Novel porous phosphorus-calcium-magnesium coatings on titanium with copper or zinc obtained by DC plasma electrolytic oxidation: Fabrication and characterization

materials Article Novel Porous Phosphorus–Calcium–Magnesium Coatings on Titanium with Copper or Zinc Obtained by DC Plasma Electrolytic Oxidation: Fabrication and Characterization Krzysztof Rokosz 1,*, Tadeusz Hryniewicz 1, Sofia Gaiaschi 2, Patrick Chapon 2, Steinar Raaen 3, Dalibor Matýsek 4, Łukasz Dudek 1and Kornel Pietrzak 1 1Division of BioEngineering and Surface Electrochemistry, Department of Engineering and Informatics Systems, Koszalin University of Technology, Racławicka 15-17, PL 75-620 Koszalin, Poland; T[email protected] (T.H.); [email protected] (Ł.D.); [email protected] (K.P.) 2HORIBA FRANCE S.A.S., Avenue de la Vauve, Passage Jobin Yvon, 91120 Palaiseau, France; [email protected] (S.G.); [email protected] (P.C.) 3Department of Physics, Norwegian University of Science and Technology (NTNU), Realfagbygget E3-124 Høgskoleringen 5, 7491 NO Trondheim, Norway; steinar[email protected] 4 Institute of Geological Engineering, Faculty of Mining and Geology, VŠB—Technical University of Ostrava, 708 33 Ostrava, Czech Republic; dalibor[email protected] *Correspondence: [email protected]; Tel.: +48-501-989-332 Received: 2 August 2018; Accepted: 7 September 2018; Published: 11 September 2018   Abstract: In this paper, the characteristics of new porous coatings fabricated at three voltages in electrolytes based on H 3 PO 4 with calcium nitrate tetrahydrate, magnesium nitrate hexahydrate, and copper(II) nitrate trihydrate are presented. The SEM, energy dispersive spectroscopy (EDS), glow discharge optical emission spectroscopy (GDOES), X-ray photoelectron spectroscopy (XPS), and XRD techniques for coating identification were used. It was found that the higher the plasma electrolytic oxidation (PEO) (micro arc oxidation (MAO)) voltage, the thicker the porous coating with higher amounts of built-in elements coming from the electrolyte and more amorphous phase with signals from crystalline Ca(H 2 PO 4 ) 2· H 2 O and/or Ti(HPO 4 ) 2· H 2 O. Additionally, the external parts of the obtained porous coatings formed on titanium consisted mainly of Ti 4+ , Ca 2+ , Mg 2+ and PO 43− , HPO 42− , H 2 PO 4− , P 2 O 74− as well as Zn 2+ or copper Cu + /Cu 2+ . The surface should be characterized by high biocompatibility, due to the presence of structures based on calcium and phosphates, and have bactericidal properties, due to the presence of zinc and copper ions. Furthermore, the addition of magnesium ions should accelerate the healing of postoperative wounds, which could lead to faster patient recovery. Keywords: micro arc oxidation; plasma electrolytic oxidation; DC PEO; DC MAO; titanium; calcium nitrate tetrahydrate; magnesium nitrate hexahydrate; copper(II) nitrate trihydrate; 85% phosphoric acid 1. Introduction The phenomenon of luminescence occurring on the surface of metals during the galvanic process was first observed by Sluginov in 1880 [ 1 ], and the information was published for the first time by Braun in 1898 [ 2 ]. In 1929, Dufford showed that during the electrolysis of metals such as aluminum, zinc, silver, tantalum, tungsten, magnesium, cerium, antimony, and mercury in selected electrolytes, the phenomenon of luminescence was observed [ 3 ]. In addition, he noticed that this phenomenon Materials 2018,11, 1680; doi:10.3390/ma11091680 www.mdpi.com/journal/materials Materials 2018,11, 1680 2 of 20 was not observed with lead, iron, copper, nickel, molybdenum, tin, and platinum. Such luminescence phenomenon was developed, inter alia, by McNeill, Gruss, Yerokhin, and Nie [ 4 , 5 ]. In addition, Yerokhin at al. proposed a definition of that process, that it is “an electrolytic treatment of anodically polarized metal surfaces carried out above the dielectric breakdown voltage of the growing oxide film”, as well as an explanation of the formation of porous coatings [ 5 , 6 ]. According to this theory, during small voltage polarization, the passive layer is forming, which may be dissolved with the voltage increase. Further increases of voltage result in film repassivation and growth of a new porous oxide film. In the next step of voltage increase, the electric field strength in the oxide film reaches a critical value, after which the film is broken through due to impact or tunneling ionization (here, luminescent sparks are observed). A further increase of voltage results in the following: thermal ionization, larger arc discharges, film fusing and alloying with electrolyte elements, microdischarges penetrating through to the substrate, powerful arcs, destructive effects, and thermal cracking of the coating. It should be also pointed out that on the same plasma electrolytic oxidation (PEO) treated surface, more than one of the described processes may occur concurrently [ 5 ]. Over the years, the phenomenon of galvanic luminescence occurring during the anodic polarization of selected metals and electrolytes has been defined as microplasma oxidation, anode spark electrolysis, plasma electrolytic anode treatment, plasma electrolytic oxidation, or micro arc oxidation [ 7 ]. Systematic studies proposed coating growth [ 8 – 10 ] and discharging [ 11 – 14 ] behaviors as well as electrolyte influence on the ignition of plasma electrolytic oxidation processes [ 15 ]. Meanwhile, Curran and Clyne described the thermophysical properties of plasma electrolytic oxidation [ 16 ] and the porosity of PEO coatings [ 17 ]. Other authors studied oxygen evolution during PEO [ 18 ] and the effects of electrical parameters on that process [ 19 ] as well as phase formation in ceramic PEO coatings [ 20 , 21 ] and their corrosion resistance [ 22 ]. The PEO process has been performed by researchers under different conditions, i.e., DC [ 23 , 24 ], AC [ 25 , 26 ], and pulse [ 27 , 28 ] currents, which result in different surface morphology [ 29 , 30 ] and chemical composition [ 31 , 32 ] as well as mechanical [ 33 , 34 ] and electrochemical properties [ 35 , 36 ]. For the most frequently treated materials by PEO processes, aluminum [ 37 – 40 ] and its alloys [ 40 – 44 ], magnesium [ 45 ] and its alloys [ 46 – 54 ], titanium [ 55 – 60 ] and its alloys [ 61 – 67 ], zirconium [ 68 – 77 ] and its alloys [ 78 , 79 ], tantalum [ 80 – 83 ], niobium [ 84 – 88 ], and hafnium [ 89 ] should be included. In the present paper, PEO coatings obtained on titanium in novel electrolytes, which have never been used or described in the literature until now, are presented (Table 1). These electrolytes are based on orthophosphoric acid and three different nitrates. Table 1. Electrolytes and conditions of the plasma electrolytic oxidation (PEO) process. Electrolytes Voltage Current Density Ref. H2O, NaAlO2, Na3PO4·12H2O, KOH, NaCl 100–900 mA·cm−2(f = 50 Hz) [7] H 2 O, Na 3 PO 4· 12H 2 O, KOH, Na 2 SO 4 , (HOCH 2 ) 3 CNH 2 , (NH 4 ) 2 HPO 4 , C 2 H 7 NO 270 mA·cm−2(f = 50 Hz) [23] H3PO4, Ca(NO3)2·4H2O, Mg(NO3)2·6H2O, Cu(NO3)2·3H2O, Zn(NO3)2·6H2O 500, 575, 650 V [24] H3PO4, Cu(NO3)2·3H2O 450 V [32] H2O, Na2SiO3, (NaPO3)6, NaAlO2microparticle 80 mA·cm−2(f = 300 Hz) [33] H3PO4, Mg(NO3)2·6H2O, Zn(NO3)2·6H2O 500–650 V [35] H2O, Na3PO4, FeSO4350 V (f = 100 Hz) [55] H2O, NaAlO2, KOH 400 V (f = 2000 Hz) [56] H2O, (CH3COO)2Ca·H2O, NaH2PO4·2H2O 300, 390 V (f = 900 Hz) [58] H2O, Ca(CH3COO)2,Sr(CH3COO)2400, 450 V (f = 100 Hz) [59] H3PO4, Ca(NO3)2·4H2O 500, 575, 650 V [60] H2O, Na3PO4, Co(CH3COO)2350 V (f = 100 Hz) [61] H2O, Na3PO4·12H2O, Na2B4O7·10H2O, Na3WO4·2H2O50 mA·cm−2[62] H2O, Na2SiO3, Na2CO3, NaOH 12 mA·cm−2(f = 100 Hz) [63] H2O, C6H18O24P6, KOH, EDTA-Na2, Ca(CH3COO)220, 50, 80 V [64] H2O, NaAlO2, Na2SiO3, (NaPO3)6550 V [65] H2O, Na2HPO4, C4H6O4Ca·H2O +400 V/−80 V (f = 250 Hz) [66] H2O, C3H9O6P, C4H6O4Ca·H2O +400 V/−80 V (f = 250 Hz) [66] H2O, Na2HPO4, C3H7CaO6P·H2O +400 V/−80 V (f = 250 Hz) [66] H2O, (CH3COO)2Ca·H2O, NaH2PO4·H2O 350–500 V (f = 1000 Hz) [67] H2O, Ca(CH3COO)2·H2O 300 V (f = 1000 Hz) [90] H2O, (CH1COO)2Ca, C3H7Na2O6P 250–400 V (f = 100 Hz) [91] H2O, (CH3COO)2Ca·H2O, C3H7Na2O6P·5H2O 450 V (f = 100 Hz) [92] Materials 2018,11, 1680 3 of 20 Table 1. Cont. Electrolytes Voltage Current Density Ref. H2O, (CH3COO)2Ca·H2O, C3H7Na2O6P·5H2O 250–500 V (f = 1000 Hz) [93,94] H2O, Ca(CH3COO)2·H2O, CaC3H7O6P 190–600 V (f = 660 Hz) [95,96] H2O, (CH3COO)2Ca·H2O, C3H7Na2O6P·5H2O 200–500 V (f = 900 Hz) [97] H2O, Na4P2O7·10H2O and KOH, NaAlO20–300 V [98] Na2B4O7·10H2O, (CH3COO)2Mn·4H2O 450–500 V [99] H2O, (CH3COO)2Ca·H2O 230 V [100] H2O, (CH3COO)2Ca·H2O, NaH2PO4·2H2O 260–420 V [101] H2O, CaHPO4, Ca(H2PO4)2, Na6P6O18, Ca(CH3COO)220, 100 mA·cm−2[102] H2O, KOH 290 V (f = 100–200 Hz) [103] H2O, KOH 350 V (f = 1000 Hz) [104] H2O, (NaPO3)6, NaF, NaAlO2150–200 V [105] H2O, K2Al2O4, Na3PO4, NaOH 400 V [106] H2O, CaCl2and KH2PO4320–340 V [107] H2O, H2SO4and Ti2(SO4)31100 V [108] H2O, Na2(EDTA), CaO, Ca(H2PO4)2, Na2SiO3·H2O 350 V (f = 200 Hz) [109] H2O, Na2SiO3, NaOH 280 V [110] H2O, CaO, Na6P6O18, Na2H2EDTA·5.5H2O, KOH AC 0.5–2 mA·cm−2[111] 2O, (NaPO3)6, NaF, NaAlO260 mA·cm−2(f = 100, 600 Hz) [112] H2O, Na3PO4, FeSO4, Co(CH3COO)2, Ni(CH3COO)2, K2ZrF6350 V (f = 100 Hz) [113] H2O, Ca(CH3COO)2·H2O, C3H7Na2O6P 150 V [114] H2O, Na2SiO3·9H2O, Na3PO4·12H2O, Na2SiO3·9H2O, Na3PO4·12H2O80 mA·cm−2(f = 150 Hz) [115] H2O, Na3PO4·12H2O, α-Al2O3nanoparticles 20 mA·cm−2[116] It should also be pointed out that in hydroxyapatite-like structures it is possible to substitute the Ca 2+ ions for Ca 2+ , Mg 2+ , Cu 2+ , and Zn 2+ , as well as OH – for Cu + , which will be used in the fabrication of novel PEO coatings. The porous calcium–phosphate coatings obtained on titanium [ 117 – 119 ] and enriched with biocompatible magnesium, which causes faster wound healing [ 120 – 125 ], as well as antibacterial zinc [ 126 – 132 ] and copper [ 133 – 136 ], may be used as biomaterial, which will be fully accepted by the tissue environment. However, without results inter alia presented in those papers, it is not possible to predict the real possibility of that substitution during plasma treatment in electrolyte in which the ions are present, as well as the thickness and porosity of the PEO coatings. Therefore, in the present paper, the results of x-ray photoelectron spectroscopy (XPS) (10 top nanometers) will be helpful in explaining the oxidation states of those chemical elements as well as chemical composition for all volumes, thicknesses, and pore shapes of obtained coatings by energy dispersive spectroscopy (EDS), XRD, glow discharge optical emission spectroscopy (GDOES), and SEM. 2. Materials and Methods Porous coatings obtained on titanium samples (10 × 10 × 2 mm) by PEO treatment in electrolyte (constant volume of 500 mL for each experiment) containing phosphoric acid (85% w/w) with the addition of calcium nitrate tetrahydrate Ca(NO 3 ) 2· 4H 2 O, magnesium nitrate hexahydrate Mg(NO 3 ) 2· 6H 2 O, and copper(II) nitrate trihydrate Cu(NO 3 ) 2· 3H 2 O in weight ratios of 1:1:1 (Table 2) at 3 voltages, 500 V (PEO time: 3 min), 575 V (PEO times: 1, 3, 5 min), and 600 V (PEO time: 3 min), using a PWR 1600H power supply (KIKUSUI Electronics Corp., Yokohama, Kanagawa, Japan) were fabricated. For their characterization, the complementary measurement methods SEM, EDS, GDOES, XPS, and XRD [ 137 – 141 ] were used. Descriptions of the setups are presented in Table 3and are detailed in reference [24]. Table 2. Experimental plan and code sample names. Sample Name Voltage Electrolyte Type Electrolyte Composition Salts Salt Concentrations (g/L) Ti_CaMgZn_500V 500 V Electrolyte 1 Ca(NO3)2·4H2O and Mg(NO3)2·6H2O & Zn(NO3)2·6H2O166.7 + 166.7 + 166.7 Ti_CaMgZn_575V 575 V Ti_CaMgZn_650V 650 V Ti_CaMgCu_500V 500 V Electrolyte 2 Ca(NO3)2·4H2O and Mg(NO3)2·6H2O & Cu(NO3)2·3H2O166.7 + 166.7 + 166.7 Ti_CaMgCu_575V 575 V Ti_CaMgCu_650V 650 V Materials 2018,11, 1680 4 of 20 Table 3. Setups of SEM, energy dispersive spectroscopy (EDS), x-ray photoelectron spectroscopy (XPS), glow discharge optical emission spectroscopy (GDEOS), and XRD equipment. Technique Equipment Manufacturer SEM Quanta 650 FEI Field Electron and Iron Company, Hillsboro, OR, USA EDS Noran System Six EDS, Silicon Drift Detectors: Keith Thompson, Thermo Fisher Scientific, Madison, WI, USA XPS SCIENCE SES 2002 Scienta AB, Scienta Omicron, Uppsala, Sweden GDOES GD Profiler 2 HORIBA Scientific, Palaiseau, France XRD Bruker-AXS D8 Advance Bruker Corporation, Billerica, MA, USA 3. Results Figure 1shows the surface morphologies of coating surfaces formed on titanium at 500 V, 575 V, and 650 V in two different electrolytes based on phosphoric acid. Two solutions were used: Electrolyte 1, containing H 3 PO 4 with the addition of calcium nitrate tetrahydrate Ca(NO 3 ) 2· 4H 2 O, magnesium nitrate hexahydrate Mg(NO 3 ) 2· 6H 2 O, and zinc nitrate hexahydrate Zn(NO 3 ) 2· 6H 2 O; and Electrolyte 2, with additions of calcium nitrate tetrahydrate Ca(NO 3 ) 2· 4H 2 O, magnesium nitrate hexahydrate Mg(NO 3 ) 2· 6H 2 O, and copper(II) nitrate trihydrate Cu(NO 3 ) 2· 3H 2 O. It should be pointed out that all the obtained coatings were porous and had a well-developed surface. In Figure 2and Table 4, the EDS semiquantitative results for samples obtained in Electrolyte 1 are presented as Ca/P, Mg/P, Zn/P, and M/P ratios. The Ca/P ratios were equal to 0.051 ± 0.003 natural units (n.u.), 0.063 ± 0.003 n.u., and 0.069 ± 0.003 n.u. for 500, 575, and 650 V, respectively. The Mg/P ratios were equal to 0.051 ± 0.004 n.u. (500 V), 0.058 ± 0.003 n.u. (575 V), and 0.060 ± 0.006 n.u. (650 V). The Zn/P ratios for 500, 575, and 650 V were equal to 0.052 ± 0.004 n.u., 0.065 ± 0.005 n.u., and 0.071 ± 0.010 n.u., respectively. The M/P ratios were equal to 0.153 ± 0.008, 0.187 ± 0.006, and 0.200 ±0.020 for 500, 575, and 650 V, respectively. Figure 3and Table 5present the EDS semiquantitative results for samples obtained in Electrolyte 2 as Ca/P, Mg/P, Cu/P, and M/P. The Ca/P ratios were equal to 0.062 ± 0.003 n.u., 0.068 ± 0.004 n.u., and 0.071 ± 0.003 n.u. for 500, 575, and 650 V, respectively. The Mg/P ratios were equal to 0.058 ± 0.002 n.u., 0.059 ± 0.003 n.u., and 0.064 ± 0.003 n.u. for 500, 575, and 650 V, respectively. The Cu/P ratios for samples obtained at 500, 575, and 650 V were equal to 0.039 ± 0.003 n.u., 0.048 ± 0.002 n.u., and 0.062 ± 0.005 n.u., respectively. The M/P ratios for samples obtained at 500, 575, and 650 V were equal to 0.158 ±0.006 n.u., 0.175 ±0.006 n.u., and 0.197 ±0.004 n.u., respectively. Table 4. Statistical description of EDS of coatings formed in Electrolyte 1. n.u., no units. Ratios Voltage xσQ1Q2Q3 Ca/P n.u. 500 V 0.051 0.003 0.050 0.052 0.052 575 V 0.063 0.003 0.062 0.064 0.065 650 V 0.069 0.003 0.068 0.071 0.071 Mg/P n.u. 500 V 0.051 0.004 0.049 0.051 0.053 575 V 0.058 0.003 0.057 0.060 0.060 650 V 0.060 0.006 0.057 0.063 0.063 Zn/P n.u. 500 V 0.052 0.004 0.050 0.053 0.054 575 V 0.065 0.005 0.063 0.068 0.068 650 V 0.071 0.010 0.065 0.075 0.075 M/P n.u. 500 V 0.153 0.008 0.149 0.151 0.157 575 V 0.187 0.006 0.184 0.188 0.190 650 V 0.200 0.015 0.192 0.195 0.206 Materials 2018,11, 1680 5 of 20 Materials 2018, 11, x FOR PEER REVIEW 5 of 19 Figure 1. Surface morphologies of surfaces after PEO processing. Figure 2. Atomic ratios (EDS) of coatings formed in Electrolyte 1. Figure 1. Surface morphologies of surfaces after PEO processing. Materials 2018, 11, x FOR PEER REVIEW 5 of 19 Figure 1. Surface morphologies of surfaces after PEO processing. Figure 2. Atomic ratios (EDS) of coatings formed in Electrolyte 1. Figure 2. Atomic ratios (EDS) of coatings formed in Electrolyte 1. Materials 2018,11, 1680 6 of 20 Materials 2018, 11, x FOR PEER REVIEW 6 of 19 Figure 3. Atomic ratios (EDS) of coatings formed in Electrolyte 2. Table 5. Statistical description of EDS of coatings formed in Electrolyte 2. n.u., no units. Ratios Voltage 𝒙  σ Q1 Q2 Q3 Ca/P n.u. 500 V 0.062 0.003 0.060 0.061 0.062 575 V 0.068 0.004 0.066 0.068 0.071 650 V 0.071 0.003 0.068 0.072 0.073 Mg/P n.u. 500 V 0.058 0.002 0.057 0.057 0.059 575 V 0.059 0.003 0.056 0.060 0.061 650 V 0.064 0.003 0.064 0.064 0.066 Cu/P n.u. 500 V 0.039 0.003 0.037 0.040 0.040 575 V 0.048 0.002 0.047 0.048 0.050 650 V 0.062 0.005 0.059 0.061 0.063 M/P n.u. 500 V 0.158 0.006 0.156 0.156 0.159 575 V 0.175 0.006 0.172 0.176 0.177 650 V 0.197 0.004 0.195 0.196 0.197 The diffraction data of PEO coatings formed in Electrolytes 1 and 2 at three voltages are presented in Figure 4. For both electrolytes, similar phenomena were observed, i.e., for samples oxidized at 500 and 575 V, only signal from titanium as metal matrix was detected, while for 650 V other crystalline phases, such as Ca(H2PO4)2∙H2O and Ti(HPO4)2∙H2O for samples obtained in Electrolyte 1 and Ca(H2PO4)2∙H2O for samples obtained in Electrolyte 2, were recorded. It was also found that voltage growth in PEO coatings caused amorphous phase accretion as well. GDOES data of PEO coatings formed in Electrolyte 1 at 500, 575, and 650 V are presented in Figure 5. The top and porous sublayers, which are enriched in Zn, P, and O and depleted in Ca, Mg, and Ti, have thicknesses of about 200, 300, and 500 s of sputtering time for 500, 575, and 650 V, respectively, while the thickness of the second (semiporous) one, which was enriched in calcium, magnesium, zinc, phosphorus, and oxygen and depleted in titanium, was in the range of 700 s (500 V) up to 2000 s (650 V) of sputtering time. On the other hand, the thicknesses of the third (transition) sublayers, in which a decrease of all signals, except titanium, was observed, increased from 800 s (500 V) up to 2000 s (650 V) of sputtering time. In Figure 6, the GDOES results of PEO coatings formed in Electrolyte 2 at the same three voltages are presented. The top and porous sublayers, which are enriched in P and O and depleted in Ca, Mg, Cu, and Ti, have thicknesses related to sputtering times equal to about 100, 300, and 600 s for 500, 575, and 650 V, respectively, while the thickness of the second (semiporous) layer, which is enriched in Ca, Mg, Cu, P, and O and depleted in Ti, is in the range of 600 s (500 V) up to 1900 s (650 V) of sputtering Figure 3. Atomic ratios (EDS) of coatings formed in Electrolyte 2. Table 5. Statistical description of EDS of coatings formed in Electrolyte 2. n.u., no units. Ratios Voltage xσQ1Q2Q3 Ca/P n.u. 500 V 0.062 0.003 0.060 0.061 0.062 575 V 0.068 0.004 0.066 0.068 0.071 650 V 0.071 0.003 0.068 0.072 0.073 Mg/P n.u. 500 V 0.058 0.002 0.057 0.057 0.059 575 V 0.059 0.003 0.056 0.060 0.061 650 V 0.064 0.003 0.064 0.064 0.066 Cu/P n.u. 500 V 0.039 0.003 0.037 0.040 0.040 575 V 0.048 0.002 0.047 0.048 0.050 650 V 0.062 0.005 0.059 0.061 0.063 M/P n.u. 500 V 0.158 0.006 0.156 0.156 0.159 575 V 0.175 0.006 0.172 0.176 0.177 650 V 0.197 0.004 0.195 0.196 0.197 The diffraction data of PEO coatings formed in Electrolytes 1 and 2 at three voltages are presented in Figure 4. For both electrolytes, similar phenomena were observed, i.e., for samples oxidized at 500 and 575 V, only signal from titanium as metal matrix was detected, while for 650 V other crystalline phases, such as Ca(H 2 PO 4 ) 2· H 2 O and Ti(HPO 4 ) 2· H 2 O for samples obtained in Electrolyte 1 and Ca(H 2 PO 4 ) 2· H 2 O for samples obtained in Electrolyte 2, were recorded. It was also found that voltage growth in PEO coatings caused amorphous phase accretion as well. GDOES data of PEO coatings formed in Electrolyte 1 at 500, 575, and 650 V are presented in Figure 5. The top and porous sublayers, which are enriched in Zn, P, and O and depleted in Ca, Mg, and Ti, have thicknesses of about 200, 300, and 500 s of sputtering time for 500, 575, and 650 V, respectively, while the thickness of the second (semiporous) one, which was enriched in calcium, magnesium, zinc, phosphorus, and oxygen and depleted in titanium, was in the range of 700 s (500 V) up to 2000 s (650 V) of sputtering time. On the other hand, the thicknesses of the third (transition) sublayers, in which a decrease of all signals, except titanium, was observed, increased from 800 s (500 V) up to 2000 s (650 V) of sputtering time. In Figure 6, the GDOES results of PEO coatings formed in Electrolyte 2 at the same three voltages are presented. The top and porous sublayers, which are enriched in P and O and depleted in Ca, Mg, Cu, and Ti, have thicknesses related to sputtering times equal to about 100, 300, and 600 s for 500, 575, and 650 V, respectively, while the thickness of the second (semiporous) layer, which is enriched in Ca, Mg, Cu, P, and O and depleted in Ti, is in the range of 600 s (500 V) up to 1900 s (650 V) of sputtering time. Here, the thicknesses of the transition sublayers are in the range from 600 s (500 V) up to 1500 s (650 V) of sputtering time. The part of C, N, and O signals may originate in the first top sublayers from Materials 2018,11, 1680 7 of 20 contamination (from air and cleaning compounds). In addition, the H signals maxima, which are always placed in third-transition sublayers, is the end of the coating porosity. It should also be noted that the accretion of voltage caused an increase in coating thickness. In Figures 7and 8, the XPS spectra of PEO coatings formed in Electrolytes 1 and 2 are presented. Based on the obtained results, it can be concluded that the top external 10 nm layers of the PEO coating consist mainly of phosphorus, oxygen, nitrogen, titanium, calcium, magnesium, and zinc (Electrolyte 1) or copper (Electrolyte 2). The bindings of C with O and N with O can be interpreted as contaminants (cleaning process and adsorbed air). The phosphorus (P 2p) and oxygen (O 1s) spectra were in the range of 133.6–134 eV and 531.3–531.5 eV, respectively, which can be interpreted as the groups PO 43− , HPO 42− , H 2 PO 4− , and P 2 O 74− . The Cu 2p spectra maxima (331.1–932.9 eV and 934.5–935.8 eV) and Auger Cu LMM (566–567.2 eV) suggest the presence of Cu + and Cu 2+ , while Ca 2+ is proved by the binding energy (BE) in the range of 347.4 − 347.7 eV. The BE of Zn 2p (1021.9–1022.4 eV) and Zn LMM (497.9–501.5 eV) proves the existence of Zn 2+ , while BE in the range of 89.1–92.8 eV (Mg 2s) and 306.2–306.9 eV (Mg KLL) indicates the existence of Mg 2+ . The BE of titanium Ti 2p 3/ is in the range of 459.9 − 460.2 eV, which means that titanium is on the fourth oxidation state (Ti 4+ ). Based on the quantitative XPS of the top 10 nm of PEO coatings obtained in Electrolytes 1 and 2 at three voltages, two ratios, Ca:Mg:Zn and Ca:Mg:Zn, were found. The Ca:Mg:Zn ratios are equal to 8:32:1 n.u. (500 V), 8:28:1 n.u. (575 V), and 14:45:1 n.u. (650 V), while the Ca:Mg:Zn ratios are equal to 5:5:1 n.u. (500 V), 4:7:1 n.u. (575 V), and 6:5:1 n.u. (650 V). (Ca + Mg + Zn)/P and (Ca + Mg + Cu)/P have their maxima equal to 0.48 n.u. and 0.21 n.u., respectively, at 575 V. The same trend was observed for single Me/P ratios, where M ∈ {Ca, Mg, Zn, Cu}, i.e., the maxima were recorded for PEO coatings obtained at 575 V. Materials 2018, 11, x FOR PEER REVIEW 7 of 19 time. Here, the thicknesses of the transition sublayers are in the range from 600 s (500 V) up to 1500 s (650 V) of sputtering time. The part of C, N, and O signals may originate in the first top sublayers from contamination (from air and cleaning compounds). In addition, the H signals maxima, which are always placed in third-transition sublayers, is the end of the coating porosity. It should also be noted that the accretion of voltage caused an increase in coating thickness. In Figures 7 and 8, the XPS spectra of PEO coatings formed in Electrolytes 1 and 2 are presented. Based on the obtained results, it can be concluded that the top external 10 nm layers of the PEO coating consist mainly of phosphorus, oxygen, nitrogen, titanium, calcium, magnesium, and zinc (Electrolyte 1) or copper (Electrolyte 2). The bindings of C with O and N with O can be interpreted as contaminants (cleaning process and adsorbed air). The phosphorus (P 2p) and oxygen (O 1s) spectra were in the range of 133.6–134 eV and 531.3–531.5 eV, respectively, which can be interpreted as the groups PO43−, HPO42−, H2PO4−, and P2O74−. The Cu 2p spectra maxima (331.1–932.9 eV and 934.5–935.8 eV) and Auger Cu LMM (566–567.2 eV) suggest the presence of Cu+ and Cu2+, while Ca2+ is proved by the binding energy (BE) in the range of 347.4−347.7 eV. The BE of Zn 2p (1021.9–1022.4 eV) and Zn LMM (497.9–501.5 eV) proves the existence of Zn2+, while BE in the range of 89.1–92.8 eV (Mg 2s) and 306.2–306.9 eV (Mg KLL) indicates the existence of Mg2+. The BE of titanium Ti 2p3/is in the range of 459.9−460.2 eV, which means that titanium is on the fourth oxidation state (Ti4+). Based on the quantitative XPS of the top 10 nm of PEO coatings obtained in Electrolytes 1 and 2 at three voltages, two ratios, Ca:Mg:Zn and Ca:Mg:Zn, were found. The Ca:Mg:Zn ratios are equal to 8:32:1 n.u. (500 V), 8:28:1 n.u. (575 V), and 14:45:1 n.u. (650 V), while the Ca:Mg:Zn ratios are equal to 5:5:1 n.u. (500 V), 4:7:1 n.u. (575 V), and 6:5:1 n.u. (650 V). (Ca + Mg + Zn)/P and (Ca + Mg + Cu)/P have their maxima equal to 0.48 n.u. and 0.21 n.u., respectively, at 575 V. The same trend was observed for single Me/P ratios, where M ∈ {Ca, Mg, Zn, Cu}, i.e., the maxima were recorded for PEO coatings obtained at 575 V. Figure 4. Diffractogram XRD results of PEO coatings obtained in (a) Electrolyte 1 and (b) Electrolyte 2. Figure 4. Diffractogram XRD results of PEO coatings obtained in ( a ) Electrolyte 1 and ( b ) Electrolyte 2. Materials 2018,11, 1680 8 of 20 Figure 5. GDEOS signals (black), first derivatives (red continuous line), and second derivatives (brown dashed line) for samples formed in Electrolyte 1. Materials 2018,11, 1680 9 of 20 Figure 6. GDEOS signals (black), first derivatives (red continuous line), and second derivatives (brown dashed line) for samples formed in Electrolyte 2. Materials 2018,11, 1680 16 of 20 57. Rokosz, K.; Hryniewicz, T.; Dudek, Ł.; Matysek, D.; Valíˇcek, J.; Harniˇcárová, M. SEM and EDS analysis of surface layer formed on titanium after plasma electrolytic oxidation in H 3 PO 4 with the addition of Cu(NO 3 ) 2 . J. Nanosci. Nanotechnol. 2016,16, 7814–7817. [CrossRef] 58. Liu, S.M.; Yang, X.J.; Cui, Z.D.; Zhu, S.L.; Wei, Q. One-step synthesis of petal-like apatite/titania composite coating on a titanium by micro-arc treating. Mater. Lett. 2011,65, 1041–1044. [CrossRef] 59. Yan, Y.Y.; Sun, J.F.; Han, Y.; Li, D.C.; Cui, K. Microstructure and bioactivity of Ca, P and Sr doped TiO 2 coating formed on porous titanium by micro-arc oxidation. Surf. Coat. Technol. 2010,205, 1702–1713. [CrossRef] 60. Rokosz, K.; Hryniewicz, T.; Gaiaschi, S.; Chapon, P.; Raaen, S.; Pietrzak, K.; Malorny, W. Characterisation of calciumand phosphorus-enriched porous coatings on CP Titanium Grade 2 fabricated by plasma electrolytic oxidation. Metals 2017,7, 354. [CrossRef] 61. Tang, H.; Sun, Q.; Xin, T.Z.; Yi, C.G.; Jiang, Z.H.; Wang, F.P. Influence of Co(CH 3 COO) 2 concentration on thermal emissivity of coatings formed on titanium alloy by micro-arc oxidation. Curr. Appl. Phys. 2012 , 12, 284–290. [CrossRef] 62. Rudnev, V.S.; Yarovaya, T.P.; Egorkin, V.S.; Sinebryukov, S.L.; Gnedenkov, S.V. Properties of coatings formed on titanium by plasma electrolytic oxidation in a phosphate-borate electrolyte. Russ. J. Appl. Chem. 2010 , 83, 664–670. [CrossRef] 63. Xu, Y.J.; Yao, Z.P.; Jia, F.Z.; Wang, Y.L.; Jiang, Z.H.; Bu, H.T. Preparation of PEO ceramic coating on Ti alloy and its high temperature oxidation resistance. Curr. Appl. Phys. 2010,10, 698–702. [CrossRef] 64. Qiao, L.P.; Lou, J.; Zhang, S.F.; Qu, B.; Chang, W.H.; Zhang, R.F. The entrance mechanism of calcium and phosphorus elements into micro arc oxidation coatings developed on Ti6Al4V alloy. Surf. Coat. Technol. 2016 , 285, 187–196. [CrossRef] 65. Wang, Y.H.; Liu, Z.G.; Ouyang, J.H.; Wang, Y.M.; Zhou, Y. Influence of electrolyte compositions on structure and high-temperature oxidation resistance of microarc oxidation coatings formed on Ti2AlNb alloy. J. Alloys Compd. 2015,647, 431–437. [CrossRef] 66. Karbowniczek, J.; Muhaffel, F.; Cempura, G.; Cimenoglu, H.; Filemonowicz, A.C. Influence of electrolyte composition on microstructure, adhesion and bioactivity of micro-arc oxidation coatings produced on biomedical Ti6Al7Nb alloy. Surf. Coat. Technol. 2017,321, 97–107. [CrossRef] 67. Montazeri, M.; Dehghanian, C.; Shokouhfar, M.; Baradaran, A. Investigation of the voltage and time effects on the formation of hydroxyapatite-containing titania prepared by plasma electrolytic oxidation on Ti-6Al-4V alloy and its corrosion behavior. Appl. Surf. Sci. 2011,257, 7268–7275. [CrossRef] 68. Simka, W.; Sowa, M.; Socha, R.P.; Maciej, A.; Michalska, J. Anodic oxidation of zirconium in silicate solutions. Electrochim. Acta 2013,104, 518–525. [CrossRef] 69. Sowa, M.; Dercz, G.; Suchanek, K.; Simka, W. Investigation of anodic oxide coatings on zirconium after heat treatment. Appl. Surf. Sci. 2015,346, 534–542. [CrossRef] 70. Matykina, E.; Arrabal, R.; Skeldon, P.; Thompson, G.E.E.; Wang, P.; Wood, P. Plasma electrolytic oxidation of a zirconium alloy under AC conditions. Surf. Coat. Technol. 2010,204, 2142–2151. [CrossRef] 71. Cengiz, S.; Uzunoglu, A.; Stanciu, L.; Tarakci, M.; Gencer, Y. Direct fabrication of crystalline hydroxyapatite coating on zirconium by single-step plasma electrolytic oxidation process. Surf. Coat. Technol. 2016 , 301, 74–79. [CrossRef] 72. Fidan, S.; Muhaffel, F.; Riool, M.; Cempura, G.; de Boer, L.; Zaat, S.A.J.; Filemonowicz, A.C.; Cimenoglu, H. Fabrication of oxide layer on zirconium by micro-arc oxidation: Structural and antimicrobial characteristics. Mater. Sci. Eng. C 2017,71, 565–569. [CrossRef] [PubMed] 73. Cengiz, S.; Gencer, Y. The characterization of the oxide based coating synthesized on pure zirconium by plasma electrolytic oxidation. Surf. Coat. Technol. 2014,242, 132–140. [CrossRef] 74. Cengiz, S.; Azakli, Y.; Tarakci, M.; Stanciu, L.; Gencer, Y. Microarc oxidation discharge types and bio properties of the coating synthesized on zirconium. Mater. Sci. Eng. C 2017 ,77, 374–383. [CrossRef] [PubMed] 75. Ha, J.-Y.Y.; Tsutsumi, Y.; Doi, H.; Nomura, N.; Kim, K.-H.H.; Hanawa, T. Enhancement of calcium phosphate formation on zirconium by micro-arc oxidation and chemical treatments. Surf. Coat. Technol. 2011 , 205, 4948–4955. [CrossRef] 76. Sandhyarani, M.; Rameshbabu, N.; Venkateswarlu, K.; Rama Krishna, L. Fabrication, characterization and in-vitro evaluation of nanostructured zirconia/hydroxyapatite composite film on zirconium. Surf. Coat. Technol. 2014,238, 58–67. Materials 2018,11, 1680 17 of 20 77. Sandhyarani, M.; Prasadrao, T.; Rameshbabu, N. Role of electrolyte composition on structural, morphological and in-vitro biological properties of plasma electrolytic oxidation films formed on zirconium. Appl. Surf. Sci. 2014,317, 198–209. 78. Apelfeld, A.V.; Betsofen, S.Y.; Borisov, A.M.; Vladimirov, B.V.; Savushkina, S.V.; Knyazev, E.V. Stabilization of the high-temperature phases in ceramic coatings on zirconium alloy produced by plasma electrolytic oxidation. J. Phys. Conf. Ser. 2016,748, 12019. [CrossRef] 79. Apelfeld, A.V.; Ashmarin, A.A.; Borisov, A.M.; Vinogradov, A.V.; Savushkina, S.V.; Shmytkova, E.A. Formation of zirconia tetragonal phase by plasma electrolytic oxidation of zirconium alloy in electrolyte comprising additives of yttria nanopowder. Surf. Coat. Technol. 2017,328, 513–517. [CrossRef] 80. Sowa, M.; Simka, W. Electrochemical Impedance and Polarization Corrosion Studies of Tantalum Surface Modified by DC Plasma Electrolytic Oxidation. Materials 2018,11, 545. [CrossRef] [PubMed] 81. Sowa, M.; Woszczak, M.; Kazek-K˛esik, A.; Dercz, G.; Korotin, D.M.; Zhidkov, I.S.; Kurmaev, E.Z.; Cholakh, S.O.; Basiaga, M.; Simka, W. Influence of process parameters on plasma electrolytic surface treatment of tantalum for biomedical applications. Appl. Surf. Sci. 2017,407, 52–63. [CrossRef] 82. Rokosz, K.; Hryniewicz, T.; Chapon, P.; Raaen, S.; Ricardo Zschommler Sandim, H. XPS and GDOES characterization of porous coating enriched with copper and calcium obtained on tantalum via plasma electrolytic oxidation. J. Spectrosc. 2016,2016, 7093071. [CrossRef] 83. Fattah-alhosseini, A.; Pourmahmoud, M. Passive and semiconducting properties assessment of commercially pure tantalum in Hank’s physiological solution. J. Mater. Eng. Perform. 2018,27, 116–123. [CrossRef] 84. Sowa, M.; Worek, J.; Dercz, G.; Korotin, D.M.; Kukharenko, A.I.; Kurmaev, E.Z.; Cholakh, S.O.; Basiaga, M.; Simka, W. Surface characterisation and corrosion behaviour of niobium treated in a Caand P-containing solution under sparking conditions. Electrochim. Acta 2016,198, 91–103. [CrossRef] 85. Sowa, M.; Simka, W. Electrochemical behavior of plasma electrolytically oxidized niobium in simulated physiological environment. Surf. Coat. Technol. 2018,344, 21–131. [CrossRef] 86. Rokosz, K.; Hryniewicz, T. Comparative SEM and EDX analysis of surface coatings created on niobium and titanium alloys after Plasma Electrolytic Oxidation (PEO). Tehniˇcki Vjesnik-Technical Gazette 2017 ,24, 465–472. 87. Gomes, M.A.B.; Onofre, S.; Juanto, S.; Bulhões, L.O.S. Anodization of niobium in sulphuric acid media. J. Appl. Electrochem. 1991,21, 1023–1026. [CrossRef] 88. Pereira, B.L.; Lepienski, C.M.; Mazzaro, I.; Kuromoto, N.K. Apatite grown in niobium by two-step plasma electrolytic oxidation. Mater. Sci. Eng. C Mater. 2017,77, 1235–1241. [CrossRef] [PubMed] 89. Stojadinovi´c, S.; Tadi´c, N.; Vasilic, R. Plasma electrolytic oxidation of hafnium. J. Refract. Met. Hard Mater. 2017,69, 153–157. [CrossRef] 90. Han, Y.; Hong, S.H.; Xu, K.W. Synthesis of nanocrystalline titania films by micro-arc oxidation. Mater. Lett. 2002,56, 744–747. [CrossRef] 91. Han, Y.; Xu, K. Photoexcited formation of bone apatite-like coatings on micro-arc oxidized titanium. J. Biomed. Mater. Res. 2004,71A, 608–614. [CrossRef] [PubMed] 92. Huang, P.; Xu, K.W.; Han, Y. Preparation and apatite layer formation of plasma electrolytic oxidation film on titanium for biomedical application. Mater. Lett. 2005,59, 185–189. [CrossRef] 93. Song, W.H.; Jun, Y.K.; Han, Y.; Hong, S.H.; Kim, H.E.; Heo, S.J.; Koak, J.Y. Biomimetic apatite coatings on micro-arc oxidized titania. Biomaterials 2004,25, 3341–3349. [CrossRef] [PubMed] 94. Zhang, Y.M.; Bataillon-Linez, P.; Huang, P.; Zhao, Y.M.; Han, Y.; Traisnel, M.; Xu, K.W.; Hildebrand, H.F. Surface analyses of micro-arc oxidized and hydrothermally treated titanium and effect on osteoblast behavior. J. Biomed. Mater. Res. 2003,68, 383–391. [CrossRef] [PubMed] 95. Li, L.H.; Kong, Y.M.; Kim, H.W.; Kim, Y.W.; Kim, H.E.; Heo, S.J.; Koak, J.Y. Improved biological performance of Ti implants due to surface modification by micro-arc oxidation. Biomaterials 2004 ,25, 2867–2875. [CrossRef] [PubMed] 96. Lee, S.H.; Kim, H.W.; Lee, E.J.; Li, L.H.; Kim, H.E. Hydroxyapatite–TiO 2 hybrid coating on Ti implants. J. Biomater. Appl. 2006,20, 194–208. [CrossRef] [PubMed] 97. Han, Y.; Hong, S.H.; Xu, K.W. Structure and in vitro bioactivity of titania-based films by micro-arc oxidation. Surf. Coat. Technol. 2003,168, 249–258. [CrossRef] 98. Teh, T.H.; Berkani, A.; Mato, S.; Skeldon, P.; Thompson, G.E.; Habazaki, H.; Shimizu, K. Initial stages of plasma electrolytic oxidation of titanium. Corros. Sci. 2003,45, 2757–2768. [CrossRef] Materials 2018,11, 1680 18 of 20 99. Rudnev, V.S.; Vasilyeva, M.S.; Kondrikov, N.B.; Tyrina, L.M. Plasma-electrolytic formation, composition and catalytic activity of manganese oxide containing structures on titanium. Appl. Surf. Sci. 2005 ,252, 1211–1220. [CrossRef] 100. Ryu, H.S.; Song, W.H.; Hong, S.H. Biomimetic apatite induction on Ca-containing titania. Curr. Appl. Phys. 2005,5, 512–515. [CrossRef] 101. Chen, J.Z.; Shi, Y.L.; Wang, L.; Yan, F.Y.; Zhang, F.Y. Preparation and properties of hydroxyapatite-containing titania coating by micro-arc oxidation. Mater. Lett. 2006,60, 2538–2543. [CrossRef] 102. Matykina, E.; Montuori, M.; Gough, J.; Monfort, F.; Berkani, A.; Skeldon, P.; Thompson, G.E.; Habazaki, H. Spark anodising of titanium for biomedical applications. Trans. IMF 2006,84, 125–133. [CrossRef] 103. Han, I.H.; Choi, J.H.; Zhao, B.H.; Baik, H.K.; Lee, I.S. Effects of electrical wave form on pore size of micro-arc oxidized TiO2film. Key Eng. Mater. 2006,309–311, 375–378. [CrossRef] 104. Shokouhfar, M.; Dehghanian, C.; Montazeri, M.; Baradaran, A. 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. [CrossRef] 105. Zhu, L.; Ye, X.; Tang, G.; Zhao, N.; Gong, Y.; Zhao, Y.; Zhao, J.; Zhang, X. Corrosion test, cell behavior test, and in vivo study of gradient TiO 2 layers produced by compound electrochemical oxidation. J. Biomed. Mater. Res. A 2006,78, 515–522. [CrossRef] [PubMed] 106. Habazaki, H.; Onodera, T.; Fushimi, K.; Konno, H.; Toyotake, K. Spark anodizing of β -Ti alloy for wear-resistant coating. Surf. Coat. Technol. 2007,201, 8730–8737. [CrossRef] 107. Kim, M.S.; Ryu, J.J.; Sung, Y.M. One-step approach for nano-crystalline hydroxyapatite coating on titanium via micro-arc oxidation. Electrochem. Commun. 2007,9, 1886–1891. [CrossRef] 108. Ragaleviˇcius, R.; Stalnionis, G.; Niaura, G.; Jagminas, A. Micro-arc oxidation of Ti in a solution of sulfuric acid and Ti+3 salt. Appl. Surf. Sci. 2008,254, 1608–1613. [CrossRef] 109. Zhang, W.; Du, K.; Yan, C.; Wang, F. Preparation and characterization of a novel Si-incorporated ceramic film on pure titanium by plasma electrolytic oxidation. Appl. Surf. Sci. 2008,254, 5216–5223. [CrossRef] 110. Lebukhova, N.V.; Rudnev, V.S.; Kirichenko, E.A.; Chigrin, P.G.; Lukiyanchuk, I.V.; Yarovaya, T.P. Effect of the structure of the oxidized titanium surface on the particle size and properties of the deposited copper-molybdate catalyst. Prot. Met. Phys. Chem. Surf. 2016,52, 1024–1030. [CrossRef] 111. Terleeva, O.P.; Sharkeev, Y.P.; Slonova, A.I.; Mironov, I.V.; Legostaeva, E.V.; Khlusov, I.A.; Matykina, E.; Skeldon, P.; Thompson, G.E. Effect of microplasma modes and electrolyte composition on micro-arc oxidation coatings on titanium for medical applications. Surf. Coat. Technol. 2010,205, 1723–1729. [CrossRef] 112. Wang, Y.M.; Lei, T.Q.; Jiang, B.L.; Guo, L.X. Growth, microstructure and mechanical properties of microarc oxidation coatings on titanium alloy in phosphate-containing solution. Appl. Surf. Sci. 2004 ,233, 258–267. [CrossRef] 113. Tang, H.; Sun, Q.; Yi, C.G.; Jiang, Z.H.; Wang, F.P. High emissivity coatings on titanium alloy prepared by micro-arc oxidation for high temperature application. J. Mater. Sci. 2012,47, 2162–2168. [CrossRef] 114. Santos, A.D.; Araujo, J.R.; Landi, S.M.; Kuznetsov, A.; Granjeiro, J.M.; Sena, L.A.D.; Achete, C.A. A study of the physical, chemical and biological properties of TiO 2 coatings produced by micro-arc oxidation in a Ca-P-based electrolyte. J. Mater. Sci. Mater. Med. 2014,25, 1769–1780. [CrossRef] [PubMed] 115. Li, Q.B.; Yang, W.B.; Liu, C.C.; Wang, D.A.; Liang, J. Correlations between the growth mechanism and properties of micro-arc oxidation coatings on titanium alloy: Effects of electrolytes. Surf. Coat. Technol. 2017 , 316, 162–170. [CrossRef] 116. Sharifi, H.; Aliofkhazraei, M.; Darband, G.; Rouhaghdam, A.S. Characterization of PEO nanocomposite coatings on titanium formed in electrolyte containing atenolol. Surf. Coat. Technol. 2016 ,304, 438–449. [CrossRef] 117. Zhu, X.L.; Chen, J.; Scheideler, L.; Reichl, R.; Geis-Gerstorfer, J. Effects of topography and composition of titanium surface oxides on osteoblast responses. Biomaterials 2004,25, 4087–4103. [CrossRef] [PubMed] 118. Geetha, M.; Singh, A.K.; Asokamani, R.; Gogia, A.K. Ti based biomaterials, the ultimate choice for orthopaedic implants—A review. Prog. Mater. Sci. 2009,54, 397–425. [CrossRef] 119. Jayaraman, M.; Meyer, U.; Bühner, M.; Joos, U.; Wiesmann, H.P. Influence of titanium surfaces on attachment of osteoblast-like cells in vitro. Biomaterials 2004,25, 625–631. [CrossRef] Materials 2018,11, 1680 19 of 20 120. Cai, Y.L.; Zhang, J.J.; Zhang, S.; Venkatraman, S.S.; Zeng, X.T.; Du, H.J.; Mondal, D. Osteoblastic cell response on fluoridated hydroxyapatite coatings: The effect of magnesium incorporation. Biomed. Mater. 2010 , 5, 054114. [CrossRef] [PubMed] 121. Piocha, D.; Zima, A.; Paszkiewicz, Z.; Slosarczyk, A. Physicochemical properties of the novel biphasic hydroxyapatite–magnesium phosphate biomaterial. Acta Bioeng. Biomech. 2013,15, 53–63. 122. Grigolato, R.; Pizzi, N.; Brotto, M.C.; Corrocher, G.; Desando, G.; Grigolo, B. Magnesium-enriched hydroxyapatite as bone filler in an ameloblastoma mandibular defect. Int. J. Clin. Exp. Med. 2015 ,8, 281–288. [PubMed] 123. Nygren, H.; Bigdeli, N.; Ilver, L.; Malmberg, P. Mg-corrosion, hydroxyapatite, and bone healing. Biointerphases 2017,12, 02C407. [CrossRef] [PubMed] 124. Laurencin, D.; Almora-Barrios, N.; de Leeuw, N.H.; Gervais, C.; Bonhomme, C.; Mauri, F.; Chrzanowski, W.; Knowles, J.C.; Newport, R.J.; Wong, A.; et al. Magnesium incorporation into hydroxyapatite. Biomaterials 2011,32, 1826–1837. [CrossRef] [PubMed] 125. Farzadi, A.; Bakhshi, F.; Solati-Hashjin, M.; Asadi-Eydivand, M.; Abu Osman, A.A. Magnesium incorporated hydroxyapatite: Synthesis and structural properties characterization. Ceram. Int. 2014 ,40, 6021–6029. [CrossRef] 126. Hu, H.; Zhang, W.; Qiao, Y.; Jiang, X.; Liu, X.; Ding, C. Antibacterial activity and increased bone marrow stem cell functions of Zn-incorporated TiO 2 coatings on titanium. Acta Biomater. 2012 ,8, 904–915. [CrossRef] [PubMed] 127. Zhang, X.; Wang, H.; Li, J.; He, X.; Hang, R.; Huang, X.; Tian, L.; Tang, B. Corrosion behavior of Zn-incorporated antibacterial TiO 2 porous coating on titanium. Ceram. Int. 2016 ,42, 17095–17100. [CrossRef] 128. Zhang, L.; Gao, Q.; Han, Y. Zn and Ag co-doped anti-microbial TiO 2 coatings on Ti by micro-arc oxidation. J. Mater. Sci. Technol. 2016,32, 919–924. [CrossRef] 129. Zhang, J.Y.; Ai, H.J.; Qi, M. Osteoblast growth on the surface of porous Zn-containing HA/TiO 2 hybrid coatings on Ti substrate by MAO plus sol-gel methods. Surf. Coat. Technol. 2013 ,228, S202–S205. [CrossRef] 130. Zhao, B.H.; Zhang, W.; Wang, D.N.; Feng, W.; Liu, Y.; Lin, Z.; Du, K.Q.; Deng, C.F. Effect of Zn content on cytoactivity and bacteriostasis of micro-arc oxidation coatings on pure titanium. Surf. Coat. Technol. 2013 , 228, S428–S432. [CrossRef] 131. Jin, G.D.; Qin, H.; Cao, H.L.; Qian, S.; Zhao, Y.C.; Peng, X.C.; Zhang, X.L.; Liu, X.Y.; Chu, P.K. Synergistic effects of dual Zn/Ag ion implantation in osteogenic activity and antibacterial ability of titanium. Biomaterials 2014,35, 7699–7713. [CrossRef] [PubMed] 132. Huang, Y.; Zhang, X.J.; Mao, H.H.; Li, T.T.; Zhao, R.L.; Yan, Y.J.; Pang, X.F. Osteoblastic cell responses and antibacterial efficacy of Cu/Zn co-substitued hydroxyapatite coatings on pure titanium using electrodeposition method. RSC Adv. 2015,5, 17076–17086. [CrossRef] 133. Rokosz, K.; Hryniewicz, T.; Gaiaschi, S.; Chapon, P.; Raaen, S.; Malorny, W.; Matysek, W.; Pietrzak, K. Development of porous coatings enriched with magnesium and zinc obtained by DC Plasma Electrolytic Oxidation. Micromachines 2018,9, 332. [CrossRef] 134. Zhao, L.Z.; Chu, P.K.; Zhang, Y.M.; Wu, Z.F. Antibacterial coatings on titanium implants. J. Biomed. Mater. Res. Part B 2009,91, 470–480. [CrossRef] [PubMed] 135. Zhu, W.; Zhang, Z.; Gu, B.; Sun, J.; Zhu, L. Biological activity and antibacterial property of nano-structured TiO 2 coating incorporated with Cu prepared by micro-arc oxidation. J. Mater. Sci. Technol. 2013 ,29, 237–244. [CrossRef] 136. Zhang, L.; Guo, J.; Huang, X.; Zhang, Y.; Han, Y. The dual function of Cu-doped TiO 2 coatings on titanium for application in percutaneous implants. J. Mater. Chem. B 2016,4, 3788–3800. [CrossRef] 137. Moulder, J.F.; Stickle, W.F.; Sobol, P.E.; Bomben, K.D. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer Corporation: Eden Prairie, MN, USA, 1992. 138. Wagner, C.D.; Naumkin, A.V.; Kraut-Vass, A.; Allison, J.W.; Powell, C.J.; Rumble, J.R., Jr. NIST Standard Reference Database 20, Version 3.4. 2003. Available online: http://srdata.nist.gov/xps (accessed on 1 July 2018). 139. Casa Software Ltd. CasaXPS: Processing Software for XPS, AES, SIMS and More. 2009. Available online: http://www.casaxps.com (accessed on 1 July 2018). Materials 2018,11, 1680 20 of 20 140. Gaiaschi, S.; Richard, S.; Chapon, P.; Acher, O. Real-time measurement in glow discharge optical emission spectrometry via differential interferometric profiling. J. Anal. Atom. Spectrom. 2017 ,32, 1798–1804. [CrossRef] 141. Pulsed RF Glow Discharge Optical Emission Spectrometry—Ultra Fast Elemental Depth Profiling, HORIBA Scientific, Printed in France— © HORIBA Jobin Yvon. 2014, p. 7. Available online: http://www.horiba.com/ scientific/products/atomic-emission-spectroscopy/glow-discharge/ (accessed on 1 July 2018). © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).