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Effect of ZrO2 particle on the performance of micro-arc oxidation coatings on Ti6Al4V
Applied Surface Science 342 (2015) 183–190
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Effect of ZrO2 particle on the performance of micro-arc oxidation coatings on Ti6Al4V Hong Li, Yezi Sun, Jin Zhang ∗ Institute for Advanced Materials Technology, University of Science and Technology Beijing, Beijing 100083, China
a r t i c l e
i n f o
Article history: Received 8 October 2014 Received in revised form 7 March 2015 Accepted 10 March 2015 Available online 18 March 2015 Keywords: Ti6Al4V Micro-arc oxidation ZrO2 particle Oxidaton resistance Wear resistance
a b s t r a c t This paper investigates the effect of ZrO2 particle on the oxidation resistance and wear properties of coatings on a Ti6Al4V alloy generated using the micro-arc oxidation (MAO) technique. Different concentrations micron ZrO2 particles were added in phosphate electrolyte and dispersed by magnetic stirring apparatus. The composition of coating was characterized using X-ray diffraction and energy dispersive spectrum, and the morphology was examined using SEM. The high temperature oxidation resistance of the coating sample at 700 ◦ C was investigated. Sliding wear behaviour was tested by a wear tester. The results showed that the coating consisted of ZrTiO4 , ZrO2 , TiO2 . With ZrO2 particle addition, the ceramic coating’s forming time reduced by the current dynamic curve. It was shown that the addition of ZrO2 particles (3 g/L, 6 g/L) expressed an excellent oxidation resistance at 700 ◦ C and wear resistance. © 2015 Published by Elsevier B.V.
1. Introduction Titanium alloys are attractive materials due to their low density, good corrosion resistance, and high relative strength. However, their safety working temperature is generally below 500 ◦ C and the resistance to frictional wear is poor. Surface modification techniques are widely used to improve the high temperature oxidation resistance [1–4] and wear resistance of titanium alloys [5–7]. Micro-arc-oxidation (MAO), often also referred to as plasma electrolytic oxidation (PEO), is an electrochemical formation of anodic films on valve metals (Al, Ti, Mg, Ta, W, Zn and Zr) by spark/arc micro-discharges. The qualities of MAO coatings were controlled by the composition of the substrate, the nature of the electrolyte, and the electrical source etc. on. Effect of process parameters on coating properties, thermo-mechanical, corrosion resistance and tribological had been studied in references [7–13]. The MAO coating containing ZrO2 coating plays an important role on the performance of wear resistance [14] and corrosion resistance [15]. Generally, there are two ways to prepare ZrO2 coatings by MAO process. The first way is to prepare ZrO2 coatings in zirconate electrolytic solution on magnesium, aluminium, titanium alloy, the second way is to prepare on Zr substrate [14–18]. It is rarely reported to prepare containing ZrO2 coating on titanium alloy by MAO technique as the zirconate is little used and
∗ Corresponding author. Tel.: +86 1082377393; fax: +86 1088490065. E-mail address: [email protected] (J. Zhang). http://dx.doi.org/10.1016/j.apsusc.2015.03.051 0169-4332/© 2015 Published by Elsevier B.V.
expensive. In this paper, a different simple way to form ZrO2 coating on titanium alloy is introduced. The different content of powders were added and uniformly dispersed in phosphate electrolyte to prepare the ZrO2 /TiO2 composite coating. The microstructure, oxidation and wear resistance are discussed. 2. Experimental The substrate material used for this investigation was Ti6Al4V titanium alloy with a chemical composition (wt%) of 6.3 Al, 4.2 V, 0.15 O, 0.11 Fe, 0.03 C, 0.02 N, 0.001 H and Ti balance. Samples with size of 20 mm × 15 mm × 2 mm were ground using 60# , 120# , 400# , 600# , 1000# , 1500# grit silicon carbide papers, cleaned using distilled water and acetone, and then dried in air. A pulse power was employed for MAO treatment. A Ti6Al4V plate was used as an anode electrode while a graphite plate used as a cathode in an electrolytic cell. The ZrO2 particles with 3, 6, 9, 12 g/L was dispersed respectively in sodium phosphate electrolyte with the concentration of 0.3 mol/L by magnetic stirrer. During the MAO treatment, the parameters list in Table 1, the temperature of the electrolyte was hold below 45 ◦ C. After the MAO treatment, the obtained samples were washed with distilled water and dried at room temperature. The particle size of ZrO2 was measured by laser diffraction particle size analyser (LMS-30). The current was noted and the growth kinetics curves were plotted. The weight gain of the sample was recorded by an analytical balance (METTLER AE240 analytical, China) with the accuracy of 10−5 g. The thicknesses of the coatings were measured by an eddy current coating thickness gauge (TT260,
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Fig. 1. The current–time (a) and thickness–time (b) characteristic curve of MAO process.
Table 1 The preparation parameters of MAO coating. Voltage/V
Pulse frequency/HZ
Duty ratio/%
Time/min
500
2000
60
15
Time Group, Beijing). The phase component of the MAO-treated samples was analysed with X-ray diffraction (XRD, using a Cu K␣ radiation DMAX-2500RB, scanning from 20◦ to 80◦ , with step of 0.02◦ , scan rate 2◦ /min, voltage 40 kV, current 200,150 mA). Scanning electron microscopy (SEM, FEI Quanta250 Environmental Scanning Electron Microscope) was employed to observe the morphologies. The tribological performance of the MAO coatings was evaluated using a ball-on-disk with a tribometer (CETR UMT-3). A steel ball slides in a circle against a flat specimen located beneath the ball with 3 mm diameter. The load applied downward through the ball against the specimen was 3 N, with a radius of gyration of 6 mm. The friction coefficient was monitored during the tests. The profile of the wear scars was examined by SEM, and the volume-loss of samples was calculated. The high-temperature cyclic oxidation was performed in a kryptol heater furnace in air for 100 h. Ceramic crucibles were preheated before holding sample. The uncoated and coated samples (in Table 2) were heated first at 700 ◦ C for 10 h, and then taken out to weight again after cooling in air to the room temperature. Then, the samples were put back to the furnace again for another cycle. Ten times thermal cycle oxidation for 100 h had been conducted. The kinetic curve of weight variant per unit area versus to time has been got. 3. Results and discussion 3.1. Coating growth kinetics and evolution of the coating morphology The current–time and thickness–time characteristic of all the samples during MAO process are revealed in Fig. 1. It can be seen
that three main regions are identifiable. In the first region, the current declines linearly with time, with a high slope (Fig. 1(a)), the opposite regularity is found in thickness–time (Fig. 1(b)). Micro-discharge was observed distributing evenly over the whole metal surface and moving slowly from the starting of the MAO process as the setting voltage was over the breakdown voltage [19]. In the first region the thickness of the coating increased quickly causing to the drop of the current according to the Ohm law. In the second region, the current declines more and more slowly. The density and size of the micro-discharge became gradually smaller. The reducing of micro-discharge lead to the slowly increasing of thickness and the drop of the current. In the third region, the current and the thickness gradually reached a constant value. Small and moving slowly micro-discharge sporadically distributed on the surface of the samples. The phenomenon in the whole process seems to be a symmetry inverse process compared with the constant current density set in the MAO process [20]. The current of the sample without ZrO2 particles in electrolyte is obviously larger than that prepared with ZrO2 particle during the MAO process, and holds at a higher current. With increasing concentration of particles, the thickness in the three regions gradually increased while the current reduced gradually. 3.2. The weight gain of coating with the addition of ZrO2 particle The particle size distribution of ZrO2 is from 0.2 m to 10.0 m and mainly between 0.5 m and 7.1 m expressed in Fig. 2. The percentage increase in weight of different samples is shown in Fig. 3. The weight of the samples increases linearly with ZrO2 particles due to the ZrO2 involved in the reaction. The ZrO2 particles were uniformly dispersed in the electrolyte with the bottom of magnetic stirrer. During the process of sparking, the part temperature could be nearly 4000 K [21], the moving ZrO2 particles were adsorbed on the Ti6Al4V surface and penetrated into the coating [22]. With the increase of the ZrO2 particle added in electrolyte, more ZrO2 particles were involved in the coatings in the process of partial melting recrystallization.
Table 2 Code of samples. Implication
Uncoated sample
Without ZrO2
3 g/L ZrO2
6 g/L ZrO2
9 g/L ZrO2
12 g/L ZrO2
Code
Ti6Al4V
TMP
TMPZr3
TMPZr6
TMPZr9
TMPZr12
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3.3. Effect of ZrO2 particles on the coating roughness and thickness
Fig. 2. The particle size distribution of ZrO2 particle.
Fig. 4 reveals the roughness and thickness of MAO coatings on Ti–6Al–4V. It can be seen that the thickness increases with the concentration of ZrO2 . The thickness gauge works by inducing eddy currents in the metallic substrate beneath the coating, and measuring the magnetic field that is set up [23]. Eddy current gauge are performed for the measurement of the thickness of the non-conductive coating on a substrate of non-ferromagnetic metal. High-frequency electromagnetic AC signal generated in the coil probe, while the probe is close to the substrate, a vortex is formed in them. Probe from the closer the substrate, the greater the eddy, the greater the reflected impedance. The distance between the probe and the conductive substrate is characterized by the size of the feedback amount, which is the thickness of the coating on the conductive substrate. The thickness by eddy current can reach resolution of 0.1 m, tolerance of 1%, the range of 10 mm. This is an accurate technique and less affected by the roughness of the surface than the micrometer, since it only measures the thickness of one coating surface, and consequentially has less error in its measurements. However, the roughness of the coatings with ZrO2 particle added in changed compared to the TMP sample. The roughness of TMPZr3 is about 4.8 m lower than the sample of TMP 5.3 m, TMPZr6 is almost with TMP. The roughness increases with the concentration of ZrO2 . 3.4. Effect of ZrO2 particles on the composition of the coatings
Fig. 3. The weight gain versus different concentration of ZrO2 particle.
Fig. 5 shows the surface micrograph of the samples. The surface of TMP reveals numerous, micron-sized pores and some obvious cracks. The pores are typical of MAO coatings on Ti alloys which were mentioned in other papers [24–26], and are possibly formed by the emission of gas through the molten materials that are generated by the high-temperature of the discharge columns. The cracks are due to release of stress generated by the coating growth processes. Gas emission is an intrinsic feature of the MAO, the breakdown of formed coating leads an efficient reduction of coating growth [24]. There are many small cracks and micro-pores in a relatively uniform distribution on the surface of TMPZr3. Increasing the
Fig. 4. The thickness and roughness of the coatings with different samples.
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Fig. 5. Surface micrographs for the coatings by SEM (A: TMP, B: TMPZr3, C: TMPZr6, D: TMPZr9, E: TMPZr12).
Table 3 Relative content of main elements on the surface of the coating by EDS analysis (wt%). Sample
ZrL
TiK
AlK
OK
PK
TMP TMPZr3 TMPZr6 TMPZr9 TMPZr12
– 37.24 42.56 43.75 45.52
60.72 31.34 24.51 22.99 19.93
03.26 01.55 01.71 01.21 02.09
22.90 22.03 23.29 25.12 25.37
11.34 6.87 7.03 6.26 6.01
concentration of ZrO2 particles, the cracks and micro-pores were reduced and the coating became rough. The relative content of the main elements Zr, Ti, Al, O and P on the surface of the coating was analysed by EDS shown in Table 3. Increasing the concentration of ZrO2 particles, the Zr content on the surface of the coating increased while the Ti content decreased gradually (Table 3). A small amount of Al element existed on the surface of the coating. P was found in the coating for that phosphorus is capable of penetrating into the coating [27] during MAO processes. The content of O increased slightly with the concentration of ZrO2 particles.
The cross-section scanning electron micrographs of TMP and TMPZr6 are shown in Fig. 6. Clearly, the coatings reveal porosity within the coating thickness. And compared to TMP, much less pores in the coating of TMPZr6 appeared. There is thin layer bonding the porous coating and the substrate. Fig. 7 shows the distribution of Zr content along the cross-section of the coating and it is clear that there are more Zr existed in the coating with the increasing of the ZrO2 particles in electrolyte. In the early stage of MAO process, the voltage was not so high, micro arc was fine and sparse, few ZrO2 particles got into the connecting layer. With the increasing of the voltage micro arcs covered the surface and more ZrO2 particles were captured to exist in the porous layer. 3.5. The phases of the coating The XRD patterns of Ti6Al4V and the coating samples prepared by different MAO technology parameters are shown in Fig. 8. The ceramic coating formed in phosphate electrolytes without ZrO2 particle is composed of metastable anatase TiO2 and rutile TiO2 , and Ti (Fig. 8, TMP), which is same as the reference [28]. With the addition of ZrO2 particles, new phases of t-ZrO2 and ZrTiO4 appear in the coating. The phase of t-ZrO2 increases slightly with increasing
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Fig. 6. Cross-section micrograph of the coatings of TMP and TMPZr6.
the concentration of ZrO2 particles. The electrode reaction generate in phosphate electrolyte: Anode electrode: Ti → Ti4+ + 4e− −
(1) −
4OH → O2 ↑ +2H2 O + 4e
(2)
Ti4+ + 4OH− → TiO2 + 2H2 O
(3)
Cathode electrode: 2H+ + 2e− → H2 ↑
(4)
According to the binary phase diagram system of TiO2 –ZrO2 , two eutectic reactions are followed: 1373 K
TiO2 (R) + ZrO2 (at an excess) −→ ZrTiO4 · ZrO2 1473 K
ZrO2 + TiO2 (R)(at an excess) −→ ZrTiO4 · TiO2 Fig. 7. The distribution of Zr content along the cross-section of the coating.
Fig. 8. Phase composition in different samples.
(5) (6)
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Fig. 9. The oxidation kinetics curves of the samples at 700 ◦ C for 100 h.
As stated above, during the MAO process, ZrO2 particle and melting TiO2 form ZrTiO4 .
in thermal cycle oxidation 60 h later when the concentration of ZrO2 particle is over 6 g/L.
3.6. Effect of ZrO2 on the oxidation resistance of the coatings at 700 ◦ C
3.7. Effect of ZrO2 particles on the wear resistance
Fig. 9 shows the oxidation kinetics curves at 700 ◦ C for 100 h. The weight gain of the uncoated Ti6Al4V increases quickly with oxidation time, which implies that Ti6Al4V has been oxidized seriously. Besides, TMPZr9 and TMPZr12 display low weight gain at the beginning and then the gain increase linearly. Meanwhile, the samples of TMP, TMPZr3 and TMPZr6 exhibit relatively low weight gain all the time. The MAO coating prepared in phosphate electrolyte can improve the oxidation resistance of Ti6Al4V at 700 ◦ C. The prepared MAO coating also show good anti-oxidation when the concentration of ZrO2 particle is 3 g/L, 6 g/L. But the coating begins to damage
The friction coefficient versus sliding time is shown in Fig. 10. The uncoated Ti6Al4V, the friction coefficient varies around 0.4, accompanied by oscillation (curve Ti6Al4V in Fig. 10). The oscillation shows poor wear resistance without a protective surface coating. MAO coating is an effective and economical method to deposit ceramic coatings on titanium alloys for wear resistance protection [26,28–31]. The friction coefficient of TMP varies in the range of 0.4–0.5 (curve TMP). Comparing the coatings, the sample of TMPZr12 shows high and oscillation friction coefficient (curve, TMPZr12). The friction coefficient of TMPZr9 stays steadily nearly 0.2 before 1 min and then increases gradually to 0.4 accompanied by
Fig. 10. Friction coefficient versus sliding time in different samples.
H. Li et al. / Applied Surface Science 342 (2015) 183–190
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Fig. 11. The comparison of wear tracks in different samples after wear test (A: Ti6Al4V, B: TMP, C: TMPZr3, D: TMPZr6, E: TMPZr9, F: TMPZr12).
oscillation (curve TMPZr9). As shown in curve TMPZr3 and TMPZr6, the friction coefficient remains nearly 0.2 and rather steady during the overall test. The wear of MAO coating manifested as abrasive wear [32], the shedding of hard particle and salient on surface tend to become abrasive causing the aggravating wear. The main phase of TMP are TiO2 and Ti, while ZrTiO4 and ZrO2 for TMPZr3–12 shown in Fig. 6. Chen [33] mentioned ZrTiO4 and ZrO2 had high hardness, and the hardness decrease with the pores on surface. Thus with the adding of ZrO2 particles the wear resistance improved. Chen also noted that the hardness of ZrO2 is larger than that of ZrTiO4 , with the increase of ZrO2 in coating, the ZrO2 particles accelerate the wear of the coating; also the roughness increase from TMPZr3 to TMPZr12, which resulting the decreasing of coating wear resistance. The worn trace morphology of the samples is shown in Fig. 11. Much narrower trace is presented in the samples of TMPZr3, TMPZr6 and TMPZr9, meanwhile the substrate is not found exposed
on them, which reveals better wear resistance. The samples of TMP and TMPZr12 show poor wear resistance as much wider trace comparing to the Ti6Al4V. The wear volume was calculated by the following derivations of formulas:
⎛ V = 4Rr 2 ⎝
− 4
sin 2arcsin
4
1 − l2 /4r 2
arcsin −
1 − l2 /4r 2
2
⎞ ⎠ (7)
where V is the wear volume [mm3 ], r is the radium of the ball [mm], l is the wear scar width [m], R is radius of gyration [mm]. The weight variation and wear volume of different samples are shown in Table 4.
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Table 4 The weight variation and wear volume of different samples. Sample
TMP
TMPZr3
TMPZr6
TMPZr9
TMPZr12
Weight variation (g) Wear volume (mm3 )
0.0031 3.4166
0.0017 1.8694
0.0016 1.9680
0.0017 1.8119
0.0028 3.9542
The weight variation and wear volume are consistent. Sample TMP expresses the most weight variation and wear volume, meanwhile the samples of TMPZr3, TMPZr6 and TMPZr9 show least weight variation and wear volume. TMPZr12 gets more weight variation and wear volume than that of TMPZr3–9, but less than TMP. Zirconium dioxide has excellent wear resistance and corrosion resistance and so on [34]. The tribological characteristics of ZrTiO4 coatings prepared by magnetron sputtering have been reported [35]. Combining the analysis of the friction coefficient and phases, the ZrO2 presented in the coating play a role of particle strengthening. In the process of wear, the ZrO2 shedding from the coating can be automatically filled into the pores, play the role of self-repair, while increasing the contact area and reducing the unit area loads. The fall hard particles can play “micro-ball” friction-reducing effect on the surface friction. 4. Conclusions With the addition of ZrO2 particles in sodium phosphate electrolyte, the composite coating with ZrO2 and ZrTiO4 on Ti6Al4V can be prepared by micro-arc oxidation. Less cracks, less pores and thicker coating can be got compared with micro-arc oxidation directly on Ti alloy. Perfect oxidation resistance and wear resistance can be got by MAO treatment with the appropriate addition of ZrO2 (3 g/L, 6 g/L). Over ZrO2 particles addition in electrolyte will result in the increase of roughness. Acknowledgement This work was sponsored by the “Fundamental Research Funds for the Central Universities” (Grant number: 51271030). References [1] L. Chen, J. Paulitsch, Y. Du, P.H. Mayrhofer, Thermal stability and oxidation resistance of Ti–Al–N coatings, Surf. Coat. Technol. 206 (2012) 2954–2960. [2] X. Liu, H. Wang, Microstructure, wear and high-temperature oxidation resistance of laser clad Ti5Si3/␥/TiSi composite coatings on ␥-TiAl intermetallic alloy, Surf. Coat. Technol. 200 (2006) 4462–4470. [3] J. Sun, J.S. Wu, B. Zhao, F. Wang, Microstructure, wear and high temperature oxidation resistance of nitrided TiAl based alloys, Mater. Sci. Eng. A 329–331 (2002) 713–717. [4] C.H. Zhang, X.C. Lu, H. Wang, J.B. Luo, Y.G. Shen, K.Y. Li, Microstructure, mechanical properties, and oxidation resistance of nanocomposite Ti–Si–N coatings, Appl. Surf. Sci. 252 (2006) 6141–6153. ˇ ´ D. Kakaˇs, N. Bibic, M. Rakita, Microstructural studies of TiN coatings c, [5] B. Skori prepared by PVD and IBAD, Surf. Sci. 566–568 (Part 1) (2004) 40–44. [6] D. Kuo, K. Huang, Kinetics and microstructure of TiN coatings by CVD, Surf. Coat. Technol. 135 (2001) 150–157. [7] T. Kim, Y. Park, M. Wey, Characterization of Ti–6Al–4V alloy modified by plasma carburizing process, Mater. Sci. Eng. A 361 (2003) 275–280. [8] S. Tsunekawa, Y. Aoki, H. Habazaki, Two-step plasma electrolytic oxidation of Ti–15V–3Al–3Cr–3Sn for wear-resistant and adhesive coating, Surf. Coat. Technol. 205 (2011) 4732–4740. [9] W. Song, Y. Jun, Y. Han, S. Hong, Biomimetic apatite coatings on micro-arc oxidized titania, Biomaterials 25 (2004) 3341–3349. [10] X. Sun, Z. Jiang, S. Xin, Z. Yao, Composition and mechanical properties of hard ceramic coating containing ␣-Al2 O3 produced by microarc oxidation on Ti–6Al–4V alloy, Thin Solid Films 471 (2005) 194–199. [11] J.A. Curran, T.W. Clyne, Thermo-physical properties of plasma electrolytic oxide coatings on aluminium, Surf. Coat. Technol. 199 (2005) 168–176.
[12] J. Tian, Z. Luo, S. Qi, X. Sun, Structure and antiwear behavior of micro-arc oxidized coatings on aluminum alloy, Surf. Coat. Technol. 154 (2002) 1–7. [13] W. Xue, C. Wang, R. Chen, Z. Deng, Structure and properties characterization of ceramic coatings produced on Ti–6Al–4V alloy by microarc oxidation in aluminate solution, Mater. Lett. 52 (2002) 435–441. [14] Y. Yan, Y. Han, J. Huang, Formation of Al2 O3 –ZrO2 composite coating on zirconium by micro-arc oxidation, Scripta Mater. 59 (2008) 203–206. [15] Y. Yan, Y. Han, D. Li, J. Huang, Q. Lian, Effect of NaAlO2 concentrations on microstructure and corrosion resistance of Al2 O3 /ZrO2 coatings formed on zirconium by micro-arc oxidation, Appl. Surf. Sci. 256 (2010) 6359–6366. [16] W. Mu, Y. Han, Characterization and properties of the MgF2/ZrO2 composite coatings on magnesium prepared by micro-arc oxidation, Surf. Coat. Technol. 202 (2008) 4278–4284. [17] F. Samanipour, M.R. Bayati, H.R. Zargar, F. Golestani-Fard, T. Troczynski, M. Taheri, Electrophoretic enhanced micro arc oxidation of ZrO2 –HAp–TiO2 nanostructured porous layers, J. Alloy Compd. 509 (2011) 9351–9355. [18] V. Shoaei-Rad, M.R. Bayati, F. Golestani-Fard, H.R. Zargar, J. Javadpour, Fabrication of ZrO2 -Al2 O3 hybrid nano-porous layers through micro arc oxidation process, Mater. Lett. 65 (2011) 1835–1838. [19] Z. Yao, Y. Jiang, F. Jia, Z. Jiang, F. Wang, Growth characteristics of plasma electrolytic oxidation ceramic coatings on Ti–6Al–4V alloy, Appl. Surf. Sci. 254 (2008) 4084–4091. ˇ sovic, ´ S. Stojadinovic, ´ N.M. Siˇ ´ N. Konjevic, ´ Spectroscopic study of [20] J. Jovovic, plasma during electrolytic oxidation of magnesium- and aluminium-alloy, J. Quant. Spectrosc. Radiat. Transfer 113 (2012) 1928–1937. [21] R.O. Hussein, X. Nie, D.O. Northwood, A. Yerokhin, A. Matthews, Spectroscopic study of electrolytic plasma and discharging behaviour during the plasma electrolytic oxidation (PEO) process, J. Phys. D: Appl. Phys. 43 (2010) 105203. [22] X. Li, B.L. Luan, Discovery of Al2 O3 particles incorporation mechanism in plasma electrolytic oxidation of AM60B magnesium alloy, Mater. Lett. 86 (2012) 88–91. [23] J.M. Wheeler, C.A. Collier, J.M. Paillard, J.A. Curran, Evaluation of micromechanical behaviour of plasma electrolytic oxidation (PEO) coatings on Ti–6Al–4V, Surf. Coat. Technol. 204 (2010) 3399–3409. [24] E. Matykina, A. Berkani, P. Skeldon, G.E. Thompson, Real-time imaging of coating growth during plasma electrolytic oxidation of titanium, Electrochim. Acta 53 (2007) 1987–1994. [25] Y. Han, S. Hong, K.W. Xu, Porous nanocrystalline titania films by plasma electrolytic oxidation, Surf. Coat. Technol. 154 (2002) 314–318. [26] Y. Wang, B. Jiang, T. Lei, L. Guo, Dependence of growth features of microarc oxidation coatings of titanium alloy on control modes of alternate pulse, Mater. Lett. 58 (2004) 1907–1911. [27] G. Lv, W. Gu, H. Chen, W. Feng, M.L. Khosa, L. Li, E. Niu, G. Zhang, S. Yang, Characteristic of ceramic coatings on aluminum by plasma electrolytic oxidation in silicate and phosphate electrolyte, Appl. Surf. Sci. 253 (2006) 2947–2952. [28] 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. [29] Y.M. Wang, B.L. Jiang, L.X. Guo, T.Q. Lei, Tribological behavior of microarc oxidation coatings formed on titanium alloys against steel in dry and solid lubrication sliding, Appl. Surf. Sci. 252 (2006) 2989–2998. [30] J. Chen, R. Zeng, W. Huang, Z. Zheng, Z. Wang, J. Wang, Characterization and wear resistance of macro-arc oxidation coating on magnesium alloy AZ91 in simulated body fluids, T. Nonferr. Metal. Soc. 18 (Suppl. 1) (2008) s361–s364. [31] A.L. Yerokhin, A. Leyland, A. Matthews, Kinetic aspects of aluminium titanate layer formation on titanium alloys by plasma electrolytic oxidation, Appl. Surf. Sci. 200 (2002) 172–184. [32] L.R. Krishna, A.S. Purnima, G. Sundararajan, A comparative study of tribological behavior of microarc oxidation and hard-anodized coatings, Wear 261 (2006) 1095–1101. [33] X. Miao, D. Sun, P.W. Hoo, J. Liu, Y. Hu, Y. Chen, Effect of titania addition on yttria-stabilised tetragonal zirconia ceramics sintered at high temperatures, Ceram. Int. 30 (2004) 1041–1047. [34] Z. Yao, Y. Jiang, Z. Jiang, F. Wang, Z. Wu, Preparation and structure of ceramic coatings containing zirconium oxide on Ti alloy by plasma electrolytic oxidation, J. Mater. Process. Tech. 205 (2008) 303–307. [35] D.V. Shtansky, M.I. Petrzhik, I.A. Bashkova, F.V. Kiryukhantsev-Korneev, A.N. Sheve˘ıko, E.A. Levashov, Adhesion, friction, and deformation characteristics of Ti-(Ca, Zr)-(C, N, O, P) coatings for orthopedic and dental implants, Phys. Solid State 48 (2006) 1301–1308.
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Effect of ZrO2 particle on the performance of micro-arc oxidation coatings on Ti6Al4V Hong Li, Yezi Sun, Jin Zhang ∗ Institute for Advanced Materials Technology, University of Science and Technology Beijing, Beijing 100083, China
a r t i c l e
i n f o
Article history: Received 8 October 2014 Received in revised form 7 March 2015 Accepted 10 March 2015 Available online 18 March 2015 Keywords: Ti6Al4V Micro-arc oxidation ZrO2 particle Oxidaton resistance Wear resistance
a b s t r a c t This paper investigates the effect of ZrO2 particle on the oxidation resistance and wear properties of coatings on a Ti6Al4V alloy generated using the micro-arc oxidation (MAO) technique. Different concentrations micron ZrO2 particles were added in phosphate electrolyte and dispersed by magnetic stirring apparatus. The composition of coating was characterized using X-ray diffraction and energy dispersive spectrum, and the morphology was examined using SEM. The high temperature oxidation resistance of the coating sample at 700 ◦ C was investigated. Sliding wear behaviour was tested by a wear tester. The results showed that the coating consisted of ZrTiO4 , ZrO2 , TiO2 . With ZrO2 particle addition, the ceramic coating’s forming time reduced by the current dynamic curve. It was shown that the addition of ZrO2 particles (3 g/L, 6 g/L) expressed an excellent oxidation resistance at 700 ◦ C and wear resistance. © 2015 Published by Elsevier B.V.
1. Introduction Titanium alloys are attractive materials due to their low density, good corrosion resistance, and high relative strength. However, their safety working temperature is generally below 500 ◦ C and the resistance to frictional wear is poor. Surface modification techniques are widely used to improve the high temperature oxidation resistance [1–4] and wear resistance of titanium alloys [5–7]. Micro-arc-oxidation (MAO), often also referred to as plasma electrolytic oxidation (PEO), is an electrochemical formation of anodic films on valve metals (Al, Ti, Mg, Ta, W, Zn and Zr) by spark/arc micro-discharges. The qualities of MAO coatings were controlled by the composition of the substrate, the nature of the electrolyte, and the electrical source etc. on. Effect of process parameters on coating properties, thermo-mechanical, corrosion resistance and tribological had been studied in references [7–13]. The MAO coating containing ZrO2 coating plays an important role on the performance of wear resistance [14] and corrosion resistance [15]. Generally, there are two ways to prepare ZrO2 coatings by MAO process. The first way is to prepare ZrO2 coatings in zirconate electrolytic solution on magnesium, aluminium, titanium alloy, the second way is to prepare on Zr substrate [14–18]. It is rarely reported to prepare containing ZrO2 coating on titanium alloy by MAO technique as the zirconate is little used and
∗ Corresponding author. Tel.: +86 1082377393; fax: +86 1088490065. E-mail address: [email protected] (J. Zhang). http://dx.doi.org/10.1016/j.apsusc.2015.03.051 0169-4332/© 2015 Published by Elsevier B.V.
expensive. In this paper, a different simple way to form ZrO2 coating on titanium alloy is introduced. The different content of powders were added and uniformly dispersed in phosphate electrolyte to prepare the ZrO2 /TiO2 composite coating. The microstructure, oxidation and wear resistance are discussed. 2. Experimental The substrate material used for this investigation was Ti6Al4V titanium alloy with a chemical composition (wt%) of 6.3 Al, 4.2 V, 0.15 O, 0.11 Fe, 0.03 C, 0.02 N, 0.001 H and Ti balance. Samples with size of 20 mm × 15 mm × 2 mm were ground using 60# , 120# , 400# , 600# , 1000# , 1500# grit silicon carbide papers, cleaned using distilled water and acetone, and then dried in air. A pulse power was employed for MAO treatment. A Ti6Al4V plate was used as an anode electrode while a graphite plate used as a cathode in an electrolytic cell. The ZrO2 particles with 3, 6, 9, 12 g/L was dispersed respectively in sodium phosphate electrolyte with the concentration of 0.3 mol/L by magnetic stirrer. During the MAO treatment, the parameters list in Table 1, the temperature of the electrolyte was hold below 45 ◦ C. After the MAO treatment, the obtained samples were washed with distilled water and dried at room temperature. The particle size of ZrO2 was measured by laser diffraction particle size analyser (LMS-30). The current was noted and the growth kinetics curves were plotted. The weight gain of the sample was recorded by an analytical balance (METTLER AE240 analytical, China) with the accuracy of 10−5 g. The thicknesses of the coatings were measured by an eddy current coating thickness gauge (TT260,
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Fig. 1. The current–time (a) and thickness–time (b) characteristic curve of MAO process.
Table 1 The preparation parameters of MAO coating. Voltage/V
Pulse frequency/HZ
Duty ratio/%
Time/min
500
2000
60
15
Time Group, Beijing). The phase component of the MAO-treated samples was analysed with X-ray diffraction (XRD, using a Cu K␣ radiation DMAX-2500RB, scanning from 20◦ to 80◦ , with step of 0.02◦ , scan rate 2◦ /min, voltage 40 kV, current 200,150 mA). Scanning electron microscopy (SEM, FEI Quanta250 Environmental Scanning Electron Microscope) was employed to observe the morphologies. The tribological performance of the MAO coatings was evaluated using a ball-on-disk with a tribometer (CETR UMT-3). A steel ball slides in a circle against a flat specimen located beneath the ball with 3 mm diameter. The load applied downward through the ball against the specimen was 3 N, with a radius of gyration of 6 mm. The friction coefficient was monitored during the tests. The profile of the wear scars was examined by SEM, and the volume-loss of samples was calculated. The high-temperature cyclic oxidation was performed in a kryptol heater furnace in air for 100 h. Ceramic crucibles were preheated before holding sample. The uncoated and coated samples (in Table 2) were heated first at 700 ◦ C for 10 h, and then taken out to weight again after cooling in air to the room temperature. Then, the samples were put back to the furnace again for another cycle. Ten times thermal cycle oxidation for 100 h had been conducted. The kinetic curve of weight variant per unit area versus to time has been got. 3. Results and discussion 3.1. Coating growth kinetics and evolution of the coating morphology The current–time and thickness–time characteristic of all the samples during MAO process are revealed in Fig. 1. It can be seen
that three main regions are identifiable. In the first region, the current declines linearly with time, with a high slope (Fig. 1(a)), the opposite regularity is found in thickness–time (Fig. 1(b)). Micro-discharge was observed distributing evenly over the whole metal surface and moving slowly from the starting of the MAO process as the setting voltage was over the breakdown voltage [19]. In the first region the thickness of the coating increased quickly causing to the drop of the current according to the Ohm law. In the second region, the current declines more and more slowly. The density and size of the micro-discharge became gradually smaller. The reducing of micro-discharge lead to the slowly increasing of thickness and the drop of the current. In the third region, the current and the thickness gradually reached a constant value. Small and moving slowly micro-discharge sporadically distributed on the surface of the samples. The phenomenon in the whole process seems to be a symmetry inverse process compared with the constant current density set in the MAO process [20]. The current of the sample without ZrO2 particles in electrolyte is obviously larger than that prepared with ZrO2 particle during the MAO process, and holds at a higher current. With increasing concentration of particles, the thickness in the three regions gradually increased while the current reduced gradually. 3.2. The weight gain of coating with the addition of ZrO2 particle The particle size distribution of ZrO2 is from 0.2 m to 10.0 m and mainly between 0.5 m and 7.1 m expressed in Fig. 2. The percentage increase in weight of different samples is shown in Fig. 3. The weight of the samples increases linearly with ZrO2 particles due to the ZrO2 involved in the reaction. The ZrO2 particles were uniformly dispersed in the electrolyte with the bottom of magnetic stirrer. During the process of sparking, the part temperature could be nearly 4000 K [21], the moving ZrO2 particles were adsorbed on the Ti6Al4V surface and penetrated into the coating [22]. With the increase of the ZrO2 particle added in electrolyte, more ZrO2 particles were involved in the coatings in the process of partial melting recrystallization.
Table 2 Code of samples. Implication
Uncoated sample
Without ZrO2
3 g/L ZrO2
6 g/L ZrO2
9 g/L ZrO2
12 g/L ZrO2
Code
Ti6Al4V
TMP
TMPZr3
TMPZr6
TMPZr9
TMPZr12
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3.3. Effect of ZrO2 particles on the coating roughness and thickness
Fig. 2. The particle size distribution of ZrO2 particle.
Fig. 4 reveals the roughness and thickness of MAO coatings on Ti–6Al–4V. It can be seen that the thickness increases with the concentration of ZrO2 . The thickness gauge works by inducing eddy currents in the metallic substrate beneath the coating, and measuring the magnetic field that is set up [23]. Eddy current gauge are performed for the measurement of the thickness of the non-conductive coating on a substrate of non-ferromagnetic metal. High-frequency electromagnetic AC signal generated in the coil probe, while the probe is close to the substrate, a vortex is formed in them. Probe from the closer the substrate, the greater the eddy, the greater the reflected impedance. The distance between the probe and the conductive substrate is characterized by the size of the feedback amount, which is the thickness of the coating on the conductive substrate. The thickness by eddy current can reach resolution of 0.1 m, tolerance of 1%, the range of 10 mm. This is an accurate technique and less affected by the roughness of the surface than the micrometer, since it only measures the thickness of one coating surface, and consequentially has less error in its measurements. However, the roughness of the coatings with ZrO2 particle added in changed compared to the TMP sample. The roughness of TMPZr3 is about 4.8 m lower than the sample of TMP 5.3 m, TMPZr6 is almost with TMP. The roughness increases with the concentration of ZrO2 . 3.4. Effect of ZrO2 particles on the composition of the coatings
Fig. 3. The weight gain versus different concentration of ZrO2 particle.
Fig. 5 shows the surface micrograph of the samples. The surface of TMP reveals numerous, micron-sized pores and some obvious cracks. The pores are typical of MAO coatings on Ti alloys which were mentioned in other papers [24–26], and are possibly formed by the emission of gas through the molten materials that are generated by the high-temperature of the discharge columns. The cracks are due to release of stress generated by the coating growth processes. Gas emission is an intrinsic feature of the MAO, the breakdown of formed coating leads an efficient reduction of coating growth [24]. There are many small cracks and micro-pores in a relatively uniform distribution on the surface of TMPZr3. Increasing the
Fig. 4. The thickness and roughness of the coatings with different samples.
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Fig. 5. Surface micrographs for the coatings by SEM (A: TMP, B: TMPZr3, C: TMPZr6, D: TMPZr9, E: TMPZr12).
Table 3 Relative content of main elements on the surface of the coating by EDS analysis (wt%). Sample
ZrL
TiK
AlK
OK
PK
TMP TMPZr3 TMPZr6 TMPZr9 TMPZr12
– 37.24 42.56 43.75 45.52
60.72 31.34 24.51 22.99 19.93
03.26 01.55 01.71 01.21 02.09
22.90 22.03 23.29 25.12 25.37
11.34 6.87 7.03 6.26 6.01
concentration of ZrO2 particles, the cracks and micro-pores were reduced and the coating became rough. The relative content of the main elements Zr, Ti, Al, O and P on the surface of the coating was analysed by EDS shown in Table 3. Increasing the concentration of ZrO2 particles, the Zr content on the surface of the coating increased while the Ti content decreased gradually (Table 3). A small amount of Al element existed on the surface of the coating. P was found in the coating for that phosphorus is capable of penetrating into the coating [27] during MAO processes. The content of O increased slightly with the concentration of ZrO2 particles.
The cross-section scanning electron micrographs of TMP and TMPZr6 are shown in Fig. 6. Clearly, the coatings reveal porosity within the coating thickness. And compared to TMP, much less pores in the coating of TMPZr6 appeared. There is thin layer bonding the porous coating and the substrate. Fig. 7 shows the distribution of Zr content along the cross-section of the coating and it is clear that there are more Zr existed in the coating with the increasing of the ZrO2 particles in electrolyte. In the early stage of MAO process, the voltage was not so high, micro arc was fine and sparse, few ZrO2 particles got into the connecting layer. With the increasing of the voltage micro arcs covered the surface and more ZrO2 particles were captured to exist in the porous layer. 3.5. The phases of the coating The XRD patterns of Ti6Al4V and the coating samples prepared by different MAO technology parameters are shown in Fig. 8. The ceramic coating formed in phosphate electrolytes without ZrO2 particle is composed of metastable anatase TiO2 and rutile TiO2 , and Ti (Fig. 8, TMP), which is same as the reference [28]. With the addition of ZrO2 particles, new phases of t-ZrO2 and ZrTiO4 appear in the coating. The phase of t-ZrO2 increases slightly with increasing
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Fig. 6. Cross-section micrograph of the coatings of TMP and TMPZr6.
the concentration of ZrO2 particles. The electrode reaction generate in phosphate electrolyte: Anode electrode: Ti → Ti4+ + 4e− −
(1) −
4OH → O2 ↑ +2H2 O + 4e
(2)
Ti4+ + 4OH− → TiO2 + 2H2 O
(3)
Cathode electrode: 2H+ + 2e− → H2 ↑
(4)
According to the binary phase diagram system of TiO2 –ZrO2 , two eutectic reactions are followed: 1373 K
TiO2 (R) + ZrO2 (at an excess) −→ ZrTiO4 · ZrO2 1473 K
ZrO2 + TiO2 (R)(at an excess) −→ ZrTiO4 · TiO2 Fig. 7. The distribution of Zr content along the cross-section of the coating.
Fig. 8. Phase composition in different samples.
(5) (6)
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Fig. 9. The oxidation kinetics curves of the samples at 700 ◦ C for 100 h.
As stated above, during the MAO process, ZrO2 particle and melting TiO2 form ZrTiO4 .
in thermal cycle oxidation 60 h later when the concentration of ZrO2 particle is over 6 g/L.
3.6. Effect of ZrO2 on the oxidation resistance of the coatings at 700 ◦ C
3.7. Effect of ZrO2 particles on the wear resistance
Fig. 9 shows the oxidation kinetics curves at 700 ◦ C for 100 h. The weight gain of the uncoated Ti6Al4V increases quickly with oxidation time, which implies that Ti6Al4V has been oxidized seriously. Besides, TMPZr9 and TMPZr12 display low weight gain at the beginning and then the gain increase linearly. Meanwhile, the samples of TMP, TMPZr3 and TMPZr6 exhibit relatively low weight gain all the time. The MAO coating prepared in phosphate electrolyte can improve the oxidation resistance of Ti6Al4V at 700 ◦ C. The prepared MAO coating also show good anti-oxidation when the concentration of ZrO2 particle is 3 g/L, 6 g/L. But the coating begins to damage
The friction coefficient versus sliding time is shown in Fig. 10. The uncoated Ti6Al4V, the friction coefficient varies around 0.4, accompanied by oscillation (curve Ti6Al4V in Fig. 10). The oscillation shows poor wear resistance without a protective surface coating. MAO coating is an effective and economical method to deposit ceramic coatings on titanium alloys for wear resistance protection [26,28–31]. The friction coefficient of TMP varies in the range of 0.4–0.5 (curve TMP). Comparing the coatings, the sample of TMPZr12 shows high and oscillation friction coefficient (curve, TMPZr12). The friction coefficient of TMPZr9 stays steadily nearly 0.2 before 1 min and then increases gradually to 0.4 accompanied by
Fig. 10. Friction coefficient versus sliding time in different samples.
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Fig. 11. The comparison of wear tracks in different samples after wear test (A: Ti6Al4V, B: TMP, C: TMPZr3, D: TMPZr6, E: TMPZr9, F: TMPZr12).
oscillation (curve TMPZr9). As shown in curve TMPZr3 and TMPZr6, the friction coefficient remains nearly 0.2 and rather steady during the overall test. The wear of MAO coating manifested as abrasive wear [32], the shedding of hard particle and salient on surface tend to become abrasive causing the aggravating wear. The main phase of TMP are TiO2 and Ti, while ZrTiO4 and ZrO2 for TMPZr3–12 shown in Fig. 6. Chen [33] mentioned ZrTiO4 and ZrO2 had high hardness, and the hardness decrease with the pores on surface. Thus with the adding of ZrO2 particles the wear resistance improved. Chen also noted that the hardness of ZrO2 is larger than that of ZrTiO4 , with the increase of ZrO2 in coating, the ZrO2 particles accelerate the wear of the coating; also the roughness increase from TMPZr3 to TMPZr12, which resulting the decreasing of coating wear resistance. The worn trace morphology of the samples is shown in Fig. 11. Much narrower trace is presented in the samples of TMPZr3, TMPZr6 and TMPZr9, meanwhile the substrate is not found exposed
on them, which reveals better wear resistance. The samples of TMP and TMPZr12 show poor wear resistance as much wider trace comparing to the Ti6Al4V. The wear volume was calculated by the following derivations of formulas:
⎛ V = 4Rr 2 ⎝
− 4
sin 2arcsin
4
1 − l2 /4r 2
arcsin −
1 − l2 /4r 2
2
⎞ ⎠ (7)
where V is the wear volume [mm3 ], r is the radium of the ball [mm], l is the wear scar width [m], R is radius of gyration [mm]. The weight variation and wear volume of different samples are shown in Table 4.
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Table 4 The weight variation and wear volume of different samples. Sample
TMP
TMPZr3
TMPZr6
TMPZr9
TMPZr12
Weight variation (g) Wear volume (mm3 )
0.0031 3.4166
0.0017 1.8694
0.0016 1.9680
0.0017 1.8119
0.0028 3.9542
The weight variation and wear volume are consistent. Sample TMP expresses the most weight variation and wear volume, meanwhile the samples of TMPZr3, TMPZr6 and TMPZr9 show least weight variation and wear volume. TMPZr12 gets more weight variation and wear volume than that of TMPZr3–9, but less than TMP. Zirconium dioxide has excellent wear resistance and corrosion resistance and so on [34]. The tribological characteristics of ZrTiO4 coatings prepared by magnetron sputtering have been reported [35]. Combining the analysis of the friction coefficient and phases, the ZrO2 presented in the coating play a role of particle strengthening. In the process of wear, the ZrO2 shedding from the coating can be automatically filled into the pores, play the role of self-repair, while increasing the contact area and reducing the unit area loads. The fall hard particles can play “micro-ball” friction-reducing effect on the surface friction. 4. Conclusions With the addition of ZrO2 particles in sodium phosphate electrolyte, the composite coating with ZrO2 and ZrTiO4 on Ti6Al4V can be prepared by micro-arc oxidation. Less cracks, less pores and thicker coating can be got compared with micro-arc oxidation directly on Ti alloy. Perfect oxidation resistance and wear resistance can be got by MAO treatment with the appropriate addition of ZrO2 (3 g/L, 6 g/L). Over ZrO2 particles addition in electrolyte will result in the increase of roughness. Acknowledgement This work was sponsored by the “Fundamental Research Funds for the Central Universities” (Grant number: 51271030). References [1] L. Chen, J. Paulitsch, Y. Du, P.H. Mayrhofer, Thermal stability and oxidation resistance of Ti–Al–N coatings, Surf. Coat. Technol. 206 (2012) 2954–2960. [2] X. Liu, H. Wang, Microstructure, wear and high-temperature oxidation resistance of laser clad Ti5Si3/␥/TiSi composite coatings on ␥-TiAl intermetallic alloy, Surf. Coat. Technol. 200 (2006) 4462–4470. [3] J. Sun, J.S. Wu, B. Zhao, F. Wang, Microstructure, wear and high temperature oxidation resistance of nitrided TiAl based alloys, Mater. Sci. Eng. A 329–331 (2002) 713–717. [4] C.H. Zhang, X.C. Lu, H. Wang, J.B. Luo, Y.G. Shen, K.Y. Li, Microstructure, mechanical properties, and oxidation resistance of nanocomposite Ti–Si–N coatings, Appl. Surf. Sci. 252 (2006) 6141–6153. ˇ ´ D. Kakaˇs, N. Bibic, M. Rakita, Microstructural studies of TiN coatings c, [5] B. Skori prepared by PVD and IBAD, Surf. Sci. 566–568 (Part 1) (2004) 40–44. [6] D. Kuo, K. Huang, Kinetics and microstructure of TiN coatings by CVD, Surf. Coat. Technol. 135 (2001) 150–157. [7] T. Kim, Y. Park, M. Wey, Characterization of Ti–6Al–4V alloy modified by plasma carburizing process, Mater. Sci. Eng. A 361 (2003) 275–280. [8] S. Tsunekawa, Y. Aoki, H. Habazaki, Two-step plasma electrolytic oxidation of Ti–15V–3Al–3Cr–3Sn for wear-resistant and adhesive coating, Surf. Coat. Technol. 205 (2011) 4732–4740. [9] W. Song, Y. Jun, Y. Han, S. Hong, Biomimetic apatite coatings on micro-arc oxidized titania, Biomaterials 25 (2004) 3341–3349. [10] X. Sun, Z. Jiang, S. Xin, Z. Yao, Composition and mechanical properties of hard ceramic coating containing ␣-Al2 O3 produced by microarc oxidation on Ti–6Al–4V alloy, Thin Solid Films 471 (2005) 194–199. [11] J.A. Curran, T.W. Clyne, Thermo-physical properties of plasma electrolytic oxide coatings on aluminium, Surf. Coat. Technol. 199 (2005) 168–176.
[12] J. Tian, Z. Luo, S. Qi, X. Sun, Structure and antiwear behavior of micro-arc oxidized coatings on aluminum alloy, Surf. Coat. Technol. 154 (2002) 1–7. [13] W. Xue, C. Wang, R. Chen, Z. Deng, Structure and properties characterization of ceramic coatings produced on Ti–6Al–4V alloy by microarc oxidation in aluminate solution, Mater. Lett. 52 (2002) 435–441. [14] Y. Yan, Y. Han, J. Huang, Formation of Al2 O3 –ZrO2 composite coating on zirconium by micro-arc oxidation, Scripta Mater. 59 (2008) 203–206. [15] Y. Yan, Y. Han, D. Li, J. Huang, Q. Lian, Effect of NaAlO2 concentrations on microstructure and corrosion resistance of Al2 O3 /ZrO2 coatings formed on zirconium by micro-arc oxidation, Appl. Surf. Sci. 256 (2010) 6359–6366. [16] W. Mu, Y. Han, Characterization and properties of the MgF2/ZrO2 composite coatings on magnesium prepared by micro-arc oxidation, Surf. Coat. Technol. 202 (2008) 4278–4284. [17] F. Samanipour, M.R. Bayati, H.R. Zargar, F. Golestani-Fard, T. Troczynski, M. Taheri, Electrophoretic enhanced micro arc oxidation of ZrO2 –HAp–TiO2 nanostructured porous layers, J. Alloy Compd. 509 (2011) 9351–9355. [18] V. Shoaei-Rad, M.R. Bayati, F. Golestani-Fard, H.R. Zargar, J. Javadpour, Fabrication of ZrO2 -Al2 O3 hybrid nano-porous layers through micro arc oxidation process, Mater. Lett. 65 (2011) 1835–1838. [19] Z. Yao, Y. Jiang, F. Jia, Z. Jiang, F. Wang, Growth characteristics of plasma electrolytic oxidation ceramic coatings on Ti–6Al–4V alloy, Appl. Surf. Sci. 254 (2008) 4084–4091. ˇ sovic, ´ S. Stojadinovic, ´ N.M. Siˇ ´ N. Konjevic, ´ Spectroscopic study of [20] J. Jovovic, plasma during electrolytic oxidation of magnesium- and aluminium-alloy, J. Quant. Spectrosc. Radiat. Transfer 113 (2012) 1928–1937. [21] R.O. Hussein, X. Nie, D.O. Northwood, A. Yerokhin, A. Matthews, Spectroscopic study of electrolytic plasma and discharging behaviour during the plasma electrolytic oxidation (PEO) process, J. Phys. D: Appl. Phys. 43 (2010) 105203. [22] X. Li, B.L. Luan, Discovery of Al2 O3 particles incorporation mechanism in plasma electrolytic oxidation of AM60B magnesium alloy, Mater. Lett. 86 (2012) 88–91. [23] J.M. Wheeler, C.A. Collier, J.M. Paillard, J.A. Curran, Evaluation of micromechanical behaviour of plasma electrolytic oxidation (PEO) coatings on Ti–6Al–4V, Surf. Coat. Technol. 204 (2010) 3399–3409. [24] E. Matykina, A. Berkani, P. Skeldon, G.E. Thompson, Real-time imaging of coating growth during plasma electrolytic oxidation of titanium, Electrochim. Acta 53 (2007) 1987–1994. [25] Y. Han, S. Hong, K.W. Xu, Porous nanocrystalline titania films by plasma electrolytic oxidation, Surf. Coat. Technol. 154 (2002) 314–318. [26] Y. Wang, B. Jiang, T. Lei, L. Guo, Dependence of growth features of microarc oxidation coatings of titanium alloy on control modes of alternate pulse, Mater. Lett. 58 (2004) 1907–1911. [27] G. Lv, W. Gu, H. Chen, W. Feng, M.L. Khosa, L. Li, E. Niu, G. Zhang, S. Yang, Characteristic of ceramic coatings on aluminum by plasma electrolytic oxidation in silicate and phosphate electrolyte, Appl. Surf. Sci. 253 (2006) 2947–2952. [28] 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. [29] Y.M. Wang, B.L. Jiang, L.X. Guo, T.Q. Lei, Tribological behavior of microarc oxidation coatings formed on titanium alloys against steel in dry and solid lubrication sliding, Appl. Surf. Sci. 252 (2006) 2989–2998. [30] J. Chen, R. Zeng, W. Huang, Z. Zheng, Z. Wang, J. Wang, Characterization and wear resistance of macro-arc oxidation coating on magnesium alloy AZ91 in simulated body fluids, T. Nonferr. Metal. Soc. 18 (Suppl. 1) (2008) s361–s364. [31] A.L. Yerokhin, A. Leyland, A. Matthews, Kinetic aspects of aluminium titanate layer formation on titanium alloys by plasma electrolytic oxidation, Appl. Surf. Sci. 200 (2002) 172–184. [32] L.R. Krishna, A.S. Purnima, G. Sundararajan, A comparative study of tribological behavior of microarc oxidation and hard-anodized coatings, Wear 261 (2006) 1095–1101. [33] X. Miao, D. Sun, P.W. Hoo, J. Liu, Y. Hu, Y. Chen, Effect of titania addition on yttria-stabilised tetragonal zirconia ceramics sintered at high temperatures, Ceram. Int. 30 (2004) 1041–1047. [34] Z. Yao, Y. Jiang, Z. Jiang, F. Wang, Z. Wu, Preparation and structure of ceramic coatings containing zirconium oxide on Ti alloy by plasma electrolytic oxidation, J. Mater. Process. Tech. 205 (2008) 303–307. [35] D.V. Shtansky, M.I. Petrzhik, I.A. Bashkova, F.V. Kiryukhantsev-Korneev, A.N. Sheve˘ıko, E.A. Levashov, Adhesion, friction, and deformation characteristics of Ti-(Ca, Zr)-(C, N, O, P) coatings for orthopedic and dental implants, Phys. Solid State 48 (2006) 1301–1308.