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Article

Creation of Bioceramic Coatings on the Surface of Ti–6Al–4V Alloy by Plasma Electrolytic Oxidation Followed by Gas Detonation Spraying

by
Bauyrzhan Rakhadilov
1 and
Daryn Baizhan
1,2,*
1
PlasmaScience LLP, Ust-Kamenogorsk 070010, Kazakhstan
2
Institute for Composite Materials, Ust-Kamenogorsk 070010, Kazakhstan
*
Author to whom correspondence should be addressed.
Coatings 2021, 11(12), 1433; https://doi.org/10.3390/coatings11121433
Submission received: 24 October 2021 / Revised: 7 November 2021 / Accepted: 19 November 2021 / Published: 23 November 2021
(This article belongs to the Special Issue New Advances in Thermal Spraying)
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Abstract

:
In this work, bioceramic coatings were formed on Ti6Al4V titanium alloy using a combined technique of plasma electrolytic oxidation followed by gas detonation spraying of calcium phosphate ceramics, based on hydroxyapatite. Plasma electrolytic oxidation was carried out in electrolytes with various chemical compositions, and the effect of electrolytes on the macro and microstructure, pore size and phase composition of coatings was estimated. Three types of electrolytes based on sodium compounds were used: phosphate, hydroxide, and silicate. Plasma electrolytic oxidation of the Ti–6Al–4V titanium alloy was carried out at a fixed DC voltage (270 V) for 5 min. The sample morphology and phase composition were studied with a scanning electron microscope and an X-ray diffractometer. According to the results, the most homogeneous structure with lower porousness and many crystalline anatase phases was obtained in the coating prepared in the silicate-based electrolyte. A hydroxyapatite layer was obtained on the surface of the oxide layer using detonation spraying. It was determined that the appearance of α-tricalcium phosphate phases is characteristic for detonation spraying of hydroxyapatite, but the hydroxyapatite phase is retained in the coating composition. Raman spectroscopy results indicate that hydroxyapatite is the main phase in the coatings.

1. Introduction

In modern medical practice, implants made of titanium or titanium alloys are widely used to replace damaged or defective tissue areas [1]. However, the use of implants with a significant difference between the physicochemical and mechanical properties of bone tissue and the alloy causes active rejection in the human body and, consequently, a further complication in treatment [2]. To reduce the negative impact of such factors, it is necessary to create a transition zone between the implant and the bone, which can have a strong connection with the implant material and a macro and microstructure acceptable to the body. Such a zone should be obtained in a coating with a developed morphology and a specific porosity for more effective implant engraftment. Various combinations of metallic and non-metallic structures are considered to be one of the most promising solutions. Therefore, in this work, medical titanium alloys Ti–6Al–4V are supposed to be used as substrates, on which bioceramic layers are applied in a specific sequence. As the material of such coatings, metal oxides with a specific phase composition can be used (for example, for a titanium substrate—a TiO2 anatase film) and coatings of calcium phosphate compounds based on hydroxyapatite. These coatings increase the osseointegration of the implant surface and have improved adhesive properties [3,4,5]. There are many methods for applying such coatings. Still, there is a problem of the unresolved issue of the most optimal and suitable modes of their application, and the formation of their composition. Therefore, scientific research is practical in this direction. The paper proposes to conduct a comprehensive study of the processes and develop a technology for obtaining multilayer films of bioceramics based on the most promising physical methods for solving this issue: plasma electrolytic oxidation and detonation spraying. The combination of “Ti-TiO2-HA” allows combining the high mechanical properties of the base material and the coating biological qualities, which give the implant surface properties as close as possible to the properties of bone tissue, which improves the ability of the implant to integrate with the organism.
Various surface treatment methods have improved the osseointegration and biocompatibility of titanium and titanium alloys [6], among which plasma electrolytic oxidation (PEO) has significant advantages. PEO is an eco-friendly and high-performance process that allows the obtaining of coatings with good adhesion and developed porosity to integrate osteoblast cells [7,8,9]. Recently, the PEO process has attracted considerable interest as a cost-effective, environmentally friendly, and highly efficient technology for applying porous and well-adhesive ceramic films on Ti surfaces [10,11].
The resulting porous oxide layer contains species obtained from the substrate and electrolytes [12]. Moreover, this porous oxide film consists of a dense inner layer and an outer porous layer [13]. The formed PEO coating properties depend on several factors, including processing time, electrical parameters, electrolyte composition and substrate. One of these parameters, which significantly influences the properties of PEO coatings, is the electrolyte chemical composition, which can be changed by changing the concentration of components or by adding various additives.
To obtain coatings, acid solutions are most often used as electrolytes: oxalic, phosphoric, sulfuric, and some mixtures [13,14].
A method of PEO in alternating current mode in a combined electrolyte has been developed at Penza State University. This electrolyte contains 80–120 g/L of sodium silicate, 5–10 g/L of sodium phosphate and 5–15 g/L of sodium hydroxide. The method lasts 5–80 min at the density current of 5–30 A/dm2 and the voltage of 120–220 V [15]. According to the proposed method, sodium silicate within the specified limits allows for the increase in the growth rate of the coating; sodium hydroxide increases the microhardness and reduces the porosity of the coating.
Oxidized titanium implants are biotolerant; they are guaranteed not to cause adverse reactions and do not exhibit bioactivity, providing mechanical coupling in the bioconstructions. The oxide coating improves the quality of the implants and expands the possibilities of their use in restorative medicine [12,13,14].
Based on previously published works [6,7,8,9,10,11,12,13,14,15], by changing the parameters in the PEO, films of biocompatible oxide–anatase (TiO2) can be obtained, and the surface relief can be developed to have a beneficial effect on the implant surface osseointegration in the recipient’s body.
From the point of view of modern experience, the prospects for improving the quality of implants for restorative medicine are mainly associated with technologies for obtaining bioactive implants.
Nowadays, several commercial methods of applying HA coatings to metal implants have been developed: plasma spraying, magnetron spraying, electrochemical deposition, sol-gel technology, etc. [16,17,18,19,20,21,22,23]. Each of these methods has its limitations; for example, poor coatings adhesion to the substrate, the inability to regulate their elemental composition, and limited choice of substrate material for coating formation.
Pulsed energy sources, namely, the explosion energy of an oxidizer with combustible gases mixtures, have recently been increasingly used to heat and accelerate sprayed materials. This energy method is called gas-detonation spraying (GDS) [16,20,24]. The essence of forming these coatings is to direct the fine particles of sprayed powder onto the product from the technological channel when separated from the detonation products. Pulsed devices with minimal thermal effect on the sprayed material are characterized by the simplicity of converting the explosion energy into helpful work and having high specific powers [25].
The gas-detonation spraying technology was used to obtain biocompatible coatings for medical applications [20,24,26,27]. The detonation method in the coating formation consists of creating conditions close to the natural conditions of bone growth. The authors of [28] found that the coatings obtained by detonation-gas spraying on the “Katun-M” installation consist of particles and conglomerates of particles that form a pronounced surface relief. The adhesion strength of detonation coatings based on calcium hydroxyapatite varies from 10 to 30 MPa. The physical–mechanical characteristics of the coating show that the detonation spraying method can be used to obtain high-quality biocoatings on titanium implants. In addition, the Ca/P ratio equal to ≈1.67 can be achieved by optimizing the parameters of the GDS process [29,30,31]. To achieve the required quality of the GDS coatings, a very careful selection of the technological regime and the automation of technological equipment are required in order to exclude the human factor [32]. Our previous studies have shown [33] that using powders of fractions ≥40 microns are more efficient to form detonation coatings. Different powder fractions can be a positive factor in the formation of porous coating structures. However, there are few works aimed at obtaining hydroxyapatite coatings by detonation gas spraying. In addition, in connection with the emergence of new detonation spraying devices, it is interesting to study hydroxyapatite coatings obtained by gas-detonation spraying.
Due to the above, the aim of this study was to obtain the “HA/TiO2/substrate” structure using a combined technique of plasma electrolytic oxidation followed by gas detonation spraying.

2. Materials and Methods

Square-shaped samples of Ti6Al4V titanium alloy with a size of 0.2 cm × 0.2 cm × 0.03 cm were chosen as a substrate material for combined processing. The composition of the Ti6Al4V titanium alloy is shown in Table 1. The samples were ground and polished using silicon carbide (SiC) paper with grain sizes from 100 to 2000 and using GOI paste (grinding and polishing pastes) (abrasive ability 0.3–0.1 μm), washed with distilled water, and then dried before the PEO. For the deposition of coatings, PEO baths with different compositions were used (Table 2). In addition, 4 g of potassium hydroxide (KOH) was added to all baths to increase the conductivity of the electrolyte. The oxide layer was obtained in the form of a coating by excitation on the sample’s surface, an anode (Ti6Al4V), and immersed in the corresponding electrolyte, and a bath (cathode) micro-arc discharge occurred by applying a voltage to the sample. The PEO processes were performed using a constant voltage (270 V) for 5 min. The power source was a powerful rectifier, giving the maximum output value of 360 V/100 A in the form of direct current [34]. Using the cooling system, the electrolyte temperature during the experiments was cooled below 40 °C. Samples processed by the PEO method were washed with distilled water and dried in air. The PEO installation circuit representation is presented in Figure 1.
After PEO, an oxide layer was formed on the surface of the Ti6Al4V alloy, and then HA-based coating was applied using a CCDS2000 gas detonation complex (IGiL SB RAS, Novosibirsk, Russia). Gas-detonation spraying is carried out by feeding oxidizing and combustible gases into the channel (trunk) in a ratio close to stoichiometric [35,36]. The mixture of working gases is ignited in the installation channel using an electric spark. As a result of ignition, a detonation wave occurs. At the same time, the sprayed material is injected into the detonation installation channel by a dosing device. The general view and schematic diagram of the detonation spraying process are shown in Figure 2. The working installation cycle is as follows: The combustion chamber is filled with a mixture of working gases. Together with the working chamber filling with a detonating mixture or a slight delay, the sprayed powder material is fed into the chamber. Then, an “inert” stopper is created from the phlegmatizing gas to prevent a backstroke between the ignition initiation point and the working gas mixture [37]. A spark discharge ignites the particles of the sprayed powder material in the detonating mixture. Combustion arises, which spreads with increasing speed. After passing a certain distance, combustion turns into detonation. As a result of the detonation wave interaction with the detonation products, the sprayed material particles are heated and accelerated to the substrate surface. Next, the detonation chamber is purged with a neutral gas, which displaces the remaining detonation products. The possibility of igniting the explosive mixture, which fills the chamber during the subsequent operation detonation unit cycle, is prevented [38].
Sample X-ray diffraction studies were performed on an X’PertPRO diffractometer (Philips Corporation, Amsterdam, The Netherlands). Diffractograms were taken using CuKa radiation (λ = 2.2897 A0) at a voltage of 40 kV and a current of 30 mA. The decoding of diffractograms was made manual with the standard method and the PDF-4 database, and quantitative analysis was conducted using the program of Powder Cell 2.4. The morphology of the surface was studied by scanning electron microscopy on a Tescan MIRA/LMU (Brno, Czech Republic) with an electron-probe attachment for local microanalysis. The surface area of the pores was measured using an image analyzer (Altami studio, Saint Petersburg, Russian Federation). Angular hydroxyapatite (HA) powder (99.95%, Sigma-Aldrich, Stainheim, Germany) with a particle size of 5–25 µm was used as an initial material.

3. Results and Discussion

The electrolyte chemical composition significantly affects the metal acceleration passivation, dielectric breakdown, and, consequently, thin insulating film formation [39]. The surface structures of coatings made using various chemical electrolytes are shown in Figure 3. Different electrolytes of different compounds (phosphate, silicate and hydroxide) were chosen here. The use of various additives in the base electrolyte had a significant impact on the coating surface structure. In fact, a spark, known as a combustion phenomenon, and an exothermic reaction containing an oxidizing agent, results from the absorption of a sufficient amount of oxygen. A micro-arc discharge brings the eruption of the molten substances from the discharge channels and the subsequent formation of micropores in the form of craters on the coating surface (Figure 3) [40].
Depending on the electrolyte’s chemical composition, the spark voltages were different for each electrolyte. The composition of the electrolyte affects the intensity and size of micro-charges and the volume of gas evolution of various electrolytes. The coating heterogeneity in the PEO process may result from a more substantial effect of micro-discharges on the growing layer due to increased electrolyte conductivity when the electrolyte changes composition.
The surface morphology of various coatings formed on Ti6Al4V titanium alloys are shown in Figure 3 and Figure 4. Coatings made using different electrolytes with different chemical compositions showed different microstructures. All coatings had a porous structure caused by the formation of micropores as a result of discharges accompanied by an avalanche of electrons at the interface between the electrolyte and the oxide layer, which contained micropores in the form of volcanic craters, which is one of the characteristics of PEO coatings. When molten oxides leave the discharge channels and meet the surrounding electrolyte, they quickly solidify, forming pores on the coating surface [41]. On the other hand, a significant amount of gas is formed in the discharge channels and later released into the electrolyte. At the early stage of the PEO process, the spark intensity and the amount of gas generated in the exhaust channels is much higher. Due to the gas ejection, molten oxides are thrown out of the outlet channels. They cannot fill the outlet channels, and, accordingly, many micropores are formed. Due to a decrease in the sparks’ intensity, the gas-formed volume in the discharge channels decreases to some extent [42].
In PEO processes, the characteristics of small and large sparks and the amount of gas released affect the coating surface morphology. More extensive sparks result in more and fewer micropores, while smaller sparks create more and smaller micropores and, consequently, form a more uniform structure. The average micropore size and the percentage of coating porosity is shown in Figure 5. According to Figure 4 and Figure 5, it is evident that due to numerous tiny sparks and the small amount of gas release, the electrolyte coating (S) (Figure 4c,c-1) has the lowest porosity (1.22%). In this sample, the micropores are well distributed over the surface of the coating and are identical in size. The sample prepared in the electrolyte (H) (Figure 4a,a-1) had some large micropores with inhomogeneous dispersion, as well as sizeable spherical condensation products formed by the rapid coating growth as a result of solid discharges [43]. In addition, the largest average micropore size (1.49 μm) was associated with electrolyte preparation (H). The increase in the size of the pores on the surface may be due to electrical breakdowns along the paths of least resistance that pass through the already formed pores. On the surface of the sample (P), several micropores and many microcracks were observed (Figure 4b,b-1). Microcracks appear during the coating growth process due to thermal stress and discharge activity [44]. Thus, the plasma discharge’s high temperature leads to the melting of oxides around the discharge channels, followed by rapid cooling with the electrolyte. Then, a rapid change in temperature causes the appearance of microcracks [45]. Although the size of the average micropores in the sample (P) was smaller, the percentage of porosity was higher than the sample (H). In the case of a surface obtained by a silicate electrolyte (Figure 4c,c-1), it is confirmed that it is possible to obtain a smaller pore size compared to other electrolytes.
Titanium dioxide occurs in nature in three main polymorphic modifications: metastable anatase and stable rutile have tetragonal syngony; the metastable phase is brookite with orthorhombic syngony [46]. In fact, TiO2 and especially anatase are considered biocompatible. In addition, rutile has a higher stability, high hardness and, therefore, better mechanical properties and higher density than anatase. At the first stage of the PEO process, the temperature is low. Thus, the anatase phase is formed earlier than rutile. With an increase in the applied voltage and current density, the temperature rises, and anatase is converted to rutile at 915 °C, which is a more stable TiO2 phase at high temperatures [47]. Diffraction patterns of coatings obtained in various electrolytes are shown in Figure 6. X-ray diffraction analysis showed that all coatings, (H), (P), and (S), contain oxide phases of rutile and anatase. The combination of these compounds has a positive effect on cell survival [48]. In the S coating, anatase accounts for a large proportion (Table 3), favorably affecting biocompatibility [49]. In the coatings (H) and (P), rutile is about 26% and 53% of the crystalline phase. Its content is higher than anatase. When the electrolyte electrical conductivity is less, the PEO micro-discharges are small and have a relatively low power; as a result, anatase is mainly formed under such conditions.
Based on the results obtained, the most homogeneous structure with lower porosity and a large number of crystalline phases was obtained in the coating prepared in the electrolyte based on silicate (S). We selected this sample (S) for subsequent HA coating using a CCDS2000 gas detonation complex.
Angular hydroxyapatite (HA) powder (99.95%, Sigma-Aldrich, Stainheim, Germany) with a particle size of 5–25 µm was used as an initial material (Figure 7).
Numerous literature data indicate that the success of osseointegration is influenced by the coatings phase composition [50,51]. The presence of resorbable calcium phosphate compounds initiates the growth of bone tissue, which accelerates the process of osseointegration. Implant compatibility is improved due to the approximation of the phase-structural state of the resulting coating and its properties to the bone tissue parameters. The phase state of the biocoatings of bone implants also determines the nature of their physicochemical and mechanical properties. Figure 8 shows the diffraction patterns of the HA powder and coatings based on HA obtained by the combined PEO/GDS method. The X-ray shows the characteristic peaks of hydroxyapatite according to ICDD-PDF 96-900-2216. Figure 8b shows the diffraction patterns of the HA coating obtained by gas detonation spraying. In the case of gas detonation spraying of pure HA, the appearance of α-tricalcium phosphate phases (ICDD-PDF 96-210-5286) is characteristic, but the HA phase (ICDD-PDF 96-900-2216) is the main phase in the composition of the coatings. The characteristic sharp peaks of HA demonstrate good crystallinity of the HA phase (Table 4). However, no calcium oxide (CaO) was usually present in other coatings of HA with thermal spraying [3,14,23]. This confirms good phase transplantation during the formation of the HA coating by the GDS method [52,53,54].
Figure 9 shows the coating Raman spectrum obtained by the PEO/GDS method on the surface of an oxide layer prepared in the silicate-based electrolyte. The most intense in the obtained coating Raman spectrum is the band with a frequency shift of 961 cm−1, which indicates that HA is the main phase in the coatings. This band belongs to the P–O symmetric extension mode (ν1) of the PO4 group and is the most characteristic carbonized apatite band. The sharpness of this band confirms the excellent HA coating crystallinity, which is also confirmed by other authors [55]. Similarly, the bands associated with the antisymmetric stretching mode (ν3) of PO4 groups show a shift from 1045 to 1033 cm−1 (Table 5). In addition, it should be emphasized that this change in the carbonate content in the coatings is closely related to changes in the growth morphology and crystallite size, which are known to occur with a change in a temperature-higher atomic disorder corresponding to smaller crystal sizes [56,57].
The morphology of the hydroxyapatite coating showed the formation of a layered porous structure. The coating consists of hydroxyapatite particles (Figure 10), which in some cases are melted under the detonation flow influence. Coatings contain pores. This may result from limited spray deformation HA due to little melting during the GDS process and minor compaction of the coating surface layer caused by the impact of falling particles. It is known that the success of osseointegration is also influenced by the porous structure, which contributes to the intensive ingrowth of bone tissue into the implant surface, which ensures its reliable fixation. The elemental composition of coatings on medical implants is an important characteristic and determines the degree of biodegradation of the layer and the creation of conditions for osteoinduction. The coating must ultimately be absent of toxic products. Having performed a spectral analysis on the calcium phosphate detonation coating surface, it can be concluded that only chemical elements of the initial calcium hydroxyapatite powder are present in the coatings (Table 6). Based on the results of elemental analysis, it can be argued that detonation spraying did not cause changes in the chemical composition of the coating, which is critical for the biocompatibility and preservation of the coating. The Ca/P ratio in coatings is one of the main parameters determining bioactivity. Elemental analysis allows us to compare the concentrations of elements that make up the coatings and calculate the Ca/P ratio. The results of the study of the chemical composition showed that the ratio of calcium and phosphorus in the sprayed coating is Ca/P—1.56, which is close to the original powder—Ca/P—1.67. Figure 10 shows an Scanning electron microscopy image and elemental analysis of the detonation coating based on HA [58,59].

4. Conclusions

Bioceramic “HA/TiO2/substrate” coatings were obtained on Ti6Al4V titanium alloy using a combined technique of plasma electrolytic oxidation followed by gas detonation spraying of calcium phosphate ceramics based on hydroxyapatite. The results of PEO showed that the most homogeneous structure with lower porosity and a large number of crystalline anatase phases was obtained in the coating prepared in the silicate-based electrolyte. HA coating was obtained on the surface of the oxide layer using gas detonation spraying. XRD and Raman spectroscopy results indicated that most of the HA phase remained in the HA/TiO2 coatings, in addition to a slight degradation of the HA to α-TCP. The stoichiometric ratio was Ca/P ≈ 1.56–1.86. This HA/TiO2 bilayer coating demonstrates potential applications in load bearing biomaterials. A conclusion is formed about the prospects of the proposed complex technology of applying bioceramic coatings to titanium implants.

Author Contributions

B.R. and D.B. designed the experiments; D.B. performed the experiments; B.R. analyzed the data; B.R. and D.B. wrote, reviewed and edited the paper. All authors have read and agreed to the published version of the manuscript.

Funding

This research has been funded by the Science Committee of the Ministry of Education and Science of the Republic of Kazakhstan (Grant No. AP09563455).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare that there is no conflict of interest regarding the publication of this manuscript.

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Coatings 11 01433 g001 550
Figure 1. Schematic representation of the installation for PEO processing.
Figure 1. Schematic representation of the installation for PEO processing.
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Coatings 11 01433 g002 550
Figure 2. The CCDS2000 setup schematic diagram: 1—controlling computer; 2—distribution company; 3—mixing chamber; 4—spark plug; 5—barrel damper; 6—fuel line; 7—oxygen line; 8—gas valves; 9—gas distribution unit; 10—barrel indicated part; 11—powder feeder; 12—workpiece; 13—manipulator; 14—the barrel muzzle; F1—acetylene; F2—propane–butane; O2—oxygen; N2—nitrogen.
Figure 2. The CCDS2000 setup schematic diagram: 1—controlling computer; 2—distribution company; 3—mixing chamber; 4—spark plug; 5—barrel damper; 6—fuel line; 7—oxygen line; 8—gas valves; 9—gas distribution unit; 10—barrel indicated part; 11—powder feeder; 12—workpiece; 13—manipulator; 14—the barrel muzzle; F1—acetylene; F2—propane–butane; O2—oxygen; N2—nitrogen.
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Coatings 11 01433 g003 550
Figure 3. PEO coating surface morphology of various electrolytes ×1500: (a) H; (b) P; (c) S, and ×3000: (a-1) H; (b-1) P; (c-1) S.
Figure 3. PEO coating surface morphology of various electrolytes ×1500: (a) H; (b) P; (c) S, and ×3000: (a-1) H; (b-1) P; (c-1) S.
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Coatings 11 01433 g004 550
Figure 4. Surface morphology of PEO coatings of various electrolytes: (a) H; (b) P; (c) S, and pore distribution of PEO coatings in various electrolytes: (a-1) H; (b-1) P; (c-1) S, obtained using an image analyzer by Altami studio.
Figure 4. Surface morphology of PEO coatings of various electrolytes: (a) H; (b) P; (c) S, and pore distribution of PEO coatings in various electrolytes: (a-1) H; (b-1) P; (c-1) S, obtained using an image analyzer by Altami studio.
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Figure 5. Average size of micropores and percentage of porosity of PEO coatings in various electrolytes.
Figure 5. Average size of micropores and percentage of porosity of PEO coatings in various electrolytes.
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Figure 6. X-ray diffraction patterns of PEO coatings obtained in various electrolytes: (a) H; (b) S; (c) P.
Figure 6. X-ray diffraction patterns of PEO coatings obtained in various electrolytes: (a) H; (b) S; (c) P.
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Figure 7. Morphology of HA. Morphology of HA ×1000 (a) and ×10,000 (b).
Figure 7. Morphology of HA. Morphology of HA ×1000 (a) and ×10,000 (b).
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Figure 8. Diffraction patterns of HA powder (a) and HA coating obtained by PEO/GDS methods (b).
Figure 8. Diffraction patterns of HA powder (a) and HA coating obtained by PEO/GDS methods (b).
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Figure 9. Raman spectrum of the hydroxyapatite coating obtained by the PEO/GDS method.
Figure 9. Raman spectrum of the hydroxyapatite coating obtained by the PEO/GDS method.
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Figure 10. SEM image and elemental analysis of hydroxyapatite coating.
Figure 10. SEM image and elemental analysis of hydroxyapatite coating.
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Table
Table 1. Ti6Al4V alloy chemical composition (percent of weight).
Table 1. Ti6Al4V alloy chemical composition (percent of weight).
TiAlVFeCONH
88.5–92.55.5–6.53.5–4.5<0.25<0.08<0.13<0.05<0.012
Table
Table 2. Electrolyte composition for the PEO processes.
Table 2. Electrolyte composition for the PEO processes.
Electrolyte CodeElectrolyte CompositionCurrent Density A/cm2
H20 g/L NaOH + 4 g/L KOH25 A/cm2
P20 g/L Na3PO4 + 4 g/L KOH25 A/cm2
S20 g/L Na2SiO3 + 4 g/L KOH25 A/cm2
Table
Table 3. X-ray phase analysis results.
Table 3. X-ray phase analysis results.
SampleDetected PhasesPhase Structure Data in the Powder Cell DatabaseStructure TypePhase Content wt.%
Grid TypeSpatial Group
PTihexagonalP63/mmc (194)D6h432
TiO2 (Anatase)tetragonalI41amd (141)D4h1915
TiO2 (Rutile)tetragonalP42/mnm (136)D4h1953
STihexagonalP63/mmc (194)D6h446
TiO2 (Anatase)tetragonalI41amd (141)D4h1948
TiO2 (Rutile)tetragonalP42/mnm (136)D4h196
HTihexagonalP63/mmc (194)D6h456
TiO2 (Anatase)tetragonalI41amd (141)D4h1918
TiO2 (Rutile)tetragonalP42/mnm (136)D4h1926
Table
Table 4. X-ray phase analysis results.
Table 4. X-ray phase analysis results.
SampleDetected PhasesPhase Structure Data in the Powder Cell DatabaseStructure TypePhase Content wt.%
Grid TypeSpatial Group
HA powderHydroxyapatitehexagonalP63/mmc (176)D6h4100
HA coating obtained by PEO/GDS methodsHydroxyapatitehexagonalP63/mmc (176)D6h476
α-TCPmonoclinicP121/c1
(14)		D4h1924
Table
Table 5. Results of the hydroxyapatite coating with different frequencies of corresponding lines.
Table 5. Results of the hydroxyapatite coating with different frequencies of corresponding lines.
Raman Frequency Shift, cm−1Fragment, Wobble
154(Ti-O)—(Anatase)
278(Ti-O)—(Anatase)
423(PO4)3−(ν2) (P-O vibrational)
585(PO4)3−(ν4) (P-O deformation)
711(PO4)3−(ν4) (P-O deformation)
950–965(PO4)3−(ν3) (P-O asymmetric valence)
1030–1045(PO4)3−(ν3) (P-O asymmetric valence)
Table
Table 6. Elemental analysis of hydroxyapatite coating.
Table 6. Elemental analysis of hydroxyapatite coating.
SpectrumElementWeightAtomic
Spectrum 1O46.3155.74
P19.1817.29
Ca34.5126.97
Spectrum 2O45.3556.66
P19.7617.35
Ca34.8925.99
Spectrum 3O46.9855.98
P19.7616.88
Ca33.2627.14
Spectrum 4O44.7957.06
P19.7618.32
Ca34.4524.62
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Rakhadilov, B.; Baizhan, D. Creation of Bioceramic Coatings on the Surface of Ti–6Al–4V Alloy by Plasma Electrolytic Oxidation Followed by Gas Detonation Spraying. Coatings 2021, 11, 1433. https://doi.org/10.3390/coatings11121433

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Rakhadilov B, Baizhan D. Creation of Bioceramic Coatings on the Surface of Ti–6Al–4V Alloy by Plasma Electrolytic Oxidation Followed by Gas Detonation Spraying. Coatings. 2021; 11(12):1433. https://doi.org/10.3390/coatings11121433

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Rakhadilov, Bauyrzhan, and Daryn Baizhan. 2021. "Creation of Bioceramic Coatings on the Surface of Ti–6Al–4V Alloy by Plasma Electrolytic Oxidation Followed by Gas Detonation Spraying" Coatings 11, no. 12: 1433. https://doi.org/10.3390/coatings11121433

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