Available online at www.worldscientificnews.com

( Received 20 January 2018; Accepted 07 February 2018; Date of Publication 08 February 2018 )

WSN 94(2) (2018) 99-114 EISSN 2392-2192

Porous PEO coatings on titanium, obtained under DC regime, enriched in magnesium, calcium, zinc,

and copper

Krzysztof Rokosza, *Tadeusz Hryniewiczb, Kornel Pietrzakc, Łukasz Dudekd

Division of BioEngineering and Surface Electrochemistry, Department of Engineering and Informatics Systems, Faculty of Mechanical Engineering, Koszalin University of Technology,

Racławicka 15-17, PL 75-620 Koszalin, Poland

a-dE-mail address: rokosz@tu.koszalin.pl , Tadeusz.Hryniewicz@tu.koszalin.pl ,

kornel.pietrzak@s.tu.koszalin.pl , lukasz.dudek@tu.koszalin.pl,

*Corresponding authors: Tadeusz.Hryniewicz@tu.koszalin.pl

ABSTRACT

In the present paper, the analysis of SEM and EDS results of porous and enriched in calcium,

magnesium, zinc, copper and phosphorus coatings, obtained during 3-minute treatments performed by

Plasma Electrolytic Oxidation (PEO)/Micro Arc Oxidation (MAO) processes on CP Titanium Grade

2, is presented. The PEO process was carried out at DC potentials of 500 VDC, 575 VDC, and 650 VDC

in electrolytes containing 125 g Ca(NO3)2·4H2O, and 125 g Mg(NO3)2∙6H2O, and 125 g

Zn(NO3)2∙6H2O, and 125 g Cu(NO3)2∙3H2O in 1 L H3PO4. It was found that obtained coatings have

pores with different shapes and diameters and/or size/dimentions. The Ca/P, Mg/P, Zn/P, Cu/P, and

M/P ratios (M=Ca+Mg+Zn+Cu) were equal to 0.074, 0.046, 0.056, 0.042, and 0.218, respectively.

The highest value of each of these ratios was recorded for 650 VDC. It may be concluded that the

obtained PEO coatings structures most likely are similar to hydroxyapatite-like structures, in which the

Ca2+

may be replaced with Zn2+

, Mg2+

, Cu2+

.

Keywords: Plasma Electrolytic Oxidation (PEO); Micro Arc Oxidation (MAO); CP Titanium Grade 2;

calcium nitrate Ca(NO3)2·4H2O; magnesium nitrate Mg(NO3)2∙6H2O; zinc nitrate Zn(NO3)2∙6H2O;

copper nitrate Cu(NO3)2∙3H2O

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1. INTRODUCTION

Metal surface treatments by electrochemical processing have been steadily developed

since many decades. Standard electropolishing (EP) [1-5], considered as a surface finishing

performed on a plateau current density level has been recently modified by extending the

treatment conditions to the high-current density electropolishing (HCEP) [6-8] or high-

voltage electropolishing (HVEP) [9] with the improved results concerning nano-films of

nano-coatings. Another electrochemical surface technique acquires a magnetic field [10] with

the process named magnetoelectropolishing (MEP) [10-24] in which, apart from improving

surface finishing [13-24], nanohardness [12], corrosion resistance [13-16], biological response

[26], and fundamental about 90-percent de-hydrogenation is obtained [25].

On the other hand, Plasma Electrolytic Oxidation (PEO) [27] also known as Micro Arc

Oxidation (MAO) has been developed to form micro-coatings on metals and alloys [28-64].

These coatings are usually porous and may be enriched in chemical elements, such as

phosphorus and calcium to form the hydroxyapatite-like structure [38-46]. Moreover, another

elements, such as bactericidal copper [47-56], magnesium, which may accelerate the healing

of wounds [41,58,59] as well as zinc with antibacterial properties [40,60-63], capable for

improving osteogenic characteristics and bone regenerating capacity [64-67], may be

introduced into the porous coatings.

The aim of this work is to characterize porous and biocompatible hydroxyapatite-like

surface coatings (calcium-phosphorus structures) created on titanium by Plasma Electrolytic

Oxidation; these coatings are enriched with antibacterial zinc and copper as well as with

magnesium. The additives, used to enrich the porous coatings on metallic implants, are

helpful with wound healing.

2. METHOD

The samples of CP Titanium Grade 2, with dimensions 10 10 2 mm, were treated by

Plasma Electrolytic Oxidation (Micro Arc Oxidation) for the surface studies. The plasma

electrolytic oxidation (PEO) was performed at the voltages of 500 VDC, 575 VDC and

650 VDC. For the studies, the electrolyte containing 125 g calcium nitrate Ca(NO3)2·4H2O,

and 125 g magnesium nitrate Mg(NO3)2∙6H2O, and 125 g zinc nitrate Zn(NO3)2∙6H2O, and

125 g copper nitrate Cu(NO3)2∙3H2O, in 1 L concentrated 85% analytically pure

orthophosphoric acid H3PO4, was used. For each run, the electrolytic cell made of glass was

used, containing up to 500 ml of the electrolyte.

For the studies, the scanning electron microscope Quanta 250 FEI with Low Vacuum

and Environmental Scanning Electron Microscope (ESEM) mode and a field emission

cathode as well as the Energy-Dispersive X-ray Spectroscopy (EDS) system in a Noran

System Six with nitrogen-free silicon drift detector, were employed.

3. RESULTS AND DISCUSSION

In Figures 1 and 2, the SEM images of coating formed on Titanium after PEO

treatment at 500 VDC in electrolyte containing of 125 g Ca(NO3)2·4H2O, 125 g

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Mg(NO3)2∙6H2O, 125 g Zn(NO3)2∙6H2O, and 125 g Cu(NO3)2∙3H2O in 1 L H3PO4, are

presented.

Fig. 1. SEM pictures with magnification 500 times spectrum for coating formed on Titanium

after PEO treatment for 3 min at 500 VDC in electrolyte containing of 125 g Ca(NO3)2·4H2O,

125 g Mg(NO3)2∙6H2O, 125 g Zn(NO3)2∙6H2O, and 125 g Cu(NO3)2∙3H2O in 1 L H3PO4

It may be seen that during Plasma Electrolytic Oxidation process, irregular surface with

common micrometers size pores were developed which in general is beneficial to bone tissue

growth [59] and can be employed as local drug delivery systems where polymer matrix

containing bioactive substances is applied as thin layer also filling pores [60-62]. (This is also

known as smart materials). In Figure 3 and Tables 1 and 2, EDS results of coating formed on

Titanium after PEO treatment at 500 VDC in electrolyte containing of 125 g Ca(NO3)2·4H2O,

and 125 g Mg(NO3)2∙6H2O, and 125 g Zn(NO3)2∙6H2O, and 125 g Cu(NO3)2∙3H2O in 1 L

H3PO4, are displayed.

These EDS peaks of phosphorus, titanium, calcium, magnesium, zinc and copper show

that formed PEO coating is built mainly of phosphorus-titanium-calcium and calcium

substituted by magnesium zinc and copper compounds what may suggest the hydroxyapatite-

like structure has been formed. In PEO coating, apart from the titanium (36.3 ± 0.2 at%)

which is substrate, and of which signal may partly come from matrix, phosphorus (54.4 ± 0.4

at%), calcium (1.8 ± 0.1 at%), magnesium (2.0 ± 0.1 at%), zinc (1.9 ± 0.2 at%) and copper

(1.8 ± 0.2 at%) were also recorded. Thus metal-to-phosphorus ratios for calcium, magnesium,

zinc, copper and sum of metals equal 0.066, 0.037, 0.034, 0.033, and 0.170, respectively.

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Fig. 2. SEM pictures with magnification 10 000 times spectrum for coating formed on

Titanium after PEO treatment for 3 min at 500 VDC in electrolyte containing of 125 g

Ca(NO3)2·4H2O, 125 g Mg(NO3)2∙6H2O, 125 g Zn(NO3)2∙6H2O, and 125 g Cu(NO3)2∙3H2O

in 1 L H3PO4

Fig. 3. EDS spectrum for coating formed on titanium after PEO treatment for 3 min at 500

VDC in electrolyte containing of 125 g Ca(NO3)2·4H2O, 125 g Mg(NO3)2∙6H2O, 125 g

Zn(NO3)2∙6H2O, 125 g Cu(NO3)2∙3H2O in 1 L H3PO4

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Table 1. EDS results of coatings formed on Titanium after PEO treatment for 3 min at 500

VDC in electrolyte containing of 125 g Ca(NO3)2·4H2O, 125 g Mg(NO3)2∙6H2O, 125 g

Zn(NO3)2∙6H2O, 125 g Cu(NO3)2∙3H2O in 1 L H3PO4

Atomic Concentration [%]

P Ca Mg Zn Cu Ti

54.4 ± 0.4 3.6 ± 0.1 2.0 ± 0.1 1.9 ± 0.2 1.8 ± 0.2 36.3 ± 0.2

Table 2. Metal-to-phosphorus ratios of coatings formed on Titanium after PEO treatment for

3 min at 500 VDC in electrolyte containing of 125 g Ca(NO3)2·4H2O, 125 g Mg(NO3)2∙6H2O,

125 g Zn(NO3)2∙6H2O, 125 g Cu(NO3)2∙3H2O in 1 L H3PO4

Metal-to-phosphorus ratio

Ca/P Mg/P Zn/P Cu/P M/P

0.066 0.037 0.034 0.033 0.170

Table 3. EDS results of coatings formed on Titanium after PEO treatment for 3 min at 575

VDC in electrolyte containing of 125 g Ca(NO3)2·4H2O, 125 g Mg(NO3)2∙6H2O, 125 g

Zn(NO3)2∙6H2O, 125 g Cu(NO3)2∙3H2O in 1 L H3PO4

Atomic Concentration [%]

P Ca Mg Zn Cu Ti

49.4 ± 0.4 2.6 ± 0.1 2.2 ± 0.1 2.0 ± 0.3 1.8 ± 0.2 42.0 ± 0.3

Table 4. Metal-to-phosphorus ratios of coatings formed on Titanium after PEO treatment for

3 min at 575 VDC in electrolyte containing of 125 g Ca(NO3)2·4H2O, 125 g Mg(NO3)2∙6H2O,

125 g Zn(NO3)2∙6H2O, 125 g Cu(NO3)2∙3H2O in 1 L H3PO4

Metal-to-phosphorus ratio

Ca/P Mg/P Zn/P Cu/P M/P

0.053 0.043 0.043 0.039 0.177

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In Figures 4 and 5, the SEM pictures of coating formed on Titanium after PEO

treatment at 575 VDC in electrolyte containing of 125 g Ca(NO3)2·4H2O, and 125 g

Mg(NO3)2∙6H2O, and 125 g Zn(NO3)2∙6H2O, and 125 g Cu(NO3)2∙3H2O in 1 L H3PO4, are

presented. One may notice, that during plasma electrolytic oxidation process, irregular surface

with common micrometers size pores were developed, which in general is beneficial to bone

tissue growth [59] and can be employed as local drug delivery systems where polymer matrix

containing bioactive substances is applied as thin layer also filling pores [60-62].

In Figure 6 and Tables 3 and 4, EDS results of coating formed on Titanium after PEO

treatment at 575 VDC in the electrolyte containing of 125 g Ca(NO3)2·4H2O, and 125 g

Mg(NO3)2∙6H2O, and 125 g Zn(NO3)2∙6H2O, and 125 g Cu(NO3)2∙3H2O in 1 L H3PO4, are

presented. These EDS peaks of phosphorus, titanium, calcium, magnesium, zinc and copper

show that formed PEO coating is built mainly of phosphorus-titanium-calcium and calcium

substituted by magnesium zinc and copper compounds what may suggest, the hydroxyapatite-

like structure may exist.

In the PEO coating, apart from the titanium (42.0 ± 0.3 at%) which is substrate, and of

which signal may partly come from matrix, phosphorus (49.4 ± 0.4 at%), calcium (2.6 ± 0.1

at%), magnesium (2.2 ± 0.1 at%), zinc (2.0 ± 0.3 at%) and copper (1.8 ± 0.2 at%), were also

recorded. Thus metal to phosphorus ratios for calcium, magnesium, zinc, copper and sum of

metals equal 0.053, 0.043, 0.043, 0.039, and 0.177, respectively.

Fig. 4. SEM pictures with magnification 500 times spectrum for coating formed on Titanium

after PEO treatment for 3 min at 575 VDC in electrolyte containing of 125 g Ca(NO3)2·4H2O,

125 g Mg(NO3)2∙6H2O, 125 g Zn(NO3)2∙6H2O, 125 g Cu(NO3)2∙3H2O in 1 L H3PO4

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Fig. 5. SEM pictures with magnification 10 000 times spectrum for coating formed on

Titanium after PEO treatment for 3 min at 575 VDC in electrolyte containing of 125 g

Ca(NO3)2·4H2O, 125 g Mg(NO3)2∙6H2O, 125 g Zn(NO3)2∙6H2O, 125 g Cu(NO3)2∙3H2O in

1 L H3PO4

Fig. 6. EDS spectrum for coating formed on titanium after PEO treatment for 3 min at 575

VDC in electrolyte containing of 125 g Ca(NO3)2·4H2O, 125 g Mg(NO3)2∙6H2O, 125 g

Zn(NO3)2∙6H2O, 125 g Cu(NO3)2∙3H2O in 1 L H3PO4

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In Figures 7 and 8, the SEM images of coating formed on Titanium after PEO

treatment at 575 VDC in electrolyte containing of 125 g Ca(NO3)2·4H2O, and 125 g

Mg(NO3)2∙6H2O, and 125 g Zn(NO3)2∙6H2O, and 125 g Cu(NO3)2∙3H2O in 1 L H3PO4, are

presented. One can easily notice, that during plasma electrolytic oxidation process, irregular

surface with common micrometers size pores were developed, which in general is beneficial

to bone tissue growth [59] and can be employed as local drug delivery systems where polymer

matrix containing bioactive substances is applied as thin layer also filling pores [65-67].

In Figure 9 and Tables 5-6, EDS results of coating formed on Titanium after PEO

treatment at 575 VDC in electrolyte containing of 125 g Ca(NO3)2·4H2O, and 125 g

Mg(NO3)2∙6H2O, and 125 g Zn(NO3)2∙6H2O, and 125 g Cu(NO3)2∙3H2O in 1 L H3PO4, are

presented. These EDS peaks of phosphorus, titanium, calcium, magnesium, zinc and copper

show that formed PEO coating is built mainly of phosphorus-titanium-calcium and calcium

substituted by magnesium zinc and copper compounds, what may suggest the hydroxyapatite-

like structure is formed. In the PEO coating, apart from the titanium (38.5 ± 0.2 at%) which is

a substrate, and of which signal may partly come from matrix, phosphorus (50.3 ± 0.4 at%),

calcium (4.7 ± 0.1 at%), magnesium (2.2 ± 0.1 at%), zinc (2.4 ± 0.3 at%) and copper (1.9 ±

0.3 at%), were also recorded. Thus metal-to-phosphorus ratios for calcium (Ca/P), magnesium

(Mg/P), zinc (Zn/P), copper (Cu/P), and the ratio of sum of these metals (M/P) were equal to

0.074, 0.046, 0.056, 0.042, and 0.218, respectively.

Fig. 7. SEM pictures with magnification 500 times spectrum for coating formed on Titanium

after PEO treatment for 3 min at 650 VDC in electrolyte containing of 125 g Ca(NO3)2·4H2O,

125 g Mg(NO3)2∙6H2O, 125 g Zn(NO3)2∙6H2O, 125 g Cu(NO3)2∙3H2O in 1 L H3PO4

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Fig. 8. SEM pictures with magnification 10 000 times spectrum for coating formed on

Titanium after PEO treatment for 3 min at 650 VDC in electrolyte containing of 125 g

Ca(NO3)2·4H2O, 125 g Mg(NO3)2∙6H2O, 125 g Zn(NO3)2∙6H2O, 125 g Cu(NO3)2∙3H2O in

1 L H3PO4

Fig. 9. SEM pictures with magnification (a) 500 times, (b) 10 000 times and (c) EDS

spectrum for coatings formed on Titanium after PEO treatment for 3 min at 650 VDC in

electrolyte containing of 125 g Ca(NO3)2·4H2O, 125 g Mg(NO3)2∙6H2O, 125 g

Zn(NO3)2∙6H2O, 125 g Cu(NO3)2∙3H2O in 1 L H3PO4

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In Figure 10, the summary EDS results for 500 VDC, 575 VDC and 650 VDC, are

presented. One can note that all of M/P ratios (for M = Ca, Mg, Zn, Cu) decrease for 500 VDC

and 575 VDC, with some mixed results referred to 650 VDC. On the other hand, however, the

ratio of the sum of all four metals (Ca+Mg+Zn+Cu) to phosphorus increases with the growth

of PEO voltage from 500 up to 650 VDC.

Fig. 10. EDS results for coatings formed on Titanium after PEO treatment for 3 min in

electrolyte containing of 125 g Ca(NO3)2·4H2O, 125 g Mg(NO3)2∙6H2O, 125 g

Zn(NO3)2∙6H2O, 125 g Cu(NO3)2∙3H2O in 1 L H3PO4

Table 5. EDS results of coatings formed on Titanium after PEO treatment for 3 min at 650

VDC in electrolyte containing of 125 g Ca(NO3)2·4H2O, 125 g Mg(NO3)2∙6H2O, 125 g

Zn(NO3)2∙6H2O, 125 g Cu(NO3)2∙3H2O in 1 L H3PO4

Atomic Concentration [%]

P Ca Mg Zn Cu Ti

50.3 ± 0.4 4.7 ± 0.1 2.2 ± 0.1 2.4 ± 0.3 1.9 ± 0.3 38.5 ± 0.2

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Table 6. Metal-to-phosphorus ratios of coatings formed on Titanium after PEO treatment for

3 min at 650 VDC in electrolyte containing of 125 g Ca(NO3)2·4H2O, 125 g Mg(NO3)2∙6H2O,

125 g Zn(NO3)2∙6H2O, 125 g Cu(NO3)2∙3H2O in 1 L H3PO4

Metal-to-phosphorus ratio

Ca/P Mg/P Zn/P Cu/P M/P

0.074 0.046 0.056 0.042 0.218

4. CONCLUSIONS

The following conclusions may be drawn from the studies:

it is possible to obtain the porous surface on titanium, enriched in magnesium, calcium,

zinc and copper as well as phosphorus in electrolyte containing: 125 g Ca(NO3)2·4H2O,

and 125 g Mg(NO3)2∙6H2O, and 125 g Zn(NO3)2∙6H2O, and 125 g Cu(NO3)2∙3H2O, in 1

L H3PO4 at the voltage ranging from 500 VDC until 650 VDC during 3 min PEO process

the ratios Ca/P, Mg/P, Zn/P, Cu/P, and M/P, where M=Ca+Mg+Zn+Cu, were equal to

0.074, 0.046, 0.056, 0.042, 0.218, respectively

the highest values of all those ratios were recorded for 650 VDC

most likely the obtained coatings are similar to hydroxyapatite-like structures, in which

the Ca2+

may be replaced with Zn2+

, Mg2+

, Cu2+

.

Acknowledgements

This work was supported by subsidizing by Grant OPUS 11 of National Science Centre, Poland, with

registration number 2016/21/B/ST8/01952, titled "Development of models of new porous coatings obtained on

titanium by Plasma Electrolytic Oxidation in electrolytes containing phosphoric acid with addition of calcium,

magnesium, copper and zinc nitrates".

Prof. Winfried Malorny from Hochschule Wismar-University of Applied Sciences Technology, Business and

Design, Faculty of Engineering, DE 23966 Wismar, Germany, is given thanks for providing access to the

SEM/EDS apparatus allowing to perform the studies.

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