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Effect of nitrogen-ion implantation on the corrosion resistance of OT-4-0 titanium alloy in 0.9% NaCl environment
Surface and Coatings Technology 111 (1999) 86–91
Effect of nitrogen-ion implantation on the corrosion resistance of OT-4-0 titanium alloy in 0.9% NaCl environment D. Krupa a,*, J. Baszkiewicz a, E. Jezierska, J. Mizera a, T. Wierzchon´ a, A. Barcz b, R. Fillit c a Department of Materials Science and Engineering, Warsaw University of Technology, Narbutta 85, 02-524 Warsaw, Poland b Institute of Electron Technology, Al. Lotniko´w 46, 02-668 Warsaw, Poland c Ecole Nationale Superieure des Mines de Saint-Etienne, Saint-Etienne, France Received 28 November 1997; accepted 12 September 1998
Abstract This work presents data on the properties of the surface layer of OT-4-0 titanium alloy after nitrogen-ion implantation. Polished samples were implanted with nitrogen doses of 1×1016, 1×1017, 6×1017 and 1×1018 N+ cm−2. The corrosion resistance was examined by the electrochemical methods in 0.9% NaCl at a temperature of 37 °C. Structural examinations were made with a transmission electron microscope. Depth profiles of nitrogen were investigated by secondary-ion mass spectrometry. The phase composition of the implanted layers and the residual stresses induced in them during the implantation were examined by the X-ray diffraction method using a Dosophatex system. It was found that the increase of the corrosion resistance depends on the nitrogen dose employed; the maximum improvement of the corrosion resistance was observed with a dose of 1×1017 N+ cm−2. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Corrosion; Ion implantation; Nitrogen; Titanium; Transmission electron microscopy; X-ray diffraction
1. Introduction Advantageous physical and mechanical properties, as well as their high resistance to corrosion, make titanium and it alloys suitable for use not only in industry but also in medicine as implants into the human body. A drawback is the poor wear resistance of these materials, but this can be obviated by appropriate surface modification. Sioshansi [1] suggests that the most suitable method for modifying the surface of biomaterials is ion implantation. The surface of titanium and its alloys is usually implanted with nitrogen, carbon or oxygen ions. The literature reports on the effect of ion implantation on the tribological properties of these materials, but data about its effect on the corrosion resistance are scarce. The effect of implanting nitrogen ions (1×1017 to 1.4×1018 N+ cm−2) on the electrochemical behavior of Ti–6A1–4V alloy in the 1 M H SO environment was 2 4 examined by Becdelievre et al. [2,3]. They observed that * Corresponding author: Fax: +48 22 484875; e-mail: [email protected]
the oxide layers formed on implanted samples during polarization were thinner and contained a smaller number of defects than those formed on non-implanted samples. These changes in the properties of the oxide layers were attributed to the presence of TiN precipitates that appeared during the nitrogen implantation. An advantageous effect of nitrogen-ion implantation upon the corrosion resistance of OT-4-0 ( Ti–0.7Mn–0.7Al ) titanium alloy was also observed by Krupa et al. [4]. The experiments were carried out with the two nitrogen doses, 1×1017 and 6×1017 N+ cm−2, and the samples were examined in 1 M H SO and 2 4 0.5 M NaCl solutions. In both test environments the corrosion resistance of the alloy increased after implantation, the increase being greater in the samples implanted with 1×1017 N+ cm−2. This improvement in the corrosion resistance was attributed to the changes in the structure of the surface layer that took place during nitrogen-ion implantation. It was found that the layer thus formed was composed of TiN precipitates dispersed with a-Ti. The number, size and distribution of the TiN precipitates depended on the dose of implanted nitrogen. From the point of view of the application of titanium
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and its alloys as implants in the human body, it is interesting to know how the implanted samples behave in physiological environments. Such experiments were carried out by Leita˘o et al. [5,6 ], who studied the electrochemical behavior of Ti–6Al–4V [5] and Ti–5Al–2.5Fe alloys [6 ], implanted with nitrogen ions, in a simulated physiological solution (HBSS — Hank’s balanced salt solution) at a temperature of 37 °C. Samples implanted with 1015, 1016 and 1017 N+ cm−2 were examined by the potentiodynamic method. With Ti–6Al–4V alloy [5], the corrosion current was smallest in the samples implanted with 1×1016 N+ cm−2. At a dose of 1×1017 N+ cm−2 the corrosion current was increased. In the opinion of the authors, the changes in the corrosion resistance of the Ti–6Al–4V alloy subjected to nitrogen-ion implantation could be attributed to the formation of TiN and TiC N precipitates during x x y this process. Examinations of the Ti–5Al–2.5Fe alloy [6 ] after nitrogen implantation have shown an increased corrosion resistance compared with that of nonimplanted samples. An increase in the corrosion resistance (examined in the 0.9% NaCl environment) of Ti–6Al–4V alloy after nitrogen-ion implantation (dose of 3×1017 N+ cm−2) was also observed by Yu et al. [7]. These authors report a decreased anodic current density in the implanted samples compared with that observed for non-implanted samples. The aim of the present study was to attempt to determine the relationship between the structure of the implanted layer and its resistance to corrosion.
2. Examination methods The chemical composition of the alloy examined is given in Table 1. Disk-shaped test samples 23 mm in diameter and 2 mm thick were polished on one side to a mirror finish. Their surfaces were then implanted with nitrogen ions at doses of 1×1016, 1×1017, 6×1017 and 1×1018 N+ cm−2. The ion energy was 50 keV. During implantation, the temperature of the samples did not exceed 70 °C. The implantation process was carried out in a Balzers MPB-202 RP implanter. The resistance to corrosion was examined in nondeaerated 0.9% NaCl solution at a temperature of 37 °C using the two methods: the linear polarization procedure (Stern’s method ) for measuring the polarization resistance (R ) and the potentiodynamic method to determine p the polarization curves. Prior to the measurements the samples were exposed to the test conditions for 24 h so as to stabilize the corrosion potential (E ). The meascorr urements were started at a potential of 20 mV above the corrosion potential (E ) and then the potential corr was decreased in the cathodic direction until the potential lower by 20 mV than E was achieved. After each corr
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potential decrement, the electric current and potential were allowed to stabilize for about 5 min. The polarization resistance was calculated by the least-squares method. The polarization curve being measured, the samples were polarized in the anodic direction beginning from a potential of −700 mV up to a potential of 5000 mV. The potential variation rate was 100 mV min−1. The reference electrode was a saturated calomel electrode. The electric charge (Q) that passed through the anodic region during the polarization from 0 to 5000 mV was also calculated. After the polarization, the samples were examined with an optical microscope. The chemical composition of the surface layers formed during implantation was examined by secondary-ion mass spectrometry (SIMS). The chemical composition profiles were determined at an argon-beam energy of 4 keV. The surface area scanned was about 1 mm2 and the rate at which material was removed was 0.15 nm s−1. Structural examinations were made by transmission electron microscopy ( TEM ) using a Jeol JEM-100B instrument. The TEM samples were cut by the electrospark method and a wire saw, and then their non-implanted surfaces were polished electrochemically until perforation occurred. The phase composition of the implanted layers and the residual stresses (s ) induced in them during implantR ation were measured by the X-ray diffraction method (with Co K radiation) using a Dosophatex system [8,9]. a This system operates in the classical H–2H arrangement and is additionally equipped with a holder that allows the measurement of thin-film specimens. The examinations were performed at the Ecole des Mines de SaintEtienne, France. The residual stresses were calculated by the sin2 Y method.
3. Results 3.1. Corrosion resistance The results of electrochemical examinations are given in Table 2, together with the amount of electric charge, Q, that flows through the sample in the anodic region from 0 to 5 V. This latter parameter has been introduced with the aim of better differentiating between the properties of the surface of the alloy examined. The polarization curves for various nitrogen doses are shown in Fig. 1. These results show that the nitrogen ions implanted into the surface of the OT-4-0 titanium alloy increase its corrosion resistance in the 0.9% NaCl environment. This can be inferred from the increased polarization resistance, R ( Table 2), the increased corrosion potenp tial, E , the shapes of the anodic polarization curves corr
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D. Krupa et al. / Surface and Coatings Technology 111 (1999) 86–91
Table 1 Chemical composition of OT-4-0 titanium alloy Element
C
Mn
Si
S
Cr
Ni
Cu
Co
Al
V
N
Fe
Ti
Amount (wt%)
<0.01
0.71
0.01
<0.001
0.01
0.03
0.002
0.01
0.67
0.04
0.0059
0.09
98
(Fig. 1) and the decreased amount of the electric charge, Q. The degree of improvement in the corrosion resistance depends on the dose of implanted nitrogen. A dose of 1×1016 N+ cm−2 does not change the corrosion resistance in a significant manner. The observed increase in R and in E can probably be ascribed to the effect of p corr cleaning the sample surfaces that accompanies the implantation. The most pronounced effect was observed with the 1×1017 N+ cm−2 dose: the polarization resistance increased 25 times compared with that measured in non-implanted samples. When comparing the anodic polarization curves obtained for non-implanted and implanted samples (Fig. 1) we also see that the reduction in the anodic current density is most pronounced with the dose of 1×1017 N+ cm−2. The peak within the 2–3.5 V range vanishes, and the anodic current density begins to increase at a potential of about 1.2 V. Implantation with a dose of 1×1016 N+ cm−2 doubles the value of the polarized resistance, whereas the shape of the anodic polarization curves remains the same as measured for non-implanted samples but shifted towards positive potentials. The anodic current density begins to increase at a potential of about 0.5 V. In samples implanted with 6×1017 N+ cm−2 dose, the polarization resistance increases by a factor of 15. The potential at which the anodic current density begins to increase is about 0.75 V. The shape of the polarization curve changes: the peak between 2 and 3.5 V vanishes and two new peaks appear with maxima at 2.5 and 3.5 V. Increasing the nitrogen dose to 1×1018 N+ cm−2 decreases the corrosion potential and the polarization resistance compared with those obtained with the two smaller doses. The anodic current begins to increase at the same potential as in samples implanted with 1×1016 N+ cm−2. In the polarization curves, instead of the peak between 2 and 3 V, we observe three peaks:
two between 1.5 and 2.5 V and the third between 3.5 and 4 V. Summing up the results presented above, we can say that, as the nitrogen dose increases above 1×1017 N+ cm−2, the effect of implantation upon the corrosion resistance becomes weaker. 3.2. TEM results The results of TEM examinations of the nonimplanted material and of the structure of the surface layers implanted with 1×1017 and 6×1017 N+ cm−2 were reported in [4]. It was found that implanted layers consist of fine TiN particles dispersed in a deformed matrix of a-Ti. The surface layer formed by implanting the 1×1017 N+ cm−2 dose was uniform and showed good adherence to the substrate. The surface layer produced by implanting the 6×1017 N+ cm−2 dose shows morphological non-uniformity with respect to its shape, distribution and size of the nitride particles, as well as the depth of nitrogen penetration into the substrate. In addition, some morphologically complicated forms can be observed, such as TiN nanocrystals (below 30 nm) distributed concentrically and forming spherical regions 1 mm in diameter [4]. The structure obtained by implanting 1×1018 N+ cm−2 is shown in Fig. 2. This dose gives a layer which, like the layers implanted with lower doses,
Table 2 Results of electrochemical experiments Implanted dose
E (mV ) corr
R (MV cm2) p
Q (mC cm−2)
Non-implanted 1×1016 N+ cm−2 1×1017 N+ cm−2 6×1017 N+ cm−2 1×1018 N+ cm−2
−95 85 233 230 160
2.5 6.6 62.9 40.3 4.8
1063 943 362 477 401
Fig. 1. Anodic polarization measured for the OT-4-0 alloy in a solution 0.9% NaCl: 1, non-implanted specimen; 2, specimen implanted with 1×1016 N+ cm−2; 3, specimen implanted with 1×1017 N+ cm−2; 4, specimen implanted with 6×1017 N+ cm−2; 5, specimen implanted with 1×1018 N+ cm−2.
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Fig. 2. TEM images and electron diffraction patterns of OT-4-0 implanted with a nitrogen dose of 1×1018 N+ cm−2.
is composed of nanocrystalline TiN (10–150 nm in diameter). This layer also contains TiN nanocrystallites of complex morphology, arranged concentrically so as to form 1 mm broad spherical regions (Fig. 2(b)). The layer is non-uniform with respect to its shape, size (10–150 nm) and the distribution of the TiN precipitates present in it. Moreover, it does not adhere well to the substrate, especially within the regions where the concentric TiN forms appear. It should be noted that such concentric forms also occur in the samples implanted with 6×1017 N+ cm−2 [4]. Within these regions we observe an accumulation of stresses that results in layer cracking, especially along the peripheries of the concentric titanium nitride forms ( Fig. 2(b)). The samples implanted with 1×1016 N+ cm−2 were not examined by TEM since no Ti–N bonds had been found in the layers in earlier XPS examinations.
is shown in Fig. 3. Its characteristic is that the nitrogen concentration exhibits a plateau extending to a depth of ~50 nm, which is in contrast to the expected Gaussianlike distribution. Such a behavior is indicative of a mechanism in which implantation can be considered to be not a purely non-equilibrium process but as being influenced by the chemical forces that dominate at the end of the collision cascades, which leads to the formation of a layer of fixed composition.
3.3. SIMS results After the implantation with a dose of 1×1016 N+ cm−2, the profile of nitrogen concentration versus depth remained unchanged. The profiles after 1×1017 and 6×1017 N+ cm−2 have been reported in [4]. The SIMS in-depth profile of nitrogen in the sample implanted with the highest fluence of 1×1018 N+ cm−2
Fig. 3. Measured nitrogen concentration depth profile for the specimen implanted with a nitrogen dose of 1×1018 N+ cm−2.
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D. Krupa et al. / Surface and Coatings Technology 111 (1999) 86–91
3.4. XRD results Analysis of the phase composition was made with the use of a low-angle camera. The TiN phase was identified in the samples implanted with 6×1017 and 1×1018 N+ cm−2. The samples implanted with 1×1017 N+ cm−2 only gave titanium peaks, whereas those implanted with 6×1017 and 1×1018 N+ cm−2 gave both titanium and TiN peaks. The residual stresses, s , were calculated for TiN R (6×1017 and 1×1018 N+ cm−2 implanted samples) and for titanium (1×1017, 6×1017 and 1×1018 N+ cm−2 implanted samples). For the sake of comparison, the stresses were also analyzed in non-implanted samples. All the results are given in Table 3. From these results we can see that, in the implanted layer, the level of residual stresses, compressive in character, was increased compared with that in non-implanted samples. In the TiN phase being formed, the stresses are also compressive. At a dose of 6×1017 N+ cm−2 they are considerably smaller than those present in the matrix; at a dose of 1×1018 N+ cm−2, the magnitude of the stresses increases threefold.
4. Discussion Examination of the corrosion resistance has shown that nitrogen-ion implantation increases the corrosion resistance of the OT-4-0 alloy in the 0.9% NaCl environment. The best corrosion resistance was achieved with a dose of 1×1017 N+ cm−2. A similar improvement was observed in the nitrogen-implanted OT-4-0 alloy examined in 0.5 M NaCl and 1.5 M H SO environments [4]. 2 4 The corrosion resistance increased after the implantation, the greatest increase being observed with a dose of 1×1017 N+ cm−2. An advantageous effect of nitrogen-ion implantation on the electrochemical behavior of Ti–6Al–4V alloy in the 1 M H SO environment was 2 4 reported by Becdelievre et al. [2,3]. They observed that, after nitrogen implantation, the anodic dissolution peak vanished and the peak between 2.2 and 3.8 V became lower — the more so when the samples were implanted with smaller doses of 1×1017 and 2.8×1017 N+ cm−2.
Table 3 Residual stresses Specimen
Non-implanted 1×1017 N+ cm−2 6×1017 N+ cm−2 1×1018 N+ cm−2
s ±Ds (MPa) R R Ti
TiN
−1228±141 −1554±77 −1628±33 −1600±198
– – −489±161 −1280±245
At these smaller doses, the anodic current densities in the passive region also decreased. The effect of the dose of implanted nitrogen ions on the corrosion resistance of Ti–6Al–4V titanium alloy immersed in simulated physiological solution (HBSS ) was investigated by Leita˜o et al. [5]. They observed that the lowest corrosion current occurred in the 1×1016 N+ cm−2 implanted samples. The results obtained the present study differ from those reported by Leita˜o et al. [5] as regards the most advantageous implantation dose. This difference may be attributed to the differences in the chemical compositions of the alloys examined, in the implantation process parameters and in the chemical composition of the solution in which the samples were tested. Hanawa et al. [10,11] showed that calcium phosphate layers formed on the surface of titanium and its alloys immersed in HBSS. Leita˜o et al. [5] found calcium and phosphorus ions on the surface of the samples examined, with the greatest concentration of these ions occurring at a dose of 1×1016 N+ cm−2. The [Ca]/[P] ratio was 1.78, which suggested that a hydroxyapatite layer had formed on the surface. The authors observed that, at the 1×1017 N+ cm−2 dose, the calcium and phosphorus ion concentrations on the sample surface were lower and formation of the hydroxyapatite layer was inhibited. It was probably this layer that reduced the corrosion current in the 1×1016 N+ cm−2 implanted samples. To explain how the hydroxyapatite layer affects the corrosion rate of titanium, this parameter should be measured by the impedance method. In the present study the observed changes in corrosion resistance due to nitrogenion implantation can be explained in terms of the structure of the implanted surface being changed. The increase in corrosion resistance can be attributed to the presence of the TiN precipitates, whose corrosion resistance is greater than that of titanium. The extent to which the corrosion resistance will increase after implantation depends on the number and size of the TiN precipitates, which in turn depend on the nitrogenion dose applied. TEM examinations of the surface layers formed by implanting 1×1017, 6×1017 and 1×1018 N+ cm−2 have shown that the sizes of the TiN precipitates increase with increasing nitrogen dose. With the 1×1017 N+ cm−2 dose the sizes of the TiN precipitates range from 3 to 5 nm, whereas with the 1×1018 N+ cm−2 dose, they fall within the range from 10 to 150 nm. The differences in the properties of the surface layers result from the differences in size of these precipitates. The results obtained are consistent with those reported by Qiu et al. [12], who examined the relationship between the implanted nitrogen dose and the microstructure of the surface layer formed during implantation. They implanted four nitrogen doses, 9×1016, 3×1017, 6.6×1017 and 1×1018 N+ cm−2, into the surface of
D. Krupa et al. / Surface and Coatings Technology 111 (1999) 86–91
Ti–6Al–4V alloy. The energy of the beam was 50 keV. At a dose of 9×1016, TiN just began to form — TEM examinations only showed early stages of its formation. As the dose increased, the number and size of the precipitates increased. A similar relationship between the size and number of the precipitates and the implanted nitrogen dose was also observed by Hohmuth et al. [13]. The layer obtained with a dose of 1×1017 N+ cm−2 is uniform, tight and adheres well to the substrate. With the higher doses, the structure of the surface layer remains the same, but the layer becomes highly nonuniform and contains regions that show no adhesion nor tightness. This is due to the TiN precipitates that form the 1 mm broad spherical regions ( Fig. 2), within which we observe an accumulation of stresses and, as a result, cracking of the layer. This is why, above a dose of 6×1017 N+ cm−2, the corrosion resistance decreases. The TEM results have been confirmed by calculations of compressive stresses ( Table 3), which appear to increase with increasing nitrogen dose. As a result, the implanted layer cracks and the corrosion resistance decreases. Leita˜o et al. [5] also indicated a relationship between the increased corrosion resistance after implantation and the size and number of the TiN precipitates per unit surface area. They suggest that large titanium nitride precipitates would tend to detach from the surrounding metallic matrix, and the area of attack would be considerably greater than that with smaller, coherent precipitates. The coherence between the precipitates and the matrix may be good enough to avoid detachment from the matrix and allows the formation of more protective films.
5. Conclusions (1) Implantation of nitrogen ions into the surface of OT-4-0 titanium alloy improves its corrosion resistance. (2) The increase in corrosion resistance depends on the nitrogen dose employed; the maximum improve-
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ment of the corrosion resistance was observed with a dose of 1×1017 N+ cm−2. (3) The increase in corrosion resistance depends on the structure of the surface layer. If the nanocrystalline TiN precipitates formed during implantation are coherent with the basic titanium lattice and uniformly distributed throughout the surface layer, the effect of implantation is most advantageous — the increase in the corrosion resistance is the greatest.
References [1] P. Sioshansi, Medical application of ion beam processes, Nucl. Inst. Meth. Phys. Res. B 19/20 (1987) 204. [2] A.M. de Becdelievre, Y. Arnauld, N. Mesbahi, M. Brunel, J. de Becdelievre, M. Romand, Oxydation anodique de l’alliage de titane TA6V en milieu acide sulfurique 1 M: influence de l’implantation en ions azote, J. Chim. Phys. 2 (86) (1989) 365. [3] A.M. de Becdelievre, D. Fleche, J. de Becdelievre, Effect of nitrogen ion implantation on the electrochemical behaviour of TA6V in sulphuric medium, Electrochim. Acta 33 (8) (1988) 1067. [4] D. Krupa, E. Jezierska, J. Baszkiewicz, M. Kamin´ski, T. Wierzchon´, A. Barcz, Effect of nitrogen ion implantation on the structure and corrosion resistance of OT-4-0 titanium alloy, Surf. Coat. Technol. 79 (1996) 240. [5] E. Leita˜o, C. Sa´, R.A. Silva, M.A. Barbosa, H. Ali, Electrochemical and surface modifications on N+-ion implanted Ti–6Al–4V immersed in HBSS, Corros. Sci. 37 (11) (1995) 1861. [6 ] E. Leita˜o, R.A. Silva, M.A. Barbosa, Electrochemical and surface modification on N+-ion implanated Ti–5Al–2.5Fe immersed in HBSS, Corros. Sci. 39 (2) (1997) 377. [7] J. Yu, J. Zhao, L.X. Li, Corrosion fatigue resistances of surgical implant stainless steels and titanium alloy, Corros. Sci. 35 (1)2)3)4) (1993) 587. [8] R. Fillit, H. Bruyans, E. Patay, Europatent No. 84 500 954.5, 1987. [9] R. Fillit, A.J. Perry, J. Pol Doddelle, in: C.D. Ruud ( Ed.), Nondestructive Characterization of Materials, Plenum Press, New York, 1991. [10] T. Hanawa, M. Ota, Calcium phosphate naturally formed on titanium in electrolyte solution, Biomaterials 12 (1991) 767. [11] T. Hanawa, M. Ota, Characterization of surface film formed on titanium in electrolyte using XPS, Appl. Surf. Sci. 55 (1992) 269. [12] X. Qiu, R.A. Dodd, J.R. Conrad, A. Chen, F.J. Worzala, Microstructural study of nitrogen-implanted Ti–6Al–4V alloy, Nucl. Instr. Meth. Phys. Res. B 59/60 (1991) 951. [13] K. Hohmuth, B. Rauschenbach, High fluence implantation of nitrogen ions into titanium, Mater. Sci. Eng. 69 (1985) 4889.
Effect of nitrogen-ion implantation on the corrosion resistance of OT-4-0 titanium alloy in 0.9% NaCl environment D. Krupa a,*, J. Baszkiewicz a, E. Jezierska, J. Mizera a, T. Wierzchon´ a, A. Barcz b, R. Fillit c a Department of Materials Science and Engineering, Warsaw University of Technology, Narbutta 85, 02-524 Warsaw, Poland b Institute of Electron Technology, Al. Lotniko´w 46, 02-668 Warsaw, Poland c Ecole Nationale Superieure des Mines de Saint-Etienne, Saint-Etienne, France Received 28 November 1997; accepted 12 September 1998
Abstract This work presents data on the properties of the surface layer of OT-4-0 titanium alloy after nitrogen-ion implantation. Polished samples were implanted with nitrogen doses of 1×1016, 1×1017, 6×1017 and 1×1018 N+ cm−2. The corrosion resistance was examined by the electrochemical methods in 0.9% NaCl at a temperature of 37 °C. Structural examinations were made with a transmission electron microscope. Depth profiles of nitrogen were investigated by secondary-ion mass spectrometry. The phase composition of the implanted layers and the residual stresses induced in them during the implantation were examined by the X-ray diffraction method using a Dosophatex system. It was found that the increase of the corrosion resistance depends on the nitrogen dose employed; the maximum improvement of the corrosion resistance was observed with a dose of 1×1017 N+ cm−2. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Corrosion; Ion implantation; Nitrogen; Titanium; Transmission electron microscopy; X-ray diffraction
1. Introduction Advantageous physical and mechanical properties, as well as their high resistance to corrosion, make titanium and it alloys suitable for use not only in industry but also in medicine as implants into the human body. A drawback is the poor wear resistance of these materials, but this can be obviated by appropriate surface modification. Sioshansi [1] suggests that the most suitable method for modifying the surface of biomaterials is ion implantation. The surface of titanium and its alloys is usually implanted with nitrogen, carbon or oxygen ions. The literature reports on the effect of ion implantation on the tribological properties of these materials, but data about its effect on the corrosion resistance are scarce. The effect of implanting nitrogen ions (1×1017 to 1.4×1018 N+ cm−2) on the electrochemical behavior of Ti–6A1–4V alloy in the 1 M H SO environment was 2 4 examined by Becdelievre et al. [2,3]. They observed that * Corresponding author: Fax: +48 22 484875; e-mail: [email protected]
the oxide layers formed on implanted samples during polarization were thinner and contained a smaller number of defects than those formed on non-implanted samples. These changes in the properties of the oxide layers were attributed to the presence of TiN precipitates that appeared during the nitrogen implantation. An advantageous effect of nitrogen-ion implantation upon the corrosion resistance of OT-4-0 ( Ti–0.7Mn–0.7Al ) titanium alloy was also observed by Krupa et al. [4]. The experiments were carried out with the two nitrogen doses, 1×1017 and 6×1017 N+ cm−2, and the samples were examined in 1 M H SO and 2 4 0.5 M NaCl solutions. In both test environments the corrosion resistance of the alloy increased after implantation, the increase being greater in the samples implanted with 1×1017 N+ cm−2. This improvement in the corrosion resistance was attributed to the changes in the structure of the surface layer that took place during nitrogen-ion implantation. It was found that the layer thus formed was composed of TiN precipitates dispersed with a-Ti. The number, size and distribution of the TiN precipitates depended on the dose of implanted nitrogen. From the point of view of the application of titanium
0257-8972/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII S 02 5 7 -8 9 7 2 ( 9 8 ) 0 0 71 4 - 2
D. Krupa et al. / Surface and Coatings Technology 111 (1999) 86–91
and its alloys as implants in the human body, it is interesting to know how the implanted samples behave in physiological environments. Such experiments were carried out by Leita˘o et al. [5,6 ], who studied the electrochemical behavior of Ti–6Al–4V [5] and Ti–5Al–2.5Fe alloys [6 ], implanted with nitrogen ions, in a simulated physiological solution (HBSS — Hank’s balanced salt solution) at a temperature of 37 °C. Samples implanted with 1015, 1016 and 1017 N+ cm−2 were examined by the potentiodynamic method. With Ti–6Al–4V alloy [5], the corrosion current was smallest in the samples implanted with 1×1016 N+ cm−2. At a dose of 1×1017 N+ cm−2 the corrosion current was increased. In the opinion of the authors, the changes in the corrosion resistance of the Ti–6Al–4V alloy subjected to nitrogen-ion implantation could be attributed to the formation of TiN and TiC N precipitates during x x y this process. Examinations of the Ti–5Al–2.5Fe alloy [6 ] after nitrogen implantation have shown an increased corrosion resistance compared with that of nonimplanted samples. An increase in the corrosion resistance (examined in the 0.9% NaCl environment) of Ti–6Al–4V alloy after nitrogen-ion implantation (dose of 3×1017 N+ cm−2) was also observed by Yu et al. [7]. These authors report a decreased anodic current density in the implanted samples compared with that observed for non-implanted samples. The aim of the present study was to attempt to determine the relationship between the structure of the implanted layer and its resistance to corrosion.
2. Examination methods The chemical composition of the alloy examined is given in Table 1. Disk-shaped test samples 23 mm in diameter and 2 mm thick were polished on one side to a mirror finish. Their surfaces were then implanted with nitrogen ions at doses of 1×1016, 1×1017, 6×1017 and 1×1018 N+ cm−2. The ion energy was 50 keV. During implantation, the temperature of the samples did not exceed 70 °C. The implantation process was carried out in a Balzers MPB-202 RP implanter. The resistance to corrosion was examined in nondeaerated 0.9% NaCl solution at a temperature of 37 °C using the two methods: the linear polarization procedure (Stern’s method ) for measuring the polarization resistance (R ) and the potentiodynamic method to determine p the polarization curves. Prior to the measurements the samples were exposed to the test conditions for 24 h so as to stabilize the corrosion potential (E ). The meascorr urements were started at a potential of 20 mV above the corrosion potential (E ) and then the potential corr was decreased in the cathodic direction until the potential lower by 20 mV than E was achieved. After each corr
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potential decrement, the electric current and potential were allowed to stabilize for about 5 min. The polarization resistance was calculated by the least-squares method. The polarization curve being measured, the samples were polarized in the anodic direction beginning from a potential of −700 mV up to a potential of 5000 mV. The potential variation rate was 100 mV min−1. The reference electrode was a saturated calomel electrode. The electric charge (Q) that passed through the anodic region during the polarization from 0 to 5000 mV was also calculated. After the polarization, the samples were examined with an optical microscope. The chemical composition of the surface layers formed during implantation was examined by secondary-ion mass spectrometry (SIMS). The chemical composition profiles were determined at an argon-beam energy of 4 keV. The surface area scanned was about 1 mm2 and the rate at which material was removed was 0.15 nm s−1. Structural examinations were made by transmission electron microscopy ( TEM ) using a Jeol JEM-100B instrument. The TEM samples were cut by the electrospark method and a wire saw, and then their non-implanted surfaces were polished electrochemically until perforation occurred. The phase composition of the implanted layers and the residual stresses (s ) induced in them during implantR ation were measured by the X-ray diffraction method (with Co K radiation) using a Dosophatex system [8,9]. a This system operates in the classical H–2H arrangement and is additionally equipped with a holder that allows the measurement of thin-film specimens. The examinations were performed at the Ecole des Mines de SaintEtienne, France. The residual stresses were calculated by the sin2 Y method.
3. Results 3.1. Corrosion resistance The results of electrochemical examinations are given in Table 2, together with the amount of electric charge, Q, that flows through the sample in the anodic region from 0 to 5 V. This latter parameter has been introduced with the aim of better differentiating between the properties of the surface of the alloy examined. The polarization curves for various nitrogen doses are shown in Fig. 1. These results show that the nitrogen ions implanted into the surface of the OT-4-0 titanium alloy increase its corrosion resistance in the 0.9% NaCl environment. This can be inferred from the increased polarization resistance, R ( Table 2), the increased corrosion potenp tial, E , the shapes of the anodic polarization curves corr
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Table 1 Chemical composition of OT-4-0 titanium alloy Element
C
Mn
Si
S
Cr
Ni
Cu
Co
Al
V
N
Fe
Ti
Amount (wt%)
<0.01
0.71
0.01
<0.001
0.01
0.03
0.002
0.01
0.67
0.04
0.0059
0.09
98
(Fig. 1) and the decreased amount of the electric charge, Q. The degree of improvement in the corrosion resistance depends on the dose of implanted nitrogen. A dose of 1×1016 N+ cm−2 does not change the corrosion resistance in a significant manner. The observed increase in R and in E can probably be ascribed to the effect of p corr cleaning the sample surfaces that accompanies the implantation. The most pronounced effect was observed with the 1×1017 N+ cm−2 dose: the polarization resistance increased 25 times compared with that measured in non-implanted samples. When comparing the anodic polarization curves obtained for non-implanted and implanted samples (Fig. 1) we also see that the reduction in the anodic current density is most pronounced with the dose of 1×1017 N+ cm−2. The peak within the 2–3.5 V range vanishes, and the anodic current density begins to increase at a potential of about 1.2 V. Implantation with a dose of 1×1016 N+ cm−2 doubles the value of the polarized resistance, whereas the shape of the anodic polarization curves remains the same as measured for non-implanted samples but shifted towards positive potentials. The anodic current density begins to increase at a potential of about 0.5 V. In samples implanted with 6×1017 N+ cm−2 dose, the polarization resistance increases by a factor of 15. The potential at which the anodic current density begins to increase is about 0.75 V. The shape of the polarization curve changes: the peak between 2 and 3.5 V vanishes and two new peaks appear with maxima at 2.5 and 3.5 V. Increasing the nitrogen dose to 1×1018 N+ cm−2 decreases the corrosion potential and the polarization resistance compared with those obtained with the two smaller doses. The anodic current begins to increase at the same potential as in samples implanted with 1×1016 N+ cm−2. In the polarization curves, instead of the peak between 2 and 3 V, we observe three peaks:
two between 1.5 and 2.5 V and the third between 3.5 and 4 V. Summing up the results presented above, we can say that, as the nitrogen dose increases above 1×1017 N+ cm−2, the effect of implantation upon the corrosion resistance becomes weaker. 3.2. TEM results The results of TEM examinations of the nonimplanted material and of the structure of the surface layers implanted with 1×1017 and 6×1017 N+ cm−2 were reported in [4]. It was found that implanted layers consist of fine TiN particles dispersed in a deformed matrix of a-Ti. The surface layer formed by implanting the 1×1017 N+ cm−2 dose was uniform and showed good adherence to the substrate. The surface layer produced by implanting the 6×1017 N+ cm−2 dose shows morphological non-uniformity with respect to its shape, distribution and size of the nitride particles, as well as the depth of nitrogen penetration into the substrate. In addition, some morphologically complicated forms can be observed, such as TiN nanocrystals (below 30 nm) distributed concentrically and forming spherical regions 1 mm in diameter [4]. The structure obtained by implanting 1×1018 N+ cm−2 is shown in Fig. 2. This dose gives a layer which, like the layers implanted with lower doses,
Table 2 Results of electrochemical experiments Implanted dose
E (mV ) corr
R (MV cm2) p
Q (mC cm−2)
Non-implanted 1×1016 N+ cm−2 1×1017 N+ cm−2 6×1017 N+ cm−2 1×1018 N+ cm−2
−95 85 233 230 160
2.5 6.6 62.9 40.3 4.8
1063 943 362 477 401
Fig. 1. Anodic polarization measured for the OT-4-0 alloy in a solution 0.9% NaCl: 1, non-implanted specimen; 2, specimen implanted with 1×1016 N+ cm−2; 3, specimen implanted with 1×1017 N+ cm−2; 4, specimen implanted with 6×1017 N+ cm−2; 5, specimen implanted with 1×1018 N+ cm−2.
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Fig. 2. TEM images and electron diffraction patterns of OT-4-0 implanted with a nitrogen dose of 1×1018 N+ cm−2.
is composed of nanocrystalline TiN (10–150 nm in diameter). This layer also contains TiN nanocrystallites of complex morphology, arranged concentrically so as to form 1 mm broad spherical regions (Fig. 2(b)). The layer is non-uniform with respect to its shape, size (10–150 nm) and the distribution of the TiN precipitates present in it. Moreover, it does not adhere well to the substrate, especially within the regions where the concentric TiN forms appear. It should be noted that such concentric forms also occur in the samples implanted with 6×1017 N+ cm−2 [4]. Within these regions we observe an accumulation of stresses that results in layer cracking, especially along the peripheries of the concentric titanium nitride forms ( Fig. 2(b)). The samples implanted with 1×1016 N+ cm−2 were not examined by TEM since no Ti–N bonds had been found in the layers in earlier XPS examinations.
is shown in Fig. 3. Its characteristic is that the nitrogen concentration exhibits a plateau extending to a depth of ~50 nm, which is in contrast to the expected Gaussianlike distribution. Such a behavior is indicative of a mechanism in which implantation can be considered to be not a purely non-equilibrium process but as being influenced by the chemical forces that dominate at the end of the collision cascades, which leads to the formation of a layer of fixed composition.
3.3. SIMS results After the implantation with a dose of 1×1016 N+ cm−2, the profile of nitrogen concentration versus depth remained unchanged. The profiles after 1×1017 and 6×1017 N+ cm−2 have been reported in [4]. The SIMS in-depth profile of nitrogen in the sample implanted with the highest fluence of 1×1018 N+ cm−2
Fig. 3. Measured nitrogen concentration depth profile for the specimen implanted with a nitrogen dose of 1×1018 N+ cm−2.
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3.4. XRD results Analysis of the phase composition was made with the use of a low-angle camera. The TiN phase was identified in the samples implanted with 6×1017 and 1×1018 N+ cm−2. The samples implanted with 1×1017 N+ cm−2 only gave titanium peaks, whereas those implanted with 6×1017 and 1×1018 N+ cm−2 gave both titanium and TiN peaks. The residual stresses, s , were calculated for TiN R (6×1017 and 1×1018 N+ cm−2 implanted samples) and for titanium (1×1017, 6×1017 and 1×1018 N+ cm−2 implanted samples). For the sake of comparison, the stresses were also analyzed in non-implanted samples. All the results are given in Table 3. From these results we can see that, in the implanted layer, the level of residual stresses, compressive in character, was increased compared with that in non-implanted samples. In the TiN phase being formed, the stresses are also compressive. At a dose of 6×1017 N+ cm−2 they are considerably smaller than those present in the matrix; at a dose of 1×1018 N+ cm−2, the magnitude of the stresses increases threefold.
4. Discussion Examination of the corrosion resistance has shown that nitrogen-ion implantation increases the corrosion resistance of the OT-4-0 alloy in the 0.9% NaCl environment. The best corrosion resistance was achieved with a dose of 1×1017 N+ cm−2. A similar improvement was observed in the nitrogen-implanted OT-4-0 alloy examined in 0.5 M NaCl and 1.5 M H SO environments [4]. 2 4 The corrosion resistance increased after the implantation, the greatest increase being observed with a dose of 1×1017 N+ cm−2. An advantageous effect of nitrogen-ion implantation on the electrochemical behavior of Ti–6Al–4V alloy in the 1 M H SO environment was 2 4 reported by Becdelievre et al. [2,3]. They observed that, after nitrogen implantation, the anodic dissolution peak vanished and the peak between 2.2 and 3.8 V became lower — the more so when the samples were implanted with smaller doses of 1×1017 and 2.8×1017 N+ cm−2.
Table 3 Residual stresses Specimen
Non-implanted 1×1017 N+ cm−2 6×1017 N+ cm−2 1×1018 N+ cm−2
s ±Ds (MPa) R R Ti
TiN
−1228±141 −1554±77 −1628±33 −1600±198
– – −489±161 −1280±245
At these smaller doses, the anodic current densities in the passive region also decreased. The effect of the dose of implanted nitrogen ions on the corrosion resistance of Ti–6Al–4V titanium alloy immersed in simulated physiological solution (HBSS ) was investigated by Leita˜o et al. [5]. They observed that the lowest corrosion current occurred in the 1×1016 N+ cm−2 implanted samples. The results obtained the present study differ from those reported by Leita˜o et al. [5] as regards the most advantageous implantation dose. This difference may be attributed to the differences in the chemical compositions of the alloys examined, in the implantation process parameters and in the chemical composition of the solution in which the samples were tested. Hanawa et al. [10,11] showed that calcium phosphate layers formed on the surface of titanium and its alloys immersed in HBSS. Leita˜o et al. [5] found calcium and phosphorus ions on the surface of the samples examined, with the greatest concentration of these ions occurring at a dose of 1×1016 N+ cm−2. The [Ca]/[P] ratio was 1.78, which suggested that a hydroxyapatite layer had formed on the surface. The authors observed that, at the 1×1017 N+ cm−2 dose, the calcium and phosphorus ion concentrations on the sample surface were lower and formation of the hydroxyapatite layer was inhibited. It was probably this layer that reduced the corrosion current in the 1×1016 N+ cm−2 implanted samples. To explain how the hydroxyapatite layer affects the corrosion rate of titanium, this parameter should be measured by the impedance method. In the present study the observed changes in corrosion resistance due to nitrogenion implantation can be explained in terms of the structure of the implanted surface being changed. The increase in corrosion resistance can be attributed to the presence of the TiN precipitates, whose corrosion resistance is greater than that of titanium. The extent to which the corrosion resistance will increase after implantation depends on the number and size of the TiN precipitates, which in turn depend on the nitrogenion dose applied. TEM examinations of the surface layers formed by implanting 1×1017, 6×1017 and 1×1018 N+ cm−2 have shown that the sizes of the TiN precipitates increase with increasing nitrogen dose. With the 1×1017 N+ cm−2 dose the sizes of the TiN precipitates range from 3 to 5 nm, whereas with the 1×1018 N+ cm−2 dose, they fall within the range from 10 to 150 nm. The differences in the properties of the surface layers result from the differences in size of these precipitates. The results obtained are consistent with those reported by Qiu et al. [12], who examined the relationship between the implanted nitrogen dose and the microstructure of the surface layer formed during implantation. They implanted four nitrogen doses, 9×1016, 3×1017, 6.6×1017 and 1×1018 N+ cm−2, into the surface of
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Ti–6Al–4V alloy. The energy of the beam was 50 keV. At a dose of 9×1016, TiN just began to form — TEM examinations only showed early stages of its formation. As the dose increased, the number and size of the precipitates increased. A similar relationship between the size and number of the precipitates and the implanted nitrogen dose was also observed by Hohmuth et al. [13]. The layer obtained with a dose of 1×1017 N+ cm−2 is uniform, tight and adheres well to the substrate. With the higher doses, the structure of the surface layer remains the same, but the layer becomes highly nonuniform and contains regions that show no adhesion nor tightness. This is due to the TiN precipitates that form the 1 mm broad spherical regions ( Fig. 2), within which we observe an accumulation of stresses and, as a result, cracking of the layer. This is why, above a dose of 6×1017 N+ cm−2, the corrosion resistance decreases. The TEM results have been confirmed by calculations of compressive stresses ( Table 3), which appear to increase with increasing nitrogen dose. As a result, the implanted layer cracks and the corrosion resistance decreases. Leita˜o et al. [5] also indicated a relationship between the increased corrosion resistance after implantation and the size and number of the TiN precipitates per unit surface area. They suggest that large titanium nitride precipitates would tend to detach from the surrounding metallic matrix, and the area of attack would be considerably greater than that with smaller, coherent precipitates. The coherence between the precipitates and the matrix may be good enough to avoid detachment from the matrix and allows the formation of more protective films.
5. Conclusions (1) Implantation of nitrogen ions into the surface of OT-4-0 titanium alloy improves its corrosion resistance. (2) The increase in corrosion resistance depends on the nitrogen dose employed; the maximum improve-
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ment of the corrosion resistance was observed with a dose of 1×1017 N+ cm−2. (3) The increase in corrosion resistance depends on the structure of the surface layer. If the nanocrystalline TiN precipitates formed during implantation are coherent with the basic titanium lattice and uniformly distributed throughout the surface layer, the effect of implantation is most advantageous — the increase in the corrosion resistance is the greatest.
References [1] P. Sioshansi, Medical application of ion beam processes, Nucl. Inst. Meth. Phys. Res. B 19/20 (1987) 204. [2] A.M. de Becdelievre, Y. Arnauld, N. Mesbahi, M. Brunel, J. de Becdelievre, M. Romand, Oxydation anodique de l’alliage de titane TA6V en milieu acide sulfurique 1 M: influence de l’implantation en ions azote, J. Chim. Phys. 2 (86) (1989) 365. [3] A.M. de Becdelievre, D. Fleche, J. de Becdelievre, Effect of nitrogen ion implantation on the electrochemical behaviour of TA6V in sulphuric medium, Electrochim. Acta 33 (8) (1988) 1067. [4] D. Krupa, E. Jezierska, J. Baszkiewicz, M. Kamin´ski, T. Wierzchon´, A. Barcz, Effect of nitrogen ion implantation on the structure and corrosion resistance of OT-4-0 titanium alloy, Surf. Coat. Technol. 79 (1996) 240. [5] E. Leita˜o, C. Sa´, R.A. Silva, M.A. Barbosa, H. Ali, Electrochemical and surface modifications on N+-ion implanted Ti–6Al–4V immersed in HBSS, Corros. Sci. 37 (11) (1995) 1861. [6 ] E. Leita˜o, R.A. Silva, M.A. Barbosa, Electrochemical and surface modification on N+-ion implanated Ti–5Al–2.5Fe immersed in HBSS, Corros. Sci. 39 (2) (1997) 377. [7] J. Yu, J. Zhao, L.X. Li, Corrosion fatigue resistances of surgical implant stainless steels and titanium alloy, Corros. Sci. 35 (1)2)3)4) (1993) 587. [8] R. Fillit, H. Bruyans, E. Patay, Europatent No. 84 500 954.5, 1987. [9] R. Fillit, A.J. Perry, J. Pol Doddelle, in: C.D. Ruud ( Ed.), Nondestructive Characterization of Materials, Plenum Press, New York, 1991. [10] T. Hanawa, M. Ota, Calcium phosphate naturally formed on titanium in electrolyte solution, Biomaterials 12 (1991) 767. [11] T. Hanawa, M. Ota, Characterization of surface film formed on titanium in electrolyte using XPS, Appl. Surf. Sci. 55 (1992) 269. [12] X. Qiu, R.A. Dodd, J.R. Conrad, A. Chen, F.J. Worzala, Microstructural study of nitrogen-implanted Ti–6Al–4V alloy, Nucl. Instr. Meth. Phys. Res. B 59/60 (1991) 951. [13] K. Hohmuth, B. Rauschenbach, High fluence implantation of nitrogen ions into titanium, Mater. Sci. Eng. 69 (1985) 4889.