Characterisation of Calcium-and Phosphorus-Enriched Porous Coatings on CP Titanium Grade 2 Fabricated by Plasma Electrolytic Oxidation

In the paper, Scanning Electron Microscopy (SEM), Energy-dispersive X-ray Spectroscopy (EDS), X-ray Photoelectron Spectroscopy (XPS), and Glow Discharge Optical Emission Spectroscopy (GDOES) analyses of calciumand phosphorus-enriched coatings obtained on commercial purity (CP) Titanium Grade 2 by plasma electrolytic oxidation (PEO), known also as micro arc oxidation (MAO), in electrolytes based on concentrated phosphoric acid with calcium nitrate tetrahydrate, are presented. The preliminary studies were performed in electrolytes containing 10, 300, and 600 g/L of calcium nitrate tetrahydrate, whereas for the main research the solution contained 500 g/L of the same hydrated salt. It was found that non-porous coatings, with very small amounts of calcium and phosphorus in them, were formed in the solution with 10 g/L Ca(NO3)2·4H2O, whereas the other coatings, fabricated in the consecutive electrolytes containing from 300 up to 650 g/L Ca(NO3)2·4H2O, were porous. Based on the GDOES data, it was also found that the obtained porous PEO coating may be divided into three sub-layers: the first, top, porous layer was the thinnest; the second, semi-porous layer was about 12 times thicker than the first; and the third, transition sub-layer was about 10 times thicker than the first. Based on the recorded XPS spectra, it was possible to state that the top 10-nm layer of porous PEO coatings included chemical compounds containing titanium (Ti4+), calcium (Ca2+), as well as phosphorus and oxygen (PO4 and/or HPO4 and/or H2PO4, and/or P2O7).

The main advantage of the PEO process is the formation of porous coatings which are enriched in ions originating from the electrolyte used [55].This is a very important characteristic, because the biomaterials' top layers (in this case the PEO coating) act as transition layers between bone structure and metal, and they should mimic the bone tissue.Therefore, the coatings' bone-like structure should be non-stoichiometric and enriched mainly in calcium (Ca 2+ ) and phosphorus (H 2 PO 4 − , HPO 4 2− , PO 4  3− ).It should be noted that in biological apatites, the Ca 2+ ions may be substituted by other ions, e.g., Mg 2+ , Sr 2+ , Na + , K + , Si 4+ , whereas PO 4 3− groups may be substituted by CO 3 2− [56,57].It is also possible to find in the literature a chemical formula of bone, i.e., Ca 8.3 (PO 4 ) 4.3 (CO 3 ) x (HPO 4 ) y (OH) 3 , where x + y ≈ 1.7, knowing that x increases and y decreases with age [56].Additionally, it may be stated that other chemical elements may be used in substitution of the calcium Ca 2+ ions.These ions, among others, may be bactericidal copper (Cu 2+ , Cu + ) [22,[58][59][60][61][62] or silver (Ag + ) [63][64][65][66].In addition, it was determined that the roughness parameters [67,68] may be used to describe the porosity of the top surface of porous coatings, i.e., the higher the voltage used in the PEO process, the higher the roughness will be [19,61], which may be explained by the fact that for large pores, a high surface roughness is recorded.
In the present paper, new porous coatings enriched in calcium and phosphorus and obtained in an electrolyte based on concentrated phosphoric 85% H 3 PO 4 acid and calcium nitrate tetrahydrate Ca(NO 3 ) 2 •4H 2 O under DC voltage conditions, with and without pulsation, is presented.Analysis of the available literature shows that most commonly used electrolytes containing calcium ions, which may be used in PEO treatments of titanium and its alloys, contain in themselves inter alia calcium dihydrohypophosphite [34], calcium acetate hydrate [69], disodium hydrogen phosphate [70], Ca-β-glycerophosphate [71], calcium acetate [72], and tricalcium phosphate [73].

Method
The plasma electrolytic oxidation (micro arc oxidation) process was used for the treatment of samples of commercial purity (CP) Titanium Grade 2 with dimensions of 10 mm × 10 mm × 2 mm.The plasma electrolytic oxidation (PEO) during the preliminary studies was performed with an average voltage of 450 ± 46 V, and pulsation at a frequency of 300 Hz during 3 min of treatment by using a three-phase transformer with six diodes of Greatz Bridge in electrolytes containing 10, 300, and 600 g/L of calcium nitrate tetrahydrate dissolved in 1000 mL concentrated 85% analytically pure H 3 PO 4 (98 g/mole).The main studies were performed at 500 V DC, 575 V DC , 650 V DC voltages without any pulsation by using a commercial DC power supply PWR 1600 H, Multi Range DC Power Supply 1600 W, 0-650 V/0-8 A. The electrolyte, used in the main studies, consisted of a concentrated 85% analytically pure H 3 PO 4 (98 g/mole) acid, 1000 mL, with 500 g of calcium nitrate Ca(NO 3 ) 2 •4H 2 O dissolved in it.
A scanning electron microscope Quanta 250 FEI (Field Electron and Iron Company, Hillsboro, OR, USA) with Low Vacuum and ESEM mode and a field emission cathode, as well as an Energy-dispercive X-ray Spectroscopy (EDS, Silicon Drift Detectors: Keith Thompson, Thermo Fisher Scientific, Madison, WI, USA), system in a Noran System Six with nitrogen-free silicon drift detector, were used.
The X-ray Photoelectron Spectroscopy (XPS) measurements on studied samples' surfaces were performed by means of a SCIENCE SES 2002 instrument (SCIENTA AB, ScientaOmicron, Uppsala, Sweden) using a monochromatic (Gammadata-Scienta) Al Kα (hν = 1486.6eV) X-ray source (18.7 mA, 13.02 kV).Scan analyses were carried out with an analysis area of 1 × 3 mm and a pass energy of 500 eV with the energy step 0.2 eV and step time 200 ms.The binding energy of the spectrometer was calibrated by the position of the Fermi level on a clean metallic sample.The power supplies were stable and of high accuracy.The experiments were carried out in an ultra-high vacuum system with a base pressure of about 6 × 10 −8 Pa.The XPS spectra were recorded in normal emission.For the XPS analyses, CasaXPS 2.3.14 software (Shirley background type) [77], with the help of XPS tables [78], was used.All the binding energy values presented in this paper were charge corrected to C 1s at 284.8 eV.

Results
In Figure 1, the SEM images with 500× (a), 1000× (b), 5000× (c), and 10,000× (d) magnifications as well as the EDS spectrum (e) of the porous coating formed on CP Titanium Grade 2 after PEO treatment at a voltage of 450 V with a pulsation of 300 Hz after 3 min in an electrolyte containing 10 g of calcium nitrate tetrahydrate Ca(NO 3 ) 2 •4H 2 O in 1000 mL of concentrated 85% phosphoric acid H 3 PO 4 , are presented.The obtained surface is not porous with small islands within a dozen of micrometers, which contain phosphorus (ca.0.5 at %) and calcium (ca.0.1 at %) compounds, resulting in a calcium-to-phosphorus Ca/P ratio equal to ca. 0.2.However, the EDS analysis shows that the amount of calcium in most places, besides the mentioned islands, is null.The X-ray Photoelectron Spectroscopy (XPS) measurements on studied samples' surfaces were performed by means of a SCIENCE SES 2002 instrument (SCIENTA AB, ScientaOmicron, Uppsala, Sweden) using a monochromatic (Gammadata-Scienta) Al Kα (hν = 1486.6eV) X-ray source (18.7 mA, 13.02 kV).Scan analyses were carried out with an analysis area of 1 × 3 mm and a pass energy of 500 eV with the energy step 0.2 eV and step time 200 ms.The binding energy of the spectrometer was calibrated by the position of the Fermi level on a clean metallic sample.The power supplies were stable and of high accuracy.The experiments were carried out in an ultra-high vacuum system with a base pressure of about 6 × 10 −8 Pa.The XPS spectra were recorded in normal emission.For the XPS analyses, CasaXPS 2.3.14 software (Shirley background type) [77], with the help of XPS tables [78], was used.All the binding energy values presented in this paper were charge corrected to C 1s at 284.8 eV.

Results
In Figure 1, the SEM images with 500× (a), 1000× (b), 5000× (c), and 10,000× (d) magnifications as well as the EDS spectrum (e) of the porous coating formed on CP Titanium Grade 2 after PEO treatment at a voltage of 450 V with a pulsation of 300 Hz after 3 min in an electrolyte containing 10 g of calcium nitrate tetrahydrate Ca(NO3)2•4H2O in 1000 mL of concentrated 85% phosphoric acid H3PO4, are presented.The obtained surface is not porous with small islands within a dozen of micrometers, which contain phosphorus (ca.0.5 at %) and calcium (ca.0.1 at %) compounds, resulting in a calcium-to-phosphorus Ca/P ratio equal to ca. 0.2.However, the EDS analysis shows that the amount of calcium in most places, besides the mentioned islands, is null.In Figure 2, the SEM images with 500× (a), 1000× (b), 5000× (c), and 10,000× (d) magnifications as well as the EDS spectrum (e) of the porous coating formed on CP Titanium Grade 2 after PEO treatment at a voltage of 450 V with a pulsation of 300 Hz after 3 min in an electrolyte containing 300 g of calcium nitrate tetrahydrate Ca(NO3)2•4H2O in 1000 mL of concentrated phosphoric acid H3PO4, are presented.The obtained coatings are porous and contain calcium (3.7 ± 0.6 at %; median: 3.8 at %, range: 1.9 at %), phosphorus (45 ± 0.9 at %; median: 45.1 at %, range: 2.7 at %), and titanium (51.2 ± 1.3 at %; median: 51.1 at %, range: 3.8 at %), which may have originated both from coatings as well as from the matrix.Based on the EDS data, calcium-to-phosphorus Ca/P ratios equal to 0.08 ± 0.01 (median: 0.09, range: 0.04) were found.In Figure 2, the SEM images with 500× (a), 1000× (b), 5000× (c), and 10,000× (d) magnifications as well as the EDS spectrum (e) of the porous coating formed on CP Titanium Grade 2 after PEO treatment at a voltage of 450 V with a pulsation of 300 Hz after 3 min in an electrolyte containing 300 g of calcium nitrate tetrahydrate Ca(NO 3 ) 2 •4H 2 O in 1000 mL of concentrated phosphoric acid H 3 PO 4 , are presented.The obtained coatings are porous and contain calcium (3.7 ± 0.6 at %; median: 3.8 at %, range: 1.9 at %), phosphorus (45 ± 0.9 at %; median: 45.1 at %, range: 2.7 at %), and titanium (51.2 ± 1.3 at %; median: 51.1 at %, range: 3.8 at %), which may have originated both from coatings as well as from the matrix.Based on the EDS data, calcium-to-phosphorus Ca/P ratios equal to 0.08 ± 0.01 (median: 0.09, range: 0.04) were found.In Figure 2, the SEM images with 500× (a), 1000× (b), 5000× (c), and 10,000× (d) magnifications as well as the EDS spectrum (e) of the porous coating formed on CP Titanium Grade 2 after PEO treatment at a voltage of 450 V with a pulsation of 300 Hz after 3 min in an electrolyte containing 300 g of calcium nitrate tetrahydrate Ca(NO3)2•4H2O in 1000 mL of concentrated phosphoric acid H3PO4, are presented.The obtained coatings are porous and contain calcium (3.7 ± 0.6 at %; median: 3.8 at %, range: 1.9 at %), phosphorus (45 ± 0.9 at %; median: 45.1 at %, range: 2.7 at %), and titanium (51.2 ± 1.3 at %; median: 51.1 at %, range: 3.8 at %), which may have originated both from coatings as well as from the matrix.Based on the EDS data, calcium-to-phosphorus Ca/P ratios equal to 0.08 ± 0.01 (median: 0.09, range: 0.04) were found.In Figure 3, the SEM images with 500× (a), 1000× (b), 5000× (c), and 10,000× (d) magnifications as well as the EDS spectrum (e) of the porous coating formed on CP Titanium Grade 2 after PEO treatment at a voltage of 450 V with a pulsation of 300 Hz after 3 min in an electrolyte containing 600 g of calcium nitrate tetrahydrate Ca(NO3)2•4H2O in 1000 mL of concentrated phosphoric acid H3PO4, are presented.The obtained coatings are porous and contain calcium (7.4 ± 0.6 at %; median: 7.6 at %, range: 1.7 at %), phosphorus (51.8 ± 1.5 at %; median: 51.9 at %, range: 3.3 at %), and titanium (40.7 ± 2.0 at %; median: 40.1 at %, range: 4.6 at %), which may have originated both from the coatings as well as from the matrix.Based on the EDS data, calcium-to-phosphorus Ca/P ratios equal to 0.15 ± 0.01 (median: 0.15, range: 0.03) were found.The decreasing amount of titanium from the EDS analysis indicates that with increasing amount of calcium nitrate tetrahydrate from 10 g/L up to 600 g/L in the electrolyte, an increase in the coating thickness is observed.In addition, it was found that the ratio Ca/P can be expressed as Ca/P = 2.4 × 10 −4 × x, where x (g/L) is the amount of calcium nitrate tetrahydrate in the electrolyte.In Figure 3, the SEM images with 500× (a), 1000× (b), 5000× (c), and 10,000× (d) magnifications as well as the EDS spectrum (e) of the porous coating formed on CP Titanium Grade 2 after PEO treatment at a voltage of 450 V with a pulsation of 300 Hz after 3 min in an electrolyte containing 600 g of calcium nitrate tetrahydrate Ca(NO 3 ) 2 •4H 2 O in 1000 mL of concentrated phosphoric acid H 3 PO 4 , are presented.The obtained coatings are porous and contain calcium (7.4 ± 0.6 at %; median: 7.6 at %, range: 1.7 at %), phosphorus (51.8 ± 1.5 at %; median: 51.9 at %, range: 3.3 at %), and titanium (40.7 ± 2.0 at %; median: 40.1 at %, range: 4.6 at %), which may have originated both from the coatings as well as from the matrix.Based on the EDS data, calcium-to-phosphorus Ca/P ratios equal to 0.15 ± 0.01 (median: 0.15, range: 0.03) were found.The decreasing amount of titanium from the EDS analysis indicates that with increasing amount of calcium nitrate tetrahydrate from 10 g/L up to 600 g/L in the electrolyte, an increase in the coating thickness is observed.In addition, it was found that the ratio Ca/P can be expressed as Ca/P = 2.4 × 10 −4 × x, where x (g/L) is the amount of calcium nitrate tetrahydrate in the electrolyte.In Figure 3, the SEM images with 500× (a), 1000× (b), 5000× (c), and 10,000× (d) magnifications as well as the EDS spectrum (e) of the porous coating formed on CP Titanium Grade 2 after PEO treatment at a voltage of 450 V with a pulsation of 300 Hz after 3 min in an electrolyte containing 600 g of calcium nitrate tetrahydrate Ca(NO3)2•4H2O in 1000 mL of concentrated phosphoric acid H3PO4, are presented.The obtained coatings are porous and contain calcium (7.4 ± 0.6 at %; median: 7.6 at %, range: 1.7 at %), phosphorus (51.8 ± 1.5 at %; median: 51.9 at %, range: 3.3 at %), and titanium (40.7 ± 2.0 at %; median: 40.1 at %, range: 4.6 at %), which may have originated both from the coatings as well as from the matrix.Based on the EDS data, calcium-to-phosphorus Ca/P ratios equal to 0.15 ± 0.01 (median: 0.15, range: 0.03) were found.The decreasing amount of titanium from the EDS analysis indicates that with increasing amount of calcium nitrate tetrahydrate from 10 g/L up to 600 g/L in the electrolyte, an increase in the coating thickness is observed.In addition, it was found that the ratio Ca/P can be expressed as Ca/P = 2.4 × 10 −4 × x, where x (g/L) is the amount of calcium nitrate tetrahydrate in the electrolyte.In Figure 4, the GDOES signals with their derivates of calcium (a), phosphorus (b), oxygen (c), hydrogen (d), carbon (e), nitrogen (f), and titanium (g) of the porous coating formed on CP Titanium Grade 2 after PEO treatment at a voltage of 450 V in 10 g, 300 g, and 600 g Ca(NO3)2•4H2O in 1000 mL H3PO4 electrolytes, are presented.At first glance, it can be observed that the thickest porous coating enriched with calcium, phosphorus, and oxygen was obtained with the PEO treatment in an electrolyte containing 600 g/L of calcium nitrate tetrahydrate.As it was presented in the EDS results (Figure 1) with the coating/layer obtained from the solution with 10 g/L Ca(NO3)2•4H2O, the GDOES signals of oxygen, carbon, and nitrogen may suggest that the formed surface layer consists of organic contaminations with very small amounts of calcium-phosphate-titanium compounds.It is worth noting [19,22] that the obtained coatings may be divided into three sub-layers, i.e., the first one with open and organically contaminated pores; the second semi-porous one, which has a different thickness dependent on calcium nitrate tetrahydrate amount in the electrolyte used; and the third transition sub-layer.The thicknesses of the first sub-layer for all obtained layers/coatings are about the same and correspond to 40 s of GDOES sputtering time.For the sample treated in the electrolyte with 10 g/L Ca(NO3)2•4H2O, only the two sub-layers (the first and third) were found, while for the next two treatments, i.e., with 300 g/L and 600 g/L of Ca(NO3)2•4H2O, all three sub-layers were recorded.It has to be noted that the second sub-layer has a different thickness, and depends on the electrolyte used.The higher the amount of calcium nitrate tetrahydrate in the solution, the thicker the second/third sub-layer will be, i.e., corresponding with 360 s/350 s and 510 s/400 s of sputtering time for 300 and 600 g/L of Ca(NO3)2•4H2O, respectively.Based on carbon and hydrogen GDOES signals and their derivates, it is possible to determine the point at which the porosity decreases down to null.They are observed as the local maxima in carbon and hydrogen signals, what may be explained as organic contaminations from air.Thus, it has to be inferred that all pores are connected together and the end of porosity is located inside the third-transitional layer of PEO coating.That place/dimple may be used e.g., for drug delivery.All of the sub-layers are enriched in calcium, phosphorus, oxygen, and small amounts of nitrogen and titanium.Taking into account the fact that calcium nitrate tetrahydrate Ca(NO3)2•4H2O and phosphoric acid H3PO4 were used for the PEO treatment of the electrolyte, it should be noted that the coatings are built of titanium (Ti 4+ ), calcium (Ca 2+ ), and phosphate (PO4 3− ) and/or hydrogen phosphate (HPO4 2− ) and/or dihydrogen phosphate (H2PO4 − ) and/or pyrophosphates (P2O7 4− ).
In Figure 5, the SEM images with 500× (a), 1000× (b), 5000× (c), and 10,000× (d) magnifications as well as the EDS spectrum (e) of the porous coating formed on CP Titanium Grade 2 after PEO treatment at a voltage of 450 VDC without any pulsation after 3 min in an electrolyte containing 500 g of calcium nitrate tetrahydrate Ca(NO3)2•4H2O in 1000 mL of concentrated phosphoric acid H3PO4, are presented.The obtained coatings are porous with "volcano pores", which contain calcium (8.8 ± 0.3 at %; median: 8.8 at %, range: 1.5 at %), phosphorus (49.9 ± 0.9 at %; median: 50.6 at %, range: 4.1 at %), and titanium (40.5 ± 1.2 at %; median: 40.7 at %, range: 5.2 at %), and may have originated both from the coatings as well as from the matrix.Based on the EDS data, a calcium-to-phosphorus Ca/P ratio equal to 0.18 ± 0.01 (median: 0.18, range: 0.01) was found.Therefore, it follows that with using DC voltage (450 VDC) without any pulsation and an electrolyte with a smaller amount (500 g/L) of In Figure 4, the GDOES signals with their derivates of calcium (a), phosphorus (b), oxygen (c), hydrogen (d), carbon (e), nitrogen (f), and titanium (g) of the porous coating formed on CP Titanium Grade 2 after PEO treatment at a voltage of 450 V in 10 g, 300 g, and 600 g Ca(NO 3 ) 2 •4H 2 O in 1000 mL H 3 PO 4 electrolytes, are presented.At first glance, it can be observed that the thickest porous coating enriched with calcium, phosphorus, and oxygen was obtained with the PEO treatment in an electrolyte containing 600 g/L of calcium nitrate tetrahydrate.As it was presented in the EDS results (Figure 1) with the coating/layer obtained from the solution with 10 g/L Ca(NO 3 ) 2 •4H 2 O, the GDOES signals of oxygen, carbon, and nitrogen may suggest that the formed surface layer consists of organic contaminations with very small amounts of calcium-phosphate-titanium compounds.It is worth noting [19,22] that the obtained coatings may be divided into three sub-layers, i.e., the first one with open and organically contaminated pores; the second semi-porous one, which has a different thickness dependent on calcium nitrate tetrahydrate amount in the electrolyte used; and the third transition sub-layer.The thicknesses of the first sub-layer for all obtained layers/coatings are about the same and correspond to 40 s of GDOES sputtering time.For the sample treated in the electrolyte with 10 g/L Ca(NO 3 ) 2 •4H 2 O, only the two sub-layers (the first and third) were found, while for the next two treatments, i.e., with 300 g/L and 600 g/L of Ca(NO 3 ) 2 •4H 2 O, all three sub-layers were recorded.It has to be noted that the second sub-layer has a different thickness, and depends on the electrolyte used.The higher the amount of calcium nitrate tetrahydrate in the solution, the thicker the second/third sub-layer will be, i.e., corresponding with 360 s/350 s and 510 s/400 s of sputtering time for 300 and 600 g/L of Ca(NO 3 ) 2 •4H 2 O, respectively.Based on carbon and hydrogen GDOES signals and their derivates, it is possible to determine the point at which the porosity decreases down to null.They are observed as the local maxima in carbon and hydrogen signals, what may be explained as organic contaminations from air.Thus, it has to be inferred that all pores are connected together and the end of porosity is located inside the third-transitional layer of PEO coating.That place/dimple may be used e.g., for drug delivery.All of the sub-layers are enriched in calcium, phosphorus, oxygen, and small amounts of nitrogen and titanium.Taking into account the fact that calcium nitrate tetrahydrate Ca(NO 3 ) 2 •4H 2 O and phosphoric acid H 3 PO 4 were used for the PEO treatment of the electrolyte, it should be noted that the coatings are built of titanium (Ti 4+ ), calcium (Ca 2+ ), and phosphate (PO In Figure 5, the SEM images with 500× (a), 1000× (b), 5000× (c), and 10,000× (d) magnifications as well as the EDS spectrum (e) of the porous coating formed on CP Titanium Grade 2 after PEO treatment at a voltage of 450 V DC without any pulsation after 3 min in an electrolyte containing 500 g of calcium nitrate tetrahydrate Ca(NO 3 ) 2 •4H 2 O in 1000 mL of concentrated phosphoric acid H 3 PO 4 , are presented.The obtained coatings are porous with "volcano pores", which contain calcium (8.8 ± 0.3 at %; median: 8.8 at %, range: 1.5 at %), phosphorus (49.9 ± 0.9 at %; median: 50.6 at %, range: 4.1 at %), and titanium (40.5 ± 1.2 at %; median: 40.7 at %, range: 5.2 at %), and may have originated both from the coatings as well as from the matrix.Based on the EDS data, a calcium-to-phosphorus Ca/P ratio equal to 0.18 ± 0.01 (median: 0.18, range: 0.01) was found.Therefore, it follows that with using DC voltage (450 V DC ) without any pulsation and an electrolyte with a smaller amount (500 g/L) of calcium nitrate tetrahydrate, it is possible to obtain more calcium in coatings than it was using the three-phase transformer with six diodes of Greatz Bridge and the solution with 600 g/L of Ca(NO 3 ) 2 •4H 2 O.
In Figure 6, the SEM images with 500× (a), 1000× (b), 5000× (c), and 10,000× (d) magnifications as well as the EDS spectrum (e) of the porous coating formed on CP Titanium Grade 2 after PEO treatment at a voltage of 500 V DC without any pulsation after 3 min in an electrolyte containing 500 g of calcium nitrate tetrahydrate Ca(NO 3 ) 2 •4H 2 O in 1000 mL of concentrated phosphoric acid H 3 PO 4 , are presented.The obtained coatings are porous, and more "volcano pores" with bigger diameters were found on them than on the coatings treated at 450 V DC .Based on the EDS results, it is possible to state that the porous coatings contain calcium (11 ± 1.5 at %; median: 11 at %, range: 5.8 at %), phosphorus (50.5 ± 1.0 at %; median: 50.4 at %, range: 3.4 at %), and titanium (38.5 ± 1.1 at %; median: 38.7 at %, range: 4.3 at %). Calcium-to-phosphorus Ca/P ratios equal to 0.22 ± 0.03 (median: 0.22, range: 0.12) were obtained.From the results, it seems that the PEO treatment at 500 V DC allows the formation of coatings with higher amount of calcium compared to those described before.Therefore, the other two higher potentials, i.e., 550 V DC and 650 V DC , were used in order to find even higher Ca/P ratios.
In Figure 7, the SEM images with 500× (a), 1000× (b), 5000× (c), and 10,000× (d) magnifications as well as the EDS spectrum (e) of the porous coating formed on Titanium Grade 2 after PEO treatment at a voltage of 550 V DC without any pulsation after 3 min in an electrolyte containing 500 g of calcium nitrate tetrahydrate Ca(NO 3 ) 2 •4H 2 O in 1000 mL of concentrated phosphoric acid H 3 PO 4 , are presented.The obtained coatings are porous and look similar to those obtained after PEO treatment at 500 V DC .The porous coatings formed at 550 V DC contain calcium (10 ± 0.7 at %; median: 10 at %, range: 2.4 at %), phosphorus (53.6 ± 1.8 at %; median: 53.1 at %, range: 6.2 at %), and titanium (36.4 ± 1.8 at %; median: 36.6 at %, range: 5.6 at %).The calcium-to-phosphorus Ca/P ratios were found to be 0.9 ± 0.02 (median: 0.19, range: 0.05).From this, it follows that further increasing the voltage leads to a decrease of the amount of calcium in the PEO coating.
In Figure 8, the SEM images with 500× (a), 1000× (b), 5000× (c), and 10,000× (d) magnifications as well as the EDS spectrum (e) of the porous coating formed on CP Titanium Grade 2 after PEO treatment at a voltage of 650 V DC without any pulsation after 3 min in an electrolyte containing 500 g of calcium nitrate tetrahydrate Ca(NO 3 ) 2 •4H 2 O in 1000 mL of concentrated phosphoric acid H 3 PO 4 , are presented.The obtained coatings are porous but it should be noted that they exhibit the most developed area among all of those surveyed, and look similar to those obtained after PEO treatment at 500 V DC .The chemical composition of the formed PEO coating was as follows: calcium (9.8 ± 0.3 at %; median: 9.8 at %, range: 1.3 at %), phosphorus (50.1 ± 0.7 at %; median: 50.4 at %, range: 2.6 at %), and titanium (40.1 ± 0.9 at %; median: 39.8 at %, range: 3.0 at %).The calcium-to-phosphorus Ca/P ratios were found to be equal to 0.19 ± 0.01 (median: 0.2, range: 0.02).This confirms that further increasing voltage up to 650 V DC results in a decrease of the amount of calcium in the formed PEO coating.
In Figure 9, XPS spectra of coatings formed on CP Titanium Grade 2 after PEO treatment at voltages of 450 V DC , 500 V DC , and 650 V DC after 3 min in 500 g/L of Ca(NO 3 ) 2 •4H 2 O in 1000 mL H 3 PO 4 electrolyte, are presented.The obtained XPS results clearly show that in the top layer (10 nm) there is titanium (Ti 4+ ), calcium (Ca 2+ ), as well as phosphorus and oxygen with hydrogen, most likely as PO 4 3− and/or HPO 4 2− and/or H 2 PO 4 − , and/or P 2 O 7 4− present, which is confirmed by the binding energies, i.e., Ti 2p 3/2 (460-460.4eV), Ca 2p 3/2 (347.4-347.7 eV), P 2p (133.9-134.4eV), and O 1s (531.5-531.6 eV).In addition, based on the peaks of oxygen O 1s and phosphorus P 2p, the oxygen-to-phosphorus O/P ratios, which are within the range of 1.4-1.7,for all three coatings were found.Taking into account the information from the EDS studies for DC voltages without pulsation, which indicated that Ca/P ratios for all coatings were equal to ca. 0.2, one may conclude that the obtained PEO coating compounds are not stoichiometric.In Figure 9, XPS spectra of coatings formed on CP Titanium Grade 2 after PEO treatment at voltages of 450 VDC, 500 VDC, and 650 VDC after 3 min in 500 g/L of Ca(NO3)2•4H2O in 1000 mL H3PO4 electrolyte, are presented.The obtained XPS results clearly show that in the top layer (10 nm) there is titanium (Ti 4+ ), calcium (Ca 2+ ), as well as phosphorus and oxygen with hydrogen, most likely as PO4 3− and/or HPO4 2− and/or H2PO4 − , and/or P2O7 4− present, which is confirmed by the binding energies, i.e., Ti 2p3/2 (460-460.4eV), Ca 2p3/2 (347.4-347.7 eV), P 2p (133.9-134.4eV), and O 1s (531.5-531.6 eV).In addition, based on the peaks of oxygen O 1s and phosphorus P 2p, the oxygen-to-phosphorus O/P ratios, which are within the range of 1.4-1.7,for all three coatings were found.Taking into account the information from the EDS studies for DC voltages without pulsation, which indicated that Ca/P ratios for all coatings were equal to ca. 0.2, one may conclude that the obtained PEO coating compounds are not stoichiometric.Plasma electrolytic oxidation (micro arc oxidation) was developed on CP Titanium Grade 2 to obtain porous, calcium-and phosphorus-enriched, coatings to be used as biocompatible surfaces as well as for automotive and industrial catalysts.SEM, EDS, XPS, and GDOES techniques were used to study the PEO coatings.In the experiments, two types of DC voltages, i.e., with and without pulsation, were used to fabricate these PEO coatings.The preliminary studies were performed on samples, on which the porous coatings were formed in electrolytes containing 10, 300, and 600 g/L of calcium nitrate tetrahydrate with the use of a three-phase transformer with six diodes of Greatz Bridge.The obtained results clearly show that the coating formed in the solution with 10 g/L Ca(NO 3 ) 2 •4H 2 O is not porous and that there are islands on it containing mainly calcium, phosphorus, and oxygen, for which calcium-to-phosphate Ca/P ratio is equal to 0.2.The coatings obtained in electrolytes with 300 and 600 g/L calcium nitrate tetrahydrate are porous and their Ca/P ratios are equal to 0.08 and 0.15, respectively.Based on the analysis of the GDOES results, one may conclude that the obtained PEO coatings may be divided into sub-layers, i.e., the first layer with open sharp edges of pores (40 s of sputtering time), the second, semi-porous layer, enriched in calcium with thicknesses corresponding to sputtering time from 360 s to 510 s, and the third transition layer (350-400 s).It was also found that the higher the amount of calcium nitrate tetrahydrate in the solution, the thicker the second and third sub-layers become.Based on the preliminary results, a new experimental plan of PEO coatings fabrication in an electrolyte with 500 g/L Ca(NO 3 ) 2 •4H 2 O with the use of a commercial DC power supply was designed and conducted.It was found that all formed coatings were porous and enriched in calcium and phosphorus, with a calcium-to-phosphorus ratio of about 0.2.The XPS results showed that the top 10-nm layer consists mainly of compounds containing titanium (Ti 4+ ), calcium (Ca 2+ ), as well as phosphorus and oxygen (PO  Plasma electrolytic oxidation (micro arc oxidation) was developed on CP Titanium Grade 2 to obtain porous, calcium-and phosphorus-enriched, coatings to be used as biocompatible surfaces as well as for automotive and industrial catalysts.SEM, EDS, XPS, and GDOES techniques were used to study the PEO coatings.In the experiments, two types of DC voltages, i.e., with and without pulsation, were used to fabricate these PEO coatings.The preliminary studies were performed on

Conclusions
Calcium-and phosphorus-enriched PEO coatings on CP Titanium Grade 2, obtained in electrolytes containing dissolved calcium nitrate tetrahydrate Ca(NO 3 ) 2 •4H 2 O in concentrated 85% analytically pure H 3 PO 4 (98 g/mole) acid, may be characterized as follows: (1) The coating obtained in the electrolyte with 10 g/L of calcium nitrate tetrahydrate in it at 450 ± 46 V with a pulsation of 300 Hz is not porous, whereas the coatings formed in the solutions with 300 and 600 g/L Ca(NO 3 ) 2 •4H 2 O are porous.(2) The Ca/P ratio of the coatings, obtained by using a commercial DC power supply without pulsation, at 450 V, in an electrolyte containing 500 g/L of Ca(NO 3 ) 2 •4H 2 O, is equal to 0.18 ± 0.01, which is slightly higher than that calculated for the coating formed at 450 ± 46 V with a pulsation of 300 Hz (0.15 ± 0.01).(3) In the PEO coatings, three different sub-layers may be distinguished, i.e., the first with open pores, the second that is semi-porous and enriched in calcium, and the third, a transition sub-layer.(4) The higher the amount of calcium nitrate tetrahydrate dissolved in an electrolyte, the thicker the second and third sub-layers become.(5) The top surface of the PEO coatings consists of titanium (Ti 4+ ), calcium (Ca 2+ ), as well as phosphorus and oxygen (PO

4 3 −
and/or HPO 4 2− and/or H 2 PO 4 − ,and/or P 2 O 7 4− ).To summarize, it should be noted that the obtained PEO porous coatings, which are enriched in calcium and phosphorus, may be used in the production of automotive and industrial catalysts and as biocompatible surfaces.