Materials Science and Engineering C 51 (2015) 57–63

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Crystalline hydroxyapatite coatings synthesized under hydrothermal conditions on modified titanium substrates Katarzyna Suchanek a,⁎, Amanda Bartkowiak a, Agnieszka Gdowik b, Marcin Perzanowski a, Sławomir Kąc d, Barbara Szaraniec e, Mateusz Suchanek c, Marta Marszałek a a

The Henryk Niewodniczański Institute of Nuclear Physics, Polish Academy of Sciences, Radzikowskiego Street 152, 31-342 Krakow, Poland Faculty of Physics and Applied Computer Science, AGH University of Science and Technology, Mickiewicza 30, 30-059 Krakow, Poland Department of Chemistry and Physics, University of Agriculture in Krakow, Mickiewicza 21, 31-120 Krakow, Poland d Faculty of Metals Engineering and Industrial Computer Science, AGH University of Science and Technology, Mickiewica 30, 30-059 Krakow e Department of Biomaterials, AGH University of Science and Technology, Mickiewicza 30, 30-059 Krakow, Poland b c

a r t i c l e

i n f o

Article history: Received 17 September 2014 Received in revised form 5 December 2014 Accepted 23 February 2015 Available online 25 February 2015 Keywords: Hydroxyapatite coatings Hydrothermal method Titanium substrate

a b s t r a c t Hydroxyapatite coatings were successfully produced on modified titanium substrates via hydrothermal synthesis in a Ca(EDTA)2− and (NH4)2HPO4 solution. The morphology of modified titanium substrates as well as hydroxyapatite coatings was studied using scanning electron microcopy and phase identification by X-ray diffraction, and Raman and FTIR spectroscopy. The results show that the nucleation and growth of hydroxyapatite needle-like crystals with hexagonal symmetry occurred only on titanium substrates both chemically and thermally treated. No hydroxyapatite phase was detected on only acid etched Ti metal. This finding demonstrates that only a particular titanium surface treatment can effectively induce the apatite nucleation under hydrothermal conditions. © 2015 Elsevier B.V. All rights reserved.

1. Introduction High requirements for dental and orthopedic implants have pushed the research towards complex and intelligent biomaterials. To overcome the disadvantages of a single phase implant, such as low biocompatibility for metals and poor mechanical properties for ceramics, a significant amount of research has focused on the development of their combination, i.e. covering load-bearing metallic implants with thin bioactive ceramic-like films. Titanium and its alloys, with excellent mechanical properties, high corrosion resistance and biocompatibility, have been widely used as leading materials for hard tissue replacement [1]. However, as biologically inactive materials, once implanted into the human body they get encapsulated by a fibrous tissue to isolate them from surrounding bone, which prolongs the healing time and may cause implant loosening. As long-term complications, corrosion and abrasive wear of the implant can produce metallic debris which builds up in the surrounding soft tissues leading to metallosis. Other side effects of using metals in arthroplasty concern: osteolysis, heterotropic ossification and avascular necrosis. On the other hand, the structural similarity of hydroxyapatite [Ca10(PO4)6(OH)2, or HAp] with the hard tissues makes HAp an attractive coating material to accelerate bone growth around the implant. Numerous studies have shown that HAp bonds directly to the host bone tissue after implantation without the ⁎ Corresponding author. E-mail address: [email protected] (K. Suchanek).

http://dx.doi.org/10.1016/j.msec.2015.02.029 0928-4931/© 2015 Elsevier B.V. All rights reserved.

intervention of fibrous tissue at the implant–tissue interface [2,3]. Its favorable osteoconductive and bioactive properties made HAp a preferred biomaterial for both dental and orthopedic applications. Moreover, as a ceramic coating, it provides protection against corrosion and metallic debris release and thus reducing implant rejection. Many methods have been developed to deposit a HAp layer onto biomedical metal surface including plasma spraying [4–6], sputtering process [7,8], sol–gel method [9], pulsed laser deposition [10–12], electrophoretic deposition [13], electrochemical deposition [14] and hydrothermal synthesis [15–18]. Amongst those techniques, only plasma spraying is commercially used to produce HAp coatings on titanium implants. However, it is limited by: poor uniformity in coating thickness and its adherence to substrate, phase impurity, low crystallinity and degradation of bending strength and fatigue life caused by the dissolution of HAp phase in long-term contact with body fluids [6]. To confront the disadvantages of plasma spraying technique, the alternative coating methods have been extensively developed and tested. One of the well-known techniques to produce different forms of HAp powders with excellent crystal quality and Ca/P ratio close to the stoichiometric value is hydrothermal method [19,20]. The synthesis is based on a chemical precipitation from an aqueous solution containing calcium and phosphate sources, carried out in an autoclave at elevated temperature and high vapor pressure. An organic modifier is often added, usually a chelating agent such as carboxylic acid or ethylenediamine tetraacetic acid (EDTA), to control supersaturation of HAp and thus the crystal growth rate. Upon heating, calcium

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chelate decomposes and calcium ions are slowly and continuously released so that the degree of supersaturation is constantly low during the deposition process [21]. First attempts to hydrothermally produce HAp coatings on metallic substrates were taken by Fujishiro [15]. The results showed that the crystal nucleation on unmodified titanium surface is limited; HAp precipitated non-uniformly, forming separate needle-like aggregates on the titanium plate. Therefore, to effectively produce HAp coatings under hydrothermal conditions, a two step approach is necessary. The first step is (i) to produce on titanium surface an intermediate layer rich in calcium and/or phosphorus ions, or (ii) to modify the titanium surface in another manner to induce good coating adherence. The second step is the hydrothermal synthesis carried out on a modified titanium substrate. As a result, continuous and relatively thick films are produced. So far, the surface enrichment with calcium and/or phosphorus preceding the hydrothermal deposition of HAp was made in several ways: through hydrothermal synthesis of CaTiO3 and TiO2 films [22], anodization of titanium in electrolytic aqueous solution of dissolved calcium acetate and sodium glycerophosphate [16], electrochemical deposition of brushit [23] or hydroksyapatite [17,18] as a seeding layer, sol–gel preparation of bioglass coatings [24] or plasmasprayed HAp coating [25]. In the context of the aforementioned studies, a much easier way to prepare the titanium substrate for hydrothermal process is to modify its surface by sand-blasting and subsequent acid etching [26]. Recently, a nano-sized HAp film was hydrothermally synthesized on TiNb alloy, although the coating thickness was of nanometers and the assigned XRD signal was poor [27]. In the present study, highly crystalline HAp coatings were successfully produced through EDTA-assisted hydrothermal synthesis directly on titanium substrates which were modified accordingly to Kokubo method [28]. The HAp nucleation is known to be a spontaneous reaction greatly influenced by the surface properties of materials where the crystals are intended to grow. According to many previous studies, only particular functional groups, such as negatively charged hydroxyl or carboxyl groups, attached to the titanium surface can catalyzed and accelerate the HAp nucleation [29,30]. Kokubo et al. [28] proposed a new principle for obtaining bioactive titanium. The method is based on positively charged surface achieved by the absorption of acid groups through a H2SO4/HCl mixed acid treatment and by the formation of positively charged TiO2 film on Ti surface by a subsequent heat treatment. The ability to hydrothermally form HAp on titanium surface subjected to different heat and acid treatment was investigated. The results indicate that the surface modification in acid and 650 °C was the most effective method inducing the formation of good quality HAp coatings during the hydrothermal synthesis. 2. Experimental 2.1. Preparation of Ti substrates Commercially available pure titanium sheet (BIMO Metals, Wrocław, Poland) with a thickness of 3 mm was cut into square samples with a dimension of 10 × 10 mm2. The samples were cleaned in acetone and alcohol in an ultrasonic bath and rinsed with distilled water. The cleaned Ti plates were firstly soaked in a mixture of 66.3% H2SO4 (w/w) solution (POCH, Poland) and 10.6% HCl (w/w) solution (POCH, Poland) in a weight ratio of 1:1 at 60 °C for 1 h, then treated in oxidizing conditions at 400 °C and 650 °C according to the method described in [28]. One of the substrates was left without heating as a reference. 2.2. Hydrothermal synthesis For hydrothermal synthesis, an aqueous solution of Ca(NO3)2·4 H2O (0.2 M) (POCH, Poland) and Na2EDTA·2H2O (0.2 M) (POCH, Poland) was prepared in 35 ml of distilled water. (NH4)2HPO4 (0.12 M)

(POCH, Poland) was dissolved in same amount of distilled water in a separate beaker. After the reactants were completely mixed, the two solutions were combined together and subsequently stirred at room temperature for 30 min. The pH was adjusted to pH = 9 by adding dropwise an appropriate amount of ammonium hydroxide. The final stock solution was transferred to 200 ml glass vessel and put into hydrothermal reactor (Carl Roth 2098.1). The synthesis was performed on three modified titanium substrates at the same time. They were placed on a special titanium holder inside the glass vessel. The hydrothermal autoclave was sealed and set up to 220 °C for 7 h. 2.3. Surface characterization The Raman spectrometer used in the study was a high resolution, confocal micro-spectrometer Almega XR of Thermo Electron Corp. The chosen excitation light wavelength was 532 nm, objective magnification 100×, and a pinhole aperture of 25 μm. The Raman scattered light was energy-dispersed by diffraction grating and registered in the CCD camera. The spectrometer's spectral resolution was 2 cm−1. The laser spot size was about 1 μm and the confocal depth of the microscope was about 2 μm, thus the sample volume probed in each spectrum collection run was about 2 μm3. The data were recorded in the spectral range from about 100 cm− 1 up to 4000 cm− 1. HAp coatings were characterized with an ATR-FTIR system (PerkinElmer, Spectrum BX) in the spectral range from 525 cm−1 up to 4000 cm−1 with a spectral resolution of 4 cm−1. Scans were repeated 64 times. The X-ray diffraction (XRD) measurements were performed with PANalytical X'Pert Pro diffractometer operating at 30 mA and 40 kV. The radiation wavelength (Cu Kα) was 1.54 Å. X-ray diffraction patterns were taken over the 2Θ range of 20°–60° with a 0.05° step size. The sample's morphology and cross-section view was examined using a scanning electron microscope (SEM) (JSM-6460 LV JEOL). The surface roughness of coated and uncoated specimens was measured using mechanical contact surface profilometry (T-500, Hommelwerke, Germany). The parameters evaluated were: Ra (the arithmetic mean of the area between the roughness profile and its mean line); Rz (the arithmetic mean of the five highest peaks plus the depth of the five deepest valleys over the evaluation length); and Rt (the length of the highest peak plus the depth of the deepest valley over the evaluation length). For each sample, the mean value of Ra, Rz and Rt and standard deviation (SD) were computed along six horizontal lines randomly chosen from the individual area. One-way variance analysis (ANOVA) was used to identify the most significant differences (p b 0.05). The differences between the groups were determined using t-test analysis with two population comparison. The scratch test characterizing the mechanical properties of HAp coatings was performed in Nano-Scratch Test System (CSM Instruments). A linearly progressive normal load was applied with a Rockwell (diamond) indenter of 2 μm radius. The test was conducted in the 25–100 mN load range with a loading rate of 25 mN/min. To allow total delamination to occur within the scratch span, a scratch length of 3 mm was selected. The critical normal force (critical load) at which adhesive failure was first detected (with a sudden increase in friction forces) was used as a measure of adhesion. 3. Results and discussion 3.1. Structure and morphology of modified titanium surface with acid and heat treatment The SEM images of modified titanium surfaces are depicted in Fig. 1a–c. There is no obvious difference between the first two images showing the morphology of only the etched sample and the sample with additional heat treatment at 400 °C (Fig. 1a and b, respectively). In both cases the modifications led to the formation of rough surface with similar micro-sized craters. Annealing at 650 °C slightly changed

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Fig. 1. SEM images of etched Ti surface after heat treatment at different temperatures: (a) without, (b) 400 °C and (c) 650 °C.

Table 1 Surface roughness parameters of modified Ti metal samples before and after hydrothermal synthesis. Type of material

Roughness parameters [μm] mean value (standard deviation) n = 6 Ra

Rt

Rz

TiNTa Ti400 Ti650 HApTiNT HApTi400 HApTi650

0.78 (0.03) 0.87 (0.03) 0.74 (0.02) 1.35 (0.05) 2.03 (0.19) 1.80 (0.29)

6.25 (1.06) 6.65 (0.87) 6.00 (0.42) 10.29 (0.87) 20.57 (2.5) 19.22 (2.33)

5.14 (0.42) 5.37 (0.36) 4.91 (0.23) 8.58 (0.46) 15.17 (0.75) 13.74 (1.48)

a TiNT, Ti400, Ti650 denotes acid etched Ti sample without subsequent annealing, annealed at 400 °C, and at 650 °C, respectively.

The Raman spectra of etched Ti surfaces as a function of annealing temperature are presented in Fig. 3. The Raman spectrum of only chemically treated sample (Fig. 3a) does not show any active Raman bands. After the thermal treatment at 400 °C (Fig. 3b), distinct peaks at 610, 446 and 242 cm− 1 are developed. The observed peaks are consistent with the presence of rutile phase. Rutile has a space group D4h (P4 2/mnm) containing two molecules in the unit cell. From the group analysis, there are four Raman active modes for rutile, A1g + B1g + B2g + Eg, which were identified at 143 cm−1 (B1g), 447 cm− 1 (Eg), 612 cm− 1 (A1g), and 826 cm−1 (B2g) [31]. The band centered around 240 cm−1 is assigned to O–O interaction involving three- or four- coordinate oxygen atoms [31]. However, Raman bands observed for the sample annealed at 400 °C are characterized by low intensity and large line width. The weak Raman signal is commensurate with the low degree of crystallinity indicating the dominance of amorphous form of TiO2 in the formed layer. These results are consistent with XRD analysis, where for the discussed sample no rutile phase was detected. When the annealing temperature increased and reached 650 °C (Fig. 3c), the broad peaks observed previously become sharper and more intense, signifying an increase in crystalline phase content in the formed layer. 3.2. Effect of titanium surface modification on hydrothermal HAp nucleation The surfaces morphology of differently modified titanium plates after immersion in Ca(EDTA)2− and (NH4)2HPO4 solution under hydrothermal conditions are shown in Figs. 4–6. No HAp precipitates were observed on only acid etched titanium surfaces after the hydrothermal process (Fig. 4a–c). Fig. 4 shows low and high magnifications of only Ti TiH TiO2 TiO.325

(c) Intensity [arb.u]

the morphology of the surface decreasing its roughness. As we can see in Fig. 1c, pores are smoother and the surface appears to be gently melted. The three types of samples modified under different conditions were also examined for surface roughness. Table 1 collects results for roughness parameters Ra, Rz, and Rt and their standard deviations. Statistical analysis showed that Ti surface modified with acid etching and subsequent annealing at 400 °C exhibited a significantly rougher surface (R a = 0.87 ± 0.03 μm) than the one without annealing (Ra = 0.78 ± 0.03 μm) and heated at 650 °C (Ra = 0.74 ± 0.02 μm). There were no significant differences between the roughness values determined for titanium without annealing and titanium heated at 650 °C. The lowest values of the roughness parameters were collected for titanium etched and annealed at 650 °C, which correlates well with SEM results. The crystallographic phases identified for modified titanium substrates by X-ray diffraction are shown in Fig. 2. The XRD patterns of only chemically treated surface (Fig. 2a) show characteristic peaks for hexagonal α-titanium, as expected. Additionally, there are peaks originating from the titanium hydride phase (TiHx, x = 1.5 according to ICSD No. 063427) derived from the reaction between the titanium plate and the sulfuric and hydrochloric acids. During the etching process the passive titanium oxide layer dissolved and hydrogen ions diffused into the unprotected metal. The surface enrichment in hydrogen resulted in the formation of titanium hydride film. Fig. 2b shows the XRD pattern of sample etched and subsequently heated in air at 400 °C for 1 h. During the heating process, TiH decomposed and the only remaining phase was α-titanium. Simultaneously with dehydrogenation, the oxidation process takes place, however at 400 °C only a trace amount of titanium oxide forms, hence it was not detected during XRD measurements. Fig. 2c shows further surface transformation occurring during the annealing. When the etched substrate is heated up to 650 °C, crystalline TiO2 with tetragonal rutile structure (ICSD No. 088624) as well as nonstoichiometric titanium oxide (TiOx, x = 0.325) (ICSD No. 024080) can be observed.

(b)

(a) 20

24

28

32

36

40

44

48

52

56

60

2 [degree] Fig. 2. XRD diffraction patterns of etched Ti surface after heat treatment at different temperatures: (a) without heating, (b) 400 °C and (c) 650 °C.

Raman Intensity [arb. u]

233

443

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610

60

(c)

(b) (a)

1000

800

600

400

200

-1

Wavenumber [cm ] Fig. 3. Raman spectra of Ti surface after (a) acid etching, acid and heat treatment at (b) 400 °C and (c) 650 °C.

etched sample and no whiskers characteristic for HAp were found in SEM analysis. However, this specimen exhibits higher surface roughness (Table 1), as Ra is higher compared to the sample before synthesis and changes from Ra = 0.78 ± 0.03 μm to Ra = 1.35 ± 0.03 μm. This indicates that under hydrothermal conditions the etched surface is being modified and it might be an initial stage of titanium oxide formation (which is shown by further Raman analysis). Crystalline HAp coatings were found only on titanium with acid and heat treatment at 400 °C and 650 °C (Fig. 5 a–b, respectively). For both samples we observe elongated HAp crystals with needle-like shape and hexagonal symmetry, as shown in Fig. 6, as an example. The HAp crystals grown on etched Ti surface and heated at 400 °C are slightly smaller in length (6–8 μm) than the ones grown on Ti surface treated at 650 °C (5–10 μm). In both cases the annealed Ti substrates were uniformly and entirely covered by the HAp precipitates. Morphology

of HAp coatings was analyzed with respect to surface roughness. It is proven that topography of HAp coatings surface influence cellular activity, enhancing the cell proliferation and differentiation [32]. Table 1 shows the mean values and standard deviations of Ra parameter obtained from the measurements. Hydrothermal synthesis leads to micrometer-size roughness, ranged from Ra = 1.80 ± 0.29 μm to Ra = 2.03 ± 0.19 μm for surface obtained on Ti with acid and heat treatment at 650 °C and 400 °C, respectively. Those results are common in the literature. The coating's thickness was thoroughly characterized using crosssections shown in Fig. 5c–d. The thicknesses were assessed directly from SEM images at 60 μm and 170 μm for Ti substrate annealed at 400 °C and 650 °C, respectively. The scratch test was further performed to examine the adhesion strength of the coatings. The critical scratch load was calculated from the abrupt change in the signal of the sensor output, which indicates complete detachment of the layer. The adhesion strengths was around 85 mN and 93 mN for the HAp coatings hydrothermally deposited on annealed samples at 400 °C and 650 °C, respectively. The X-ray diffraction patterns of as-prepared HAp coatings on modified titanium plates are shown in Fig. 7. No characteristic peaks for hydroxyapatite phase were found in XRD spectrum for hydrothermally treated sample on only acid etched Ti substrate (Fig. 7a). The results are in agreement with SEM observations (Fig. 4). α-Titanium was the only phase detected for this sample. The absence of TiH phase is possibly due to its decomposition under hydrothermal conditions. Fig. 7b–c show the diffraction patterns of coatings synthesized on Ti substrates treated with acid and subsequent annealing. Inspection of both figures reveals the presence of a hexagonal hydroxyapatite phase with space group P63/m, being consistent with ICSD No. 016742 database card. No other calcium phosphate phase, such as TCP or OCP, were detected. Along with the HAp phase, we observe α-Ti for both samples and crystalline TiO2 with tetragonal rutile structure and nonstoichiometric form of titanium oxide (TiOx, x = 0.325) for sample annealed at 650 °C (Fig. 7c). The titanium compounds originate from the substrate and their relatively strong signal indicates small thickness of the coating. The observed XRD pattern of the HAp coatings exhibit an enhancement in the intensity of the peak at 2Θ of about 32.84° in comparison to the intensity reported in the database. This reflection corresponds to (300) crystallographic plane and its increase may indicate that c-axes of the crystals are dominantly oriented which is consistent with many previous studies [33,34]. The lattice parameters of hydroxyapatite precipitates were determined using the Bragg equation and the equation for interplanar distance in hexagonal structure [35]. The calculated values were a = 9.44 ± 0.51 Å and c = 6,89 ± 0.51 Å and correspond well to the values given in database (a = 9.432 Å, c = 6.881 Å). For HAp, the vibration spectra, both Raman and FTIR, display a strong molecular character associated with the internal modes of the PO3− 4

Fig. 4. SEM images at low (a) and high (c) magnifications of acid etched titanium surface after hydrothermal synthesis in 220 °C for 7 h.

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Fig. 5. SEM images of HAp coatings obtained after hydrothermal synthesis in 220 °C for 7 h on modified titanium substrates. Surface and cross-sectional view of HAp coatings obtained on titanium with acid and heat treatment at 400 °C (a, c) and at 650 °C (b, d).

group. The vibrational modes of PO3− 4 group contribute to four different bands. The ν1 frequency represent the symmetric stretching vibration of P–O bond; ν 2 mode is assigned to the doubly degenerated bending vibration of O–P–O bonds; ν4 frequency results from the triply degenerated bending mode of O–P–O bonds, and finally the ν3 mode are attributed to the triply degenerated stretching vibration of P–O bonds [36]. Besides these features the vibration spectra can be enriched with modes arising from the vibrations of hydroxyl group [35]. The Raman analysis of HAp coatings synthesized under hydrothermal conditions are depicted in Fig. 8. Raman results show that in the

case of only etched titanium, soaked in Ca/P rich solution, we do not observe any Raman active bands characteristic for HAp (Fig. 8a). The small and sharp peak observed around 144 cm−1 can be assigned to anatase phase of TiO2 [37], however the low intensity indicates its trace amount. Fig. 8b–c shows Raman spectra of HAp coatings deposited on samples with acid and heat treatment at 400 °C and 650 °C. Both spectra appear to be similar expect for slight differences in the intensity of OH− symmetric stretching mode centered at 3572 cm−1. Furthermore, bands Ti HAp TiO2 TiO.325

Intensity [arb.u]

(c)

(b)

(a) 20

24

28

32

36

40

44

48

52

56

60

2 [degree]

Fig. 6. Enlargement (5000×) of SEM image of HAp coating synthesized on acid etched and annealed at 650 °C Ti substrate.

Fig. 7. XRD patterns of the samples after hydrothermal synthesis in 220 °C for 7 h on: (a) acid etched Ti substrate, acid etched and annealed at (b) 400 °C and (c) 650 °C Ti substrate.

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1100 cm− 1 to ν3 mode. Positions of all peaks are in good agreement with data reported by other authors on HAp phase [34]. FTIR spectroscopy provides a complementary information to Raman. Fig. 9 presents ATR-FTIR spectra of all hydrothermally synthesized coatings. None of the absorption bands related to hydroxyapatite are seen in Fig. 9a. Other two spectra presented in Fig. 9b and c, collected for HAp coatings synthesized on Ti substrates treated with acid and annealing, are consistent with published data for HAp phase [38,27]. Most of the observed peaks are assigned to the vibrations of phosphate group and are located around 1090 and 1022 cm−1 (ν3), 961 cm−1 (ν1), 602 and 560 cm−1 (ν4). Absorption bands centered at 630 cm−1 are attributed to the librational mode of the hydroxyl group. The further investigation of FTIR spectra shows an existence of low intensity band in position of 865 cm−1. This band can be attributed to the vibrations of CO23 − group, indicating the incorporation of carbonate ions into hydroxyapatite crystal structure [38], however the other characteristic bands of this anions around 1400–1550 cm−1 do not appear. Therefore, more group (attributed likely the band at 865 cm−1 is derived from HPO2− 4 to P–OH vibrations) indicating this ion incorporation into the HAp structure [34].

Raman Intensity [arb. u]

1

3572

(c)

2

4

3

(b)

(a) 4000

3500

1200

1000

800

600

400

3.3. Discussion on the mechanism of EDTA-assisted hydrothermal synthesis of HAp coatings

200

-1

Wavenumber [cm ] Fig. 8. Raman spectra of coatings obtained under hydrothermal condition on modified Ti substrate: (a) acid etched, acid and heat treated at (b) 400 °C and (c) 650 °C.

3(PO43-)

IR Intensity [arb. u.]

4(PO43-)

1(PO43-) (OH)

(c)

24

(HPO )

(b) (a)

2000

1750

1500

1250

1000

750

500

-1

Wavenumber [cm ] Fig. 9. ATR-FTIR spectra of coatings obtained under hydrothermal condition on modified Ti substrate: (a) acid etched, acid and heat treated at (b) 400 °C and (c) 650 °C.

The experimental results of this work, provide clear evidence that hydrothermal synthesis of HAp coatings requires both acid and heat treatment. During the etching process pits and grooves are developed leading to rough surface. As a result, micro-sized roughness forms and remains almost unchanged after the annealing. This fact leads to a conclusion, that roughness is not the critical parameter intensifying the HAp nucleation during the hydrothermal synthesis. Most likeable theory provides Kokubo et.al [28], suggesting that the ability to form apatites on thermochemically modified titanium is assigned to the existence of positive surface charge. Fig. 10 shows a schematic chemical reaction occurring on the modified Ti surface during the hydrothermal synthesis of HAp coating. As starting materials for the stock solution, we used ethylenediamine tetraacetic acid (EDTA) as a chelating agent, calcium nitrate Ca (NO3)2 and diammonium phosphate (NH4)2HPO4. At room temperature, EDTA immediately chelates calcium ions and forms stable Ca–EDTA complex, depleting the stock solution in free Ca2+. In the first stage of hydrothermal process, the negatively charged phosphate ions are drawn to the positively charged substrate and bond with its surface. As the PO34 − ions accumulate, the surface charge changes to negative and is able to attract Ca2+ ions which are slowly released from thermally decomposing Ca–EDTA complexes. The reaction leads to the nucleation and growth of HAp crystals. The proposed method allows to produce well-crystallized HAp coatings with the thickness of 170 μm and decent adhesion. 4. Conclusions

derived from the vibrations of PO−3 groups are observed in both spec4 tra. The most intense of them is ν1 mode centered at 961 cm−1. Bands located in the position of 390–470 cm− 1 are assigned to ν2, bands between 560 and 625 cm− 1 to ν4 , and bands between 1010 and

We examined the effect of different surface treatment of Ti metal on HAp crystallization in Ca(EDTA)2 − and (NH4)2HPO4 solution under hydrothermal conditions. The results showed that the surface chemistry

Fig. 10. Schematic view of the chemical reactions involved in the processes of HAp-coating formation under hydrothermal conditions.

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of titanium substrate is a key factor inducing the growth of crystalline hydroxyapatite coating during the synthesis. As discussed in [28], the positive charge of a surface, which can be achieved by superficial formation of crystalline TiO2 during the chemical and thermal treatment, can induce the heterogeneous nucleation of HAp. The positively charged layer of titanium oxide attracts the negatively charged phosphate ions present in the solution and as they accumulate on the surface, they absorb calcium ions. As a consequence, the crystalline apatite coating can be formed. According to XRD, Raman and FTIR spectroscopy, no HAp crystals were present on only etched surface of titanium. However, when etching was followed by annealing process, the Ti surface was fully covered by HAp particles. Moreover, HAp was the only calcium phosphate phase detected after the hydrothermal process. Synthesized HAp crystals were needle-like in shape with well-defined hexagonal symmetry and elongated along the c-axis. As for coatings thickness, the highest value estimated from SEM images of cross-sectional view was 170 μm for the substrate etched and annealed at 650 °C. The best adhesion strength of HAp coatings was also achieved for the same specimen. Our study showed that titanium prepared in the described way is a very promising substrate to hydrothermally produce good quality HAp coatings, as it promotes the nucleation of apatite. Further work is needed to improve the uniform coverage of Ti substrates under hydrothermal conditions. References [1] M. Long, H.J. Rack, Titanium alloys in total joint replacement — a materials science perspective, Biomaterials 19 (1998) 1621–1639. [2] L.L. Hench, Bioceramics: from concept to clinic, J. Am. Ceram. Soc. 74 (1991) 1487–1510. [3] T. Kokubo, H. Takadama, How useful is SBF in predicting in vivo bone bioactivity? Biomaterials 27 (2006) 2907–2915. [4] R. Gadow, A. Killinger, N. Stiegler, Hydroxyapatite coatings for biomedical applications deposited by different thermal spray techniques, Surf. Coat. Technol. 205 (2010) 1157–1164. [5] M. Roy, A. Bandyopadhyay, S. Bose, Induction plasma sprayed nano hydroxyapatite coatings on titanium for orthopaedic and dental implants, Surf. Coat. Technol. 205 (2011) 2785–2792. [6] T. Laonapakul, A.R. Nimkerdphol, Y. Otsuka, Failure behavior of plasma-sprayed Hap coating on commercially pure titanium substrate In simulated body fluid (SBF) under bending load, J. Mech. Behav. Biomed. Mater. 15 (2012) 153–166. [7] V. Nelea, C. Morosanu, M. Iliescu, I.N. Mihailescu, Microstructure and mechanical properties of hydroxyapatite thin films grown by RF magnetron sputtering, Surf. Coat. Technol. 173 (2003) 315–322. [8] Y. Yang, K.-H. Kim, J.L. Ong, A review on calcium phosphate coatings produced using a sputtering process — an alternative to plasma spraying, Biomaterials 26 (2005) 327–337. [9] P. Choudhury, D.C. Agrawal, Sol–gel derived hydroxyapatite coatings on titanium substrates, Surf. Coat. Technol. 206 (2011) 360–365. [10] C.F. Koch, S. Johnson, D. Kumar, M. Jelinek, D.B. Chrisey, A. Doraiswamy, C. Jin, R.J. Narayan, I.N. Mihailescu, Pulsed laser deposition of hydroxyapatite thin films, Mater. Sci. Eng. C 27 (2007) 484–494. [11] M. Sygnatowicz, A. Tiwari, Controlled synthesis of hydroxyapatite-based coatings for biomedical application, Mater. Sci. Eng. C 29 (2009) 1071–1076. [12] G.P. Dinda, J. Shin, J. Mazumder, Pulsed laser deposition of hydroxyapatite thin films on Ti–6Al–4V: effect of heat treatment on structure and properties, Acta Biomater. 5 (2009) 1821–1830. [13] A.R. Boccaccini, S. Keim, R. Ma, Y. Li, I. Zhitomirsky, Electrophoretic deposition of biomaterials, J. R. Soc. Interface 7 (2010) S581–S613.

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Crystalline hydroxyapatite coatings synthesized under hydrothermal conditions on modified titanium substrates.

Hydroxyapatite coatings were successfully produced on modified titanium substrates via hydrothermal synthesis in a Ca(EDTA)(2-) and (NH4)2HPO4 solutio...
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