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Influence of the Ca- and P-enriched oxide layers produced on titanium and the Ti6Al4V alloy by the IBAD method upon the corrosion resistance of these materials
Vacuum 70 (2003) 163–167
Influence of the Ca- and P-enriched oxide layers produced on titanium and the Ti6Al4V alloy by the IBAD method upon the corrosion resistance of these materials J. Baszkiewicza,*, D. Krupaa, J.A. Kozubowskia, B. Rajchelb, M. Miturab, d ! osarczyk ! A. Barczc, A. Sl , Z. Paszkiewiczd, Z. Puffe a
Warsaw University of Technology, Wo!oska 141, Warsaw 02-507, Poland Institute of Nuclear Physics, Radzikowskiego 152, Cracow 31-342, Poland c ! 46, Warsaw 02-668, Poland Institute of Electron Technology, Al. Lotnikow d University of Mining and Metallurgy, Mickiewicza 30, Cracow 30-059, Poland e Warsaw University of Technology, Noakowskiego 3, Warsaw 00-664, Poland b
Abstract The paper presents the results of examination of the properties of the layers enriched with calcium and phosphorus produced by the ion beam assisted deposition (IBAD) method. Transmission electron microscopy (TEM) was used to investigate the microstructure of the surface layers. The chemical composition of the layers was examined by the secondary ion mass spectrometry (SIMS) and Rutherford backscattering (RBS). The corrosion resistance was measured electrochemically in a simulated body fluid (SBF) at a temperature of 371C. Prior to the measurements, the samples were exposed to the test conditions for 13 or 1000 h. A SIMS analysis indicates that the layers formed on titanium and the Ti6Al4V alloy contain calcium, oxygen and phosphorus. TEM results show that the surface layers have an amorphous structure, irrespective of the substrate and the kind of the auxiliary target. Corrosion resistance of the surface layers depends on the time of exposure in SBF. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: Titanium; Ti6Al4V; Hydroxyapatite; IBAD; Corrosion
1. Introduction Among the materials used for implants into the human body, titanium and hydroxyapatite have the best properties. Within the human organism, titanium shows a high chemical stability whereas hydroxyapatite is able to bind chemically with the *Corresponding author. Tel.: +48-22-660-7449; fax: +4822-628-1983. E-mail address: [email protected] (J. Baszkiewicz).
bone tissue. Ceramic materials (such as hydroxyapatite) cannot be used alone since they are brittle and their impact strength is poor. Titanium and its alloys, on the other hand, have good mechanical properties, but their bioactivity is not as good as that of ceramic materials. The properties of titanium and ceramics can be combined by coating titanium with bioactive layers. The method, which is most often used, for covering titanium implants with hydroxyapatite coatings is plasma spraying. There have also been studies on other
0042-207X/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0042-207X(02)00636-X
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methods such as ion sputtering, electrochemical deposition, electrophoresis and sol–gel process. Recently, several studies have been published concerning the formation of hydroxyapatite layers by the IBAD method [1–5]. The studies cited in the present paper are only concerned with the examination of the chemical composition, structure and solubility of the layers. However, there are no data concerning the behaviour of the substrate metal. The present study was aimed at determining the effect of the Ca- and P-enriched oxide layers produced by the IBAD method on the corrosion resistance of titanium and the Ti6Al4V alloy.
method) and potentiodynamic method. The chemical composition of the test solution was (in mM): Na+—142, K+—5, Ca2+—2.5, Mg2+— 11.5, HPO4 2—1, HCO3 1—4.2, SO4 2—0.5, Cl 1— 192.8 plus a Trizma Base 50. The reference electrode was a saturated calomel electrode. Prior to the electrochemical examinations, the samples were exposed to the test conditions for 13 and 1000 h. The polarization of the samples in the anodic direction started from a potential of 800 mV and was gradually increased to a potential of +5000 mV. The potential variation rate was 20 mV/min. The polarization resistance was calculated by the least-squares method (R2 > 0:985). Unmodified samples were also examined for the sake of comparison.
2. Materials and methods The material examined was titanium of a purity of 99.6% and Ti6Al4V. The composition of this alloy is: C—0.01%, Al—6.4%, V—4.05%, N— 0.006%, Fe—0.09%, Ti—balance. The test samples in the form of discs 14 mm in diameter were polished one-side to a mirror finish. The oxide coatings were produced by the two-beam IBAD method in which the surface of a flat-pressed hydroxyapatite plate or a calcium oxide plate (auxiliary targets) was sputtered by an argon ion beam of an energy of 25 keV. The argon ion beam strokes the plate at an angle of 671 with respect to the normal. Additionally, the coating being formed was bombarded with a phosphorus ion beam of an energy of 25 keV incident on the surface in parallel to the normal. The process was conducted for 10 h. According to the auxiliary target employed, the samples were denoted as (HA) or (CaO). The structure of the surface layers was examined by TEM and their chemical composition by SIMS and RBS. The beam used in the RBS examinations was an He+ beam of an energy of 1 MeV, directed perpendicularly to the sample surface, whereas the detector was positioned at an angle of 1001 to the beam direction. The corrosion resistance in a non-deaerated, physiological fluid simulating solution at a temperature of 371C was measured using the two methods: linear polarization method (Stern’s
3. Results 3.1. TEM results TEM examinations of the surface layers have shown that, irrespective of the substrate and the auxiliary target, the layer has an amorphous structure (Fig. 1).
Fig. 1. Microstructure and diffraction pattern of the surface layer formed on Ti6Al4V alloy. Auxiliary target—hydroxyapatite.
J. Baszkiewicz et al. / Vacuum 70 (2003) 163–167
3.2. SIMS results Fig. 2 shows the concentration profiles of calcium, phosphorus, oxygen, and titanium in the layer formed on titanium surface by the IBAD method. We can see that, irrespective of the auxiliary target, the profiles are similar to one another. The layers are composed of a top layer and a transition-implanted layer and their thickness is about 1 mm.
3.3. RBS results The layers produced on titanium with the use of an auxiliary hydroxyapatite target appeared to be composed of the two zones: the outer of the composition: 52.6% Ca, 31.6% P, 10.5% O, and 5.25% Ti and the inner, implanted layer of the composition: 33.3% Ca, 20% P, 6.7% O and 40% Ti. The layers produced with the use of an auxiliary CaO target also contain two zones, namely, the outer of the composition: 61.5% Ca, 15.4% P, 6.7% Ti and 16.4% O, and the inner,
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implanted layer of the composition: 26.4% Ca, 16.7% P, 46.3% Ti and 10.0% O.
3.4. Results of electrochemical examinations These results are shown in Figs. 3 and 4 and Tables 1 and 2.
3.4.1. Results obtained for titanium In all the samples after short-term exposures in an SBF, irrespective of the kind of the auxiliary target employed, the IBAD modification of titanium surface increases the polarisation resistance and decreases the corrosion potentials. A prolongation of the exposure time to 1000 h results in an increase of both the polarisation resistance and corrosion potential in all the samples examined. We can see from the anodic polarisation curves of the 13 h exposed samples that the anodic currents are higher in the modified samples within the potential range from Ecorr to 0. After a 1000 h exposure of these samples, on the other hand, the
Fig. 2. SIMS depth profiles of the elements in surface layer on titanium: (A) auxiliary target—CaO, (B) auxiliary target— hydroxyapatite.
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Fig. 3. Anodic polarization curves for titanium and titanium modified by IBAD method.
Fig. 4. Anodic polarization curves for unmodified Ti6Al4V alloy and Ti6Al4V alloy modified by IBAD method.
anodic current densities decrease within the entire potential range. 3.4.2. Results obtained for Ti6Al4V alloy The results obtained for the Ti6Al4V alloy (Table 2) exposed in an SBF for 13 h show that, irrespective of the kind of the auxiliary target, the IBAD surface modification decreases the polarisa-
tion resistance and the corrosion potentials. The prolongation of the exposure to 1000 h results in an increase of these two parameters in all the samples examined. The anodic polarisation curves measured for the 13 h exposed samples show that, within the potential range from Ecorr to 200 mV, the anodic currents are higher in modified samples. Moreover, the samples modified with the use of a
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Table 1 Results of electrochemical measurements for titanium Material Time (h) 13 1000
Titanium Ecorr (mV) 261 33
Ti(CaO) 2
Rp (MO cm )
Ti(HA) 2
Ecorr (mV)
8.8 40.9
Rp (MO cm )
351 247
14.5 54.5
Ecorr (mV) 555 185
Rp (MO cm2) 11.0 60.5
Table 2 Results of electrochemical measurements for Ti6Al4 V alloy Material Time (h) 13 1000
Ti6Al4V Ecorr (mV) 182 7
Ti6Al4V(CaO) 2
Rp (MO cm ) 21.8 46.9
Ecorr (mV) 619 227
CaO target underwent pitting corrosion during the polarisation. The breakdown potential was about 3.5 V. After the 1000 h exposure, on the other hand, the anodic current densities in the modified samples decreased within the entire potential range.
4. Discussion of the results and conclusions The results obtained for titanium indicate that the oxide layers, enriched with calcium and phosphorus ions, produced on its surface by the IBAD method increase the corrosion resistance of this material, irrespective of whether exposed to an SBF for a short or long time. This increase in corrosion resistance can be attributed to the presence of the intermediate layer, formed due to the implantation [6]. In the layer produced with the use of an auxiliary hydroxyapatite target, the [Ca]/[P] ratio is the same as that in hydroxyapatite. The high corrosion resistance of the layer and the value of the [Ca]/[P] ratio suggest that the IBAD method is suitable for modifying the surfaces of titanium implants. The results obtained for the Ti6Al4V alloy are not so unequivocally advantageous. After shortterm exposures in an SBF, the corrosion resistance of the modified alloy appears to be lower than that of the unmodified alloy, but it increases with
Ti6Al4V(HA) 2
Rp (MO cm ) 12.2 38.2
Ecorr (mV) 505 99
Rp (MO cm2) 6.8 49.3
increasing exposure time. This effect is particularly pronounced in the layer produced with the use of the auxiliary hydroxyapatite target. The increase of the corrosion resistance after a long-term exposure is most probably due to a rebuilding of the surface layer. Why this resistance decreases after short-term exposures is difficult to explain and requires further examinations. The surface layers formed by the IBAD method have amorphous structures irrespective of the kind of the substrate and of the auxiliary target. Acknowledgements This work was supported by the State Committee for Scientific Research project no. 7T 08C 00317. References [1] Yoshinari M, Ohtasuka Y, D!erand T. Biomaterials 1994;15(7):529–35. [2] Ohtsuka Y, Matasuura M, Chida N, Yashinari M, Sumii T, D!erand T. Surf Coat Technol 1994;65:224–30. [3] Ektessabi AM. Nucl Instrum Methods B 1997;127/ 128:1008–14. [4] Choi J, Kim H, Lee I. Biomaterials 2000;21:469–73. [5] Wang Ch, Chen Z, Guan L, Liu Z, Wang P, Zheng S, Liao X. Surf Coat Technol 2000;130:39–45. [6] Krupa D, Baszkiewicz J, Kozubowski JA, Barcz A, Sobczak ! JW, Bilinski A, Rajchel B. Vacuum 2001;63:715–9.
Influence of the Ca- and P-enriched oxide layers produced on titanium and the Ti6Al4V alloy by the IBAD method upon the corrosion resistance of these materials J. Baszkiewicza,*, D. Krupaa, J.A. Kozubowskia, B. Rajchelb, M. Miturab, d ! osarczyk ! A. Barczc, A. Sl , Z. Paszkiewiczd, Z. Puffe a
Warsaw University of Technology, Wo!oska 141, Warsaw 02-507, Poland Institute of Nuclear Physics, Radzikowskiego 152, Cracow 31-342, Poland c ! 46, Warsaw 02-668, Poland Institute of Electron Technology, Al. Lotnikow d University of Mining and Metallurgy, Mickiewicza 30, Cracow 30-059, Poland e Warsaw University of Technology, Noakowskiego 3, Warsaw 00-664, Poland b
Abstract The paper presents the results of examination of the properties of the layers enriched with calcium and phosphorus produced by the ion beam assisted deposition (IBAD) method. Transmission electron microscopy (TEM) was used to investigate the microstructure of the surface layers. The chemical composition of the layers was examined by the secondary ion mass spectrometry (SIMS) and Rutherford backscattering (RBS). The corrosion resistance was measured electrochemically in a simulated body fluid (SBF) at a temperature of 371C. Prior to the measurements, the samples were exposed to the test conditions for 13 or 1000 h. A SIMS analysis indicates that the layers formed on titanium and the Ti6Al4V alloy contain calcium, oxygen and phosphorus. TEM results show that the surface layers have an amorphous structure, irrespective of the substrate and the kind of the auxiliary target. Corrosion resistance of the surface layers depends on the time of exposure in SBF. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: Titanium; Ti6Al4V; Hydroxyapatite; IBAD; Corrosion
1. Introduction Among the materials used for implants into the human body, titanium and hydroxyapatite have the best properties. Within the human organism, titanium shows a high chemical stability whereas hydroxyapatite is able to bind chemically with the *Corresponding author. Tel.: +48-22-660-7449; fax: +4822-628-1983. E-mail address: [email protected] (J. Baszkiewicz).
bone tissue. Ceramic materials (such as hydroxyapatite) cannot be used alone since they are brittle and their impact strength is poor. Titanium and its alloys, on the other hand, have good mechanical properties, but their bioactivity is not as good as that of ceramic materials. The properties of titanium and ceramics can be combined by coating titanium with bioactive layers. The method, which is most often used, for covering titanium implants with hydroxyapatite coatings is plasma spraying. There have also been studies on other
0042-207X/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0042-207X(02)00636-X
164
J. Baszkiewicz et al. / Vacuum 70 (2003) 163–167
methods such as ion sputtering, electrochemical deposition, electrophoresis and sol–gel process. Recently, several studies have been published concerning the formation of hydroxyapatite layers by the IBAD method [1–5]. The studies cited in the present paper are only concerned with the examination of the chemical composition, structure and solubility of the layers. However, there are no data concerning the behaviour of the substrate metal. The present study was aimed at determining the effect of the Ca- and P-enriched oxide layers produced by the IBAD method on the corrosion resistance of titanium and the Ti6Al4V alloy.
method) and potentiodynamic method. The chemical composition of the test solution was (in mM): Na+—142, K+—5, Ca2+—2.5, Mg2+— 11.5, HPO4 2—1, HCO3 1—4.2, SO4 2—0.5, Cl 1— 192.8 plus a Trizma Base 50. The reference electrode was a saturated calomel electrode. Prior to the electrochemical examinations, the samples were exposed to the test conditions for 13 and 1000 h. The polarization of the samples in the anodic direction started from a potential of 800 mV and was gradually increased to a potential of +5000 mV. The potential variation rate was 20 mV/min. The polarization resistance was calculated by the least-squares method (R2 > 0:985). Unmodified samples were also examined for the sake of comparison.
2. Materials and methods The material examined was titanium of a purity of 99.6% and Ti6Al4V. The composition of this alloy is: C—0.01%, Al—6.4%, V—4.05%, N— 0.006%, Fe—0.09%, Ti—balance. The test samples in the form of discs 14 mm in diameter were polished one-side to a mirror finish. The oxide coatings were produced by the two-beam IBAD method in which the surface of a flat-pressed hydroxyapatite plate or a calcium oxide plate (auxiliary targets) was sputtered by an argon ion beam of an energy of 25 keV. The argon ion beam strokes the plate at an angle of 671 with respect to the normal. Additionally, the coating being formed was bombarded with a phosphorus ion beam of an energy of 25 keV incident on the surface in parallel to the normal. The process was conducted for 10 h. According to the auxiliary target employed, the samples were denoted as (HA) or (CaO). The structure of the surface layers was examined by TEM and their chemical composition by SIMS and RBS. The beam used in the RBS examinations was an He+ beam of an energy of 1 MeV, directed perpendicularly to the sample surface, whereas the detector was positioned at an angle of 1001 to the beam direction. The corrosion resistance in a non-deaerated, physiological fluid simulating solution at a temperature of 371C was measured using the two methods: linear polarization method (Stern’s
3. Results 3.1. TEM results TEM examinations of the surface layers have shown that, irrespective of the substrate and the auxiliary target, the layer has an amorphous structure (Fig. 1).
Fig. 1. Microstructure and diffraction pattern of the surface layer formed on Ti6Al4V alloy. Auxiliary target—hydroxyapatite.
J. Baszkiewicz et al. / Vacuum 70 (2003) 163–167
3.2. SIMS results Fig. 2 shows the concentration profiles of calcium, phosphorus, oxygen, and titanium in the layer formed on titanium surface by the IBAD method. We can see that, irrespective of the auxiliary target, the profiles are similar to one another. The layers are composed of a top layer and a transition-implanted layer and their thickness is about 1 mm.
3.3. RBS results The layers produced on titanium with the use of an auxiliary hydroxyapatite target appeared to be composed of the two zones: the outer of the composition: 52.6% Ca, 31.6% P, 10.5% O, and 5.25% Ti and the inner, implanted layer of the composition: 33.3% Ca, 20% P, 6.7% O and 40% Ti. The layers produced with the use of an auxiliary CaO target also contain two zones, namely, the outer of the composition: 61.5% Ca, 15.4% P, 6.7% Ti and 16.4% O, and the inner,
165
implanted layer of the composition: 26.4% Ca, 16.7% P, 46.3% Ti and 10.0% O.
3.4. Results of electrochemical examinations These results are shown in Figs. 3 and 4 and Tables 1 and 2.
3.4.1. Results obtained for titanium In all the samples after short-term exposures in an SBF, irrespective of the kind of the auxiliary target employed, the IBAD modification of titanium surface increases the polarisation resistance and decreases the corrosion potentials. A prolongation of the exposure time to 1000 h results in an increase of both the polarisation resistance and corrosion potential in all the samples examined. We can see from the anodic polarisation curves of the 13 h exposed samples that the anodic currents are higher in the modified samples within the potential range from Ecorr to 0. After a 1000 h exposure of these samples, on the other hand, the
Fig. 2. SIMS depth profiles of the elements in surface layer on titanium: (A) auxiliary target—CaO, (B) auxiliary target— hydroxyapatite.
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Fig. 3. Anodic polarization curves for titanium and titanium modified by IBAD method.
Fig. 4. Anodic polarization curves for unmodified Ti6Al4V alloy and Ti6Al4V alloy modified by IBAD method.
anodic current densities decrease within the entire potential range. 3.4.2. Results obtained for Ti6Al4V alloy The results obtained for the Ti6Al4V alloy (Table 2) exposed in an SBF for 13 h show that, irrespective of the kind of the auxiliary target, the IBAD surface modification decreases the polarisa-
tion resistance and the corrosion potentials. The prolongation of the exposure to 1000 h results in an increase of these two parameters in all the samples examined. The anodic polarisation curves measured for the 13 h exposed samples show that, within the potential range from Ecorr to 200 mV, the anodic currents are higher in modified samples. Moreover, the samples modified with the use of a
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167
Table 1 Results of electrochemical measurements for titanium Material Time (h) 13 1000
Titanium Ecorr (mV) 261 33
Ti(CaO) 2
Rp (MO cm )
Ti(HA) 2
Ecorr (mV)
8.8 40.9
Rp (MO cm )
351 247
14.5 54.5
Ecorr (mV) 555 185
Rp (MO cm2) 11.0 60.5
Table 2 Results of electrochemical measurements for Ti6Al4 V alloy Material Time (h) 13 1000
Ti6Al4V Ecorr (mV) 182 7
Ti6Al4V(CaO) 2
Rp (MO cm ) 21.8 46.9
Ecorr (mV) 619 227
CaO target underwent pitting corrosion during the polarisation. The breakdown potential was about 3.5 V. After the 1000 h exposure, on the other hand, the anodic current densities in the modified samples decreased within the entire potential range.
4. Discussion of the results and conclusions The results obtained for titanium indicate that the oxide layers, enriched with calcium and phosphorus ions, produced on its surface by the IBAD method increase the corrosion resistance of this material, irrespective of whether exposed to an SBF for a short or long time. This increase in corrosion resistance can be attributed to the presence of the intermediate layer, formed due to the implantation [6]. In the layer produced with the use of an auxiliary hydroxyapatite target, the [Ca]/[P] ratio is the same as that in hydroxyapatite. The high corrosion resistance of the layer and the value of the [Ca]/[P] ratio suggest that the IBAD method is suitable for modifying the surfaces of titanium implants. The results obtained for the Ti6Al4V alloy are not so unequivocally advantageous. After shortterm exposures in an SBF, the corrosion resistance of the modified alloy appears to be lower than that of the unmodified alloy, but it increases with
Ti6Al4V(HA) 2
Rp (MO cm ) 12.2 38.2
Ecorr (mV) 505 99
Rp (MO cm2) 6.8 49.3
increasing exposure time. This effect is particularly pronounced in the layer produced with the use of the auxiliary hydroxyapatite target. The increase of the corrosion resistance after a long-term exposure is most probably due to a rebuilding of the surface layer. Why this resistance decreases after short-term exposures is difficult to explain and requires further examinations. The surface layers formed by the IBAD method have amorphous structures irrespective of the kind of the substrate and of the auxiliary target. Acknowledgements This work was supported by the State Committee for Scientific Research project no. 7T 08C 00317. References [1] Yoshinari M, Ohtasuka Y, D!erand T. Biomaterials 1994;15(7):529–35. [2] Ohtsuka Y, Matasuura M, Chida N, Yashinari M, Sumii T, D!erand T. Surf Coat Technol 1994;65:224–30. [3] Ektessabi AM. Nucl Instrum Methods B 1997;127/ 128:1008–14. [4] Choi J, Kim H, Lee I. Biomaterials 2000;21:469–73. [5] Wang Ch, Chen Z, Guan L, Liu Z, Wang P, Zheng S, Liao X. Surf Coat Technol 2000;130:39–45. [6] Krupa D, Baszkiewicz J, Kozubowski JA, Barcz A, Sobczak ! JW, Bilinski A, Rajchel B. Vacuum 2001;63:715–9.