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Surface morphology, corrosion resistance and in vitro bioactivity of P containing ZrO2 films formed on Zr by plasma electrolytic oxidation
Journal of Alloys and Compounds 553 (2013) 324–332
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Surface morphology, corrosion resistance and in vitro bioactivity of P containing ZrO2 films formed on Zr by plasma electrolytic oxidation M. Sandhyarani a, N. Rameshbabu a,⇑, K. Venkateswarlu a,b, D. Sreekanth a, Ch. Subrahmanyam c a
Department of Metallurgical and Materials Engineering, National Institute of Technology, Tiruchirappalli 620 015, Tamilnadu, India Department of Physics, National Institute of Technology, Tiruchirappalli 620 015, Tamilnadu, India c Department of Chemistry, Indian Institute of Technology, Hyderabad 502 205, Andhra Pradesh, India b
a r t i c l e
i n f o
Article history: Received 25 September 2012 Received in revised form 13 November 2012 Accepted 23 November 2012 Available online 29 November 2012 Keywords: Zirconium Plasma electrolytic oxidation Thin films Simulated body fluid Corrosion Bioactivity
a b s t r a c t The present work was aimed at developing the corrosion resistant and bioactive oxide film on zirconium by plasma electrolytic oxidation in phosphate electrolyte. The effect of plasma electrolytic oxidation treatment time on surface morphology and corrosion resistance of the oxide films was further investigated. The phase composition, surface morphology, thickness and elemental composition of the oxide films were analyzed by X-ray diffraction and scanning electron microscopy equipped with energydispersive X-ray spectroscopy. The corrosion behavior of substrate and oxide films in simulated body fluid environment was studied by open circuit potential and potentiodynamic polarization tests. The apatite forming ability of the oxide film was evaluated after immersing in simulated body fluid for 14 days. X-ray diffraction patterns show that the oxide films predominantly comprised of monoclinic zirconia with a small amount of tetragonal zirconia. With prolonging treatment time, phase transformation of tetragonal to monoclinic zirconia was observed. Scanning electron microscopy results show that for a treatment time of 2–8 min, uniform and highly dense oxide films, thickness varying from 3 to 14 lm with no obvious pores were formed and the phosphorous content in the films was found to be in the range of 2.8–6.8 at.%. Corrosion test results reveal that all oxide films improved their corrosion resistance especially in terms of pitting potential and showed superior passivity in simulated body fluid environment. Bioactivity test results confirm that plasma electrolytic oxidation treated zirconium was fully covered by apatite layer in simulated body fluid medium. The incorporation of phosphorous in oxide film during coating process significantly enhanced the apatite forming ability of zirconium. In conclusion, among all the plasma electrolytic oxidation coated samples, the 6 min coated zirconium with high corrosion resistance and bioactivity is a potential candidate as orthopedic implants. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Owing to their excellent mechanical properties, high corrosion resistance and biocompatibility, zirconium (Zr) and its alloys are promising alternative materials in orthopedic and dental restoration fields [1]. Zr belongs to the same group (IVB) as that of titanium (Ti) in the periodic table and exhibits similar chemical and mechanical properties. In addition, Zr possesses unique properties such as low magnetic susceptibility (13.8 106 cm3 mol1, in oxide form) which makes it suitable for magnetic resonance imaging during orthopedic and brain surgery [2]. The Zr implants with low elastic modulus (92 GPa, much close to that of living bone) are envisaged to more evenly disperse mechanical loading than Ti, which in turn minimizes ‘‘stress shielding’’ of the host
⇑ Corresponding author. Tel.: +91 431 2503464; fax: +91 431 2500133. E-mail addresses: [email protected], [email protected] (N. Rameshbabu). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.11.147
bone, resulting in less bone atrophy and beneficial to bone healing and remodeling [3]. It is well known that the high corrosion resistance and the biocompatibility of Zr are attributed to the formation of a native oxide film on its surface. However, the native oxide film is very thin (at most several tens of nanometers) and can be lost soon when Zr is used in load bearing implant applications over a prolonged period of time. This results in an increase in corrosion rate and in turn reduces the efficiency and service time of the Zr implant. In addition, this native oxide film on Zr is stable against electrolytes and do not lead to apatite formation in body fluids [4]. This could be a disadvantage because an early integration between biomaterials and bone is desirable for most orthopedic and dental materials, such as stems of artificial joints and dental implants. Thus, there is a need to modify Zr surface to improve its corrosion resistance and bioactivity in body fluids environment. A few surface treatments, e.g., sol–gel deposition [5], plasma spray [6] and plasma electrolytic oxidation (PEO) [7] on Zr have been
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investigated for this purpose. Among these methods, PEO appears to be superior to others due to its simplicity of processing and easy production of porous and highly adherent oxide films on valve metals (such as Al, Ti, Ta, Mg and Zr) and its alloys [7–11]. PEO combines the chemical and morphological modification of valve metal surfaces in a single process step [12]. Further, the thickness and morphology of the oxide film formed during PEO can be controlled over a wide range by changing the electrolyte composition and PEO processing parameters (such as volt-ampere modes and treatment time) [13,14]. PEO coatings on Al, Ti, Mg and their alloys have been widely investigated, whereas surface modification of Zr and its alloys through PEO are still quite limited. Yan and his co-workers [15] have studied the bioactivity and osteoblast response of micro arc oxidized (MAO) Zr in a calcium based electrolyte. However, the formed oxide films in calcium electrolyte are highly porous, composed of cubic zirconia (c-ZrO2) phase and their corrosion resistance is unexplored. Further, Yan et al. [16] and Ha et al. [4] reported that PEO surface modification followed by acid/alkaline chemical treatment is an effective way to enhance the apatite formation on Zr. They found that the chemical treatments produced Zr–OH groups on the Zr surface and thus improved the bioactivity of the coated Zr. Han et al. [17] studied the bioactivity of ultraviolet (UV) irradiated PEO treated Zr. PEO treated Zr when photo excited by UV light of energy greater than band gap of Zr, resulted in abundant basic Zr–OH groups on the surface and exhibited an enhanced apatite formation during immersion in SBF solution. Although chemical treatment in acid/alkaline solutions and/or UV irradiation can enhance the apatite formation on PEO treated Zr by producing Zr–OH groups on Zr surface, the influence of chemical species present on the oxide film on Zr surface in inducing apatite formation is still unexplored. To the best of the author’s knowledge, the surface modification of Zr using PEO in phosphate electrolyte has not been investigated by other researchers and there is no report on corrosion resistance of PEO treated Zr in SBF environment. Thus the present study is focused on the formation of oxide film on Zr in phosphate electrolyte by varying the PEO treatment time, keeping the electrolytic concentration and current density constant. In addition, the effect of electrolyte borne element phosphorous (P) in oxide film on apatite forming ability of Zr is investigated. Further, an optimized treatment time is established for the formation of corrosion resistant and bioactive oxide film on Zr under physiological conditions for biomedical implant applications. 2. Experimental
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2.2. Phase analysis, morphology and elemental composition The phases present in the oxide films were characterized by a Rigaku Ultima III X-ray diffractometer (40 kV, 30 mA) with a Cu Ka radiation over a 2h range from 20° to 70° with a scan speed of 1° min1 and a step size of 0.05°. The formed phases were identified by matching relevant data from the JCPDS (Joint Committee on Powder Diffraction Standards) cards. From the X-ray diffraction (XRD) patterns, the volume ratio (Vt) of tetragonal zirconia (t-ZrO2) to monoclinic zirconia (m-ZrO2) phase in all the oxide films was calculated according to the formula [18]:
Vt ¼
Xt Cð1 X t Þ þ X t
where C = 1.381 for Cu Ka radiation,integral intensity ratio of the diffraction intensity (Xt) is
Xt ¼
Ið1 0 1Þt 1 1Þ þ Ið1 1 1Þ þ ð1 0 1Þ Ið1 m m
1 1Þ and I(1 1 1)m are specific peak intensities of (1 0 1) tetragonal where I(1 0 1)t, I(1 m 1 1Þ and (1 1 1) are monoclinic diffraction peaks, respectively. The diffraction peak, ð1 surface morphology, film thickness, elemental composition and elemental mapping of the oxide films were examined using scanning electron microscope (SEM, Hitachi – S3000N) equipped with energy-dispersive X-ray spectroscopy. 2.3. In-vitro tests 2.3.1. Electrochemical corrosion study To evaluate the in vitro corrosion properties of the substrate and the PEO fabricated ZrO2 films, Tafel and potentiodynamic polarization (PDP) tests were conducted under simulated body fluid (SBF) condition (7.4 pH and 37 °C). The SBF test medium was prepared following the procedure suggested by Kokubo et al. [19]. The Tafel and PDP plots were obtained using a computer controlled ACM Gill AC corrosion testing unit (ACM Instruments, Cumbria, UK). A three electrode cell, with sample as working electrode, saturated calomel electrode as reference electrode and platinum foil as counter electrode, was employed in the present study. During the test, the sample with an exposed area of 0.5 cm2 was kept in contact with the test solution. Prior to these tests all the samples were immersed in the test solution for 4 h to attain a stable open circuit electrode potential (OCP). The OCP measurements were conducted for every 5 min during the immersion time of 4 h. The corrosion current density and the corrosion rate of the samples were determined by Tafel extrapolation method by using the Tafel plots obtained, over a potential range of ±200 mV with reference to OCP employing a scan rate of 0.166 mV/s. The polarization resistance (Rp) of all the test samples is calculated using Stern–Geary equation [20]:
Rp ¼
b a bc 2:303 jcorr ðba þ bc Þ
where ba, bc are slopes of anodic and cathodic Tafel plots, respectively, and jcorr is the corrosion current density. From the obtained Rp values, the protection efficiency of all the oxide films was calculated according to the formula [21]:
Protection efficiency ð%Þ ¼
RP ðPEO treated ZrÞ Rp ðuntreated ZrÞ 100 Rp ðPEO treated ZrÞ
Further, the passivation behavior of all the samples in the SBF medium was studied over a potential range of 500 to 3000 mV by performing PDP test.
2.1. Film preparation Commercially available Zr (purity >99.5 wt.%) coupons with the dimensions of 20 mm 15 mm 1.5 mm were used. They were ground with abrasive papers, and then cleaned with acetone and deionized water in an ultrasonic bath prior to PEO treatment. Two stage cleaning is followed to ensure that the surface is clean before the PEO treatment. A DC power supply unit (Milman Thin Films Pvt. Ltd, Pune, India) with a maximum peak voltage of 900 V and a maximum output current of 15 A was employed to carry out the PEO process. The DC power supply unit was specially designed to deliver the stable current between 0.5 and 15 A. The Zr coupons were then treated in an aqueous electrolyte solution containing 5 g/L of tri-sodium ortho phosphate (TSOP; Na3PO412H2O, Merck India Pvt. Ltd) for 2, 4, 6 and 8 min, respectively, at a constant current of 1 A corresponding to a current density of about 150 mA/cm2 at the work piece. The applied frequency and duty cycle were 50 Hz and 95%, respectively. The electrolyte bath was water cooled during the process to control the bath temperature close to the room temperature thereby avoiding thermally driven growth process. The electrolyte solution was kept under continuous stirring during the process by a digital magnetic stirrer (Q 20A, REMI, India) to ensure uniform electrolyte concentration and dissipation of heat generated. The breakdown voltage was recorded by a careful observation of the appearance of the initial micro spark on the anodic surface. This was triplicated and the average value was reported in the present study. Similarly, the final voltages observed at the end of 2, 4, 6 and 8 min were also recorded. After PEO treatment, the treated samples were cleaned with deionized water and air dried at room temperature.
2.3.2. In-vitro bioactivity study The apatite forming ability of the substrate and PEO fabricated ZrO2 films were evaluated by immersing the samples in Kokubo SBF solution [19]. This solution has an ion concentration similar to human blood plasma and has been extensively used to evaluate the in vitro behavior of biomaterials [5,9]. The solution was buffered to the pH of 7.4 with tris (hydroxymethyl) amino methane and concentrated HCl at 36.5 °C. Each sample was immersed in a plastic vial containing 62 mL of SBF solution and was kept under static conditions inside a biological thermostat at 37 °C. The sample area (in mm2) to the solution volume (in ml) ratio was set equal to 10 [19]. The SBF solution was refreshed every 24 h, so that the lack of ions would not inhibit the apatite formation. After immersed for 14 days, the samples were taken out from the SBF, gently washed with deionized water and dried in an oven at 60 °C for 12 h.
3. Results and discussion 3.1. Voltage–time behavior during PEO process on Zr During the PEO treatment, the voltage variation Vs process time was recorded. The voltage–time response of Zr at 150 mA/cm2 for 8 min is shown in Fig. 1. At the initial stages during anodic
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Fig. 1. Voltage–time response during PEO process of Zr at 0.150 A/cm2 in 5 g/L Na3PO412H2O electrolyte for 8 min.
which is attesting to the fact that no further increase in anodic voltage is required to maintain the same current value. This may be attributed to the significant increase in the fraction of electronic current in the total current associated with the dielectric breakdown and the constant resistance of the film. At this state, strong but largely separated concentric discharges appear with increased size and duration. Further, discharges appear to remain at some locations for relatively long periods, while other regions of the sample were discharge free. The critical voltage during this treatment, beyond which the anodic voltage reaches a comparatively stable value, is 577 V which is very close to that reached at 6 min treatment time. The voltage reached at the end of 2, 4, 6 and 8 min in the present study are 559, 571, 576 and 577 V, respectively. The identification codes for the PEO treated Zr samples at different treatment time, their final voltages, relative amounts of formed phases and the respective film thickness values obtained were presented in Table 1. The PEO treated Zr samples were further referred to with these identification codes and the untreated Zr was referred to as ‘‘Z’’. 3.2. Phase structure of the PEO fabricated ZrO2 films
oxidation state, the voltage increases relatively rapidly at a rate of 45 V/s to approximately 455 V in about 10 s. This initial rapid rise in voltage is due to the development of an initial insulating oxide film at the interface of anode and electrolyte by conventional anodizing [7]. When the voltage reached 463 V (breakdown voltage), numerous micro-sparks are observed on the whole sample surface which indicates a start of the PEO process from where the chemical reactions between the components of plasma and the Zr substrate in a discharge channel take place. Based on the difference in the process voltage rising rate and sparking behavior, the whole PEO process in Fig. 1 is divided into three states namely the dynamic PEO (sparking) state, the near steady-state PEO (arcing) and the steady-state PEO (beyond critical voltage). In the first state, during dynamic PEO, the process voltage increases with average rise of about 1 V/s in the interval between 463 and 559 V. This state is characterized by numerous sparks moving rapidly over the whole sample surface area. The low rate of voltage rise compared to anodic oxidation state is due to the occurrence of small and dense micro sparks associated with the electron current. During anodic oxidation, the current passed through the grown oxide film could be represented by ionic current only, whereas in the first state of PEO, during dynamic state, both ionic species and electrons contribute to the current. Thus the total current is represented by the sum of the ionic current and the electronic current caused by the sparking [22]. Hence, a relatively low voltage is sufficient to maintain the same current value compared with the anodic oxidation stage. In the second state, during near steady-state PEO, the process voltage raises slowly with a rise of about 0.1 V/s in the interval between 559 and 571 V. The rate of voltage rise then reduces to 0.04 V/s in the interval between 571 and 576 V. At this state, a significant increase in intensity of micro-sparks that are moving rapidly over the surface is observed. In the third state, during steady state PEO, no further significant increase in voltage with respect to time is observed
The XRD patterns of the PEO fabricated ZrO2 films and untreated Zr is shown in Fig. 2. It can be seen from Fig. 2, the intensity of Zr (JCPDS card no. 05-0665) substrate peaks indexed by the letter ‘Z’ gradually decreases with an increasing PEO treatment time suggesting that the developed oxide films could prevent the incident X-ray beam from reaching the substrate. The decrease in intensity of the substrate diffraction peaks (indexed by letter ‘z’) from Z4 to Z6 can be attributed to increase in thickness of the oxide films grow for 2–8 min treatment time which is evidenced by the measured thickness values reported in Table 1 and interpreted in Section 3.3. In addition to the substrate diffraction peaks, all the oxide films show the presence of monoclinic zirconia (m-ZrO2, JCPDS card no. 37-1484) phase with a trace of tetragonal zirconia (t-ZrO2, JCPDS card no. 42-1164) phase. Further, intensity of t-ZrO2 peaks (indexed by letter ‘t’) gradually decreases with an increasing PEO treatment time. The vol.% of t-ZrO2 to m-ZrO2 phases in all PEO fabricated ZrO2 films is calculated and reported in Table 1. As the PEO treatment time increases from 2 to 6 min, the volume ratio of t-ZrO2 to m-ZrO2 gradually reduces from 0.07 to 0.025 and at 8 min, t-ZrO2 is completely transformed to m-ZrO2 with volume ratio 0.009. 3.2.1. Formation of the t-ZrO2 phase and its transformation to m-ZrO2 phase It is well known that the crystalline ZrO2 has three known polymorphs under atmospheric pressure, namely monoclinic (temperature below 1000 °C), tetragonal (temperature between 1000 and 1500 °C), and cubic (temperature above 1500 °C), in which the m-ZrO2 is thermodynamically the most stable phase at ambient temperature, and t-ZrO2 and c-ZrO2 phases are stable at high-temperatures [15]. These transformation temperatures are influenced by the dopant’s concentration in ZrO2. The formation of t-ZrO2 is due to the fact that the instant temperature of plasma electrolytic
Table 1 Process parameters for preparation of oxide films with their sample identification codes, final voltages, phase contents and their average film thicknesses. Sample code
PEO treating time (min)
Final voltage (V)
Film thickness (lm)
m-ZrO2 (vol.%)
t-ZrO2 (vol.%)
Z2 Z4 Z6 Z8
2 4 6 8
559 571 576 577
3 6 12 14
93 96 97 99
7 4 3 1
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Fig. 2. XRD patterns of (a) substrate, and PEO treated (b) Z2, (c) Z4, (d) Z6, (e) Z8 samples.
discharge region is up to 103–106 °C during PEO processing [23]. Further, the higher applied voltage (>550 V in 2 min treatment time) can yield higher localized temperature in the PEO discharge channels, enhancing the transformation of m-ZrO2 to t-ZrO2 because the latter is a high temperature phase. Thus all the PEO treated samples showed t-ZrO2 phase and the maximum vol.% of t-ZrO2 obtained is 7% at 2 min treatment time. However, further transformation is restricted and the vol.% of t-ZrO2 gradually reduces after 2 min. In general, the localized heating increases with increasing PEO treatment time, causes t-ZrO2 to sinter and form larger crystallites. Once a critical size is reached, transformation from tetragonal to monoclinic ZrO2 occurs as the crystallite size of the latter being larger than the tetragonal crystallite size [24]. Thus, m-ZrO2 is a major phase in all PEO fabricated ZrO2 films with a trace of t-ZrO2 under the present process conditions as shown in Fig. 2. However, such phase transformation, which results in existence of both types of ZrO2 phases, has a positive effect on the mechanical properties of the layers. It has been reported that zirconia-based materials exhibit exceptional toughness due to the martensitic transformations between t-ZrO2 and m-ZrO2 [25]. Thus, all the PEO fabricated ZrO2 films in the present study can be expected to exhibit good mechanical properties. 3.3. Surface morphology and quantitative analysis of the oxide films on Zr The surface morphology and corresponding elemental composition of the samples Z2, Z4, Z6 and Z8 obtained by scanning electron microscopy with energy-dispersive X-ray spectroscopy are presented in Fig. 3. All the samples present globally a smooth surface with very fine pore morphology in TSOP electrolyte which is singularly different to that observed elsewhere treated in other electrolytes by PEO [4,15–16]. The SEM micrographs (Fig. 3) clearly indicate the presence of fine discharge channels appearing circular spots distributed all over the surface of the oxide films. It is also apparent that the number of discharge channels decreases and discharge channel diameter increases with treatment time which can be attributed to different discharge characteristics followed during the film growth process as interpreted in Section 3.1. However, at higher voltages and treatment time, the morphology rapidly changes. When voltage reached critical voltage, discharge channel
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number decreases but their intensity increases resulting in nonuniform film structure (Fig. 3(d)). Further, they present a lot of internal damages, such as micro cracks begin to appear on the oxide film (shown by arrows) and localized delamination of the oxide film (shown by circle) is observed at some locations that are caused by thermal stress due to rapid solidification of molten oxide in the relatively cool electrolyte. Thus the PEO treatment time played a significant role on the surface characteristics of the samples. The film thickness of the samples Z2, Z4, Z6 and Z8 were measured using cross-sectional SEM micrographs and are reported in Table 1. At 2 min of treatment time, a highly dense and uniform film with 3 lm thickness is obtained. Further, the thickness increased with PEO treatment time, and the final thickness attained at the end of 4, 6 and 8 min of treatment time is 6, 12 and 14 lm, respectively. This increase in thickness is due to an increase in spark voltages at higher treatment times. A high spark voltage causes sparks with high level of energy which generates molten materials, so that it is easier for the high temperature molten materials to erupt and form a thick oxide film. The scanning electron microscopy with energy-dispersive X-ray spectroscopy analysis results show that all the films contain Zr, O and P elements. Among all the samples, the content of Zr is much higher (34.25 at.%) for Z2 sample (Fig. 3(a)). The higher amount of Zr in sample Z2 is attributed to the X-ray signals arising from both the coating and the substrate due to its low film thickness. All the oxide films maintained O/Zr atomic ratio between 1.85 and 2, which is very close to stoichiometric ZrO2. From Fig. 3, it is observed that the electrolyte borne element P is incorporated into oxide films during the film growth process and the content of P tends to increase with an increase in treatment time. This is in accordance with the reported fact that higher PEO treatment time results in large discharges which can sinter more elements arising from the electrolyte into the film [26]. Further, the distribution of Zr, O and P elements are shown by the elemental mapping result (Fig. 4) obtained for Z6 film for instance. The elemental mapping result shown in Fig. 4 suggested that P is uniformly distributed over the surface of the oxide film. Though the scanning electron microscopy with energy-dispersive X-ray spectroscopy analysis results show a significant amount of P in all oxide films, no separate phosphate phases are found in the XRD patterns (Fig. 2) indicating that P is doped into the ZrO2 film or the phosphate phase, if any, might be below the detection limit of X-ray diffractometer. 3.4. Electrochemical corrosion characteristics 3.4.1. Open circuit potential (OCP) analysis The thermodynamic tendency of implant material to undergo electrochemical reactions within the body fluid environment is essential in order to understand their stability in the human body. One simple way to study the film formation and the passivation of implants in a solution is to monitor the OCP as a function of time [27]. The obtained OCP curves for the untreated Zr and Z2, Z4, Z6 and Z8 samples tested for 4 h in SBF test medium are provided in Fig. 5. As can be seen from Fig. 5, the OCP values shifted to the noble direction for all the PEO fabricated ZrO2 films compared to untreated Zr which indicates their thermodynamic stability in taking part in electrochemical reactions. All the PEO fabricated ZrO2 films showed small instability in the initial 60 min of immersion time irrespective of their PEO treatment time, and then maintained almost stable OCP values up to 4 h. The instability in the initial period (10 min to 1 h) of all the oxide films can be attributed to the increased activity [28] due to the defective nature of the newly formed oxide film at the metal/oxide interface, and the stable OCP values attained at long exposure times (1 h to 4 h) might be attributed to the stable nature exhibited by the formed oxide films. The OCP values shifted to noble direction as PEO
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Fig. 3. The surface SEM micrographs and scanning electron microscopy with energy-dispersive X-ray dispersive spectroscopy analysis of (a) Z2, (b) Z4, (c) Z6, (d) Z8 samples.
treatment time increases from 2 to 6 min. In contrast, 8 min treated Zr showed lower OCP value compared to 6 min treated Zr, this can be due to the presence of surface cracks (Fig. 3(d)) on Z8 film which makes the surface more active towards dissolution in corrosive medium compared to Z6 film. Thus from the OCP results, the thermodynamic tendency of PEO coated samples towards the electrochemical oxidation reaction decreases in the
order of Z > Z2 > Z4 > Z8 > Z6 and among all the samples, Z6 has shown more noble behavior towards the anodic reactions. 3.4.2. Tafel extrapolation method The Tafel plots obtained over a potential range of ±200 mV with reference to the stable OCP are shown in Fig. 6 and the obtained kinetic parameter values are reported in Table 2. After PEO
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Fig. 4. The elemental mapping micrographs with a scanning electron microscopy with energy-dispersive X-ray dispersive spectroscopy of Z6 film.
Rp increases from 409 kX cm2 to 21,600 kX cm2 depicting the highest corrosion resistance for 6 min treated Zr. Further, Rp of Z6 sample (21,600 kX cm2) is two orders of magnitude higher that of untreated Zr (274 kX cm2). This increase in Rp with treatment time is due to the increase in film thickness from 3 to 12 lm (Table 1). However, when the treatment time increases to 8 min, Rp value decreases to 523 kX cm2. The decrease in Rp for Z8 sample is attributed to the presence of cracks and the defective film formation (Fig. 3(d)). Thus the highly corrosive Cl ions that are present in the SBF solution can easily enter into the oxide film along the cracks, thereby reducing the corrosion resistance of the Z8 sample. The protection efficiency of Z2, Z4, Z6 and Z8 calculated from Tafel data are found to be 33%, 96%, 99% and 48%, respectively. Thus by considering the jcorr, Rp and PE of fabricated ZrO2 films, the Z6 film shows highest corrosion resistance over a potential range of ±200 mV with reference to the stable OCP.
Fig. 5. Open circuit potential curves of Z, Z2, Z4, Z6 and Z8 samples for an immersion period of 4 h in 7.4 pH SBF medium.
Fig. 6. Tafel polarization curves of Z, Z2, Z4, Z6 and Z8 samples in 7.4 pH SBF medium.
treatment, the corrosion potential (Ecorr) of the Zr samples increases and their jcorr significantly reduce than that of the untreated Zr. From the reported values in Table 2, it can be seen that Rp of untreated Zr is 274 KX cm2. Among the PEO fabricated ZrO2 films, as the treatment time increases from 2 min to 6 min,
3.4.3. Potentiodynamic polarization (PDP) studies The passivation behavior of the untreated Zr and the PEO fabricated ZrO2 films were studied by conducting PDP test. The PDP curves of the Z, Z2, Z4, Z6 and Z8 samples are displayed in Fig. 7 and their pitting potential values are reported in Table 2. As shown in Fig. 7, the untreated Zr shows an active state in the beginning and then pitting corrosion happens immediately. The pitting potential (Epitt) of untreated Zr is 341.54 mV with the protection interval, defined as the difference Epitt Ecorr is 833.52 mV and when the polarization potential is over 757.23 mV, passivation at the surface takes place. Among the PEO fabricated ZrO2 films, Z2, Z4 and Z6 films showed excellent pitting corrosion resistance over a potential range of 500 to 3000 mV. Additionally, the PEO fabricated ZrO2 films show higher corrosion resistance than untreated Zr, which means that the ZrO2 films are less susceptible to corrosion in the SBF environment. However, when the treatment time increased to 8 min, the corrosion resistance decreases and Z8 film shows a similar trend with the untreated Zr. It can be noticed from Table 2 and Fig. 7 that the Z8 film shows a passivation stage in the beginning and undergoes pitting corrosion at 692.509 mV with protection interval (Epitt Ecorr) of 839.485 mV. When the polarization potential is over 730.759 mV, the repassivation takes place at the film/Zr substrate interface under the cracked oxide film in case of Z8 sample. Although, Z8 sample undergoes pitting attack similar to untreated Zr, its Epitt value improves about 351 mV compared to the untreated Zr. Thus by considering the Epitt, Epitt Ecorr and the repassivation potential of the untreated Zr and PEO fabricated ZrO2 films, the corrosion resistance properties of all the samples can be ranked as: Z6 > Z4 > Z2 > Z8 > Z and the Z6 sample treated at 6 min shows the highest corrosion resistance without pitting attack over a potential range of 500 to 3000 mV in 7.4 pH SBF environment. This improvement on the corrosion resistance of the Z6 sample is strongly dependent on the microstructure of the oxide film. Denser and thicker films are helpful to improve the corrosion resistance by
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Table 2 The OCP and kinetic parameter values of Z, Z2, Z4, Z6 and Z8 samples. Sample code
ba (mV/dec)
|bc| (mV/dec)
Ecorr (mV)
jcorr (lA/cm2)
Rp (kX.cm2)
Epitt (mV)
Epitt Ecorr (mV)
PE (%)
Z Z2 Z4 Z6 Z8
584.78 838.12 1006.84 816.40 995.25
308.95 238.35 244.73 213.39 186.30
491.98 231.87 200.01 65.87 142.32
0.32 0.197 0.012 0.0034 0.139
274 409 7180 21,600 490
341.54 – – – 692.51
833.52 – – – 839.48
– 33 96 99 48
inhibiting the attack from Cl ions in the 7.4 pH SBF solution. According to the surface (Fig. 3(c)) and cross-sectional SEM micrograph results, a 12 lm thick and uniform oxide film with no obvious pores is formed on the surface of Z6 sample. As a consequence, the oxide film on Z6 shows highest protection efficiency against the corrosive ions present in physiological environment. However, for the treatment time of 8 min, even though the thickness of the film increased to 14 lm, the presence of cracks and localized delamination on the surface of the oxide film reduces the protection efficiency against the corrosive ions present in SBF solution. 3.5. Bioactivity response of the PEO fabricated ZrO2 film
Fig. 7. Potentiodynamic polarization curves of Z, Z2, Z4, Z6 and Z8 samples in 7.4 pH SBF medium.
As Z6 film has shown higher corrosion resistance among all the PEO fabricated ZrO2 films, it was chosen for the bioactivity test and is compared with the bioactivity of untreated Zr. Scanning electron microscopy with energy-dispersive X-ray spectroscopy analysis is undertaken for Z6 film and untreated Zr to check the mineralization of apatite layer after immersion in SBF solution. The surface SEM micrographs and scanning electron microscopy with energydispersive X-ray spectroscopy analysis micrographs of Z6 film and untreated Zr after 14 days of immersion in SBF solution were
Fig. 8. The surface SEM micrographs and scanning electron microscopy with energy-dispersive X-ray dispersive spectroscopy analysis of (a) substrate and (b) PEO treated Z6 film after immersion in SBF for 14 days.
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shown in Fig. 8. It can be seen from Fig. 8, spherical like deposits are observed on Z6 film and the entire surface is covered by a precipitated layer indicating that the PEO treated Zr shows good bioactivity. Whereas a few spherical like deposits are observed on untreated Zr exhibiting its low bioactivity. Further, the scanning electron microscopy with energy-dispersive X-ray spectroscopy analysis micrographs results for the Z6 film after immersion in SBF shows the presence of Ca and P elements, while these signals are very weak in untreated Zr. The appearance of strong signals for Ca, Mg, Na, K and P and the decrease in the at.% for Zr (Fig. 8(b)) indicating the apatite formed on the Z6 film is thicker with approximate Ca/P ratio of 1.64. It was reported in literature [29] that the biomimetic apatites possess relatively low Ca/P ratio due to the deficiency of Ca2+ ions in the apatite crystal as the Ca2+ ions could be substituted by K+, Na+ and Mg2+ ions that are present in SBF solution. Surface SEM micrographs of untreated Zr (Fig. 8(a)) demonstrate that there is scarcely any apatite formed on untreated Zr indicating its poor capability of the apatite mineralization. The reason for the enhanced apatite forming ability of Z6 over untreated Zr is thought to be related to the presence of P species distributed uniformly on the surface of oxide film (Fig. 4). Thus, after immersing in SBF, the P species from the surface layers of Z6 film can effectively increase the ionic activity of the surrounding fluid and the phosphate ions formed on the surface of the film can selectively combine with the positively charged Ca2+ ions by electrostatic attraction in the SBF solution. As the Ca2+ ion concentration increases, the surface gets positively charged and thus combines with PO3 ions in the SBF solution. 4 This results in a higher super saturation of Ca and P at the interface of specimen and SBF solution. This enhancement of the super saturation at the interface induces the nucleation of apatite. Once the apatite nuclei are formed, they spontaneously grow by consuming more Ca2+ and PO3 ions from SBF solution. 4 As a result, apatite deposit increases on the Z6 film surface. Thus, the presence of P species in Z6 film has a significant effect on the initiation and the enhancement of the apatite formation on its surface.
4. Conclusions In summary, P incorporated oxide films were prepared on Zr using PEO technique in phosphate electrolyte. At a current density of 150 mA/cm2, uniform and highly dense oxide films with thickness ranging from 3 to 14 lm are formed on Zr at 2–8 min of treatment time. All the oxide films predominantly comprised m-ZrO2 with a small amount of t-ZrO2 and with increasing treatment time phase transformation from t-ZrO2 to m-ZrO2 takes place. The OCP of all oxide films shifted towards noble directions that are accompanied with substantial increase in Ecorr and decrease in jcorr compared to that of untreated Zr. The PDP results showed that no pitting corrosion is observed for Z2, Z4 and Z6 samples. However, the corrosion resistance of Z8 sample is significantly decreased and pitting corrosion is observed at 692 mV due to the presence of cracks over its surface. Thus, among all films under study, the oxide film grown at 6 min treatment time is found to be optimized one with its highest Rp (21,600 kX cm2) and provides beneficial protection against pitting in 7.4 pH SBF environment. Ca–P rich compounds are detected on Z6 film during immersion in SBF solution for 14 days, while very weak signals of such compounds are found on the untreated Zr. The presence of P species in oxide film greatly enhanced the bioactivity of Zr in 7.4 pH SBF solution thereby avoiding secondary treatments after PEO. Thus 6 min PEO treated Zr in phosphate electrolyte with high corrosion resistance and bioactivity is envisaged to be a promising material as orthopedic implants.
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Acknowledgements The authors would like to acknowledge the facilities procured through the grants received from the Department of Biotechnology, New Delhi (BT/PR-11731/MED/32/99/2008, dated 19-082009), and the Department of Science and Technology, New Delhi (SR/FTP/ETA-040/2009) for carrying out this research investigation. References [1] J. Chevalier, What future for zirconia as a biomaterial, Biomaterials 27 (2006) 535–543. [2] Suyalatu, R. Kondo, Y. Tsutsumi, H. Doi, N. Nomura, T. Hanawa, Effects of phase constitution on magnetic susceptibility and mechanical properties of Zr-rich Zr–Mo alloys, Acta Biomater. 7 (2011) 4259–4266. [3] D.W. Peterson, J.E. Lemons, L.C. Lucas, Comparative evaluations of surface characteristics of Cp–Ti, Ti–6Al–4V and Ti–15Mo–2.8Nb–0.2Si (Timetal 21SRx), J. ASTM Int. 2 (2005) 9. [4] J.Y. Ha, Y. Tsutsumi, H. Doi, N. Nomura, K.H. Kim, T. Hanawa, Enhancement of CaP formation on Zr by MAO and chemical treatments, Surf. Coat. Technol. 205 (2011) 4948–4955. [5] M. Uchida, H.M. Kim, T. Kokubo, K. Tanaka, T. Nakamura, Structural dependence of apatite formation on zirconia gels in a simulated body fluid, J. Ceram. Soc. Jpn. 110 (2002) 710–715. [6] G. Wang, X. Liu, C. Ding, Phase composition and in-vitro bioactivity of plasma sprayed calca stabilized zirconia coatings, Surf. Coat. Technol. 202 (2008) 5824–5831. [7] T. Pauporte, J. Finne, A. Kahn-Harari, D. Lincot, Growth by plasma electrolysis of ZrO2 films in the micrometer range, Surf. Coat. Technol. 199 (2005) 213– 219. [8] Z. Wang, L. Wu, W. Cai, A Shan, Z Jiang, Effects of fluoride on the structure and properties of microarc oxidation coating on aluminium alloy, J. Alloys Comp. 505 (2010) 188–193. [9] K. Venkateswarlu, N. Rameshbabu, A.C. Bose, V. Muthupandi, S. Subramanian, D. Mubarak Ali, N. Thajuddin, Fabrication of corrosion resistant, bioactive and antibacterial silver substituted hydroxyapatite/titania composite coating on Cp Ti, Ceram. Int. 38 (2012) 731–740. [10] S. Stojadinovic, J. Jovonic, M. Petkovic, R. Vasilic, N. Konjevic, Spectroscopic and real-time imaging investigation of tantalum plasma electrolytic oxidation, Surf. Coat. Technol. 205 (2011) 5406–5413. [11] D. Sreekanth, N. Rameshbabu, Development and characterization of MgO/ hydroxyapatite composite coating on AZ31 magnesium alloy by plasma electrolytic oxidation coupled with electrophoretic deposition, Mater. Lett. 68 (2012) 439–442. [12] K.C. Kung, T.M. Lee, T.S. Lui, Bioactivity and corrosion properties of novel coatings containing strontium by micro-arc oxidation, J. Alloys Comp. 508 (2010) 384–390. [13] L.R. Krishna, G. Poshal, G. Sundararajan, Influence of electrolyte chemistry on morphology and corrosion resistance of micro arc oxidation coatings deposited on magnesium, Metall. Mater. Trans. A 41 (2010) 3499–3508. [14] A.L. Yerokhin, X. Nie, A. Leyland, Plasma electrolysis for surface engineering, Surf. Coat. Technol. 122 (1999) 73–93. [15] Y. Han, Y. Yan, C. Lu, Y. Zhang, K. Xu, Bioactivity and osteoblast response of the MAO zirconia films, J. Biomed. Mater. Res. A 88 (2008) 117–127. [16] Y. Yan, Y. Han, C. Lu, The effect of chemical treatment on apatite forming ability of the macroporous zirconia films formed by MAO, Appl. Surf. Sci. 254 (2008) 4833–4839. [17] Y. Han, Y. Yan, C. Lu, Ultraviolet-enhanced bioactivity of ZrO2 films prepared by MAO, Thin Solid Films 517 (2009) 1577–1581. [18] J. Li, X. Bai, D. Zhang, H. Li, Characterization and structure study of the anodic oxide film on Zircaloy-4 synthesized using NaOH electrolytes at room temperature, Appl. Surf. Sci. 252 (2006) 7436–7441. [19] T. Kokubo, H. Takadamo, How useful is SBF in prediction in vivo bone bioactivity, Biomaterials 27 (2006) 2907–2915. [20] K. Venkateswarlu, N. Rameshbabu, D. Sreekanth, A.C. Bose, V. Muthupandi, S. Subramanian, Fabrication and characterization of micro arc oxidized fluoride containing titania films on Cp Ti, Ceram. Int. 39 (2013) 801–812. [21] A. Madhankumar, N. Rajendran, A promising copolymer of p-phenylendiamine and O-aminophenol: chemical and electrochemical synthesis, characterization and its corrosion protection aspect on mild steel, Synth. Met. 162 (2012) 176– 185. [22] K. Venkateswarlu, N. Rameshbabu, D. Sreekanth, A.C. Bose, V. Muthupandi, N.K. Babu, S. Subramanian, Role of electrolyte additives on in-vitro electrochemical behaviour of micro arc oxidized titania films on Cp Ti, Appl. Surf. Sci. 258 (2012) 6853–6863. [23] M.D. Klapkiv, O.S. Chunhmarev, P.Y. Sydor, V.M. Posuvailo, Thermodynamics of the interaction of Al, Mg, and Zr with components of an electrolytic plasma, Mater. Sci. 36 (2000) 66–79. [24] P.D.L. Mercera, J.G.V. Ommen, E.B.M. Doesburg, A.J. Burggraaf, J.R.H. Ross, Zirconia as a support for catalysts: evolution of the texture and structure on calcinations in air, Appl. Catal. 57 (1990) 127–148. [25] A.R. Kmita, A. Slosarczyk, Z. Paszkiewicz, Mechanical properties of HAp–ZrO2 composites, J. Eur. Ceram. Soc. 26 (2006) 1481–1488.
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Surface morphology, corrosion resistance and in vitro bioactivity of P containing ZrO2 films formed on Zr by plasma electrolytic oxidation M. Sandhyarani a, N. Rameshbabu a,⇑, K. Venkateswarlu a,b, D. Sreekanth a, Ch. Subrahmanyam c a
Department of Metallurgical and Materials Engineering, National Institute of Technology, Tiruchirappalli 620 015, Tamilnadu, India Department of Physics, National Institute of Technology, Tiruchirappalli 620 015, Tamilnadu, India c Department of Chemistry, Indian Institute of Technology, Hyderabad 502 205, Andhra Pradesh, India b
a r t i c l e
i n f o
Article history: Received 25 September 2012 Received in revised form 13 November 2012 Accepted 23 November 2012 Available online 29 November 2012 Keywords: Zirconium Plasma electrolytic oxidation Thin films Simulated body fluid Corrosion Bioactivity
a b s t r a c t The present work was aimed at developing the corrosion resistant and bioactive oxide film on zirconium by plasma electrolytic oxidation in phosphate electrolyte. The effect of plasma electrolytic oxidation treatment time on surface morphology and corrosion resistance of the oxide films was further investigated. The phase composition, surface morphology, thickness and elemental composition of the oxide films were analyzed by X-ray diffraction and scanning electron microscopy equipped with energydispersive X-ray spectroscopy. The corrosion behavior of substrate and oxide films in simulated body fluid environment was studied by open circuit potential and potentiodynamic polarization tests. The apatite forming ability of the oxide film was evaluated after immersing in simulated body fluid for 14 days. X-ray diffraction patterns show that the oxide films predominantly comprised of monoclinic zirconia with a small amount of tetragonal zirconia. With prolonging treatment time, phase transformation of tetragonal to monoclinic zirconia was observed. Scanning electron microscopy results show that for a treatment time of 2–8 min, uniform and highly dense oxide films, thickness varying from 3 to 14 lm with no obvious pores were formed and the phosphorous content in the films was found to be in the range of 2.8–6.8 at.%. Corrosion test results reveal that all oxide films improved their corrosion resistance especially in terms of pitting potential and showed superior passivity in simulated body fluid environment. Bioactivity test results confirm that plasma electrolytic oxidation treated zirconium was fully covered by apatite layer in simulated body fluid medium. The incorporation of phosphorous in oxide film during coating process significantly enhanced the apatite forming ability of zirconium. In conclusion, among all the plasma electrolytic oxidation coated samples, the 6 min coated zirconium with high corrosion resistance and bioactivity is a potential candidate as orthopedic implants. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Owing to their excellent mechanical properties, high corrosion resistance and biocompatibility, zirconium (Zr) and its alloys are promising alternative materials in orthopedic and dental restoration fields [1]. Zr belongs to the same group (IVB) as that of titanium (Ti) in the periodic table and exhibits similar chemical and mechanical properties. In addition, Zr possesses unique properties such as low magnetic susceptibility (13.8 106 cm3 mol1, in oxide form) which makes it suitable for magnetic resonance imaging during orthopedic and brain surgery [2]. The Zr implants with low elastic modulus (92 GPa, much close to that of living bone) are envisaged to more evenly disperse mechanical loading than Ti, which in turn minimizes ‘‘stress shielding’’ of the host
⇑ Corresponding author. Tel.: +91 431 2503464; fax: +91 431 2500133. E-mail addresses: [email protected], [email protected] (N. Rameshbabu). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.11.147
bone, resulting in less bone atrophy and beneficial to bone healing and remodeling [3]. It is well known that the high corrosion resistance and the biocompatibility of Zr are attributed to the formation of a native oxide film on its surface. However, the native oxide film is very thin (at most several tens of nanometers) and can be lost soon when Zr is used in load bearing implant applications over a prolonged period of time. This results in an increase in corrosion rate and in turn reduces the efficiency and service time of the Zr implant. In addition, this native oxide film on Zr is stable against electrolytes and do not lead to apatite formation in body fluids [4]. This could be a disadvantage because an early integration between biomaterials and bone is desirable for most orthopedic and dental materials, such as stems of artificial joints and dental implants. Thus, there is a need to modify Zr surface to improve its corrosion resistance and bioactivity in body fluids environment. A few surface treatments, e.g., sol–gel deposition [5], plasma spray [6] and plasma electrolytic oxidation (PEO) [7] on Zr have been
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investigated for this purpose. Among these methods, PEO appears to be superior to others due to its simplicity of processing and easy production of porous and highly adherent oxide films on valve metals (such as Al, Ti, Ta, Mg and Zr) and its alloys [7–11]. PEO combines the chemical and morphological modification of valve metal surfaces in a single process step [12]. Further, the thickness and morphology of the oxide film formed during PEO can be controlled over a wide range by changing the electrolyte composition and PEO processing parameters (such as volt-ampere modes and treatment time) [13,14]. PEO coatings on Al, Ti, Mg and their alloys have been widely investigated, whereas surface modification of Zr and its alloys through PEO are still quite limited. Yan and his co-workers [15] have studied the bioactivity and osteoblast response of micro arc oxidized (MAO) Zr in a calcium based electrolyte. However, the formed oxide films in calcium electrolyte are highly porous, composed of cubic zirconia (c-ZrO2) phase and their corrosion resistance is unexplored. Further, Yan et al. [16] and Ha et al. [4] reported that PEO surface modification followed by acid/alkaline chemical treatment is an effective way to enhance the apatite formation on Zr. They found that the chemical treatments produced Zr–OH groups on the Zr surface and thus improved the bioactivity of the coated Zr. Han et al. [17] studied the bioactivity of ultraviolet (UV) irradiated PEO treated Zr. PEO treated Zr when photo excited by UV light of energy greater than band gap of Zr, resulted in abundant basic Zr–OH groups on the surface and exhibited an enhanced apatite formation during immersion in SBF solution. Although chemical treatment in acid/alkaline solutions and/or UV irradiation can enhance the apatite formation on PEO treated Zr by producing Zr–OH groups on Zr surface, the influence of chemical species present on the oxide film on Zr surface in inducing apatite formation is still unexplored. To the best of the author’s knowledge, the surface modification of Zr using PEO in phosphate electrolyte has not been investigated by other researchers and there is no report on corrosion resistance of PEO treated Zr in SBF environment. Thus the present study is focused on the formation of oxide film on Zr in phosphate electrolyte by varying the PEO treatment time, keeping the electrolytic concentration and current density constant. In addition, the effect of electrolyte borne element phosphorous (P) in oxide film on apatite forming ability of Zr is investigated. Further, an optimized treatment time is established for the formation of corrosion resistant and bioactive oxide film on Zr under physiological conditions for biomedical implant applications. 2. Experimental
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2.2. Phase analysis, morphology and elemental composition The phases present in the oxide films were characterized by a Rigaku Ultima III X-ray diffractometer (40 kV, 30 mA) with a Cu Ka radiation over a 2h range from 20° to 70° with a scan speed of 1° min1 and a step size of 0.05°. The formed phases were identified by matching relevant data from the JCPDS (Joint Committee on Powder Diffraction Standards) cards. From the X-ray diffraction (XRD) patterns, the volume ratio (Vt) of tetragonal zirconia (t-ZrO2) to monoclinic zirconia (m-ZrO2) phase in all the oxide films was calculated according to the formula [18]:
Vt ¼
Xt Cð1 X t Þ þ X t
where C = 1.381 for Cu Ka radiation,integral intensity ratio of the diffraction intensity (Xt) is
Xt ¼
Ið1 0 1Þt 1 1Þ þ Ið1 1 1Þ þ ð1 0 1Þ Ið1 m m
1 1Þ and I(1 1 1)m are specific peak intensities of (1 0 1) tetragonal where I(1 0 1)t, I(1 m 1 1Þ and (1 1 1) are monoclinic diffraction peaks, respectively. The diffraction peak, ð1 surface morphology, film thickness, elemental composition and elemental mapping of the oxide films were examined using scanning electron microscope (SEM, Hitachi – S3000N) equipped with energy-dispersive X-ray spectroscopy. 2.3. In-vitro tests 2.3.1. Electrochemical corrosion study To evaluate the in vitro corrosion properties of the substrate and the PEO fabricated ZrO2 films, Tafel and potentiodynamic polarization (PDP) tests were conducted under simulated body fluid (SBF) condition (7.4 pH and 37 °C). The SBF test medium was prepared following the procedure suggested by Kokubo et al. [19]. The Tafel and PDP plots were obtained using a computer controlled ACM Gill AC corrosion testing unit (ACM Instruments, Cumbria, UK). A three electrode cell, with sample as working electrode, saturated calomel electrode as reference electrode and platinum foil as counter electrode, was employed in the present study. During the test, the sample with an exposed area of 0.5 cm2 was kept in contact with the test solution. Prior to these tests all the samples were immersed in the test solution for 4 h to attain a stable open circuit electrode potential (OCP). The OCP measurements were conducted for every 5 min during the immersion time of 4 h. The corrosion current density and the corrosion rate of the samples were determined by Tafel extrapolation method by using the Tafel plots obtained, over a potential range of ±200 mV with reference to OCP employing a scan rate of 0.166 mV/s. The polarization resistance (Rp) of all the test samples is calculated using Stern–Geary equation [20]:
Rp ¼
b a bc 2:303 jcorr ðba þ bc Þ
where ba, bc are slopes of anodic and cathodic Tafel plots, respectively, and jcorr is the corrosion current density. From the obtained Rp values, the protection efficiency of all the oxide films was calculated according to the formula [21]:
Protection efficiency ð%Þ ¼
RP ðPEO treated ZrÞ Rp ðuntreated ZrÞ 100 Rp ðPEO treated ZrÞ
Further, the passivation behavior of all the samples in the SBF medium was studied over a potential range of 500 to 3000 mV by performing PDP test.
2.1. Film preparation Commercially available Zr (purity >99.5 wt.%) coupons with the dimensions of 20 mm 15 mm 1.5 mm were used. They were ground with abrasive papers, and then cleaned with acetone and deionized water in an ultrasonic bath prior to PEO treatment. Two stage cleaning is followed to ensure that the surface is clean before the PEO treatment. A DC power supply unit (Milman Thin Films Pvt. Ltd, Pune, India) with a maximum peak voltage of 900 V and a maximum output current of 15 A was employed to carry out the PEO process. The DC power supply unit was specially designed to deliver the stable current between 0.5 and 15 A. The Zr coupons were then treated in an aqueous electrolyte solution containing 5 g/L of tri-sodium ortho phosphate (TSOP; Na3PO412H2O, Merck India Pvt. Ltd) for 2, 4, 6 and 8 min, respectively, at a constant current of 1 A corresponding to a current density of about 150 mA/cm2 at the work piece. The applied frequency and duty cycle were 50 Hz and 95%, respectively. The electrolyte bath was water cooled during the process to control the bath temperature close to the room temperature thereby avoiding thermally driven growth process. The electrolyte solution was kept under continuous stirring during the process by a digital magnetic stirrer (Q 20A, REMI, India) to ensure uniform electrolyte concentration and dissipation of heat generated. The breakdown voltage was recorded by a careful observation of the appearance of the initial micro spark on the anodic surface. This was triplicated and the average value was reported in the present study. Similarly, the final voltages observed at the end of 2, 4, 6 and 8 min were also recorded. After PEO treatment, the treated samples were cleaned with deionized water and air dried at room temperature.
2.3.2. In-vitro bioactivity study The apatite forming ability of the substrate and PEO fabricated ZrO2 films were evaluated by immersing the samples in Kokubo SBF solution [19]. This solution has an ion concentration similar to human blood plasma and has been extensively used to evaluate the in vitro behavior of biomaterials [5,9]. The solution was buffered to the pH of 7.4 with tris (hydroxymethyl) amino methane and concentrated HCl at 36.5 °C. Each sample was immersed in a plastic vial containing 62 mL of SBF solution and was kept under static conditions inside a biological thermostat at 37 °C. The sample area (in mm2) to the solution volume (in ml) ratio was set equal to 10 [19]. The SBF solution was refreshed every 24 h, so that the lack of ions would not inhibit the apatite formation. After immersed for 14 days, the samples were taken out from the SBF, gently washed with deionized water and dried in an oven at 60 °C for 12 h.
3. Results and discussion 3.1. Voltage–time behavior during PEO process on Zr During the PEO treatment, the voltage variation Vs process time was recorded. The voltage–time response of Zr at 150 mA/cm2 for 8 min is shown in Fig. 1. At the initial stages during anodic
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Fig. 1. Voltage–time response during PEO process of Zr at 0.150 A/cm2 in 5 g/L Na3PO412H2O electrolyte for 8 min.
which is attesting to the fact that no further increase in anodic voltage is required to maintain the same current value. This may be attributed to the significant increase in the fraction of electronic current in the total current associated with the dielectric breakdown and the constant resistance of the film. At this state, strong but largely separated concentric discharges appear with increased size and duration. Further, discharges appear to remain at some locations for relatively long periods, while other regions of the sample were discharge free. The critical voltage during this treatment, beyond which the anodic voltage reaches a comparatively stable value, is 577 V which is very close to that reached at 6 min treatment time. The voltage reached at the end of 2, 4, 6 and 8 min in the present study are 559, 571, 576 and 577 V, respectively. The identification codes for the PEO treated Zr samples at different treatment time, their final voltages, relative amounts of formed phases and the respective film thickness values obtained were presented in Table 1. The PEO treated Zr samples were further referred to with these identification codes and the untreated Zr was referred to as ‘‘Z’’. 3.2. Phase structure of the PEO fabricated ZrO2 films
oxidation state, the voltage increases relatively rapidly at a rate of 45 V/s to approximately 455 V in about 10 s. This initial rapid rise in voltage is due to the development of an initial insulating oxide film at the interface of anode and electrolyte by conventional anodizing [7]. When the voltage reached 463 V (breakdown voltage), numerous micro-sparks are observed on the whole sample surface which indicates a start of the PEO process from where the chemical reactions between the components of plasma and the Zr substrate in a discharge channel take place. Based on the difference in the process voltage rising rate and sparking behavior, the whole PEO process in Fig. 1 is divided into three states namely the dynamic PEO (sparking) state, the near steady-state PEO (arcing) and the steady-state PEO (beyond critical voltage). In the first state, during dynamic PEO, the process voltage increases with average rise of about 1 V/s in the interval between 463 and 559 V. This state is characterized by numerous sparks moving rapidly over the whole sample surface area. The low rate of voltage rise compared to anodic oxidation state is due to the occurrence of small and dense micro sparks associated with the electron current. During anodic oxidation, the current passed through the grown oxide film could be represented by ionic current only, whereas in the first state of PEO, during dynamic state, both ionic species and electrons contribute to the current. Thus the total current is represented by the sum of the ionic current and the electronic current caused by the sparking [22]. Hence, a relatively low voltage is sufficient to maintain the same current value compared with the anodic oxidation stage. In the second state, during near steady-state PEO, the process voltage raises slowly with a rise of about 0.1 V/s in the interval between 559 and 571 V. The rate of voltage rise then reduces to 0.04 V/s in the interval between 571 and 576 V. At this state, a significant increase in intensity of micro-sparks that are moving rapidly over the surface is observed. In the third state, during steady state PEO, no further significant increase in voltage with respect to time is observed
The XRD patterns of the PEO fabricated ZrO2 films and untreated Zr is shown in Fig. 2. It can be seen from Fig. 2, the intensity of Zr (JCPDS card no. 05-0665) substrate peaks indexed by the letter ‘Z’ gradually decreases with an increasing PEO treatment time suggesting that the developed oxide films could prevent the incident X-ray beam from reaching the substrate. The decrease in intensity of the substrate diffraction peaks (indexed by letter ‘z’) from Z4 to Z6 can be attributed to increase in thickness of the oxide films grow for 2–8 min treatment time which is evidenced by the measured thickness values reported in Table 1 and interpreted in Section 3.3. In addition to the substrate diffraction peaks, all the oxide films show the presence of monoclinic zirconia (m-ZrO2, JCPDS card no. 37-1484) phase with a trace of tetragonal zirconia (t-ZrO2, JCPDS card no. 42-1164) phase. Further, intensity of t-ZrO2 peaks (indexed by letter ‘t’) gradually decreases with an increasing PEO treatment time. The vol.% of t-ZrO2 to m-ZrO2 phases in all PEO fabricated ZrO2 films is calculated and reported in Table 1. As the PEO treatment time increases from 2 to 6 min, the volume ratio of t-ZrO2 to m-ZrO2 gradually reduces from 0.07 to 0.025 and at 8 min, t-ZrO2 is completely transformed to m-ZrO2 with volume ratio 0.009. 3.2.1. Formation of the t-ZrO2 phase and its transformation to m-ZrO2 phase It is well known that the crystalline ZrO2 has three known polymorphs under atmospheric pressure, namely monoclinic (temperature below 1000 °C), tetragonal (temperature between 1000 and 1500 °C), and cubic (temperature above 1500 °C), in which the m-ZrO2 is thermodynamically the most stable phase at ambient temperature, and t-ZrO2 and c-ZrO2 phases are stable at high-temperatures [15]. These transformation temperatures are influenced by the dopant’s concentration in ZrO2. The formation of t-ZrO2 is due to the fact that the instant temperature of plasma electrolytic
Table 1 Process parameters for preparation of oxide films with their sample identification codes, final voltages, phase contents and their average film thicknesses. Sample code
PEO treating time (min)
Final voltage (V)
Film thickness (lm)
m-ZrO2 (vol.%)
t-ZrO2 (vol.%)
Z2 Z4 Z6 Z8
2 4 6 8
559 571 576 577
3 6 12 14
93 96 97 99
7 4 3 1
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Fig. 2. XRD patterns of (a) substrate, and PEO treated (b) Z2, (c) Z4, (d) Z6, (e) Z8 samples.
discharge region is up to 103–106 °C during PEO processing [23]. Further, the higher applied voltage (>550 V in 2 min treatment time) can yield higher localized temperature in the PEO discharge channels, enhancing the transformation of m-ZrO2 to t-ZrO2 because the latter is a high temperature phase. Thus all the PEO treated samples showed t-ZrO2 phase and the maximum vol.% of t-ZrO2 obtained is 7% at 2 min treatment time. However, further transformation is restricted and the vol.% of t-ZrO2 gradually reduces after 2 min. In general, the localized heating increases with increasing PEO treatment time, causes t-ZrO2 to sinter and form larger crystallites. Once a critical size is reached, transformation from tetragonal to monoclinic ZrO2 occurs as the crystallite size of the latter being larger than the tetragonal crystallite size [24]. Thus, m-ZrO2 is a major phase in all PEO fabricated ZrO2 films with a trace of t-ZrO2 under the present process conditions as shown in Fig. 2. However, such phase transformation, which results in existence of both types of ZrO2 phases, has a positive effect on the mechanical properties of the layers. It has been reported that zirconia-based materials exhibit exceptional toughness due to the martensitic transformations between t-ZrO2 and m-ZrO2 [25]. Thus, all the PEO fabricated ZrO2 films in the present study can be expected to exhibit good mechanical properties. 3.3. Surface morphology and quantitative analysis of the oxide films on Zr The surface morphology and corresponding elemental composition of the samples Z2, Z4, Z6 and Z8 obtained by scanning electron microscopy with energy-dispersive X-ray spectroscopy are presented in Fig. 3. All the samples present globally a smooth surface with very fine pore morphology in TSOP electrolyte which is singularly different to that observed elsewhere treated in other electrolytes by PEO [4,15–16]. The SEM micrographs (Fig. 3) clearly indicate the presence of fine discharge channels appearing circular spots distributed all over the surface of the oxide films. It is also apparent that the number of discharge channels decreases and discharge channel diameter increases with treatment time which can be attributed to different discharge characteristics followed during the film growth process as interpreted in Section 3.1. However, at higher voltages and treatment time, the morphology rapidly changes. When voltage reached critical voltage, discharge channel
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number decreases but their intensity increases resulting in nonuniform film structure (Fig. 3(d)). Further, they present a lot of internal damages, such as micro cracks begin to appear on the oxide film (shown by arrows) and localized delamination of the oxide film (shown by circle) is observed at some locations that are caused by thermal stress due to rapid solidification of molten oxide in the relatively cool electrolyte. Thus the PEO treatment time played a significant role on the surface characteristics of the samples. The film thickness of the samples Z2, Z4, Z6 and Z8 were measured using cross-sectional SEM micrographs and are reported in Table 1. At 2 min of treatment time, a highly dense and uniform film with 3 lm thickness is obtained. Further, the thickness increased with PEO treatment time, and the final thickness attained at the end of 4, 6 and 8 min of treatment time is 6, 12 and 14 lm, respectively. This increase in thickness is due to an increase in spark voltages at higher treatment times. A high spark voltage causes sparks with high level of energy which generates molten materials, so that it is easier for the high temperature molten materials to erupt and form a thick oxide film. The scanning electron microscopy with energy-dispersive X-ray spectroscopy analysis results show that all the films contain Zr, O and P elements. Among all the samples, the content of Zr is much higher (34.25 at.%) for Z2 sample (Fig. 3(a)). The higher amount of Zr in sample Z2 is attributed to the X-ray signals arising from both the coating and the substrate due to its low film thickness. All the oxide films maintained O/Zr atomic ratio between 1.85 and 2, which is very close to stoichiometric ZrO2. From Fig. 3, it is observed that the electrolyte borne element P is incorporated into oxide films during the film growth process and the content of P tends to increase with an increase in treatment time. This is in accordance with the reported fact that higher PEO treatment time results in large discharges which can sinter more elements arising from the electrolyte into the film [26]. Further, the distribution of Zr, O and P elements are shown by the elemental mapping result (Fig. 4) obtained for Z6 film for instance. The elemental mapping result shown in Fig. 4 suggested that P is uniformly distributed over the surface of the oxide film. Though the scanning electron microscopy with energy-dispersive X-ray spectroscopy analysis results show a significant amount of P in all oxide films, no separate phosphate phases are found in the XRD patterns (Fig. 2) indicating that P is doped into the ZrO2 film or the phosphate phase, if any, might be below the detection limit of X-ray diffractometer. 3.4. Electrochemical corrosion characteristics 3.4.1. Open circuit potential (OCP) analysis The thermodynamic tendency of implant material to undergo electrochemical reactions within the body fluid environment is essential in order to understand their stability in the human body. One simple way to study the film formation and the passivation of implants in a solution is to monitor the OCP as a function of time [27]. The obtained OCP curves for the untreated Zr and Z2, Z4, Z6 and Z8 samples tested for 4 h in SBF test medium are provided in Fig. 5. As can be seen from Fig. 5, the OCP values shifted to the noble direction for all the PEO fabricated ZrO2 films compared to untreated Zr which indicates their thermodynamic stability in taking part in electrochemical reactions. All the PEO fabricated ZrO2 films showed small instability in the initial 60 min of immersion time irrespective of their PEO treatment time, and then maintained almost stable OCP values up to 4 h. The instability in the initial period (10 min to 1 h) of all the oxide films can be attributed to the increased activity [28] due to the defective nature of the newly formed oxide film at the metal/oxide interface, and the stable OCP values attained at long exposure times (1 h to 4 h) might be attributed to the stable nature exhibited by the formed oxide films. The OCP values shifted to noble direction as PEO
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Fig. 3. The surface SEM micrographs and scanning electron microscopy with energy-dispersive X-ray dispersive spectroscopy analysis of (a) Z2, (b) Z4, (c) Z6, (d) Z8 samples.
treatment time increases from 2 to 6 min. In contrast, 8 min treated Zr showed lower OCP value compared to 6 min treated Zr, this can be due to the presence of surface cracks (Fig. 3(d)) on Z8 film which makes the surface more active towards dissolution in corrosive medium compared to Z6 film. Thus from the OCP results, the thermodynamic tendency of PEO coated samples towards the electrochemical oxidation reaction decreases in the
order of Z > Z2 > Z4 > Z8 > Z6 and among all the samples, Z6 has shown more noble behavior towards the anodic reactions. 3.4.2. Tafel extrapolation method The Tafel plots obtained over a potential range of ±200 mV with reference to the stable OCP are shown in Fig. 6 and the obtained kinetic parameter values are reported in Table 2. After PEO
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Fig. 4. The elemental mapping micrographs with a scanning electron microscopy with energy-dispersive X-ray dispersive spectroscopy of Z6 film.
Rp increases from 409 kX cm2 to 21,600 kX cm2 depicting the highest corrosion resistance for 6 min treated Zr. Further, Rp of Z6 sample (21,600 kX cm2) is two orders of magnitude higher that of untreated Zr (274 kX cm2). This increase in Rp with treatment time is due to the increase in film thickness from 3 to 12 lm (Table 1). However, when the treatment time increases to 8 min, Rp value decreases to 523 kX cm2. The decrease in Rp for Z8 sample is attributed to the presence of cracks and the defective film formation (Fig. 3(d)). Thus the highly corrosive Cl ions that are present in the SBF solution can easily enter into the oxide film along the cracks, thereby reducing the corrosion resistance of the Z8 sample. The protection efficiency of Z2, Z4, Z6 and Z8 calculated from Tafel data are found to be 33%, 96%, 99% and 48%, respectively. Thus by considering the jcorr, Rp and PE of fabricated ZrO2 films, the Z6 film shows highest corrosion resistance over a potential range of ±200 mV with reference to the stable OCP.
Fig. 5. Open circuit potential curves of Z, Z2, Z4, Z6 and Z8 samples for an immersion period of 4 h in 7.4 pH SBF medium.
Fig. 6. Tafel polarization curves of Z, Z2, Z4, Z6 and Z8 samples in 7.4 pH SBF medium.
treatment, the corrosion potential (Ecorr) of the Zr samples increases and their jcorr significantly reduce than that of the untreated Zr. From the reported values in Table 2, it can be seen that Rp of untreated Zr is 274 KX cm2. Among the PEO fabricated ZrO2 films, as the treatment time increases from 2 min to 6 min,
3.4.3. Potentiodynamic polarization (PDP) studies The passivation behavior of the untreated Zr and the PEO fabricated ZrO2 films were studied by conducting PDP test. The PDP curves of the Z, Z2, Z4, Z6 and Z8 samples are displayed in Fig. 7 and their pitting potential values are reported in Table 2. As shown in Fig. 7, the untreated Zr shows an active state in the beginning and then pitting corrosion happens immediately. The pitting potential (Epitt) of untreated Zr is 341.54 mV with the protection interval, defined as the difference Epitt Ecorr is 833.52 mV and when the polarization potential is over 757.23 mV, passivation at the surface takes place. Among the PEO fabricated ZrO2 films, Z2, Z4 and Z6 films showed excellent pitting corrosion resistance over a potential range of 500 to 3000 mV. Additionally, the PEO fabricated ZrO2 films show higher corrosion resistance than untreated Zr, which means that the ZrO2 films are less susceptible to corrosion in the SBF environment. However, when the treatment time increased to 8 min, the corrosion resistance decreases and Z8 film shows a similar trend with the untreated Zr. It can be noticed from Table 2 and Fig. 7 that the Z8 film shows a passivation stage in the beginning and undergoes pitting corrosion at 692.509 mV with protection interval (Epitt Ecorr) of 839.485 mV. When the polarization potential is over 730.759 mV, the repassivation takes place at the film/Zr substrate interface under the cracked oxide film in case of Z8 sample. Although, Z8 sample undergoes pitting attack similar to untreated Zr, its Epitt value improves about 351 mV compared to the untreated Zr. Thus by considering the Epitt, Epitt Ecorr and the repassivation potential of the untreated Zr and PEO fabricated ZrO2 films, the corrosion resistance properties of all the samples can be ranked as: Z6 > Z4 > Z2 > Z8 > Z and the Z6 sample treated at 6 min shows the highest corrosion resistance without pitting attack over a potential range of 500 to 3000 mV in 7.4 pH SBF environment. This improvement on the corrosion resistance of the Z6 sample is strongly dependent on the microstructure of the oxide film. Denser and thicker films are helpful to improve the corrosion resistance by
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Table 2 The OCP and kinetic parameter values of Z, Z2, Z4, Z6 and Z8 samples. Sample code
ba (mV/dec)
|bc| (mV/dec)
Ecorr (mV)
jcorr (lA/cm2)
Rp (kX.cm2)
Epitt (mV)
Epitt Ecorr (mV)
PE (%)
Z Z2 Z4 Z6 Z8
584.78 838.12 1006.84 816.40 995.25
308.95 238.35 244.73 213.39 186.30
491.98 231.87 200.01 65.87 142.32
0.32 0.197 0.012 0.0034 0.139
274 409 7180 21,600 490
341.54 – – – 692.51
833.52 – – – 839.48
– 33 96 99 48
inhibiting the attack from Cl ions in the 7.4 pH SBF solution. According to the surface (Fig. 3(c)) and cross-sectional SEM micrograph results, a 12 lm thick and uniform oxide film with no obvious pores is formed on the surface of Z6 sample. As a consequence, the oxide film on Z6 shows highest protection efficiency against the corrosive ions present in physiological environment. However, for the treatment time of 8 min, even though the thickness of the film increased to 14 lm, the presence of cracks and localized delamination on the surface of the oxide film reduces the protection efficiency against the corrosive ions present in SBF solution. 3.5. Bioactivity response of the PEO fabricated ZrO2 film
Fig. 7. Potentiodynamic polarization curves of Z, Z2, Z4, Z6 and Z8 samples in 7.4 pH SBF medium.
As Z6 film has shown higher corrosion resistance among all the PEO fabricated ZrO2 films, it was chosen for the bioactivity test and is compared with the bioactivity of untreated Zr. Scanning electron microscopy with energy-dispersive X-ray spectroscopy analysis is undertaken for Z6 film and untreated Zr to check the mineralization of apatite layer after immersion in SBF solution. The surface SEM micrographs and scanning electron microscopy with energydispersive X-ray spectroscopy analysis micrographs of Z6 film and untreated Zr after 14 days of immersion in SBF solution were
Fig. 8. The surface SEM micrographs and scanning electron microscopy with energy-dispersive X-ray dispersive spectroscopy analysis of (a) substrate and (b) PEO treated Z6 film after immersion in SBF for 14 days.
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shown in Fig. 8. It can be seen from Fig. 8, spherical like deposits are observed on Z6 film and the entire surface is covered by a precipitated layer indicating that the PEO treated Zr shows good bioactivity. Whereas a few spherical like deposits are observed on untreated Zr exhibiting its low bioactivity. Further, the scanning electron microscopy with energy-dispersive X-ray spectroscopy analysis micrographs results for the Z6 film after immersion in SBF shows the presence of Ca and P elements, while these signals are very weak in untreated Zr. The appearance of strong signals for Ca, Mg, Na, K and P and the decrease in the at.% for Zr (Fig. 8(b)) indicating the apatite formed on the Z6 film is thicker with approximate Ca/P ratio of 1.64. It was reported in literature [29] that the biomimetic apatites possess relatively low Ca/P ratio due to the deficiency of Ca2+ ions in the apatite crystal as the Ca2+ ions could be substituted by K+, Na+ and Mg2+ ions that are present in SBF solution. Surface SEM micrographs of untreated Zr (Fig. 8(a)) demonstrate that there is scarcely any apatite formed on untreated Zr indicating its poor capability of the apatite mineralization. The reason for the enhanced apatite forming ability of Z6 over untreated Zr is thought to be related to the presence of P species distributed uniformly on the surface of oxide film (Fig. 4). Thus, after immersing in SBF, the P species from the surface layers of Z6 film can effectively increase the ionic activity of the surrounding fluid and the phosphate ions formed on the surface of the film can selectively combine with the positively charged Ca2+ ions by electrostatic attraction in the SBF solution. As the Ca2+ ion concentration increases, the surface gets positively charged and thus combines with PO3 ions in the SBF solution. 4 This results in a higher super saturation of Ca and P at the interface of specimen and SBF solution. This enhancement of the super saturation at the interface induces the nucleation of apatite. Once the apatite nuclei are formed, they spontaneously grow by consuming more Ca2+ and PO3 ions from SBF solution. 4 As a result, apatite deposit increases on the Z6 film surface. Thus, the presence of P species in Z6 film has a significant effect on the initiation and the enhancement of the apatite formation on its surface.
4. Conclusions In summary, P incorporated oxide films were prepared on Zr using PEO technique in phosphate electrolyte. At a current density of 150 mA/cm2, uniform and highly dense oxide films with thickness ranging from 3 to 14 lm are formed on Zr at 2–8 min of treatment time. All the oxide films predominantly comprised m-ZrO2 with a small amount of t-ZrO2 and with increasing treatment time phase transformation from t-ZrO2 to m-ZrO2 takes place. The OCP of all oxide films shifted towards noble directions that are accompanied with substantial increase in Ecorr and decrease in jcorr compared to that of untreated Zr. The PDP results showed that no pitting corrosion is observed for Z2, Z4 and Z6 samples. However, the corrosion resistance of Z8 sample is significantly decreased and pitting corrosion is observed at 692 mV due to the presence of cracks over its surface. Thus, among all films under study, the oxide film grown at 6 min treatment time is found to be optimized one with its highest Rp (21,600 kX cm2) and provides beneficial protection against pitting in 7.4 pH SBF environment. Ca–P rich compounds are detected on Z6 film during immersion in SBF solution for 14 days, while very weak signals of such compounds are found on the untreated Zr. The presence of P species in oxide film greatly enhanced the bioactivity of Zr in 7.4 pH SBF solution thereby avoiding secondary treatments after PEO. Thus 6 min PEO treated Zr in phosphate electrolyte with high corrosion resistance and bioactivity is envisaged to be a promising material as orthopedic implants.
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Acknowledgements The authors would like to acknowledge the facilities procured through the grants received from the Department of Biotechnology, New Delhi (BT/PR-11731/MED/32/99/2008, dated 19-082009), and the Department of Science and Technology, New Delhi (SR/FTP/ETA-040/2009) for carrying out this research investigation. References [1] J. Chevalier, What future for zirconia as a biomaterial, Biomaterials 27 (2006) 535–543. [2] Suyalatu, R. Kondo, Y. Tsutsumi, H. Doi, N. Nomura, T. Hanawa, Effects of phase constitution on magnetic susceptibility and mechanical properties of Zr-rich Zr–Mo alloys, Acta Biomater. 7 (2011) 4259–4266. [3] D.W. Peterson, J.E. Lemons, L.C. Lucas, Comparative evaluations of surface characteristics of Cp–Ti, Ti–6Al–4V and Ti–15Mo–2.8Nb–0.2Si (Timetal 21SRx), J. ASTM Int. 2 (2005) 9. [4] J.Y. Ha, Y. Tsutsumi, H. Doi, N. Nomura, K.H. Kim, T. Hanawa, Enhancement of CaP formation on Zr by MAO and chemical treatments, Surf. Coat. Technol. 205 (2011) 4948–4955. [5] M. Uchida, H.M. Kim, T. Kokubo, K. Tanaka, T. Nakamura, Structural dependence of apatite formation on zirconia gels in a simulated body fluid, J. Ceram. Soc. Jpn. 110 (2002) 710–715. [6] G. Wang, X. Liu, C. Ding, Phase composition and in-vitro bioactivity of plasma sprayed calca stabilized zirconia coatings, Surf. Coat. Technol. 202 (2008) 5824–5831. [7] T. Pauporte, J. Finne, A. Kahn-Harari, D. Lincot, Growth by plasma electrolysis of ZrO2 films in the micrometer range, Surf. Coat. Technol. 199 (2005) 213– 219. [8] Z. Wang, L. Wu, W. Cai, A Shan, Z Jiang, Effects of fluoride on the structure and properties of microarc oxidation coating on aluminium alloy, J. Alloys Comp. 505 (2010) 188–193. [9] K. Venkateswarlu, N. Rameshbabu, A.C. Bose, V. Muthupandi, S. Subramanian, D. Mubarak Ali, N. Thajuddin, Fabrication of corrosion resistant, bioactive and antibacterial silver substituted hydroxyapatite/titania composite coating on Cp Ti, Ceram. Int. 38 (2012) 731–740. [10] S. Stojadinovic, J. Jovonic, M. Petkovic, R. Vasilic, N. Konjevic, Spectroscopic and real-time imaging investigation of tantalum plasma electrolytic oxidation, Surf. Coat. Technol. 205 (2011) 5406–5413. [11] D. Sreekanth, N. Rameshbabu, Development and characterization of MgO/ hydroxyapatite composite coating on AZ31 magnesium alloy by plasma electrolytic oxidation coupled with electrophoretic deposition, Mater. Lett. 68 (2012) 439–442. [12] K.C. Kung, T.M. Lee, T.S. Lui, Bioactivity and corrosion properties of novel coatings containing strontium by micro-arc oxidation, J. Alloys Comp. 508 (2010) 384–390. [13] L.R. Krishna, G. Poshal, G. Sundararajan, Influence of electrolyte chemistry on morphology and corrosion resistance of micro arc oxidation coatings deposited on magnesium, Metall. Mater. Trans. A 41 (2010) 3499–3508. [14] A.L. Yerokhin, X. Nie, A. Leyland, Plasma electrolysis for surface engineering, Surf. Coat. Technol. 122 (1999) 73–93. [15] Y. Han, Y. Yan, C. Lu, Y. Zhang, K. Xu, Bioactivity and osteoblast response of the MAO zirconia films, J. Biomed. Mater. Res. A 88 (2008) 117–127. [16] Y. Yan, Y. Han, C. Lu, The effect of chemical treatment on apatite forming ability of the macroporous zirconia films formed by MAO, Appl. Surf. Sci. 254 (2008) 4833–4839. [17] Y. Han, Y. Yan, C. Lu, Ultraviolet-enhanced bioactivity of ZrO2 films prepared by MAO, Thin Solid Films 517 (2009) 1577–1581. [18] J. Li, X. Bai, D. Zhang, H. Li, Characterization and structure study of the anodic oxide film on Zircaloy-4 synthesized using NaOH electrolytes at room temperature, Appl. Surf. Sci. 252 (2006) 7436–7441. [19] T. Kokubo, H. Takadamo, How useful is SBF in prediction in vivo bone bioactivity, Biomaterials 27 (2006) 2907–2915. [20] K. Venkateswarlu, N. Rameshbabu, D. Sreekanth, A.C. Bose, V. Muthupandi, S. Subramanian, Fabrication and characterization of micro arc oxidized fluoride containing titania films on Cp Ti, Ceram. Int. 39 (2013) 801–812. [21] A. Madhankumar, N. Rajendran, A promising copolymer of p-phenylendiamine and O-aminophenol: chemical and electrochemical synthesis, characterization and its corrosion protection aspect on mild steel, Synth. Met. 162 (2012) 176– 185. [22] K. Venkateswarlu, N. Rameshbabu, D. Sreekanth, A.C. Bose, V. Muthupandi, N.K. Babu, S. Subramanian, Role of electrolyte additives on in-vitro electrochemical behaviour of micro arc oxidized titania films on Cp Ti, Appl. Surf. Sci. 258 (2012) 6853–6863. [23] M.D. Klapkiv, O.S. Chunhmarev, P.Y. Sydor, V.M. Posuvailo, Thermodynamics of the interaction of Al, Mg, and Zr with components of an electrolytic plasma, Mater. Sci. 36 (2000) 66–79. [24] P.D.L. Mercera, J.G.V. Ommen, E.B.M. Doesburg, A.J. Burggraaf, J.R.H. Ross, Zirconia as a support for catalysts: evolution of the texture and structure on calcinations in air, Appl. Catal. 57 (1990) 127–148. [25] A.R. Kmita, A. Slosarczyk, Z. Paszkiewicz, Mechanical properties of HAp–ZrO2 composites, J. Eur. Ceram. Soc. 26 (2006) 1481–1488.
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