Effect of electric mode of micro-arc oxidation on structural and phase state of calcium-phosphate coating

The micro-arc oxidation method has been used to obtain calcium-phosphate coatings on titanium surface, expected to have bioactive properties. The effect of the pulse current duty cycle and voltage of micro-arc oxidation on the morphology, elemental and phase composition of the coating has been studied. Decrease in the pulse duty cycle during micro-arc oxidation results in the formation of flake, spheroidal and lamellar structures. It has been shown that the Ca/P ratio and surface roughness of the coating increases regardless of the pulse duty cycle with increase of applied voltage. Depending on the application mode, the Ca/P ratio and the roughness of calcium phosphate coatings ranged from 0.44 to 0.67 and from 4.2 to 6.8 μm, respectively. It was found that change of the pulse current duty cycle and increase of the voltage up to 600 V results in the formation of the main crystalline phases Ca(H2PO4)2(H2O) and CaPO3(OH) in the coatings.


Introduction
Over the past decades, extensive research has been conducted to develop several orthopedic biomaterials that can be used as implants in the human body for bone healing [1,2].Typically, the surfaces of metal implants interact with the physiological environment of the body.The surface properties of these implants directly effect their osteoconductive and osteointegration properties [3,4].Therefore, biocompatible calcium phosphate (CaP) coatings are applied to their surface in order to increase the osteoconductivity of the implant.
The deposition of CaP coatings on the implant surface increases biocompatibility and accelerates the fixation of the implant material in the bone.There are a number of fabrication methods for the deposition of CaP coatings: sol-gel coating [5], plasma spraying [6], electrophoretic deposition [7], micro-arc oxidation (MAO) [8], chemical vapor deposition, [9] pulsed laser deposition [10], ion beam deposition [11] and magnetron sputtering [12].Some researchers [13,14] have suggested to combine these methods for deposition of CaP coatings.Micro-arc oxidation is a method based on traditional anodic oxidation [15,16].Recent progress in the use of MAO method shows that it is a cost-effective, simple and environmentally friendly method to create CaP coatings on Ti and its alloys [17].The bioactivity of CaP coatings depends on their physical and chemical parameters such as roughness, porosity, phase and elemental composition, and bond strength [18].
High quality bioactive and biocompatible CaP coatings can be obtained with the variation of the electrical mode of the process and the composition of the MAO electrolyte.Aqueous electrolytes containing dissolved compounds of the elements Ca -and PO 4 -can be used for the synthesis of CaP coatings on Ti and its alloys in the MAO process [4,19,20].They are usually divided into the following two groups: the first one is based on orthophosphoric acid and calcium containing powders (hydroxyapatite-HA, CaO, etc); the second one is based on calcium acetate, calcium glycerophosphate or other esters of phosphoric acid.The atomic ratio of Ca/P in MAO coatings does not exceed 1.0.According to previous studies, Kim et al [21] the coating thickness did not change when the concentration of CaCl 2 in the electrolyte composition was changed.However, increase in the concentration of electrolyte CaCl 2 may reduce the adhesive strength.Ni et al [22] established a relationship between pore size and concentration of (CH previous study [23] showed that titanium MAO in phosphoric acid electrolytes at pH ∼ 3-1 is promising from the point of view to obtain bioactive coatings.Electrical modes in the process of MAO plays a determining role in the growth of CaP coatings on titanium, including current mode, spark voltage, current density, pulse frequency, pulse duty cycle and pulse duration.In most cases, it is observed that the pore size of CaP coatings increases with increase in voltage.In [24], CaP coatings were obtained at low voltages ranging from 250 to 450 V.In elemental analysis, the manifestation of Ca and P was observed at voltages above 350 V. Sun et al [25] synthesized a HA layer on Ti6Al4V alloy in an electrolyte containing 0.2 M calcium acetate and 0.02 M glycerine phosphate at different voltages (400 V, 430 V, 450 V and 480 V) for 20 min of the MAO.On the X-ray spectra of the coatings, peaks associated with the HA phase appeared at an applied voltage of 430 Montazeri et al [26] developed HA/TiO 2 coatings by MAO method at applied voltages of 350, 400, 450 and 500 V.The diffraction peaks of HA appeared only at the maximum applied voltage (500 V), while the peaks of the titanium alloy substrate were not found at this voltage.Besides, several works [27][28][29] on the effect of pulse current duty cycle on the microstructure and properties of CaP coatings have been reported.Chen et al [27] studied the effects of different duty cycles (10 and 50%) and oxidation times (10 and 30 min) on coating morphology, structure and hardness.The results of the authors show that the surface morphology of the coatings changes from crater-like micropores to foam-like protrusions with increase in the duty cycle and oxidation time.The hardness increases with increase in oxidation time and duty cycle.The work [28] indicates that the porosity of the coatings increased from 5.85% to 14.38% due to an increase in the porosity from 10% to 40%.According to the results of [29], it was found that at higher duty cycle (80%), the coating had a homogeneous surface with lower porosity percentage and greater thickness.
As discussed above, the electrolyte composition and electrical synthesis parameters determine the morphology, composition and properties of CaP coatings on titanium surface during the MAO process.Despite the fact that extensive research has been performed in these works, little information has yet been reported on the effect of pulse current duty cycle on the structural and phase state and patterns of formation of CaP coatings on a titanium substrate.In this study, the CaP coating was obtained on the surface of VT1-0 grade titanium by the MAO process.The structures and morphology of the coating obtained at different pulse current duty cycles and voltages were studied.

Experimental part 2.1. Materials
Cylindrical specimens of ∅10 × 15 mm were made of VT1-0 grade titanium (equivalent to titanium GRADE 2).The specimens were mechanically processed before oxidizing.Preparation of the titanium surface for coating included cylinder cutting, grinding and polishing of the end surface with diamond paste, degreasing with hexane and washing in distilled water.A mixture of the following composition was prepared as an electrolyte: 30% aqueous solution of orthophosphoric acid Н 3 РО 4 (thermal grade A) + HA (60 g l −1 ) + CaSiO 3 (75 g l −1 ).HA used was made by Sigma Aldrich with a dispersion of <5 μm.CaSiO 3 wollastonite was added to the electrolyte composition to increase the biological activity of the CaP coatings.The choice of wollastonite is justified by the fact that it has excellent bone regeneration ability and biodegradability [30].Besides, the wollastonite phase in natural or synthetic form was used in studies [31,32] to produce various composites where its presence improved some mechanical properties as well as biological activity and porosity of the composites.The electrolyte was kept for two days in order to complete the chemical reactions in the bath.

MAO of titanium
A micro-arc unit was assembled to conduct this work.A stainless steel ultrasonic bath with a power of 150 W was used in it as the electrolysis bath.A specially assembled power supply was used as a pulse current source.It had a constant pulse duration of 300 μs and allowed the voltage to vary from 0 to 700 V, and the duty cycle-0.0102,0.0049 and 0.0022 s.MAO was conducted in anodic mode at voltages from 300 to 600 V in 100 V steps with a treatment duration of 10 min.Three series of MAO experiments for the surface of titanium samples at different pulse current duty cycles (mode A, B, C) were performed in the electrolyte.They are schematically shown in figure 1.The processed samples were designated in accordance with the mode and voltage during processing A, B, C-(V), respectively: A-300 V.The electrolyte was poured into the ultrasonic bath, where the solution was additionally stirred using an electric stirrer.

Study methods for CaP coatings
Titanium samples after MAO were studied by x-ray phase analysis (XRPA), scanning electron microscopy (SEM) and profilometry.The study of surface morphology, thickness and microanalysis was performed on a JXA-8230 microprobe analyzer (JEOL, Japan) at an accelerating voltage of 25 kV and an electron beam current of up to 8 nA at various magnifications.The phase composition of the obtained samples was studied by XRPA with the use of a D8 Advance diffractometer (BRUKER, Germany) with α-Cu radiation (λ ≈ 1.54 Å); U = 40 kV, I = 40 mA; the shooting speed was 0-1 degrees/min; angle interval 2Θ 4°-90 °with scanning step 0.01°.X-ray shooting was performed with Bragg-Brentano focusing.The PDF 2 database was used for phase analysis.The crystallite size of CaP coatings was calculated from the peaks of the dominant phase using the Scherrer equation.Surface roughness was measured with a Diavite DH-5 profilometer (DIAVITE AG, Switzerland).Each sample was measured 5 times and the average roughness result of the five measurements is presented in the paper.The measurement length was 4 mm.

Results
CaP coatings formed after MAO in all processing modes are characterized by a developed structure with different surface morphology, which changes with increase of duty cycle and voltage of current pulses.Figure 2 shows the SEM micrographs of the surface of the obtained coatings.As shown in figures 2(a) and (b), the morphology of the CaP coatings synthesized at 400 and 600 V in A mode (figure 1) was flaky with spheroids.Flower-shaped grains were observed in sparse positions.The thickness of the walls of the flaky structures is in the range of 200 nm − 1 μm (average 794 nm).In coatings synthesized at mode B (figures 2(c) and (d)), the average wall thickness of the flaky structures decreased to 208 nm for 400 V and 329 nm for 600 V.The flakes were formed inside and outside the spheres in contrast to coatings deposited at mode A. The morphology of coatings deposited at mode C (figures 2(e) and (f)) is mainly formed by colonies of lamellar structures with an average thickness of 620 nm at 400 V and 1.3 μm at 600 V. Spherical particles (figures 2(e) and (f)) observed under other deposition modes (figures 2(a)-(d), were not found in the structure of these coatings.In contrast, the surface morphology of the coatings was formed of lamellar crystals that uniformly covered the surface of the titanium substrate, under deposition conditions at 600 V. Thus, a decrease in the pulse current duty cycle (A→C) during MAO results in the formation of morphologies, such as: a flaky structure with spheroids; spheroidal structures, the internal cavity of which is filled with dispersed scales of a smaller size; a lamellar structure without spheroids or with a smaller number of them.
The chemical composition of the coating, in turn, determines the biodegradation degree of the coating.For all coatings, microanalysis measurements were performed on surface area at 320 × 320 μm 2 .The elemental composition of the CaP coatings (table 1) obtained in the electrolyte composition: 30% aqueous solution of orthophosphoric acid Н 3 РО 4 + HA (60 g l −1 ) + CaSiO 3 (75 g l −1 ), consisted of the major elements Ca, P, O, and Ti.The presence of the latter is related to the substrate material.The Ca/P ratio varied from 0.38 to 0.67.Most likely, in terms of the Ca/P ratio, the resulting coatings belong to bioresorbable phases.Calcium phosphates with a lower Ca/P ratio compared to HA (Ca/P = 1.67) should be considered as resorbable phases [33].
Figure 3 shows the results of elemental EDS mapping and cross-sectional SEM images for three samples processed at three modes A-400 V, B-600 V, and C-600 V.The element mapping shows the main elements of the  coating except for titanium.For coatings A-400 V and C-600 V, the results show that the coating consists predominantly of Ca, P, O, and Si.The overall distribution of these elements can be said to be uniform, except for small aggregations of calcium in sample A-400 V and oxygen in sample C-600 V.The coating in sample B-600 V is characterized by a uniform distribution of the elements Ca, P, and O, which are the main elements of bone apatite.According to the mapping results, silicon is noticeable in all coatings due to the presence of wallostonite in the electrolyte.However, the CaP coating B-600 V has an accumulation of silicon in the central part of the coating and on the substrate.This is probably due to the contamination of the sample with calcium carbide at the sample preparation stage.The surface roughness of the coating plays an important role in better osseointegration with bone tissue.The roughness parameter values lying in the interval 4 < Ra < 7 microns are optimal, because there is an enhanced osteogenic differentiation on such rough titanium surfaces [34,35].Figure 4(a) shows the dependence of roughness on electrical modes of MAO.The roughness of CaP coatings on titanium in all cases increases from 4.2 to 6.8 μm with increase in voltage.
As it can be seen in figure 4(b), the coating thickness interval ranged from 65 to 185 μm in total.As the voltage increases, the coating thickness increases with increase in voltage, and it also increases with increase in number of pulses (A→C).However, the thickness of coatings is about 185 μm at a voltage of 600 V in all modes, that is, in modes B and C there is a decrease in coating thickness instead of an increase.It is probably due to partial flaking of the coating due to accompanying transition of micro-arc discharges to arc discharges resulted in melting, burning and destruction of the CaP coating.As the applied voltage increased from 300 to 600 V, the half-widths of the peaks from the MCPM and titanium phosphate Ti(PO 3 ) 3 phases narrowed, indicating an increase in the crystallinity of the coating.The main phase Ca(H 2 PO 4 ) 2 (H 2 O) is biocompatible according to literature data [36,37].
Figure 6 shows the diffractograms of the CaP coatings formed at 300-600 V in the B mode of MAO.The results of an x-ray phase study of the phase composition showed that the coatings contain crystalline phases such as monetite CaPO 3 (OH) (PDF #00-009-0080), titanium phosphate and oxide, and titanium hydrogen phosphate.The diffractogram coatings, obtained at 300, 400, 500 V, includes a blurred halo from the amorphous phase of the CaP compound.The crystalline phase of CaPO 3 (OH) monotite is formed in the CaP coatings formed at 300 and 400 V.An increase in voltage to 600 V results in an increase in temperature in the discharge channels and, as a consequence, in recrystallization of the amorphous phase into the crystalline phase Ca(H 2 PO 4 ) 2 (H 2 O).
The results of the x-ray spectra of the CaP coatings obtained by mode C presented in figure 7 were similar to those of mode A. The main phase of these coatings is MCPM, whose diffraction peaks were clearly identified.Literature data repeatedly states that MCPM is stable, biocompatible and similar to the composition of natural human teeth and bones [36,37].The structure of all coatings obtained by MAO in mode C from 300 to 600 V contained an amorphous phase.
Table 2 presents the XRPA results, which show the phase fractions of all obtained samples of modes A, B and C.
To carry out a standard x-ray quantitative analysis of the obtained CaP coatings, the dominant phase of calcium phosphate in each sample was selected.Therefore, x-ray spectra were collected in the range 2 theta = 4-60 degrees.Quantitative data of CaP coatings are given in table 3. CaP coatings obtained in modes A and C mainly show a predominant orientation (111).When deposited according to mode B, coatings are

Discussion
The MAO process is schematically represented in figure 8. Electrical breakdown occurs in the first stages when a voltage is applied between the electrodes.Small oxygen bubbles are observed on the surface of the titanium substrate creating a layer of bubbles due to electrolysis.An oxide layer is formed further due to this process where many micro-discharge channels appear.Microspark movement is observed along the oxide film.The anionic compounds of the electrolyte are drawn into the microdischarge channels by means of the electric field.As a result, a porous oxide film is formed.It gradually spreads over the entire surface.Plasma channels are gradually formed due to the high temperature.And plasma chemical reactions take place there.Extreme temperatures and pressures develop in the discharge channels.The electrolyte ions diffuse to the anode where complex compounds are further formed, resulting in the formation of a thick solid CaP coating.A detailed description of the complex MAO processes is available in the following sources [19,[38][39][40].
Among the various discharge parameters considered herein, the current duty cycle had the greatest effect on the surface structure and morphology of the coatings, while the voltage determined the particle size.Porous structure with flake formations, spheroidal structure (volcanic-like MAO structure) and lamellar structure are formed on the coating surface at the considered duty cycles.The morphology of the coatings exhibits flowershaped structures in some places at duty cycle A (figure 2(a)).Low stress results in the formation of flowershaped structure, it is also correlated with results of the work [39].The flake-like structure occurs (figures 2(a)-(b)) when crystal growth is controlled by the inclusion of only those atoms and molecules that have reached the steps of the growing crystal face [41].It results in an increase in the CaP connections along the face.Further, increase in the pulse duty cycle during MAO (B, C) results in a change in morphology.As the application mode changes from A to C, the frequency of the current pulses increases.This results in an increase in the synthesis temperature and thickness of the deposited CaP coating (figure 4(b)).The spheroidal structure (figures 2(c)-(d)) occurs due to electrical breakdown and surface reconstruction of titanium [42].It is a similar structure to the MAO, however, small flakes are present inside the spheroidal formations.In Mode C, the morphology of the CaP coating has a relatively coarse lamellar surface structure.The possible reason for it is the reaction time of the coating.As a result of more intense pulses compared to modes A and B, the micro-arc discharge occurs earlier and with greater intensity.That is, it provides more energy and time for the formation of structures, which  The Ca/P ratio increased from 0.44 to 0.67 with increase of MAO voltage due to intensified precipitation of Ca 2+ ions from the electrolyte.This result conforms also with the works [45][46][47].In addition, the increase in the Ca/P ratio as a function of voltage occurred regardless of the duty cycle of the pulse current (A, B and C).However, a comparatively high Ca/P ratio of 0.67 is recorded in the CaP coating obtained in the B mode at 600 V.This was caused by the presence of monetite in the phase composition, in which the Ca/P ratio is higher than that of monocalcium phosphate monohydrate [48].
Phase formation depending on the duty cycle of the current does not change significantly.An increase in current voltage did not radically affect the phase composition of the coating, but it did lead to a change in the percentage of phases.The XRPA results (figures 5-7) show that change of the pulse current (A, B, and C) allows to obtain CaP coatings consisting of the main crystalline phases -MCPM and monetite, which, according to the literature, are biocompatible compounds.MCPM is one of the most studied calcium phosphate [37].It can be noted that a readily soluble monetite phase is additionally formed in mode B. Its presence in CaP coatings may indicate their high rate of biosorption.Monetite also promotes the formation and growth of bone apatite with subsequent mineralization [49,50].

Conclusion
It has been established that the pulse duty cycle and processing voltage have an important effect on the surface morphology, structure, and composition of CaP coatings.Based on the results of experimental studies, the following conclusions can be drawn that: 1.The surface morphology of deposited calcium phosphate coatings was highly dependent on the duty cycle and increasing voltage of the pulse current.In particular, coatings of mode A had a scaly morphology with spheroids and local flower-like structures, coatings B as part of MAO had a similar volcanic structure with pores, while the morphology of mode C was characterized by lamellar structures.The resulting coatings have a developed surface.It will serve for their improved fusion with bone tissue.
2. The duty cycle of the mode B current leads to the formation of a coating with uniformly distributed apatite elements (Ca, P, and O) and a relatively high Ca/P ratio.With increasing voltage at the studied duty cycle of the pulsed current, the thickness and Ca/P ratio in the composition of the deposited coatings increases from 65 to 185 μm and from 0.44 to 0.67, respectively.A decrease in the duty cycle of the pulsed current leads to a decrease in the surface roughness of deposited calcium phosphate coatings from 6.8 to 4.2 μm.
3. Processing of titanium alloy substrates under the found modes allows to obtain calcium-phosphate coatings consisting of the main crystalline phases Ca(H 2 PO 4 ) 2 (H 2 O) and CaPO 3 (OH), which, according to literature data, are biocompatible compounds.The ratio of crystallinity to amorphism in the coatings, regardless of the duty cycle and MAO voltage, was approximately 1:1.The crystallite size in the crystalline part of the coating was in the range of 76 to 166 nm.
4. Biocompatibility and other properties of the resulting layers require further research, however, samples of coatings synthesized using mode B seem attractive due to their porous morphology with scales, optimal roughness and potentially biocompatible phase composition.

Figure 1 .
Figure 1.Schematic diagram of MAO and characteristics of electric pulse current applied to the anode at different synthesis modes of coatings.

Figure 2 .
Figure 2. SEM images of the surface morphology of the CaP coatings.

Figure 3 .
Figure 3. Elemental EDS mapping of the cross section of CaP coatings.

Figure 4 .
Figure 4. Arithmetic mean deviation of the surface profile (a) and thickness (b) of CaP coatings.

Figure 5 .
Figure 5. Diffractograms of CaP coatings on titanium obtained at different voltages in A mode.

Figure 6 .
Figure 6.Diffractograms of CaP coatings on titanium, obtained at different voltages in mode B.

Figure 7 .
Figure 7. Diffractograms of CaP coatings on titanium, obtained at different voltages in mode C.

Table 1 .
Elemental composition of CaP coatings on titanium and Ca/P ratio.