Control of the physical properties of anodic coatings obtained by plasma electrolytic oxidation on Ti6Al4V alloy

    Control of the Physical Properties of Anodic Coatings Obtained by Plasma Electrolytic Oxidation on Ti6Al4V Alloy D. Quintero, O. Galvis, J.A. Calder´on, M.A. G´omez, J.G. Casta˜no, F. Echeverr´ıa, H. Habazaki PII: DOI: Reference:

S0257-8972(15)30347-9 doi: 10.1016/j.surfcoat.2015.10.052 SCT 20671

To appear in:

Surface & Coatings Technology

Received date: Revised date: Accepted date:

18 June 2015 20 October 2015 23 October 2015

Please cite this article as: D. Quintero, O. Galvis, J.A. Calder´on, M.A. G´ omez, J.G. Casta˜ no, F. Echeverr´ıa, H. Habazaki, Control of the Physical Properties of Anodic Coatings Obtained by Plasma Electrolytic Oxidation on Ti6Al4V Alloy, Surface & Coatings Technology (2015), doi: 10.1016/j.surfcoat.2015.10.052

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ACCEPTED MANUSCRIPT Control of the Physical Properties of Anodic Coatings Obtained by

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Plasma Electrolytic Oxidation on Ti6Al4V Alloy

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D. Quintero1a, O. Galvisa, J.A Calderóna, M.A Gómeza, J.G Castañoa, F. Echeverríaa, H. Habazakib,c a

Centro de Investigación, Innovación y Desarrollo de Materiales – CIDEMAT, Universidad

Division of Materials Chemistry, Faculty of Engineering, Hokkaido University, Sapporo,

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b

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de Antioquia UdeA; Calle70 N° 52 – 21, Medellín, Colombia

Hokkaido 060-8628, Japan

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Graduate School of Chemical Sciences and Engineering, Hokkaido University, Sapporo,

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Hokkaido 060-8628, Japan

Abstract

Spark anodizing on Ti6Al4V has been performed in three alkaline solutions using different electrical parameters in order to study the coating formation. The surface features show a dependence on the shape and distribution of the electric microdischarges. In addition, the surface features and the chemical composition of the coatings are dependent on the anodizing solution employed. The tribological properties of the coatings formed are correlated with the morphology and the internal structure of the coatings. The variation of the internal structure of

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Corresponding Author Tel: + 574 2196617 Fax: + 574 2196565 Email: [email protected]

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ACCEPTED MANUSCRIPT the coatings was evidenced by EIS analysis. Results indicate that it is possible controlling the physical properties of the anodic film by an adequate selection of the process parameters. A porous structure is obtained using a solution mainly composed of hypophosphite, which

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exhibit a good tribological performance. Low porosity and compact structure can be obtained

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in the anodic film by using an anodizing solution composed of hypophosphite and

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metasilicate; furthermore, these coatings exhibit a good corrosion protection. Highly porous structure is achieved by using an anodizing solution composed of hypophosphite and sulfate.

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The best wear resistances were observed in coatings formed at potentiostatic mode, as demonstrated by the results of ball-on-disc wear tests.

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Keywords: PEO, titanium alloy, tribological behavior, alkaline solutions

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ACCEPTED MANUSCRIPT 1. Introduction

The surface modification of titanium and its alloys has increased the number of potential

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applications of these materials in different fields and industries, such as aerospace, marine,

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chemical industry, automotive and biomaterials [1–4]. Among the different surface modification techniques (CVD, PVD, ion implantation, electroplating, plasma nitriding, thermal oxidation), anodic oxidation, especially the plasma electrolytic oxidation (PEO)

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process, has become increasingly important since it has advantages over other methods of surface modification [5–7]. Anodic oxidation is a low cost and easy control process, it is eco-

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friendly and versatile; the electrolyte composition is quite variable allowing to obtain different types of coatings [8–12]. Although titanium and its alloys exhibit good properties (low

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density, low elastic modulus, biocompatibility, relatively high melting point and excellent

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corrosion resistance), the anodizing process increases the corrosion resistance in harsh

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environments, the mechanical and tribological properties [1,9,13–15], this process shows an interesting potential to obtain biocompatible coatings [16–21] and the anodic coatings exhibit

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good adhesion [22–24].

PEO involves anodizing at potentials above the dielectric breakdown of the coating. During the process, short-lived microdischarges are generated on the surface of the material, accompanied by gas evolution [23,25]. Local heating and internal compressive stresses generate crystallization of the anodic coating, while ionic incorporation alters both chemical composition and crystalline structure of the oxide. As the coating growth besides increasing in thickness, its surface topography also changes, which gives the possibility to obtain specific surface features looking a given property [26–28]. According to literature reports, the formation mechanism, the morphological changes and some properties of the coatings are influenced by the shape and distribution of the microdischarges during the anodizing process 3

ACCEPTED MANUSCRIPT [29–32]. Therefore, these surface characteristics can be controlled by variation of the anodic process parameters, within the most important: Concentration and nature of the anodizing solution, pH, temperature, time and electrical parameters (voltage and current density) [33–

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36].

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Different alkaline solutions have been widely employed, looking to improve the tribological properties of titanium alloys due to the formation of thick coatings [3,15,37,38]. Literature on the formation of thin anodic films with good mechanical and tribological

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behavior by anodizing in alkaline solutions is very scarce. Otherwise, phosphorus species in the anodizing solution allow the formation of coatings with low friction coefficient;

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nevertheless, these coatings do not show good tribological performance [1,3]. Moreover, there are few reports in the literature where sodium hypophosphite was employed as a phosphorus

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source in the anodizing of coatings for tribological applications. Furthermore, some authors

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indicate that, in order to improve the tribological performance, it is important controlling the

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morphology during the formation process to obtain coatings with low porosity, better adhesion and without pores in the metal/oxide interface [15,38,39]. In this paper, anodic films

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on Ti6Al4V were obtained in sodium hypophosphite electrolytes with different electrical perturbations, in both galvanostatic and potentiostatic modes. EIS analysis was performed with the purpose of correlating the electrical properties of the coatings with internal structure changes and plasma characteristics. Results indicate that, by an appropriate selection of the process parameters, it is possible to control the physical properties of the anodic coatings, with potentiostatic coatings exhibiting the best tribological performance. Moreover, the plasma characteristics are influenced by changes in the electrical properties of the coating generated by elements incorporated from the anodizing solution. This issue takes special interest in titanium alloy processing, looking for an accurate control of the anodic film formation.

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ACCEPTED MANUSCRIPT 2. Experimental Titanium alloy Ti6Al4V was used as substrate with dimensions of 10 x 10 x 1 mm. The samples were mechanically polished down to # 240 SiC paper (average roughness Sa =

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188.84 nm). This pretreatment generates a similar roughness value to that of machined

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surfaces. After this, the samples were cleaned in an ultrasound bath using acetone during 900

(25g/l) /

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s. To clean the sample surface it is used a chemical attack in an alkaline solution of H2O2 NaOH (32 g/l) for 900 s at 60 ºC. Finally, the samples were washed with distilled water

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and dried in a cold air stream.

The anodizing process was carried out in a Kepco Power Supply BHK 500-0.4 MG in an

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electrochemical cell of 100 ml immersed in a cooled water bath, using stainless steel as cathode. Table 1 shows the anodizing solution composition and the electrical parameters

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employed to obtain the anodic coatings. Ca is added in the P solution seeking its incorporation

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into the coating for biomedical applications where wear resistance is also required [16,40–43].

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In the P-Si solution, silicate addition looks to improve the wear resistance as well as induce bone formation, as demonstrated in previous studies by other authors [18,44–47]. Sulfate ions

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in P-S solutions have been reported promoting formation of the rutile phase, which improves wear resistance and behaves like a solid lubricant [1,9,48]. The coatings were obtained both in galvanostatic and potentiostatic mode. Galvanostatic coatings were obtained applying a constant current and recording the potential variation with time. Two current densities, which were higher than that needed to obtain sparking, were applied for anodizing in each electrolyte composition. The anodizing process was carried out until high fluctuations of potential or current appeared; therefore, the process time was different in each anodizing electrolyte. For potentiostatic coatings, after a short galvanostatic surge (at the maximum current provided by the power supply), current changes were recorded. During the process, the microdischarge behavior were monitored using a Sony alpha a37 camera with a 50 mm F2.8 Macro lens, filming at 60 frames per second. Once anodic oxidation was completed, the 5

ACCEPTED MANUSCRIPT samples were taken out from the electrolyte, washed with deionized water in an ultrasonic bath for 15 minutes to clean the surfaces and then dried in cold air stream. All the experiments were performed for triplicate to ensure reproducibility of the anodizing process. Cross –

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sections of the coated samples were mounted in resin and polished with alumina 0.3 µm to

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mirror finish.

Surfaces and cross-sections of the samples were observed using a scanning electron microscope JEOL JSM 6490 LV, equipped with microwave energy dispersive X-ray (EDS).

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Atomic Force Microscope (Nanosurf Easyscan 2 in tapping mode) was used to study the surface roughness of the coatings. The chemical composition of the anodic coatings was

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evaluated using micro – Raman Spectroscopy (Micro-Raman Jovin Yvon Horiba, Model Labram High Resolution) and X – ray diffraction (Empyream PANalytical) using Cu K

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radiation by scanning in the 2θ = 20 – 80º range. Coating hardness was assessed with a Shimadzu Seisakusho Ltd. Hardness Tester Microscope

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with a Knoop indenter. To obtain the hardness of the anodic coatings the following loads were applied during 15 s: 1000, 500, 300, 200, 100, 50 and 25 g. The micro-hardness values were

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plotted vs. applied load in order to identify the micro-hardness value in which the substrate has a minimum influence. For hardness measure, samples was polished with alumina 0.3 μm in order to reduce surface roughness and to obtain a complete indentation of the Knoop indenter. The tribological performance of PEO coatings was evaluated using a ball-on-disc tribometer. The test was carried out under unlubricated condition at ambient atmosphere, with a sliding speed of 60 rpm, a wear track with a radius of 2 mm, a sliding distance of 22 m, a load of 4 N and using a stainless ball AISI 420 of 6 mm diameter as counterbody. The wear rate was calculated by weight loss using a micro balance Mettler Toledo UMX5 with an accuracy of ± 0.1 μg. To calculate the wear rate, the following equation (1) was used:

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Where m is the weight loss during the test, d is the sliding distance and F is the normal force

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applied. All tribological tests were stopped before reaching the substrate. The electrical properties of the anodic coatings were determined by electrochemical impedance

spectroscopy

(EIS).

EIS

measurements

were

performed

using

a

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Potentiostat/Galvanostat IM6e BAS Zahner at open circuit potential (OCP), in potentiostatic

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mode with a 10 mV of amplitude perturbation and a frequency scan from 105 to 0.005 Hz. Measurements were performed at room temperature and potentials were measured with respect to an electrode of Ag/AgCl. The electrolyte used was NaCl 0.018 mol L-1. The EIS

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results were analyzed and fitted using the software Gamry Echem Analyst version 6.11 which

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fits the experimental results using a least-squares approximation.

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ACCEPTED MANUSCRIPT 3. Results 3.1. PEO procedure The anodic coatings were obtained using the alkaline solutions mentioned in Table 1. The

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solution conductivity changed depending on the chemical species in the anodizing solution.

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During the plasma electrolytic oxidation process, gas evolution and electric microdischarges were observed on the substrate surface [23]. The maximum voltage reached during the process was different for all anodizing solutions; different researchers [49] have reported on

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the relationship between solution conductivity and breakdown potential. The lower the solution conductivity, the higher the breakdown potential value reached during anodizing, as

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observed for the anodizing solutions evaluated.

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In PEO process, the nature of the anodizing solution has a significant effect in the size

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and shape of the spark discharges, as observed in Fig. 1. For the anodic coatings obtained in the same anodizing solution, the spark discharges show a similar appearance; nevertheless,

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the spark density changes according to the anodizing conditions. At initial stages of the PEO process a surface color change is observed due to the formation of an insulating thin film.

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Then, gas evolution is observed on the titanium anode surface for about 30 s and after this, formation of sparks takes place on the surface for all anodizing solutions. Nevertheless, some changes in the sparks appearance are observed according to the solution; in P and P-Si solutions, small sparks distributed over the surface of the material are observed. However, In P-S solution, the sparks appear inside a large bubble and bubbles grow up until explosion. Also, during the process, a bubble can divide itself in two or more bubbles with the same number of sparks within them. On the other hand, the lifetime of the microdischarges is higher for the anodizing process in P-S solution. In the final stages of the process, sparks increase in size with time and their color changes into yellow. In the anodic coatings obtained in P-Si solution, the spark size and density are greater. In the potentiostatic process, the sparks 8

ACCEPTED MANUSCRIPT density is reduced notably, especially during the transition from the initial galvanostatic process into potentiostatic mode, whereas spark sizes do not show significant change during the process.

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3.2. Morphology of the coatings

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The surface morphology of the anodic coatings depends on both the nature of the anodizing solution and the electrical parameters employed to obtain the coatings [28]. Fig. 2 shows SEM micrographs of the surface morphology of the anodic coatings obtained,

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evidencing the formation of a porous structure, typical of the PEO process [8,23,38]. Fig. 2 (a-d) show the surface morphology of coatings obtained in P solution, revealing circulars

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pores for galvanostatic coatings, with a decrease of porosity as current density increases (Figs. 2(a) and 2(b)). As observed in Figs. 2(c) and 2(d), circular pores (with some coalescent pores)

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are also formed in potentiostatic coatings; however, pore density in this case appears higher

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than for galvanostatic coatings. Fig. 2 (e-h) show the surface morphology of the anodic

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coatings obtained in P-Si solution. In general, addition of silicate to the hypophosphite solution decreases the coatings surface porosity; in particular, the galvanostatic coatings

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formed in this electrolyte, exhibit the lower surface porosity of all coatings studied here. Fig. 2 (i-l) show the anodic coatings obtained in P-S solution; sulfate addition allows the formation of a porous structure with a similar appearance to bone structure. For this electrolyte, the coatings surface morphology does not show significant changes when varying anodizing parameters; although the surface porosity of galvanostatic coatings appears to decrease as current density increases. Regarding surface roughness, different values are observed as surface morphology changes; the values measured by AFM are reported in Table 1. Smooth surfaces are formed in anodic coatings obtained in P solution, while anodic coatings obtained in P - S solution exhibit the higher roughness values. For anodic coatings obtained in the same anodizing solution, surface roughness does not show significant changes despite variation in other anodizing conditions. 9

ACCEPTED MANUSCRIPT Fig. 3 shows the cross-sections of the anodic coatings formed and Table 1 shows the average thickness for all anodic coatings. It is observed that the nature of the anodizing solution and the anodizing parameters have a significant effect in the coating thickness and

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internal porosity. Anodic coatings obtained in sulfate solution (P-S) have the higher coating

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thicknesses and potentiostatic coatings exhibit lower thicknesses than galvanostatic coatings.

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From the cross-section SEM images, the formation of a dual-layer structure is evidenced, due to the existence of temperature gradients between the inner and outer layer, (a dense inner

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layer and a porous outer layer) and the development of pores and holes associated with dielectric breakdown, accompanying the micro-discharges during the PEO [50]. Crossing

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pores and big pores concentrated towards the metal/oxide interface are formed in coatings obtained in P solution (Fig. 3 (a-d)). Potentiostatic coatings, especially those obtained at 250

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V, show a decrease in porosity near the metal/oxide interface (Fig 3(c)). Dense coatings are

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formed in P-Si solution (Fig 3(e-h)); nevertheless, for all anodizing parameters in this

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electrolyte, some pores and cracks (possibly formed during cross-section polishing) near the metal/oxide interface are observed. Coatings formed in P-S solution shows increase in both porosity and film thickness (Fig 3(i-l)); cross-section images show the formation of crossing

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pores and big pores distributed along the whole thickness. Fig. 4 shows EDS line scan analysis across the coatings formed. Incorporation of elements from the anodizing solutions is observed through the coating and an inhomogeneous distribution of some elements is evident. All the anodic coatings have incorporation of phosphorous, located mainly in the middle of the coating whereas oxygen signal indicates a homogenous distribution across the film. Calcium is also observed in the outer part of anodic coatings formed in P solution (Fig 4(a)). Likewise, the anodic coatings obtained in P-Si solution exhibit an enrichment of silicon in the outer part of the coating (Fig 4(b)). The coatings obtained in P-S solution show a higher concentration of titanium in the outer part of the coating. 10

ACCEPTED MANUSCRIPT 3.3. Chemical composition The chemical composition of the anodic coatings evaluated by micro-Raman spectroscopy is shown in Fig. 5. The micro-Raman spectra evidence the formation of titanium dioxide both

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in anatase and rutile phases. For the anodic coatings formed in P solution, the anatase phase is

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the main crystalline phase, with an increase in the intensity of the anatase peaks when either

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current or potential is increased (Fig 5(a)). In the anodic coatings formed in P-Si solution (Fig 5(b)), a similar effect of potential is observed, whereas when current density is increased from

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50 to 100 mAcm-2, anatase intensity peaks slightly decreases and rutile bands start appearing. The anodic coatings obtained in P-S solution show the formation of rutile as the main phase

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with anatase phase in minor proportion (Fig 5(c)).

The crystal structure of the anodic coatings assessed by XRD is shown in Fig. 6. The

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XRD patterns show the characteristic peaks of titanium dioxide both in anatase and rutile

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phases; additionally, peaks of the substrate are also observed. In the anodic coatings obtained

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in P and P-Si, anatase is the main crystalline phase and rutile is observed in minor proportion (Fig 6(a,b)). Nevertheless, the XRD patterns show rutile phase peaks in all anodic coatings,

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unlike the micro-Raman spectra; this could be due to the high intensity of anatase phase bands in micro-Raman spectroscopy, which prevent observation of the rutile phase bands and are only seen in the thickest coating obtained in P-Si solution. The XRD patterns of the anodic coatings obtained in P-S solution evidenced the formation of rutile as the main crystalline phase with smaller amounts of anatase phase (Fig 6(c)). The XRD results are congruent with the micro-Raman characterization regarding the rutile/anatase ratio for all the anodizing conditions. 3.4. Hardness The Knoop hardness of the anodic coatings obtained in this study, are shown in Fig. 7. Fig. 7(a) show the Knoop hardness for the anodic coatings obtained in P solution; although no 11

ACCEPTED MANUSCRIPT significant changes are observed in general, the coating formed at 250 V exhibit a higher value than the other coatings. Coatings obtained in P-Si solution (see Fig. 7(b)), show the highest hardness values of all, with similar values for the various anodizing conditions; same

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situation is observed in Fig. 7(c) for the anodic coatings obtained in P-S solution.

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3.5. Tribological performance

Fig. 8 shows the friction coefficient obtained for the anodic coatings and the surface appearance of the sample surface and the counterbody after the tribological test. A stable

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friction coefficient was observed for all coatings since the beginning of the tests and only the P-Si coatings exhibit some fluctuations. P-Si coatings formed at 400 V show the lower

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friction coefficients values (Fig. 8(b)) and potentiostatic P-S coatings display lower friction coefficients than galvanostatic coatings (Fig. 8(c)). SEM analysis of the wear path after the

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test, clearly show, for all coatings, that the wear test does not reach the substrate and the

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counterbody showed wear after the test, as observed in Fig. 8(e).

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The tribological performance of the anodic coatings is shown in Fig. 9. All the anodic coatings improve the wear resistance of the substrate, with the potentiostatic coatings showing

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a better tribological performance than galvanostatic coatings. The lowest wear rates were measured in the anodic coatings formed at 250 V in P solution and 400 V in P-Si solution. All coatings exhibit an increase in wear rate with the increase of the current density and the potential applied. The negative value of wear rate for the P coating formed at 250 V (see Fig. 9 (a)) is due to adhesion of counterbody debris to the coating surface, which increases the weight of the sample. As the anodic coatings obtained in P solution, the P-Si potentiostatic coatings, especially the coating formed at 400 V, show a better tribological performance (see Fig. 9(b)). The highest wear rate is observed in the anodic coatings obtained in P-S solution (Fig. 7(c)); similar to the other anodic coatings, those obtained in potentiostatic mode exhibit lower wear rate and lower friction coefficient. 12

ACCEPTED MANUSCRIPT Results of SEM analysis for some coatings after wear tests are shown in Fig. 10. The EDS analysis (Fig. 10(b)) on the wear path of the coating formed at 100 mAcm-2 in P solution, confirmed the presence of debris from the counterbody on the surface of the anodic coating;

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which looks brighter in the wear path (See Fig. 10(c)). In the anodic coating formed at 100

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mAcm-2 in P solution (see Fig. 10(a)), adhesion and mainly spalling are the wear mechanisms

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observed. Nonetheless, in the anodic coating obtained at 250 V, adhesion is the main wear mechanism; due to anodic coating does not show significant surface damage after the wear

galvanostatic and potentiostatic coatings.

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test, as shown in Fig. 10(c); this result evidences different tribological behavior between

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For the coatings formed in P-Si solution, a similar behavior is observed; the galvanostatic coatings show spalling and adhesion as the wear mechanisms (Fig. 10(d)) and the

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potentiostatic coatings show adhesion as the main wear mechanism (Fig. 10(e)). In the anodic

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coatings formed in P-S solutions, for both galvanostatic and potentiostatic coatings, spalling

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and adhesion are the wear mechanisms (Fig. 10(f)). 3.6. Electrical properties of the anodic coatings assessed by Electrochemical Impedance

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Spectroscopy (EIS)

Fig. 11 shows the Nyquist and Bode diagrams of the anodic coatings formed in the solutions mentioned in Table 1. The bode diagram shows two capacitive loops, indicating the formation of a duplex structure in the anodic coatings (a dense inner layer and a porous outer layer) as has been mentioned by other authors [50–53]. The two capacitive loops are observed more clearly in the coatings formed in P-Si solution (Fig. 11(b)), suggesting more uncoupled processes during the relaxation of the coatings, as consequence of significant differences of the physical properties of both inner and outer coating layers. According to literature reports, the first capacitive loop corresponds to the porous outer layer and the second one corresponds to both the dense inner layer and the relaxation of the electrical double layer [54]. The 13

ACCEPTED MANUSCRIPT relaxation processes of both the inner layer and the electrical double layer have similar values of time constant, so that only one capacitive loop at low frequencies appears in the impedance diagram. Fig. 11(a) shows the impedance spectra for the coatings formed in P solution. The

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potentiostatic coatings, especially the coating formed at 250 V, exhibit a higher impedance

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module than the galvanostatic coatings as shown in the bode diagram. Moreover, the time

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constant of the porous layer (high frequencies) shows variations in its characteristic frequency, indicating changes in the electric properties of the coating. Fig. 11(b) shows the

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impedance spectra of the coatings formed in P-Si solution. Like the coatings formed in P solution, the impedance module is higher for the potentiostatic coatings, especially the coating

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formed at 400 V. Time constants in both the porous layer and the inner dense layer do not show changes in the characteristic frequency as a result of variations in anodizing parameters.

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Fig. 11(c) show the impedance spectra of the coatings formed in P-S solution. The impedance

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module show similar values for all the coatings, despite the difference in the coatings

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thicknesses. Like the coatings formed in P-Si solution, these coatings do not show significant changes in the frequency for both the porous layer and the inner dense layer. Finally, the highest impedance modules are observed in the coatings formed in the P-Si solution and the

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characteristic frequency of the porous layer is greater than in other electrolytes, which allows that the capacitive loop of the porous layer can be more clearly observed. The impedance data was analyzed using the equivalent circuit as in a previous paper [28]. In the equivalent circuit a cascade three RC arrangement is considered, in which the elements of porous layer, inner dense layer and electrical double layer are observed, giving an adequate physical meaning of the experimental electrochemical impedance. For both the inner layer and porous layer, constant phase elements (CPE) were used for simulation of the experimental results. Then, the effective capacitance were calculated from the parameters of the CPE as reported by Bryan Hirschorn et al [55]. There are two equations to calculate the effective capacitance from the parameters of the CPE, the surface distribution equation and the normal distribution equation. 14

ACCEPTED MANUSCRIPT To calculate the effective capacitance of the inner layer the surface distribution is used, given that the electrode surface (roughness in the interface metal/oxide) influences the admittance. To calculate the effective capacitance of the porous layer, the normal distribution equation is

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used, given the heterogeneous distribution of the porosity in the coating. The values of the

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electrical parameters obtained by fitting the electric response of the equivalent circuit for

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anodic coatings formed on Ti6AlV are shown in Table 2; where, Rp is the porous layer resistance, Rb is the inner layer resistance, Rtc is the charge transfer resistance, Cp is the

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porous layer capacitance, Cb is the inner layer capacitance, Cdc is the capacitance of the electrical double layer and n is one of the parameters of the phase constant element. Bode

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diagrams in Fig. 11 show a good correlation between experimental and theoretical values of electrochemical impedance (see goodness of fit values in Table 2). The energy related to the

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coating electrical charging was obtained using the calculated capacitances of the coatings

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[56,57]. To calculate this energy equation 2 was used:

Where U is the potential at the end of the process, CT is the total capacitance of the coating, which was calculated with the capacitance values both of the porous layer and the inner dense layer, considering both layers as a succession of capacitances in parallel, t is the total time of the anodizing process and Rs is the electrolyte resistance. The results of these calculations for each anodizing condition are shown in Table 3. Looking to the results shown in Tables 2 and 3, the capacitance of the barrier layer is greater than the capacitance of the porous layer, mainly in the coating obtained in the P-Si electrolyte; where the capacitance of the barrier layer is 2 orders of magnitude higher than that of the 15

ACCEPTED MANUSCRIPT porous layer. In this electrolyte, Cb determines the value of the total equivalent capacitance of the coating (CT). Additionally, CT of the coating obtained in the P-Si solution is significantly lower (6 orders of magnitude) than the capacitance of coatings obtained in other electrolytes.

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This means that the energy required for coating charging will be significantly lower, forcing

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the system to release a greater amount of energy by plasma formation, as observed

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experimentally (see Fig. 1). These results suggest that the intensity and duration of the plasma sparks are directly related to the electrical characteristics of the coating, the more compact and

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thinner the layer, the less energy will require to electrical charge of the coating and higher

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energy will be released during the plasma process through formation of sparks.

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ACCEPTED MANUSCRIPT 4. Discussion Real-time images of the PEO process evidenced that the chemical composition of the anodizing solution affects the appearance of microdischarges (Fig. 1), as well as the surface

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morphology and the internal porous structure during the coating formation (Figs 2 and 3)

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[58]. Larger microdischarges are formed in P-Si solution in the final steps of the process;

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several researches have reported the formation of large and intense sparks by using metasilicate in the anodizing solution [1,29,59]. Otherwise, the P-S solution exhibits the

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formation of microdischarges inside a bubble, which is not clearly observable in the microdischarges of other solutions. Based on Optical Emission Spectroscopy data [58],

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temperature analysis during spark discharges show the formation of two microregions: A hot core with temperatures up to 8000 to 10000 K and a relatively cold bubble (2000 K)

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separating the core from the electrolyte. Sulfate ions in the anodizing electrolyte probed to

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increase the content of rutile in the coating in agreement with other authors [60,61]. In the

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present study, addition of metasilicate in the P-Si anodizing solution do not generate the formation of SiO2 phases as has been reported by other researchers [1,59,62]. In both microRaman and XRD spectra (Fig. 4-5), the increase in the intensity of the characteristic bands of

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anatase or rutile phases as a result of increasing either current density or potential, is related to the increase in coating thickness as evidenced in cross-section SEM images (Fig. 3). The EDS analysis shows the incorporation of some elements from the anodizing solution in the anodic coatings (Fig. 4); the phosphorus found mainly in the inner parts of the coatings is the result of phosphate anions being incorporated through the discharge channels under a high electric field [8]. In addition, silicon species are also observed on the surface of the coatings formed in P-Si solution and calcium species are evidenced in the coatings obtained in P solution as reported in the literature [63,64]. EDS analysis does not clearly show incorporation of aluminum or vanadium in the coating; according to literature reports, high solubility of vanadium oxides or vanadium compounds is the cause of low incorporation of vanadium into 17

ACCEPTED MANUSCRIPT the film and the low concentration of aluminium is not conducive to formation of aluminium oxides [14,65]. Porosity in the anodic coatings formed in P solution varied with anodizing conditions, both at

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the surface (Fig. 2) and inside of the anodic film (Fig. 3 and Table 2), showing the lower

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porosity the coating obtained at 250 V. The later can be explained since the potentiostatic mode allows controlling the size and density of microdischarges, especially at low potentials. Analysis by EIS evidenced structural changes in the coatings due to changes in the electrical

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parameters of the coating layers (Table 2). In all P coatings, the overall resistance depends mainly on the dense inner layer given that the resistance of this layer (Rb) is greater than the

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porous layer resistance (Rp), in agreement with other researchers [39,50,66]. For the galvanostatic P coatings, Table 2 shows that Rp decreases by increasing the current density

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due to the increase in the porosity of the coatings (Fig. 3), consequently causing the increase

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of Cp. Regarding the inner layer, by increasing the current density a decrease in Rb is observed

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since the thickness of the inner layer decreases and correspondingly Cb rises. The charge transfer resistance represents the inner layer blocking electron transfer. For that reason, R tc

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and Cdc show a similar behavior than Rb and Cb. The electrical parameters of the potentiostatic P coatings show a similar behavior; nevertheless, Rp, Rb and Rtc are greater, especially for the coating formed at 250 V, due to the formation of a dense coating. All P coatings appears to be similar according to np parameter of CPE, which refers to uniformity of porosity through the coating (see Table 2). On the other hand, nb refers to the roughness in the metal/oxide interface; nb values in Table 2, indicate that this interface is smoother for potentiostatic coatings, due to the smaller microdischarges occurring in this anodizing condition [54]. These internal changes might influence the mechanical and tribological properties of the coating. Hardness does not show significant changes for all the anodizing conditions since the surface morphology is similar for all cases; the hardness values measured here are similar to values of the porous anodic coatings reported in the literature [1]. Potentiostatic coatings show a better 18

ACCEPTED MANUSCRIPT tribological performance, especially those obtained at low potentials, due the formation of denser coatings (Fig. 9) and the wear mechanism of these coatings is mainly adhesion, which is less severe than the wear mechanisms showed by the galvanostatic coatings. Besides, Rb is

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greater for the coating obtained at 250 V, indicating a dense inner layer and consequently

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improving coating adhesion to the substrate. Similar results have been reported, where a

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reduction in porosity in the metal/oxide interface improved the wear performance [38]. Additionally, the literature suggest that the greater the coating thickness, the weaker the

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strength of adhesion of the coating to the substrate [39]. The similar friction coefficients observed for all coatings are related to little variation on surface roughness (Table 1).

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In the coatings formed in P-Si solution, high electrical resistances of these coatings indicate that these coatings exhibit higher corrosion resistances (Table 2), as already reported by other

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authors [29,52]. These coatings show differences in electrical parameters for the inner and the

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outer layer, more significant than the coatings obtained in P and P-S solutions, as revealed by

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the clear separation of the two capacitive arcs in the EIS tests (Fig. 11(b)). Concerning the outer porous layer in galvanostatic P-Si coatings, since surface porosity is lower (Fig. 2 (e,f)),

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the respective Rp values are higher. In addition, as coating thickness decreases when current density increases (Table 1), Rp also becomes lower whilst Cp rises. Similarly, Rp increases and Cb decreases as the applied potential is raised for potentiostatic coatings due to the increase in the coating thickness. Regarding the dense inner layer, the electrical parameters show an analogous behavior than the potentiostatic coatings formed in P solution. Correspondingly, the highest impedance module is obtained in the coating formed at lower potential (400 V) (Table 2). As in the coatings formed in P solution, the heterogeneity of the porous layer is similar for all the coatings and the interface metal/oxide is smoother for the potentiostatic coatings according to parameters of CPE (nb, np). Likewise, the potentiostatic coatings show a better tribological performance, especially the coating formed at lower potential (Fig. 9), which also exhibits the lower friction coefficient. As the coatings obtained in P solution, the 19

ACCEPTED MANUSCRIPT wear mechanism of the potentiostatic coatings is less severe than the wear mechanisms of the galvanostatic coatings. Althought, the coating hardness does not show significant changes for all anodizing conditions, it shows an increase, compared to the coatings obtained in P

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solution, due to formation of coatings with lower surface porosity (Fig. 7).

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In the anodic coatings formed in P-S solution, shaped sponge structures are observed (Fig. 2). The higher lifetime of the microdischarges in this solution generates a greater dissolution of the coating, which leads to the formation of a greater porosity both in surface and internally.

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The cross-section images show an increase in the coating thickness by increasing either current density or applied potential (Fig. 3). This behavior explains the increase of R p with

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current density and the descending behavior of Cp. As the np parameter of CPE refers to uniformity of porosity throughout the coating, data in Table 2 show that the coating formed at

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200 V has a more uniform porosity than coatings formed in other electrolytes. Besides, nb

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does not show significant changes with neither current density nor applied voltage.

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Concerning the inner layer, its electrical parameters show the same behavior as the other coatings. Nevertheless, the impedance module values do not have significant differences,

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despite changes in the coatings thickness; this is probably is due to the high porosity of these coatings. In spite of these coatings have the poorest tribological performance, their morphology, which is similar to bone structure, could be of interest for biomedical applications. Nevertheless, the potentiostatic coatings have better tribological behavior as in the other cases and a lower friction coefficient (Fig. 9). Finally, the coating hardness does not show significant changes with the variation of the anodizing conditions. In the present investigation, it has been found that anodic coatings obtained in P and P-Si solutions exhibit good tribological performance, especially anodic coatings formed in potentiostatic mode. Besides, potentiostatic coatings showed a better corrosion resistance due to the increase of the resistance of the inner layer (Rb), especially coatings formed at low 20

ACCEPTED MANUSCRIPT potential, indicating the formation of a denser and thicker inner layer. This is due to the size and the lifetime of the microdischarges in potentiostatic mode, which are smaller than in galvanostatic mode. Xuelin Zhang et al [54] analyzed by EIS the grow of anodic coatings

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with time, finding that a potential increase is not beneficial for the growth of the inner layer

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due to the increase in both the size of the microdischarges and the porosity. Large discharges

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contribute to the formation of some deep pores inside the coating and also decreases the smoothness of the coating/substrate interface. These results indicate that density, intensity and

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lifetime of sparks can be related to surface morphology of the coating, particularly to the porosity. Moreover, variation of electrical parameters in the coating by fitting of the EIS

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changing the anodizing conditions.

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experimental results, evidence internal structural changes generated in the coating by

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ACCEPTED MANUSCRIPT 5. Conclusions 1. Plasma electrolytic oxidation on Ti6Al4V carried out in different alkaline solutions indicate that both shape and size of the spark discharges together with the chemical

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composition of the anodizing solution have a significant influence on the surface

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morphology of the coatings.

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2. Anodic coatings obtained in P-Si solution had lower porosity respect to the other anodic coatings obtained and, consequently, higher corrosion resistance and higher hardness. On

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the other hand, the coatings obtained in P-S solution showed a high surface porosity, exhibiting a coating morphology like bone structure whereas in P solution the formation

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of a porous structure of circular pores was observed. 3. For the anodic coatings formed in P and P-Si electrolytes, anatase crystalline phase was

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the main structure formed whilst rutile phase was detected in minor proportion.

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Conversely, in the anodic coatings formed in P-S solution, rutile phase was the main

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crystal phase observed.

4. Potentiostatic coatings showed a better tribological performance than coatings grown under galvanostatic control; it is especially true for the anodic coatings obtained in P (250

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V) and P-Si (400 V) solutions, which showed the lowest wear rate values. 5. EIS analysis allowed characterization of the electrical properties of the anodic coatings. These electrical parameters were found to be closely related to changes in the coating internal structure, which can be achieved by variation of the anodizing conditions.

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ACCEPTED MANUSCRIPT Acknowledgements The authors are pleased to acknowledge the financial assistance of the “Fundación para la Promoción de la Investigación y la Tecnología - Banco de la República” through the project

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3099 and “Estrategia de Sostenibilidad 2014-2015 de la Universidad de Antioquia”.

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ACCEPTED MANUSCRIPT References

A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, Characterisation of oxide films produced by plasma electrolytic oxidation of a Ti–6Al–4V alloy, Surf. Coatings Technol. 130 (2000) 195–206. doi:http://dx.doi.org/10.1016/S0257-8972(00)00719-2.

[2]

X. Sun, Z. Jiang, S. Xin, Z. Yao, Composition and mechanical properties of hard ceramic coating containing [alpha]-Al2O3 produced by microarc oxidation on Ti-6Al4V alloy, Thin Solid Films. 471 (2005) 194–199. http://www.sciencedirect.com/science/article/B6TW0-4CXMVVDC/2/77cd1ee5ba487ee4109718a22fa36251.

[3]

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. Coatings Technol. 204 (2010) 3399–3409. doi:http://dx.doi.org/10.1016/j.surfcoat.2010.04.006.

[4]

M. Shokouhfar, C. Dehghanian, M. Montazeri, A. Baradaran, Preparation of ceramic coating on Ti substrate by plasma electrolytic oxidation in different electrolytes and evaluation of its corrosion resistance: Part II, Appl. Surf. Sci. 258 (2012) 2416–2423. doi:http://dx.doi.org/10.1016/j.apsusc.2011.10.064.

[5]

C. Fei, Z. Hai, C. Chen, X. Yangjian, Study on the tribological performance of ceramic coatings on titanium alloy surfaces obtained through microarc oxidation, Prog. Org. Coatings. 64 (2009) 264–267. doi:http://dx.doi.org/10.1016/j.porgcoat.2008.08.034.

[6]

I. Cvijović-Alagić, Z. Cvijović, S. Mitrović, V. Panić, M. Rakin, Wear and corrosion behaviour of Ti–13Nb–13Zr and Ti–6Al–4V alloys in simulated physiological solution, Corros. Sci. 53 (2011) 796–808. http://www.sciencedirect.com/science/article/pii/S0010938X10005597.

[7]

R.O. Hussein, X. Nie, D.O. Northwood, A spectroscopic and microstructural study of oxide coatings produced on a Ti–6Al–4V alloy by plasma electrolytic oxidation, Mater. Chem. Phys. 134 (2012) 484–492. doi:http://dx.doi.org/10.1016/j.matchemphys.2012.03.020.

[8]

H. Habazaki, T. Onodera, K. Fushimi, H. Konno, K. Toyotake, Spark anodizing of β-Ti alloy for wear-resistant coating, Surf. Coatings Technol. 201 (2007) 8730–8737. doi:http://dx.doi.org/10.1016/j.surfcoat.2006.05.041.

[9]

A.F. Yetim, Investigation of wear behavior of titanium oxide films, produced by anodic oxidation, on commercially pure titanium in vacuum conditions, Surf. Coatings Technol. 205 (2010) 1757–1763. doi:http://dx.doi.org/10.1016/j.surfcoat.2010.08.079.

AC

CE P

TE

D

MA

NU

SC R

IP

T

[1]

[10] X. Liu, P.K. Chu, C. Ding, Surface modification of titanium, titanium alloys, and related materials for biomedical applications, Mater. Sci. Eng. R Reports. 47 (2004) 49–121. doi:http://dx.doi.org/10.1016/j.mser.2004.11.001. [11] L. Wu, B.C. Holloway, D.P. Beesabathina, C. Kalil, D.M. Manos, Analysis of diamond-like carbon and Ti/MoS2 coatings on Ti–6Al–4V substrates for applicability 24

ACCEPTED MANUSCRIPT to turbine engine applications, Surf. Coatings Technol. 130 (2000) 207–217. http://www.sciencedirect.com/science/article/pii/S0257897200007209.

T

[12] F. Yildiz, A.F. Yetim, A. Alsaran, A. Çelik, Plasma nitriding behavior of Ti6Al4V orthopedic alloy, Surf. Coatings Technol. 202 (2008) 2471–2476. http://www.sciencedirect.com/science/article/pii/S0257897207008778.

SC R

IP

[13] Y. Fu, N.L. Loh, J. Wei, B. Yan, P. Hing, Friction and wear behaviour of carbon nitride films deposited on plasma nitrided Ti–6Al–4V, Wear. 237 (2000) 12–19. doi:http://dx.doi.org/10.1016/S0043-1648(99)00313-0. [14] I. Cvijović-Alagić, Z. Cvijović, S. Mitrović, V. Panić, M. Rakin, Wear and corrosion behaviour of Ti–13Nb–13Zr and Ti–6Al–4V alloys in simulated physiological solution, Corros. Sci. 53 (2011) 796–808. doi:http://dx.doi.org/10.1016/j.corsci.2010.11.014.

MA

NU

[15] H. Habazaki, S. Tsunekawa, E. Tsuji, T. Nakayama, Formation and characterization of wear-resistant PEO coatings formed on β-titanium alloy at different electrolyte temperatures, Appl. Surf. Sci. 259 (2012) 711–718. doi:http://dx.doi.org/10.1016/j.apsusc.2012.07.104.

TE

D

[16] H.-Y. Wang, R.-F. Zhu, Y.-P. Lu, G.-Y. Xiao, X.-C. Zhao, K. He, et al., Preparation and properties of plasma electrolytic oxidation coating on sandblasted pure titanium by a combination treatment, Mater. Sci. Eng. C. 42 (2014) 657–664. doi:http://dx.doi.org/10.1016/j.msec.2014.06.005.

CE P

[17] T. Akatsu, Y. Yamada, Y. Hoshikawa, T. Onoki, Y. Shinoda, F. Wakai, Multifunctional porous titanium oxide coating with apatite forming ability and photocatalytic activity on a titanium substrate formed by plasma electrolytic oxidation, Mater. Sci. Eng. C. 33 (2013) 4871–4875. doi:http://dx.doi.org/10.1016/j.msec.2013.08.003.

AC

[18] A. Kazek-Kęsik, M. Krok-Borkowicz, E. Pamuła, W. Simka, Electrochemical and biological characterization of coatings formed on Ti–15Mo alloy by plasma electrolytic oxidation, Mater. Sci. Eng. C. 43 (2014) 172–181. doi:http://dx.doi.org/10.1016/j.msec.2014.07.021. [19] A. Lugovskoy, S. Lugovskoy, Production of hydroxyapatite layers on the plasma electrolytically oxidized surface of titanium alloys, Mater. Sci. Eng. C. 43 (2014) 527– 532. doi:http://dx.doi.org/10.1016/j.msec.2014.07.030. [20] M. Echeverry-Rendón, O. Galvis, D. Quintero Giraldo, J. Pavón, J. López-Lacomba, E. Jiménez-Piqué, et al., Osseointegration improvement by plasma electrolytic oxidation of modified titanium alloys surfaces, J. Mater. Sci. Mater. Med. 26 (2015) 1–18. doi:10.1007/s10856-015-5408-4. [21] K.R. Shin, Y.S. Kim, H.W. Yang, Y.G. Ko, D.H. Shin, In vitro biological response to the oxide layer in pure titanium formed at different current densities by plasma electrolytic oxidation, Appl. Surf. Sci. 314 (2014) 221–227. doi:http://dx.doi.org/10.1016/j.apsusc.2014.06.121. [22] Y.M. Wang, B.L. Jiang, T.Q. Lei, L.X. Guo, Microarc oxidation coatings formed on Ti6Al4V in Na2SiO3 system solution: Microstructure, mechanical and tribological 25

ACCEPTED MANUSCRIPT properties, Surf. Coatings Technol. 201 (2006) 82–89. doi:http://dx.doi.org/10.1016/j.surfcoat.2005.10.044.

T

[23] M. Khorasanian, A. Dehghan, M.H. Shariat, M.E. Bahrololoom, S. Javadpour, Microstructure and wear resistance of oxide coatings on Ti–6Al–4V produced by plasma electrolytic oxidation in an inexpensive electrolyte, Surf. Coatings Technol. 206 (2011) 1495–1502. doi:http://dx.doi.org/10.1016/j.surfcoat.2011.09.038.

SC R

IP

[24] Y.M. Wang, D.C. Jia, L.X. Guo, T.Q. Lei, B.L. Jiang, Effect of discharge pulsating on microarc oxidation coatings formed on Ti6Al4V alloy, Mater. Chem. Phys. 90 (2005) 128–133. doi:http://dx.doi.org/10.1016/j.matchemphys.2004.10.025.

NU

[25] E. Matykina, R. Arrabal, P. Skeldon, G.E. Thompson, H. Habazaki, Influence of grain orientation on oxygen generation in anodic titania, Thin Solid Films. 516 (2008) 2296– 2305. doi:http://dx.doi.org/10.1016/j.tsf.2007.08.104.

MA

[26] J.-H. Xing, Z.-B. Xia, J.-F. Hu, Y.-H. Zhang, L. Zhong, Growth and crystallization of titanium oxide films at different anodization modes, J. Electrochem. Soc. 160 (2013) C239–C246. [27] M.A. Henderson, A surface science perspective on photocatalysis, Surf. Sci. Rep. 66 (2011) 185–297. doi:http://dx.doi.org/10.1016/j.surfrep.2011.01.001.

CE P

TE

D

[28] D. Quintero, O. Galvis, J.A. Calderón, J.G. Castaño, F. Echeverría, Effect of electrochemical parameters on the formation of anodic films on commercially pure titanium by plasma electrolytic oxidation, Surf. Coatings Technol. 258 (2014) 1223– 1231. doi:http://dx.doi.org/10.1016/j.surfcoat.2014.06.058.

AC

[29] S. Stojadinović, R. Vasilić, M. Petković, B. Kasalica, I. Belča, A. Žekić, et al., Characterization of the plasma electrolytic oxidation of titanium in sodium metasilicate, Appl. Surf. Sci. 265 (2013) 226–233. doi:http://dx.doi.org/10.1016/j.apsusc.2012.10.183. [30] 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. doi:http://dx.doi.org/10.1016/j.electacta.2007.08.074. [31] S. Stojadinović, J. Jovović, M. Petković, R. Vasilić, N. Konjević, Spectroscopic and real-time imaging investigation of tantalum plasma electrolytic oxidation (PEO), Surf. Coatings Technol. 205 (2011) 5406–5413. doi:http://dx.doi.org/10.1016/j.surfcoat.2011.06.013. [32] M. Fazel, H.R. Salimijazi, M.A. Golozar, M.R. Garsivaz jazi, A comparison of corrosion, tribocorrosion and electrochemical impedance properties of pure Ti and Ti6Al4V alloy treated by micro-arc oxidation process, Appl. Surf. Sci. 324 (2015) 751–756. doi:10.1016/j.apsusc.2014.11.030. [33] C.S. Dunleavy, Development of quantitative techniques for the study of discharge events during PEO processes, University of Cambridge, 2010.

26

ACCEPTED MANUSCRIPT [34] D. Capek, M.P. Gigandet, M. Masmoudi, M. Wery, O. Banakh, Long-time anodisation of titanium in sulphuric acid, Surf. Coatings Technol. 202 (2008) 1379–1384. doi:http://dx.doi.org/10.1016/j.surfcoat.2007.06.027.

IP

T

[35] X.L. Zhang, Z.H. Jiang, Z.P. Yao, Z.D. Wu, Electrochemical study of growth behaviour of plasma electrolytic oxidation coating on Ti6Al4V: Effects of the additive, Corros. Sci. 52 (2010) 3465–3473. http://www.sciencedirect.com/science/article/B6TWS-50F3V8C5/2/34fb57d1330d02c9afd4ccbc45bee205.

SC R

[36] M.V. Diamanti, M. Ormellese, M. Pedeferri, Alternating current anodizing of titanium in halogen acids combined with Anodic Spark Deposition: Morphological and structural variations, Corros. Sci. 52 (2010) 1824–1829. doi:http://dx.doi.org/10.1016/j.corsci.2010.01.036.

MA

NU

[37] X. Sun, Z. Jiang, S. Xin, Z. Yao, Composition and mechanical properties of hard ceramic coating containing α-Al2O3 produced by microarc oxidation on Ti–6Al–4V alloy, Thin Solid Films. 471 (2005) 194–199. http://www.sciencedirect.com/science/article/pii/S004060900400865X.

D

[38] S. Tsunekawa, Y. Aoki, H. Habazaki, Two-step plasma electrolytic oxidation of Ti– 15V–3Al–3Cr–3Sn for wear-resistant and adhesive coating, Surf. Coatings Technol. 205 (2011) 4732–4740. doi:10.1016/j.surfcoat.2011.04.060.

CE P

TE

[39] H. Dong, Surface Engineering of Light Alloys: Aluminium, Magnesium and Titanium Alloys, CRC Press, UK, 2010. http://books.google.com.co/books?id=wAPfQwAACAAJ. [40] C.A.H. Laurindo, R.D. Torres, S.A. Mali, J.L. Gilbert, P. Soares, Incorporation of Ca and P on anodized titanium surface: Effect of high current density, Mater. Sci. Eng. C. 37 (2014) 223–231. doi:http://dx.doi.org/10.1016/j.msec.2014.01.006.

AC

[41] R. Narayanan, S.K. Seshadri, T.Y. Kwon, K.H. Kim, Calcium phosphate-based coatings on titanium and its alloys, J. Biomed. Mater. Res. Part B Appl. Biomater. 85B (2008) 279–299. doi:10.1002/jbm.b.30932. [42] A. Krząkała, K. Służalska, G. Dercz, A. Maciej, A. Kazek, J. Szade, et al., Characterisation of bioactive films on Ti–6Al–4V alloy, Electrochim. Acta. 104 (2013) 425–438. doi:http://dx.doi.org/10.1016/j.electacta.2012.12.081. [43] D. Wei, Y. Zhou, D. Jia, Y. Wang, Chemical treatment of TiO2-based coatings formed by plasma electrolytic oxidation in electrolyte containing nano-HA, calcium salts and phosphates for biomedical applications, Appl. Surf. Sci. 254 (2008) 1775–1782. doi:10.1016/j.apsusc.2007.07.144. [44] J.R. Henstock, L.T. Canham, S.I. Anderson, Silicon: the evolution of its use in biomaterials., Acta Biomater. 11 (2015) 17–26. doi:10.1016/j.actbio.2014.09.025. [45] S. Wang, X. Wang, F.G. Draenert, O. Albert, H.C. Schröder, V. Mailänder, et al., Bioactive and biodegradable silica biomaterial for bone regeneration., Bone. 67 (2014) 292–304. doi:10.1016/j.bone.2014.07.025. 27

ACCEPTED MANUSCRIPT [46] W. Zhang, K. Du, C. Yan, F. Wang, Preparation and characterization of a novel Siincorporated ceramic film on pure titanium by plasma electrolytic oxidation, Appl. Surf. Sci. 254 (2008) 5216–5223. doi:http://dx.doi.org/10.1016/j.apsusc.2008.02.047.

T

[47] W. Simka, R.P. Socha, G. Dercz, J. Michalska, A. Maciej, A. Krząkała, Anodic oxidation of Ti–13Nb–13Zr alloy in silicate solutions, Appl. Surf. Sci. 279 (2013) 317– 323. doi:10.1016/j.apsusc.2013.04.091.

SC R

IP

[48] A. Bloyce, P.-Y. Qi, H. Dong, T. Bell, Surface modification of titanium alloys for combined improvements in corrosion and wear resistance, Surf. Coatings Technol. 107 (1998) 125–132. doi:10.1016/S0257-8972(98)00580-5.

NU

[49] I. Han, J.H. Choi, B.H. Zhao, H.K. Baik, I.-S. Lee, Micro-arc oxidation in various concentration of KOH and structural change by different cut off potential, Curr. Appl. Phys. 7, Supplem (2007) e23–e27. doi:http://dx.doi.org/10.1016/j.cap.2006.11.008.

MA

[50] S. Fadl-Allah, R. El-Sherief, W. Badawy, Electrochemical formation and characterization of porous titania (TiO2) films on Ti, J. Appl. Electrochem. 38 (2008) 1459–1466. doi:10.1007/s10800-008-9590-7.

TE

D

[51] W.A. Badawy, A.M. Fathi, R.M. El-Sherief, S.A. Fadl-Allah, Electrochemical and biological behaviors of porous titania (TiO2) in simulated body fluids for implantation in human bodies, J. Alloys Compd. 475 (2009) 911–916. doi:http://dx.doi.org/10.1016/j.jallcom.2008.08.061.

CE P

[52] S.A. Fadl-allah, Q. Mohsen, Characterization of native and anodic oxide films formed on commercial pure titanium using electrochemical properties and morphology techniques, Appl. Surf. Sci. 256 (2010) 5849–5855. doi:http://dx.doi.org/10.1016/j.apsusc.2010.03.058.

AC

[53] M.E.P. Souza, M. Ballester, C.M.A. Freire, EIS characterisation of Ti anodic oxide porous films formed using modulated potential, Surf. Coatings Technol. 201 (2007) 7775–7780. doi:http://dx.doi.org/10.1016/j.surfcoat.2007.03.005. [54] X. Zhang, Z. Yao, Z. Jiang, Y. Zhang, X. Liu, Investigation of the plasma electrolytic oxidation of Ti6Al4V under single-pulse power supply, Corros. Sci. 53 (2011) 2253– 2262. doi:http://dx.doi.org/10.1016/j.corsci.2011.03.005. [55] B. Hirschorn, M.E. Orazem, B. Tribollet, V. Vivier, I. Frateur, M. Musiani, Determination of effective capacitance and film thickness from constant-phase-element parameters, Electrochim. Acta. 55 (2010) 6218–6227. doi:http://dx.doi.org/10.1016/j.electacta.2009.10.065. [56] L. Zubieta, R. Bonert, Characterization of double-layer capacitors for power electronics applications, Ind. Appl. IEEE Trans. 36 (2000) 199–205. doi:10.1109/28.821816. [57] N. Kularatna, Energy Storage Devices for Electronic Systems - Rechargeable Batteries and Supercapacitors, (n.d.). http://app.knovel.com/hotlink/toc/id:kpESDESRB5/energy-storage-devices/energystorage-devices.

28

ACCEPTED MANUSCRIPT [58] 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.

T

[59] S. Aliasghari, P. Skeldon, G.E. Thompson, Plasma electrolytic oxidation of titanium in a phosphate/silicate electrolyte and tribological performance of the coatings, Appl. Surf. Sci. 316 (2014) 463–476. doi:http://dx.doi.org/10.1016/j.apsusc.2014.08.037.

SC R

IP

[60] H.-J. Oh, J.-H. Lee, Y. Jeong, Y.-J. Kim, C.-S. Chi, Microstructural characterization of biomedical titanium oxide film fabricated by electrochemical method, Surf. Coatings Technol. 198 (2005) 247–252. doi:http://dx.doi.org/10.1016/j.surfcoat.2004.10.029.

NU

[61] X. Cui, H.M. Kim, M. Kawashita, L. Wang, T. Xiong, T. Kokubo, et al., Preparation of bioactive titania films on titanium metal via anodic oxidation, Dent. Mater. 25 (2009) 80–86. http://linkinghub.elsevier.com/retrieve/pii/S0109564108001437?showall=true.

MA

[62] Y.M. Wang, B.L. Jiang, T.Q. Lei, L.X. Guo, Microarc oxidation coatings formed on Ti6Al4V in Na2SiO3 system solution: Microstructure, mechanical and tribological properties, Surf. Coatings Technol. 201 (2006) 82–89. http://www.sciencedirect.com/science/article/B6TVV-4J021BJ4/2/4705e23fa84d95a528097eef18a71849.

TE

D

[63] M. Fini, A. Cigada, G. Rondelli, R. Chiesa, R. Giardino, G. Giavaresi, et al., In vitro and in vivo behaviour of Ca- and P-enriched anodized titanium, Biomaterials. 20 (1999) 1587–1594. doi:10.1016/S0142-9612(99)00060-5.

CE P

[64] V.. Frauchiger, F. Schlottig, B. Gasser, M. Textor, Anodic plasma-chemical treatment of CP titanium surfaces for biomedical applications, Biomaterials. 25 (2004) 593–606. doi:10.1016/S0142-9612(03)00560-X.

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[65] M. Nakajima, Y. Miura, K. Fushimi, H. Habazaki, Spark anodizing behaviour of titanium and its alloys in alkaline aluminate electrolyte, Corros. Sci. 51 (2009) 1534– 1539. http://www.sciencedirect.com/science/article/B6TWS-4TVJNNH1/2/97536e0cd0da37e43e9cd9c7740f43a1. [66] W.F. Cui, L. Jin, L. Zhou, Surface characteristics and electrochemical corrosion behavior of a pre-anodized microarc oxidation coating on titanium alloy, Mater. Sci. Eng. C. 33 (2013) 3775–3779. doi:http://dx.doi.org/10.1016/j.msec.2013.05.011.

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ACCEPTED MANUSCRIPT Tables

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Electrolyte concentration (g/l) NaH2PO2 : 10g/l EDTANa2 : 7,44 g/l (CH3COO)2Ca : 1,78 g/l NaOH : 4,04 g/l

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Conductivity (mScm-1)

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22.3

NaH2PO2: 10 g/l Na2SiO3: 5 g/l

13.7

10.77

P-S

NaH2PO2 : 10 g/l Al2(SO4)3 : 10,6 g/l NaOH : 8,08 g/l

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26.8

Time (s) 600 300 600 400 1000 600 900 900 1000 1000 1000 1000

Maximum voltage (V) 300 350 ----430 440 ----230 250 -----

Average thickness(μm) 7.63 6.36 2.37 5.65 6.32 5.35 4.40 5.71 10.17 21.70 6.19 9.66

Roughness RMS (nm) 426.3 ± 25.6 509.3 ± 33.8 521.7 ± 17.7 497.5 ± 37.5 760.5 ± 99.5 661.1 ± 60.3 737.7 ± 79.7 802.7 ± 35.3 893.6 ± 46.5 853.3 ± 53.3 796.5 ± 76.4 814.1 ± 94.7

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Anodizing parameters 50 mAcm-2 100 mAcm-2 250 V 280 V 50 mAcm-2 100 mAcm-2 400 V 420 V 100 mAcm-2 150 mAcm-2 200 V 240 V

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Table 1 Anodizing solutions and electrical parameters used to obtain the anodic coatings on Ti6Al4V

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Cp (µF/cm-2) 0.896 1.387 0.056 0.081 1.36x10-8 1.53x10-8 4.43x10-9 4.19x10-9 27.357 156.106 144.876 17.633

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Rtc (MΩ.cm2) 1.85 0.82 19.42 12.02 4.71 6.62 105.05 71.39 0.18 0.17 0.15 0.09

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Rb (MΩ.cm2) 0.024 0.020 3.370 0.647 9.34x105 2.74x106 9.17x107 3.28x107 0.12 0.08 0.15 0.13

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Rp (kΩ.cm2) 0.35 0.22 37.25 21.04 6.30x105 5.57x105 1.72x105 2.36x105 0.27 0.67 0.17 0.83

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50 mAcm-2 100 mAcm-2 250V 280 V 50mAcm-2 100 mAcm-2 400 V 420 V 100 mAcm-2 150 mAcm-2 200 V 240 V

Electrical parameter provided by the EIS fitting

0.55 0.61 0.89 0.86 0.77 0.77 0.82 0.86 0.46 0.47 0.41 0.56

Cb (µF/cm-2) 1.344 1.957 0.054 0.435 6.46x10-7 4.10x10-7 2.50x10-7 3.36x10-7 1.203 1.336 1.053 1.236

np 0.60 0.59 0.46 0.50 0.61 0.62 0.60 0.58 0.57 0.66 0.73 0.53

Cdc (µF/cm2) 0.495 0.535 0.012 0.043 24.74 9.31 48.52 62.86 0.17 0.51 0.55 1.16

Zf=0.005 (KΩ.cm2) 22.33 16.02 367.43 153.41 2.74x105 3.16x105 7.89x105 4.84x105 14.35 13.26 11.70 13.14

Goodness of fit x 10-3 0.23 0.15 0.20 0.39 1.22 1.51 1.25 1.96 0.60 0.13 0.20 0.11

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Table 2 Electrical parameters of the coatings obtained by fitting of the EIS experimental results using the equivalent circuit for the anodic coatings formed on Ti6Al4V

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P – Si

E (J)

3.56 x 10-7 3.17 x 10-7 1.80 x 10-8 8.20 x 10-8 1.05 x 10-13 6.76 x 10-14 1.29 x 10-13 5.42 x 10-14 4.55 x 10-6 2.50 x 10-5 2.32 x 10-5 3.02 x 10-6

0.016 0.019 0.001 0.003 9.72 x10-9 6.55 x 10-9 3.24 x 10-9 4.78 x 10-9 0.120 0.783 0.464 0.086

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CT (F)

37.37 37.37 37.37 37.37 77.38 77.38 77.38 77.38 31.09 31.09 31.09 31.09

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Rs (ohm)

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Table 3 Energy associated to the electrical charging of the coating for each anodizing solution

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Figure 1 Microdischarges appearance at various stages of PEO process of the anodic films

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P-S Solution.

Figure 2 Surface SEM micrographs of the anodic coatings formed in: P Solution (Figures (a-

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Figure 3 Cross - sections SEM images of the coatings formed in: P solution (Figures (a-d)),

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Figure 4 EDS Analysis of the cross-section of the coatings formed in: a) P solution (100

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Figure 5 Micro-Raman spectra of the anodic coatings formed on Ti6Al4V in the following

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Figure 6 XRD patterns of the anodic coatings formed on Ti6Al4V using the following anodizing solutions: a) P solution, b) P - Si solution and c) P – S solution

Figure 7 Knoop hardness of PEO coatings formed in: a) P solution, b) P– Si solution and c) P - S solution

Figure 8 Friction coefficient register of the anodic coatings obtained in: a) P solution and b) P-Si solution and c) P-S solution and d) SEM image of the surface of the anodic coating 33

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Figure 9 Tribological performance of the anodic coatings obtained on Ti6Al4V in: a) P

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Figure 10 SEM micrographs of wear paths of the anodic coatings obtained in: a) P solution

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(100mAcm-2), b) the elemental mapping of the wear path of the anodic coating obtained in P solution (100 mAcm-2), c) P solution (250 V), d) P-Si Solution (100 mAcm-2), e) P-Si

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Solution (400 V) and f) P-S solution (200V)

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Figure 11 Nyquist and Bode curves for the anodic coatings formed in the following solution:

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Highlights Morphological film characteristics (morphology, porosity, thickness and internal

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The shape and distribution of the microdischarges dependent on the anodizing

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structure) were controlled by changing anodizing parameters.

solution, which influences the final morphology of the coatings. Changes in the electrical properties of the anodic films were due to the incorporation of elements from the electrolyte.

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