Phase Transformation of Calcium Phosphates
by Electrodeposition and Heat Treatment
WEI-JEN SHIH, MOO-CHIN WANG, KUO-MING CHANG, CHENG-LI WANG, SZU-HAO WANG, WANG-LONG LI, and HONG-HSIN HUANG
The effect of heat treatment on the calcium phosphate deposited on Ti-6Al-4V substrate using an electrolytic process is investigated. The calcium phosphate was deposited in a 0.04 M Ca(H2PO4)2ÆH2O (MCPM) solution on a Ti-6Al-4V substrate at 333 K (60C), 10 V, and 80
Torr for 1 hour, and calcined at various temperatures for 4 hours. The X-ray diffraction (XRD) results demonstrate that the phases are dicalcium phosphate (CaHPO4, DCPD) and
hydrox-yapatile [Ca(PO4)6(OH)2, HAP] for the as-deposited samples. When the deposited sample was
calcined at 873 K (600C) for 4 hours, the XRD results show that the transformation of DCPD to HAP occurs. Moreover, HAP converts to b-TCP, CPP, and CaO. For the sample calcined at 1073 K (800C) for 4 hours, the scanning electron microscopy (SEM) micrograph reveals that the crack of the calcined sample propagates with a width of about 3 lm. This result is due to HAP becoming decomposed and converting to b-TCP, CPP, CaO, and H2O. The vaporization
of H2O within the calcined sample promotes the crack propagation and growth.
DOI: 10.1007/s11661-010-0417-x
The Minerals, Metals & Materials Society and ASM International 2010
I. INTRODUCTION
H
YDROXYAPATITE ceramic (Ca10(PO4)6(OH)2, hereafter HAP) is used in orthopaedics and dental implant surgery, either alone or in combination with other materials or substrates, as a coating on metal implants[1,2]and to fill bone defects.[2–4]A lot of effort has been made in recent years to develop processing methods, such as plasma spraying,[5,6] electrophoretic methods,[7,8] and electrochemical methods,[9–12] for depositing calcium phosphate ceramics on the implant substrate alloys in order to have high strength, good processability, suitable specific density, and excellent corrosion resistance in the living body.Although the most widely applied HAP coating procedure is the plasma spray technique,[5,6]the major problem of the decomposition and phase transformation of HAP during the spray coating process still exists. Hence, the electrochemical deposition of calcium phos-phate bioceramic coatings has attracted considerable attention[9–12] because of its many advantages. Specifi-cally, composition and coating structure controls are
possible due to the relatively low processing temperature, and highly irregular objects can be coated relatively quickly.[13] Since electrochemical deposition can be the result of increasing pH at the interface, which is attrib-uted to electron incorporation to form OH–ions and H2 through water reduction, H2gas evolution at the interface leads to a heterogeneous coating.[13] Recent work has used organic solutions to avoid the negative effects of H2 gas, and more homogeneous coatings have been reported by Chen et al.[14]In addition, Wang et al.[11]pointed out that when calcium phosphate coatings are deposited on a Ti-6Al-4V substrate using an electrolytic method under 80 Torr, bubbles quickly lift from the cathode surface, making the deposit regular and integrated.
When the electrolyte contains Ca2+ and H2PO41–, it produces calcium phosphate powders, such as mono-calcium phosphate monohydrate (MCPM, Ca(H2PO4)2Æ H2O), dicalcium phosphate dihydrate (DCPD, CaHPO4Æ 2H2O), octacalcium phosphate, amorphous calcium phosphate, and hydroxyapatite (HAP), depending on the Ca/P ratio of raw materials and the reaction that occurs.[15,16]Among these, HAP is the most interesting form of calcium phosphate, and has been electrochem-ically deposited from several solutions by a number of researchers.[9,11,17–20] However, the effect of heat treat-ment on HAP formation on a Ti-6Al-4V substrate in a 0.04 M MCPM solution using an electrolytic process has not yet been discussed in detail.
In the present study, a 0.04 M MCPM solution was used for the synthesis of HAP on a Ti-6Al-4V substrate using an electrolytic process. The main objective of this investigation is to study the effect of heat treatment on HAP formation on a Ti-6Al-4V substrate by differential thermal and thermogravimetric analyses (DTA/TGA), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscopy WEI-JEN SHIH, Engineer, is with the Metal Industries Research
and Development Center, Kaohsiung 81160, Taiwan R.O.C. MOO-CHIN WANG, Professor, is with the Head of Department of Fragrance and Cosmetic Science, Kaohsiung Medical University, Kaohsiung 80782, Taiwan R.O.C. Contact e-mail: mcwang@kmu. edu.tw KUO-MING CHANG, Professor, CHENG-LI WANG, PhD Student, and SZU-HAO WANG, Engineer, are with the Department of Mechanical Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 80782, Taiwan R.O.C. WANG-LONG LI, Professor, is with the Institute of Nanotechnology and Micro-systems Engineering, National Cheng Kung University, Tainan 70101, Taiwan R.O.C. HONG-HSIN HUANG, Professor, is with the Department of Electrical Engineering, Cheng Shiu University, Niaosong, Kaohsiung 83347, Taiwan R.O.C.
Manuscript submitted July 8, 2009. Article published online October 5, 2010
(SEM), transmission electron microscopy (TEM), and electron diffraction (ED).
The purposes of this study are to (1) determine the thermal behavior of the converted calcium phosphate that is deposited, (2) investigate the phase transforma-tion of the as-deposited and postcalcined calcium phosphate samples, and (3) observe the microstructure of the calcium phosphate deposited and postcalcined at various temperatures.
II. EXPERIMENTAL PROCEDURE
A. Substrate Preparation
A Ti-6Al-4V alloy plate (ASTM standard F-136) and a platinum plate were used as the cathode and anode, respectively. A 15 9 15 9 3 mm Ti-6Al-4V alloy plate was mechanically ground with SiC papers from 120 to 1200 grit and polished with 0.3-lm Al2O3powders to a mirror finish. The Ti-6Al-4V plate was then washed thoroughly with running distilled water before being ultrasonically degreased with acetone and dried at 333 K (60C).
B. Electrolytic Deposition
The saturated 0.04 M electrolyte was prepared by adding 1 g of analytical grade monocalcium phosphate monohydrate (Ca(H2PO4)2ÆH2O, MCPM; supplied by Showa Chemical Co. Ltd., Tokyo) into 100 mL of water. The electrolyte was stirred with a magnetic stirrer for 1 hour to enhance the dissolution of the calcium phosphate. The pH of the electrolyte was about 3.0. Electrolytic deposition was carried out at 333 K (60C) for 20 to 120 minutes under a cathode voltage of 4 to 10 V. The distance between the electrodes and the cathode area was maintained at 3 cm and 1.057 cm2, respectively. The ambient pressure of 80 Torr was selected for the electrolysis to improve the assembly of the experimental setup, which is shown in Figure 1. After deposition, the sample was washed in distilled water and dried in air at room temperature.
C. Sample Characterization
DTA/TGA was conducted on a 5.0-mg powder sam-ple at a heating rate of 10 C/min in air (Simultaneous
symmetrical thermoanalyzer, TGA24, SETARAM, Caluire, France) with Al2O3 powders as a reference material.
The chemical behavior and molecular bonding struc-ture of the converted HAP were evaluated using a Fourier transform infrared spectroscope (PerkinElmer Spectrum One FT-IR spectrometer, Boston, MA). Each sample was mixed with KBr (sample: KBr = 1 : 99 in mass ratio) and was pressed into 200-mg pellets, 13 mm in diameter, for taking infrared adsorption spectra at a frequency range of 400 to 4000 cm1. A spectral resolution of 4 cm1 was chosen, and the composite spectrum for each sample was represented by the average of 64 scans, normalized to the spectrum of the blank KBr pellets.
The crystalline phases of the dried and postcalcined samples were examined using XRD (Rigaku D-Max/ IIIV, Tokyo). Monochromatic Cu Karadiation and a Ni filter were selected. The operating tube voltage and current were 30 kV and 20 mA, respectively. The scanning angle (2h) of the sample was from 20 to 55 deg, with a scanning speed of 4 deg/min.
The coating microstructure and morphology were investigated using a scanning electron microscope (Hitachi S2700 SEM, Hitachi, Tokyo). A transmission electron microscope (Hitachi FE-2000) was used to determine the crystal structure at 200 kV. The TEM sample was prepared by dispersing the HAP powders in an ultrasonic bath and then collecting them on a copper grid.
III. RESULTS AND DISCUSSION
A. Thermal Behavior of the Converted Calcium Phosphate Deposits
Figure2shows the DTA/TGA curves of the calcium phosphate deposited under 10 V at 333 K (60C) for 1 hour and measured at a heating rate of 10C/min in air. This figure indicates that the endothermic peaks at 398 K and 461 K (125C and 188 C) accompanied by weight losses of 3.0 and 4.8 pct, respectively, are attributed to the vaporization of water. An endothermic peak at around 703 K (430C) accompanied by a
Fig. 2—DTA/TGA curves measured at a heating rate of 10C/min in air for calcium phosphate powders deposited at 333 K (60C) and 10 V for 1 h.
Fig. 1—Assembly diagram of low-pressure electrolytic deposition.
weight loss of 1.2 pct (total weight loss of 14.8 pct) is due to the release of crystal water of dicalcium phos-phate dihydrate (DCPD, CaHPO4Æ2H2O).[19] An exo-thermic reaction peak at 793 K (520 C) is attributed to the crystallization of HAP. The weak broad arc-form continuum of the exothermic reaction between 1023 K and 1123 K (750 C and 850 C) is due to the formation of calcium pyrophosphate (Ca2P2O7) and b-tricalcium phosphate (Ca3(PO4)2, b-TCP).
The FT-IR spectra for the calcium phosphate pow-ders deposited at 333 K (60C) under various applied voltages for 1 hour are shown in Figure 3. For an applied voltage of 4 V (Figure3(a)), the absorptions at 3522 and 3488 cm1 are attributed to absorbed water. The characteristic bands around 1653 and 682 cm1are consistent with H-O-H bonding vibration.[19,20] The band at 1653 cm1 is also due to water molecules and the oxidized titanium layer on the metal.[21]The absorp-tions at 1061, 1220, and 1135 cm1 are due to P = O associated stretching vibrations.[19]The P = O stretch-ing vibration in PO43–ions at 987 cm1is observed. The bands located in the range of 1090 to 1030 cm1and at 960 cm1are consistent with phosphate group absorp-tion in HAP, as reported by Manso et al.[20]The bands at 875 and 799 cm1 are due to the P-O-P asymmetric stretching vibration in the HPO42– group.[19,21] The bands located at 578 and 527 cm1 can be assigned to the P-O mode of the PO43– characteristic peak. How-ever, in the bands at 500 to 700 cm1, the most intense peak observed at 600 cm1is not found in DCPD, but it appears in all amorphous calcium phosphates, including amorphous dicalcium phosphate.[22] In the present study, the bands observed in this range could be associated with the DCPD. When the applied voltage is increased from 5 to 10 V (Figures3(b) through (e)), the band at 603 cm1 represents the O-P-O bonding vibrations of the PO4 group in the phosphate
deposits.[23] The intensity of the 578 and 527 cm1 peaks increases with applied voltage. This is because in well-crystallized DCPD, the DCPD phosphate peaks become progressively more clearly defined and intense, and a spectrum analogous to that of well-crystallized DCPD is eventually obtained.[22]
B. Phase Transformation of the As-Deposited and Postcalcined Calcium Phosphate Samples
XRD patterns of the calcium phosphate samples deposited at 333 K (60C) under 10 V for various durations are shown in Figure4. With a 20-minute deposition time (Figure4(a)), DCPD and the Ti (sub-strate) are the dominant phases and the HAP is the minor phase.
When the deposition time is increased from 20 to 60 and 120 minutes (Figures4(b) and (c)), the phase does not change, but the reflection intensity of HAP increases with the deposition time. Figure4also indicates that the reflection intensity of Ti decreases with increasing deposition time. This is because the thickness of the deposits increases along with the deposition time, which reduces Ti reflections at 2h = 37.8 and 39.6 deg.
The formation mechanism of DCPD and HAP can be explained as follows. MCPM is the most soluble and acidic among the calcium phosphates. The dissolution of MCPM at temperatures from 298 K to 373 K (25C to 100C) has been expressed as the following reaction equation:[24]
Ca Hð 2PO4Þ2H2Oþ xH2O
!H2O
CaHPO4þ H3PO4þ 1 þ xð ÞH2O ½1
The stepwise dissociation of H3PO4 acid[25] is as follows:
H3PO4 ! Hþþ H2PO4 KI¼ 7:5 103 ½2
H2PO4 ! H þþ H
2PO4 KII¼ 6:2 108 ½3
Fig. 3—FT-IR spectra for the calcium phosphate powders deposited at 333 K (60C) for 1 h under various applied voltages.
Fig. 4—XRD patterns of the calcium phosphate samples deposited at 333 K (60C) under 10 V for various durations: (a) 20 min, (b) 60 min, and (c) 120 min (D: DCPD, H: HAP, and T: Ti).
structures occur more readily in rapidly transforming ceramic materials.
IV. CONCLUSIONS
Calcium phosphate was deposited in a 0.04 M Ca(H2PO4)2ÆH2O (MCPM) solution on a Ti-6Al-4V substrate at 333 K (60C), 10 V, and 80 Torr for 1 hour and calcined at various temperatures for 4 hours. The effect of heat treatment on the calcium phosphate deposits was investigated using DTA/TGA, FT-IR, XRD, SEM, TEM, and ED. DTA results show that an exothermic reaction peak at 793 K (520 C) can be attributed to the crystallization of HAP. The weak broad arc-form continuum of the exothermic reaction that exists between 1023 K and 1123 K (750 C and 850 C) is due to the formation of calcium pyrophos-phate (Ca2P2O7, CPP) and b-tricalcium phosphate [Ca(PO4)2, b-TCP]. The XRD results show that the as-deposited sample contains phases of DCPD and HAP. When a sample is calcined at 1073 K (800C) for 4 hours, the crystallized phases are composed of the major phases of b-TCP and CPP, and minor phases of HAP, CaO, anatase, and rutile. When the deposited sample is calcined at 1273 K (1000 C) for 4 hours, the reflection intensity of HAP and CPP decreases, but that of b-TCP increases. The surface image of the as-deposited sample shows that the DCPD crystals have a platelike morphology with a smooth, flat, and sharp edge. After being calcined at 873 K (600C) for 4 hours, the morphology of HAP crystals becomes platelike. Granular b-TCP is also observed, which is caused by the granular b-TCP crystals being converted from the platelike crystals.
ACKNOWLEDGMENTS
The authors acknowledge the financial support pro-vided by the National Science Council Taiwan, Republic of China (Contact No. NSC93-2216-E-151-005). We also thank Mr. H.Y. Yao for TEM/EDS experiments, Mr. F.C. Wu for SEM photography, and Professor M.P. Hung for suggestions on the manu-script preparation.
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