hydroxyapatite composite coating fabricated by cold spraying

Materials Letters 250 (2019) 197–201

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A novel bioactive Ta/hydroxyapatite composite coating fabricated by cold spraying Junrong Tang a,b, Zhipo Zhao a,b, Housheng Liu a,b, Xinyu Cui a, Jiqiang Wang a,⇑, Tianying Xiong a,⇑ a b

Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, PR China School of Materials Science and Engineering, University of Science and Technology of China, Shenyang 110016, PR China

a r t i c l e

i n f o

Article history: Received 26 February 2019 Received in revised form 4 April 2019 Accepted 30 April 2019 Available online 30 April 2019 Keywords: Biomaterials Thick films Cold spraying Composite coating Ta/hydroxyapatite Simulated body fluid

a b s t r a c t To avoid the decomposition of hydroxyapatite (HA) and thus further increase its bioactivity, a novel bioactive Ta/HA composite coating (23 lm) is firstly fabricated by cold spraying mechanical milling Ta/HA composite powders. The results indicate that HA without decomposition and phase transformation is well contained and evenly distributed in the composite powders and coating. The simulated body fluid (SBF) test shows that bone-like apatite layer is mineralized on the surface of composite coating after 1– 3 days. It demonstrates that the as-prepared Ta/HA composite coating has high bioactivity. Ó 2019 Elsevier B.V. All rights reserved.

1. Introduction Hydroxyapatite [Ca10(PO4)6(OH)2, shortened to HA] has been widely used in dental and orthopedic implants due to its chemical and crystallographic similarity with bone minerals [1–3]. Studies reveal that once HA is implanted, it has the ability to bond directly to the bone, achieving earlier and greater fixation and then reducing the healing time. Various techniques have been employed to deposit HA coatings. Among them, plasma spraying is one of the most common ways to achieve bioactive surfaces for orthopedic implant prosthesis. Although it offers high deposition rates with low cost, the long term stability of plasma sprayed HA is still questionable [4,5]. This is mainly because HA may be converted into other calcium phosphate phases and the crystallinity of HA may also be lowered due to the high temperature of plasma spray process [6,7]. Moreover, the significance of coating resorption is controversial. It has been suggested that resorption disintegrates the coating and reduces the bonding strength between implant and

bone and the strength of the coating-implant interface, which might lead to implant loosening, coating delamination and acceleration of third body wear processes [8,9]. Compared to plasma spray, the temperature of cold spraying is well below the melting points of the sprayed materials [10,11]. It overcomes a number of traditional thermal spray ‘shortcomings’, such as undesirable phase transitions and decomposition [6,7]. Unfortunately, HA being a kind of ceramic lacks deformability and cannot be directly deposited by cold spraying. Therefore, it is a suitable method by cold spraying HA with a ductile metal. Among many kinds of metals, tantalum (Ta) is gaining more attention recently as a new metallic biomaterial [12,13]. It has excellent chemical properties and corrosion resistance. Meanwhile, Ta can be adapted to biological cells with excellent affinity [14–16]. Therefore, it is feasible to prepare Ta/HA composite coating by cold spraying. Furthermore, the prepared composite coating is expected to have excellent bioactivity. 2. Experimental

⇑ Corresponding authors. E-mail addresses: [email protected] (J. Wang), [email protected] (T. Xiong). https://doi.org/10.1016/j.matlet.2019.04.123 0167-577X/Ó 2019 Elsevier B.V. All rights reserved.

The powders used in experiments are mixture of commercial pure Ta powders (Beijing Xing Rong Yuan Technology Co., Ltd., Beijing, China) and hydrothermal agglomerated (prepared in our laboratory) nanocrystalline (NC) HA powders (Nanjing Ai Pu Rui Nano-materials Co., Ltd., China), obtained by mechanical milling

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using agate mortar for 20 min, in which the mass percentage of HA was about 10 wt%. The Ti6Al4V (TC4) substrate was sandblasted using SiO2 grit prior to cold spray deposition. A self-assembly facility was used for cold spraying. Compressed air was used as the process gas as well as the powder carrier gas. The process gas pressure and temperature were maintained at 2.2 ± 0.2 MPa and 450 ± 10 °C, respectively. A pure Ta buffer layer (200 lm) was also prepared by cold spraying under the same parameters to improve the adhesion between Ta/HA composite coating and substrate. The bonding strength between the coating and the substrate was determined using a tensile adhesive test [17]. The simulated body fluid (SBF) test [18] was adopted for a preliminary bioactivity evaluation. The preparation and composition of SBF can be referred to references [18] and [19]. Then 10 mm  10 mm  2 mm specimens were soaked in 30 mL of SBF. A scanning electron microscope (SEM, FEI INSPECT F50) equipped with energy dispersive X-ray (EDX) unit was used for observation and element analysis of the coating and powders. Size distributions of powders were examined by a laser particle size analyzer (MASTERSIZER 2000, Malvern, UK). X-Ray Diffraction (XRD, Philips X’Pert MPD) and X-ray photoelectron spectroscopy (XPS, ESCALAB250) analysis of the feedstock powder and coatings were carried out. Three-dimension (3-D) surface topography of the obtained coating was measured using a laser scanning confocal microscope (LSCM, ZEISS LSM700). 3. Results and discussions SEM pictures of the original Ta and HA powders are shown in Fig. 1a and b, respectively. It indicates that both Ta and HA powders have a sponge-like surface morphology, with a particle size

of 35 + 8 lm and 10 + 5 lm, respectively. After mechanical milling, the prepared powders show an irregular morphology (Fig. 1c), with a particle size of 25 + 5 lm. HA particles are embedded in Ta particles as shown in Fig. 1d. The composite powders obtained from milling are more advantageous than simply mixture of two powders. Conventional hard/soft mixture powders are created by dispersion of hard/soft particles and coatings are deposited with hard particles embedding in a softer material, thus the content of hard particles in composite coating would be greatly reduced resulted from rebound in the process of cold spraying mixture powders [20,21]. However, composite powders could improve above problem. Fig. 2a shows a comparison of the XRD spectra of the feedstock and the resultant coating. It is clear that Ta did not appear oxidation and the decomposition and phase transformation of HA did not take place during the deposition as the working temperature of cold spraying is kept low enough. XPS analysis is used to further determine the chemical composition of the composite coating. Fig. 2b shows a comparison of the XPS survey spectra of the composite powders and composite coatings and core-level spectra of Ca2p, P2p, O1s and Ta4f of the composite coating. All peaks are corrected by C1s(284.6 eV). It indicates that Ca, P, O and Ta are the major elements of the coating. The Ca2p core-level spectrum exhibits doublet peaks located at 347.4 eV and 351.0 eV, respectively, which is the fingerprint for Ca2p in standard HA [22]. Similarly, the P2p peak is positioned at 133.2 eV, assigned as PO34 groups of HA. Through the peak processing, it can be obtained that O exists in the form of PO34 (531.6 eV) and OH (531.1 eV) in Ca10(PO4)6(OH)2 and a little Ta2O5(530.6 eV) passivate film on Ta surface, while Ta can be found of Ta4f5/2(23.5 eV), Ta4f7/2(21.6 eV) and Ta2O5(26 eV).

Fig. 1. SEM pictures of (a) Ta powders, (b) NC HA powders, (c) Ta/HA composite powders and (d) high magnification of a single Ta/HA composite particle.

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indicates that Ta and HA are evenly distributed in the coating. Fig. 3c shows the cross-section of the coating. It can be seen that the buffer layer of pure Ta acts as a connection between the Ta/ HA composite coating and substrate. The bonding strength between the coating and the substrate was approximately 36 MPa. At the top of pure Ta buffer coating, a thin layer with 23 lm can be observed and its corresponding EDX spectrum in the blue rectangle is shown in Fig. 3d. It indicates that Ta, Ca, P and O elements are contained in this composite layer. Porosity in the Ta/HA composite particles absorbed the particle kinetic energy and reduced the probability of rebounding [24,25]. HA are retained by being embedded in the pores of the Ta/HA composite powders. SEM examination of the surfaces of Ta/HA composite coatings soaked in SBF for various days is shown in Fig. 4. It is obvious that a covered thin apatite layer become larger and larger from 1 to 3 days. The corresponding EDX analysis indicated Ta peak decreased while Ca and P peaks increased. Micro-cracks caused by drying can be seen in corresponding high magnifications. However, no sediment was found on the surface of TC4 substrate after 3 days. It has been shown for various types of bioactive materials that the essential condition which various types of bioactive materials can bond to living bone is the formation of an apatite layer on their surfaces in human body [26]. This means the rate of the apatite nucleation on the surfaces of the composite coating is much higher than that of the TC4 substrate because the dissolutions of the Ca2+ and PO34 from the composite coating increase the degree of supersaturation of surrounding fluid with respect to the apatite. In addition, the island-like surface provides specific favorable sites for the apatite nucleation. Therefore, it demonstrates that the island-like surface and addition of HA in composite coating do benefit SBF to mineralize. Whether these are acceptable has to be verified by in vitro cell experiments and in vivo experiments.

Fig. 2. XRD (a) and XPS (b) spectra of HA, Ta and Ta/HA composite powders and Ta/ HA composite coatings.

4. Conclusion

Fig. 3a is the SEM photo of the surface of Ta/HA composite coating with a 3-D surface topography. The surface of the coating is rough with pores (marked with a yellow arrow). LSCM is better in distinguishing the 3-D surfaces than SEM. It shows that an island-like structure, which is the result of particles impacting in the deposition, is more clearly reflected in the color mapping. Researches [23] prove that rough surfaces are more conducive to biological fixation. So, as a biomaterial, the rough surface and proper amount of pores are beneficial to the combination of human tissues. Fig. 3b shows elemental mappings of Ta, Ca, P and O. It

In summary, Ta/HA composite powders were prepared by mechanical milling pure Ta and NC HA powders. The as-prepared composite coating was successfully deposited by cold spraying without oxidation, decomposition and phase transformation. The SBF test shows that bone-like apatite layer is mineralized on the surface of Ta/HA composite coating after 1–3 days, which indicates that the as-prepared Ta/HA composite coating has high bioactivity. This is mainly because that the evenly distributed HA increase the degree of supersaturation of surrounding fluid with respect to the apatite and the island-like rough surface provides specific favorable sites for the apatite nucleation.

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Fig. 3. SEM photo with a 3-D topography of the surface (a), elemental mapping (b), cross-section (c) and corresponding EDX spectrum (d) of Ta/HA composite coating.

Fig. 4. SEM photographs of the surfaces of Ta/HA composite coatings with corresponding EDX spectra soaked in SBF for 1 (a and d), 2 (b and e) and 3 (c and f) days.

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Conflict of interest The authors have no conflict of interest to disclose. References [1] L.M. Sun, C.C. Berndt, K.A. Gross, A. Kucuk, J. Biomed. Mater. Res. 58 (5) (2001) 570–592. [2] W. Suchanek, M. Yoshimura, J. Mater. Res. 13 (1) (1998) 94–117. [3] L.L. Hench, J. Am. Ceram. Soc. 81 (7) (1998) 1705–1728. [4] E. Mohseni, E. Zalnezhad, A.R. Bushroa, Int. J. Adhes. Adhes. 48 (2014) 238–257. [5] W.S.W. Harun, R.I.M. Asri, J. Alias, F.H. Zulkifli, K. Kadirgama, S.A.C. Ghani, J.H. M. Shariffuddin, Ceram. Int. 44 (2) (2018) 1250–1268. [6] R.B. Heimann, Surf. Coat. Technol. 201 (5) (2006) 2012–2019. [7] R.B. Heimann, R. Wirth, Biomaterials 27 (6) (2006) 823–831. [8] S. Overgaard, Acta Orthop. Scand. 71 (2000). [9] S. Overgaard, K. Soballe, M. Lind, C. Bunger, J. Bone Joint Surg. Br. 79B (4) (1997) 654–659. [10] T. Stoltenhoff, H. Kreye, H.J. Richter, J. Therm. Spray Technol. 11 (4) (2002) 542–550. [11] R.C. Dykhuizen, M.F. Smith, J. Therm. Spray Technol. 7 (2) (1998) 205–212. [12] B.R. Levine, S. Sporer, R.A. Poggie, C.J. Della Valle, J.J. Jacobs, Biomaterials 27 (27) (2006) 4671–4681.

201

[13] V.K. Balla, S. Bodhak, S. Bose, A. Bandyopadhyay, Acta Biomater. 6 (8) (2010) 3349–3359. [14] T. Miyazaki, H.M. Kim, T. Kokubo, C. Ohtsuki, H. Kato, T. Nakamura, Biomaterials 23 (3) (2002) 827–832. [15] H. Matsuno, A. Yokoyama, F. Watari, M. Uo, T. Kawasaki, Biomaterials 22 (11) (2001) 1253–1262. [16] D. Ding, Y.T. Xie, K. Li, L.P. Huang, X.B. Zheng, Materials 11 (4) (2018) 546. [17] Y.M. Xiong, W.M. Zhuang, M.X. Zhang, Surf. Coat. Technol. 270 (2015) 259– 265. [18] T. Kokubo, H. Takadama, Biomaterials 27 (15) (2006) 2907–2915. [19] T. Kokubo, H. Kushitani, S. Sakka, T. Kitsugi, T. Yamamuro, J. Biomed. Mater. Res. 24 (6) (1990) 721–734. [20] X. Qiu, N.U. Tariq, J.Q. Wang, J.R. Tang, L. Gyansah, Z.P. Zhao, T.Y. Xiong, Surf. Coat. Technol. 350 (2018) 391–400. [21] K.I. Triantou, D.I. Pantelis, V. Guipont, M. Jeandin, Wear 336 (2015) 96–107. [22] T. Hanawa, M. Ota, Biomaterials 12 (8) (1991) 767–774. [23] A.M. Vilardell, N. Cinca, N. Garcia-Giralt, S. Dosta, I.G. Cano, X. Nogues, J.M. Guilemany, J. Mater. Sci.-Mater. Med. 29 (2) (2018) 19. [24] S. Yin, E.J. Ekoi, T.L. Lupton, D.P. Dowling, R. Lupoi, Mater. Des. 126 (2017) 305– 313. [25] P.H. Gao, Y.G. Li, C.J. Li, G.J. Yang, C.X. Li, J. Therm. Spray Technol. 17 (5–6) (2008) 742–749. [26] E.G. Nordstrom, O.L.S. Munoz, Bio-Med. Mater. Eng. 11 (3) (2001) 221–231.