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Structural and morphological study of electrodeposited calcium phosphate materials submitted to thermal treatment
Accepted Manuscript Structural and morphological study of electrodeposited calcium phosphate materials submitted to thermal treatment Richard Drevet, Joël Faure, Hicham Benhayoune PII: DOI: Reference:
S0167-577X(17)31145-X http://dx.doi.org/10.1016/j.matlet.2017.07.101 MLBLUE 22943
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
6 March 2017 28 June 2017 23 July 2017
Please cite this article as: R. Drevet, J. Faure, H. Benhayoune, Structural and morphological study of electrodeposited calcium phosphate materials submitted to thermal treatment, Materials Letters (2017), doi: http://dx.doi.org/ 10.1016/j.matlet.2017.07.101
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Structural and morphological study of electrodeposited calcium phosphate materials submitted to thermal treatment
Richard DREVET*, Joël FAURE, Hicham BENHAYOUNE Université de Reims Champagne-Ardenne, Laboratoire Ingénierie et Sciences des Matériaux, EA 4695, Bât. 6, Moulin de la Housse, BP 1039, 51687 Reims Cedex 2, France * Corresponding author E-mail: [email protected] Tel.: +33 326 91 36 60
ABSTRACT The effect of thermal treatments on electrodeposited calcium phosphate materials (CaP) is investigated. For this purpose, several temperatures up to 1000°C are applied to powders obtained by scratching the synthesized coatings from their metallic substrate. The goal is to assess the structural and the morphological modifications of the powders that are known to strongly influence the in vivo bioactivity of the CaP materials. The morphology of the CaP materials is made of needles at room temperature that agglomerate to form rounded particles when the treatment temperature exceeds 800°C. Simultaneously, the crystallinity increases with the treatment temperature until reaching a fully crystalline biphasic structure at 1000°C. This biphasic structure consists in 80% Hydroxyapatite (HAP) and 20% tricalcium phosphate ( -TCP).
Keywords: calcium phosphate; pulsed electrodeposition; transmission electron microscopy; thermal treatment; hydroxyapatite; tricalcium phosphate
1
1- Introduction The titanium alloys, such as Ti6Al4V, are widely used as implant materials in orthopaedic and dental surgery. To improve their bioactivity inside the body, they are partially coated with bioactive bioceramics that belong to the calcium phosphate family (CaP) [1]. Several methods can be employed to produce this coating as plasma spray [2], sol–gel [3] or electrodeposition [4]. This last one is very attractive to synthesize homogeneous coatings with a full control of their chemical composition and thickness even on complex surface substrates. However, as this process is carried out in aqueous medium, a thermal treatment is required after the deposition to evaporate the solvent trapped inside the coating at the outlet of the electrochemical cell. This thermal treatment also aims at improving the mechanical properties of the coating mainly its compactness and its adhesion to the substrate [5]. Depending on the expected properties after the thermal treatment, various temperatures are described in the literature. For example, Shirkhanzadeh et al. applied 425°C and described a coating with a low crystallinity and much porosity [6]. Wang et al. used temperatures up to 650°C to improve the bonding strength between the coating and the substrate [7]. At last, Kuo and Yen described at 700°C, improved mechanical properties of the calcium phosphate coating and some phases’ changes observed from TGA/TDA results [8]. In a recent work, we have demonstrated that the treatment temperature is limited to 550°C to avoid the deterioration of the Ti6Al4V substrate due to its surface oxidation [9]. Therefore, to study the structure and the morphology of the CaP materials at higher temperatures it is necessary to use the powder scratched from the substrate. These properties are known to impact extremely the in vivo bioactivity of the material thus it is necessary to characterize them with the greatest accuracy. The present work aims at studying the influence of several temperatures up to 1000°C on the structural and morphological properties of the electrodeposited CaP materials. For that
2
purpose, X-ray diffraction (XRD), Transmission Electron Microscopy associated to Selected Area Electron Diffraction (TEM-SAED) are used.
2- Materials and Methods 2-1 Electrodeposition and thermal treatment of the calcium phosphate materials The CaP materials are electrodeposited on Ti6Al4V substrates in an electrolyte solution at 60°C prepared by dissolving 0.042 M Ca(NO3)2.4H2O and 0.025 M NH4(H2PO4) in ultra-pure water mixed with hydrogen peroxide (H2O2, 6% in volume). The current density is applied in 5 pulsed cycles, each one at -15 mA/cm² during 1 min followed by a break (0 mA/cm²) during 2 min. These breaks during deposition strongly reduce the H2 bubble emission at the working electrode and allow ion concentrations to be homogenized in the solution. This process is known to produce uniform and adherent calcium phosphate coatings on titanium alloy substrate [10-12]. After electrodeposition, the CaP materials are scratched from the substrates. The obtained powders are thermally treated in two steps according to the protocol proposed by Vallet-Regi and Gonzalez-Calbet [13]. A first plateau at 120°C during 1 hour is followed by a second one at the selected temperature (300, 600, 800 or 1000 °C) during 1 hour. After treatment, the powders are cooled inside the furnace until room temperature.
2-2 Characterization of the calcium phosphate materials The phase composition and the crystallinity of the powders are investigated by XRD from a Bruker D8 Advance using a monochromatic Cu Kα radiation. The phases are identified from the Powder Diffraction Files (PDF) provided by the International Centre for Diffraction Data (ICDD), # 09-0432 for hydroxyapatite (HAP) and # 09-0169 for -tricalcium phosphate (TCP). The crystallinity of the materials is assessed according to the international standard ISO 13779-3 by comparison with a fully crystalline reference (its crystallinity rate is 100%) [14].
3
This international standard also provides a specific method to quantify from the XRD patterns the weight amount of the identified phases (minimum 10%) and then the corresponding Ca/P atomic ratio. Firstly several standard mixtures of commercial HAP and commercial -TCP are characterized by XRD (Fig.1a). For each XRD pattern, the coefficient R is determined from the equation below as the ratio between the areas under the most intense and isolated peak of each phase. According to international standard, it corresponds to the (2.1.0.) peak for HAP at 2θ = 28.9° and the (0.2.10.) peak for -TCP at 2θ = 31.0° [14].
Next, each obtained R value is reported inside a graph as a function of its corresponding TCP weight amount. From these points, the abacus is built (Fig.1b). Similarly, the abacus that provides the Ca/P atomic ratio is built by knowing the amount of HAP (Ca/P = 1.67) and TCP (Ca/P = 1.5) in each mixture (Fig.1c). At last, the morphology of the powders is studied at a nanometre level with a TEM Philips CM30. The powders are collected on copper grids, coated with a conductive layer of carbon and then observed in STEM and TEM modes by using a voltage of 250 kV. Furthermore, SAED is also carried out to study the crystallinity of the material at a nanometre level.
3- Results The XRD patterns presented in Fig.2 indicate that without any thermal treatment, the asdeposited material has a very low crystallinity. The synthesized CaP material has an apatite structure the crystallinity of which progressively increases with the temperature used during the thermal treatment (Table.1). This low crystalline phase is commonly described as calcium-deficient hydroxyapatite (Ca-def HAP) [6]. The corresponding STEM micrographs indicate that from room temperature to 600°C the Ca-def HAP material is made of grains with
4
some small needles of about 0.2 µm in size on its top. After the thermal treatment at 800°C and 1000°C, the XRD patterns of the CaP materials highlight the emergence of new crystalline peaks. Their identification indicates the formation of a new phase (-TCP) together with a well-defined HAP. Particularly at 1000°C, the diffraction peaks of the two phases are clearly apparent, pointing out the almost fully crystalline nature of the biphasic CaP material. The corresponding coefficient R is reported inside the abaci of Fig.1. The obtained value is R = 0.72. Thus, the CaP material is identified to be a mixture of 80% HAP and 20% -TCP, i.e. its Ca/P atomic ratio is 1.62. The STEM images of Fig.2 show that the morphology of the CaP needles previously observed has been modified into rounded particles with a smooth surface. More particularly these images evidence the increase of the grain size of the CaP material from 800°C to 1000°C as observed from the corresponding narrowing of the XRD peaks that became very thin after the thermal treatment at 1000°C. The TEM micrographs of Fig.3 also illustrate the effect of the thermal treatment on the morphology of the CaP materials. This accurate characterization indicates that without any thermal treatment, the CaP material is made of submicrometric needles stacked one upon another. On the other hand, after the thermal treatment at 1000°C, the shape of the particles is modified; they become rounded but remain submicrometric in size. Moreover, before the thermal treatment, the corresponding SAED pattern is composed of diffused rings slightly pointed that are characteristics of the low crystallinity of the powder. After the thermal treatment at 1000°C, the SAED pattern reveals numerous well pointed diffraction rings thinner than those previously observed. This observation clearly indicates the increasing crystallinity of the powder that became a biphasic polycrystalline material after the thermal treatment at high temperature. These localized observations confirm the global crystalline structure changes highlighted from the XRD results. Indeed, the number of diffraction plans similarly increases due to the improvement of the crystallinity and due to the biphasic nature of the obtained material. Moreover, the SAED
5
pattern reveals an increase of the diffraction rings number related to the increase of the diffraction plans number as observed by XRD. The diffraction rings are better defined after thermal treatment, becoming thinner like the diffraction peaks observed by XRD. Thus, SAED clearly reveals that the crystallinity of the CaP materials is improved after the thermal treatment.
4- Discussion This work describes the structural and morphological modifications of a CaP powder induced by thermal treatments up to 1000°C. Since the powder is obtained by scratching an electrodeposited coating, the same morphological and structural modifications are expected for this coating made of agglomerated grains of powders. Therefore, the highlighted modifications will occur similarly at the same temperatures. It is observed that the crystallinity of the material increases with the temperature until reaching a fully crystalline biphasic structure. The assessment of the crystallinity of the material is essential since it is well established in the literature that the amorphous CaP materials are rapidly dissolved in the physiological environment [15]. If the dissolution of the CaP material is too fast, the chemical interactions between the implant and the bone tissue can’t take place adequately, resulting in some anchorage failures to the bone [16]. According to the thermal treatment temperature, the level of crystallinity of the CaP material can be selected, providing a control of the kinetics of the chemical interactions inside the body. More particularly from 800°C, the biphasic nature of the material is evidenced with the observation of two CaP phases, HAP and -TCP (80% and 20% respectively). The following reaction intends to describe the chemical process that occurs from this temperature [17,18]:
6
Ca10-x(HPO4)x(PO4)6-x(OH)2-x Ca-def HAP
(1-x) Ca10(PO4)6(OH)2 + (3x) Ca3(PO4)2 + (x) H2O (1-x) HAP
+ (3x) -TCP
+ (x) H2O
with 0 ≤ x ≤ 1
These two CaP phases are known to have different solubility product constants in physiological environment [16]. The dissolution of -TCP is faster than that of HAP. Therefore the biphasic material supports an appropriate re-precipitation of the bone-like apatite on the surface on the implant. At last, the bone cells growth completes the bioactivity process for a total integration of the implant inside the body [15]. The morphology of the material is also greatly modifies by the thermal treatment. This aspect has to be considered with a careful attention since its influence on the bone cell growth is established [19]. Lee et al. have shown that the cell adhesion, spreading, proliferation, and differentiation depend on the CaP surface morphology. They point out that the cellular response is more active and more pronounced on the smoother surfaces than on the rough surfaces [20]. More specifically, Cairns et al. have demonstrated that a regular smooth topography significantly increases the osteocalcin expression and the alkaline phosphatase activity, promoting the bone cell growth in comparison with rough surfaces made of needles [21]. Consequently, according to these observations, thermal treatment temperatures upper than 800°C provide a more suitable morphology of the electrodeposited CaP material. Moreover, many interesting processes are developed and described in literature to reach temperatures up to 1000°C without any severe deterioration of titanium alloys [22,23]. In this way, we develop a new procedure under controlled atmosphere without scratching the powder called THUCA [24]. In the next future, this process will be applied to the system presented in this work to obtain biphasic coatings (HAP and -TCP) on Ti6Al4V substrates.
7
5- Conclusion This work has shown that the thermal treatment of electrodeposited CaP materials is optimized for temperatures upper than 800°C. From this temperature, the crystallinity and the morphology of the material are more appropriate to promote the best biological response and cell behaviours at the surface of the prosthesis material in vivo. Consequently, the electrochemical process is adequately completed with this thermal treatment to produce a suitable implant with the best physico-chemical properties.
References [1] S.V. Dorozhkin, Mater. Sci. Eng. C 55 (2015) 272-326. [2] R.B. Heimann, Surf. Coat. Technol. 201 (2006) 2012-2019. [3] A. Bigi, E. Boanini, K. Rubini, J. Solid State Chem. 177 (2004) 3092-3098. [4] M. Ibrahim Coşkun, I.H. Karahan, Y. Yücel, T.D. Golden, Surf. Coat. Technol. 301 (2016) 42-53. [5] E. Mohseni, E. Zalnezhad, A.R. Bushroa, Int. J. Adhes. Adhes 48 (2014) 238-257. [6] M. Shirkhanzadeh, J. Mater. Sci.: Mater. Med. 6 (1995) 90-93. [7] J. Wang, Y. Chao, Q. Wan, Z. Zhu, H. Yu, Acta Biomater. 5 (2009) 1798-1807. [8] M.C. Kuo, S.K. Yen, Mater. Sci. Eng. C 20 (2002) 153-160. [9] R. Drevet, J. Fauré, H. Benhayoune, Adv. Eng. Mater. 14 (2012) 377-382. [10] H. Benhayoune, R. Drevet, J. Fauré, S. Potiron, T. Gloriant, H. Oudadesse, D. LaurentMaquin, Adv. Eng. Mater. 12 (2010) B192-B199. [11] R. Drevet, H. Benhayoune, L. Wortham, S. Potiron, J. Douglade, D. Laurent-Maquin, Mater. Charact 61 (2010) 786-795.
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[12] R. Drevet, H. Benhayoune, Electrochemical deposition of calcium phosphate coatings on a prosthetic titanium alloy substrate, in: R.B. Heimann (Eds.), Calcium Phosphate: Structure, synthesis, properties and applications, Nova Science Publishers Inc., New York, 2012, pp. 231-252. [13] M. Vallet-Regi, J.M. Gonzalez-Calbet, Progr. Solid State Chem. 32 (2004) 1–31. [14] International Organization for Standardization. Implants for surgery - Hydroxyapatite Part 3, ISO 13779-3 (2008) 1-19. [15] H. Yang, K. Xia, T. Wang, J. Niu, Y. Song, Z. Xiong, K. Zheng, S. Wei, W. Lu, J. Alloys Compd. 672 (2016) 366-373. [16] S.V. Dorozhkin, Biomaterials 31 (2010) 1465-1485. [17] R.Z. Legeros, S. Lin, R. Rohanizadeh, D. Mijares, J.P. Legeros, J. Mater. Sci.: Mater. Med. 14 (2003) 201-209. [18] S.V. Dorozhkin, Acta Biomater. 8 (2012) 963-977. [19] T. Iwamoto, Y. Hieda, Y. Kogai, Mater. Sci. Eng. C 69 (2016) 1263-1267. [20] W.K. Lee, S.M. Lee, H.M. Kim, J. Ind. Eng. Chem. 15 (2009) 677-682. [21] M.L. Cairns, B.J. Meenan, G.A. Burke, A.R. Boyd, Colloids Surf. B Biointerfaces 78 (2010) 283-290. [22] M. Wang, Y. Wu, S. Lu, T. Chen, Y. Zhao, H. Chen, Z. Tang, Prog. Nat. Sci. Mater. Int. 26 (2016) 671-677. [23] S. Leuders, M. Thöne, A. Riemer, T. Niendorf, T. Tröster, H.A. Richard, H.J. Maier, Int. J. Fatigue 48 (2013) 300-307. [24] N. Ben Jaber, R. Drevet, J. Faure, C. Demangel, S. Potiron, A. Tara, A. Ben Cheikh Larbi, H. Benhayoune, Adv. Eng. Mater. 17 (2015) 1608-1615.
9
Figure captions
Fig.1. (a) XRD patterns of the four mixtures used to build the two abaci to determine (b) the weight amount of -TCP and (c) the Ca/P atomic ratio of a HAP / -TCP mixture.
Fig.2. XRD patterns and STEM micrographs of the electrodeposited calcium phosphate material as a function of the thermal treatment temperature.
Fig.3. TEM micrographs and SAED patterns of the electrodeposited calcium phosphate material (a) before the thermal treatment and (b) after the thermal treatment at 1000 °C.
10
Fig.1.
11
Fig.2.
12
Fig.3.
13
Table.1. Structural and morphological properties of the electrodeposited calcium phosphate materials as a function of the thermal treatment temperature
Thermal treatment temperature
Crystalline phase
Level of crystallinity
1000 °C
HAP + -TCP
96 %
800 °C
HAP + -TCP
76 %
600 °C
Ca-def HAP
64 %
Needles
300 °C
Ca-def HAP
57 %
Needles
no thermal treatment
Ca-def HAP
29 %
Needles
14
Morphology Rounded particles with a smooth surface Rounded particles with a smooth surface
Highlights -
The influence of the thermal treatment temperature on electrodeposited calcium phosphate materials is assessed.
-
The morphology of the calcium phosphate materials is modified from 800°C.
-
The crystallinity of the calcium phosphate materials increases with the temperature.
-
A fully crystalline biphasic structure made of HAP and -TCP is obtained at 1000°C.
15
S0167-577X(17)31145-X http://dx.doi.org/10.1016/j.matlet.2017.07.101 MLBLUE 22943
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
6 March 2017 28 June 2017 23 July 2017
Please cite this article as: R. Drevet, J. Faure, H. Benhayoune, Structural and morphological study of electrodeposited calcium phosphate materials submitted to thermal treatment, Materials Letters (2017), doi: http://dx.doi.org/ 10.1016/j.matlet.2017.07.101
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Structural and morphological study of electrodeposited calcium phosphate materials submitted to thermal treatment
Richard DREVET*, Joël FAURE, Hicham BENHAYOUNE Université de Reims Champagne-Ardenne, Laboratoire Ingénierie et Sciences des Matériaux, EA 4695, Bât. 6, Moulin de la Housse, BP 1039, 51687 Reims Cedex 2, France * Corresponding author E-mail: [email protected] Tel.: +33 326 91 36 60
ABSTRACT The effect of thermal treatments on electrodeposited calcium phosphate materials (CaP) is investigated. For this purpose, several temperatures up to 1000°C are applied to powders obtained by scratching the synthesized coatings from their metallic substrate. The goal is to assess the structural and the morphological modifications of the powders that are known to strongly influence the in vivo bioactivity of the CaP materials. The morphology of the CaP materials is made of needles at room temperature that agglomerate to form rounded particles when the treatment temperature exceeds 800°C. Simultaneously, the crystallinity increases with the treatment temperature until reaching a fully crystalline biphasic structure at 1000°C. This biphasic structure consists in 80% Hydroxyapatite (HAP) and 20% tricalcium phosphate ( -TCP).
Keywords: calcium phosphate; pulsed electrodeposition; transmission electron microscopy; thermal treatment; hydroxyapatite; tricalcium phosphate
1
1- Introduction The titanium alloys, such as Ti6Al4V, are widely used as implant materials in orthopaedic and dental surgery. To improve their bioactivity inside the body, they are partially coated with bioactive bioceramics that belong to the calcium phosphate family (CaP) [1]. Several methods can be employed to produce this coating as plasma spray [2], sol–gel [3] or electrodeposition [4]. This last one is very attractive to synthesize homogeneous coatings with a full control of their chemical composition and thickness even on complex surface substrates. However, as this process is carried out in aqueous medium, a thermal treatment is required after the deposition to evaporate the solvent trapped inside the coating at the outlet of the electrochemical cell. This thermal treatment also aims at improving the mechanical properties of the coating mainly its compactness and its adhesion to the substrate [5]. Depending on the expected properties after the thermal treatment, various temperatures are described in the literature. For example, Shirkhanzadeh et al. applied 425°C and described a coating with a low crystallinity and much porosity [6]. Wang et al. used temperatures up to 650°C to improve the bonding strength between the coating and the substrate [7]. At last, Kuo and Yen described at 700°C, improved mechanical properties of the calcium phosphate coating and some phases’ changes observed from TGA/TDA results [8]. In a recent work, we have demonstrated that the treatment temperature is limited to 550°C to avoid the deterioration of the Ti6Al4V substrate due to its surface oxidation [9]. Therefore, to study the structure and the morphology of the CaP materials at higher temperatures it is necessary to use the powder scratched from the substrate. These properties are known to impact extremely the in vivo bioactivity of the material thus it is necessary to characterize them with the greatest accuracy. The present work aims at studying the influence of several temperatures up to 1000°C on the structural and morphological properties of the electrodeposited CaP materials. For that
2
purpose, X-ray diffraction (XRD), Transmission Electron Microscopy associated to Selected Area Electron Diffraction (TEM-SAED) are used.
2- Materials and Methods 2-1 Electrodeposition and thermal treatment of the calcium phosphate materials The CaP materials are electrodeposited on Ti6Al4V substrates in an electrolyte solution at 60°C prepared by dissolving 0.042 M Ca(NO3)2.4H2O and 0.025 M NH4(H2PO4) in ultra-pure water mixed with hydrogen peroxide (H2O2, 6% in volume). The current density is applied in 5 pulsed cycles, each one at -15 mA/cm² during 1 min followed by a break (0 mA/cm²) during 2 min. These breaks during deposition strongly reduce the H2 bubble emission at the working electrode and allow ion concentrations to be homogenized in the solution. This process is known to produce uniform and adherent calcium phosphate coatings on titanium alloy substrate [10-12]. After electrodeposition, the CaP materials are scratched from the substrates. The obtained powders are thermally treated in two steps according to the protocol proposed by Vallet-Regi and Gonzalez-Calbet [13]. A first plateau at 120°C during 1 hour is followed by a second one at the selected temperature (300, 600, 800 or 1000 °C) during 1 hour. After treatment, the powders are cooled inside the furnace until room temperature.
2-2 Characterization of the calcium phosphate materials The phase composition and the crystallinity of the powders are investigated by XRD from a Bruker D8 Advance using a monochromatic Cu Kα radiation. The phases are identified from the Powder Diffraction Files (PDF) provided by the International Centre for Diffraction Data (ICDD), # 09-0432 for hydroxyapatite (HAP) and # 09-0169 for -tricalcium phosphate (TCP). The crystallinity of the materials is assessed according to the international standard ISO 13779-3 by comparison with a fully crystalline reference (its crystallinity rate is 100%) [14].
3
This international standard also provides a specific method to quantify from the XRD patterns the weight amount of the identified phases (minimum 10%) and then the corresponding Ca/P atomic ratio. Firstly several standard mixtures of commercial HAP and commercial -TCP are characterized by XRD (Fig.1a). For each XRD pattern, the coefficient R is determined from the equation below as the ratio between the areas under the most intense and isolated peak of each phase. According to international standard, it corresponds to the (2.1.0.) peak for HAP at 2θ = 28.9° and the (0.2.10.) peak for -TCP at 2θ = 31.0° [14].
Next, each obtained R value is reported inside a graph as a function of its corresponding TCP weight amount. From these points, the abacus is built (Fig.1b). Similarly, the abacus that provides the Ca/P atomic ratio is built by knowing the amount of HAP (Ca/P = 1.67) and TCP (Ca/P = 1.5) in each mixture (Fig.1c). At last, the morphology of the powders is studied at a nanometre level with a TEM Philips CM30. The powders are collected on copper grids, coated with a conductive layer of carbon and then observed in STEM and TEM modes by using a voltage of 250 kV. Furthermore, SAED is also carried out to study the crystallinity of the material at a nanometre level.
3- Results The XRD patterns presented in Fig.2 indicate that without any thermal treatment, the asdeposited material has a very low crystallinity. The synthesized CaP material has an apatite structure the crystallinity of which progressively increases with the temperature used during the thermal treatment (Table.1). This low crystalline phase is commonly described as calcium-deficient hydroxyapatite (Ca-def HAP) [6]. The corresponding STEM micrographs indicate that from room temperature to 600°C the Ca-def HAP material is made of grains with
4
some small needles of about 0.2 µm in size on its top. After the thermal treatment at 800°C and 1000°C, the XRD patterns of the CaP materials highlight the emergence of new crystalline peaks. Their identification indicates the formation of a new phase (-TCP) together with a well-defined HAP. Particularly at 1000°C, the diffraction peaks of the two phases are clearly apparent, pointing out the almost fully crystalline nature of the biphasic CaP material. The corresponding coefficient R is reported inside the abaci of Fig.1. The obtained value is R = 0.72. Thus, the CaP material is identified to be a mixture of 80% HAP and 20% -TCP, i.e. its Ca/P atomic ratio is 1.62. The STEM images of Fig.2 show that the morphology of the CaP needles previously observed has been modified into rounded particles with a smooth surface. More particularly these images evidence the increase of the grain size of the CaP material from 800°C to 1000°C as observed from the corresponding narrowing of the XRD peaks that became very thin after the thermal treatment at 1000°C. The TEM micrographs of Fig.3 also illustrate the effect of the thermal treatment on the morphology of the CaP materials. This accurate characterization indicates that without any thermal treatment, the CaP material is made of submicrometric needles stacked one upon another. On the other hand, after the thermal treatment at 1000°C, the shape of the particles is modified; they become rounded but remain submicrometric in size. Moreover, before the thermal treatment, the corresponding SAED pattern is composed of diffused rings slightly pointed that are characteristics of the low crystallinity of the powder. After the thermal treatment at 1000°C, the SAED pattern reveals numerous well pointed diffraction rings thinner than those previously observed. This observation clearly indicates the increasing crystallinity of the powder that became a biphasic polycrystalline material after the thermal treatment at high temperature. These localized observations confirm the global crystalline structure changes highlighted from the XRD results. Indeed, the number of diffraction plans similarly increases due to the improvement of the crystallinity and due to the biphasic nature of the obtained material. Moreover, the SAED
5
pattern reveals an increase of the diffraction rings number related to the increase of the diffraction plans number as observed by XRD. The diffraction rings are better defined after thermal treatment, becoming thinner like the diffraction peaks observed by XRD. Thus, SAED clearly reveals that the crystallinity of the CaP materials is improved after the thermal treatment.
4- Discussion This work describes the structural and morphological modifications of a CaP powder induced by thermal treatments up to 1000°C. Since the powder is obtained by scratching an electrodeposited coating, the same morphological and structural modifications are expected for this coating made of agglomerated grains of powders. Therefore, the highlighted modifications will occur similarly at the same temperatures. It is observed that the crystallinity of the material increases with the temperature until reaching a fully crystalline biphasic structure. The assessment of the crystallinity of the material is essential since it is well established in the literature that the amorphous CaP materials are rapidly dissolved in the physiological environment [15]. If the dissolution of the CaP material is too fast, the chemical interactions between the implant and the bone tissue can’t take place adequately, resulting in some anchorage failures to the bone [16]. According to the thermal treatment temperature, the level of crystallinity of the CaP material can be selected, providing a control of the kinetics of the chemical interactions inside the body. More particularly from 800°C, the biphasic nature of the material is evidenced with the observation of two CaP phases, HAP and -TCP (80% and 20% respectively). The following reaction intends to describe the chemical process that occurs from this temperature [17,18]:
6
Ca10-x(HPO4)x(PO4)6-x(OH)2-x Ca-def HAP
(1-x) Ca10(PO4)6(OH)2 + (3x) Ca3(PO4)2 + (x) H2O (1-x) HAP
+ (3x) -TCP
+ (x) H2O
with 0 ≤ x ≤ 1
These two CaP phases are known to have different solubility product constants in physiological environment [16]. The dissolution of -TCP is faster than that of HAP. Therefore the biphasic material supports an appropriate re-precipitation of the bone-like apatite on the surface on the implant. At last, the bone cells growth completes the bioactivity process for a total integration of the implant inside the body [15]. The morphology of the material is also greatly modifies by the thermal treatment. This aspect has to be considered with a careful attention since its influence on the bone cell growth is established [19]. Lee et al. have shown that the cell adhesion, spreading, proliferation, and differentiation depend on the CaP surface morphology. They point out that the cellular response is more active and more pronounced on the smoother surfaces than on the rough surfaces [20]. More specifically, Cairns et al. have demonstrated that a regular smooth topography significantly increases the osteocalcin expression and the alkaline phosphatase activity, promoting the bone cell growth in comparison with rough surfaces made of needles [21]. Consequently, according to these observations, thermal treatment temperatures upper than 800°C provide a more suitable morphology of the electrodeposited CaP material. Moreover, many interesting processes are developed and described in literature to reach temperatures up to 1000°C without any severe deterioration of titanium alloys [22,23]. In this way, we develop a new procedure under controlled atmosphere without scratching the powder called THUCA [24]. In the next future, this process will be applied to the system presented in this work to obtain biphasic coatings (HAP and -TCP) on Ti6Al4V substrates.
7
5- Conclusion This work has shown that the thermal treatment of electrodeposited CaP materials is optimized for temperatures upper than 800°C. From this temperature, the crystallinity and the morphology of the material are more appropriate to promote the best biological response and cell behaviours at the surface of the prosthesis material in vivo. Consequently, the electrochemical process is adequately completed with this thermal treatment to produce a suitable implant with the best physico-chemical properties.
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[12] R. Drevet, H. Benhayoune, Electrochemical deposition of calcium phosphate coatings on a prosthetic titanium alloy substrate, in: R.B. Heimann (Eds.), Calcium Phosphate: Structure, synthesis, properties and applications, Nova Science Publishers Inc., New York, 2012, pp. 231-252. [13] M. Vallet-Regi, J.M. Gonzalez-Calbet, Progr. Solid State Chem. 32 (2004) 1–31. [14] International Organization for Standardization. Implants for surgery - Hydroxyapatite Part 3, ISO 13779-3 (2008) 1-19. [15] H. Yang, K. Xia, T. Wang, J. Niu, Y. Song, Z. Xiong, K. Zheng, S. Wei, W. Lu, J. Alloys Compd. 672 (2016) 366-373. [16] S.V. Dorozhkin, Biomaterials 31 (2010) 1465-1485. [17] R.Z. Legeros, S. Lin, R. Rohanizadeh, D. Mijares, J.P. Legeros, J. Mater. Sci.: Mater. Med. 14 (2003) 201-209. [18] S.V. Dorozhkin, Acta Biomater. 8 (2012) 963-977. [19] T. Iwamoto, Y. Hieda, Y. Kogai, Mater. Sci. Eng. C 69 (2016) 1263-1267. [20] W.K. Lee, S.M. Lee, H.M. Kim, J. Ind. Eng. Chem. 15 (2009) 677-682. [21] M.L. Cairns, B.J. Meenan, G.A. Burke, A.R. Boyd, Colloids Surf. B Biointerfaces 78 (2010) 283-290. [22] M. Wang, Y. Wu, S. Lu, T. Chen, Y. Zhao, H. Chen, Z. Tang, Prog. Nat. Sci. Mater. Int. 26 (2016) 671-677. [23] S. Leuders, M. Thöne, A. Riemer, T. Niendorf, T. Tröster, H.A. Richard, H.J. Maier, Int. J. Fatigue 48 (2013) 300-307. [24] N. Ben Jaber, R. Drevet, J. Faure, C. Demangel, S. Potiron, A. Tara, A. Ben Cheikh Larbi, H. Benhayoune, Adv. Eng. Mater. 17 (2015) 1608-1615.
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Figure captions
Fig.1. (a) XRD patterns of the four mixtures used to build the two abaci to determine (b) the weight amount of -TCP and (c) the Ca/P atomic ratio of a HAP / -TCP mixture.
Fig.2. XRD patterns and STEM micrographs of the electrodeposited calcium phosphate material as a function of the thermal treatment temperature.
Fig.3. TEM micrographs and SAED patterns of the electrodeposited calcium phosphate material (a) before the thermal treatment and (b) after the thermal treatment at 1000 °C.
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Fig.1.
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Fig.2.
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Fig.3.
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Table.1. Structural and morphological properties of the electrodeposited calcium phosphate materials as a function of the thermal treatment temperature
Thermal treatment temperature
Crystalline phase
Level of crystallinity
1000 °C
HAP + -TCP
96 %
800 °C
HAP + -TCP
76 %
600 °C
Ca-def HAP
64 %
Needles
300 °C
Ca-def HAP
57 %
Needles
no thermal treatment
Ca-def HAP
29 %
Needles
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Morphology Rounded particles with a smooth surface Rounded particles with a smooth surface
Highlights -
The influence of the thermal treatment temperature on electrodeposited calcium phosphate materials is assessed.
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The morphology of the calcium phosphate materials is modified from 800°C.
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The crystallinity of the calcium phosphate materials increases with the temperature.
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A fully crystalline biphasic structure made of HAP and -TCP is obtained at 1000°C.
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