Plasma Electrolytic Oxidation as a modern method to … Electrolytic Oxidation as a modern method ... The modification in nanoscale is possible by using electropolishing [1-6], magnetoelectropolishing

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WSN 35 (2016) 44-61 EISSN 2392-2192

Plasma Electrolytic Oxidation as a modern method to form porous coatings enriched in phosphorus

and copper on biomaterials

Krzysztof Rokosza, Tadeusz Hryniewiczb

Division of Surface Electrochemistry and Engineering, Koszalin University of Technology, Racławicka 15-17, PL 75-620 Koszalin, Poland

a,bE-mail address: [email protected] , [email protected]

ABSTRACT

In the paper, the porous coatings obtained on niobium and two titanium alloys (Ti6Al4V, and

TNZ) after Plasma Electrolytic Oxidation (PEO), known also as Micro Arc Oxidation, were studied.

The samples were treated at the voltage of 450 V for 3 minutes in the electrolyte consisting of 300 g

and 600 g of copper nitrate Cu(NO3)2 in 1 litre of concentrated phosphoric acid H3PO4, consecutively.

SEM and EDS studies were performed on the samples. Based on the obtained results it may be

concluded that enriched in copper porous coatings on all studied materials were created in the

electrolyte within copper nitrate amounting for 600 g. The proposed by the Authors factor to evaluate

the obtained coatings, i.e. copper-to-phosphorus ratio, which for the studied materials amounted to

0.21, clearly indicates that the performed electrochemical PEO treatment for surface modification

especially of bioimplants may be advised.

Keywords: Plasma Electrolytic Oxidation (PEO); Niobium; Ti6Al4V; TNZ; Porous coatings

mailto:[email protected]

mailto:[email protected]

World Scientific News 35 (2016) 44-61

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1. INTRODUCTION

The modification in nanoscale is possible by using electropolishing [1-6],

magnetoelectropolishing [7-14] as well as high-current density electropolishing [15-17]

processes, while in microscale – by Plasma Electrolytic Oxidation (PEO), known also as

Micro Arc Oxidation [18-21] of surface layer. Nowadays the surface modification by Plasma

Electrolytic Oxidation of biomaterials, such as titanium, niobium, zirconium, tantalum as well

as on their alloys, is widely used in medical techniques. By this method of electrolytic

treatment it is possible to obtain the porous coatings having a thickness of about 10

micrometers and consisting mainly of phosphorus within some desirable chemical elements,

resulting in their biocompatibility [22]. Most important feature of the present studies is the

possibility to obtain the coating with antibacterial copper [23-28] inside and with small

amount of toxic elements such as aluminum [29-34], vanadium [25-41] and tin [42].

2. METHOD

2. 1. Material

The Niobium and two titanium alloys (Ti6Al4V, TNZ) samples cut off from the cold-

rolled plate served for the study (Table 1). The samples were prepared in the form of

rectangular specimens of dimensions 5301.3 mm and cleaned with acetone before the PEO

treatment.

Table 1. Nominal composition of Ti alloys/materials used in the study, wt %.

Ti Nb Zr V Al

Ti6Al4V 90 0 0 4 6

TNZ 74 20 60 0 0

2. 2. PEO set up and parameters

Fig. 1. Set up for Plasma Electrolytic Oxidation.


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The plasma electrolytic oxidation (PEO) was performed at the average voltage of 450 V,

according to the machining set up, which is presented in Figure 1. The shapes of voltage

pulsations and current are visible in Figure 2. The main elements of the set-up were:

a processing cell, a dc three-phase power supply, the electrodes and connecting wiring. The

studies were carried out in the electrolyte of initial temperature of 20 2 C. For the studies,

the electrolyte composed of orthophosphoric (85%) acid with an addition of copper II nitrate

was used. For each run, the electrolytic cell made of glass was used, containing up to 500 ml

of the electrolyte.

Fig. 2. Shapes of voltage and current during Plasma Electrolytic Oxidation process.

2. 3. Set up of SEM/EDS

The scanning electron microscope Quanta 250 FEI with Low Vacuum and ESEM mode

and a field emission cathode as well as the energy dispersive EDX system in a Noran System

Six with nitrogen-free silicon drift detector were used. The magnification of 6000 times for

SEM photos and EDS analyses were used.

3. RESULTS

3. 1. Niobium – electrolyte of lower Cu(NO3)2 concentration

In Figures 3 through 8 and in Tables 2-3 there are presented results of Niobium sample

after PEO treatment at 450 V for 3 minutes in the electrolyte consisting of 300 g of Cu(NO3)2

in 1 L H3PO4. Based on the SEM picture, presented in Figure 3, it is possible to conclude that

the PEO coating is porous, but the pores are small and mostly closed. Additionally, the

studied surface consisted mainly of niobium and phosphorus (Fig. 4) in the amounts of 48.23

±0.8 wt% and 9.03 ±0.26 wt%, respectively. In that place of analysis it was not possible to

detect copper, what is noticeable in Table 2. In the other place of the study on the same

sample, presented in Figure 5, a bigger pore is included and in this case it was possible to

record the small amount of copper, see Table 3 (1.32 ±0.29 wt%). The other elements, which


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are dominant, i.e. niobium and phosphorus were detected in the amounts of 47.73±0.65 wt%

and 8.11 ±0.26 wt%, respectively (Table 3). In Figures 7-8, the higher magnifications of

50000 times and 75000 times are presented, to demostrate the pore diameter, which amounts

to about one micrometer.

Fig. 3. SEM image of Niobium after PEO treatment at 450 V for 3 minutes in the electrolyte

consisting of 300 g Cu(NO3)2 in 1 L H3PO4, magnification: 6000 times.

Fig. 4. EDS spectrum of Niobium after PEO treatment at 450 V for 3 minutes in the electrolyte

consisting of 300 g Cu(NO3)2 in 1 L H3PO4


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Table 2. Results of EDS analysis of Niobium after PEO treatment at 450 V for 3 minutes in the

electrolyte consisting of 300 g Cu(NO3)2 in 1 L H3PO4

Element

lines wt%

Error

wt% at%

Error

at%

C K 2.75 ± 0.15 6.48 ± 0.69

O K 39.88 --- 70.49 ± 2.39

Si K 0.11 ± 0.05 0.11 ± 0.11

P K 9.03 ± 0.26 8.24 ± 0.47

Nb L 48.23 ± 0.80 14.68 ± 0.48

Fig. 5. SEM image and EDS spectrum of Niobium after PEO treatment at 450 V for 3 minutes in the

electrolyte consisting of 300 g Cu(NO3)2 in 1 L H3PO4, magnification: 6000 times.


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Fig. 6. SEM image and EDS spectrum of Niobium after PEO treatment at 450 V for 3 minutes in the





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Element

lines wt%

Error

wt% at%

Error

at%

C K 3.18 ± 0.16 7.49 ± 0.76

O K 39.66 --- 70.02 ± 2.45

P K 8.11 ± 0.26 7.39 ± 0.47

Cu K 1.32 ± 0.29 0.59 ± 0.26

Nb L 47.73 ± 0.65 14.51 ± 0.40


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3. 2. Niobium – electrolyte of higher Cu(NO3)2 concentration


consisting of 600 g Cu(NO3)2 in 1 L H3PO4; magnification: 6000 times.

Fig. 10. EDS spectrum of Niobium after PEO treatment at 450 V for 3 minutes in the electrolyte


In Figures 9-10 and in Table 4, there are presented results of Niobium sample after

PEO treatment at 450 V for 3 minutes in the electrolyte consisting of 600 g of Cu(NO3)2 in

1 L H3PO4.


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Element

lines wt%

Error

wt% at%

Error

at%

C K 2.89 ± 0.20 6.08 ± 0.83

O K 44.04S --- 69.51 ± 2.09

P K 16.93 ± 0.31 13.81 ± 0.50

Fe K 1.83 ± 0.20 0.83 ± 0.18

Cu K 3.58 ± 0.35 1.42 ± 0.28

Nb L 30.72 ± 0.82 8.35 ± 0.44


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Based on the SEM pictures, presented in Figure 9, it is possible to conclude that the

PEO coating is very porous, but the pores are sharp and mostly opened. The studied results of

surface layer (Figure 10), containing mainly the niobium and phosphorus in the amounts of

30.72 ±0.82 wt% and 16.93 ±0.31 wt%, respectively, within the copper (3.58 ±0.35 wt%) and

iron (1.83 ±0.2 wt%) inside, are reported in Table 4. In Figure 11, the shape of the pore is

presented. Most important information is that the pores are layered, i.e. in the larger ones

there are located the smaller pores

3. 3. Titanium alloy – electrolyte of lower Cu(NO3)2 concentration

In Figures 12-13 and in Table 5, there are presented results of Ti6Al4V alloy sample


in 1 L H3PO4. Based on the SEM picture, presented in Figure 12, it is possible to conclude

that the PEO coating is very porous, but the pores are sharp and mostly opened. The studied

results of the surface layer (Figure 13), containing mainly the titanium and phosphorus in the

amounts of 26.71 ±0.34 wt% and 18.55 ±0.21 wt%, respectively, within the copper (1.72

±0.26 wt%), iron (1.05±0.14 wt%), aluminum (1.33 ±0.09 wt%), as well as calcium (0.26

±0.05 wt%) inside, are presented in Table 5.

Fig. 12. SEM image of Ti6Al4V alloy after PEO treatment at 450 V for 3 minutes in the electrolyte

consisting of 300 g Cu(NO3)2 in 1 L H3PO4; magnification: 6000 times


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Fig. 13. EDS spectrum of Ti6Al4V alloy after PEO treatment at 450 V for 3 minutes in the electrolyte


Table 5. Results of of Ti6Al4V alloy after PEO treatment at 450 V for 3 minutes in the electrolyte


Element

lines wt%

Error

wt% at%

Error

at%

C K 1.81 ± 0.16 3.39 ± 0.59

O K 48.57 --- 68.30 ± 2.18

Al K 1.33 ± 0.09 1.11 ± 0.14

P K 18.55 ± 0.21 13.47 ± 0.31

Ca K 0.26 ± 0.05 0.15 ± 0.05

Ti K 26.71 ± 0.34 12.55 ± 0.32

Fe K 1.05 ± 0.14 0.42 ± 0.11

Cu K 1.72 ± 0.26 0.61 ± 0.18

3. 4. Titanium alloys – electrolyte of higher Cu(NO3)2 concentration

In Figures 14-15 and in Table 6, there are presented results of Ti6Al4V alloy sample


in 1 L H3PO4. Based on the SEM picture, presented in Figure 14, it is possible to conclude

that the PEO coating is very porous, but the pores are also sharp and mostly opened. The

studied results of the surface layer (Figure 15), containing mainly the titanium and

phosphorus in the amounts of 22.36 ±0.31 wt% and 18.75 ±0.21 wt%, respectively, within the

copper (3.85 ±0.51 wt%), iron (0.58 ±0.13 wt %), aluminum (1.08 ±0.08 wt%) inside, have

been presented in Table 6.


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Fig. 14. SEM image of Ti6Al4V alloy after PEO treatment at 450 V for 3 minutes in the electrolyte

consisting of 600 g Cu(NO3)2 in 1 L H3PO4,

Table 6. Results of Ti6Al4V alloy after PEO treatment at 450 V for 3 minutes in the electrolyte


Element

lines wt%

Error

wt% at%

Error

at%

C K 3.42 ± 0.19 6.20 ± 0.68

O K 49.96 --- 68.02 ± 2.07

Al K 1.08 ± 0.08 0.87 ± 0.13

P K 18.75 ± 0.21 13.19 ± 0.30

Ti K 22.36 ± 0.31 10.17 ± 0.28

Fe K 0.58 ± 0.13 0.22 ± 0.10

Cu K 3.85 ± 0.51 1.32 ± 0.35


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Fig. 15. EDS spectrum of Ti6Al4V alloy after PEO treatment at 450 V for 3 minutes in the electrolyte


In Figures 16-17 and in Table 7, there are presented results of TNZ alloy sample after

PEO treatment at 450 V for 3 minutes in the electrolyte consisting of 600 g of Cu(NO3)2 in

1 L H3PO4. Based on the SEM picture, presented in Figure 16, it is possible to conclude that

the PEO coating is very porous, but the pores are sharp and mostly opened. The studied

results of the surface layer (Figure 17), containing mainly the titanium, niobium, zirconium

and phosphorus in the amounts of 17.19 ±0.22 wt%, 5.39 ±0.92 wt%, 5.89 ±2.13 wt% and

17.97 ±0.85 wt%, respectively, within the copper (3.81 ±0.32 wt %), iron (1.03 ±0.16 wt %)

and calcium (0.23 ±0.06 wt %) inside, are given in Table 7.

Table 7. Results of TNZ alloy after PEO treatment at 450 V for 3 minutes in the electrolyte consisting

of 600 g Cu(NO3)2 in 1 L H3PO4

Element

lines wt%

Error

wt% at%

Error

at%

C K 2.29 ± 0.17 4.52 ± 0.66

O K 46.20 --- 68.36 ± 2.24

P K 17.97 ± 0.85 13.74 ± 1.30

Ca K 0.23 ± 0.06 0.14 ± 0.07

Ti K 17.19 ± 0.22 8.50 ± 0.22

Fe K 1.03 ± 0.16 0.44 ± 0.14

Cu K 3.81 ± 0.32 1.42 ± 0.24

Zr L 5.89 ± 2.13 1.53 ± 1.11

Nb L 5.39 ± 0.92 1.37 ± 0.47


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Fig. 16. SEM image of TNZ alloy after PEO treatment at 450 V for 3 minutes in the electrolyte


Fig. 17. EDS spectrum of TNZ alloy after PEO treatment at 450 V for 3 minutes in the electrolyte


4. CONCLUSION

Due to the fact that all studied samples (matrices) have different chemical compositions

(wt%), the ratio of copper-to-phosphorus in the PEO coating was selected to carry out their


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comparison. For the electrolytes containing of 600 g Cu(NO3)2 in 1 L H3PO4 to all studied

materials the calculated copper-to-phosphorus ratio was equal and amounted to 0.21.

In case of less concentrated solutions, consisting of 1 L H3PO4 within 300 g Cu(NO3)2,

the copper content in PEO coating was much smaller: in case of niobium it was between 0 and

0.16, while for Ti6Al4V amounted to 0.09.

It must be concluded that to obtain a porous PEO coatings enriched in copper, it is

advisable to use the electrolyte consisting of 600 g Cu(NO3)2 in 1 L H3PO4.

ACKNOWLEDGMENTS

Dra G. Vara, and Dra M.B. García-Blanco from Fundación Cidetec. Parque Tecnológico de San Sebastián Pº

Miramón, 196 20009 Donostia-San Sebastián (Gipuzkoa), Spain, are highly acknowledged for delivering

Ti6Al4V for the PEO experiments.

Prof. F. Prima from Ecole Nationale Supérieure de Chimie de Paris, France, as well as Prof. Zschommler

Sandim, H. R. from DEMAR-EEL-USP, Universidade de São Paulo, Brazil are thankful for delivering TNZ and

Niobium materials for the PEO experiments, respectively.

Prof. Dr.-Ing. Winfried Malorny and Dr Torsten Barfels from Hochschule Wismar, Germany, are highly

acknowledged for providing access to the SEM/EDS apparatus allowing to perform the studies.

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Documents

Plasma Electrolytic Oxidation as a modern method to … Electrolytic Oxidation as a modern method ... The modification in nanoscale is possible by using electropolishing [1-6], magnetoelectropolishing