evaluate profiles of wear tracks. Wear profile values were obtained by scanning in 1000 m × 1000 m area in 3 dimensions. 3. Results and discussion 3.1. Surface morphologies of the coatings The surface morphologies of the MAO coatings are shown in Fig. 1. It is reported that the surface of the coatings produced by MAO gets rougher and more porous and the oxide layer gets thicker during MAO process [29,30]. The pores on the surface calls as micro discharge channels. The size of the micro discharge channels increases although the number of micro discharge channels decreases with increasing current density as seen in Fig. 1. The micro discharge channels on the coatings produced in the silicate solution are less than those in the phosphate solution as seen in Fig. 1. The different surface morphology of the coatings produced in the silicate and the phosphate based solutions may be related to the dissimilar characteristics of micro sparks such as the size and the number of micro sparks occurring during MAO process. Actually, higher energy is required for the current to pass through the thicker coating than the thinner one. Under this condition, the current is localized at weak points of the oxide layer formed to find its way through the coating. Thus, the number of micro sparks on the coatings produced in the phosphate solution is more than the one in the silicate solution and the coatings produced in the phosphate solution consist of greater number of pores. It is thought that the intensity of micro sparks on the coatings produced in the phosphate solution gets stronger than the one in the silicate solution during MAO process. It is seen from Fig. 1 that when high current density is applied, the number of micro discharge channels formed on the coating surface decreases in both electrolyte solutions at the same treatment time. Thus, the higher current flow, the more porosity is at the surface of coating. In addition, the surface structure of the coating has cracks owing to the fact that the thermal stresses are generated by rapid solidification during process and the cracks on the surface generally grow up with increasing time or current density. 3.2. Thickness of the coatings Fig. 2 illustrates that the thickness of the micro arc oxidized coatings changes with the current density. The average thicknesses of the micro arc oxidized coatings are 27 m, 35 m and 48 m for 0.060 A/cm2 , 0.085 A/cm2 and 0.140 A/cm2 in the silicate solution, whereas they are 45 m, 60 m and 75 m for 0.060 A/cm2 , 0.085 A/cm2 and 0.140 A/cm2 in the phosphate solution. The coating thickness increases with increasing current density as supported in literature [31]. The formation of the coatings produced in
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Fig. 1. Surface morphologies of the coatings product in the silicate electrolyte are coated for 30 min (a) 0.060 A/cm2 , (b) 0.085 A/cm2 , (c) 0.140 A/cm2 and in the phosphate electrolyte for 30 min by micro arc oxidation method (d) 0.060 A/cm2 , (e) 0.085 A/cm2 , (f) 0.140 A/cm2 .
the phosphate solution is faster than the one in the silicate solution because the coatings produced in the phosphate solution have greater growth rate than the one produced in the silicate solution. The coatings produced in the phosphate solution have many big pores and their microstructures are very loose as seen in Fig. 1. It is supported in literature that increment of thickness of the micro arc oxide coatings may be attributed to the plasma chemical reactions in the micro discharge channels [23,32]. Micro sparks with high intensity generates very thick oxide layer on the surface. As the size of sparks grows up during MAO process, many anionic and cationic ions enter into the micro discharge channels. Thus, more oxide layer is produced on the surface. In addition, high energy occurs during MAO process and this energy with very high intensity that leads to increase sparking discharge intensity provides the rapid growth of the coating [33]. As a result, strong sparks on the coatings produced in the phosphate solution contribute to increases of the growth rate. Therefore, it is concluded that the coatings produced in the phosphate solution are thicker than the ones produced in the silicate solution for the same current density. The oxide coatings produced by MAO process occur at three stages. Magnesium sample attached to the steel container is
exposed to the potential. The dielectric passive oxide film forms on metal/electrolyte interface when the sample immersed into the solution is exposed to very low anodic potential. Many micro discharge channels occur on the surface in the low electrical conductivity region with losing of dielectric stability at the first stages of coating process. Thus, the movement of micro sparks is observed on the oxide film. Local temperature in the discharge channels is about between 2000 and 10,000 K due to electron collision. Anionic compounds in the electrolyte are drawn into discharge channels owing to the existence of electrical field. The magnesium melts with high temperature and it enters into discharge channels. As a result of this process, plasma channels form on the surface. Plasma chemical reactions occur in the plasma channels at the second stage of process and the pressure in the channels increases. Micro discharge channels gradually grow up to reduce the pressure when the current is increased. At the same time, anions and cations are separated in the discharge channels due to the presence of electrical field. Cations that get out of channels by electrostatic forces are pushed into the electrolyte. At the last stage of process, micro discharge channels are cooled by electrolyte and the reaction products precipitate on the discharge channel walls. Eventually, this process causes to increase the coating thickness by repeating many times in many different places on the surface of the sample [13,20].
3.3. Phase structure of the coatings
Fig. 2. Variation of coating thicknesses of samples coated at different current densities for 30 min: (1) The silicate solution and (2) The phosphate solution.
Figs. 3 and 4 show X-ray diffraction results of magnesium and coatings produced in the silicate and phosphate solution, respectively. Major phases were found to be spinal Mg2 SiO4 and MgO for coatings produced in the silicate solution. Similarly, the coatings produced in the phosphate solution were composed of Mg3 (PO4 )2 and MgO. Peak intensities of coatings increase due to formation of thicker coatings with increasing current density. Mg transforms into the Mg2+ ions under the high voltage at the first stages of process and this magnesium ions formed during MAO process react with other ions such as O2− , SiO3 2− and PO4 3− in the micro discharge channels. Eventually, the oxide based phases occur
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Fig. 3. X-ray diffraction patterns, showing Mg, MgO, Mg2 SiO4 in the silicate solution.
on the commercial pure magnesium. The ionization mechanism of magnesium is shown below (Eq. (1)): Mg → Mg2+ + 2e−
(1)
The formation mechanism of MgO, Mg2 SiO4 and Mg3 (PO4 )2 can be explained as follows: Magnesium in the substrate and oxygen in the electrolyte ionize and they transform into Mg2+ and O2− under the high voltage, respectively. MgO is formed by outward migration of Mg2+ from substrate metal to discharge channels and inward migration of O2− from electrolyte to discharge channels under the high temperature due to the presence of electric field [17]. This reaction is shown in Eq. (2). Mg
2+
+O
2−
→ MgO
[17]
(2)
Mg2 SiO4 phase can be formed at two different reactions during MAO process. At the first reaction type, the ions react with each
other to generate the oxide based different phases such as MgO and SiO2 . These phases are sintered under high temperature and high pressure in the discharge channels. Mg2 SiO4 is generated from the fact that SiO3 2− in the silicate electrolyte penetrates into the micro discharge channels. After SiO3 2− anions lose an atom of oxygen, the reaction between SiO2 and O2 occurs. MgO and SiO2 that melt during process under high temperature react and produce the Mg2 SiO4 [34,35]. These reactions are shown in Eq. (3) and Eq. (4). At the second reaction type, Mg2+ and SiO3 2− ions react with each other to generate Mg2 SiO4 phase during MAO process. This reaction is given in Eq. (5). SiO3 2− − 2e → SiO2 + 1/2O2 2MgO + SiO2 → Mg2 SiO4
[34]
[35]
2Mg2+ + 2SiO3 2− → Mg2 SiO4 + SiO2
Fig. 4. X-ray diffraction patterns, showing Mg, MgO, Mg3 (PO4 )2 in the phosphate solution.
(3) (4) (5)
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S. Durdu, M. Usta / Applied Surface Science 261 (2012) 774–782 Table 2 Critical load results for the coating produced in different solutions after the scratch test.
Fig. 5. Average hardness of pure magnesium and the micro arc oxide coatings produced different current densities for 30 min: (1) The silicate solution and (2) The phosphate solution.
Mg3 (PO4 )2 is formed by outward migration of Mg2+ from substrate metal to discharge channels and inward migration of PO4 3− from electrolyte to discharge channels under high temperature [20]. This reaction is shown in Eq. (6). 3Mg2+ + 2PO4 3− → Mg3 (PO4 )2
[20]
(6)
3.4. Hardness of the coatings The average hardness of micro arc oxide coatings produced in the silicate and phosphate solution for 30 min is shown in Fig. 5. Average hardness of the commercial pure magnesium is approximately 40 HV. The average hardness values of the micro arc oxidized coatings are 260 HV, 370 HV and 575 HV for 0.060 A/cm2 , 0.085 A/cm2 and 0.140 A/cm2 in the silicate solution, whereas they are 175 HV, 230 HV and 300 HV for 0.060 A/cm2 , 0.085 A/cm2 and 0.140 A/cm2 in the phosphate solution. Hardness of the coating depends on the phase structure and compactness of coating
Current density (A/cm2 )
Lc (N) (The silicate solution)
Lc (N) (The phosphate solution)
0.060 0.085 0.140
58.837 62.565 83.328
72.294 78.490 85.010
structure. Thus, the average hardness of coatings increases with the current density because the coating structure becomes denser with increasing the current density for coatings produced in both solutions as shown in Fig. 6. In addition, the coatings produced in the silicate solution mainly consist of Mg2 SiO4 phase and the amount of this phase increases with increasing current density. It is generally reported [3] that Mg2 SiO4 is harder than MgO and Mg3 (PO4 )2 . Thus, the coatings produced in the silicate solution are harder than those in the phosphate solution due to compactness of coating structure and the presence of harder Mg2 SiO4 phase in the coating structure produced in the silicate solution. Hardness of coating produced at 0.140 A/cm2 in the silicate solution is maximum due to the fact that the coating is composed of Mg2 SiO4 phase with high hardness and the coating structure is denser than the other coatings. 3.5. Adhesion strength of the coatings The scratch test was used to examine adhesion strength of the coatings produced at different solutions by MAO. The scratch test results of micro arc oxide coatings on the commercial pure magnesium are given in Table 2. The critical load, Lc , is determined by optical microscopy imaging of scratch track after the test. After the scratch test, the optical microscopy images of the each coating produced at different solutions are illustrated in Fig. 7. After the scratch test of MAO coatings, a cohesive failure in the inner layer of track and an adhesive failure at the edge of track occur as seen in Fig. 7. Lc , that is necessary for detaching of the coating, is a characteristic value for adhesion strength. Lc value depends on coating hardness,
Fig. 6. Cross sectional morphologies of the coatings produced in silicate electrolyte for 30 min (a) 0.060 A/cm2 , (b) 0.085 A/cm2 , (c) 0.140 A/cm2 and in phosphate electrolyte for 30 min by micro arc oxidation method (d) 0.060 A/cm2 , (e) 0.085 A/cm2 , (f) 0.140 A/cm2 : (1) epoxy resin region, (2) the coating region and (3) the substrate.
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Fig. 7. Optical micrograph of the coatings produced in silicate electrolyte for 30 min (a) 0.060 A/cm2 , (b) 0.085 A/cm2 , (c) 0.140 A/cm2 and in phosphate electrolyte for 30 min by micro arc oxidation method (d) 0.060 A/cm2 , (e) 0.085 A/cm2 , (f) 0.140 A/cm2 after scratch test.
coating thickness and load carrying capacity of the coating and Lc increases with increasing value of these parameters as supported in the literature [36]. For coatings produced in the both solutions, the critical load and the applied load values necessary for fracture of the coatings increase with increasing the current density. In addition, it is reported that the coating produced at the higher current value adheres much firmly to the metal because the coating thickness increase with increasing current density [13]. Thus, Lc value of the coatings produced at different solutions increases with increasing current density. Maximum Lc values were only obtained for the coatings produced at 0.140 A/cm2 in both solutions. Therefore, the higher critical loads are the better adhesion strength of coatings is. Figs. 8 and 9 illustrate the load–distance curve for coatings produced in silicate and phosphate based solutions, respectively. Black linear curves corresponding to frictional forces for each coating in Figs. 8 and 9 are known as normal forces (Lc ). These forces cause the fracture of the coatings. The critical load values necessary for fracture of the coatings increase with increasing current density as seen in Figs. 8 and 9. For the coatings produced in both solutions at the same current density, the adhesion strength of coatings produced in the phosphate solution is greater than that in the silicate solution (Table 2). It is verified that the connection of oxide coating to the substrate metal get stronger with increasing coating thickness [13]. The reason of this is explained with the entering of larger
Fig. 8. Load–distance curve for the coatings produced in the silicate solution.
volume metal into the coating process [36]. However, the adhesion strength of coatings produced in the silicate solution is greater than that that of the one produced in the phosphate solution as long as the same thickness for each coating was kept constant. In order to compare the adhesion strength, it is crucial to have the similar coating thickness. From this point, the similar coating thicknesses are compared in terms of the adhesion strength. For example, the coatings produced at 0.140 A/cm2 current density in the silicate solution and at 0.060 A/cm2 current density in the phosphate solution have similar coating thicknesses of 48 and 45 m, respectively. Coating produced at 0.140 A/cm2 current density in the silicate solution has higher adhesion strength and its value is 83.328 N. The coating produced at 0.060 A/cm2 current density in the phosphate solution has lower adhesion strength and its value is 72.294 N. Thus, the hardness of the coating is dominant parameter to measure the adhesion strength. As a result, it is concluded that the coating produced in the silicate solution adheres much better to metal than the one produced in the phosphate solution. 3.6. Wear resistance of the coatings Fig. 10 illustrates the wear rates of the coatings produced in silicate and phosphate based solutions. The wear rates of micro arc oxide coatings are considerably lower than that of the uncoated
Fig. 9. Load–distance curve for the coatings produced in the phosphate solution.
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S. Durdu, M. Usta / Applied Surface Science 261 (2012) 774–782 Table 3 Wear rate results of the coatings produced in different solutions.
Fig. 10. Wear rates of commercial pure magnesium and the micro arc oxide coatings produced at different current densities during 30 min: (1) The silicate solution and (2) The phosphate solution.
magnesium. The wear resistance of the coatings depends on the phase structure and the microstructure of the coatings. The wear rate of the coatings decreases with increased current density because the hardness and compactness of the coatings increases with increased current density. Increasing current density causes to increase of the amount of forsterite. Therefore, the coatings produced at 0.140 A/cm2 in the both solutions have better wear resistance than the ones at low current density. For the same current density, the wear rate of the coatings produced in the silicate solution is considerably very low due to the presence of Mg2 SiO4
Current density (A/cm2 )
Wear rate (mm3 /N/m) (The silicate solution)
Wear rate (mm3 /N/m) (The phosphate solution)
0.060 0.085 0.140
8.5 × 10−5 7.7 × 10−5 5.6 × 10−5
119.5 × 10−5 99.0 × 10−5 19.3 × 10−5
phase with high hardness. For the same current density, although the coatings produced in the silicate solutions are thinner than the one in the phosphate solutions as seen in Fig. 6, the coatings produced in the silicate solutions have very compact structure. Therefore, for the same current density, wear rates of the coatings produced in the silicate solution is lower than that in the phosphate solution. The wear rate results of the coatings produced in different solutions are given in Table 3. The surfaces of coatings produced by micro arc oxidation method are very rough. On the other hand, the wear surfaces are smoothed out with fine crumb particles. There are many micro cracks observed within wear track. In this condition, the mechanism of wear is abrasive and associated with micro cracks [37]. It is proved that micro arc oxide coatings have excellent wear resistance compared to pure magnesium due to their higher hardness. SEM photographs of wear tracks can be seen in Figs. 11 and 12. Because the outer layer of the MAO coatings is rough and has very low hardness, the compact coating structure under the rough surface of the coating appears after the wear test as seen in Figs. 11 and 12. Abrasive wear was observed in the MAO coating. Abrasive ball removes rough coating surface during the wear test. Moreover, the abrasive types of wear due to ruptured particles are observed. As seen in
Fig. 11. Wear tracks of the coatings produced in silicate electrolyte for 30 min (a) 0.060 A/cm2 , (b) 0.085 A/cm2 , (c) 0.140 A/cm2 .
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Fig. 12. Wear tracks of the coatings produced in phosphate electrolyte for 30 min (a) 0.060 A/cm2 , (b) 0.085 A/cm2 , (c) 0.140 A/cm2 .
SEM photographs, the micro cracks appear within the wear track after the wear test. A large size of micro cracks is formed on the surface of coatings produced at low current density. The size and number of micro cracks decrease on the worn surface because the compactness of the coating increase with increasing current density. 4. Conclusions The MAO coatings produced in silicate and phosphate solutions at different current densities were systematically characterized and the mechanical and tribological properties of the coatings are investigated. The following results were obtained: 1. The coating thickness increases with increasing current density. The coatings produced in the phosphate electrolyte are thicker than ones produced in the silicate electrolyte. 2. The coatings produced in the silicate solution contain Mg2 SiO4 (Forsterite) and MgO (Periclase), whereas the coatings produced in the phosphate solution are composed of Mg3 (PO4 )2 . (Farringtonite) and MgO (Periclase). 3. The coatings produced in the silicate electrolyte have high hardness value because Mg2 SiO4 is harder than MgO and Mg3 (PO4 )2 . 4. The MAO coatings adhere much firmly to the metal. Adhesion strength of the coatings increases with increasing current density. The coatings produced in the silicate solution have greater adhesion strength than the ones produced in the phosphate solution for similar coating thicknesses. 5. The wear resistance of the coatings produced in both two solutions was significantly improved compared to the uncoated
magnesium. The wear resistances of the coatings produced in the silicate solution are greater than the ones produced in the phosphate solution due to the existence of forsterite phase with high hardness. Acknowledgements The authors would like to thank Dr. A. Polat for helping with the preparation of experiment, Mr. S. Levent Aktug for running the X-ray diffractometer, Mr. A. Nazım for helping with SEM study at Gebze Institute of Technology. This work was supported by TUBITAK project (No. 107M476). References [1] J. Liang, L. Hu, J. Hao, Characterization of microarc oxidation coatings formed on AM60B magnesium alloy in silicate and phosphate electrolytes, Appl. Surf. Sci. 253 (2007) 4490–4496. [2] Y. Zhang, C. Yan, Development of anodic film on Mg alloy AZ91D, Surf. Coat. Technol. 201 (2006) 2381–2386. [3] J. Ding, J. Liang, L.T. Hu, J.C. Hao, Q.J. Xue, Effects of sodium tungstate on characteristics of microarc oxidation coatings formed on magnesium alloy in silicate-KOH electrolyte, Trans. NonFerr. Metal. Soc. China 17 (2007) 244–249. [4] J. Liang, B. Guo, J. Tian, H. Liu, J. Zhou, T. Xu, Effect of potassium fluoride in electrolytic solution on the structure and properties of microarc oxidation coatings on magnesium alloy, Appl. Surf. Sci. 252 (2005) 345–351. [5] Y.K. Lee, K. Lee, T. Jung, Study on microarc oxidation of AZ31B magnesium alloy in alkaline metal silicate solution, Electrochem. Commun. 10 (2008) 1716–1719. [6] X.P. Zhang, Z.P. Zhao, F.M. Wu, Y.L. Wang, J. Wu, Corrosion and wear resistance of AZ91D magnesium alloy with and without microarc oxidation coating in Hank’s solution, J. Mater. Sci. 42 (2007) 8523–8528. [7] J.E. Gray, B. Luan, Protective coatings on magnesium and its alloys — a critical review, J. Alloys Compd. 336 (2002) 88–113.
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[24]
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[29]
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performance of a phosphate-based plasma electrolytic oxidation coated AM50 magnesium alloy, Appl. Surf. Sci. 256 (2010) 3928–3935. G. Rapheal, S. Kumar, C. Blawert, N.B. Dahotre, Wear behavior of plasma electrolytic oxidation (PEO) and hybrid coatings of PEO and laser on MRI 230D magnesium alloy, Wear 271 (2011) 1987–1997. J. Guo, L. Wang, J. Liang, Q. Xue, F. Yan, Tribological behavior of plasma electrolytic oxidation coating on magnesium alloy with oil lubrication at elevated temperatures, J. Alloys Compd. 481 (2009) 903–909. C. Ma, M. Zhang, Y. Yuan, X. Jing, X. Bai, Tribological behavior of plasma electrolytic oxidation coatings on the surface of Mg–8Li–1Al alloy, Tribol. Int. 47 (2012) 62–68. P.B. Srinivasan, C. Blawert, W. Dietzel, Dry sliding wear behaviour of plasma electrolytic oxidation coated AZ91 cast magnesium alloy, Wear 266 (2009) 1241–1247. P.B. Srinivasan, C. Blawert, J. Liang, W. Dietzel, Dry sliding wear behaviour of magnesium oxide and zirconium oxide plasma electrolytic oxidation coated magnesium alloy, Appl. Surf. Sci. 256 (2009) 3265–3273. Y. Wang, T. Lei, B. Jiang, L. Guo, Growth, microstructure and mechanical properties of microarc oxidation coatings on titanium alloy in phosphate-containing solution, Appl. Surf. Sci. 233 (2004) 258–267. A. Ghasemi, V.S. Raja, C. Blawert, W. Dietzel, K.U. Kainer, The role of anions in the formation and corrosion resistance of the plasma electrolytic oxidation coatings, Surf. Coat. Technol. 204 (2010) 1469–1478. J. Zhaohua, Z. Xiaobin, Y. Zhongping, Preparation of micro-arc oxidation coatings on magnesium alloy and its thermal shock resistance property, Rare Metals 25 (2006) 270–273. H. Chen, G. Lv, G. Zhang, H. Pang, X.Q. Wang, H. Lee, S.Z. Yang, Corrosion performance of plasma electrolytic oxidized AZ31 magnesium alloy in silicate solutions with different additives, Surf. Coat. Technol. 205 (2010) 32–35. J. Liang, L. Hu, J. Hao, Improvement of corrosion properties of microarc oxidation coating on magnesium alloy by optimizing current density parameters, Appl. Surf. Sci. 253 (2007) 6939–6945. S.G. Xin, L.X. Song, R.G. Zhao, X.F. Xu, Composition and thermal properties of the coating containing mullite and alumina, Mater. Chem. Phys. 97 (2006) 132–136. S.V. Gnedenkov, O.A. Khrisanfova, A.G. Zavidnaya, S.L. Sinebryukhov, V.S. Egorkin, M.V. Nistratova, A. Yerokhin, A. Matthews, PEO coatings obtained on an Mg–Mn type alloy under unipolar and bipolar modes in silicate-containing electrolytes, Surf. Coat. Technol. 204 (2010) 2316–2322. A. Polat, M. Makaraci, M. Usta, Influence of sodium silicate concentration on structural and tribological properties of microarc oxidation coatings on 2017A aluminum alloy substrate, J. Alloys Compd. 504 (2010) 519–526. J. Baxi, P. Kar, H. Liang, A. Polat, M. Usta, A.H. Ucisik, Tribological characterization of microarc oxidized alumina coatings for biological applications, Vacuum 83 (2009) 217–222.
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Characterization and mechanical properties of coatings on magnesium by micro arc oxidation Salih Durdu ∗ , Metin Usta Department of Materials Science and Engineering, Gebze Institute of Technology, Gebze, Kocaeli, 41400, Turkey
a r t i c l e
i n f o
Article history: Received 26 June 2012 Received in revised form 23 August 2012 Accepted 24 August 2012 Available online 31 August 2012 Keywords: Micro arc oxidation (MAO) Plasma electrolytic oxidation (PEO) Hardness Adhesion strength Wear resistance
a b s t r a c t The commercial pure magnesium was coated by micro arc oxidation method in different aqueous solution, containing sodium silicate and sodium phosphate. Micro arc oxidation process was carried out at 0.060 A/cm2 , 0.085 A/cm2 and 0.140 A/cm2 current densities for 30 min. The thickness, phase composition, morphology, hardness, adhesion strength and wear resistance of coatings were analyzed by eddy current, X-ray diffraction (XRD), scanning electron microscope (SEM), micro hardness tester, scratch tester and ball-on disk tribometer, respectively. The average thicknesses of the micro arc oxidized coatings ranged from 27 to 48 m for sodium silicate solution and from 45 to 75 m for sodium phosphate solution. The dominant phases formed on the pure magnesium were found to be a mixture of spinel Mg2 SiO4 (Forsterite) and MgO (Periclase) for sodium silicate solution and Mg3 (PO4 )2 (Farringtonite) and MgO (Periclase) for sodium phosphate solution. The average hardnesses of the micro arc oxidized coatings were between 260 HV and 470 HV for sodium silicate solution and between 175 HV and 260 HV for sodium phosphate solution. Adhesion strengths and wear resistances of coatings produced in sodium silicate solution were higher than those of the ones in sodium phosphate solution due to high hardness of coatings produced in sodium silicate solution. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.
1. Introduction Magnesium is the lightest metal in all construction metals [1]. Moreover, it is known that the magnesium and its alloys have good properties such as low density, high strength to weight ratio, excellent dimensional stability, good electromagnetic shielding, superior damping capacity, good machinability and recycling ability [1–5]. Because of these features, the magnesium and its alloys are widely used in many applications such as automotive, aerospace and communication fields [6]. Unfortunately, the magnesium and its alloys have very poor wear resistance and corrosion resistance especially in aggressive environment due to high chemical activity of the magnesium [7–12]. Therefore, the micro arc oxidation (MAO) is used to improve wear and corrosion resistance of magnesium [13–15]. The micro arc oxide coatings on the surface of the pure magnesium and magnesium alloys result in high thickness, high hardness, excellent adhesion strength and good wear resistance [16]. In addition, this method is preferred because of low cost and environmental characteristics in many fields [17].
∗ Corresponding author. Tel.: +90 262 6052689; fax: +90 262 6538490. E-mail address: [email protected] (S. Durdu).
The corrosion resistance of the pure magnesium coated in different electrolytic solutions by MAO method was generally investigated in the literature [18–20]. In our previous study, we investigated the corrosion behavior of the MAO coatings on the commercial pure magnesium produced in the silicate and phosphate based solution for different current densities [20]. The use of the commercial pure magnesium in the tribological applications is not practically available. However, the commercial pure magnesium coated by MAO can be used for tribological applications. The MAO of alloyed magnesium was studied in the literature [4,16,21–28]. In these studies, mainly wear properties, corrosion properties and coating structure of magnesium alloys coated by MAO were investigated. However, the effect of MAO on the mechanical and tribological behavior such as hardness, adhesion strength and wear resistance etc. was not studied for the commercial pure magnesium in the literature. In this study, unlike the literature, the commercial pure magnesium was coated in different solutions by MAO method. The MAO coatings were systematically characterized for different coating parameters such as solution, current density etc. In addition, to observe the effect of the solution and current density on the mechanical and tribological behavior such as hardness, adhesion strength, the wear resistance of the coatings on the commercial pure magnesium was investigated in detail.
0169-4332/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.08.099
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Table 1 Micro arc oxidation coating parameters for the coatings produced in different solutions. Solution code
Electrolyte composition
Time (min)
Current density (A/cm2 )
Final voltage (V) (Vcathodic /Vanodic ) 145/535 155/545 145/580 115/535 112/545 130/580
The silicate solution
5 g/L Na2 SiO3 ·5H2 O, 1 g/L KOH, distilled water
30
0.060 0.085 0.140
The phosphate solution
5 g/L Na3 PO4 , 1 g/L KOH, distilled water
30
0.060 0.085 0.140
2. Experimental 2.1. Materials and preparation of coatings Magnesium blocks with the purity of 99.96% were used as substrates. Before the micro arc oxide coatings, the samples were cut in the sizes of 50 mm × 25 mm × 5 mm, 68 mm × 25 mm × 5 mm and 75 mm × 25 mm × 5 mm. Surfaces of the magnesium samples were ground to grids of 400, 800 and 1200 by SiC papers. Then, the samples were cleaned in distilled water and acetone. The electrolyte solution which consists of 5 g/L sodium silicate (Na2 SiO3 ·5H2 O), 1 g/L potassium hydroxide (KOH) and pure water mixture was prepared and this solution was called as a silicate solution. The phosphate solution was composed of 5 g/L sodium phosphate (Na3 PO4 ), 1 g/L potassium hydroxide (KOH) and pure water. The MAO equipment of 100 kW was composed of an AC power supply, a stainless steel container as well as cooling and stirring systems. The magnesium substrate was used as the anode, while the stainless steel container was used as the cathode. Different current densities, namely 0.060 A/cm2 , 0.085 A/cm2 and 0.140 A/cm2 , were used for the anodizing. The micro arc oxidation was carried out at 0.060, 0.085 and 0.140 A/cm2 current densities in sodium silicate solutions and sodium phosphate solutions for 30 min. Electrolyte temperature did not exceed 30 ◦ C during the MAO. After the MAO, the coated Mg samples were washed with distilled water and dried out under the air. Table 1 shows the electrolyte composition, final voltages and coating parameters such as current density and duration time.
2.2. Characterization of the coatings In this study, the influence of the coating parameters such as the current density and different electrolyte types on the coating thickness, the phase composition, the microstructure, the hardness, the adhesion and the wear of the coating was investigated. For this reason, the following equipment was used in the current study. The coating thicknesses of MAO coatings were measured by using “Fischer Dualscope MP20” device. The average thickness of coating was taken from at least twenty measurements due to nonuniform coating morphology. Surface morphology of coating was analyzed by using Philips XL30 SFEG SEM. In order to analyze the phase structures of the MAO coatings, “Bruker D8 Advance” Xray diffraction (XRD) device was used between 20◦ and 80◦ angles by 2◦ /min rate. Vickers hardness of the coatings was measured by using a microhardness tester (Anton Paar MHT-10) with load of 0.3 N. At least five measurements of Vickers hardness were taken from the different locations of the coatings and the pure magnesium. After that, the average values were given in the results. In order to evaluate adhesion strengths of coatings, “Nanovea Scratch Tester” device was used. The progressive load was applied from 1 N to 175 N along 5 mm on the coatings. Wear test on the coatings was carried out with a ball-on-disk tribometer (CSM Instruments) in dry condition at room temperature. A normal load of 3 N was
applied in all tests. The coated sample and uncoated samples were reciprocated against alumina (Al2 O3 ) ball of 6 mm diameter for the maximum linear speed of 30 cm/s along 300 m. Dektak 8 Profiler was used to evaluate profiles of wear tracks. Wear profile values were obtained by scanning in 1000 m × 1000 m area in 3 dimensions. 3. Results and discussion 3.1. Surface morphologies of the coatings The surface morphologies of the MAO coatings are shown in Fig. 1. It is reported that the surface of the coatings produced by MAO gets rougher and more porous and the oxide layer gets thicker during MAO process [29,30]. The pores on the surface calls as micro discharge channels. The size of the micro discharge channels increases although the number of micro discharge channels decreases with increasing current density as seen in Fig. 1. The micro discharge channels on the coatings produced in the silicate solution are less than those in the phosphate solution as seen in Fig. 1. The different surface morphology of the coatings produced in the silicate and the phosphate based solutions may be related to the dissimilar characteristics of micro sparks such as the size and the number of micro sparks occurring during MAO process. Actually, higher energy is required for the current to pass through the thicker coating than the thinner one. Under this condition, the current is localized at weak points of the oxide layer formed to find its way through the coating. Thus, the number of micro sparks on the coatings produced in the phosphate solution is more than the one in the silicate solution and the coatings produced in the phosphate solution consist of greater number of pores. It is thought that the intensity of micro sparks on the coatings produced in the phosphate solution gets stronger than the one in the silicate solution during MAO process. It is seen from Fig. 1 that when high current density is applied, the number of micro discharge channels formed on the coating surface decreases in both electrolyte solutions at the same treatment time. Thus, the higher current flow, the more porosity is at the surface of coating. In addition, the surface structure of the coating has cracks owing to the fact that the thermal stresses are generated by rapid solidification during process and the cracks on the surface generally grow up with increasing time or current density. 3.2. Thickness of the coatings Fig. 2 illustrates that the thickness of the micro arc oxidized coatings changes with the current density. The average thicknesses of the micro arc oxidized coatings are 27 m, 35 m and 48 m for 0.060 A/cm2 , 0.085 A/cm2 and 0.140 A/cm2 in the silicate solution, whereas they are 45 m, 60 m and 75 m for 0.060 A/cm2 , 0.085 A/cm2 and 0.140 A/cm2 in the phosphate solution. The coating thickness increases with increasing current density as supported in literature [31]. The formation of the coatings produced in
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Fig. 1. Surface morphologies of the coatings product in the silicate electrolyte are coated for 30 min (a) 0.060 A/cm2 , (b) 0.085 A/cm2 , (c) 0.140 A/cm2 and in the phosphate electrolyte for 30 min by micro arc oxidation method (d) 0.060 A/cm2 , (e) 0.085 A/cm2 , (f) 0.140 A/cm2 .
the phosphate solution is faster than the one in the silicate solution because the coatings produced in the phosphate solution have greater growth rate than the one produced in the silicate solution. The coatings produced in the phosphate solution have many big pores and their microstructures are very loose as seen in Fig. 1. It is supported in literature that increment of thickness of the micro arc oxide coatings may be attributed to the plasma chemical reactions in the micro discharge channels [23,32]. Micro sparks with high intensity generates very thick oxide layer on the surface. As the size of sparks grows up during MAO process, many anionic and cationic ions enter into the micro discharge channels. Thus, more oxide layer is produced on the surface. In addition, high energy occurs during MAO process and this energy with very high intensity that leads to increase sparking discharge intensity provides the rapid growth of the coating [33]. As a result, strong sparks on the coatings produced in the phosphate solution contribute to increases of the growth rate. Therefore, it is concluded that the coatings produced in the phosphate solution are thicker than the ones produced in the silicate solution for the same current density. The oxide coatings produced by MAO process occur at three stages. Magnesium sample attached to the steel container is
exposed to the potential. The dielectric passive oxide film forms on metal/electrolyte interface when the sample immersed into the solution is exposed to very low anodic potential. Many micro discharge channels occur on the surface in the low electrical conductivity region with losing of dielectric stability at the first stages of coating process. Thus, the movement of micro sparks is observed on the oxide film. Local temperature in the discharge channels is about between 2000 and 10,000 K due to electron collision. Anionic compounds in the electrolyte are drawn into discharge channels owing to the existence of electrical field. The magnesium melts with high temperature and it enters into discharge channels. As a result of this process, plasma channels form on the surface. Plasma chemical reactions occur in the plasma channels at the second stage of process and the pressure in the channels increases. Micro discharge channels gradually grow up to reduce the pressure when the current is increased. At the same time, anions and cations are separated in the discharge channels due to the presence of electrical field. Cations that get out of channels by electrostatic forces are pushed into the electrolyte. At the last stage of process, micro discharge channels are cooled by electrolyte and the reaction products precipitate on the discharge channel walls. Eventually, this process causes to increase the coating thickness by repeating many times in many different places on the surface of the sample [13,20].
3.3. Phase structure of the coatings
Fig. 2. Variation of coating thicknesses of samples coated at different current densities for 30 min: (1) The silicate solution and (2) The phosphate solution.
Figs. 3 and 4 show X-ray diffraction results of magnesium and coatings produced in the silicate and phosphate solution, respectively. Major phases were found to be spinal Mg2 SiO4 and MgO for coatings produced in the silicate solution. Similarly, the coatings produced in the phosphate solution were composed of Mg3 (PO4 )2 and MgO. Peak intensities of coatings increase due to formation of thicker coatings with increasing current density. Mg transforms into the Mg2+ ions under the high voltage at the first stages of process and this magnesium ions formed during MAO process react with other ions such as O2− , SiO3 2− and PO4 3− in the micro discharge channels. Eventually, the oxide based phases occur
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Fig. 3. X-ray diffraction patterns, showing Mg, MgO, Mg2 SiO4 in the silicate solution.
on the commercial pure magnesium. The ionization mechanism of magnesium is shown below (Eq. (1)): Mg → Mg2+ + 2e−
(1)
The formation mechanism of MgO, Mg2 SiO4 and Mg3 (PO4 )2 can be explained as follows: Magnesium in the substrate and oxygen in the electrolyte ionize and they transform into Mg2+ and O2− under the high voltage, respectively. MgO is formed by outward migration of Mg2+ from substrate metal to discharge channels and inward migration of O2− from electrolyte to discharge channels under the high temperature due to the presence of electric field [17]. This reaction is shown in Eq. (2). Mg
2+
+O
2−
→ MgO
[17]
(2)
Mg2 SiO4 phase can be formed at two different reactions during MAO process. At the first reaction type, the ions react with each
other to generate the oxide based different phases such as MgO and SiO2 . These phases are sintered under high temperature and high pressure in the discharge channels. Mg2 SiO4 is generated from the fact that SiO3 2− in the silicate electrolyte penetrates into the micro discharge channels. After SiO3 2− anions lose an atom of oxygen, the reaction between SiO2 and O2 occurs. MgO and SiO2 that melt during process under high temperature react and produce the Mg2 SiO4 [34,35]. These reactions are shown in Eq. (3) and Eq. (4). At the second reaction type, Mg2+ and SiO3 2− ions react with each other to generate Mg2 SiO4 phase during MAO process. This reaction is given in Eq. (5). SiO3 2− − 2e → SiO2 + 1/2O2 2MgO + SiO2 → Mg2 SiO4
[34]
[35]
2Mg2+ + 2SiO3 2− → Mg2 SiO4 + SiO2
Fig. 4. X-ray diffraction patterns, showing Mg, MgO, Mg3 (PO4 )2 in the phosphate solution.
(3) (4) (5)
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Fig. 5. Average hardness of pure magnesium and the micro arc oxide coatings produced different current densities for 30 min: (1) The silicate solution and (2) The phosphate solution.
Mg3 (PO4 )2 is formed by outward migration of Mg2+ from substrate metal to discharge channels and inward migration of PO4 3− from electrolyte to discharge channels under high temperature [20]. This reaction is shown in Eq. (6). 3Mg2+ + 2PO4 3− → Mg3 (PO4 )2
[20]
(6)
3.4. Hardness of the coatings The average hardness of micro arc oxide coatings produced in the silicate and phosphate solution for 30 min is shown in Fig. 5. Average hardness of the commercial pure magnesium is approximately 40 HV. The average hardness values of the micro arc oxidized coatings are 260 HV, 370 HV and 575 HV for 0.060 A/cm2 , 0.085 A/cm2 and 0.140 A/cm2 in the silicate solution, whereas they are 175 HV, 230 HV and 300 HV for 0.060 A/cm2 , 0.085 A/cm2 and 0.140 A/cm2 in the phosphate solution. Hardness of the coating depends on the phase structure and compactness of coating
Current density (A/cm2 )
Lc (N) (The silicate solution)
Lc (N) (The phosphate solution)
0.060 0.085 0.140
58.837 62.565 83.328
72.294 78.490 85.010
structure. Thus, the average hardness of coatings increases with the current density because the coating structure becomes denser with increasing the current density for coatings produced in both solutions as shown in Fig. 6. In addition, the coatings produced in the silicate solution mainly consist of Mg2 SiO4 phase and the amount of this phase increases with increasing current density. It is generally reported [3] that Mg2 SiO4 is harder than MgO and Mg3 (PO4 )2 . Thus, the coatings produced in the silicate solution are harder than those in the phosphate solution due to compactness of coating structure and the presence of harder Mg2 SiO4 phase in the coating structure produced in the silicate solution. Hardness of coating produced at 0.140 A/cm2 in the silicate solution is maximum due to the fact that the coating is composed of Mg2 SiO4 phase with high hardness and the coating structure is denser than the other coatings. 3.5. Adhesion strength of the coatings The scratch test was used to examine adhesion strength of the coatings produced at different solutions by MAO. The scratch test results of micro arc oxide coatings on the commercial pure magnesium are given in Table 2. The critical load, Lc , is determined by optical microscopy imaging of scratch track after the test. After the scratch test, the optical microscopy images of the each coating produced at different solutions are illustrated in Fig. 7. After the scratch test of MAO coatings, a cohesive failure in the inner layer of track and an adhesive failure at the edge of track occur as seen in Fig. 7. Lc , that is necessary for detaching of the coating, is a characteristic value for adhesion strength. Lc value depends on coating hardness,
Fig. 6. Cross sectional morphologies of the coatings produced in silicate electrolyte for 30 min (a) 0.060 A/cm2 , (b) 0.085 A/cm2 , (c) 0.140 A/cm2 and in phosphate electrolyte for 30 min by micro arc oxidation method (d) 0.060 A/cm2 , (e) 0.085 A/cm2 , (f) 0.140 A/cm2 : (1) epoxy resin region, (2) the coating region and (3) the substrate.
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Fig. 7. Optical micrograph of the coatings produced in silicate electrolyte for 30 min (a) 0.060 A/cm2 , (b) 0.085 A/cm2 , (c) 0.140 A/cm2 and in phosphate electrolyte for 30 min by micro arc oxidation method (d) 0.060 A/cm2 , (e) 0.085 A/cm2 , (f) 0.140 A/cm2 after scratch test.
coating thickness and load carrying capacity of the coating and Lc increases with increasing value of these parameters as supported in the literature [36]. For coatings produced in the both solutions, the critical load and the applied load values necessary for fracture of the coatings increase with increasing the current density. In addition, it is reported that the coating produced at the higher current value adheres much firmly to the metal because the coating thickness increase with increasing current density [13]. Thus, Lc value of the coatings produced at different solutions increases with increasing current density. Maximum Lc values were only obtained for the coatings produced at 0.140 A/cm2 in both solutions. Therefore, the higher critical loads are the better adhesion strength of coatings is. Figs. 8 and 9 illustrate the load–distance curve for coatings produced in silicate and phosphate based solutions, respectively. Black linear curves corresponding to frictional forces for each coating in Figs. 8 and 9 are known as normal forces (Lc ). These forces cause the fracture of the coatings. The critical load values necessary for fracture of the coatings increase with increasing current density as seen in Figs. 8 and 9. For the coatings produced in both solutions at the same current density, the adhesion strength of coatings produced in the phosphate solution is greater than that in the silicate solution (Table 2). It is verified that the connection of oxide coating to the substrate metal get stronger with increasing coating thickness [13]. The reason of this is explained with the entering of larger
Fig. 8. Load–distance curve for the coatings produced in the silicate solution.
volume metal into the coating process [36]. However, the adhesion strength of coatings produced in the silicate solution is greater than that that of the one produced in the phosphate solution as long as the same thickness for each coating was kept constant. In order to compare the adhesion strength, it is crucial to have the similar coating thickness. From this point, the similar coating thicknesses are compared in terms of the adhesion strength. For example, the coatings produced at 0.140 A/cm2 current density in the silicate solution and at 0.060 A/cm2 current density in the phosphate solution have similar coating thicknesses of 48 and 45 m, respectively. Coating produced at 0.140 A/cm2 current density in the silicate solution has higher adhesion strength and its value is 83.328 N. The coating produced at 0.060 A/cm2 current density in the phosphate solution has lower adhesion strength and its value is 72.294 N. Thus, the hardness of the coating is dominant parameter to measure the adhesion strength. As a result, it is concluded that the coating produced in the silicate solution adheres much better to metal than the one produced in the phosphate solution. 3.6. Wear resistance of the coatings Fig. 10 illustrates the wear rates of the coatings produced in silicate and phosphate based solutions. The wear rates of micro arc oxide coatings are considerably lower than that of the uncoated
Fig. 9. Load–distance curve for the coatings produced in the phosphate solution.
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S. Durdu, M. Usta / Applied Surface Science 261 (2012) 774–782 Table 3 Wear rate results of the coatings produced in different solutions.
Fig. 10. Wear rates of commercial pure magnesium and the micro arc oxide coatings produced at different current densities during 30 min: (1) The silicate solution and (2) The phosphate solution.
magnesium. The wear resistance of the coatings depends on the phase structure and the microstructure of the coatings. The wear rate of the coatings decreases with increased current density because the hardness and compactness of the coatings increases with increased current density. Increasing current density causes to increase of the amount of forsterite. Therefore, the coatings produced at 0.140 A/cm2 in the both solutions have better wear resistance than the ones at low current density. For the same current density, the wear rate of the coatings produced in the silicate solution is considerably very low due to the presence of Mg2 SiO4
Current density (A/cm2 )
Wear rate (mm3 /N/m) (The silicate solution)
Wear rate (mm3 /N/m) (The phosphate solution)
0.060 0.085 0.140
8.5 × 10−5 7.7 × 10−5 5.6 × 10−5
119.5 × 10−5 99.0 × 10−5 19.3 × 10−5
phase with high hardness. For the same current density, although the coatings produced in the silicate solutions are thinner than the one in the phosphate solutions as seen in Fig. 6, the coatings produced in the silicate solutions have very compact structure. Therefore, for the same current density, wear rates of the coatings produced in the silicate solution is lower than that in the phosphate solution. The wear rate results of the coatings produced in different solutions are given in Table 3. The surfaces of coatings produced by micro arc oxidation method are very rough. On the other hand, the wear surfaces are smoothed out with fine crumb particles. There are many micro cracks observed within wear track. In this condition, the mechanism of wear is abrasive and associated with micro cracks [37]. It is proved that micro arc oxide coatings have excellent wear resistance compared to pure magnesium due to their higher hardness. SEM photographs of wear tracks can be seen in Figs. 11 and 12. Because the outer layer of the MAO coatings is rough and has very low hardness, the compact coating structure under the rough surface of the coating appears after the wear test as seen in Figs. 11 and 12. Abrasive wear was observed in the MAO coating. Abrasive ball removes rough coating surface during the wear test. Moreover, the abrasive types of wear due to ruptured particles are observed. As seen in
Fig. 11. Wear tracks of the coatings produced in silicate electrolyte for 30 min (a) 0.060 A/cm2 , (b) 0.085 A/cm2 , (c) 0.140 A/cm2 .
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Fig. 12. Wear tracks of the coatings produced in phosphate electrolyte for 30 min (a) 0.060 A/cm2 , (b) 0.085 A/cm2 , (c) 0.140 A/cm2 .
SEM photographs, the micro cracks appear within the wear track after the wear test. A large size of micro cracks is formed on the surface of coatings produced at low current density. The size and number of micro cracks decrease on the worn surface because the compactness of the coating increase with increasing current density. 4. Conclusions The MAO coatings produced in silicate and phosphate solutions at different current densities were systematically characterized and the mechanical and tribological properties of the coatings are investigated. The following results were obtained: 1. The coating thickness increases with increasing current density. The coatings produced in the phosphate electrolyte are thicker than ones produced in the silicate electrolyte. 2. The coatings produced in the silicate solution contain Mg2 SiO4 (Forsterite) and MgO (Periclase), whereas the coatings produced in the phosphate solution are composed of Mg3 (PO4 )2 . (Farringtonite) and MgO (Periclase). 3. The coatings produced in the silicate electrolyte have high hardness value because Mg2 SiO4 is harder than MgO and Mg3 (PO4 )2 . 4. The MAO coatings adhere much firmly to the metal. Adhesion strength of the coatings increases with increasing current density. The coatings produced in the silicate solution have greater adhesion strength than the ones produced in the phosphate solution for similar coating thicknesses. 5. The wear resistance of the coatings produced in both two solutions was significantly improved compared to the uncoated
magnesium. The wear resistances of the coatings produced in the silicate solution are greater than the ones produced in the phosphate solution due to the existence of forsterite phase with high hardness. Acknowledgements The authors would like to thank Dr. A. Polat for helping with the preparation of experiment, Mr. S. Levent Aktug for running the X-ray diffractometer, Mr. A. Nazım for helping with SEM study at Gebze Institute of Technology. This work was supported by TUBITAK project (No. 107M476). References [1] J. Liang, L. Hu, J. Hao, Characterization of microarc oxidation coatings formed on AM60B magnesium alloy in silicate and phosphate electrolytes, Appl. Surf. Sci. 253 (2007) 4490–4496. [2] Y. Zhang, C. Yan, Development of anodic film on Mg alloy AZ91D, Surf. Coat. Technol. 201 (2006) 2381–2386. [3] J. Ding, J. Liang, L.T. Hu, J.C. Hao, Q.J. Xue, Effects of sodium tungstate on characteristics of microarc oxidation coatings formed on magnesium alloy in silicate-KOH electrolyte, Trans. NonFerr. Metal. Soc. China 17 (2007) 244–249. [4] J. Liang, B. Guo, J. Tian, H. Liu, J. Zhou, T. Xu, Effect of potassium fluoride in electrolytic solution on the structure and properties of microarc oxidation coatings on magnesium alloy, Appl. Surf. Sci. 252 (2005) 345–351. [5] Y.K. Lee, K. Lee, T. Jung, Study on microarc oxidation of AZ31B magnesium alloy in alkaline metal silicate solution, Electrochem. Commun. 10 (2008) 1716–1719. [6] X.P. Zhang, Z.P. Zhao, F.M. Wu, Y.L. Wang, J. Wu, Corrosion and wear resistance of AZ91D magnesium alloy with and without microarc oxidation coating in Hank’s solution, J. Mater. Sci. 42 (2007) 8523–8528. [7] J.E. Gray, B. Luan, Protective coatings on magnesium and its alloys — a critical review, J. Alloys Compd. 336 (2002) 88–113.
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