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Open AccessArticle

Novel Porous Phosphorus–Calcium–Magnesium Coatings on Titanium with Copper or Zinc Obtained by DC Plasma Electrolytic Oxidation: Fabrication and Characterization

1
Division of BioEngineering and Surface Electrochemistry, Department of Engineering and Informatics Systems, Koszalin University of Technology, Racławicka 15-17, PL 75-620 Koszalin, Poland
2
HORIBA FRANCE S.A.S., Avenue de la Vauve, Passage Jobin Yvon, 91120 Palaiseau, France
3
Department of Physics, Norwegian University of Science and Technology (NTNU), Realfagbygget E3-124 Høgskoleringen 5, 7491 NO Trondheim, Norway
4
Institute of Geological Engineering, Faculty of Mining and Geology, VŠB—Technical University of Ostrava, 708 33 Ostrava, Czech Republic
*
Author to whom correspondence should be addressed.
Materials 2018, 11(9), 1680; https://doi.org/10.3390/ma11091680
Received: 2 August 2018 / Revised: 4 September 2018 / Accepted: 7 September 2018 / Published: 11 September 2018
(This article belongs to the Special Issue Metals Functionalization via Plasma Electrolytic Oxidation)

Abstract

In this paper, the characteristics of new porous coatings fabricated at three voltages in electrolytes based on H3PO4 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(H2PO4)2∙H2O and/or Ti(HPO4)2∙H2O. Additionally, the external parts of the obtained porous coatings formed on titanium consisted mainly of Ti4+, Ca2+, Mg2+ and PO43−, HPO42−, H2PO4, P2O74− as well as Zn2+ or copper Cu+/Cu2+. 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 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 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 [8,9,10] and discharging [11,12,13,14] 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 [37,38,39,40] and its alloys [40,41,42,43,44], magnesium [45] and its alloys [46,47,48,49,50,51,52,53,54], titanium [55,56,57,58,59,60] and its alloys [61,62,63,64,65,66,67], zirconium [68,69,70,71,72,73,74,75,76,77] and its alloys [78,79], tantalum [80,81,82,83], niobium [84,85,86,87,88], 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.
It should also be pointed out that in hydroxyapatite-like structures it is possible to substitute the Ca2+ ions for Ca2+, Mg2+, Cu2+, and Zn2+, 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 [117,118,119] and enriched with biocompatible magnesium, which causes faster wound healing [120,121,122,123,124,125], as well as antibacterial zinc [126,127,128,129,130,131,132] and copper [133,134,135,136], 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(NO3)2·4H2O, magnesium nitrate hexahydrate Mg(NO3)2·6H2O, and copper(II) nitrate trihydrate Cu(NO3)2·3H2O 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 [137,138,139,140,141] were used. Descriptions of the setups are presented in Table 3 and are detailed in reference [24].

3. Results

Figure 1 shows 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 H3PO4 with the addition of calcium nitrate tetrahydrate Ca(NO3)2·4H2O, magnesium nitrate hexahydrate Mg(NO3)2·6H2O, and zinc nitrate hexahydrate Zn(NO3)2·6H2O; and Electrolyte 2, with additions of calcium nitrate tetrahydrate Ca(NO3)2·4H2O, magnesium nitrate hexahydrate Mg(NO3)2·6H2O, and copper(II) nitrate trihydrate Cu(NO3)2·3H2O. It should be pointed out that all the obtained coatings were porous and had a well-developed surface.
In Figure 2 and 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 3 and Table 5 present 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.
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 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 Figure 7 and Figure 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.

4. Discussion

In this paper, the characteristics of new porous coatings fabricated at 500, 575, and 650 V in electrolytes based on H3PO4 and Mg(NO3)2·6H2O, Ca(NO3)2·4H2O with Cu(NO3)2·3H2O, and Zn(NO3)2·6H2O were presented. Information on the chemical composition of the PEO coatings was obtained by use of the XPS method (for the first 10 nm) and EDS and XRD (for the whole volume of the coatings). Based on EDS results, which were recorded for the whole volume of the coatings, it was found that increased PEO voltage results in an increase of the average metal-to-phosphorus ratios (Ca/P, Mg/P, Zn/P, and Cu/P), while XPS analysis of 10 nm showed that the maxima of those ratios are achieved for the values of the central voltage (575 V), which indicates that the coatings are layered, as proven by GDOES elemental profiles. All the PEO coatings can be divided into three sublayers: (i) external porous layer, enriched in P, O, and Zn (Electrolyte 1) and depleted in Ca, Mg, and Cu (Electrolyte 2) and Ti, but also the most contaminated (CO2, C2H5OH); (ii) semiporous layer, enriched in Ca, Mg, P, O, and Zn (Electrolyte 1) or Cu (Electrolyte 2), and depleted in Ti; (iii) transition layer, in which the titanium signal increases and depletion of all other elements (P, O, Ca, Mg, Zn, and Cu) is detected. On the basis of these XPS data, it was possible to conclude that the extreme surface of the coatings most likely consists of titanium (Ti4+), calcium (Ca2+), magnesium (Mg2+), and oxygen with PO43−, HPO42−, H2PO4, and P2O74− as well as Zn2+ or Cu+/Cu2+. Furthermore, the XRD analysis suggests that increasing voltage results in amorphization of the coatings, with the detection of crystalline phases such as Ca(H2PO4)2∙H2O and/or Ti(HPO4)2∙H2O.
It was also observed that using zinc ions as a bactericidal element instead of the copper ions in PEO coatings obtained on titanium substrate results in a drastic increase of magnesium incorporated into the obtained structure, combined with a slight increase of calcium ions. The results presented in this paper may be used to design biocompatible and bactericidal coatings due to the creation hydroxyapatite-like structures, in which the Ca2+ may be replaced by others, i.e., Mg2+, Zn2+, Cu2+, and the hydroxy group (OH) by Cu+ ions. It should be pointed out that while magnesium accelerates the healing of postoperative wounds, the structure composed of calcium and phosphorus is bone-like. Therefore, zinc or copper added in controlled quantities would perform antibacterial functions, which, together with magnesium, would allow faster healing of postoperative wounds.

5. Conclusions

  • It is possible to obtain porous calcium–magnesium–phosphate coatings enriched with copper or zinc.
  • The higher the voltage of PEO treatment, the thicker the porous coatings.
  • The higher the voltage of PEO treatment, the higher the amount of built-in elements coming from the electrolyte and more amorphous phase in coatings.
  • The top 10 nm layer of the studied coatings consist mainly of Ti4+, Ca2+, Mg2+ and PO43−, HPO42−, H2PO4.

Author Contributions

K.R. and T.H. conceived and designed the experiments; K.R., S.G., P.C., Ł.D., S.R., D.M., and W.M. performed the experiments; K.R., K.P., and Ł.D. analyzed the data; K.R., K.P., and Ł.D. contributed reagents, materials, and analysis tools; K.R. wrote the paper.

Funding

This work was supported by a grant from OPUS 11 of the 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”.

Conflicts of Interest

The authors declare no conflict of interest.

Nomenclature

PEOPlasma electrolytic oxidation
MAOMicro arc oxidation
SEMScanning electron microscopy
EDSEnergy dispersive spectroscopy
GDOESGlow discharge optical emission spectroscopy
XPSX-ray photoelectron spectroscopy
XRDX-ray powder diffraction
x ¯ Mean
σStandard deviation
Q1First quartile
Q2Second quartile (median)
Q3Third quartile
MMetal (here M = Ca + Mg + Zn or M = Ca+ Mg + Cu)
BEBinding energy
fFrequency
DCDirect current
ACAlternating current
n.u.no unit

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Figure 1. Surface morphologies of surfaces after PEO processing.
Figure 1. Surface morphologies of surfaces after PEO processing.
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Figure 2. Atomic ratios (EDS) of coatings formed in Electrolyte 1.
Figure 2. Atomic ratios (EDS) of coatings formed in Electrolyte 1.
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Figure 3. Atomic ratios (EDS) of coatings formed in Electrolyte 2.
Figure 3. Atomic ratios (EDS) of coatings formed in Electrolyte 2.
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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.
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Figure 5. GDEOS signals (black), first derivatives (red continuous line), and second derivatives (brown dashed line) for samples formed in Electrolyte 1.
Figure 5. GDEOS signals (black), first derivatives (red continuous line), and second derivatives (brown dashed line) for samples formed in Electrolyte 1.
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Figure 6. GDEOS signals (black), first derivatives (red continuous line), and second derivatives (brown dashed line) for samples formed in Electrolyte 2.
Figure 6. GDEOS signals (black), first derivatives (red continuous line), and second derivatives (brown dashed line) for samples formed in Electrolyte 2.
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Figure 7. XPS spectra for titanium samples after PEO treatment in Electrolyte 1.
Figure 7. XPS spectra for titanium samples after PEO treatment in Electrolyte 1.
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Figure 8. XPS spectra for titanium samples after PEO treatment in Electrolyte 2.
Figure 8. XPS spectra for titanium samples after PEO treatment in Electrolyte 2.
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Table 1. Electrolytes and conditions of the plasma electrolytic oxidation (PEO) process.
Table 1. Electrolytes and conditions of the plasma electrolytic oxidation (PEO) process.
ElectrolytesVoltage Current DensityRef.
H2O, NaAlO2, Na3PO4·12H2O, KOH, NaCl100–900 mA·cm−2 (f = 50 Hz)[7]
H2O, Na3PO4·12H2O, KOH, Na2SO4, (HOCH2)3CNH2, (NH4)2HPO4, C2H7NO270 mA·cm−2 (f = 50 Hz)[23]
H3PO4, Ca(NO3)2·4H2O, Mg(NO3)2·6H2O, Cu(NO3)2·3H2O, Zn(NO3)2·6H2O500, 575, 650 V[24]
H3PO4, Cu(NO3)2·3H2O450 V[32]
H2O, Na2SiO3, (NaPO3)6, NaAlO2 microparticle80 mA·cm−2 (f = 300 Hz)[33]
H3PO4, Mg(NO3)2·6H2O, Zn(NO3)2·6H2O500–650 V[35]
H2O, Na3PO4, FeSO4350 V (f = 100 Hz)[55]
H2O, NaAlO2, KOH400 V (f = 2000 Hz)[56]
H2O, (CH3COO)2Ca·H2O, NaH2PO4·2H2O300, 390 V (f = 900 Hz)[58]
H2O, Ca(CH3COO)2,Sr(CH3COO)2400, 450 V (f = 100 Hz)[59]
H3PO4, Ca(NO3)2·4H2O500, 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, NaOH12 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·H2O350–500 V (f = 1000 Hz)[67]
H2O, Ca(CH3COO)2·H2O300 V (f = 1000 Hz)[90]
H2O, (CH1COO)2Ca, C3H7Na2O6P250–400 V (f = 100 Hz)[91]
H2O, (CH3COO)2Ca·H2O, C3H7Na2O6P·5H2O450 V (f = 100 Hz)[92]
H2O, (CH3COO)2Ca·H2O, C3H7Na2O6P·5H2O250–500 V (f = 1000 Hz)[93,94]
H2O, Ca(CH3COO)2·H2O, CaC3H7O6P190–600 V (f = 660 Hz)[95,96]
H2O, (CH3COO)2Ca·H2O, C3H7Na2O6P·5H2O200–500 V (f = 900 Hz)[97]
H2O, Na4P2O7·10H2O and KOH, NaAlO20–300 V[98]
Na2B4O7·10H2O, (CH3COO)2Mn·4H2O450–500 V[99]
H2O, (CH3COO)2Ca·H2O230 V[100]
H2O, (CH3COO)2Ca·H2O, NaH2PO4·2H2O260–420 V[101]
H2O, CaHPO4, Ca(H2PO4)2, Na6P6O18, Ca(CH3COO)220, 100 mA·cm−2[102]
H2O, KOH290 V (f = 100–200 Hz)[103]
H2O, KOH350 V (f = 1000 Hz)[104]
H2O, (NaPO3)6, NaF, NaAlO2150–200 V[105]
H2O, K2Al2O4, Na3PO4, NaOH400 V[106]
H2O, CaCl2 and KH2PO4320–340 V[107]
H2O, H2SO4 and Ti2(SO4)31100 V[108]
H2O, Na2(EDTA), CaO, Ca(H2PO4)2, Na2SiO3·H2O 350 V (f = 200 Hz)[109]
H2O, Na2SiO3, NaOH280 V[110]
H2O, CaO, Na6P6O18, Na2H2EDTA⋅5.5H2O, KOHAC 0.5–2 mA·cm2[111]
2O, (NaPO3)6, NaF, NaAlO260 mA·cm2 (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, C3H7Na2O6P150 V[114]
H2O, Na2SiO3·9H2O, Na3PO4·12H2O, Na2SiO3·9H2O, Na3PO4·12H2O80 mA·cm2 (f = 150 Hz)[115]
H2O, Na3PO4·12H2O, α-Al2O3 nanoparticles20 mA·cm2[116]
Table 2. Experimental plan and code sample names.
Table 2. Experimental plan and code sample names.
Sample NameVoltageElectrolyte TypeElectrolyte Composition
SaltsSalt Concentrations (g/L)
Ti_CaMgZn_500V500 VElectrolyte 1Ca(NO3)2·4H2O and Mg(NO3)2·6H2O & Zn(NO3)2·6H2O166.7 + 166.7 + 166.7
Ti_CaMgZn_575V575 V
Ti_CaMgZn_650V650 V
Ti_CaMgCu_500V500 VElectrolyte 2Ca(NO3)2·4H2O and Mg(NO3)2·6H2O & Cu(NO3)2·3H2O166.7 + 166.7 + 166.7
Ti_CaMgCu_575V575 V
Ti_CaMgCu_650V650 V
Table 3. Setups of SEM, energy dispersive spectroscopy (EDS), x-ray photoelectron spectroscopy (XPS), glow discharge optical emission spectroscopy (GDEOS), and XRD equipment.
Table 3. Setups of SEM, energy dispersive spectroscopy (EDS), x-ray photoelectron spectroscopy (XPS), glow discharge optical emission spectroscopy (GDEOS), and XRD equipment.
TechniqueEquipmentManufacturer
SEMQuanta 650 FEIField Electron and Iron Company, Hillsboro, OR, USA
EDSNoran System SixEDS, Silicon Drift Detectors: Keith Thompson, Thermo Fisher Scientific, Madison, WI, USA
XPSSCIENCE SES 2002Scienta AB, Scienta Omicron, Uppsala, Sweden
GDOESGD Profiler 2HORIBA Scientific, Palaiseau, France
XRDBruker-AXS D8 AdvanceBruker Corporation, Billerica, MA, USA
Table 4. Statistical description of EDS of coatings formed in Electrolyte 1. n.u., no units.
Table 4. Statistical description of EDS of coatings formed in Electrolyte 1. n.u., no units.
RatiosVoltage x ¯ σQ1Q2Q3
Ca/P
n.u.
500 V0.0510.0030.0500.0520.052
575 V0.0630.0030.0620.0640.065
650 V0.0690.0030.0680.0710.071
Mg/P
n.u.
500 V0.0510.0040.0490.0510.053
575 V0.0580.0030.0570.0600.060
650 V0.0600.0060.0570.0630.063
Zn/P
n.u.
500 V0.0520.0040.0500.0530.054
575 V0.0650.0050.0630.0680.068
650 V0.0710.0100.0650.0750.075
M/P
n.u.
500 V0.1530.0080.1490.1510.157
575 V0.1870.0060.1840.1880.190
650 V0.2000.0150.1920.1950.206
Table 5. Statistical description of EDS of coatings formed in Electrolyte 2. n.u., no units.
Table 5. Statistical description of EDS of coatings formed in Electrolyte 2. n.u., no units.
RatiosVoltage x ¯ σQ1Q2Q3
Ca/P
n.u.
500 V0.0620.0030.0600.0610.062
575 V0.0680.0040.0660.0680.071
650 V0.0710.0030.0680.0720.073
Mg/P
n.u.
500 V0.0580.0020.0570.0570.059
575 V0.0590.0030.0560.0600.061
650 V0.0640.0030.0640.0640.066
Cu/P
n.u.
500 V0.0390.0030.0370.0400.040
575 V0.0480.0020.0470.0480.050
650 V0.0620.0050.0590.0610.063
M/P
n.u.
500 V0.1580.0060.1560.1560.159
575 V0.1750.0060.1720.1760.177
650 V0.1970.0040.1950.1960.197
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