Development of Porous Coatings Enriched with Magnesium and Zinc

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Jul 2, 2018 - novel porous coatings, which may be used, e.g., in micromachine's biocompatible ... strong nonlinear conditions of plasma discharges with high temperature, .... Statistical description of Mg/P and Zn/P ratios based on atomic percent. ... For samples obtained in electrolyte with 300 g/L of Mg(NO3)2·6H2O, the ...
micromachines Article

Development of Porous Coatings Enriched with Magnesium and Zinc Obtained by DC Plasma Electrolytic Oxidation Krzysztof Rokosz 1, *, Tadeusz Hryniewicz 1 , Sofia Gaiaschi 2 , Patrick Chapon 2 , Steinar Raaen 3 , Winfried Malorny 4 , Dalibor Matýsek 5 and Kornel Pietrzak 1 1

2 3 4 5

*

Division of BioEngineering and Surface Electrochemistry, Department of Engineering and Informatics Systems, Koszalin University of Technology, Racławicka 15-17, PL 75-620 Koszalin, Poland; [email protected] (T.H.); [email protected] (K.P.) HORIBA France S.A.S., Avenue de la Vauve-Passage Jobin Yvon, 91120 Palaiseau, France; [email protected] (S.G.); [email protected] (P.C.) Department of Physics, Norwegian University of Science and Technology (NTNU), Realfagbygget, E3-124 Høgskoleringen 5, 7491 NO Trondheim, Norway; [email protected] Faculty of Engineering, Hochschule Wismar-University of Applied Sciences Technology, Business and Design, DE 23966 Wismar, Germany; [email protected] Institute of Geological Engineering, Faculty of Mining and Geology, VŠB—Technical University of Ostrava, 708 33 Ostrava, Czech Republic; [email protected] Correspondence: [email protected]; Tel.: +48-501-989-332

Received: 5 June 2018; Accepted: 27 June 2018; Published: 2 July 2018

 

Abstract: Coatings with developed surface stereometry, being based on a porous system, may be obtained by plasma electrolytic oxidation, PEO (micro arc oxidation, MAO). In this paper, we present novel porous coatings, which may be used, e.g., in micromachine’s biocompatible sensors’ housing, obtained in electrolytes containing magnesium nitrate hexahydrate Mg(NO3 )2 ·6H2 O and/or zinc nitrate hexahydrate Zn(NO3 )2 ·6H2 O in concentrated phosphoric acid H3 PO4 (85% w/w). Complementary techniques are used for coatings’ surface characterization, such as scanning electron microscopy (SEM), for surface imaging as well as for chemical semi-quantitative analysis via energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), glow discharge optical emission spectroscopy (GDOES), and X-ray powder diffraction (XRD). The results have shown that increasing contents of salts (here, 250 g/L Mg(NO3 )2 ·6H2 O and 250 g/L Zn(NO3 )2 ·6H2 O) in electrolyte result in increasing of Mg/P and Zn/P ratios, as well as coating thickness. It was also found that by increasing the PEO voltage, the Zn/P and Mg/P ratios increase as well. In addition, the analysis of XPS spectra revealed the existence in 10 nm top of coating magnesium (Mg2+ ), zinc (Zn2+ ), titanium (Ti4+ ), and phosphorus compounds (PO4 3− , or HPO4 2− , or H2 PO4 − , or P2 O7 4− ). Keywords: plasma electrolytic oxidation; micro arc oxidation; DC PEO; titanium; zinc nitrate hexahydrate Zn(NO3 )2 ·6H2 O; magnesium nitrate hexahydrate Mg(NO3 )2 ·6H2 O; 85% phosphoric acid H3 PO4

1. Introduction In the literature, the terms plasma electrolytic oxidation (PEO) or micro arc oxidation (MAO) refer to the electrochemical method of surface treatment of lightweight metals and their alloys, which leads to the spontaneous development of an oxide layer on their surfaces. These alloys usually consist of elements/metals, which may be found in the fourth and fifth B groups of the periodic table, i.e., titanium [1], zirconium [2,3], hafnium [4,5], niobium [6,7], tantalum [8,9], though the first works

Micromachines 2018, 9, 332; doi:10.3390/mi9070332

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related to PEO (MAO) technique were carried out on aluminum, magnesium, and their alloys [10–14]. This method leads to locally strong nonlinear conditions of plasma discharges with high temperature, that results in the formation, on treated material, of a new porous coating [15] enriched with both electrolyte and substrate elements. It should also be noted that the use of the PEO (MAO) treatment creates coatings enriched with selected chemical elements in micro scale, while the nanolayers may be fabricated with the use of standard electropolishing [16], magnetoelectropolishing [17], or high current electropolishing [18]. Thicknesses of the coatings vary from 1 µm up to hundreds of micrometers [7,19,20], which results in different properties of the surface coating/passive layer compared to the metallic matrix. These phenomena are used in a variety of applications, including catalysts [21], biomedical implantable devices, joint prostheses, fracture fixation devices and dental implants [22], aerospace [23], chemical sensors [24], and wear-resistant materials [25]. The chemical composition, corrosion resistance of PEO coatings, as well as their thickness and porosity, depend on the electrolyte composition and used voltage or current regime (DC, AC, pulse). In addition, the frequency and shape of the voltage have influence on the properties of the obtained PEO coatings. As reported by Yong Han and Kewei Xu, amorphous titanium coatings may be obtained in electrolyte containing dicalcium phosphate CaHPO4 , while the nanocrystallized structures may be formed in electrolytic solutions containing sodium carbonate Na2 CO3 and acetate monohydrate (CH3 COO)2 Ca [26]. To fabricate the porous PEO coatings containing titanium oxides (anatase and/or apatite and/or rutile), a voltage from 250 V up to 450 V with pulse resume fixed at 100 Hz (duty cycle equaling from 3% [27]) up to 20% [28] or voltage of 250–500 V with pulse frequency of 1000 Hz (duty circle equals to 60%) [29,30] in aqueous electrolytes containing acetate monohydrate (CH3 COO)2 Ca and β-glycerophosphate disodium salt pentahydrate C3 H7 Na2 O6 P, can be used. The team of Long-Hao Li, Young-Min Kong at al. [31] published information that the use of DC voltage lower than 250 V, i.e., 190–600 V, with frequency 660 Hz (duty cycle equaling from 10%) in aqueous electrolyte containing of calcium acetate monohydrate (CH3 COO)2 Ca·H2 O and calcium glycerophosphate CaC3 H7 O6 P [32] may be used to create the porous PEO coatings of titanium oxides. It is also reported that, for PEO oxidation, the other electrolytes and voltages listed in Table 1, were used. Table 1. Examples of aqueous electrolytes used in plasma electrolytic oxidation (PEO) treatment with voltage conditions. Electrolytes Na2 CO3 and Na3 PO4 , and (CH3 COO)2 Ca·H2 O (CH3 COO)2 Ca·H2 O and C3 H7 Na2 O6 P·5H2 O Na4 P2 O7 ·10H2 O and KOH, NaAlO2 Na2 B4 O7 ·10H2 O and (CH3 COO)2 Mn·4H2 O (CH3 COO)2 Ca·H2 O (CH3 COO)2 Ca·H2 O and NaH2 PO4 ·2H2 O NH4 H2 PO4 , CaCl2 , NaH2 PO4 , (CH3 COO)2 Ca KOH KOH (NaPO3 )6 , NaF and NaAlO2 K2 Al2 O4 , Na3 PO4 , NaOH CaCl2 and KH2 PO4 H2 SO4 and Ti2 (SO4 )3 Na2 (EDTA) and CaO, Ca(H2 PO4 )2 and Na2 SiO3 ·H2 O Na2 SiO3 , and NaOH

Voltages

Ref.

200–500 V (900 Hz)

[33]

0–300 V 450–500 V 230 V 260–420 V 0–500 V 290 V (100–200 Hz) 350 V (1000 Hz) 150–200 V 400 V 320–340 V 1100 V 350 V (200 Hz) 280 V

[34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46]

Based on the chemical composition of electrolytes reported in literature, the authors decided to propose a new electrolyte containing magnesium (Mg2+ ), zinc (Zn2+ ), and phosphate PO4 3− ions. Previous authors’ studies [47–50] clearly indicate that it is possible to obtain the porous coatings by plasma electrolytic oxidation on titanium [47,48] and its alloys [49,50] in electrolytes based on concentrated orthophosphoric acid with selected nitrates. The conducted research indicates that the porosity is gained in an entire volume of the obtained coatings, which are enriched with elements

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originating mainly from the electrolyte. In the present paper, we will present porous coatings obtained at voltages in the range of 450–650 V using new solutions, and their characterization by complementary techniques. Such results could lead to establishing a novel knowledge to be used in any micromachines’ applications. 2. Materials and Methods Samples of commercially pure titanium grade 2, with dimensions of 10 × 10 × 2 mm, were used for plasma electrolytic oxidation (PEO) process, and then characterized by scanning electron microscope (SEM), energy dispersive X-ray Spectroscopy (EDS), X-ray powder diffraction (XRD), glow discharge optical emission spectroscopy (GDEOS), and X-ray photoelectron spectroscopy (XPS). In the first part of the studies (preliminary tests), the three-phase transformer with six diodes of Graetz bridge as the voltage source of 450 ± 46 V with pulsation frequency of 300 Hz was used, while during the second part (main studies), the PWR 1600 H (KIKUSUI ELECTRONICS CORP., Yokohama, Japan), Multi Range DC Power Supply 1600 W, 0–650 V/0–8 A as a power source of three voltages, i.e., 500 V, 575 V and 600 V, was used. All the PEO treatments were performed for 3 min in 500 mL of electrolytes containing magnesium nitrate hexahydrate Mg(NO3 )2 ·6H2 O and/or zinc nitrate hexahydrate Zn(NO3 )2 ·6H2 O in phosphoric acid (85% w/w). In details, all the solutions are presented in Table 2. The methodology of scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), glow discharge optical emission spectroscopy (GDOES), and X-ray photoelectron spectroscopy (XPS) [51–55] are presented in Table 3, and in detail, in the authors’ reference [48]. Table 2. Electrolytes used to oxidize the titanium by (PEO) treatment. Power Supply

Voltage (V)

Three-phase transformer with Graetz Bridge

PWR 1600 H, Multi Range DC Power Supply

Salt

Salt Concentration (g/L)

Mg(NO3 )·6H2 O

10 300 600

Zn(NO3 )2 ·6H2 O

10 300 600

500 575 650

Mg(NO3 )2 ·6H2 O

500

500 575 650

Zn(NO3)2 ·6H2 O

500

500 575 650

Mg(NO3 )2 ·6H2 O & Zn(NO3 )2 ·6H2 O

250 + 250

450 ± 46

Table 3. Set-up descriptions of SEM, EDS, XPS, GDEOS, and XRD equipment. SEM & EDS

XPS

GDOES

XRD

Quanta 250 & 650 FEG (SEM: Field Electron and Iron Company, Hillsboro, OR, USA EDS: Thermo Fisher Scientific, Madison, WI, USA)

SCIENCE SES 2002 (SCIENTA AB, ScientaOmicron, Uppsala, Sweden)

Horiba Scientific GD Profiler 2 (HORIBA Scientific, Palaiseau, France)

Bruker-AXS D8 Advance (BRUKER Corporation, Billerica, MA, USA)

High Vacuum

monochromatic (Gammadata-Scienta) Al K(alpha) X-ray source

radio frequency (RF) pulsed source

2Θ/Θ geometry

ESEM mode

(hν = 1486.6 eV) (18.7 mA, 13.02 kV)

pressure: 700 Pa, power: 40 W

radiation CuKα Ni filter

EDS Noran System Six

energy step 0.2 eV

frequency: 3000 Hz, duty cycle: 0.25

voltage 40 kV current 40 mA

ETD & BSED detectors

step time 200 ms

anode diameter: 4 mm

step by step mode of 0.014 2Θ with an interval of 0.25 s per step

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3. 3. Results In In Figures Figures 11 and and 2, 2, SEM SEM micrographs micrographs of of coatings’ coatings’ surfaces surfaces produced produced on on titanium titanium with with the the use use of of three-phase transformer with six diodes of Graetz Bridge, are presented. Some surface morphology three-phase transformer with six diodes of Graetz Bridge, are presented. Some surface morphology changes, increasingof ofsalt saltconcentration concentrationforfor both electrolytes, with Mg(NO 6H or changes, with increasing both electrolytes, i.e.,i.e., with Mg(NO 3)2·6H or2 O with 3 )22·O with Zn(NO canobserved. be observed. results a bettersurface surfacedevelopment development from from island-like Zn(NO 3)2·6H can ThisThis results in in a better island-like 3 2)O, 2 ·6H 2 O,be structure of salt salt in in electrolyte). electrolyte). structure(10 (10g/L g/L of salt in electrolyte), electrolyte), to to microporous microporousone one(300 (300g/L g/L and 600 g/L g/L of The The layers layers obtained obtained in in 10 10g/L g/L of magnesium-containing magnesium-containing solution seem to be be more more morphologically morphologically developed than those obtained with the zinc-containing solution. Differences can also noticed developed those obtained with the zinc-containing solution. Differences can be be also noticed for for concentrations of 300 g/L and 600 g/L, where coatings obtained in Zn(NO ) · 6H O have more concentrations of 300 g/L and 600 g/L, where coatings obtained in Zn(NO3)2·6H23O2 have2 more porous porous morphology. In addition, it should be noted cracks porouscoating coating obtained obtained in surfacesurface morphology. In addition, it should be noted thatthat cracks onon a aporous in electrolyte containing 600 g/L magnesium nitrate, are recorded. This phenomenon is very unfavorable, electrolyte containing 600 g/L magnesium nitrate, are recorded. This phenomenon is very due to the propagation of cracks, which may lead may to a lead faster of the coating. The EDS unfavorable, due to the propagation of cracks, which to acrumble faster crumble of the coating. The semi-quantitative results, presented as metal-to-phosphorus ratios (Mg/P Zn/P), shown EDS semi-quantitative results, presented as metal-to-phosphorus ratios or (Mg/P or have Zn/P),been have been in Figure 3 and Table For4.both samples obtained in electrolytes with concentrations shown in Figure 3 and 4. Table For both samples obtained in electrolytes with concentrationsofof10 10g/L, g/L, the the EDS EDS results results indicate indicate the the presence presence of of both both magnesium magnesium or or zinc zinc and and phosphorus phosphorus in in the the studied studied coatings, due to tothe thesmall smallsignals signalsofofzinc zinc and magnesium, qualitative analysis with coatings, however, due and magnesium, qualitative analysis with the the use use of that method cannot be performed. The Mg/P ratio for samples obtained in the electrolyte of that method cannot be performed. The Mg/P ratio for samples obtained in the electrolyte composed composed ·6H in H300 300 g/Lsalt of that saltsolution in the solution is equal to 0.080 ± 0.002 of Mg(NOof 3)2Mg(NO ·6H2O in 3PO 4 for that in the is equal to 0.080 ± 0.002 (first 3 )2H 2O 3 POg/L 4 forof (first quartile 0.078; third quartile 0.082), while 600 g/L of the same salt equals 0.165 ± 0.024 (first quartile 0.078; third quartile 0.082), while 600 g/L of the same salt equals 0.165 ± 0.024 (first quartile quartile 0.140;quartile third quartile The Zn/P ratiothe forsamples the samples obtained in electrolyte composed 0.140; third 0.195). 0.195). The Zn/P ratio for obtained in electrolyte composed of of Zn(NO )2 ·6H O in g/L that saltininthe thesolution solutionequals equals 0.054 0.054 ±±0.004 0.004 (first quartile Zn(NO 3)23 ·6H 2O 2in H3H PO 4 for 300300 g/L of of that salt quartile 3 PO 4 for 0.050; 0.050; third third quartile quartile 0.058), 0.058), while while for for600 600g/L g/L of of the the same same salt, salt, ititequals equals0.089 0.089±± 0.016 0.016 (first (first quartile quartile 0.075; 0.075; third third quartile quartile 0.105). 0.105). For both types of electrolytes, it was observed that the more salt salt in in the the electrolyte Zn/P)ratios ratiosthat thatwere wereobtained. obtained. electrolyte used, used, the the higher higher the the metal-to-phosphorus metal-to-phosphorus(Mg/P (Mg/P and Zn/P)

Figure 1. Cont.

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Figure 1.1.SEM SEM micrographs of samples surfaces afterprocessing PEO processing in electrolytes Figure micrographs of samples surfaces after PEO obtained obtained in electrolytes composed composed 3and ) 2·6H 2O and Zn(NO 3)2·6H 2O PO in H 3in PO 4 PEO in three concentrations 10 g/L, 300 g/L, and and Figure 1. 3of SEM of samples surfaces after processing obtained in electrolytes ·6Hmicrographs · of Mg(NO )2Mg(NO O Zn(NO ) 6H O in H three concentrations 10 g/L, 300 g/L, 2 3 2 2 3 4 600 g/L, at a voltage of 450 ± 46 V, with a pulsation frequency of 300 Hz. Magnification 1000 times. composed of Mg(NO 3 ) 2 ·6H 2 O and Zn(NO 3 ) 2 ·6H 2 O in H 3 PO 4 in three concentrations 10 g/L, 300 g/L, and 600 g/L, at a voltage of 450 ± 46 V, with a pulsation frequency of 300 Hz. Magnification 1000 times. 600 g/L, at a voltage of 450 ± 46 V, with a pulsation frequency of 300 Hz. Magnification 1000 times.

Figure 2. SEM micrographs of samples surfaces after PEO processing obtained in electrolytes composed of Mg(NO 3)2·6H2O and 3)2·6H 2O in H 3PO4 PEO in three concentrations 10 g/L, g/L, and Figure SEM micrographs of Zn(NO samples surfaces after processing in 300 electrolytes Figure 2.2.SEM micrographs of samples surfaces after PEO processing obtainedobtained in electrolytes composed 600 g/L, at a voltage of 450 ± 46 V, with a pulsation frequency of 300 Hz. Magnification 10,000 times. composed of Mg(NO 3 ) 2 ·6H 2 O and Zn(NO 3 ) 2 ·6H 2 O in H 3 PO 4 in three concentrations 10 g/L, 300 g/L, and of Mg(NO ) ·6H O and Zn(NO ) ·6H O in H PO in three concentrations 10 g/L, 300 g/L, and 3 2

2

3 2

2

3

4

600 g/L, 600 g/L,atataavoltage voltageof of450 450±±4646V,V,with withaapulsation pulsationfrequency frequencyofof300 300Hz. Hz.Magnification Magnification10,000 10,000times. times.

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Figure 3. EDS results of coatings after PEO processing obtained in electrolytes composed of Figure 3. EDS results of coatings after PEO processing obtained in electrolytes composed of Mg(NO3)2·6H2O and Zn(NO3)2·6H2O in H3PO4 at concentrations of 300 g/L and 600 g/L at a voltage of 450 Mg(NO3 )2 ·6H2 O and Zn(NO3 )2 ·6H2 O in H3 PO4 at concentrations of 300 g/L and 600 g/L at a voltage ± 46 V, with a pulsation frequency of 300 Hz. of 450 ± 46 V, with a pulsation frequency of 300 Hz. Table 4. Statistical description of Mg/P and Zn/P ratios based on atomic percent. Table 4. Statistical description of Mg/P and Zn/P ratios based on atomic percent. Salt Salt Mg(NO3)2·6H2O Mg(NO3 )2 ·6H2 O Zn(NO3)2·6H2O Zn(NO3 )2 ·6H2 O

Concentration Concentration 300 g/L 600 g/L 300 g/L g/L 600300 g/L 600 g/L 300 g/L 600 g/L

Mean Mean 0.080 0.165 0.080 0.054 0.165 0.089 0.054 0.089

Stand. Dev. Stand. 0.002 Dev. 0.024 0.002 0.004 0.024 0.016 0.004 0.016

First Quartile First Quartile 0.078 0.140 0.078 0.050 0.140 0.075 0.050 0.075

Third Quartile Third Quartile 0.082 0.195 0.082 0.058 0.195 0.105 0.058 0.105

In Figures 4 and 5, GDEOS results of PEO coatings i.e., signals of magnesium (285 nm), zinc (481 nm), phosphorus (178 nm), oxygen (130 nm), hydrogen (122 nm), carbon (156 nm), nitrogen (149 nm), and In Figures 4 and 5, GDEOS results of PEO coatings i.e., signals of magnesium (285 nm), zinc titanium (365 nm), fabricated in electrolyte, which is composed of 10 g/L, 300 g/L, and 600 g/L of (481 nm), phosphorus (178 nm), oxygen (130 nm), hydrogen (122 nm), carbon (156 nm), nitrogen Mg(NO3)2·6H2O or Zn(NO3)2·6H2O in H3PO4 at 450 ± 46 V with pulsation frequency of 300 Hz, are (149 nm), and titanium (365 nm), fabricated in electrolyte, which is composed of 10 g/L, 300 g/L, presented. The elements, such as magnesium, phosphorus, and oxygen, which originate from and 600 g/L of Mg(NO3 )2 ·6H2 O or Zn(NO3 )2 ·6H2 O in H3 PO4 at 450 ± 46 V with pulsation electrolyte, should be treated as the main components of the PEO coating. The titanium and frequency of 300 Hz, are presented. The elements, such as magnesium, phosphorus, and oxygen, magnesium signals are the smallest ones in the external top-layers, and they increase. The hydrogen which originate from electrolyte, should be treated as the main components of the PEO coating. and carbon signals may originate from molecules of orthophosphoric acid, water, or organic The titanium and magnesium signals are the smallest ones in the external top-layers, and they contaminations absorbed from the air or from cleaning process (alcohol molecules), while part of increase. The hydrogen and carbon signals may originate from molecules of orthophosphoric acid, nitrogen signals should be interpreted as a contamination, and partly as the component of coatings, water, or organic contaminations absorbed from the air or from cleaning process (alcohol molecules), originating from nitrates of magnesium. Based on GDEOS data, the total thickness of the layers, while part of nitrogen signals should be interpreted as a contamination, and partly as the component measured as sputtering time, for magnesium- or zinc-enriched coatings, increases with increasing of of coatings, originating from nitrates of magnesium. Based on GDEOS data, the total thickness Mg(NO3)2·6H2O or Zn(NO3)2·6H2O concentration from 10 g/L up to 600 g/L in H3PO4. For a of the layers, measured as sputtering time, for magnesium- or zinc-enriched coatings, increases concentration of 10 g/L of Mg(NO3)2·6H2O or Zn(NO3)2·6H2O, no clear sublayers of obtained PEO with increasing of Mg(NO3 )2 ·6H2 O or Zn(NO3 )2 ·6H2 O concentration from 10 g/L up to 600 g/L in coatings are observed, while for samples obtained in electrolyte containing 300 g/L and 600 g/L of H3 PO4 . For a concentration of 10 g/L of Mg(NO3 )2 ·6H2 O or Zn(NO3 )2 ·6H2 O, no clear sublayers of Mg(NO3)2·6H2O or Zn(NO3)2·6H2O, three sublayers can be distinguished as clearly visible. For obtained PEO coatings are observed, while for samples obtained in electrolyte containing 300 g/L and samples obtained in electrolyte with 300 g/L of Mg(NO3)2·6H2O, the thicknesses of the first, second, and 600 g/L of Mg(NO3 )2 ·6H2 O or Zn(NO3 )2 ·6H2 O, three sublayers can be distinguished as clearly visible. third sublayers are equal to about 50 s, 350 s, and 400 s by sputtering time, respectively, while for those For samples obtained in electrolyte with 300 g/L of Mg(NO3 )2 ·6H2 O, the thicknesses of the first, treated in the solution with 600 g/L salt, the thicknesses of the first, second, and third sublayers second, and third sublayers are equal to about 50 s, 350 s, and 400 s by sputtering time, respectively, correspond with the times of 50 s, 400 s, and 450 s, respectively. For samples obtained in the while for those treated in the solution with 600 g/L salt, the thicknesses of the first, second, and third electrolyte with 300 g/L of Zn(NO3)2·6H2O, thicknesses of the first, second, and third sublayers are sublayers correspond with the times of 50 s, 400 s, and 450 s, respectively. For samples obtained in the equal to about 50 s, 350 s, and 350 s by sputtering time, respectively, while for those ones treated in electrolyte with 300 g/L of Zn(NO3 )2 ·6H2 O, thicknesses of the first, second, and third sublayers are the solution with 600 g/L salt in it, the thicknesses of the first, second, and third sublayers correspond equal to about 50 s, 350 s, and 350 s by sputtering time, respectively, while for those ones treated in the with the times of 150 s, 700 s, and 700 s, respectively.

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solution with 600 g/L salt in it, the thicknesses of the first, second, and third sublayers correspond 7 of 19 s, 700 s, and 700 s, respectively.

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Figure Figure 4.4. GDEOS GDEOSsignals signals(black) (black)with withfirst first(red (redcontinuous continuousline) line)and andsecond second(brown (browndashed dashedline) line) derivativesfor forsamples samplesafter after PEO PEO processing processing obtained obtained in electrolyte composed of Mg(NO33)22·6H ·6H22OOinin derivatives concentrations ofg/L, 10 g/L, 300and g/L,600 and 600 at a of voltage ofV, 450 ± 46 V, with afrequency pulsation HH3PO 4 at concentrations of 10 300 g/L, g/L, at g/L, a voltage 450 ± 46 with a pulsation 3 PO 4 at frequency of 300 Hz. of 300 Hz.

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Figure5.5. GDEOS GDEOS signals signals (black) (black) with with first first(red (redcontinuous continuousline) line)and andsecond second(brown (browndashed dashedline) line) Figure derivatives for samples after PEO processing obtained in electrolyte composed of Zn(NO 3 ) 2 ·6H 2 derivatives for samples after PEO processing obtained in electrolyte composed of Zn(NO3 )2 ·6H2 OOinin 3PO4 at concentrationsof of10 10g/L, g/L, 300 300g/L, g/L, and and 600 600 g/L g/L at a voltage voltage of of450 450±± 46 V, V,with withaapulsation pulsation HH3 PO 4 atconcentrations frequencyofof300 300Hz. Hz. frequency

XPS results for samples after PEO processing obtained in electrolyte composed of XPS results for samples after PEO processing obtained in electrolyte composed of Mg(NO3 )2 ·6H2 O Mg(NO3)2·6H2O in H3PO4 at a concentration of 600 g/L at the voltage of 450 ± 46 V with pulsation in H3 PO4 at a concentration of 600 g/L at the voltage of 450 ± 46 V with pulsation frequency of 300 Hz frequency of 300 Hz are presented in Figure 6. The XPS results show that the top 10 nm layer is are presented in Figure 6. The XPS results show that the top 10 nm layer is enriched in magnesium enriched in magnesium (Mg2+), phosphorus (as PO43−, or HPO42−, or H2PO4−, or P2O74–), nitrogen (Mg2+ ), phosphorus (as PO4 3− , or HPO4 2− , or H2 PO4 − , or P2 O7 4– ), nitrogen (organic contamination), (organic contamination), titanium (Ti4+), as confirmed by the binding energies of Mg 2s (89.4 eV), Mg titanium (Ti4+ ), as confirmed by the binding energies of Mg 2s (89.4 eV), Mg KLL (306.9 eV), O 1s KLL (306.9 eV), O 1s (531.5 eV), P 2p (134 eV), Ti 2p2/3 (460.0 eV). XPS results for samples after PEO (531.5 eV), P 2p (134 eV), Ti 2p2/3 (460.0 eV). XPS results for samples after PEO processing obtained processing obtained in electrolyte, composed of Zn(NO3)2·6H2O in H3PO4 at a concentration of 600 in electrolyte, composed of Zn(NO3 )2 ·6H2 O in H3 PO4 at a concentration of 600 g/L at the voltage of g/L at the voltage of 450 ± 46 V with pulsation frequency of 300 Hz, are presented in Figure 7. The 450 ± 46 V with pulsation frequency of 300 Hz, are presented in Figure 7. The XPS spectra show that XPS spectra show that the top 10 nm layer is enriched in zinc (Zn2+), phosphorus (PO43−, or HPO42−, or the top 10 nm layer is enriched in zinc (Zn2+ ), phosphorus (PO4 3− , or HPO4 2− , or H2 PO4 − , or P2 O7 4− ), H2PO4−, or P2O74−), and titanium (Ti4+), as confirmed by the binding energies of Zn 2p (1022.2 eV), Zn LMM (500 eV and 497.4 eV), O 1s (531.3 eV), P 2p (133.8 eV), Ti 2p2/3 (460.1 eV).

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and titanium (Ti4+ ), as confirmed by the binding energies of Zn 2p (1022.2 eV), Zn LMM (500 eV and 497.4 eV), O 1s (531.3 eV), P 2p (133.8 eV), Ti 2p2/3 (460.1 eV). Micromachines 2018, 9, x 9 of 19

Figure 6. 6. XPS electrolyte composed composed of of Figure XPS results results for for samples samples after after PEO PEO processing processing obtained obtained in in electrolyte 3 ) 2 ·6H 2 O in H 3 PO 4 at a concentration of 600 g/L at a voltage of 450 ± 46 V, with a pulsation Mg(NO Mg(NO3 )2 ·6H2 O in H3 PO4 at a concentration of 600 g/L at a voltage of 450 ± 46 V, with a pulsation frequency of of 300 300 Hz. Hz. frequency

The next step of analysis was to present the characterization of porous coatings obtained at three The next step of analysis was to present the characterization of porous coatings obtained at voltages, i.e., 500 V, 575 V, and 650 V, in electrolytes composed of Mg(NO3)2·6H2O in H3PO4 or three voltages, i.e., 500 V, 575 V, and 650 V, in electrolytes composed of Mg(NO3 )2 ·6H2 O in H3 PO4 or Zn(NO3)2·6H2O in H3PO4, with the use of commercial power supply, where no voltage pulsations Zn(NO3 )2 ·6H2 O in H3 PO4 , with the use of commercial power supply, where no voltage pulsations were recorded. A part of these results (SEM, EDS, GDOES, corrosion studies) related to were recorded. A part of these results (SEM, EDS, GDOES, corrosion studies) related to characterization characterization of these coatings were presented in reference [48]. However, for a comprehensive of these coatings were presented in reference [48]. However, for a comprehensive full analysis, the full analysis, the XRD analysis results should be also added, and they are presented in Figure 8. For XRD analysis results should be also added, and they are presented in Figure 8. For coatings formed coatings formed in both electrolytes at voltages of 500 V and 575 V, only titanium as crystalline phase in both electrolytes at voltages of 500 V and 575 V, only titanium as crystalline phase (a signal from (a signal from substrate) was detected. Only for samples obtained at the highest voltage, i.e., 650 V, substrate) was detected. Only for samples obtained at the highest voltage, i.e., 650 V, was the Ti2 P2 O7 was the Ti2P2O7 crystalline phase was detected. It is worth noting that the increasing of PEO voltages crystalline phase was detected. It is worth noting that the increasing of PEO voltages results in the results in the increasing of amorphous phase in coatings’ structures. increasing of amorphous phase in coatings’ structures.

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Figure 7.7. XPS for samples samples after after PEO PEO processing processing obtained obtained in in electrolyte electrolyte composed composed of of Figure XPS results results for 2O in H3PO4 at a concentration of 600 g/L and at a voltage of 450 ± 46 V with pulsation Zn(NO3))2··6H Zn(NO 3 2 6H2 O in H3 PO4 at a concentration of 600 g/L and at a voltage of 450 ± 46 V with pulsation frequency of 300Hz. Hz. frequency of 300

In the following part, we present the possibility of manufacturing a porous coating obtained in In the following part, we present the possibility of manufacturing a porous coating obtained electrolytes composed of 250 g/L Mg(NO3)2·6H2O and 250 g/L Zn(NO3)2·6H2O in H3PO4 at the same in electrolytes composed of 250 g/L Mg(NO3 )2 ·6H2 O and 250 g/L Zn(NO3 )2 ·6H2 O in H3 PO4 at the three voltages 500 V, 575 V, 650 V, which were used in [48]. In Figure 9, the SEM micrographs of the same three voltages 500 V, 575 V, 650 V, which were used in [48]. In Figure 9, the SEM micrographs of surfaces after PEO processing, at four magnifications, are presented. The EDS results of samples the surfaces after PEO processing, at four magnifications, are presented. The EDS results of samples obtained in electrolytes composed of 250 g/L Mg(NO3)2·6H2O and 250 g/L Zn(NO3)2·6H2O in H3PO4 obtained in electrolytes composed of 250 g/L Mg(NO3 )2 ·6H2 O and 250 g/L Zn(NO3 )2 ·6H2 O in H3 PO4 as metal-to-phosphorus atomic ratios (Mg/P and Zn/P and M/P, where M = Mg + Zn), are presented as metal-to-phosphorus atomic ratios (Mg/P and Zn/P and M/P, where M = Mg + Zn), are presented in Figure 10 and Table 5. For coatings obtained at 500 V, the Mg/P, Zn/P and M/P ratios are as follows: in Figure 10 and Table 5. For coatings obtained at 500 V, the Mg/P, Zn/P and M/P ratios are as 0.073 ± 0.003 (first quartile: 0.070, third quartile: 0.075), 0.071 ± 0.003 (first quartile: 0.069, third follows: 0.073 ± 0.003 (first quartile: 0.070, third quartile: 0.075), 0.071 ± 0.003 (first quartile: 0.069, quartile: 0.074), 0.145 ± 0.005 (first quartile: 0.141, third quartile: 0.148), respectively. For the coatings third quartile: 0.074), 0.145 ± 0.005 (first quartile: 0.141, third quartile: 0.148), respectively. For the obtained at 575 V, Mg/P, Zn/P and M/P (where M = Mg + Zn) are as follows: 0.084 ± 0.004 (first coatings obtained at 575 V, Mg/P, Zn/P and M/P (where M = Mg + Zn) are as follows: 0.084 ± 0.004 quartile: 0.081, third quartile: 0.088), 0.089 ± 0.004 (first quartile: 0.086, third quartile: 0.091), 0.173 ± (first quartile: 0.081, third quartile: 0.088), 0.089 ± 0.004 (first quartile: 0.086, third quartile: 0.091), 0.007 (first quartile: 0.168, third quartile: 0.178), respectively. For the coatings obtained at 650 V, Mg/P, 0.173 ± 0.007 (first quartile: 0.168, third quartile: 0.178), respectively. For the coatings obtained at 650 V, Zn/P and M/P (where M = Mg + Zn) are as follows: 0.087 ± 0.007 (first quartile: 0.082, third quartile: Mg/P, Zn/P and M/P (where M = Mg + Zn) are as follows: 0.087 ± 0.007 (first quartile: 0.082, third 0.091), 0.102 ± 0.005 (first quartile: 0.098, third quartile: 0.106), 0.188 ± 0.010 (first quartile: 0.178, third quartile: 0.091), 0.102 ± 0.005 (first quartile: 0.098, third quartile: 0.106), 0.188 ± 0.010 (first quartile: quartile: 0.196), respectively. Both metals, i.e., magnesium and zinc, are built-in into the coating in ca. 0.178, third quartile: 0.196), respectively. Both metals, i.e., magnesium and zinc, are built-in into the 1:1 atomic proportion, moreover, all calculated ratios show a positive correlation with applied coating in ca. 1:1 atomic proportion, moreover, all calculated ratios show a positive correlation with voltage. It is also worth noting that with the increasing applied voltage, the reproducibility decreases, as indicated by calculated standard deviations.

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applied voltage. It is also worth noting that with the increasing applied voltage, the reproducibility decreases, as indicated by calculated standard deviations. Micromachines 2018, 9, x 11 of 19 Micromachines 2018, 9, x

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Figure 8. XRD results of coatings after PEO processing obtained in electrolytes composed of Figure 8. XRD results of coatings after PEO processing obtained in electrolytes composed of Figure3)28.·6H XRD coatings after processing obtained in at electrolytes composed of 2O orresults Zn(NOof 3)2·6H 2O in H 3PO4PEO at a concentration of 500 g/L three voltages, 500 V, 575 Mg(NO Mg(NO3 )2 ·6H2 O or Zn(NO3 )2 ·6H2 O in H3 PO4 at a concentration of 500 g/L at three voltages, 500 V, 3 ) 2 ·6H 2 O or Zn(NO 3 ) 2 ·6H 2 O in H 3 PO 4 at a concentration of 500 g/L at three voltages, 500 V, 575 Mg(NO V, and 650 V. 575V,V,and and650 650V.V.

Figure 9. Cont.

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Figure 9. SEM micrographs of samples surfaces after PEO processing obtained in electrolytes Figure 9. SEM micrographs of samples surfaces after PEO obtained in electrolytes composed Figure 9. SEM micrographs of samples surfaces after processing PEO processing obtained in electrolytes composed of 250 g/L Mg(NO3)2·6H2O and 250 g/L Zn(NO3)2·6H2O in H3PO4 at three voltages, 500 V, of composed 250 g/L Mg(NO )2 ·6H 2502O g/L Zn(NO H32PO voltages, 500 V, 575 ·6H2 O 3in of 250 3g/L Mg(NO 3)2·6H and 250 g/L )2·6H O in Hthree 3PO4 at three voltages, 500VV,and 2 O and 3 )2Zn(NO 4 at 575 V and 650 V. Magnifications 500, 1000, 5000, and 10,000 times. 575 and 650 V. Magnifications 500, 1000, 5000, and 10,000 times. 650 V.VMagnifications 500, 1000, 5000, and 10,000 times.

Figure 10. EDS results for samples after PEO processing obtained in electrolytes composed of 250 g/L Figure EDSresults resultsfor forsamples samples after after PEO PEO processing ofof 250 g/Lg/L Figure 10.10.EDS processingobtained obtainedininelectrolytes electrolytescomposed composed 250 Mg(NO3)2·6H2O and 250 g/L Zn(NO3)2·6H2O in H3PO4 at three voltages, 500 V, 575 V, and 650 V. Mg(NO 3)2·6H2O and 250 g/L Zn(NO3)2·6H2O in H3PO4 at three voltages, 500 V, 575 V, and 650 V. Mg(NO3 )2 ·6H2 O and 250 g/L Zn(NO3 )2 ·6H2 O in H3 PO4 at three voltages, 500 V, 575 V, and 650 V. Table 5 Statistical description of Mg/P, Zn/P and M/P ratios based on atomic percent (M = Mg + Zn). 5 Statistical description of Mg/P, Zn/P and M/P ratios based on atomic percent (M = Mg + Zn). TableTable 5. Statistical description of Mg/P, Zn/P and M/P ratios based on atomic percent (M = Mg + Zn). Ratios Voltage Mean Stand. Dev. First Quartile Third Quartile Ratios Voltage Mean Stand. Dev. First Quartile Third Quartile 500 V 0.073 0.003 0.070 0.075 Ratios Mean Stand. Dev. First Third Quartile 500Voltage V 0.073 0.003 0.070Quartile 0.075 Mg/P 575 V 0.084 0.004 0.081 0.088 Mg/P 575500 V V 0.0840.073 0.0040.003 0.0810.070 0.088 0.075 650 V V 0.087 0.007 0.082 0.091 Mg/P 0.088 650575 V 0.0870.084 0.0070.004 0.0820.081 0.091 500 V V 0.071 0.003 0.069 0.074 0.091 500650 V 0.0710.087 0.0030.007 0.0690.082 0.074 Zn/P 575 V 0.089 0.004 0.086 0.091 0.074 Zn/P 575500 V V 0.0890.071 0.0040.003 0.0860.069 0.091 650 V V 0.102 0.005 0.098 0.106 0.091 Zn/P 650575 V 0.1020.089 0.0050.004 0.0980.086 0.106 500650 VV 0.145 0.005 0.141 0.148 0.102 0.005 0.098 0.106 500 V 0.145 0.005 0.141 0.148 M/P 575500 VV 0.173 0.007 0.168 0.178 0.148 M/P 575 V 0.1730.145 0.0070.005 0.1680.141 0.178 650 V V 0.188 0.010 0.178 0.196 0.178 M/P 650575 V 0.1880.173 0.0100.007 0.1780.168 0.196 650 V

0.188

0.010

0.178

0.196

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In Figure Figure 11, 11, GDEOS GDEOS signals signals of of samples samples and and their their first first and and second second derivatives derivatives for for the the samples samples In after PEO PEO processing processing obtained obtained in inelectrolyte electrolytecomposed composedofof250 250g/L g/LMg(NO Mg(NO 3)2·6H2O and 250 g/L after 3 )2 ·6H2 O and 250 g/L Zn(NO 3 ) 2 ·6H 2 O in H 3 PO 4 at three voltages, 500 V, 575 V, 650 V, are presented. The total thicknesses Zn(NO3 )2 ·6H2 O in H3 PO4 at three voltages, 500 V, 575 V, 650 V, are presented. The total thicknesses of of layers, measured as a sputtering time, for magnesiumand zinc-enriched coatings, are equal to layers, measured as a sputtering time, for magnesium- and zinc-enriched coatings, are equal to about abouts,2400 24004700 s and s. three For allvoltages, three voltages, three sublayer is applicable. For the 2400 2400 s,s and s. 4700 For all the threethe sublayer model ismodel applicable. For the samples samples obtained at 500 V and 575 V, the thicknesses of the first, second, and third sublayers obtained at 500 V and 575 V, the thicknesses of the first, second, and third sublayers are at about are 400 at s, about 400 1100 s, 900 and 1100 s,For respectively. For coatings obtainedofat the of 650 V, the 900 s, and s, s, respectively. coatings obtained at the voltage 650 V, voltage the thicknesses of the thicknesses thethird, first, second, and third, transition are 500, 2400, and 1800 respectively. first, second,ofand transition sublayers are 500, sublayers 2400, and 1800 s, respectively. Fors,all the porous For all the porous castings obtained at voltages which are in the range of 500–650 V, the first sublayers castings obtained at voltages which are in the range of 500–650 V, the first sublayers are enriched in zinc, are enriched oxygen, in zinc, phosphorus, oxygen, hydrogen, nitrogen, and depleted in magnesium phosphorus, hydrogen, carbon, nitrogen, andcarbon, depleted in magnesium and titanium, while and titanium, while the second sublayers can be characterized as a semi-plateau region with nonthe second sublayers can be characterized as a semi-plateau region with non-increasing trend of all increasing trend of all signals, except for the titanium one. In addition, the maximum for magnesium signals, except for the titanium one. In addition, the maximum for magnesium signal in that sublayer signal that sublayer is alsosignal observed, andinlower signal of zincthe in comparison with the firstThe sublayer is also in observed, and lower of zinc comparison with first sublayer is visible. third, is visible. The third, transition sublayer is characterized by an increase of titanium signal and a transition sublayer is characterized by an increase of titanium signal and a decrease in magnesium, decrease in magnesium, oxygen, nitrogen signals. The peakswhich in carbon and zinc, phosphorus, oxygen,zinc, and phosphorus, nitrogen signals. The and peaks in carbon and hydrogen, originate hydrogen, which originate most likely from the contamination related to the cleaning process, should most likely from the contamination related to the cleaning process, should be interpreted as the end of be interpreted the endcoatings. of porosity of the obtained coatings. porosity of the as obtained

Figure 11. Cont.

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Figure GDEOS signals (black) with first (red continuous line) and second (brown dashed line) Figure11. Figure 11. 11. GDEOS GDEOS signals signals (black) (black) with with first first (red (red continuous continuous line) line) and and second second (brown (brown dashed dashed line) line) derivatives derivatives for for samples samples after after PEO PEO processing processing obtained obtained in in the the electrolyte electrolyte composed composed of of 250 250 g/L g/L derivatives for samples after PEO processing obtained in the electrolyte composed of 250 g/L Mg(NO 3 ) 2 ·6H 2 O and 250 g/L Zn(NO 3 ) 2 ·6H 2 O in H 3 PO 4 at three voltages, 500 V, 575 V, and 650 V. Mg(NO3 )2 ·6H2 O and 250 g/L Zn(NO3 )2 ·6H2 O in H3 PO4 at three voltages, 500 V, 575 V, and 650 V. Mg(NO3)2·6H2O and 250 g/L Zn(NO3)2·6H2O in H3PO4 at three voltages, 500 V, 575 V, and 650 V.

The XPS results for samples after PEO processing obtained in the electrolyte of 250 g/L The The XPS XPS results results for for samples samples after after PEO PEO processing processing obtained obtained in in the the electrolyte electrolyte of of 250 250 g/L g/L Mg(NO 3)2·6H2O and 250 g/L Zn(NO3)2·6H2O in 1L H3PO4 at three voltages, 500 V, 575 V, and 650 V, Mg(NO ·6H22OOand and 250 250 g/L g/LZn(NO Zn(NO3)32)·6H · 6H O in 1L H PO at three voltages, 500 V, 575 V, and 650 V, Mg(NO33)22·6H 2 O in 1L H 3 PO 4 at three voltages, 500 V, 575 V, and 650 V, 2 2 3 4 are presented in Figure 12. The XPS results show that the top 10 nm layer of all the obtained coatings are are presented presented in in Figure Figure 12. 12. The The XPS XPS results results show show that that the the top top 10 10 nm nm layer layer of of all all the obtained obtained coatings coatings 2+), zinc (Zn2+), phosphorus (as PO43−, or HPO42−, or H2PO4−, or P2O74−), are enriched ininmagnesium (Mg 2+ ), zinc (Zn − , or2−HPO 2− , or − 2+), are magnesium (Mg ), phosphorus (as43−PO H2PPO are enriched in magnesium (Mg zinc (Zn2+), 2+ phosphorus (as PO , or4 3HPO 4 , or H 2O744−),, 42PO4−, or 4+), as confirmed by the binding energies of Mg 2s (89.3–89.4 eV), Mg KLL (306.6– and titanium (Ti 4 − 4+ 4+), as confirmed or P2titanium O7 ), and (Ti ), asbyconfirmed by energies the binding energies of Mg 2s (89.3–89.4 Mg and (Tititanium the binding of Mg 2s (89.3–89.4 eV), Mg KLLeV), (306.6– 306.7 eV), Zn 2p (1021.9–1022.2 eV), Zn LMM (500 eV & 497 eV), O 1s (531.2–531.5 eV), P 2p (133.7– KLL eV), Zn 2p (1021.9–1022.2 eV), ZneV LMM (500 eVO&1s497 eV), O 1s (531.2–531.5 eV), 306.7(306.6–306.7 eV), Zn 2p (1021.9–1022.2 eV), Zn LMM (500 & 497 eV), (531.2–531.5 eV), P 2p (133.7– 134.8 Ti 2p2/3 (459.9–460 eV). In FigureeV). 13, theFigure XRD 13, results for samples after PEO processing P134.8 2p eV), (133.7–134.8 Ti 2p2/3 (459.9–460 the XRD results for samples after PEO eV), Ti 2p2/3eV), (459.9–460 eV). In Figure 13,Inthe XRD results for samples after PEO processing obtained in the electrolyte 250 g/L Mg(NO )2·6HMg(NO 2O and 250 g/L Zn(NO3)2·6H2O in 1L H3PO4 at three processing in theofof electrolyte of 250 3g/L and 250 )23·PO 6H42 O 1L obtained inobtained the electrolyte 250 g/L Mg(NO 3)2·6H2O and g/L 3)2g/L ·6H2Zn(NO O in 1L3H at in three 3 )250 2 ·6H 2 OZn(NO voltages, 500 V, 575 V, 650500 V, are presented. the coatingsFor formed at threeformed voltages, titanium H three 575 V, 650 V,For are the coatings at only three voltages, V,voltages, 575 V, 650 V,V,are presented. Forpresented. the coatings formed at three voltages, only voltages, titanium 3 PO4 at 500 as a crystallineasphase (a signal from(amatrix), alikematrix), in the case of a coating the solution only a crystalline phase from case ofobtained aobtained coatingin obtained in the as a titanium crystalline phase (a signal from signal matrix), alike in thealike case in ofthe a coating in the solution with single salts at voltages of 500 V and 575 V, was detected. solution with single salts at voltages 500 V and 575detected. V, was detected. with single salts at voltages of 500 V of and 575 V, was

Figure 12. Cont.

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Figure (Mg 2s, Mg KLL), zinc (Zn 2p 3/2, Zn LMM) carbon (C 1s), oxygen XPS signals signals of magnesium (Mg (Mg 2s, 2s, Mg MgKLL), KLL),zinc zinc(Zn (Zn2p 2p3/2 3/2,, Zn Figure 12. 12. XPS of magnesium magnesium Zn LMM) LMM) carbon carbon (C 1s), oxygen (O 1s), phosphorus (P 2p) and titanium (Ti 2p) for samples after PEO obtained in 1s),phosphorus phosphorus titanium (Tifor2p) for samples after PEO processing processing obtained in (O 1s), (P (P 2p)2p) andand titanium (Ti 2p) samples after PEO processing obtained in electrolyte 3)2·6H2O and 250 g/L Zn(NO3)2·6H2O in H3PO4 at three electrolyte composed of 250 g/L Mg(NO )2·6H 2O Zn(NO and 250 )2·6H 2Othree in Hvoltages, 3PO4 at 500 three electrolyteofcomposed of 2503 )g/L composed 250 g/L Mg(NO g/L 6H2Zn(NO O in H33PO V, 2 ·6HMg(NO 2 O and 3250 3 )2 ·g/L 4 at voltages, 575 V, and500 650V, voltages, 500 V,V.575 575 V, V, and and 650 650 V. V.

Figure 13. results of coatings after PEO processing obtained in electrolytes Figure 13. 13. XRD XRD electrolytes composed composed of of Figure XRD results results of of coatings coatings after after PEO PEO processing processing obtained obtained in in electrolytes composed of Mg(NO 3)2·6H2O and Zn(NO3)2·6H2O in H3PO4 at a concentration of 500 g/L at three voltages 500 V, Mg(NO33))22·6H and Zn(NO3)2·6H2O in H3PO4 at a concentration of 500 g/L at three voltages 500 V, Mg(NO ·6H2O 2 O and Zn(NO3 )2 ·6H2 O in H3 PO4 at a concentration of 500 g/L at three voltages 575 and 650 V. 575 V, V, 575 andV, 650 V.650 V. 500 V, and

4. 4. Discussion Discussion 4. Discussion The technologies the The development development of of technologies at at the micro micro scale scale provides provides the the opportunity opportunity to to increase increase The development of technologies at the micro scale provides the opportunity to increase applications of micromachines, most often in medicine. In the available literature, it is possible to applications of micromachines, most often in medicine. In the available literature, it is possible to find find applications of micromachines, most often in medicine. In the available literature, it is possible to find out some examples, such as multiplexed microfluidic platform for bone marker measurement [56] or out some examples, such as multiplexed microfluidic platform for bone marker measurement [56] or out some examples, such as multiplexed microfluidic platform for bone marker measurement [56] or integrated microfluidic devices, e.g., for DNA analysis, cell handling, sorting, and general analysis integrated microfluidic devices, e.g., for DNA analysis, cell handling, sorting, and general analysis [57]. [57]. It It should should be be pointed pointed out out that that the the main main novelty novelty of of this this work work is is fabrication fabrication and and characterization characterization of of

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integrated microfluidic devices, e.g., for DNA analysis, cell handling, sorting, and general analysis [57]. It should be pointed out that the main novelty of this work is fabrication and characterization of new porous coatings enriched in magnesium and zinc in new electrolytes, based on concentrated phosphoric acid, with the addition of magnesium and zinc nitrates, which may be used as biocompatible sensors’ housing. They may be put into the bone to monitor, for instance, the amount of bacteria in wounds. In addition, porous housing may be used, e.g., as a drug delivery polymer system. The preliminary studies have turned the attention on the possibility to create porous coatings in electrolytes containing single magnesium or zinc nitrate, as well as in those with two salts. It was found that the coatings obtained in 10 g/L of magnesium-containing solution are characteristic, with more developed surface than those ones formed in electrolyte with the same amount of zinc nitrate. However, the truly porous coatings have been obtained for the salt contents in a solution of 300 and 600 g/L, respectively. The other critical issues of some obtained surfaces are coatings’ cracking, which has been visible especially on these samples formed in electrolyte containing 600 g/L magnesium nitrate. That case is very unfavorable due to the propagation of cracks, which may lead to faster coat crumbling, e.g., during exploitation. Based on the EDS results of Mg/P and Zn/P ratios, it was concluded that the building-in of the magnesium ions into the phosphate structure is more probable than the zinc ones. Generally, it should be noted that the more total amount of salts in electrolyte, the higher the metal-to-phosphorus ratios in coatings that are observed. The XPS studies, which complement the information on the chemical composition of the 10 nm depth coating, allowed it to be revealed that the external (top) coatings’ part is composed mainly of magnesium (Mg2+ ), (Zn2+ ), titanium (Ti4+ ), and phosphorous (PO4 3− , or HPO4 2− , or H2 PO4 − , or P2 O7 4− ). The depth profiles, which were performed by GDOES, have clearly displayed that for the concentration of 10 g/L of Mg(NO3 )2 ·6H2 O or Zn(NO3 )2 ·6H2 O in electrolytes, no clear sublayers of the obtained PEO coatings were observed, while for samples obtained in the electrolyte with 300 g/L and 600 g/L of the same salts, three sublayers could be detected. It was also observed that increasing the amount of salt in electrolyte solution results in the formation of thicker coatings, while the increasing of PEO voltage, for the same electrolyte, results in growing the amorphous phase, as well as increasing the Zn/P and Mg/P ratios. 5. Conclusions (a) (b) (c) (d) (e)

The more salt (Mg(NO3 )2 ·6H2 O and Zn(NO3 )2 ·6H2 O) in electrolyte, the higher the metal-to-phosphorus (Mg/P and Zn/P) ratios that are obtained. The more salt (Mg(NO3 )2 ·6H2 O and/or Zn(NO3 )2 ·6H2 O) in electrolyte, the thicker the coating that is formed. The increase of PEO voltages results in the increase of amorphous phase in the coatings’ structures. The higher voltage of PEO treatment, the higher are Zn/P and Mg/P ratios in coatings obtained in the electrolytes containing Mg(NO3 )2 ·6H2 O and 250 g/L Zn(NO3 )2 ·6H2 O. The top 10 nm layers of the studied coatings are composed of magnesium (Mg2+ ), zinc (Zn2+ ), phosphorous (PO4 3− , or HPO4 2− , or H2 PO4 − , or P2 O7 4− ), and titanium (Ti4+ ).

Author Contributions: K.R. and T.H. conceived and designed the experiments; K.R., S.G., P.C., S.R., D.M. and W.M. performed the experiments; K.R. and K.P. analyzed the data; K.R. and K.P. contributed reagents, materials, analysis tools; K.R. wrote the paper. Funding: 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”. Conflicts of Interest: The authors declare no conflict of interest.

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