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Alternative technique for hydroxyapatite coatings
Surface & Coatings Technology 201 (2007) 9587 – 9593 www.elsevier.com/locate/surfcoat
Alternative technique for hydroxyapatite coatings R.M. Trommer ⁎, L.A. Santos, C.P. Bergmann Federal University of Rio Grande do Sul, Department of Materials Engineering, Av. Osvaldo Aranha 99/705, 90035190 Porto Alegre, RS, Brazil Received 20 November 2006; accepted in revised form 10 April 2007 Available online 18 April 2007
Abstract Flame Assisted Chemical Vapor Deposition (FACVD), a novel technique that shows an enormous potential in porous oxides deposition, was employed for the first time aiming to obtain hydroxyapatite (HA) coatings on 316 L stainless steel metallic substrates. Calcium acetate and ammonium phosphate diluted in ethanol were employed as precursor salts. A Ca/P molar ratio of 1.66 was employed in precursor solution, which is equivalent to stoichiometric hydroxyapatite. A porous coating, formed by an open and interconnected network, was observed by scanning electronic microscopy (SEM) and associated with homogenous reactions. Thickness of hydroxyapatite coating was 412 ± 3 μm. X-ray diffraction (XRD) analysis indicated the presence of crystalline coatings, mainly constituted by hydroxyapatite phase and traces of tricalcium phosphate (β-TCP). Carbonate in the hydroxyapatite coatings was identified by Fourier transform-infrared (FTIR) spectroscopy. © 2007 Elsevier B.V. All rights reserved. Keywords: Biomaterials; Hydroxyapatite; Coatings; Chemical vapour deposition
1. Introduction Bioactive materials have been studied as coatings on bioinert materials since 1970 [1]. Thus, it is possible to combine the bioactivity of materials such as calcium phosphate ceramics with the excellent mechanical properties of metals and its alloys. Hydroxyapatite (HA) – Ca10(PO4)6(OH)2 – was the first calcium phosphate studied for surgical procedures, mainly due to its good compatibility with the inorganic components of bones and teeth [2]. Moreover, HA is a bioactive material that does not cause side effects in the human body. Hydroxyapatite belongs to the apatite family, a group of materials with similar structures, but not necessarily the same composition [3]. Calcium phosphates are usually classified according to the molar ratio between calcium and phosphorus, and HA has a molar ratio equal to 1.66 [4]. HA is a low toughness brittle material, what limits its use when high mechanical stresses are required. For this reason, HA is widely employed as coating on metallic prosthesis and implants, usually over titanium and its alloys or stainless steels.
⁎ Corresponding author. Tel.: +55 51 3316 3637; fax: +55 51 3316 3405. E-mail address: [email protected] (R.M. Trommer). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.04.028
Plasma spray is the commercial technique used to produce HA coatings. However, it has several disadvantages, mainly the use of high cost equipments, production of low crystalline coatings and presence of other phases like α- and β-TCP. For this reason, heat treatment is necessary after plasma spray deposition to avoid solubilization and detachment of HA coating. In this work, Flame Assisted Chemical Vapor Deposition (FACVD) was employed for the first time as an alternative process for the production of crystalline HA coatings on AISI 316 L stainless steel substrates. This technique was firstly described by Hunt and his colleagues [5] in 1993, when the authors obtained ZrO2, BaTiO 3 , YBa 2 Cu 2 O x and Y2BaCuO5 films. FACVD is a relatively new technique, and its main advantage is the use of precursors and low cost equipment. The principles of this process are based on conventional Chemical Vapor Deposition (CVD). Summarily, this deposition process involves the combustion of liquid or gaseous precursors in a flame, where they are vaporized and decomposed, leading to chemical reactions that leads to the formation of a coating over a substrate [6]. The flame plays an important role in the depositions process, and its main function is to supply the necessary energy to promote chemical reactions. The flame is responsible for warming substrate and allows diffusion, leading to a relatively dense coating [5]. When
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compared to conventional CVD techniques, FACVD does not require the use of a reactor or a chamber to promote chemical reactions. Thus, for the specific case of hydroxyapatite, a coating produced with this novel technique probably has low cost when compared with that one obtained with conventional CVD or plasma spray, for example. 2. Experimental 2.1. Substrate preparation AISI 316 L stainless steel discs (19 ± 0.2 mm diameter and 2 ± 0.2 mm thickness) were prepared in a precision cutting machine (Buehler Isomet 2000 model). An orifice was made in their middle in order to place a type-K thermocouple inside it, so the substrate temperature could be monitored during the deposition process. Substrates were polished with a sequence of sandpapers (#80, #160, #320 and #600 grits) to obtain an average roughness (Ra) lower than 0.1 μ. Before depositions they were cleaned with ethanol to avoid detachment during depositions due to contamination. Stainless steel was chosen as substrate because it stands the deposition temperature without oxidizing, it is cheaper than titanium, and it is considered a biomaterial. It can also be used in some cases with or without a coating [1–3,7,8]. 2.2. Precursor solution preparation Calcium acetate (Ca(CH3COO)2 U H2O) (Synth®), in a concentration of 0.0833 M, and ammonium phosphate ((NH4)2HPO4) (Synth®) in a concentration of 0.0500 M were employed as precursors salts. Ca/P molar ratio in the precursor solution was equivalent to biological hydroxyapatite: 1.66. To avoid ammonium phosphate precipitation, nitric acid (HNO3) (Synth®) was used. Ethanol (95%) was employed to dilute precursor salts up to the desired concentration and as fuel in the flame, increasing available energy necessary for chemical reactions.
2.3. Deposition equipment The apparatus developed in laboratory to obtain hydroxyapatite coatings with FACVD technique is shown in Fig. 1. Due to oscillations in the flame caused by the atomization device, it was necessary to control the temperature of the substrate during deposition process, so a cooling system that uses compressed air was employed. The cooler system was useful as a holder for the substrate too. To monitor substrate temperature, a type-K thermocouple linked to a data acquisition (Novus — Field Logger model and Field Chart software) was used. The thermocouple tip was placed at the same level of the substrate surface. Thus, the temperature on the substrate surface was estimated, and the cooling allowed keeping the deposition temperature within a pre-determined range. A Cole–Parmer (Masterflex model) peristaltic pump was employed to control solution precursor flux. Precursor solution was processed in an atomization device, composed of two perpendicular needles. Precursor solution flows through the inner needle (0.6 mm diameter) and the atomization gas (compressed air) flows through the bigger one (1.5 mm diameter). As mentioned before, FACVD employs a flame to supply the required energy for oxide depositions [5]. Thus, a Bunsen burner was used as pilot flame, using propane as combustible and air as oxygen source. Many authors reported the use of an oxy-acetylene flame, but aiming to obtain Diamond like-carbon (DLC) films or coatings [9–11]. For hydroxyapatite depositions, it was not necessary to use an oxyacetylene flame, because the Bunsen burner supplies the necessary energy to promote chemical reactions. When the atomized precursor solution intercepts the pilot flame, another flame is produced, which is called main flame and is obtained from the combustion of precursor solution that contains ethanol. The role of this secondary flame is to supply the required energy to promote chemical reactions that leads to the formation of the coatings. Moreover, the ions calcium and phosphorous are dissolved in the precursor solution, and their combustion is
Fig. 1. FACVD apparatus developed to obtain hydroxyapatite coatings.
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necessary to promote the chemical reactions that leads to the production of hydroxyapatite. After that, a coating is produced on the surface of the substrate. It must be highlighted that most components of the apparatus developed in laboratory have low cost when compared with commercial equipments generally used to produce hydroxyapatite coatings. 2.4. Deposition parameters The distance between the atomization device and the substrate was kept constant (140 mm), and it was defined in pre-tests. This distance was chosen because when the atomization device is placed far from the substrate the temperature on substrate surface is low. This can make the diffusion on the surface or in the first layers of deposited coating difficult. When the atomization device is placed close to the substrate, the combustion of the precursor solution is not complete, and the coating produced is powdery and not well adhered to the substrate [12,13]. The deposition temperature on the substrate was kept constant at 500 ± 5 °C, and it was chosen based on another work developed by the authors with plasma spray [14]. A higher deposition temperature can probably decompose hydroxyapatite to tricalcium phosphate (β-TCP). On the other hand, a low deposition temperature on the substrate can produce an amorphous coating [15,16]. Both β-TCP and amorphous phase can lead to the loss of implant, due to the absorption of the coating by the organism [1–3,7,8]. Deposition time was fixed at 10 min and precursor solution flux was 12 mL/min, both defined in pre-tests. Compressed air was employed as atomization gas, at the pressure of 4 MPa. Deposition parameters are summarized in Table 1. 2.5. Characterization techniques X-ray diffraction analyses were performed to identify mineralogical phases, and a Philips diffractometer (X'Pert MPD model) equipped with a graphite monochromator and a rotative anode of copper operating a 40 kV and 40 mA was used. All measurements were made using CuKα radiation. Scanning rate was 0.05°/min, with a time-step of 5 s in the 20–60° range. Fourier transform infrared (FTIR) spectroscopy was used to identify typical hydroxyapatite chemical groups. Analyses were performed in the range of 400 to 4000 cm− 1, with a Perkin Elmer (Spectrum 1000 model) equipment. It was necessary to remove the coating from the substrate before analysis. After this, samples were prepared as KBr tablets for the analysis. Hydroxyapatite coatings microstructure was evaluated by means of scanning electronic microscopy (SEM) with a Jeol (JSM 5800 model) equipment. The coatings cross sections were Table 1 Deposition parameters employed for hydroxyapatite coatings deposition Atomization device–substrate distance Deposition temperature Deposition time Precursor solution flux Atomization gas pressure
140 mm 500 ± 5 °C 10 min 12 mL/min 4 MPa
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analyzed. Since hydroxyapatite is a brittle material, it was necessary to apply a coating of epoxy resin (Comfibras LTDA) to avoid its fracture during cutting. After the setting of epoxy resin, coatings were cut in a precision cutting machine (Buehler model Isomet 2000), polished and metalized with a thin gold layer to observe its cross section. UTHSCSA Image Tool version 3.00 software, developed by University of Texas Health Science Center, was employed to determine the thickness of the coatings. First, it was necessary to generate a calibration standard by the use of scale bars provided in the SEM micrographs. 3. Results and discussion 3.1. Crystalline phases From the XRD analyses it was possible to identify the major phase hydroxyapatite (JCPDS 09-0432) as well as the austenitic phase of substrate (JCPDS 33-0397), as shown Fig. 2. XRD pattern confirmed that an initial Ca/P molar ratio of 1.66 in the precursor solution led to the production of a hydroxyapatite coating. We believe that the Ca/P molar ratio of the precursor solution is the main reason for obtaining the mineralogical phases in the coatings, considering the specific deposition parameters used in this work and when FACVD was employed. XRD diffraction results confirmed that with the use of low cost precursor salts such as calcium acetate and ammonium phosphate it was possible to obtain hydroxyapatite coating using FACVD technique. Thus, it is not necessary to use starting powders of hydroxyapatite, which are in some cases expensive and need a specific range of particle size and deposition parameters (in the case of plasma spray technique, for example) [17,18]. During hydroxyapatite deposition, calcium ions are supplied by calcium acetate, while phosphate ions are available from ammonium phosphate employed as precursor salt. However, a small amount of β-tricalcium phosphate (βTCP) (JCPDS 09-0169) in the hydroxyapatite coatings was identified, as shown Fig. 2. It is believed that β-TCP phase was produced due to the high temperature in the flame employed in FACVD. Choy [6] reported that FACVD has a wide range of temperatures in the flame, between 1727 and 2727 °C, associated to the deposition parameters. Charojrochkul et al. [13] mentioned that the maximum temperature in the flame during the deposition of porous lanthanum strontium manganese oxide (LSM) to produce the cathode for a solid oxide fuel cell (SOFC) was 1200 °C. Xu et al. [19] showed in their work that the flame temperature depends on the distance between the substrate and the atomization device, and it can range from 1100 to 2400 °C. When the flame temperature is high, thermal decomposition, dehydroxylation and transformation of hydroxyapatite can occur [20]. β-TCP phase is usually identified in hydroxyapatite coatings produced by plasma spray technique [14], due to the high plasma temperature (5000–20,000 K) [21,22]. Hydroxyapatite decomposition reaction in β-TCP is described by Eq. (1). This reaction can usually occur in a temperature range from 1200 to 1450 °C. Ca10 ðPO4 Þ6 ðOHÞ2 Y3Ca3 ðPO4 Þ2 þCaO þ H2 O
ð1Þ
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Fig. 2. X-ray diffraction pattern of a coating produced by FACVD, showing the major phase hydroxyapatite and traces of tricalcium phosphate.
Thus, it was possible that some hydroxyapatite particles were already formed in the flame and during the course of deposition on the substrate surface, being decomposed to β-TCP due to the high flame temperature. 3.2. Microstructure Microstructure of as-deposited hydroxyapatite coating is shown in Fig. 3. It is possible to observe open pores in the coatings, forming an interconnected network. This microstructure is characteristic of coatings and films produced by FACVD, and many authors associated this microstructure to homogenous reactions [13,14]. Briefly, chemical reactions that happen during CVD deposition can be subdivided in two categories: heterogeneous or homogenous. The first one is characterized by reactions that occur near/on the surface of substrate, leading to a dense coating. The further is characterized by reactions that happen in the flame, and produces a porous coating, usually with poor adhesion to the substrate. Thus, after precursor solution combustion, chemical reactions already occurred in the flame, leading to the production of HA particles in the flame. These particles reached the substrate surface, forming agglomerates that led to the formation of a porous coating. Homogenous reactions are favored because precursor solution is basically composed of ethanol. Combustion of ethanol supplies more thermal energy to chemical reactions that produce HA particles, which reach the substrate surface and produces a powdery coating. However, an open structure can allow future penetration of bone tissue into hydroxyapatite coating, creating a stronger biointegration and better mechanical stability of prosthesis or implant with the host tissue [23,24]. Porosity increases surface area of hydroxyapatite coatings, leading to a higher reactivity when compared to a denser coating [1,2]. Thus, it is possible to substitute hydroxyapatite coating for bone tissue, leading to a faster recuperation of the patient. Moreover, a porous coating allows the use of medicine like anti-inflammatory and antibiotics, minimizing inflammatory response of living tissue [4,7,8].
With higher magnification, SEM micrographs revealed that hydroxyapatite coatings produced by FACVD had a microstructure composed of lamellas and spherical particles, as shown in Fig. 4. This microstructure is very similar to thermally sprayed ceramic coatings, composed of lamellas and melted particles [25]. Hydroxyapatite coatings had this microstructure because, as discussed before, some particles were formed in the flame (homogenous reactions) and due to the high temperature these particles melted or were subdivided due to high plasticity (caused by temperature and fluid dynamics in the flame). After the particles reached the substrate or layers of deposited coating with high velocity, they splashed, leading to a structure characterized by lamellas and spherical particles. Coarse and fine pores were observed in the microstructure of hydroxyapatite coatings obtained with FACVD. The first one can be described as macroporosity, while the second one is associated to microporosity. Macroporosity formation mechanism is very similar to plasma spray process, and it is associated to the incomplete contact between particles or lamellas that are already deposited and those ones that reach the metal and/or coating substrate
Fig. 3. SEM image of as-deposited hydroxyapatite coating. It is possible to observe a porous structure, creating an interconnected network. (Magnification: 1000X).
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of 500 ± 5 °C had a thickness of 412 ± 3 μm. For hydroxyapatite coatings, a wide range of thickness is reported in the literature, depending on the technique employed. Usually, hydroxyapatite coatings produced by plasma spray have a thickness between 30 and 200 μm [26]. Wang et al. [27], using electrophoretic deposition, produced hydroxyapatite coatings with a maximum thickness of 50 μm. Blind et al. [28] employed pulsed laser deposition to produce coatings with thickness of 100 nm. Based on deposition time and thickness of the coating, it is possible to calculate deposition rate, expressed by Eq. (2). deposition rate ðμm=minÞ ¼ Fig. 4. SEM image of as-deposited hydroxyapatite coating showing a structure composed of lamellas and spherical particles that are formed in the flame and reached the surface of the substrate. (Magnification: 5000X).
[17,25]. Microporosity is formed by evolution of gases. Since the precursor solution is basically composed of ethanol, and there are chemical reactions in the flame or even near the substrate surface, it is possible that some gases were present in the coatings for a short time. Thus, these gases vaporized during deposition, creating micropores in the coatings. Fig. 5a and b shows the cross section of a hydroxyapatite coating, confirming the presence of macropores or voids in the coatings. As discussed before, macroporosity is associated to the coalescence of the first deposited particles, forming an interconnected network. As the coating becomes thicker, some voids or macropores inside the coating can still exist. Furthermore, we must consider that initial deposition temperature is relatively low. Deposition temperature in the substrate was kept constant (500 ± 5 °C), but at the beginning of deposition it was about 100 °C, increasing up to the desired deposition temperature. This low temperature in the first moments of deposition can benefit the presence of macropores or voids [13,15]. In fact, when the cross section of coatings is analyzed (see Fig. 5b), it is possible to observe that macropores are mainly located close to the substrate surface. It is believed that in the first moments of deposition, atomic diffusion and viscous flux of particles are inhibited due to low temperature in the substrate, which can avoid the coalescence and densification of the first layers [14]. During deposition, more energy is provided to the system because more precursor solution, and consequently more ethanol, is available in the flame. Consequently, temperature in the substrate increases, decreasing the amount of macropores and voids. We believe that a higher temperature at the beginning of deposition can benefit diffusion and viscous flux, allowing the coalescence of particles already deposited and formation of denser coatings with no macropores. Moreover, we must consider that during coating deposition, more reactive species can be incorporated into the coatings, and this is another factor that influences the porosity of coatings. Cross section micrographs revealed that there are no cracks in the HA coatings produced by FACVD, which are typical of plasma sprayed coatings [17]. Hydroxyapatite coatings obtained in 10 min, with a precursor solution flux of 12 mL/min and substrate temperature
thickness ðμmÞ deposition time ðminÞ
ð2Þ
Deposition rate of HA coating produced by FACVD technique was 41 μm/min, considering specific deposition parameters and a constant deposition rate. The value of deposition rate observed in HA coating can be compared to coatings obtained by plasma spray process, which have high deposition rates. This indicates that FACVD is a very efficient process. 3.3. Functional groups Typical functional groups that are associated to hydroxyapatite structure were identified by FTIR spectroscopy, as shown Fig. 6. There are four vibrational modes associated to PO4− 3 ion
Fig. 5. Cross section of hydroxyapatite coating showing macropores or voids near the substrate (a) Magnification: 150X and (b) Magnification: 500X.
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Fig. 6. FTIR spectroscopy identifying characteristic chemical groups of hydroxyapatite compounds.
that can be observed in hydroxyapatite FTIR spectrum [29,30]. Absorption bands ν2 and ν3 at 1000–1200 cm− 1 are attributed to asymmetric stretching, while ν1 absorption band at 950– 960 cm− 1 is associated to symmetric stretching and ν4 at 560– 570 cm− 1 absorption band is attributed to asymmetric bending vibrations. The wide absorption band observed at 3400– 3100 cm− 1 is attributed to water presence in the sample, while the absorption band at 2400 cm− 1 is usually associated to potassium bromide (KBr) used in sample preparation. The small absorption band located at 3670–3570 cm− 1 is associated to OH− ion stretching. Thus, the main characteristic chemical groups of hydroxyapatite were identified by FTIR analysis. However, carbonate groups were identified in the coatings, characterized by absorption bands at 1500–1400 cm− 1 and 875 cm− 1, which are associated to asymmetric stretching and out of plane bending mode, respectively. Table 2 shows the absorption bands, their respective wavenumber and chemical bending. Presence of carbonate in the coatings produced by FACVD is not a problem. It is well known that hydroxyapatite is the most stable and less soluble calcium phosphate material found in human body [1–4]. This property can avoid or decrease bone tissue regeneration when a hydroxyapatite coated
Table 2 Absorption bands and wavenumber of functional groups identified in hydroxyapatite coatings Wavenumber (cm− 1)
Chemical bending
OH H2O KBr
3670–3570 3400–3100 2400
CO2− 3
1500–1400 875 1200–1000 960–950 570–560
Ion stretching Water Weak absorption band attributed to KBr Asymmetric stretching Out of plane bending mode Asymmetric stretching Symmetric stretching Asymmetric bending vibrations
Absorption band −
PO3− 4
prosthesis or implant is present. Usually, a material that has certain solubility is employed, because they allow bone tissue regeneration. This is possible by the use of a carbonated hydroxyapatite, which is a material with similar properties of biological hydroxyapatite [32]. Recent in vivo studies have shown that carbonated hydroxyapatite dissolution rate is intermediate between pure hydroxyapatite (lowest rate) and TCP-β (highest rate) [31]. Carbonate group is probably present in hydroxyapatite coatings due to the fact that depositions are performed in open atmosphere. For this reason, carbonate is incorporated into the coating due to CO2 present in the environment, which reacts with OH − present in precursor solution or atmospheric humidity, as stated in Eq. (3) [33]. CO2 þ 2OH X CO2 3 þ H2 O
ð3Þ
4. Conclusions It was possible to obtain hydroxyapatite coatings using Flame Assisted Chemical Vapor Deposition. Precursor solution composed of calcium acetate and ammonium phosphate, with a Ca/P molar ratio of 1.66 led to the formation of crystalline coatings, with the presence of a major phase of hydroxyapatite and traces of tricalcium phosphate. Hydroxyapatite coatings have an open structure, formed basically by interconnected pores and associated with homogenous reactions. HA coatings had a thickness of 412 ± 3 μm, and the deposition rate was 41 μm/min. Carbonate groups were identified in the hydroxyapatite coatings. Acknowledgements The authors gratefully acknowledge the financial support of the Brazilian Agency CNPq — Conselho Nacional de Desenvolvimento Cientifico e Tecnológico.
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References [1] H. Aoki, Science and Medical Applications of Hydroxyapatite, Japan Association of Apatite Science, Takayama Press Co., Tokyo, Japan, 1991, p. 28. [2] A. Ravaglioli, A. Krajewski, Bioceramics: Materials, Properties, Applications, Chapman and Hall, New York, USA, 1992, p. 197. [3] R.Z. Le Geros, Clin. Mater. 14 (1993) 65. [4] L.L. Hench, J. Wilson, An Introduction to Bioceramics, World Scientific, London, United Kingdom, 1999, p. 213. [5] A.T. Hunt, W.B. Carter, J.K. Cochran, Appl. Phys. Lett. 63 (1993) 266. [6] K.L. Choy, Prog. Mater Sci. 48 (2003) 57. [7] L.L. Hench, Biomaterials 19 (1998) 1419. [8] B.D. Ratner, et al., Biomaterials Science: An Introduction to Materials in Medicine, Academic Press, New york, USA, 1996, p. 121. [9] R.T. Rozbicki, V.K. Sarin, Thin Solid Films 332 (1998) 87. [10] J.J. Schermer, Diam. Relat Mater. 2 (1993) 1149. [11] I. Doi, M.S. Haga, Y.E. Nagai, Diam. Relat. Mater 8 (1999) 1682. [12] S. Charojrochkul, K.L. Choy, B.C.H. Steele, Solid State Ionics 121 (1999) 107. [13] S. Charojrochkul, K.L. Choy, B.C.H. Steele, J. Eur. Ceram. Soc. 24 (2004) 2515. [14] C.P. Bergmann, M.C. Valente, A.S. Takimi, M.D. Lima, A.H. Bressiani, Adv. Powder Technol. II 189 (2001) 623.
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Alternative technique for hydroxyapatite coatings R.M. Trommer ⁎, L.A. Santos, C.P. Bergmann Federal University of Rio Grande do Sul, Department of Materials Engineering, Av. Osvaldo Aranha 99/705, 90035190 Porto Alegre, RS, Brazil Received 20 November 2006; accepted in revised form 10 April 2007 Available online 18 April 2007
Abstract Flame Assisted Chemical Vapor Deposition (FACVD), a novel technique that shows an enormous potential in porous oxides deposition, was employed for the first time aiming to obtain hydroxyapatite (HA) coatings on 316 L stainless steel metallic substrates. Calcium acetate and ammonium phosphate diluted in ethanol were employed as precursor salts. A Ca/P molar ratio of 1.66 was employed in precursor solution, which is equivalent to stoichiometric hydroxyapatite. A porous coating, formed by an open and interconnected network, was observed by scanning electronic microscopy (SEM) and associated with homogenous reactions. Thickness of hydroxyapatite coating was 412 ± 3 μm. X-ray diffraction (XRD) analysis indicated the presence of crystalline coatings, mainly constituted by hydroxyapatite phase and traces of tricalcium phosphate (β-TCP). Carbonate in the hydroxyapatite coatings was identified by Fourier transform-infrared (FTIR) spectroscopy. © 2007 Elsevier B.V. All rights reserved. Keywords: Biomaterials; Hydroxyapatite; Coatings; Chemical vapour deposition
1. Introduction Bioactive materials have been studied as coatings on bioinert materials since 1970 [1]. Thus, it is possible to combine the bioactivity of materials such as calcium phosphate ceramics with the excellent mechanical properties of metals and its alloys. Hydroxyapatite (HA) – Ca10(PO4)6(OH)2 – was the first calcium phosphate studied for surgical procedures, mainly due to its good compatibility with the inorganic components of bones and teeth [2]. Moreover, HA is a bioactive material that does not cause side effects in the human body. Hydroxyapatite belongs to the apatite family, a group of materials with similar structures, but not necessarily the same composition [3]. Calcium phosphates are usually classified according to the molar ratio between calcium and phosphorus, and HA has a molar ratio equal to 1.66 [4]. HA is a low toughness brittle material, what limits its use when high mechanical stresses are required. For this reason, HA is widely employed as coating on metallic prosthesis and implants, usually over titanium and its alloys or stainless steels.
⁎ Corresponding author. Tel.: +55 51 3316 3637; fax: +55 51 3316 3405. E-mail address: [email protected] (R.M. Trommer). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.04.028
Plasma spray is the commercial technique used to produce HA coatings. However, it has several disadvantages, mainly the use of high cost equipments, production of low crystalline coatings and presence of other phases like α- and β-TCP. For this reason, heat treatment is necessary after plasma spray deposition to avoid solubilization and detachment of HA coating. In this work, Flame Assisted Chemical Vapor Deposition (FACVD) was employed for the first time as an alternative process for the production of crystalline HA coatings on AISI 316 L stainless steel substrates. This technique was firstly described by Hunt and his colleagues [5] in 1993, when the authors obtained ZrO2, BaTiO 3 , YBa 2 Cu 2 O x and Y2BaCuO5 films. FACVD is a relatively new technique, and its main advantage is the use of precursors and low cost equipment. The principles of this process are based on conventional Chemical Vapor Deposition (CVD). Summarily, this deposition process involves the combustion of liquid or gaseous precursors in a flame, where they are vaporized and decomposed, leading to chemical reactions that leads to the formation of a coating over a substrate [6]. The flame plays an important role in the depositions process, and its main function is to supply the necessary energy to promote chemical reactions. The flame is responsible for warming substrate and allows diffusion, leading to a relatively dense coating [5]. When
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compared to conventional CVD techniques, FACVD does not require the use of a reactor or a chamber to promote chemical reactions. Thus, for the specific case of hydroxyapatite, a coating produced with this novel technique probably has low cost when compared with that one obtained with conventional CVD or plasma spray, for example. 2. Experimental 2.1. Substrate preparation AISI 316 L stainless steel discs (19 ± 0.2 mm diameter and 2 ± 0.2 mm thickness) were prepared in a precision cutting machine (Buehler Isomet 2000 model). An orifice was made in their middle in order to place a type-K thermocouple inside it, so the substrate temperature could be monitored during the deposition process. Substrates were polished with a sequence of sandpapers (#80, #160, #320 and #600 grits) to obtain an average roughness (Ra) lower than 0.1 μ. Before depositions they were cleaned with ethanol to avoid detachment during depositions due to contamination. Stainless steel was chosen as substrate because it stands the deposition temperature without oxidizing, it is cheaper than titanium, and it is considered a biomaterial. It can also be used in some cases with or without a coating [1–3,7,8]. 2.2. Precursor solution preparation Calcium acetate (Ca(CH3COO)2 U H2O) (Synth®), in a concentration of 0.0833 M, and ammonium phosphate ((NH4)2HPO4) (Synth®) in a concentration of 0.0500 M were employed as precursors salts. Ca/P molar ratio in the precursor solution was equivalent to biological hydroxyapatite: 1.66. To avoid ammonium phosphate precipitation, nitric acid (HNO3) (Synth®) was used. Ethanol (95%) was employed to dilute precursor salts up to the desired concentration and as fuel in the flame, increasing available energy necessary for chemical reactions.
2.3. Deposition equipment The apparatus developed in laboratory to obtain hydroxyapatite coatings with FACVD technique is shown in Fig. 1. Due to oscillations in the flame caused by the atomization device, it was necessary to control the temperature of the substrate during deposition process, so a cooling system that uses compressed air was employed. The cooler system was useful as a holder for the substrate too. To monitor substrate temperature, a type-K thermocouple linked to a data acquisition (Novus — Field Logger model and Field Chart software) was used. The thermocouple tip was placed at the same level of the substrate surface. Thus, the temperature on the substrate surface was estimated, and the cooling allowed keeping the deposition temperature within a pre-determined range. A Cole–Parmer (Masterflex model) peristaltic pump was employed to control solution precursor flux. Precursor solution was processed in an atomization device, composed of two perpendicular needles. Precursor solution flows through the inner needle (0.6 mm diameter) and the atomization gas (compressed air) flows through the bigger one (1.5 mm diameter). As mentioned before, FACVD employs a flame to supply the required energy for oxide depositions [5]. Thus, a Bunsen burner was used as pilot flame, using propane as combustible and air as oxygen source. Many authors reported the use of an oxy-acetylene flame, but aiming to obtain Diamond like-carbon (DLC) films or coatings [9–11]. For hydroxyapatite depositions, it was not necessary to use an oxyacetylene flame, because the Bunsen burner supplies the necessary energy to promote chemical reactions. When the atomized precursor solution intercepts the pilot flame, another flame is produced, which is called main flame and is obtained from the combustion of precursor solution that contains ethanol. The role of this secondary flame is to supply the required energy to promote chemical reactions that leads to the formation of the coatings. Moreover, the ions calcium and phosphorous are dissolved in the precursor solution, and their combustion is
Fig. 1. FACVD apparatus developed to obtain hydroxyapatite coatings.
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necessary to promote the chemical reactions that leads to the production of hydroxyapatite. After that, a coating is produced on the surface of the substrate. It must be highlighted that most components of the apparatus developed in laboratory have low cost when compared with commercial equipments generally used to produce hydroxyapatite coatings. 2.4. Deposition parameters The distance between the atomization device and the substrate was kept constant (140 mm), and it was defined in pre-tests. This distance was chosen because when the atomization device is placed far from the substrate the temperature on substrate surface is low. This can make the diffusion on the surface or in the first layers of deposited coating difficult. When the atomization device is placed close to the substrate, the combustion of the precursor solution is not complete, and the coating produced is powdery and not well adhered to the substrate [12,13]. The deposition temperature on the substrate was kept constant at 500 ± 5 °C, and it was chosen based on another work developed by the authors with plasma spray [14]. A higher deposition temperature can probably decompose hydroxyapatite to tricalcium phosphate (β-TCP). On the other hand, a low deposition temperature on the substrate can produce an amorphous coating [15,16]. Both β-TCP and amorphous phase can lead to the loss of implant, due to the absorption of the coating by the organism [1–3,7,8]. Deposition time was fixed at 10 min and precursor solution flux was 12 mL/min, both defined in pre-tests. Compressed air was employed as atomization gas, at the pressure of 4 MPa. Deposition parameters are summarized in Table 1. 2.5. Characterization techniques X-ray diffraction analyses were performed to identify mineralogical phases, and a Philips diffractometer (X'Pert MPD model) equipped with a graphite monochromator and a rotative anode of copper operating a 40 kV and 40 mA was used. All measurements were made using CuKα radiation. Scanning rate was 0.05°/min, with a time-step of 5 s in the 20–60° range. Fourier transform infrared (FTIR) spectroscopy was used to identify typical hydroxyapatite chemical groups. Analyses were performed in the range of 400 to 4000 cm− 1, with a Perkin Elmer (Spectrum 1000 model) equipment. It was necessary to remove the coating from the substrate before analysis. After this, samples were prepared as KBr tablets for the analysis. Hydroxyapatite coatings microstructure was evaluated by means of scanning electronic microscopy (SEM) with a Jeol (JSM 5800 model) equipment. The coatings cross sections were Table 1 Deposition parameters employed for hydroxyapatite coatings deposition Atomization device–substrate distance Deposition temperature Deposition time Precursor solution flux Atomization gas pressure
140 mm 500 ± 5 °C 10 min 12 mL/min 4 MPa
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analyzed. Since hydroxyapatite is a brittle material, it was necessary to apply a coating of epoxy resin (Comfibras LTDA) to avoid its fracture during cutting. After the setting of epoxy resin, coatings were cut in a precision cutting machine (Buehler model Isomet 2000), polished and metalized with a thin gold layer to observe its cross section. UTHSCSA Image Tool version 3.00 software, developed by University of Texas Health Science Center, was employed to determine the thickness of the coatings. First, it was necessary to generate a calibration standard by the use of scale bars provided in the SEM micrographs. 3. Results and discussion 3.1. Crystalline phases From the XRD analyses it was possible to identify the major phase hydroxyapatite (JCPDS 09-0432) as well as the austenitic phase of substrate (JCPDS 33-0397), as shown Fig. 2. XRD pattern confirmed that an initial Ca/P molar ratio of 1.66 in the precursor solution led to the production of a hydroxyapatite coating. We believe that the Ca/P molar ratio of the precursor solution is the main reason for obtaining the mineralogical phases in the coatings, considering the specific deposition parameters used in this work and when FACVD was employed. XRD diffraction results confirmed that with the use of low cost precursor salts such as calcium acetate and ammonium phosphate it was possible to obtain hydroxyapatite coating using FACVD technique. Thus, it is not necessary to use starting powders of hydroxyapatite, which are in some cases expensive and need a specific range of particle size and deposition parameters (in the case of plasma spray technique, for example) [17,18]. During hydroxyapatite deposition, calcium ions are supplied by calcium acetate, while phosphate ions are available from ammonium phosphate employed as precursor salt. However, a small amount of β-tricalcium phosphate (βTCP) (JCPDS 09-0169) in the hydroxyapatite coatings was identified, as shown Fig. 2. It is believed that β-TCP phase was produced due to the high temperature in the flame employed in FACVD. Choy [6] reported that FACVD has a wide range of temperatures in the flame, between 1727 and 2727 °C, associated to the deposition parameters. Charojrochkul et al. [13] mentioned that the maximum temperature in the flame during the deposition of porous lanthanum strontium manganese oxide (LSM) to produce the cathode for a solid oxide fuel cell (SOFC) was 1200 °C. Xu et al. [19] showed in their work that the flame temperature depends on the distance between the substrate and the atomization device, and it can range from 1100 to 2400 °C. When the flame temperature is high, thermal decomposition, dehydroxylation and transformation of hydroxyapatite can occur [20]. β-TCP phase is usually identified in hydroxyapatite coatings produced by plasma spray technique [14], due to the high plasma temperature (5000–20,000 K) [21,22]. Hydroxyapatite decomposition reaction in β-TCP is described by Eq. (1). This reaction can usually occur in a temperature range from 1200 to 1450 °C. Ca10 ðPO4 Þ6 ðOHÞ2 Y3Ca3 ðPO4 Þ2 þCaO þ H2 O
ð1Þ
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Fig. 2. X-ray diffraction pattern of a coating produced by FACVD, showing the major phase hydroxyapatite and traces of tricalcium phosphate.
Thus, it was possible that some hydroxyapatite particles were already formed in the flame and during the course of deposition on the substrate surface, being decomposed to β-TCP due to the high flame temperature. 3.2. Microstructure Microstructure of as-deposited hydroxyapatite coating is shown in Fig. 3. It is possible to observe open pores in the coatings, forming an interconnected network. This microstructure is characteristic of coatings and films produced by FACVD, and many authors associated this microstructure to homogenous reactions [13,14]. Briefly, chemical reactions that happen during CVD deposition can be subdivided in two categories: heterogeneous or homogenous. The first one is characterized by reactions that occur near/on the surface of substrate, leading to a dense coating. The further is characterized by reactions that happen in the flame, and produces a porous coating, usually with poor adhesion to the substrate. Thus, after precursor solution combustion, chemical reactions already occurred in the flame, leading to the production of HA particles in the flame. These particles reached the substrate surface, forming agglomerates that led to the formation of a porous coating. Homogenous reactions are favored because precursor solution is basically composed of ethanol. Combustion of ethanol supplies more thermal energy to chemical reactions that produce HA particles, which reach the substrate surface and produces a powdery coating. However, an open structure can allow future penetration of bone tissue into hydroxyapatite coating, creating a stronger biointegration and better mechanical stability of prosthesis or implant with the host tissue [23,24]. Porosity increases surface area of hydroxyapatite coatings, leading to a higher reactivity when compared to a denser coating [1,2]. Thus, it is possible to substitute hydroxyapatite coating for bone tissue, leading to a faster recuperation of the patient. Moreover, a porous coating allows the use of medicine like anti-inflammatory and antibiotics, minimizing inflammatory response of living tissue [4,7,8].
With higher magnification, SEM micrographs revealed that hydroxyapatite coatings produced by FACVD had a microstructure composed of lamellas and spherical particles, as shown in Fig. 4. This microstructure is very similar to thermally sprayed ceramic coatings, composed of lamellas and melted particles [25]. Hydroxyapatite coatings had this microstructure because, as discussed before, some particles were formed in the flame (homogenous reactions) and due to the high temperature these particles melted or were subdivided due to high plasticity (caused by temperature and fluid dynamics in the flame). After the particles reached the substrate or layers of deposited coating with high velocity, they splashed, leading to a structure characterized by lamellas and spherical particles. Coarse and fine pores were observed in the microstructure of hydroxyapatite coatings obtained with FACVD. The first one can be described as macroporosity, while the second one is associated to microporosity. Macroporosity formation mechanism is very similar to plasma spray process, and it is associated to the incomplete contact between particles or lamellas that are already deposited and those ones that reach the metal and/or coating substrate
Fig. 3. SEM image of as-deposited hydroxyapatite coating. It is possible to observe a porous structure, creating an interconnected network. (Magnification: 1000X).
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of 500 ± 5 °C had a thickness of 412 ± 3 μm. For hydroxyapatite coatings, a wide range of thickness is reported in the literature, depending on the technique employed. Usually, hydroxyapatite coatings produced by plasma spray have a thickness between 30 and 200 μm [26]. Wang et al. [27], using electrophoretic deposition, produced hydroxyapatite coatings with a maximum thickness of 50 μm. Blind et al. [28] employed pulsed laser deposition to produce coatings with thickness of 100 nm. Based on deposition time and thickness of the coating, it is possible to calculate deposition rate, expressed by Eq. (2). deposition rate ðμm=minÞ ¼ Fig. 4. SEM image of as-deposited hydroxyapatite coating showing a structure composed of lamellas and spherical particles that are formed in the flame and reached the surface of the substrate. (Magnification: 5000X).
[17,25]. Microporosity is formed by evolution of gases. Since the precursor solution is basically composed of ethanol, and there are chemical reactions in the flame or even near the substrate surface, it is possible that some gases were present in the coatings for a short time. Thus, these gases vaporized during deposition, creating micropores in the coatings. Fig. 5a and b shows the cross section of a hydroxyapatite coating, confirming the presence of macropores or voids in the coatings. As discussed before, macroporosity is associated to the coalescence of the first deposited particles, forming an interconnected network. As the coating becomes thicker, some voids or macropores inside the coating can still exist. Furthermore, we must consider that initial deposition temperature is relatively low. Deposition temperature in the substrate was kept constant (500 ± 5 °C), but at the beginning of deposition it was about 100 °C, increasing up to the desired deposition temperature. This low temperature in the first moments of deposition can benefit the presence of macropores or voids [13,15]. In fact, when the cross section of coatings is analyzed (see Fig. 5b), it is possible to observe that macropores are mainly located close to the substrate surface. It is believed that in the first moments of deposition, atomic diffusion and viscous flux of particles are inhibited due to low temperature in the substrate, which can avoid the coalescence and densification of the first layers [14]. During deposition, more energy is provided to the system because more precursor solution, and consequently more ethanol, is available in the flame. Consequently, temperature in the substrate increases, decreasing the amount of macropores and voids. We believe that a higher temperature at the beginning of deposition can benefit diffusion and viscous flux, allowing the coalescence of particles already deposited and formation of denser coatings with no macropores. Moreover, we must consider that during coating deposition, more reactive species can be incorporated into the coatings, and this is another factor that influences the porosity of coatings. Cross section micrographs revealed that there are no cracks in the HA coatings produced by FACVD, which are typical of plasma sprayed coatings [17]. Hydroxyapatite coatings obtained in 10 min, with a precursor solution flux of 12 mL/min and substrate temperature
thickness ðμmÞ deposition time ðminÞ
ð2Þ
Deposition rate of HA coating produced by FACVD technique was 41 μm/min, considering specific deposition parameters and a constant deposition rate. The value of deposition rate observed in HA coating can be compared to coatings obtained by plasma spray process, which have high deposition rates. This indicates that FACVD is a very efficient process. 3.3. Functional groups Typical functional groups that are associated to hydroxyapatite structure were identified by FTIR spectroscopy, as shown Fig. 6. There are four vibrational modes associated to PO4− 3 ion
Fig. 5. Cross section of hydroxyapatite coating showing macropores or voids near the substrate (a) Magnification: 150X and (b) Magnification: 500X.
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Fig. 6. FTIR spectroscopy identifying characteristic chemical groups of hydroxyapatite compounds.
that can be observed in hydroxyapatite FTIR spectrum [29,30]. Absorption bands ν2 and ν3 at 1000–1200 cm− 1 are attributed to asymmetric stretching, while ν1 absorption band at 950– 960 cm− 1 is associated to symmetric stretching and ν4 at 560– 570 cm− 1 absorption band is attributed to asymmetric bending vibrations. The wide absorption band observed at 3400– 3100 cm− 1 is attributed to water presence in the sample, while the absorption band at 2400 cm− 1 is usually associated to potassium bromide (KBr) used in sample preparation. The small absorption band located at 3670–3570 cm− 1 is associated to OH− ion stretching. Thus, the main characteristic chemical groups of hydroxyapatite were identified by FTIR analysis. However, carbonate groups were identified in the coatings, characterized by absorption bands at 1500–1400 cm− 1 and 875 cm− 1, which are associated to asymmetric stretching and out of plane bending mode, respectively. Table 2 shows the absorption bands, their respective wavenumber and chemical bending. Presence of carbonate in the coatings produced by FACVD is not a problem. It is well known that hydroxyapatite is the most stable and less soluble calcium phosphate material found in human body [1–4]. This property can avoid or decrease bone tissue regeneration when a hydroxyapatite coated
Table 2 Absorption bands and wavenumber of functional groups identified in hydroxyapatite coatings Wavenumber (cm− 1)
Chemical bending
OH H2O KBr
3670–3570 3400–3100 2400
CO2− 3
1500–1400 875 1200–1000 960–950 570–560
Ion stretching Water Weak absorption band attributed to KBr Asymmetric stretching Out of plane bending mode Asymmetric stretching Symmetric stretching Asymmetric bending vibrations
Absorption band −
PO3− 4
prosthesis or implant is present. Usually, a material that has certain solubility is employed, because they allow bone tissue regeneration. This is possible by the use of a carbonated hydroxyapatite, which is a material with similar properties of biological hydroxyapatite [32]. Recent in vivo studies have shown that carbonated hydroxyapatite dissolution rate is intermediate between pure hydroxyapatite (lowest rate) and TCP-β (highest rate) [31]. Carbonate group is probably present in hydroxyapatite coatings due to the fact that depositions are performed in open atmosphere. For this reason, carbonate is incorporated into the coating due to CO2 present in the environment, which reacts with OH − present in precursor solution or atmospheric humidity, as stated in Eq. (3) [33]. CO2 þ 2OH X CO2 3 þ H2 O
ð3Þ
4. Conclusions It was possible to obtain hydroxyapatite coatings using Flame Assisted Chemical Vapor Deposition. Precursor solution composed of calcium acetate and ammonium phosphate, with a Ca/P molar ratio of 1.66 led to the formation of crystalline coatings, with the presence of a major phase of hydroxyapatite and traces of tricalcium phosphate. Hydroxyapatite coatings have an open structure, formed basically by interconnected pores and associated with homogenous reactions. HA coatings had a thickness of 412 ± 3 μm, and the deposition rate was 41 μm/min. Carbonate groups were identified in the hydroxyapatite coatings. Acknowledgements The authors gratefully acknowledge the financial support of the Brazilian Agency CNPq — Conselho Nacional de Desenvolvimento Cientifico e Tecnológico.
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