Development and SEM/EDS characterisation of porous coatings enriched in magnesium and copper obtained on titanium by PEO with ramp voltage

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1 Available online at WSN 80 (2017) EISSN Development and SEM/EDS characterisation of porous coatings enriched in magnesium and copper obtained on titanium by PEO with ramp voltage Krzysztof Rokosz a, *Tadeusz Hryniewicz b, Kornel Pietrzak c, Łukasz Dudek d Division of Bioengineering and Surface Electrochemistry, Department of Engineering and Informatics Systems, Faculty of Mechanical Engineering, Koszalin University of Technology, Racławicka 15-17, PL Koszalin, Poland address: a rokosz@tu.koszalin.pl, b Tadeusz.Hryniewicz@tu.koszalin.pl, c kornel.pietrzak@s.tu.koszalin.pl, d lukasz.dudek@tu.koszalin.pl, *Corresoponding author: Tadeusz.Hryniewicz@tu.koszalin.pl ABSTRACT In the present paper, the SEM and EDS results of porous and enriched in calcium and/or zinc coatings, which were obtained during 3-minute treatments by Plasma Electrolytic Oxidation/ Micro Arc Oxidation processes on CP Titanium Grade 2 at ramp potentials (liner polarization) from 0 up to 650 V DC in electrolytes containing 500 g Mg(NO 3 ) 2 6H 2 O and/or 500 g Cu(NO 3 ) 2 3H 2 O in 1 L H 3 PO 4, are reported. It was found that the obtained coatings, dependent on the PEO process conditions, have pores with different shapes and diameters. The Mg/P and Cu/P ratios by atomic concentration are the same and equal to 0.09±0.01 (by wt %) 0.11±0.01 (by at%) and/or 0.40±0.08 (by wt %) 0.19±0.04 (by at%), respectively. That may testify the hydroxyapatite-like structures of Mg-Ti-PO 4 3 and/or Cu- Ti-PO 4 3 have been identified to occur. Keywords: Plasma Electrolytic Oxidation (PEO), Micro Arc Oxidation (MAO), CP Titanium Grade 2, calcium nitrate Mg(NO 3 ) 2 6H 2 O, zinc nitrate Ca(NO 3 ) 2 3H 2 O, ramp voltage, linear polarization

2 1. INTRODUCTION Titanium and its alloys are key biomaterials used for medical devices like orthopaedic and dental implants. This is because titanium holds a unique combination of surface and bulk properties, which includes high surface TiO 2 stability, Young s modulus closer to that of human bone and high specific strength [1-3]. The surface topography and composition of surface layer play important roles on guiding the bone integration between the implant surface and cells. The manufacturing process can improve the corrosion resistance, wear behavior and affect various other functional properties. The surface modification of titanium and its alloys has increased the number of potential applications of these materials in different fields and industries, such as aerospace, marine, chemical industry, automotive and biomaterials [3-5]. Metals have poor biocompatibility because metals or alloys tend to release metal ions that cause pathological changes in cells, alter genes, and form cancer cells. An appropriate surface treatment is critically required for the biomedical implants such as dental implants, artificial hips and knee components to modify and improve their bioactivity and mechanical properties. To improve the chemical features and biocompatible abilities, numerous surface modification techniques have been progressively developed and applied to different metals, alloys or ceramics for biomedical purposes. Moreover, enhancing the corrosion resistance of titanium and its alloys are crucial to increasing the biocompatibility. The life quality of patients could be improved by using biomaterials that may interact with biological systems having minimal flaws and long service. Therefore, thanks to improved properties, they may be used for manufacturing human body implants [3-6]. This paper is a continuation of the works on fabrication and surface characterization of porous coatings which were obtained on Titanium by Plasma Electrolytic Oxidation process in electrolytes containing selected nitrates, what was first related partly in reference [7]. It has to be pointed out that with use of electrochemical methods it may be possible to obtain both nano-layers as well as micro-layers (micro-coatings). Therefore a standard electropolishing (EP) [8-11], magnetoelectropolishing (MEP) [12-21] or high-current density electropolishing (HDEP) [22-24] may be used to form nano-layers on metals and alloys, whereas the micro coatings may be obtained by the Plasma Electrolytic Oxidation (PEO) also known as Micro Arc Oxidation (MAO) [25-44]. Among the different surface modification techniques (CVD, PVD, ion implantation, electroplating, plasma nitriding, thermal oxidation), anodic oxidation, especially the plasma electrolytic oxidation (PEO) process has become increasingly important since it has advantages over other methods of surface modification [1-6, 25-44]. The PEO process creates porous coatings on titanium [2, 7, 25-33] and its alloys [2-6, 34-39], which may be enriched in bactericidal copper [45-55] as well as in magnesium, which may accelerate the healing of wounds [56-57]. The aim of this paper is the development and SEM/EDS characterisation of PEO porous coatings, enriched in magnesium and copper, obtained on titanium. The three primary factors of biocompatibility on the cellular level are genotoxicity, carcinogenicity, and cytotoxicity. The pore size and surface roughness formed by PEO play an important role in the adsorption of proteins, adhesion of cells, and the rate of osseointegration. -30-

3 2. METHOD The samples of CP Titanium Grade 2 with dimensions mm were treated by Plasma Electrolytic Oxidation (Micro Arc Oxidation) for the surface studies. The plasma electrolytic oxidation (PEO) was performed at the ramp voltages from 0 up to 650 V DC. For the studies, the electrolyte based on orthophosphoric acid H 3 PO 4 with 500 g/l of calcium nitrate Mg(NO 3 ) 2 6H 2 O or copper nitrate Cu(NO 3 ) 2 3H 2 O was used. For each run, the electrolytic cell made of glass was used, containing up to 500 ml of the electrolyte. Scanning Electron Microscope (SEM) FEI Quanta 650 FEG equipped with Energy- Dispersive X-ray Spectroscopy (EDS) for surface analysis was used. The microscope operated under the following conditions: voltage 15 kv, current 8-10 na, beam diameter 6 μm, decreased vacuum in the chamber with the pressure of 50 Pa. The identification of spectral lines was performed by means of a spectral decomposition using the holographic peak deconvolution function. 3. RESULTS AND DISCUSSION In Figures 1-3, the SEM pictures of coating formed on Titanium after PEO treatment at ramp voltages from 0 till 650 V DC (linear polarization) in electrolyte containing of 500 g Mg(NO 3 ) 2 6H 2 O in 1 L H 3 PO 4, are presented. The EDS spectrum of obtained coating is shown in Figure 4. These EDS peaks of phosphorus, titanium and magnesium show that formed PEO coating is built mainly of phosphorus-titanium-magnesium compounds, what may suggest the existence of hydroxyapatite-like structure enriched in magnesium, which replaced the calcium in that structure. In the PEO coating, apart from the titanium (40.3±1.1 wt% 29.9±0.8 at%), which is a substrate, and which signal may partly come from matrix, phosphorus (54.9±0.7 wt% 63.1±0.6 at%) and magnesium (4.8±0.4 wt% 7.0±0.5 at%) were also recorded, what is presented in Figure 5. In addition, the median and range of results were found out. Thus the medians of magnesium, phosphorus and titanium were equal to 4.8 wt% (6.7 at%), 55 wt% (63.1 at%) and 40.1 wt% (29.7 at%), respectively. The highest range of obtained results were observed for titanium (2.9 wt% 2.2 at%), while the smallest one for magnesium (0.9 wt% 1.3 at %). In Figures 6-8, the SEM pictures of coating formed on Titanium after PEO treatment at ramp voltages from 0 till 650 V DC (linear polarization) in electrolyte containing of 500 g Cu(NO 3 ) 2 3H 2 O in 1 L H 3 PO 4, are presented. The EDS spectrum of obtained coating is shown in Figure 9. These EDS peaks of phosphorus, titanium and magnesium show that formed PEO coating is built mainly of phosphorus-titanium-copper compounds, what may suggest the existence of hydroxyapatite-like structure enriched in copper, which replaced the calcium in that structure. In the PEO coating behind the titanium (52.3±4.9 wt% 45.4±4.9 at%), which is a substrate, and which signal may partly come from matrix, phosphorus (34.2±4.0 wt% 45.7±4.5 at%) and copper (13.5±2.2 wt% 8.8±1.4 at%) were also recorded, what is presented in Figure 10. Thus the medians of copper, phosphorus and titanium were equal to 13.8 wt% (8.9 at%), 33.4 wt% (44.7 at%) and 53.3 wt% (46.7 at%), respectively. The highest range of obtained results were observed for titanium (11.6 wt% 11.1 at%), while the lowest one for copper (4.7 wt% 3.3 at%). -31-

4 Fig. 1. SEM picture of coating formed on Titanium after PEO treatment at ramp voltages from 0 till 650 V DC in electrolyte containing of 500 g Mg(NO 3 ) 2 6H 2 O in 1 L H 3 PO 4. Magnification times Fig. 2. SEM picture of coating formed on Titanium after PEO treatment at voltages from 0 till 650 V DC in electrolyte containing of 500 g Mg(NO 3 ) 2 6H 2 O in 1 L H 3 PO 4. Magnification times -32-

5 Fig. 3. SEM picture of coating formed on Titanium after PEO treatment at voltages from 0 till 650 V DC in electrolyte containing of 500 g Mg (NO 3 ) 2 6H 2 O in 1 L H 3 PO 4. Magnification times Fig. 4. SEM picture of coating formed on Titanium after PEO treatment at voltages from 0 till 650 V DC in electrolyte containing of 500 g Mg(NO 3 ) 2 6H 2 O in 1 L H 3 PO 4-33-

6 Fig. 5. Comparison of mean values of EDS results of coating formed on Titanium after PEO treatment at voltages from 0 till 650 V DC in electrolyte containing of 500 g Mg(NO 3 ) 2 6H 2 O in 1 L H 3 PO 4 Fig. 6. SEM picture of coating formed on Titanium after PEO treatment at voltages from 0 till 650 V DC in electrolyte containing of 500 g Cu(NO 3 ) 2 3H 2 O in 1 L H 3 PO 4. Magnification times -34-

7 Fig. 7. SEM picture of coating formed on Titanium after PEO treatment at voltages from 0 till 650 V DC in electrolyte containing of 500 g Cu(NO 3 ) 2 3H 2 O in 1 L H 3 PO 4. Magnification times Fig. 8. SEM picture of coating formed on Titanium after PEO treatment at voltages from 0 till 650 V DC in electrolyte containing of 500 g Cu(NO 3 ) 2 3H 2 O in 1 L H 3 PO 4. Magnification times -35-

8 Fig. 9. EDS result of coating formed on Titanium after PEO treatment at voltages from 0 till 650 V DC in electrolyte containing of 500 g Cu(NO 3 ) 2 3H 2 O in 1 L H 3 PO 4 Fig. 10. Comparison of mean values of EDS results of coating formed on Titanium after PEO treatment at voltages from 0 till 650 V DC in electrolyte containing of 500 g Cu(NO 3 ) 2 3H 2 O in 1 L H 3 PO 4-36-

9 To characterize the metal surface after PEO treatment, the two ratios (Figure 11), i.e. Mg/P and Cu/P, which were equal to 0.09±0.01 (by wt%) 0.11±0.01 (by at%) and 0.40±0.08 (by wt%) 0.19±0.04 (by at%), were calculated. Based on the experimental results it may be concluded that in hydroxyapatite-like structures obtained, there is about two times more atoms of copper than those ones of magnesium forming a similar structure. Fig. 11. Mg/P and Cu/P ratios of EDS results of coating formed on Titanium after PEO treatment at voltages from 0 till 650 V DC in electrolyte containing of 500 g MgNO 3 ) 2 6H 2 O or CuNO 3 ) 2 3H 2 O in 1 L H 3 PO 4 4. CONCLUSIONS At this stage of the PEO studies, the following conclusions may be drawn: during the PEO process with use of a liner polarization (ramp voltage) from 0 till 650 V DC, it is possible to obtain the porous surface enriched in calcium and phosphorus in electrolyte containing 500 g Mg(NO 3 ) 2 6H 2 O and/or 500 g Cu(NO 3 ) 2 3H 2 O in 1 L H 3 PO 4 the Mg/P and Cu/P ratios by atomic concentration are the same and equal to 0.09±0.01 (by wt %) 0.11±0.01 (by at %) and 0.40±0.08 (by wt %) 0.19±0.04 (by at %), respectively; such a composition of porous coating suggests the hydroxyapatite-like structure consisting with Mg-Ti-PO 4 3 and/or Cu-Ti-PO

10 This way it is proved the PEO has been becoming a promising technique for surface modifications. 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". Assoc. Prof. Jan Valíček and Dr Dalibor Matýsek from Vysoká škola báňská - Technická univerzita Ostrava - VŠB-TUO, Czech Republic, are given thanks for providing access to the SEM/EDS apparatus allowing to perform the studies. References [1] Isabella da Silva Vieira Marques,Nilson Cristino da Cruz, Richard Landers, Judy Chia- Chun Yuan, Marcelo Ferraz Mesquita, Cortino Sukotjo, Mathew T. Mathew, and Valentin Ricardo Barao, Incorporation of Ca, P, and Si on bioactive coatings produced by plasma electrolytic oxidation: The role of electrolyte concentration and treatment duration, Biointerphases, 11 (2016) ; [2] Yavari S.A., Necula B.S., Fratila-Apachitei L.E., Duszczyk J., Apatichei I., Biofunctional surfaces by plasma electrolytic oxidation on titanium biomedical alloys, Surface Engineering, 32(6) (2016) ; DOI: / Y [3] Quintero D., Galvis O., Calderón J.A., Gómez M.A., Castaño J.G., Echeverría F., Habazaki H., Control of the physical properties of anodic coatings obtained by plasma electrolytic oxidation on Ti6Al4V alloy, Surface and Coatings Technology, 283 (2015) [4] Davis J.R., Handbook of materials for medical devices, Chapter 3, Metallic materials, 21 50; 2003, Materials Park, OH, ASM International. [5] Ming-Tzu Tsai, Yin-Yu Chang, Henh-Li Huang, Yu-Hsuan Wu, Tzong-Ming Shieh, Micro-arc oxidation treatment enhanced the biological performance of human osteosarcoma cell line and human skin fibroblasts cultured on titanium zirconium films, Surface and Coatings Technology, 303A (2016) [6] Mónica Echeverry-Rendón, Oscar Galvis, David Quintero Giraldo, Juan Pavón, Jose Luis López-Lacomba, Emilio Jimenez-Pique, Marc Anglada, Sara M. Robledo, Juan G. Castaño, Felix Echeverrıa, Osseointegration improvement by plasma electrolytic oxidation of modified titanium alloys surfaces, Journal of Materials Science: Materials in Medicine, 72 (2015) 26 (18 pages); DOI /s [7] Rokosz K., Hryniewicz T., Pietrzak K., SEM and EDS studies of porous coatings enriched in calcium and zinc obtained by PEO with ramp voltage, World Scientific News, 77(2) (2017) [8] Hryniewicz T., Physico-chemical and technological fundamentals of electropolishing steels (Fizykochemiczne i technologiczne podstawy procesu elektropolerowania stali), -38-

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