- Email: [email protected]
Atom ingress from synthetic body fluid into nanoporous layers formed in titanium by helium ion-implantation
Current Applied Physics 6 (2006) 327–330 www.elsevier.com/locate/cap www.kps.or.kr
Atom ingress from synthetic body fluid into nanoporous layers formed in titanium by helium ion-implantation Peter B. Johnson a
a,c,* ,
V. John Kennedy
b,c
, Andreas Markwitz
b,c
School of Chemical and Physical Sciences, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand b Rafter Research Centre, Institute of Geological and Nuclear Sciences, P.O. Box 31312, Lower Hutt, New Zealand c The MacDiarmid Institute for Advanced Materials and Nanotechnology, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand Available online 27 December 2005
Abstract Helium ion-implantation can be used to form a buried layer of nanoscale cavities in Ti. This offers the potential for diffusing dopant atoms of interest for biomedical applications, into a Ti surface. Disc specimens were taken from a He-implanted foil and a small spot in the centre was eroded to a different depth on each disc before treatment with synthetic biological fluid. IBA techniques (including RBS, NRA and PIXE) were used to determine the uptake of specific elements in two regions: the central eroded spot and the outer (un-eroded) region. The most significant effect was for oxygen. A small amount of prior erosion resulted in a threefold increase in the oxygen in both regions. Whereas, greater erosion caused a rapid reduction in the uptake in the centre, the strong enhancement in the outer region remained a striking feature. This provides convincing evidence that oxygen can move laterally over large distances through the nanoporous layer. Ó 2005 Elsevier B.V. All rights reserved. PACS: 61.82.Bg; 68.35. p; 68.37.Hk; 68.55.Ln; 81.20. n; 52.77.Dq Keywords: Bio-nanotechnology; Nanoporous surfaces; Plasma-immersion ion-implantation; Helium; Titanium; Ion-beam analysis
1. Introduction Titanium and titanium-based alloys are preferred materials for use as medical implants because of their low density, excellent fatigue strength and machining qualities, and good biocompatibility. The biocompatibility is related to the native oxide that forms readily at the Ti surface. Methods for increasing the thickness and bonding of the nearsurface oxide are of developing interest, e.g., Ref. [1]. Titanium based metals also have the potential to form modified surfaces with sufficient bioactivity to promote in vivo bone *
Corresponding author. Address: The MacDiarmid Institute for Advanced Materials and Nanotechnology, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand. Tel.: +64 4 463 5996; fax: +64 4 463 5237. E-mail address: [email protected] (P.B. Johnson). 1567-1739/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2005.11.011
growth and osseo-integration through the incorporation of dopant atoms such as Ca and O [1–3]. Here we investigate the use of nanoporosity produced by He ion-implantation as a means of enhancing the ingress of such atoms. We have shown in previous work that He-implantation can be used to introduce nanoscale cavities in very high concentrations into the surface of materials [4–11]. This allows the possibility of subsequent modification of the surface by diffusing (and/or implanting) desired dopant atoms such as O into the cavity structure. There are a number of potential advantages, including the ability to tailor a smooth change in physical and chemical properties over a relatively broad depth range so as to integrate the modified layer into the surface rather than form it on top of the surface. In the present work the He is implanted using pulsed plasma-immersion ion-implantation (PI3TM) [12]. The
328
P.B. Johnson et al. / Current Applied Physics 6 (2006) 327–330
combination of energies and fluences used produces the cavity structures in a buried layer and we investigate the effect of removing some of the overlying surface as a means of enhancing the ingress of dopant atoms. Previously we have used thermal diffusion [4–11] or ion-implantation [5,7] to introduce the desired elements. Here we investigate a chemical treatment based on exposing specimens to synthetic body fluid (SBF). Following SBF treatment elemental depth-profiles were determined by RBS and nuclear reaction analysis (NRA). 2. Experimental The Ti target was in the form of polished, annealed, high-purity polycrystalline foil. The PI3TM He-implantation was at 300 °C and used a sequence of three different energies: 40, 20 and 6 keV to give a broad He distribution centred on a depth 250 nm. The fluences were respectively (in units of 1017 He cm 2) 8, 6, and 4. In previous work [7– 11] the depth-profiles for H, He, C, N, and O were measured using heavy-ion elastic recoil detection analysis (HERDA) [13], employing an analysing beam comprised 77 MeV iodine ions. However, it has been found that a HERDA beam can severely erode Ti targets that contain high levels of He [11]. For this reason RBS and NRA depth-profiling techniques have been preferred in this work. Discs, 3 mm in diameter, were punched from Ti foil. A small spot, 1 mm in diameter was eroded in the centre of each disc by means of ion-beam milling in a Gatan precision ion-polisher. Each disc was eroded to a different depth and then treated with SBF under the same controlled conditions. The specimens and the various treatments used in their preparation are summarised in Table 1. (For comparison purposes Ti #0 received the same polishing, annealing and bath treatment as the other specimens, but was not implanted and not ion-eroded.) The depths of erosion in arbitrary units (unless stated) are as follows: Ti #0, #8, #9—zero; Ti #1—200 nm; Ti #2—100; Ti #3—200; Ti #4—250; Ti #5—300; Ti #6— 350; Ti #7—400. (These depths are subject to large uncertainties owing to the varying porosity of the Ti, and possible differences in the ion-beam polishing characteristics, from one disc to another.) Ti #1 has had the entire surface
Table 1 Summary of disc treatments Specimen
He-implantation
Spot erosion
Bath treatment
Ti Ti Ti Ti Ti
Yes Yes No Yes Yes
HERDA Yes No No No
Yes Yes Yes Yes No
#1 #2–7 #0 #8 #9
Discs Ti #2–Ti #7 have been ion polished to varying depths in a central spot. Ti #1 has had the entire surface removed to a depth of 200 nm as the result of HERDA analysis.
Fig. 1. SEM micrograph of the nanoporous layer exposed in the centre of Ti #7 by ion polishing shows the larger scale cavities and Ti grain size. (Nanoscale cavities are too small to be seen here.)
removed to a depth 200 nm (estimated from step-height measurements) as the result of a HERDA determination of elemental composition [11]. The SEM micrograph of Fig. 1 shows the larger scale (micron-sized) cavity structure revealed by ion-beam polishing. (The smaller nanoscale cavities occurring in high concentration are only evident in TEM micrographs, see Ref. [7] for example). The as-prepared discs were placed in microporous capsules and soaked for fourteen-days in 100 ml of an acellular SBF [2] maintained at 37 °C and pH 7.4. The initial ionic concentrations (in mM) were: Na+ 142.0; K+ 5.0; Mg2+ 1.5; Ca2+ 2.5; Cl 147.8; HCO3 4.2; HPO24 1.0; SO24 0.5. (These concentrations are nearly equal to those in human blood plasma.) At the end of the fourteen-day period, the discs were taken from the fluid, rinsed with distilled water and air-dried. 3. Results and discussion For specimens Ti #2–Ti #7, the uptake of specific elements in two separate regions on each disc: the central eroded spot and the surrounding outer (un-eroded) region, was determined with IBA using RBS, NRA and particle induced X-ray analysis (PIXE). The RBS spectra have been unfolded with an iterative simulation process using RUMP [14]. For Ti #1 measurements were in the centre of the HERDA eroded area. For Ti #0, Ti #8 and Ti #9, which were not eroded, a single measurement was made in the centre of each disc. For Ti #8 a measurement was also made towards the edge of the disc to confirm that the results were the same as for the centre. 3.1. PIXE measurements In both regions of the discs, PIXE measurements show that the levels of the following elements are very low (in
P.B. Johnson et al. / Current Applied Physics 6 (2006) 327–330
some cases below the detection limit): F, Na, Mg, Al, Si, P, S, Cl, K, Fe, and Ca. NRA measurements show significant levels of O in both regions but only low levels of C and N. For this reason, the RUMP simulations have included only the elements He, O and Ti. It is clear He-implantation has not enhanced the uptake of either Ca or P, two elements that, in addition to O, are of particular interest.
329
3.2. RBS measurements Titanium concentration profiles for Ti #0, Ti #8 and Ti #1 deduced from RBS spectra are shown in Fig. 2. Representative RBS Ti concentration profiles for selected other discs are shown in Fig. 3. The best-fit data from the simulations are given in Table 2.
100
Ti concentration (at.%)
80
60 Ti#0-Unimplanted Ti#8 Implanted Ti#1 HERDA spot
40
20
0
50
100
150
200
250
300
350
400
Depth (nm) Fig. 2. Ti concentration profiles deduced from RBS spectra. (Simulation data summarised in Table 2). The differences between the He-implanted targets and the unimplanted target are owing to the presence of He and O. The 200 nm displacement of the Ti #1 profile relative to the Ti #8 profile reflects the removal of the Ti #1 surface to this depth in a prior HERDA analysis.
120
100
Ti concentration (at.%)
Ti#0 Unimplanted Ti#8 implanted 80
Ti#2 Centre Ti#2 outside Ti#4 centre Ti#4 outside
60
Ti#6 centre Ti#6 outside Ti#3 centre
40
20
0
50
100
150
200
250
300
350
400
450
500
550
600
Depth (nm) Fig. 3. Ti concentration profiles for representative specimens deduced from RBS spectra. (Simulation data summarised in Table 2). With the exception of Ti #2, erosion moves the Ti profile, for the eroded area, to the left by an amount corresponding to the depth of erosion. Note that the profiles for Ti #2 (centre spot), and for the outside regions of all discs eroded in the centre, show the effect of substantial oxygen uptake.
330
P.B. Johnson et al. / Current Applied Physics 6 (2006) 327–330
Table 2 Best-fit simulation data for He and O content of representative specimens obtained from the RUMP simulation analyses of RBS spectra Disc Ti#
0u 8u 1e 2e 2o 4e 4o 6e 6o
Bin width (nm) 1
2
3
20 20 20 88 88 90 90 90 90
– 142 80 107 150 70 90 70 100
– 107 – 71 80 – 90 – 90
Helium (at.%) – 6 1 17 19 18 20 1 20
– 32 21 24 24 9 25 1 25
Oxygen (at.%) – 22 – 25 25 – 25 – 30
64 62 64 27 19 7 30 5 31
– 3 5 27 28 1 25 1 21
– 4 – 10 10 – 10 – 7
RHe
RO
0 7018 1700 5839 7272 2250 6300 160 7000
1280 2094 1680 5975 6672 700 5850 520 5520
In column 1 the letters denote the following: u—un-eroded disc surface, e—eroded centre spot and o—outside eroded spot. The final two columns give the total He and O content (calculated as the sum of atomic concentration times bin width).
The results for reference disc Ti #0 showed the presence of a shallow surface oxide layer, 20 nm deep, with a stochiometric ratio consistent with titanium dioxide. A similar surface layer was present also in Ti #1 and Ti #8. In Ti #8 there was in addition O at measurable levels deeper in the target. The results for Ti #9 were very similar to those obtained for Ti #8 and so the two are listed together: they are typical for He-implanted surfaces that have not been subject to erosion. Whereas Ti #8 received the standard bath treatment, Ti #9 was subject only to casual oxidation in the normal laboratory atmosphere. It is concluded that for un-eroded specimens bath oxidation offers little advantage over casual oxidation. A comparison of Ti #8 and Ti #0, shows that pre-implantation with He causes an increase in O in the deeper regions of the implanted layer. In the case of Ti #1 the effect of the widespread surface removal has been to reduce the amounts of He and O retained in the implanted layer. This is also the case for the other heavily eroded discs, Ti #3 to Ti #7: within broad limits there is a decrease in both the He and O content in the eroded area with increasing erosion. The total O in the eroded area of each of these discs is at a low level compared with Ti #0. The striking exception is Ti #2 where in the eroded area a small amount of erosion has led to an expected small reduction in retained He, but to a threefold increase in the total O (compared with the un-eroded targets Ti #8 and Ti #9). Representative results for the outside areas of the discs are also included in Fig. 2 and Table 2. In this case the amount of retained He shows little variation for the Heimplanted specimens (Ti #2–Ti #9). However, in a striking result, it is found that the effect of erosion in the centre is to raise the total O content outside the eroded spot by factors between 2 (for Ti #7) and 3 (for Ti #2) when compared with Ti #8 and Ti #9. This provides convincing evidence of the movement of O over large distances through the nanoporous layer. In recent work (to be published) we have shown that post-implantation processing can be used to produce flat-bottomed circular craters a few microns in
diameter that are intimately linked into the buried nanoporous layer. The large lateral diffusion of O demonstrated here raises the possibility of improving the mechanical/ chemical bonding of a modified surface by lateral keying of an oxide or hydroxyapatite layer. 4. Conclusion A small amount of erosion of the He-implanted Ti surface results in a large increase in O uptake in the eroded area but further erosion leads to a rapid decrease in O. Erosion in a small region can result in a threefold increase in oxygen uptake over a large area surrounding the eroded spot. We conclude that O has moved large distances laterally through the buried nanoporous layer. This effect, when coupled with Ca incorporation (for example, by Ca ionimplantation) could offer the potential for structuring radically new bioactive titanium surfaces. Acknowledgements This work was performed under research contracts to the New Zealand Foundation for Research Science and Technology and research grants from the Australian Institute of Nuclear Science and Engineering (AINSE). We thank C. Varoy of VUW and G. Collins, K. Short and N. Dytlewski of ANSTO for useful discussions and research assistance. References [1] S. Ma¨ndl, R. Sader, G. Thorwarth, D. Krause, H.-F. Zeilhofer, H.H. Horch, B. Rauschenbach, Nucl. Instrum. Methods Phys. Res., Sect. B 206 (2003) 517. [2] H. Takadama, H.-M. Kim, T. Kokubo, T. Nakamura, J. Biomed. Mater. Res. 57 (3) (2001) 441. [3] X. Liu, R.W.Y. Poon, S.C.H. Kwok, P.K. Chu, C. Ding, Surf. Coat. Technol. 191 (2005) 43. [4] P.B. Johnson, R.W. Thomson, K. Reader, J. Nucl. Mater. 273 (1999) 117. [5] A. Markwitz, P.B. Johnson, P.W. Gilberd, G.A. Collins, D.D. Cohen, N. Dytlewski, Nucl. Instrum. Methods Phys. Res., Sect. B 161–163 (2000) 1048. [6] P.B. Johnson, A. Markwitz, P.W. Gilberd, Adv. Mater. 13 (2001) 997. [7] P.B. Johnson, P.W. Gilberd, A. Markwitz, W.J. Trompetter, G.A. Collins, K.T. Short, D.D. Cohen, N. Dytlewski, Surf. Coat. Technol. 136 (2001) 217. [8] A. Markwitz, P.B. Johnson, P.W. Gilberd, G.A. Collins, Nucl. Instrum. Methods Phys. Res., Sect. B 190 (2002) 718. [9] P.B. Johnson, V.J. Kennedy, A. Markwitz, C.R. Varoy, N. Dytlewski, K.T. Short, Nucl. Instrum. Methods Phys. Res., Sect. B 206 (2003) 1056. [10] V.J. Kennedy, P.B. Johnson, A. Markwitz, C.R. Varoy, K.T. Short, Nucl. Instrum. Methods Phys. Res., Sect. B 210 (2003) 543. [11] A. Markwitz, V.J. Kennedy, S.M. Durbin, P.B. Johnson, A. Mu¨cklich, N. Dytlewski, Surf. Interface Anal. 36 (2004) 317. [12] G.A. Collins, R. Hutchings, K.T. Short, J. Tendys, Surf. Coat. Technol. 104 (1998) 212. [13] J.W. Martin, D.D. Cohen, N. Dytlewski, D.B. Garton, H.J. Whitlow, G.J. Russell, Nucl. Instrum. Methods Phys. Res., Sect. B 94 (1994) 277. [14] L. Doolittle, Nucl. Instrum. Methods Phys. Res., Sect. B 9 (1985) 334.
Atom ingress from synthetic body fluid into nanoporous layers formed in titanium by helium ion-implantation Peter B. Johnson a
a,c,* ,
V. John Kennedy
b,c
, Andreas Markwitz
b,c
School of Chemical and Physical Sciences, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand b Rafter Research Centre, Institute of Geological and Nuclear Sciences, P.O. Box 31312, Lower Hutt, New Zealand c The MacDiarmid Institute for Advanced Materials and Nanotechnology, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand Available online 27 December 2005
Abstract Helium ion-implantation can be used to form a buried layer of nanoscale cavities in Ti. This offers the potential for diffusing dopant atoms of interest for biomedical applications, into a Ti surface. Disc specimens were taken from a He-implanted foil and a small spot in the centre was eroded to a different depth on each disc before treatment with synthetic biological fluid. IBA techniques (including RBS, NRA and PIXE) were used to determine the uptake of specific elements in two regions: the central eroded spot and the outer (un-eroded) region. The most significant effect was for oxygen. A small amount of prior erosion resulted in a threefold increase in the oxygen in both regions. Whereas, greater erosion caused a rapid reduction in the uptake in the centre, the strong enhancement in the outer region remained a striking feature. This provides convincing evidence that oxygen can move laterally over large distances through the nanoporous layer. Ó 2005 Elsevier B.V. All rights reserved. PACS: 61.82.Bg; 68.35. p; 68.37.Hk; 68.55.Ln; 81.20. n; 52.77.Dq Keywords: Bio-nanotechnology; Nanoporous surfaces; Plasma-immersion ion-implantation; Helium; Titanium; Ion-beam analysis
1. Introduction Titanium and titanium-based alloys are preferred materials for use as medical implants because of their low density, excellent fatigue strength and machining qualities, and good biocompatibility. The biocompatibility is related to the native oxide that forms readily at the Ti surface. Methods for increasing the thickness and bonding of the nearsurface oxide are of developing interest, e.g., Ref. [1]. Titanium based metals also have the potential to form modified surfaces with sufficient bioactivity to promote in vivo bone *
Corresponding author. Address: The MacDiarmid Institute for Advanced Materials and Nanotechnology, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand. Tel.: +64 4 463 5996; fax: +64 4 463 5237. E-mail address: [email protected] (P.B. Johnson). 1567-1739/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2005.11.011
growth and osseo-integration through the incorporation of dopant atoms such as Ca and O [1–3]. Here we investigate the use of nanoporosity produced by He ion-implantation as a means of enhancing the ingress of such atoms. We have shown in previous work that He-implantation can be used to introduce nanoscale cavities in very high concentrations into the surface of materials [4–11]. This allows the possibility of subsequent modification of the surface by diffusing (and/or implanting) desired dopant atoms such as O into the cavity structure. There are a number of potential advantages, including the ability to tailor a smooth change in physical and chemical properties over a relatively broad depth range so as to integrate the modified layer into the surface rather than form it on top of the surface. In the present work the He is implanted using pulsed plasma-immersion ion-implantation (PI3TM) [12]. The
328
P.B. Johnson et al. / Current Applied Physics 6 (2006) 327–330
combination of energies and fluences used produces the cavity structures in a buried layer and we investigate the effect of removing some of the overlying surface as a means of enhancing the ingress of dopant atoms. Previously we have used thermal diffusion [4–11] or ion-implantation [5,7] to introduce the desired elements. Here we investigate a chemical treatment based on exposing specimens to synthetic body fluid (SBF). Following SBF treatment elemental depth-profiles were determined by RBS and nuclear reaction analysis (NRA). 2. Experimental The Ti target was in the form of polished, annealed, high-purity polycrystalline foil. The PI3TM He-implantation was at 300 °C and used a sequence of three different energies: 40, 20 and 6 keV to give a broad He distribution centred on a depth 250 nm. The fluences were respectively (in units of 1017 He cm 2) 8, 6, and 4. In previous work [7– 11] the depth-profiles for H, He, C, N, and O were measured using heavy-ion elastic recoil detection analysis (HERDA) [13], employing an analysing beam comprised 77 MeV iodine ions. However, it has been found that a HERDA beam can severely erode Ti targets that contain high levels of He [11]. For this reason RBS and NRA depth-profiling techniques have been preferred in this work. Discs, 3 mm in diameter, were punched from Ti foil. A small spot, 1 mm in diameter was eroded in the centre of each disc by means of ion-beam milling in a Gatan precision ion-polisher. Each disc was eroded to a different depth and then treated with SBF under the same controlled conditions. The specimens and the various treatments used in their preparation are summarised in Table 1. (For comparison purposes Ti #0 received the same polishing, annealing and bath treatment as the other specimens, but was not implanted and not ion-eroded.) The depths of erosion in arbitrary units (unless stated) are as follows: Ti #0, #8, #9—zero; Ti #1—200 nm; Ti #2—100; Ti #3—200; Ti #4—250; Ti #5—300; Ti #6— 350; Ti #7—400. (These depths are subject to large uncertainties owing to the varying porosity of the Ti, and possible differences in the ion-beam polishing characteristics, from one disc to another.) Ti #1 has had the entire surface
Table 1 Summary of disc treatments Specimen
He-implantation
Spot erosion
Bath treatment
Ti Ti Ti Ti Ti
Yes Yes No Yes Yes
HERDA Yes No No No
Yes Yes Yes Yes No
#1 #2–7 #0 #8 #9
Discs Ti #2–Ti #7 have been ion polished to varying depths in a central spot. Ti #1 has had the entire surface removed to a depth of 200 nm as the result of HERDA analysis.
Fig. 1. SEM micrograph of the nanoporous layer exposed in the centre of Ti #7 by ion polishing shows the larger scale cavities and Ti grain size. (Nanoscale cavities are too small to be seen here.)
removed to a depth 200 nm (estimated from step-height measurements) as the result of a HERDA determination of elemental composition [11]. The SEM micrograph of Fig. 1 shows the larger scale (micron-sized) cavity structure revealed by ion-beam polishing. (The smaller nanoscale cavities occurring in high concentration are only evident in TEM micrographs, see Ref. [7] for example). The as-prepared discs were placed in microporous capsules and soaked for fourteen-days in 100 ml of an acellular SBF [2] maintained at 37 °C and pH 7.4. The initial ionic concentrations (in mM) were: Na+ 142.0; K+ 5.0; Mg2+ 1.5; Ca2+ 2.5; Cl 147.8; HCO3 4.2; HPO24 1.0; SO24 0.5. (These concentrations are nearly equal to those in human blood plasma.) At the end of the fourteen-day period, the discs were taken from the fluid, rinsed with distilled water and air-dried. 3. Results and discussion For specimens Ti #2–Ti #7, the uptake of specific elements in two separate regions on each disc: the central eroded spot and the surrounding outer (un-eroded) region, was determined with IBA using RBS, NRA and particle induced X-ray analysis (PIXE). The RBS spectra have been unfolded with an iterative simulation process using RUMP [14]. For Ti #1 measurements were in the centre of the HERDA eroded area. For Ti #0, Ti #8 and Ti #9, which were not eroded, a single measurement was made in the centre of each disc. For Ti #8 a measurement was also made towards the edge of the disc to confirm that the results were the same as for the centre. 3.1. PIXE measurements In both regions of the discs, PIXE measurements show that the levels of the following elements are very low (in
P.B. Johnson et al. / Current Applied Physics 6 (2006) 327–330
some cases below the detection limit): F, Na, Mg, Al, Si, P, S, Cl, K, Fe, and Ca. NRA measurements show significant levels of O in both regions but only low levels of C and N. For this reason, the RUMP simulations have included only the elements He, O and Ti. It is clear He-implantation has not enhanced the uptake of either Ca or P, two elements that, in addition to O, are of particular interest.
329
3.2. RBS measurements Titanium concentration profiles for Ti #0, Ti #8 and Ti #1 deduced from RBS spectra are shown in Fig. 2. Representative RBS Ti concentration profiles for selected other discs are shown in Fig. 3. The best-fit data from the simulations are given in Table 2.
100
Ti concentration (at.%)
80
60 Ti#0-Unimplanted Ti#8 Implanted Ti#1 HERDA spot
40
20
0
50
100
150
200
250
300
350
400
Depth (nm) Fig. 2. Ti concentration profiles deduced from RBS spectra. (Simulation data summarised in Table 2). The differences between the He-implanted targets and the unimplanted target are owing to the presence of He and O. The 200 nm displacement of the Ti #1 profile relative to the Ti #8 profile reflects the removal of the Ti #1 surface to this depth in a prior HERDA analysis.
120
100
Ti concentration (at.%)
Ti#0 Unimplanted Ti#8 implanted 80
Ti#2 Centre Ti#2 outside Ti#4 centre Ti#4 outside
60
Ti#6 centre Ti#6 outside Ti#3 centre
40
20
0
50
100
150
200
250
300
350
400
450
500
550
600
Depth (nm) Fig. 3. Ti concentration profiles for representative specimens deduced from RBS spectra. (Simulation data summarised in Table 2). With the exception of Ti #2, erosion moves the Ti profile, for the eroded area, to the left by an amount corresponding to the depth of erosion. Note that the profiles for Ti #2 (centre spot), and for the outside regions of all discs eroded in the centre, show the effect of substantial oxygen uptake.
330
P.B. Johnson et al. / Current Applied Physics 6 (2006) 327–330
Table 2 Best-fit simulation data for He and O content of representative specimens obtained from the RUMP simulation analyses of RBS spectra Disc Ti#
0u 8u 1e 2e 2o 4e 4o 6e 6o
Bin width (nm) 1
2
3
20 20 20 88 88 90 90 90 90
– 142 80 107 150 70 90 70 100
– 107 – 71 80 – 90 – 90
Helium (at.%) – 6 1 17 19 18 20 1 20
– 32 21 24 24 9 25 1 25
Oxygen (at.%) – 22 – 25 25 – 25 – 30
64 62 64 27 19 7 30 5 31
– 3 5 27 28 1 25 1 21
– 4 – 10 10 – 10 – 7
RHe
RO
0 7018 1700 5839 7272 2250 6300 160 7000
1280 2094 1680 5975 6672 700 5850 520 5520
In column 1 the letters denote the following: u—un-eroded disc surface, e—eroded centre spot and o—outside eroded spot. The final two columns give the total He and O content (calculated as the sum of atomic concentration times bin width).
The results for reference disc Ti #0 showed the presence of a shallow surface oxide layer, 20 nm deep, with a stochiometric ratio consistent with titanium dioxide. A similar surface layer was present also in Ti #1 and Ti #8. In Ti #8 there was in addition O at measurable levels deeper in the target. The results for Ti #9 were very similar to those obtained for Ti #8 and so the two are listed together: they are typical for He-implanted surfaces that have not been subject to erosion. Whereas Ti #8 received the standard bath treatment, Ti #9 was subject only to casual oxidation in the normal laboratory atmosphere. It is concluded that for un-eroded specimens bath oxidation offers little advantage over casual oxidation. A comparison of Ti #8 and Ti #0, shows that pre-implantation with He causes an increase in O in the deeper regions of the implanted layer. In the case of Ti #1 the effect of the widespread surface removal has been to reduce the amounts of He and O retained in the implanted layer. This is also the case for the other heavily eroded discs, Ti #3 to Ti #7: within broad limits there is a decrease in both the He and O content in the eroded area with increasing erosion. The total O in the eroded area of each of these discs is at a low level compared with Ti #0. The striking exception is Ti #2 where in the eroded area a small amount of erosion has led to an expected small reduction in retained He, but to a threefold increase in the total O (compared with the un-eroded targets Ti #8 and Ti #9). Representative results for the outside areas of the discs are also included in Fig. 2 and Table 2. In this case the amount of retained He shows little variation for the Heimplanted specimens (Ti #2–Ti #9). However, in a striking result, it is found that the effect of erosion in the centre is to raise the total O content outside the eroded spot by factors between 2 (for Ti #7) and 3 (for Ti #2) when compared with Ti #8 and Ti #9. This provides convincing evidence of the movement of O over large distances through the nanoporous layer. In recent work (to be published) we have shown that post-implantation processing can be used to produce flat-bottomed circular craters a few microns in
diameter that are intimately linked into the buried nanoporous layer. The large lateral diffusion of O demonstrated here raises the possibility of improving the mechanical/ chemical bonding of a modified surface by lateral keying of an oxide or hydroxyapatite layer. 4. Conclusion A small amount of erosion of the He-implanted Ti surface results in a large increase in O uptake in the eroded area but further erosion leads to a rapid decrease in O. Erosion in a small region can result in a threefold increase in oxygen uptake over a large area surrounding the eroded spot. We conclude that O has moved large distances laterally through the buried nanoporous layer. This effect, when coupled with Ca incorporation (for example, by Ca ionimplantation) could offer the potential for structuring radically new bioactive titanium surfaces. Acknowledgements This work was performed under research contracts to the New Zealand Foundation for Research Science and Technology and research grants from the Australian Institute of Nuclear Science and Engineering (AINSE). We thank C. Varoy of VUW and G. Collins, K. Short and N. Dytlewski of ANSTO for useful discussions and research assistance. References [1] S. Ma¨ndl, R. Sader, G. Thorwarth, D. Krause, H.-F. Zeilhofer, H.H. Horch, B. Rauschenbach, Nucl. Instrum. Methods Phys. Res., Sect. B 206 (2003) 517. [2] H. Takadama, H.-M. Kim, T. Kokubo, T. Nakamura, J. Biomed. Mater. Res. 57 (3) (2001) 441. [3] X. Liu, R.W.Y. Poon, S.C.H. Kwok, P.K. Chu, C. Ding, Surf. Coat. Technol. 191 (2005) 43. [4] P.B. Johnson, R.W. Thomson, K. Reader, J. Nucl. Mater. 273 (1999) 117. [5] A. Markwitz, P.B. Johnson, P.W. Gilberd, G.A. Collins, D.D. Cohen, N. Dytlewski, Nucl. Instrum. Methods Phys. Res., Sect. B 161–163 (2000) 1048. [6] P.B. Johnson, A. Markwitz, P.W. Gilberd, Adv. Mater. 13 (2001) 997. [7] P.B. Johnson, P.W. Gilberd, A. Markwitz, W.J. Trompetter, G.A. Collins, K.T. Short, D.D. Cohen, N. Dytlewski, Surf. Coat. Technol. 136 (2001) 217. [8] A. Markwitz, P.B. Johnson, P.W. Gilberd, G.A. Collins, Nucl. Instrum. Methods Phys. Res., Sect. B 190 (2002) 718. [9] P.B. Johnson, V.J. Kennedy, A. Markwitz, C.R. Varoy, N. Dytlewski, K.T. Short, Nucl. Instrum. Methods Phys. Res., Sect. B 206 (2003) 1056. [10] V.J. Kennedy, P.B. Johnson, A. Markwitz, C.R. Varoy, K.T. Short, Nucl. Instrum. Methods Phys. Res., Sect. B 210 (2003) 543. [11] A. Markwitz, V.J. Kennedy, S.M. Durbin, P.B. Johnson, A. Mu¨cklich, N. Dytlewski, Surf. Interface Anal. 36 (2004) 317. [12] G.A. Collins, R. Hutchings, K.T. Short, J. Tendys, Surf. Coat. Technol. 104 (1998) 212. [13] J.W. Martin, D.D. Cohen, N. Dytlewski, D.B. Garton, H.J. Whitlow, G.J. Russell, Nucl. Instrum. Methods Phys. Res., Sect. B 94 (1994) 277. [14] L. Doolittle, Nucl. Instrum. Methods Phys. Res., Sect. B 9 (1985) 334.