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Preparation of hydroxyapatite layer by ion beam assisted simultaneous vapor deposition
Surface and Coatings Technology 163 – 164 (2003) 362–367
Preparation of hydroxyapatite layer by ion beam assisted simultaneous vapor deposition M. Hamdia,*, A. Ide-Ektessabib a
Department of Engineering Design and Manufacture, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia b International Innovation Center, Kyoto University, Yoshida Honmachi, Sakyo-ku, 606-8501 Kyoto, Japan
Abstract Hydroxyapatite layer was prepared by using ion beam assisted simultaneous vapor deposition. The method comprised of an electron beam heater and a resistance heater vaporizing CaO and P2 O5 , respectively, while an argon ion beam was focused onto the substrate to assist the deposition. The effects of ion beam assistance during deposition were investigated. All deposited layers were amorphous, regardless of the current density level of the ion beam. Post-heat treatment was applied to crystallize the deposited coatings. The CayP ratio increased with increasing ion beam current density presumably due to the high sputtering rate of P compared to that of Ca from the layer being coated. Annealing the samples increased the ratio even further due to the evaporation of P at high temperature. Significant improvement in the bond strength between the layer and the substrate was observed. The formation of an intermixed layer at the interface due to the effect of ion beam bombardment is believed to have caused such adhesion property. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Hydroxyapatite; Calcium phosphate coating; Ion beam assisted simultaneous vapor deposition
1. Introduction Over the past few decades, synthetic hydroxyapatite (HAp) deposited on metallic substrate has been recognized as one of the most important bone substitute systems in orthopedics and dentistry w1–3x. A remarkable property of the synthetic HAp is its bioactivity, in particular the ability to form chemical bonding with the surrounding hard tissues soon after implantation in living body w4–6x. However, loosening of the coating with the substrate surface due to poor adhesion is among the problems experienced by HAp coatings w7,8x. The method used to prepare the coating is very critical to the resulting physical and chemical properties of the system. Since the mid-1970s, many surface modification techniques that are based on the method of ion bombardment, such as ion beam assisted deposition (IBAD), ion beam deposition and ion beam mixing, have been developed and are now widely used to modify the surface of materials such as metals, polymers and *Corresponding author. Tel.: q81-75-753-5259; fax: q81-75-7535259. E-mail address: [email protected] (M. Hamdi).
ceramics, including bio-ceramics materials w9–14x. IBAD is a vacuum deposition process that combines physical vapor deposition (PVD) with ion beam bombardment. The major feature of IBAD is bombardment with a certain energy ion beam during deposition of the coating. There are many parameters that affect the composition, structure, mechanical and chemical properties of the as-deposited coating in the IBAD process, among which ion bombardment is the key factor. The major processing parameters are coating materials, evaporation rate or sputtering rate, ion species, ion energy and ion beam current density w15x. The most attractive characteristics of IBAD is that it is able to prepare biocoatings with much higher adhesive strength to the substrate compared to a traditional coating method. This is assumed to be the consequence of interaction between the coating and substrate atoms that is aided by ion bombardment, which results in an atomic intermixed zone in the coating–substrate interface w15x. It also possesses the advantage of low substrate temperature and high reliability and reproducibility, without adversely affecting the bulk attributes. Another attractive feature
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produced as control samples. The process chamber vacuum was approximately 3.0=10y3 Pa. Before deposition, the Si substrates were etch-cleaned for a period of 10 min with argon ions from the same ion source. A quartz thickness gauge recorded the thickness of the deposited film. To increase the crystallinity and hence biostability w17,18x, of the films, post-deposition heat treatment of the as-deposited films was conducted at 700, 1000, 1200 8C, respectively, for 2 h in an electric furnace, followed by cooling to room temperature. Fig. 1. Schematic diagram of the IBASVD apparatus used. CaO and P2O5 were placed in the electron beam heater and resistive heater, respectively.
of the IBAD process is its superior control over coating microstructure and chemical composition. Recently, simultaneous vapor deposition (SVD) technique has been successfully employed to deposit CaP coatings w16x. This technique provides an opportunity for fine control of the surface properties of the CaP coatings, with controllable stoichiometry and morphology. In this study, IBAD method was incorporated into the SVD process in order to assist the deposition process. This has allowed the determination of the effect of ion bombardment on the physico-chemical properties of the coatings produced during the SVD process. 2. Materials and methods 2.1. Ion beam assisted simultaneous vapor deposition Illustrated in Fig. 1 is a schematic diagram of the ion beam assisted simultaneous vapor deposition (IBASVD) system. It consists mainly of two evaporator sources (electron beam and resistive heater), an electron cyclotron resonance ion source, and a rotatable sample holder in the path of the ion beam. The ions produced from ionizing argon gas are singly charged and the beams contain atomic ions. Commercially available CaO and P2O5 powders were used as Ca and P precursors, respectively. Initially, the CaO powder was pressed at 16 500 kg into cylindrical pellets and heated at 650 8C for 6 h. The annealing was continued for another 2 h at 1400 8C. Due to its low melting point, the P2O5 powder which was also exposed to the same mechanical treatment, was heated at a lower temperature of 140 8C for 1.5 h. These processes were performed to improve the vacuum condition during deposition by removing the moisture and all other volatile gasses in the sources. The simultaneously evaporated CaO (by electron gun) and P2O5 (by resistive heater) source particles were deposited onto silicon substrates on a rotating substrate holder to achieve a uniform CaP film. 1.5 keV argon ions at 180 and 260 mAycm2 were used to assist the deposition process. SVD coatings without IBAD were
2.2. Coating characterizations The structure and crystallinity of the coating samples were identified using an X-ray diffractometer (a Rigaku diffractometer RAD-C) with Cu Ka radiation at 40 kV and 20 mA, with a scanning speed of 18 2uymin from 20 to 418. The peaks obtained were compared with the standard Joint Committee on Powder Diffraction Standards (JCPDS) 噛9-0432 of stoichiometric HAp powder. Rutherford backscattering spectroscopy (RBS) was used to quantitatively profile the coating composition. He2q ions were accelerated in a Tendetron type accelerator to produce energy of 3 MeV. At this energy level, such CaP coatings with 700 nm thicknesses will produce spectra of separate Ca and P peaks. This in turn will make the analysis easier without having to resolve any overlapping peaks. A Fourier transform infrared spectrometer (FTIR—Nicolet Magna-IR 860) with a reflectance mode was used for qualitative analysis of the molecular radicals. The possible structural variation and reactions in those as-deposited and heat-treated samples were examined. The infrared spectrum with a resolution of 4ycm and a scan number of 32 was adopted with a scan range 400–4000ycm. Scratch tests were conducted to determine the adhesion strength of the coating– substrate system. A scanning scratch tester, Shimadzu SST-101, having a 100 mm radius diamond stylus was vibrated at an amplitude of 100 mm while traversing at a rate of 20 mmys under an increasing load. An average from five measurements on each sample was taken as the adhesion strength of the coating. 3. Results and discussion Concerning the preparation of HAp coating, two factors are crucial for the effectiveness of this process. One is the good adherence of the deposited layer to the substrate and the other is that it should reconstruct the characteristics of HAp ceramic. As is known, the two factors are sensitive to the coating-processing methods. Therefore, it is imperative that careful characterization of the deposited film be conducted. In the IBASVD process, films were built up by atoms or ions from the evaporated targets, and energetic ions were impinged to mix the film–substrate interface. The
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Fig. 2. Deposition rate of the coating layer as a function of ion beam current density.
application of IBAD during SVD process resulted in different deposition patterns. The deposition rates of all the approximately 700 nm thick CayP layers monitored by the quartz thickness gauge are represented in Fig. 2. The deposition rate decreased almost linearly with increasing ion beam current density. Since all other parameters in the experiments were kept constant, the reduction in deposition rate indicates that partial sputtering of the deposited atoms must have taken place when ions bombarded the deposited layer. The level of the ion beam current density seems to have no effect on the crystallinity of the as-deposited layers. All the as-deposited coatings were amorphous as usually the case for CaP films produced by the technique of PVD. Low substrate temperature during deposition does not provide the necessary energy for the growths of crystallize grains. Annealing of the coatings in an inert atmosphere leads to dramatic changes both in the elemental composition and the level of crystallinity of the coatings. The responses of the coating layers with annealing temperatures are very similar to the heattreated SVD coatings published previously w16,19x. The XRD results (not shown) indicate that the crystallinity of the films increased with increasing annealing temperature. XRD patterns of the fully crystallized layers after heat-treating at 1200 8C are shown in Fig. 3. When the ion beam was not used to assist the deposition (0 mAy cm2), a relatively strong tricalcium phosphate (TCP) phase was observed together with the HAp phase. With the ion beam assisting the deposition process at 180 mAycm2, only small TCP peaks are observed. Further increase in the ion beam current density to 260 mAy cm2 resulting in a single HAp phase but with relatively fewer HAp peaks. RBS spectra of the as-deposited 180 and 260 mAy cm2 coatings are displayed in Fig. 4. The 3 MeV ion energy used resulted in a split of Ca and P peaks, easing the analyzing process quite significantly. At ion beam
Fig. 3. XRD patterns of samples annealed at 1200 8C.
current density of 260 mAycm2, the decrease in Ca and P contents can be observed with the reduction of P being more prominent. The increase of ion beam current density reduced the P content in the coating composition. This is probably due to preferential sputtering of P by the bombarding ion as suggested by several authors w20,21x. Higher ion beam current density means the number of ions bombarding the deposited surface is higher and hence more P is sputtered away before fully residing in the lattice structure of the film. This sputtering effect is deemed to have affected the CayP ratios of the as-deposited layers. The CayP ratios obtained from the RBS spectra are revealed in Fig. 5. The as-deposited 260 mAycm2 sample has a much higher CayP ratio as compared to the 180 mAycm2 sample. Both samples show a similar increasing trend when heated at temperatures of 700 and 1000 8C. However, at 1200 8C, the CayP ratio of the 180 mAy cm2 sample leveled off but the 260 mAycm2 sample shows a small dip in the ratio. At 700 8C, the 180 mAy cm2 sample registers a CayP ratio of 1.75, which is
Fig. 4. RBS spectra of the as-deposited 180 and 260 mAycm2 coatings.
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Fig. 5. CayP ratios of IBASVD samples at different annealing temperatures.
very close to stoichiometric HAp (1.67). The effect of post-heat treatment on the Ca and P elements in the coatings is best represented in Fig. 6, which is derived
Fig. 7. FTIR spectra of the (a) 180 mAycm2, and (b) 260 mAycm2 coating layers after heat treatment.
Fig. 6. Relative amounts of Ca and P detected after heat treatments for (a) 180 mAycm2, and (b) 260 mAycm2 samples.
from the areas under the RBS elemental peaks of Ca and P. The coatings become more crystallized at higher temperature but at the expense of composition of elements. Both Ca and P were evaporated during the heating process. In both cases, the percentage reduction in P content with temperature is more than Ca, perhaps due to the volatile nature of P, especially in the case of 180 mAycm2 sample. In Fig. 6a, at 1200 8C, the content of P reduced almost 70% from the initial state as compared to 40% for Ca. Fig. 7 demonstrates the FTIR transmission spectra that indicate the change in the chemistry of HAp coatings before and after heat treatment. Significant differences can be found through the absence or presence of some peaks used for the determination of certain chemical groups. The relatively broad peaks of asdeposited samples are the characteristics of the amorphous phase present in the coatings that correspond with the results of XRD analyses. In both type of samples
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and the substrate, which was a gradient mixture of Ca, P and the element of the substrate w23–25x. Li et al. w26x explained that the unique effect of energetic ions facilitated the reaction between the substrate atoms and the migrated atoms to form an intermixed layer, which had certain properties different from the substrate and the deposited films. It is believed that higher ion beam current density resulted in a wider atomic intermixed zone, which consequently increased the adhesion. The mixed layer prevented the concentration of residual stress at the interface and was a reactive layer between the film and the substrate. 4. Conclusions Fig. 8. Adhesion strength of the IBASVD samples at different elevated temperatures.
the bands at 956ycm are observed, reflecting the v1 symmetric stretch of the trivalent PO3y group. It is 4 interesting to note that at 1000 8C heat treatment temperature, another PO3y band appears at 1092ycm 4 which are assigned to the components of the triply degenerated v3 antisymmetric P–O stretching mode. Further heating at 1200 8C results in the disappearance of 956ycm band leaving behind the 1092ycm band. The bands at 3570 and 630ycm are assigned to the stretching and hindered rotation modes of apatitic hydroxyl (OHy) group in the coatings, respectively. The OHy stretching vibration is unique for crystalline HAp and its intensity is considerably weaker compared to the strong P–O stretching vibration because of the HAp stoichiometry. In Fig. 7a, the OHy stretching mode at 3570ycm gradually appears and sharpens when the annealing temperature is increased. This is due to the increase in content of crystalline HAp. However, in Fig. 7b, the higher ion beam current density seems to affect the OHy stretching mode since this band is not visible in the spectra even at higher annealing temperature. In addition, the absence of peaks at 1649 and 3447ycm in both samples indicates that the coatings did not absorb any water. Fig. 8 shows the scratch test data of both IBASVD samples at different annealing temperatures. Similar curve patterns are observed for both types of samples, where minimum detachment forces were recorded at 700 8C annealing temperature. Maximum adhesion strengths for both samples were achieved at a temperature of 1200 8C. In all cases the 260 mAycm2 sample shows higher adhesion strength than the 180 mAycm2 sample. However, for both samples, the recorded data are much higher than the maximum adhesion strength ever achieved by the SVD samples, which was below 100 mN w22x. As suggested by many researchers, the reason for the increase in adhesion strength was due to the formation of a mixed layer between the HAp film
The findings above confirmed that HAp coatings were prepared through IBASVD method utilizing Arq as an effective energetic ion for interface mixing. The asdeposited coatings were amorphous regardless of ion beam current density level but crystallized HAp type structures were obtained upon the completion of heat treatment process. Even though the bombarding ions caused some preferential sputtering of P, this setback was overcome by the positive effect of the intermixed layer formation at the interface of the coating–substrate system. The adhesion strength of the coatings produced was greatly enhanced by the intermixed layer. Since the deposition and ion bombardment parameters can be regulated separately, this can greatly facilitate the preparation of bioactive coatings with specific properties. Acknowledgments All the samples were prepared at Sanwa Kenma Ltd. The authors are grateful to Mr Y. Tanaka for his precious assistance. The RBS measurement was performed in the ‘Quantum Science and Engineering Center’ of the Department of Nuclear Engineering, Kyoto University. We would like to thank Mr K. Yoshida for his valuable assistance during the experiments. References w1x P. Ducheyne, J.M. Cuckler, Clin. Orthop. Relat. Res. 276 (1992) 102. w2x P. Ducheyne, L.L. Hench, A. Kagan, M. Martens, A. Bursens, J.C. Mulier, J. Biomed. Mater. Res. 14 (1981) 225. w3x M. Jarcho, Clin. Orthop. Relat. Res. 157 (1981) 259. w4x W.J. Dhert, Med. Prog. Technol. 20 (1994) 143. w5x C.P.A.T. Klein, T. Patka, J.G.C. Wolke, J.M.A. de BlieckHogervorst, K. de Groot, Biomaterials 15 (1994) 146. w6x L.L. Hench, J. Am. Ceram. Soc. 74 (1991) 1487. w7x P. Frayssinet, F. Tourenne, N. Rouque, P. Conte, C. Delga, G. Bonel, J. Mat. Sci. Mater. Med. 5 (1994) 11. w8x D.M. Liu, H.M. Chou, J.D. Wu, J. Mat. Sci. Mater. Med. 5 (1994) 147. w9x Q.Y. Zhang, Z.H. Long, C.S. Ren, B.H. Guo, D.Q. Xu, T.C. Ma, Surf. Coat. Technol. 103–104 (1998) 195.
M. Hamdi, A. Ide-Ektessabi / Surface and Coatings Technology 163 – 164 (2003) 362–367 w10x A.M. Ektessabi, Nucl. Instrum. Meth. B 99 (1995) 610. w11x R. Bambauer, P. Mestres, R. Schiel, P. Sioshansi, Dialysis Transplant 24 (1995) 228. w12x A.M. Ektessabi, Nucl. Instrum. Meth. B 127y128 (1997) 1008. w13x X.M. He, W.Z. Li, H.D. Li, Y. Fan, Nucl. Instrum. Meth. B 82 (1993) 528. w14x P. Sioshansi, E.J. Tobin, Surf. Coat. Technol. 83 (1996) 175. w15x F.Z. Cui, Z.S. Luo, Surf. Coat. Technol. 112 (1999) 278. w16x M. Hamdi, S. Hakamata, A.M. Ektessabi, Thin Solid Films 377y378 (2000) 484. w17x Z. Zyman, J. Weng, X. Liu, X. Zhang, Biomaterials 15 (1994) 151. w18x J. Chen, J.G.C. Wolke, K. de Groot, Biomaterials 15 (1994) 396.
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w19x M. Hamdi, A.M. Ektessabi, J. Vac. Sci. Technol. A 19 (2001) 1566. w20x C.X. Wang, Z.Q. Chen, L.M. Guan, M. Wang, Z.Y. Liu, P.L. Wang, Nucl. Instrum. Meth. B 179 (2001) 364. w21x J.M. Choi, H.E. Kim, I.S. Lee, Biomaterials 21 (2000) 469. w22x M. Hamdi, A.M. Ektessabi, J. Vac. Sci. Technol. A 19 (2001) 1566. w23x F.Z. Cui, Z.S. Luo, Q.L. Feng, J. Mater. Sci. Mater. Med. 8 (1997) 403. w24x J.Q. Liu, Z.S. Luo, F.Z. Cui, X.F. Duan, L.M. Peng, J. Biomed. Mater. Res. 52 (2000) 115. w25x P.L. Wang, Z.Y. Liu, J.Y. Jiang, H.C. Guo, Nucl. Instrum. Meth. B 62 (1991) 234. w26x X. Li, J. Weng, W. Tong, C. Zuo, X. Zhang, P. Wang, Z. Liu, Biomaterials 18 (1997) 1487.
Preparation of hydroxyapatite layer by ion beam assisted simultaneous vapor deposition M. Hamdia,*, A. Ide-Ektessabib a
Department of Engineering Design and Manufacture, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia b International Innovation Center, Kyoto University, Yoshida Honmachi, Sakyo-ku, 606-8501 Kyoto, Japan
Abstract Hydroxyapatite layer was prepared by using ion beam assisted simultaneous vapor deposition. The method comprised of an electron beam heater and a resistance heater vaporizing CaO and P2 O5 , respectively, while an argon ion beam was focused onto the substrate to assist the deposition. The effects of ion beam assistance during deposition were investigated. All deposited layers were amorphous, regardless of the current density level of the ion beam. Post-heat treatment was applied to crystallize the deposited coatings. The CayP ratio increased with increasing ion beam current density presumably due to the high sputtering rate of P compared to that of Ca from the layer being coated. Annealing the samples increased the ratio even further due to the evaporation of P at high temperature. Significant improvement in the bond strength between the layer and the substrate was observed. The formation of an intermixed layer at the interface due to the effect of ion beam bombardment is believed to have caused such adhesion property. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Hydroxyapatite; Calcium phosphate coating; Ion beam assisted simultaneous vapor deposition
1. Introduction Over the past few decades, synthetic hydroxyapatite (HAp) deposited on metallic substrate has been recognized as one of the most important bone substitute systems in orthopedics and dentistry w1–3x. A remarkable property of the synthetic HAp is its bioactivity, in particular the ability to form chemical bonding with the surrounding hard tissues soon after implantation in living body w4–6x. However, loosening of the coating with the substrate surface due to poor adhesion is among the problems experienced by HAp coatings w7,8x. The method used to prepare the coating is very critical to the resulting physical and chemical properties of the system. Since the mid-1970s, many surface modification techniques that are based on the method of ion bombardment, such as ion beam assisted deposition (IBAD), ion beam deposition and ion beam mixing, have been developed and are now widely used to modify the surface of materials such as metals, polymers and *Corresponding author. Tel.: q81-75-753-5259; fax: q81-75-7535259. E-mail address: [email protected] (M. Hamdi).
ceramics, including bio-ceramics materials w9–14x. IBAD is a vacuum deposition process that combines physical vapor deposition (PVD) with ion beam bombardment. The major feature of IBAD is bombardment with a certain energy ion beam during deposition of the coating. There are many parameters that affect the composition, structure, mechanical and chemical properties of the as-deposited coating in the IBAD process, among which ion bombardment is the key factor. The major processing parameters are coating materials, evaporation rate or sputtering rate, ion species, ion energy and ion beam current density w15x. The most attractive characteristics of IBAD is that it is able to prepare biocoatings with much higher adhesive strength to the substrate compared to a traditional coating method. This is assumed to be the consequence of interaction between the coating and substrate atoms that is aided by ion bombardment, which results in an atomic intermixed zone in the coating–substrate interface w15x. It also possesses the advantage of low substrate temperature and high reliability and reproducibility, without adversely affecting the bulk attributes. Another attractive feature
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produced as control samples. The process chamber vacuum was approximately 3.0=10y3 Pa. Before deposition, the Si substrates were etch-cleaned for a period of 10 min with argon ions from the same ion source. A quartz thickness gauge recorded the thickness of the deposited film. To increase the crystallinity and hence biostability w17,18x, of the films, post-deposition heat treatment of the as-deposited films was conducted at 700, 1000, 1200 8C, respectively, for 2 h in an electric furnace, followed by cooling to room temperature. Fig. 1. Schematic diagram of the IBASVD apparatus used. CaO and P2O5 were placed in the electron beam heater and resistive heater, respectively.
of the IBAD process is its superior control over coating microstructure and chemical composition. Recently, simultaneous vapor deposition (SVD) technique has been successfully employed to deposit CaP coatings w16x. This technique provides an opportunity for fine control of the surface properties of the CaP coatings, with controllable stoichiometry and morphology. In this study, IBAD method was incorporated into the SVD process in order to assist the deposition process. This has allowed the determination of the effect of ion bombardment on the physico-chemical properties of the coatings produced during the SVD process. 2. Materials and methods 2.1. Ion beam assisted simultaneous vapor deposition Illustrated in Fig. 1 is a schematic diagram of the ion beam assisted simultaneous vapor deposition (IBASVD) system. It consists mainly of two evaporator sources (electron beam and resistive heater), an electron cyclotron resonance ion source, and a rotatable sample holder in the path of the ion beam. The ions produced from ionizing argon gas are singly charged and the beams contain atomic ions. Commercially available CaO and P2O5 powders were used as Ca and P precursors, respectively. Initially, the CaO powder was pressed at 16 500 kg into cylindrical pellets and heated at 650 8C for 6 h. The annealing was continued for another 2 h at 1400 8C. Due to its low melting point, the P2O5 powder which was also exposed to the same mechanical treatment, was heated at a lower temperature of 140 8C for 1.5 h. These processes were performed to improve the vacuum condition during deposition by removing the moisture and all other volatile gasses in the sources. The simultaneously evaporated CaO (by electron gun) and P2O5 (by resistive heater) source particles were deposited onto silicon substrates on a rotating substrate holder to achieve a uniform CaP film. 1.5 keV argon ions at 180 and 260 mAycm2 were used to assist the deposition process. SVD coatings without IBAD were
2.2. Coating characterizations The structure and crystallinity of the coating samples were identified using an X-ray diffractometer (a Rigaku diffractometer RAD-C) with Cu Ka radiation at 40 kV and 20 mA, with a scanning speed of 18 2uymin from 20 to 418. The peaks obtained were compared with the standard Joint Committee on Powder Diffraction Standards (JCPDS) 噛9-0432 of stoichiometric HAp powder. Rutherford backscattering spectroscopy (RBS) was used to quantitatively profile the coating composition. He2q ions were accelerated in a Tendetron type accelerator to produce energy of 3 MeV. At this energy level, such CaP coatings with 700 nm thicknesses will produce spectra of separate Ca and P peaks. This in turn will make the analysis easier without having to resolve any overlapping peaks. A Fourier transform infrared spectrometer (FTIR—Nicolet Magna-IR 860) with a reflectance mode was used for qualitative analysis of the molecular radicals. The possible structural variation and reactions in those as-deposited and heat-treated samples were examined. The infrared spectrum with a resolution of 4ycm and a scan number of 32 was adopted with a scan range 400–4000ycm. Scratch tests were conducted to determine the adhesion strength of the coating– substrate system. A scanning scratch tester, Shimadzu SST-101, having a 100 mm radius diamond stylus was vibrated at an amplitude of 100 mm while traversing at a rate of 20 mmys under an increasing load. An average from five measurements on each sample was taken as the adhesion strength of the coating. 3. Results and discussion Concerning the preparation of HAp coating, two factors are crucial for the effectiveness of this process. One is the good adherence of the deposited layer to the substrate and the other is that it should reconstruct the characteristics of HAp ceramic. As is known, the two factors are sensitive to the coating-processing methods. Therefore, it is imperative that careful characterization of the deposited film be conducted. In the IBASVD process, films were built up by atoms or ions from the evaporated targets, and energetic ions were impinged to mix the film–substrate interface. The
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Fig. 2. Deposition rate of the coating layer as a function of ion beam current density.
application of IBAD during SVD process resulted in different deposition patterns. The deposition rates of all the approximately 700 nm thick CayP layers monitored by the quartz thickness gauge are represented in Fig. 2. The deposition rate decreased almost linearly with increasing ion beam current density. Since all other parameters in the experiments were kept constant, the reduction in deposition rate indicates that partial sputtering of the deposited atoms must have taken place when ions bombarded the deposited layer. The level of the ion beam current density seems to have no effect on the crystallinity of the as-deposited layers. All the as-deposited coatings were amorphous as usually the case for CaP films produced by the technique of PVD. Low substrate temperature during deposition does not provide the necessary energy for the growths of crystallize grains. Annealing of the coatings in an inert atmosphere leads to dramatic changes both in the elemental composition and the level of crystallinity of the coatings. The responses of the coating layers with annealing temperatures are very similar to the heattreated SVD coatings published previously w16,19x. The XRD results (not shown) indicate that the crystallinity of the films increased with increasing annealing temperature. XRD patterns of the fully crystallized layers after heat-treating at 1200 8C are shown in Fig. 3. When the ion beam was not used to assist the deposition (0 mAy cm2), a relatively strong tricalcium phosphate (TCP) phase was observed together with the HAp phase. With the ion beam assisting the deposition process at 180 mAycm2, only small TCP peaks are observed. Further increase in the ion beam current density to 260 mAy cm2 resulting in a single HAp phase but with relatively fewer HAp peaks. RBS spectra of the as-deposited 180 and 260 mAy cm2 coatings are displayed in Fig. 4. The 3 MeV ion energy used resulted in a split of Ca and P peaks, easing the analyzing process quite significantly. At ion beam
Fig. 3. XRD patterns of samples annealed at 1200 8C.
current density of 260 mAycm2, the decrease in Ca and P contents can be observed with the reduction of P being more prominent. The increase of ion beam current density reduced the P content in the coating composition. This is probably due to preferential sputtering of P by the bombarding ion as suggested by several authors w20,21x. Higher ion beam current density means the number of ions bombarding the deposited surface is higher and hence more P is sputtered away before fully residing in the lattice structure of the film. This sputtering effect is deemed to have affected the CayP ratios of the as-deposited layers. The CayP ratios obtained from the RBS spectra are revealed in Fig. 5. The as-deposited 260 mAycm2 sample has a much higher CayP ratio as compared to the 180 mAycm2 sample. Both samples show a similar increasing trend when heated at temperatures of 700 and 1000 8C. However, at 1200 8C, the CayP ratio of the 180 mAy cm2 sample leveled off but the 260 mAycm2 sample shows a small dip in the ratio. At 700 8C, the 180 mAy cm2 sample registers a CayP ratio of 1.75, which is
Fig. 4. RBS spectra of the as-deposited 180 and 260 mAycm2 coatings.
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Fig. 5. CayP ratios of IBASVD samples at different annealing temperatures.
very close to stoichiometric HAp (1.67). The effect of post-heat treatment on the Ca and P elements in the coatings is best represented in Fig. 6, which is derived
Fig. 7. FTIR spectra of the (a) 180 mAycm2, and (b) 260 mAycm2 coating layers after heat treatment.
Fig. 6. Relative amounts of Ca and P detected after heat treatments for (a) 180 mAycm2, and (b) 260 mAycm2 samples.
from the areas under the RBS elemental peaks of Ca and P. The coatings become more crystallized at higher temperature but at the expense of composition of elements. Both Ca and P were evaporated during the heating process. In both cases, the percentage reduction in P content with temperature is more than Ca, perhaps due to the volatile nature of P, especially in the case of 180 mAycm2 sample. In Fig. 6a, at 1200 8C, the content of P reduced almost 70% from the initial state as compared to 40% for Ca. Fig. 7 demonstrates the FTIR transmission spectra that indicate the change in the chemistry of HAp coatings before and after heat treatment. Significant differences can be found through the absence or presence of some peaks used for the determination of certain chemical groups. The relatively broad peaks of asdeposited samples are the characteristics of the amorphous phase present in the coatings that correspond with the results of XRD analyses. In both type of samples
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and the substrate, which was a gradient mixture of Ca, P and the element of the substrate w23–25x. Li et al. w26x explained that the unique effect of energetic ions facilitated the reaction between the substrate atoms and the migrated atoms to form an intermixed layer, which had certain properties different from the substrate and the deposited films. It is believed that higher ion beam current density resulted in a wider atomic intermixed zone, which consequently increased the adhesion. The mixed layer prevented the concentration of residual stress at the interface and was a reactive layer between the film and the substrate. 4. Conclusions Fig. 8. Adhesion strength of the IBASVD samples at different elevated temperatures.
the bands at 956ycm are observed, reflecting the v1 symmetric stretch of the trivalent PO3y group. It is 4 interesting to note that at 1000 8C heat treatment temperature, another PO3y band appears at 1092ycm 4 which are assigned to the components of the triply degenerated v3 antisymmetric P–O stretching mode. Further heating at 1200 8C results in the disappearance of 956ycm band leaving behind the 1092ycm band. The bands at 3570 and 630ycm are assigned to the stretching and hindered rotation modes of apatitic hydroxyl (OHy) group in the coatings, respectively. The OHy stretching vibration is unique for crystalline HAp and its intensity is considerably weaker compared to the strong P–O stretching vibration because of the HAp stoichiometry. In Fig. 7a, the OHy stretching mode at 3570ycm gradually appears and sharpens when the annealing temperature is increased. This is due to the increase in content of crystalline HAp. However, in Fig. 7b, the higher ion beam current density seems to affect the OHy stretching mode since this band is not visible in the spectra even at higher annealing temperature. In addition, the absence of peaks at 1649 and 3447ycm in both samples indicates that the coatings did not absorb any water. Fig. 8 shows the scratch test data of both IBASVD samples at different annealing temperatures. Similar curve patterns are observed for both types of samples, where minimum detachment forces were recorded at 700 8C annealing temperature. Maximum adhesion strengths for both samples were achieved at a temperature of 1200 8C. In all cases the 260 mAycm2 sample shows higher adhesion strength than the 180 mAycm2 sample. However, for both samples, the recorded data are much higher than the maximum adhesion strength ever achieved by the SVD samples, which was below 100 mN w22x. As suggested by many researchers, the reason for the increase in adhesion strength was due to the formation of a mixed layer between the HAp film
The findings above confirmed that HAp coatings were prepared through IBASVD method utilizing Arq as an effective energetic ion for interface mixing. The asdeposited coatings were amorphous regardless of ion beam current density level but crystallized HAp type structures were obtained upon the completion of heat treatment process. Even though the bombarding ions caused some preferential sputtering of P, this setback was overcome by the positive effect of the intermixed layer formation at the interface of the coating–substrate system. The adhesion strength of the coatings produced was greatly enhanced by the intermixed layer. Since the deposition and ion bombardment parameters can be regulated separately, this can greatly facilitate the preparation of bioactive coatings with specific properties. Acknowledgments All the samples were prepared at Sanwa Kenma Ltd. The authors are grateful to Mr Y. Tanaka for his precious assistance. The RBS measurement was performed in the ‘Quantum Science and Engineering Center’ of the Department of Nuclear Engineering, Kyoto University. We would like to thank Mr K. Yoshida for his valuable assistance during the experiments. References w1x P. Ducheyne, J.M. Cuckler, Clin. Orthop. Relat. Res. 276 (1992) 102. w2x P. Ducheyne, L.L. Hench, A. Kagan, M. Martens, A. Bursens, J.C. Mulier, J. Biomed. Mater. Res. 14 (1981) 225. w3x M. Jarcho, Clin. Orthop. Relat. Res. 157 (1981) 259. w4x W.J. Dhert, Med. Prog. Technol. 20 (1994) 143. w5x C.P.A.T. Klein, T. Patka, J.G.C. Wolke, J.M.A. de BlieckHogervorst, K. de Groot, Biomaterials 15 (1994) 146. w6x L.L. Hench, J. Am. Ceram. Soc. 74 (1991) 1487. w7x P. Frayssinet, F. Tourenne, N. Rouque, P. Conte, C. Delga, G. Bonel, J. Mat. Sci. Mater. Med. 5 (1994) 11. w8x D.M. Liu, H.M. Chou, J.D. Wu, J. Mat. Sci. Mater. Med. 5 (1994) 147. w9x Q.Y. Zhang, Z.H. Long, C.S. Ren, B.H. Guo, D.Q. Xu, T.C. Ma, Surf. Coat. Technol. 103–104 (1998) 195.
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