Nanocrystal Formation in Hydroxyapatite Films Electrochemically Coated on Ti-6Al-4V Alloys
CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 1 29-32
Yun-Mo Sung* and Yeong-Keun Shin Department of Materials Science & Engineering, Daejin University, Pochun-si, Kyunggi-do 487-711, South Korea
Yong-Won Song Korea-Russia Scientific & Technological Cooperation Center, Korea Institute of Science & Technology (KIST), Seoul 130-650, South Korea
Anatoli I. Mamaev and Vera A. Mamaeva Ceramic Coatings Laboratory, Institute of Strength Physics & Materials Science (ISPMS), Russian Academy of Science, Tomsk, Russia Received April 5, 2004;
Revised Manuscript Received May 16, 2004
ABSTRACT: Hydroxyapatite (HAp) and HAp/yttria-stabilized zirconia (YSZ) nanopowders were coated on Ti-6Al-4V alloys using an electrochemical method, microarc process (MAP). Both the HAp and HAp/YSZ films were uniformly coated with ∼8-20 µm thickness on the alloys. X-ray diffraction (XRD) analysis results show that HAp film is XRD amorphous and HAp/YSZ is nanocrystalline with a rutile phase. Scanning electron microscopy (SEM) analyses on the films show uniform microporous structures, and these pores are observed as closed ones. Transmission electron microscopy (TEM) analyses reveal that HAp films are almost amorphous, containing anatase nanocrystals with ∼30-40 nm size. TEM analyses on the HAp/YSZ films show formation of rutile nanocrystals with ∼200-300 nm size as well as antase nanocrystals of ∼2030 nm size. Titanium and titanium alloys have been widely used for orthopedic and dental applications due to their superior load bearing capability and biostability.1-4 However, they show poor osteointegration properties, and their natural oxide film induces bioinertness. A coating of hydroxyapatite (HAp), a major inorganic component of bone and tooth, on the metal surface has been suggested as the most effective way to give biocompatibility and osseoconductivity.5-10 Until today, several coating techniques of HAp on titanium alloys such as plasma spray coating, sputtering, chemical vapor deposition, electrophoresis, electrochemical deposition, and dip coating, have been explored, and among these techniques plasma spray coating has mostly been highlighted for the applications.11-17 However, using plasma spray coating it is very difficult to obtain uniform HAp coating and high bond strength between HAp and Ti/Tialloy substrates. Moreover, interface fractures between HAp film and Ti/Ti-alloy substrates are often observed, which result in implant mobility and loss.1 Another disadvantage of using plasma spray coating is that the porosity of HAp film coated on Ti/Ti-alloys, enhancing the bonding rate of implants to human bones and bond strength, cannot be controlled. Microarc process (MAP) is a room-temperature electrochemical method suitable for HAp coating, and this technique may replace plasma spray coating. It has been also known as microarc oxidation (MAO), and it can be applied for the formation of oxide films on valve metals such as Al, Mg, and Ti to improve wear and corrosion resistance.18-21 MAP can also be applied to ceramic coating on the metal surfaces together with formation of own oxide layer of a matrix metal. It uses highly localized microarc discharge occurring on a metal electrode surface that is induced by high applied voltage, and the ceramic particles can be sintered and strongly bonded onto a metal electrode surface due to a very thin liquid-phase film formed on each particle * Corresponding author. Tel: +82-31-539-1985; fax: +82-31-539-1980; E-mail:
[email protected].
Figure 1. X-ray diffraction (XRD) patterns of (a) HAp and (b) HAp/YSZ films coated on Ti-6Al-4V alloys using microarc process (MAP).
surface. For HAp coating on Ti/Ti-alloys, the crystallinity and phase identity of both HAp and TiO2 layers are critical issues determining biostability and biocompatibility. Matthew et al. used a combined method of electrophoresis and MAP for HAp coating on Ti-6Al-4V alloy and obtained high-quality HAp films.22 However, the detail phase identification and analysis of crystal morphology of TiO2 layer as well as HAp layer was not performed. Also, the detailed information of the pore morphologies in HAp layer was not provided since cross-sectional view images were not investigated. In this study, we used pure HAp and 75 wt % HAp-25 wt % yttria-stabilized zirconia (HAp/YSZ) nanopowders of ∼20-30 nm particle size prepared by a chemical precipitation route for MAP coating, and the procedures for preparation of HAp and HAp/YSZ nanopowders were described
10.1021/cg049869+ CCC: $30.25 © 2005 American Chemical Society Published on Web 07/03/2004
30
Crystal Growth & Design, Vol. 5, No. 1, 2005
Communications
Figure 2. Scanning electron microscopy (SEM) (a) plan-view and (b) cross-sectional view images of HAp coated Ti-6Al-4V alloy.
Figure 4. Transmission electron microscopy (TEM) image of HApcoated Ti-6Al-4V alloy (a); its blown-up TEM image and selected area electron diffraction (SAED) pattern showing formation of nanocrystalline anatase (b). Figure 3. Scanning electron microscopy (SEM) (a) plan-view and (b) cross-sectional view images of HAp/YSZ coated Ti-6Al-4V alloy.
in detail in our previous publications.9,10 YSZ was added into HAp for the purpose of improving the mechanical properties such as wear resistance and fracture toughness. The MAP coating of HAp and HAp/YSZ powder on Ti-6Al-
4V alloys was performed at the applied voltage of 440 V and 30 A for 5 min with coating speed of ∼2 µm/min. The electrolyte used was Na2CO3 solution with 80 g/L concentration, and its temperature was ∼35 °C. The nanocrystallinity of both HAp and TiO2 layers was investigated in detail using low incidence angle (3°) X-ray diffraction (XRD: Rigaku Ultima 2000, Tokyo, Japan) with a thin-
Communications
Crystal Growth & Design, Vol. 5, No. 1, 2005 31
Figure 5. Transmission electron microscopy (TEM) image of HAp/YSZ-coated Ti-6Al-4V alloy (a); its blown-up TEM images and selected area electron diffraction (SAED) patterns showing formation of nanocrystalline anatase (b) and rutile (c), respectively.
film attachment and transmission electron microscopy (TEM: JEM4010, JEOL, Tokyo, Japan) selected area electron diffraction (SAED). Also, morphology of crystals and pores in both layers was investigated using scanning electron microscopy (SEM: XL-30 ESEM, Philips, Eindhoven, Netherlands). Smooth and uniform coating of HAp and HAp/YSZ nanopowders was achieved using MAP on the Ti-6Al-4V alloys. Although the original color of both HAp and HAp/ YSZ nanopowders was white, the color of HAp coating was pale yellow, while that of HAp/YSZ was light gray. Figure 1 shows X-ray diffraction (XRD) patterns of HAp and HAp/ YSZ films. An XRD pattern of HAp film shows XRD noncrystallinity; however, there is still the possibility for nanocrystallinty in the film to exist, which cannot be detected by XRD, but can be confirmed by TEM analysis. On the other hand, the XRD pattern of HAp/YSZ film shows formation of a rutile phase together with unknown phases. However, the crystallinity of HAp and zirconia phases was not detected in both samples. SEM plan-view images reveal microporous structure in both HAp and HAp/YSZ films as shown in Figures 2 and 3. The pore size of the HAp layer film was ∼3-4 µm, while that of HAp/YSZ was ∼2-3 µm. The pore distribution was almost uniform in both films. The cross-sectional view image of HAp-coated samples shows apparently three layers, possibly HAp, TiO2, and Ti-6Al-4V alloy, while that of HAp/YSZ-coated samples show only two layers, possibly HAp/YSZ+TiO2 and Ti-6Al-4V alloy. However, both the HAp and HAp/YSZ+TiO2 layers show a closed pore structure, which is crucially important for biomedical applications. The body fluid cannot penetrate into the surface of Ti-6Al-4V alloy, and thus HAp and HAp/YSZ layers are stable for long-term use since the pores in the layers are closed in nature. On the other hand, pores existing in surface area can cause rapid intergrowth of natural bones and thus can considerably promote strong and fast bonding with natural bones, which is another critical factor for biomedical applications and a huge advantage of using MAP for HAp coating. The pore size may be controlled by changing the operation conditions. A transmission electron microscopy (TEM) cross-sectional view image of the HAp sample shows formation of noncrystallinty of HAp and nanocrystallinity of TiO2 phase,
shown in Figure 4. Two distinct layers on Ti-6Al-4V alloys were not observed in TEM analyses. Instead, TiO2 nanocrystals were randomly distributed in HAp matrix, and their crystal size was ∼30-40 nm. SAED ring patterns, shown in Figure 4b, were identified to correspond to the diffractions from (101), (103), (200), (105), (213), and (215) planes of anatase crystals. TEM cross-sectional view image of HAp/YSZ sample also shows the formation of noncrystallinity of HAp/YSZ and nanocrystallinity of TiO2 phase as shown in Figure 5. However, in this case, TiO2 crystals were in two phases: one was anatase with ∼20-30 nm size and the other was rutile with a crystal size of ∼200 nm. The anatase phase was identified by analyzing (101), (004), (111), (211), (310), and (321) diffractions from SAED patterns. The rutile phase was also identified by analyzing (110), (101), (111), (211), (310), and (321) diffractions from SAED patterns. In TEM analyses, the existence of open pores connecting the surface of the film to the Ti-alloy surface was not detected in both HAp and HAp/YSZ samples. The formation mechanism of anatase and/or rutile nanocrystals inside the HAp or HAp/YSZ layer would be suggested as following. First, the surface of Ti-6Al-4V alloys is oxidized by microarc discharge and forms porous TiO2 nanocrystals. Second, HAp or HAp/YSZ nanopowder with 20-30 nm would penetrate into the pores between TiO2 nanocrystals. Third, the local arc-discharge temperature at the surface of Ti-alloys would be high enough to fully melt HAp and HAp/YSZ nanoparticles distributed between TiO2 nanocrystals since the particle was as fine as 20-30 nm. Finally, the fully molten HAp and HAp/YSZ can rapidly quench in the electrolyte solution at 35 °C and would form an amorphous phase containing TiO2 nanocrystals. There is a thickness difference between HAp and HAp/ YSZ films of ∼7-10 and ∼12-18 µm, respectively, and also, there is a crystalline phase difference between the two as anatase and anatase/rutile under the same processing conditions. These thickness and crystalline phase variations would come from the compositional difference between the two, and YSZ would play a key role in creating these differences. The formation of a rutile phase with a bigger size in HAp/YSZ implies that YSZ would affect the microarc, possibly more powerfully since rutile is a high-
32
Crystal Growth & Design, Vol. 5, No. 1, 2005
temperature phase. Also, YSZ may affect the microarc less frequently since the thickness of HAp/YSZ film was less than that of HAp film. The crystalline phase and thickness variation along with the YSZ content change in HAp film is under investigation. The variation in particle size of HAp and HAp/YSZ powder and applied power may also play a role in determining the final crystallinity of HAp and HAp/YSZ films coated on Ti-6Al-4V alloys. The systematic study for the control of crystallinity, crystal size, and pore size of coated films is being performed and will be reported later. This TiO2 nanocrystal formation would contribute to improvement of mechanical properties along with long-term biostability of HAP or HAp/YSZ layers for biomedical applications. Acknowledgment. This study was supported by University-Industry collaborative research consortium of 2003 from Korea Small & Medium Business Administration and Russian scientist invitational program from Korea Institute of Science & Technology (KIST).
References (1) Yang, Y.; Ong, J. L. J. Biomed. Mater. Res. 2003, 64, 509. (2) Metikos-Hukovic, M.; Tkalece, E.; Kwokal, A.; Piljac, J. Surf. Coat. Technol. 2003, 165, 40. (3) Nie, X.; Leyland, A.; Matthews, A.; Jiang, J. C.; Meletis, E. I. J. Biomed. Mater. Res. 2001, 57, 612. (4) Tkalec, E.; Sauer, M.; Nonninger, R.; Schmidt, H. J. Mater. Sci. 2001, 36, 5253. (5) Ducheyne, P.; Radin, S.; Heughebaert, M.; Heughebaert, J. C. Biomaterials 1990, 11, 244.
Communications (6) Evasic, R. Dent. Today 1993, 2, 90. (7) Furlong, R. J.; Osborn, J. F. J. Bone Joint Surg. 1991, 5, 741. (8) Svehla, M.; Morberg, P.; Zicat, B.; Bruce, W.; Sonnabend, D.; Walsh, W. R. J. Biomed. Mater. Res. 2000, 51, 15. (9) Sung, Y.-M.; Lee, J.-C.; Yang, J.-W. J. Cryst. Growth 2003 262, 467. (10) Sung, Y.-M.; Kim, D.-H. J. Cryst. Growth 2003 254, 411. (11) Radin, S. R.; Ducheyne, P. J. Mater. Sci. Mater. Med. 1992, 3, 33. (12) Zhitomirsky, I.; Galor, L. J. J. Mater. Sci. Mater. Med. 1997, 8, 213. (13) Cui, F. Z.; Luo, Z. S.; Feng, Q. L. J. Mater. Sci. Mater. Med. 1997, 8, 403. (14) Hwang, K.; Lim, Y. Surf. Coat. Technol. 1999, 115, 172. (15) Lusquinos, F.; Pou, J.; Arias, J. L.; Boutinguiza, M.; Leon, B.; Perez-Amor, M.; Driessens, F. C. M.; Gibson, I.; Best, S.; Bonfield, W. J. Appl. Phys. 2001, 90, 4231. (16) Khor, K. A.; Gu, Y. W.; Quek, C. H.; Cheang, P. Surf. Coat. Technol. 2003, 168, 195. (17) Ding, S.-J. Biomaterials 2003, 24, 4233. (18) Yerokhin, A. L.; Nie, X.; Leyland, A.; Matthews, A.; Dowey, S. J. Surf. Coat. Technol. 1999, 122, 73. (19) Yerokhin, A. L.; Nie, X.; Leyland, A.; Matthews, A. Surf. Coat. Technol. 2000, 130, 195. (20) Gnedenkov, S. V.; Khrisanfova, O. A.; Zavidnaya, A. G.; Sinebrukhov, S. L.; Gordienko, P. S.; Iwatsubo, S.; Matsui, A. Surf. Coat. Technol. 2001, 145, 146. (21) Sundarajan, G.; Krishna, L. R. Surf. Coat. Technol. 2003, 167, 269. (22) Nie, X.; Leyland, A.; Matthews, A. Surf. Coat. Technol. 2000, 125, 407.
CG049869+
