Preparation of PEO coating on Ti6Al4V in different electrolytes and evaluation of its properties

Abstract

The aim of this work is to discuss the effect of different amounts of NaF in electrolyte on structure, corrosion resistance and wear behaviour of the prepared ceramic coatings on Ti6Al4V alloy by plasma electrolytic oxidation (PEO) technique. PEO process was carried out under constant current of 2 A, frequency of 1 kHz, anodic duty cycle of 45% and cathodic duty cycle of 25% using a pulse bipolar power supply. TiO₂ based coatings were prepared from three aluminate base electrolytes containing various amount of NaF (0.75, 1.5 and 2.25 g L⁻¹). Phase composition, surface morphology and thickness of the resultant coatings were examined by means of X-ray diffraction and field emission scanning electron microscopy. Results indicated that solution composition and process time have a great effect on thickness and surface porosity of the coatings. Corrosion resistance and wear properties of the coatings were in a good accordance with structural properties.

Keywords

Plasma electrolytic oxidation Ti6Al4V alloy Corrosion resistance Wear behavior

Introduction

Titanium and its alloys are widely used in aerospace, marine and biomedicine industries due to their high strength/weight ratio and corrosion resistance along with excellent mechanical properties and biocompatibility.^1-5 However, lack of surface hardness and weak frictional and wear properties have limited tribological applications of these alloys.^6,7 The poor tribological properties of titanium alloys can be overcome by specific surface treatments. ^7–9 Many surface treatments, such as thermal oxidation,¹⁰ plasma nitriding,¹¹ ion implantation,¹² spraying,¹³ laser modification¹⁴ and plasma electrolytic oxidation (PEO),^15,16 have been used to improve tribological properties of titanium and its alloys.

PEO, also called microarc oxidation, is a relatively new process to uniformly synthesise ceramic-like oxide films on the surface of lightweight metals and alloys (aluminium, magnesium, zirconium, titanium, etc.).^15,16 During the PEO process, discrete spark discharges occur on the surface, when the supplied voltage is higher than a critical value (called dielectric breakdown voltage).¹⁷ Discharges happen as a result of a loss in dielectric stability of oxide film in a region of low conductivity. This process is accompanied by visible light emission and gas evolution.^17–20 The surface temperature and pressure are locally increased by microdischarging, which results in formation of oxide coatings due to high number of discrete spark discharges. The main part of the obtained oxide films is substrate oxides. However, they also contain complex oxides of the components of the electrolytes.^17,21

Rutile, anatase and brookite are the three polymorphs of titania. Rutile is considered as the stable and anatase as the metastable form of titania.²² These different polymorphs of titania have different corrosion and wear properties. The PEO process has numerous affecting parameters such as the solution, potential, frequency and duty cycle. These parameters directly affect the TiO₂ film morphology and thus the corrosion resistance and wear characteristics changes. On the other hand, electrolytes’ composition has a great influence on the phase composition of the coating. Therefore, the effect of different electrolyte compositions on the microstructure and phase composition of coatings needs more investigations. Aluminate base electrolytes are an important group of solutions used in plasma electrolyte oxidation of titanium and its alloys.^23–26 Coatings produced in aluminate base electrolytes contain TiAl₂O₅ complex oxide phase in addition to titania phases (rutile and anatase).²³

The aim of this study is to investigate the effects of different amounts of NaF addition to the electrolyte on the microarcing situation during the process, morphology, microstructure, corrosion resistance and wear behaviour of the prepared coatings.

Experimental

Materials and PEO procedure

Disc samples of Ti6Al4V alloy with 30 mm diameter and 4 mm thickness were first polished with SiC abrasive paper (60-1500#) and then washed with acetone and distilled water. A bipolar pulse galvanostatic power supply was employed for plasma electrolyte oxidation. The applied amperage, anodic duty cycle, cathodic duty cycle and frequency were selected to be 2 A, 45%, 25% and 1000 Hz respectively. Disc samples were used as anode and stainless steel plates were used as cathode in this electrochemical system. The anodic peak voltages were monitored by means of a Ziegler RM-16 high resistance voltmeter. During the process, the discharge behaviour was also monitored using Canon XS-1000 camera, operating at 24 frames/s and a resolution of 640×480.

In the PEO process, besides the oxide salts, electrolytes may contain substances that increase the electrolyte conductivity. In this work, different amount of NaF was added to increase the electrolyte conductivity. Chemical composition, electrochemical conductivity and pH of different electrolytes and processing time of the samples are shown in Table 1. The temperature of the electrolyte was kept below 25°C by means of a water cooling system.

Table 1

Electrolyte concentrations, conductivity, pH and process duration of different samples

Morphological and microstructural analysis

Surface morphology and cross-sections of the produced coatings were examined using a field emission scanning electron microscope (Tescan Mira3). The surface morphology images were processed by means of Image Processing Lab software. Phase composition of the coatings was also analyzed by means of X-ray diffractometer (XRD; Philips X'Pert) with Cu Ka radiation.

Corrosion tests

Corrosion properties of the coatings were investigated by potentiodynamic polarisation, using a BioLogic SP 150 potentiostat/galvanostat. All the electrochemical tests were carried out in 3.5 wt-% NaCl solution at room temperature. A platinum foil (1 cm²) was used as the counter electrode, and Ag/AgCl (3M KCl) was used as the reference electrode. Potential range for potentiodynamic polarisation tests was –300 to + 1000 mV with respect to the open circuit potential, while potential scan rate was 1 mV s⁻¹.

Hardness, scratch, roughness and tribological tests

Hardness measurements were carried out on an AMSLER D-6700 Vicker's microhardness testing machine, using an applied load of 1 N for 15 s. The mean values of at least three measurements conducted on different areas of each sample were considered.

Dry sliding ball on disc wear tests on the Ti6Al4V substrate and coated samples were carried out in a laboratory atmosphere at 30–40% relative humidity and room temperature. The rotating specimens were slide against an alumina ball with a diameter of 6 mm. Wear tests were undertaken under the normal load of 2 N, the sliding linier velocity of 0.1 m s⁻¹ and sliding distance of 300 m. The coefficient of friction was also recorded continuously during the tests. Scratch tests were performed on the coated samples using the continuous load increase method up to 25 N with a Micro Scratch Tester, with a load rate of 18.2 N min⁻¹ and an advance rate of 9.5 mm min⁻¹, resulting in a scratch of 13 mm. The tip was a diamond Rockwell C diamond stylus indenter with a radius of 200 μm. The surface roughness (Ra) was measured by means of a mechanical profiler. The surface profile included the variation on the surface topography, which filtered to obtain surface roughness profile only. The filtration was done using Gaussian filter with cutoff wavelength of 0.25 mm and measuring length of 6 mm.

Results and discussion

Formation of PEO coatings in different electrolytes

The anodic peak voltage changes with time for AN15, AN25 and AN35 are shown in Fig. 1. For all the samples, the anodic peak voltage is rapidly increased to ∼175 V at the first 20 s of experiments, and then the slope of the plots is decreased. Real time imaging of the discharges can show the influence of electrolyte NaF content. Images of the samples in different electrolytes during the PEO process are shown in Fig. 2. It can be seen that in all the samples, microarcing is not started after 15 s, while microarcs appeared after 30 s. Comparing Figs. 1 and 2, it can be concluded that microarcing started after ∼20 s. Black arrows in Fig. 2 (30 s) show relatively weak discharges. Thus, the point of change in the increasing rate of the anodic peak voltage (20 s in these samples) can be the time of dielectric breakdown and the start of microarcing process.

Figure 1 also shows that the final peak voltages (Vf) of AN15, AN25 and AN35 are 400, 300 and 315 V respectively. On the other hand, Fig. 2 shows red severe arcs on the surface of the AN1 sample after 300 s. These red powerful arcs may cause some destructive effects such as rough surface finish and thermal cracking of the film.²⁷ However, severe red arcs did not appear on the surface of AN25 and AN35 because the final peak voltages of these samples are lower than that of AN15. Therefore, it can be concluded that the limiting voltage of the PEO process in our electrolytes is between 315 and 400 V, and the process in the electrolytes containing 1.5 and 2.25 g L⁻¹ NaF can be continued to Vf reach to about these voltages. Comparing Fig. 1 and conductivity of the electrolytes (in Table 1) shows a reverse relationship between the electrolyte conductivity and the slope of voltage–time response after microarcing. This reverse relation is in accordance with Ohm's law. Continuing the process in these two electrolytes (AN27 and AN37 samples) shows that after 7 min, powerful red arcs appeared (Fig. 2 b and c, after 420 s) and the peak voltage reaches to ∼400 V. Figure 3 shows the voltage–time response of AN27 and AN37 samples. It can be seen that the final peak voltages for these two samples are 380 and 420 V respectively. So, the severe red arcs of AN27 and AN37 samples after 420 s are due to the increase of peak voltage to these higher values.

Figure 1

Changes in peak voltage of AN15, AN25 and AN35 samples during PEO treatment

Figure 2

Video images of samples during PEO process in different electrolytes: a 0.75 g L^–1; b 1.5 g L^–1; c 2.25 g L^–1 NaF

Morphology and microstructure of coatings

XRD patterns of the coated samples are depicted in Fig. 4. Two of the strongest peaks in all the patterns are related to titanium from the substrate. The main phase in all coatings is Al2TiO5. Comparing the relative intensity of Al2TiO5 peaks in AN25 with other samples reveals that the amount of this phase in AN25 is higher than the other samples. Increasing of PEO process duration from 5 to 7 min decreases the relative intensity of Al2TiO5 peaks (AN27 and AN37).

According to Fig. 4, rutile is the main titanium oxide phase in all the samples. Thermodynamic studies showed that the α-Al₂O₃ phase is usually formed at temperatures higher than 1273 K. Thus, it shows that the coating is likely exposed locally to such high temperatures due to discharges. c-Al₂O₃ phase (that is, a metastable phase) is also detected in AN27 and AN37 specimens. Aluminium species in Al₂O₃ and Al2TiO5 phases must be derived from Ti6Al4V substrate and sodium aluminate (NaAlO2) in the electrolyte.²⁸

Figure 3

Changes in peak voltage of AN27 and AN37 samples during PEO treatment

Figure 4

XRD patterns of coated samples

The surface morphologies of the prepared coatings are shown in Fig. 5 (a, c, e, g and i). Pancake-like morphology of the coatings is clearly observable. This could be due to formation of microarcs during the PEO process. On the surface of each sample, pores with different diameters are observable. This is in agreement with the fact that the intensity and size of the arcs were increased continuously during the process (Fig. 2), which in turn increased the size of the micropores diameter after cooling.1

Venkateswarlu et al. mentioned that the morphological properties such as the shape (closed or opened or interconnected), size, number and distribution of micropores over the surfaces of PEO coatings can be related to the properties of microspark discharges hat occurred during the PEO film growth process.²⁹ Further, they showed that the density of the microarcs is mainly controlled by the resistivity of the electrolyte solution, and these two are in a direct relation with each other.³⁰ Comparing Fig. 5 c and e, it is seen that the pore density on the surface of AN25 is less than that on AN35, which could be due to its lower electrolyte conductivity (because of lower NaF content). In addition, the surface of AN25 film looks more compact with well dispersed oxidation products, which close the surrounding pores. According to Fig. 4 i and 5g, some regions on the surface of AN27 and AN37 samples are totally destroyed and the pancake morphology in these regions is broken down. This phenomenon may be related to the severe red microsparks at the end of the PEO process of these samples.

Image processing is a novel technique to determine surface porosity of PEO coatings.³¹ Figure 5 (b, d, f, h and j) illustrates the image processed morphologies of surface SEM images. The porosity percentage, total quantity and average pore size of each tested sample were determined from SEM images for a surface area of 41×41 μm² and are presented in Table 2. It can be seen that AN15 has the highest porosity percentage of ∼10.1%, although the average pore size of this sample is relatively high (∼0.9 μm). This high value of porosity can be attributed to the powerful and severe red microsparks generated at the end of the PEO process of this sample (Fig. 2 a, 300 s). On the other hand, AN25 shows lower porosity (∼6.5%) and smaller pore diameter (∼0.65 μm). This can be due to the homogeneous microspark density during the PEO process of this sample (Fig. 2 b, up to 300 s).

Table 2

Porosity data derived from image processing of surface SEM images

The porosity of AN27 and AN37 samples is 8.5% and 6.0% respectively. These lower percentages of porosity can be due to the fact that there are some totally damaged regions on the surface of these two samples (which are obviously detectable in Fig. 4 g and i), which can affect the real PEO treated surface and decrease it. The average pore sizes of AN27 and AN37 are 1.0 and 0.85 μm respectively. These large pores can be generated by powerful sparks at the final stages of the PEO process of these samples (Fig. 2 b and c, 420 s).

Figure 6 shows SEM images from cross-section of the coatings. The thickness of these coatings is also shown in Fig. 7. The cross-section images illustrate that the AN15 coating is thinner than AN25 and AN35. This may be because of the powerful sparks generated on the surface of AN15. These intense red sparks can lead to generation of cracks in the coating and detachment of some parts from it. Lower thickness of AN27 as compared to AN25 (8 and 18 μm respectively) could also be due to the destructive effects of powerful sparks by continuing the PEO process for 7 min (Fig. 2 b). On the other hand, Fig. 6 a, d and e shows a gap between the coating surface and resin. These gaps can be as a result of some totally destroyed regions that are obvious on the surface images of these samples (Fig. 5).

Figure 5

Surface morphology and image processed micrographs of a, b AN15, c, d AN25, e, f AN35, g, h AN27 and i, j AN37

Figure 6

Cross-section SEM images of a AN15, b AN25, c AN35, d AN27 and e AN37

Figure 6 also shows that coatings prepared in electrolytes with higher NaF content (1.5 and 2.25 g L⁻¹ NaF) have better adhesion to the substrate. This can be due to higher electrolytic conductivity and higher microarcs density.³⁰

Corrosion resistance of coatings

An oxide film by restricting adsorption of corrosive ions shifts the surface potential to more positive potentials and reduces the charge carrier mobility at the interface of metal and electrolyte, which in turn protects the substrate from corrosion. Anti-corrosion properties of this group of coatings are affected by a combination of characteristics of the oxide film (phase composition, structural imperfections and thickness) and corrosion environment (such as concentration of aggressive ions).²

In PEO coatings, larger micropores increase the real area that is exposed to the corrosive solution. On the other hand, an increase in coating thickness increases the barrier property of coating and improves anti-corrosion properties.¹ Tafel curves of the untreated Ti6Al4V alloy and PEO coated samples in the 3.5% NaCl solution are illustrated in Fig. 8. The corrosion potential (E_corr), corrosion current density (i_corr) and polarisation resistance (Rp) obtained from these curves are presented in Table 2. Stern–Geary equation was used to obtain polarisation resistance (R_p):¹ where β_a and β_c are anodic and cathodic tafel constants. Two obvious outcomes can be obtained from the potentiodynamic polarisation curves. First, the corrosion potential (Ecorr) of the PEO coated samples is more positive than the substrate, which means lower thermodynamic tendency of the coatings for corrosion. Second, the corrosion current densities (icorr) of the PEO coated samples are lower than the substrate. This implies that the anodic reactions were restrained by PEO treatment and a higher chemical stability is obtained.¹

Figure 7

Thickness of PEO treated coatings

Figure 8

Potentiodynamic polarisation curves of untreated Ti6Al4V alloy and PEO treated samples

Table 3 indicates that the PEO treatment on the Ti6Al4V surface makes the corrosion current density to decrease by several orders of magnitude. Among the films, the icorr of AN25 is the least at a value of 3.02610⁻⁸ A cm⁻² and the icorr of AN15 is the highest at a value of 8.12610⁻⁷ A cm⁻², corroborating with their Rp values. The icorr of all tested samples decreases in the order Ti6Al4V.AN15.AN35.AN27.AN37.AN25, attesting to the fact that the corrosion resistance values of the PEO treated samples are improved considerably compared to that of the untreated substrate. The better corrosion resistance of AN25 film can be due to three reasons. (i) According to the XRD patterns of the coated samples (Fig. 4), AN25 contains higher amounts of Al2TiO5 and rutile phases. These phases are known as anti-corrosion phases because of their compact coherent structure.²³ (ii) According to the image processed surface morphology of the samples (Fig. 5), AN25 has lower porosity percentage and smaller average pore diameter among all of the prepared coatings. These micropores are free paths for diffusion of corrosive ions to the interface of substrate and coating. So, lower surface porosity and smaller pore size lead to better corrosion resistance. (iii) According to the cross-section SEM images of the coated samples (Figs. 6 and 7), AN25 has the highest thickness. Higher coating thickness causes more powerful barrier against the diffusion of corrosive ions through the metal surface.

Table 3

Corrosion parameters obtained from Tafel curves

The corrosion current density of AN35 is lower than that of AN15. The XRD patterns show that the peaks related to rutile and Al2TiO5 in AN35 has higher relative intensity as compared to the intensity of these phases in AN15. On the other hand, the thickness of AN35 is, 10% more than that of AN15. Moreover, the image processed surface micrographs show that AN35 has lower surface porosity and smaller average pore diameter as compared to AN15. The higher polarisation resistance of AN27 and AN37 compared to that of AN15 and AN35 can be a result of lower surface porosity percentage of these samples.

Microhardness and wear behaviour of coatings

The microhardness of untreated Ti6Al4V substrate and PEO treated samples is shown in Fig. 8. It is clearly evident that the PEO treatment results in a great increase in the surface microhardness. AN25 has the highest microhardness among all the PEO treated samples. This can be due to the high content of hard phases (such as Al₂TiO₅ and a-Al₂O₃) and lower porosity (and smaller pores) on the surface of this sample. Comparing Figs. 5 and 9, it can be concluded that microhardness is decreased by increasing the surface porosity.

Weight losses of the untreated Ti6Al4V alloy and the PEO treated samples are presented in Table 4. According to this table, all of the PEO treated samples have better wear resistance than the substrate. The weight losses of PEO treated samples are from 1.5 to 7 times lower than the Ti6Al4V substrate.

Table 4

Weight loss and average friction coefficient of substrate and PEO treated samples in wear tests

According to the Archard theory, wear resistance is proportional to the hardness of material.³² According to given microhardness values of the samples (Fig. 9), better wear resistance of the PEO treated coatings can be attributed to higher hardness of these samples compared to the untreated Ti6Al4V. Higher friction coefficient of Ti6Al4V is consistent with the wear results. This could be due to the low hardness of the substrate (Fig. 9), resulting in increased real contact area between alumina ball and Ti6Al4V substrate. Table 5 shows the value of adhesion and roughness of the coatings. AN15 has the highest weight loss among the coatings. It can be attributed to poor adhesion of this film to substrate. AN15 has the highest roughness and lowest adhesion and it can be due to the severe microarcing process of this sample. On the other hand, high surface porosity of AN15 can lead to surface embrittlement of this film, resulting in easier surface fracture of the film during the wear test. AN25 has the best wear resistance among the coatings. SEM cross-section images (Fig. 6) show that AN25 coating has the highest thickness, and scratch tests indicate the proper adhesion of this sample. Moreover, this sample has lowest porosity and average pore size among the coatings, and this higher compactness of AN25 results in its better wear resistance.

Table 5

Critical load values of scratch test and roughness (Ra) of treated samples

Figure 9

Microhardness of untreated Ti6Al4V alloy and PEO treated samples

Moreover, scratch and roughness tests show that AN15, AN27 and AN37 samples (which have severe microarcing process) have lower adhesion and rougher surface in comparison with AN25 and AN35 samples. These data are in agreement with wear resistance of these samples.

Al₂TiO₅ and α-Al₂O₃ are known as phases with good anti-wear properties. XRD patterns show that relative intensity of the peaks of these phases in different samples is in a good agreement with the wear resistance of the coatings.

Conclusion

In the present study, different ceramic coatings were prepared by PEO of Ti6Al4V alloy in aluminate base electrolytes with different concentrations of NaF, and their morphology, microstructure, corrosion resistance and wear behaviour were examined and compared to the Ti6Al4V substrate. The outcome of the results can be summarised as follows:

In all three different electrolytes, microarcing process starts when the anodic peak voltage of the sample increases to ∼175 V, although powerful severe red arcs are generated when the peak voltage increases to ∼400 V.

Al₂TiO₅, rutile and α-Al₂O₃ are the main phases in the prepared coatings. Relative intensity of peaks shows that the sample treated in 1.5 g L⁻¹ for 5 min (AN25) has higher amount of rutile and Al2TiO5 in its structure.

The coated sample in 1.5 g L⁻¹ NaF electrolyte for 5 min (AN25) has the lowest surface porosity (6.5%) and smallest average pore size (∼0.65 μm). The coated sample in 0.75 g L⁻¹ NaF solution for 5 min (AN15) has the highest surface porosity of ∼10.1%.

The coatings prepared in electrolytes with higher NaF content (1.5 and 2.25 g L⁻¹ NaF) have better adhesion to the substrate than that prepared in the electrolyte with 0.75 g L⁻¹ NaF.

All prepared coatings showed better corrosion resistance as compared to Ti6Al4V substrate. Among the films, the icorr of AN25 was the least at a value of 3.02×10⁻⁸ A cm⁻² and the i_corr of AN15 was the highest at a value of 8.12×10⁻⁷ A cm⁻², corroborating with their Rp values.

The weight losses of PEO treated samples were from 1.5 to 7 times lower than the Ti6Al4V substrate.

Footnotes

Acknowledgements

This funding of this project has been provided by Sarkhoon and Qeshim Gas Treating Company.

References

Shokouhfar

, Dehghanian

, Montazeri

and 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., 2012, 258, 2416–2423. doi: 10.1016/j.apsusc.2011.10.064

Yerokhin

A.L.

, Nie

, Leyland

and Matthews

: ‘Characterization of oxide films produced by plasma electrolytic oxidation of a Ti-6Al-4V alloy’, Surf. Coat. Technol., 2000, 130, 195–206. doi: 10.1016/S0257-8972(00)00719-2

Wanga

, Jiang

, Lei

and Guo

: ‘Dependence of growth features of Microarc oxidation coatings of titanium alloy on control modes of alternate pulse’, Mater. Lett., 2004, 58, 1907–1911. doi: 10.1016/j.matlet.2003.11.026

Fei

, Hai

, Chen

and Yangjian

: ‘Study on the tribological performance of ceramic coatings on titanium alloy surfaces obtained through microarc oxidation’, Prog. Org. Coat., 2009, 64, 264–267. doi: 10.1016/j.porgcoat.2008.08.034

Huang

, Wang

, Xu

and Han

: ‘Mechanical properties of titania prepared by plasma electrolytic oxidation at different voltages’, Surf. Coat. Technol., 2007, 201, 5168–5171. doi: 10.1016/j.surfcoat.2006.07.137

Kuromoto

N. K.

, Simao

R. A.

and A. Soares

: ‘Titanium oxide films produced on commercially pure titanium by anodic oxidation with different voltages’, Mater. Charact., 2007, 58, 114–121. doi: 10.1016/j.matchar.2006.03.020

, Liang

, Zhou

and Xiao

: ‘One-step preparation of TiO₂/MoS₂ composite coating on Ti6Al4V alloy by plasma electrolytic oxidation and its tribological properties’, Surf. Coat. Technol., 2013, 214, 124–130. doi: 10.1016/j.surfcoat.2012.10.079

Zhecheva

, Sha

, Malinov

and Long

: ‘Enhancing the microstructure and properties of titanium alloys through nitriding and other surface engineering methods’, Surf. Coat. Technol., 2005, 200, 2192–2207. doi: 10.1016/j.surfcoat.2004.07.115

Liu

, Chu

P. K.

and Ding

: ‘Surface modification of titanium, titanium alloys, and related materials for biomedical applications’, Mater. Sci. Eng. R, 2004, 47, 49–121. doi: 10.1016/j.mser.2004.11.001

10.

Zhang

Z. X.

, Dong

and Bell

: ‘The load bearing capacity of hydrogen-free Cr-DLC coatings on deep-case oxygen hardened Ti6Al4V’, Surf. Coat. Technol., 2006, 200, 5237–5244. doi: 10.1016/j.surfcoat.2005.06.021

11.

Sun

, Tong

W. P.

, Zou

and B. Wang

: ‘Low-temperature plasma nitriding of titanium layer on Ti/Al clad sheet’, Mater. Des., 2013, 47, 408–415. doi: 10.1016/j.matdes.2012.11.029

12.

Rautray

T. R.

, Narayanan

, Kwon

T. Y.

and Kim

K. H.

: ‘Surface modification of titanium and titanium alloys by ion implantation’, J. Biomed. Mater. Res., 2010, 93B, 581–591. doi: 10.1002/jbm.b.31596

13.

Serban

V. A.

, Rosu

R. A.

, Bucur

A. L.

and Dragus

: ‘Deposition of titanium nitride and hydroxyapatite-based biocompatible composite by reactive plasma spraying’, Appl. Surf. Sci., 2012, 258, 3871–3876. doi: 10.1016/j.apsusc.2011.12.049

14.

Guo

, Zhou

, Zhang

, Zhou

, Pu

and Pen

: ‘Microstructure and tribological properties of in situ synthesized TiN/Ti3Al intermetallic matrix composite coatings on titanium by laser cladding and laser nitriding’, Mater. Sci. Eng. A, 2008, 480, 404–410. doi: 10.1016/j.msea.2007.07.010

15.

Stojadinovic

, Vasilic

, Kasalica

, Belca

, Zekic

and Zekovic

Lj.

: ‘Characterization of the plasma electrolytic oxidation of titanium in sodium metasilicate’, Appl. Surf. Sci., 2013, 265, 226–233. doi: 10.1016/j.apsusc.2012.10.183

16.

Jafarniya Kharanagh

, Faghihi Sani

M. A.

and Rafizadeh

: ‘Effect of current frequency on coating properties formed on aluminized steel by plasma electrolytic oxidation’, Surf. Eng., 2014, 30, 224–228. doi: 10.1179/1743294413Y.0000000190

17.

Stojadinovuc

, Vasilic

, Petkovic

and Zekovic

Lj.

: ‘Plasma electrolytic oxidation in heteropolytungstate acids’, Surf. Coat. Technol., 2011, 206, 575–581. doi: 10.1016/j.surfcoat.2011.07.090

18.

Stojadinovic

, Vasilic

, Petkovic

, Nedic

, Kasalica

, Belca

and Zekovic

Lj.

: ‘Luminescence properties of oxide films formed by anodization in 12-tungestophosphoric acid’, Electrochim. Acta, 2010, 55, 3857–3863. doi: 10.1016/j.electacta.2010.02.011

19.

Snizhko

L. O.

, Yerokhin

A. L.

, Pilkington

, Gurevina

N. L.

, Misnyankin

D. O.

, Leyland

and Matthews

: ‘Anodic processes in plasma electrolytic oxidation of aluminium in alkaline solutions’, Electrochim. Acta, 2004, 49, 2085–2095. doi: 10.1016/j.electacta.2003.11.027

20.

Snizhko

L. O.

, Yerokhin

A. L.

, Gurevina

N. L.

, Patalakha

V. A.

and Matthews

: ‘Excessive oxygen evolution during plasma electrolytic oxidation of aluminium’, Thin Solid Films, 2007, 516, 460–464. doi: 10.1016/j.tsf.2007.06.158

21.

Sundararajan

and R. Krishna

: ‘Mechanisms underlying the formation of thick alumina coatings through the MAO coating technology’, Surf. Coat. Technol., 2003, 167, 269–277. doi: 10.1016/S0257-8972(02)00918-0

22.

Vangolu

, Arslan

, Totik

, Demirci

and Alsaran

: ‘Optimization of the coating parameters for micro-arc oxidation of Cp-Ti’, Surf. Coat. Technol., 2010, 205, 1764–1773. doi: 10.1016/j.surfcoat.2010.08.042

23.

Shokouhfar

, Dehghanian

and Baradaran

: ‘Preparation of ceramic coating on Ti substrate by plasma electrolytic oxidation in different electrolutes and evaluation of its corrosion resistance’, Appl. Surf. Sci., 2011, 257, 2617–2624. doi: 10.1016/j.apsusc.2010.10.032

24.

Tsunekawa

, Aoki

and Habazaki

: ‘Two-step plasma electrolytic oxidation of Ti-15V-3Al-3Cr-3Sn for wear-resistant and adhesive coating’, Surf. Coat. Technol., 2011, 205, 4732–4740. doi: 10.1016/j.surfcoat.2011.04.060

25.

Khorasanian

, Dehghan

, Shariat

M. H.

, Bahrololoom

M. E.

and Javadpour

: ‘Microstructure and wear resistance of oxide coatings on Ti-6Al-4V produced by plasma electrolytic oxidation in an inexpensive electrolyte’, Surf. Coat. Technol., 2011, 206, 1495–1502. doi: 10.1016/j.surfcoat.2011.09.038

26.

Yao

, Jiang

, Jia

, Jiang

and Wang

: ‘Growth characteristics of plasma electrolytic oxidation on Ti-6Al-4V alloy’, Appl. Surf. Sci., 2008, 254, 4084–4091. doi: 10.1016/j.apsusc.2007.12.062

27.

Yerochin

A. L.

, Nie

, Leyland

, Matthews

and J. Dowey

: ‘Plasma electrolysis for surface engineering’, Surf. Coat. Technol., 1999, 122, 73–93. doi: 10.1016/S0257-8972(99)00441-7

28.

Habazaki

, Tsunekawa

, Tsuji

and Nakayama

: ‘Formation and characterization of wear-resistant PEO coatings formed on b-titanium alloy at different electrolyte temperatures’, Appl. Surf. Sci., 2012, 259, 711–718. doi: 10.1016/j.apsusc.2012.07.104

29.

Venkateswarlu

, Rameshbabu

, Sreekanth

, Bose

A. C.

, Muthupandi

and Subramanian

: ‘Fabrication and characterization of micro-arc oxidized fluoride containing titania films on Cp Ti’, Ceram. Int., 2013, 39, 801–812. doi: 10.1016/j.ceramint.2012.07.001

30.

Venkateswarlu

, Rameshbabu

, Sreekanth

, Bose

A. C.

, Muthupandi

, Babu

N. K.

and Subramanian

: ‘Role of electrolyte additives on in-vitro electrochemical behaviour of micro arc oxidized titania films on Cp Ti’, Appl. Surf. Sci., 2012, 258, 6853–6863. doi: 10.1016/j.apsusc.2012.03.118

31.

Cheng

, Chen

and Nie

: ‘Surface morphology manipulation and wear property of bioceramic oxide coatings on titanium alloy’, Surf. Coat. Technol., 2013, 215, 253–259. doi: 10.1016/j.surfcoat.2012.07.093

32.

Archard

J. F.

: ‘Contact and rubbing of flat surfaces’, J. Appl. Phys., 1953, 24, 981–988. doi: 10.1063/1.1721448