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Abstract Current drawbacks in the hydroxyapatite (HA) thin film production for applications in bone surgery are their poor mechanical strength and limited adherence. This paper presents the ion beam implantation technique as an efficient method to improve the mechanical characteristics of HA films. Crystalline films of HA were grown by pulsed laser deposition, using a KrF* excimer laser (ls248 nm, t020 ns). The depositions were performed from pure HA targets on Ti–5Al–2.5Fe alloys coated with TiN or ZrO2 buffer layers. Samples were then implanted with Arq ions of high-energy (1.5 MeV) at a dose of 1016 cmy2. The asdeposited and implanted films were characterized by light microscopy and energy dispersive X-ray spectrometry. The mechanical properties of films were studied by nanoindentation and nano-scratch techniques using a Berkovich indenter tip. Films become harder and exhibit a higher Young modulus after implantation. The best values (5 GPa hardness and 130 GPa Young modulus, respectively) were obtained for the implanted films grown on TiN. An influence of the buffer layer nature on the mechanical behavior of films was observed. Films grown on ZrO2 are brittle and crack at moderate load (;12 mN) during scratch while these ones deposited on TiN successfully withstand loading. Residual stresses occur into the HAyZrO2 structure during processing and ion bombardment. 2003 Elsevier B.V. All rights reserved.
5 g cmy3 as supposed for ZrO2.e. This avoids gaseous argon bubbles to appear into the HA coating. HA films were grown in vacuum (.5-MeV energy and 1016 cmy2 dose. respectively.5 2. EDS microanalyses were carried out in order to determine the CayP atomic ratio in the films. Experimental details The HA films were deposited by a KrF* excimer laser (ls248 nm. Before deposition the substrates were coated with a TiN or ZrO2 buffer layer.5 2.98%) polycrystalline HA powder.8 and 4 g cmy3 as estimated for TiN and 3. This highly accurate microprobe of mechanical properties analysis has the capability of sensing continuously the load (F) and displacement (h) as indents are being made in the sample.33 1. Calculated Rp values are within the (1–1. 79% of theoretical density of HA bulk.5 mm) range with values of DRP down to 210 nm. Surface morphology Fig. 1 presents light micrographs showing the surface morphology of an as-deposited (a) and implanted HAy . i. Corp. According to the Lindhard–Scharff–Schiott (LSS) model w15x. After deposition the samples were implanted with Arq ions using a Van de Graaff-type implanter. 2. respectively.1 mm. The current density was held at 1 mA cmy2. 3. indentation depth (h). In order to show possible conformational differences between the implanted and non-implanted regions of films. as calculated by the TRIM code Multi-layer system HAyTiNyTiAlFe Density of buffer Density of HA Rp DRp layers (g cmy3) films (g cmy3) (mm) (nm) 2.5 MeV energy and 1016 cmy2 dose. / Thin Solid Films 453 – 454 (2004) 208–214 209 have extended the ion implantation technique to HA films grown on metallic substrates covered with TiN inter-layers using Arq ions w11x.5 to 15 mN. The normal applied load was linearly ramped between two imposed values. friction force (Ff) and friction coefficient (m) are simultaneously monitored. its standard deviation (DRP) and the maximal concentration (Cmax) of the implanted atoms. The length of the sliding track was approximately 500 mm.5Fe (TiAlFe) alloy bars were used as collectors. In order to choose the proper implantation energy.1. Two values of the buffer layer density were used for each system. 2. This paper presents comparative results of the ion beam implantation method applied to HA thin films grown on TiN and ZrO2 buffer layers by PLD in order to improve the mechanical properties and adherence of HA films. The hardness (H) and Young modulus (E) were determined using an ultra-low load indentation system.2.5 and 4. A well-defined trigonal diamond Berkovich indenter (tip radius less than 40 nm) is drawing over the film surface with a constant translating speed of 10 mm sy1. often observed for ionimplanted films. 3. the theoretical projection can be described by the mean projected range (Rp). To study the distribution of Ca. Results 3. F1s5 mN and F2s25 mN. The normal load (F). the Nano Indenter XP (MTS. Multiple indentations were made with load ranging from 0. Buffer layers were also grown by PLD from stoichiometric TiN and ZrO2 targets in low-pressure of nitrogen and oxygen. tests at a constant normal load of 25 mN were also performed.13x.5 2. respectively. These results show that Arq ions were generally centered at the middle of the buffer layer. Calculations were run for multi-layer systems of HAy TiNyTiAlFe and HAyZrO2 yTiAlFe using different values of thickness and density of layers. while the TiN and ZrO2 buffer layers are approximately 500 nm thick. HA films have a thickness of . simulations of ions distributions as implanted in the films depth were performed using the TRIM code w14x.5 4.18 1. Friction behavior was studied on the same apparatus equipped with a nano-scratch attachment. The mechanical properties of films were studied by nanoindentation and nano-scratch techniques. The as-deposited and implanted films were characterized by light microscopy and energy dispersive X-ray spectrometry (EDS).5 g cmy3. The ion beam was 1. Polished disks cut from Ti–5Al–2. The density of HA films was assumed to be 2.8 4 2. The laser beam was focused at 458 on a hot-pressing pellet obtained from a high-purity (99. equipped with a Berkovich Table 1 Mean projected range (Rp) and its standard deviation (DRP) of Arq ions implanted at 1.V. on substrates heated at 650 8C w12.10y4 Pa residual pressure) at room temperature of substrates and subsequently annealed in ambient air at 5508 C for 1 h.07 207 153 169 162 HAyZrO2 yTiAlFe 3. cartography analyses (elemental distribution maps) were performed.5-MeV energy and 1016 cmy2 dose. The hardness and mechanical resistance of films increase after implantation while the crystalline structure of HA was preserved.24 1.5 indenter tip.).5 1. We used a Kevex EDX analyser operated on a Philips XL 30 environmental scanning electron microscope. TRIM simulations Table 1 presents the values of Rp and DRP characteristic to a HA (1 mm)ybuffer layer (500 nm)yTiAlFe multi-layer system obtained by implantation simulations with Arq ions of 1. tFWHM020 ns). Nelea et al. P and O elements on the film surface. E and H were calculated starting from the load– displacement curves using the Oliver–Pharr model w16x.
the interaction between Arq ions and droplets and their mechanism of destruction is not very clear. characteristic to the Ti alloy substrate. P can be lost during film deposition or reevaporate from the substrate under bombardment with energetic species of plasma. 1b). An important modification of the surface aspect after implantation was observed. An irregular granular morphology suggesting an important surface roughness of non-implanted films was observed. Oxygen was found as homogenous located on the entire surface. respectively) were obtained for films grown on TiN. 3. Implanted films were harder and have a higher Young modulus. independently of the buffer layer nature and ion treatment. A depletion in P of the as-deposited films with respect to the HA target stoichiometry (CayPs1. i. Its signal should been coming from both HA structure and molecular oxygen trapped in the film surface from air. The surface is constituted by agglomerations of grain-shape particles (droplets) that are typical for pulsed laser deposited films w2x. Al and Fe.3.210 V. As shown in Fig. It was supposed that passing through the film. The number of droplets was . 2 presents the evolution of the maximum depth with the applied load obtained by nanoindentation for the as-deposited and implanted films grown on TiN (a) and ZrO2 (b) buffer layers. 1. / Thin Solid Films 453 – 454 (2004) 208–214 significantly reduced and films exhibited practically particles-free surfaces. films toughness.e. 1c. However. The curves give qualitative information on the films resistance at the indenter penetration. No particular features between the implanted and nonimplanted regions were observed. Comparing the penetration depth.8–2 range. Fig. Signals of Ti. Chemical microanalysis As measured by EDS. the implanted surface of HAyZrO2 film has a particular morphology. TiN (b) and HAyZrO2 (c) films. in contrast to the implanted one of HAyTiN (Fig. 3. Results demonstrate that a more compact structure of HA was formed by implantation. it concludes that films grown on TiN became more resistant after implantation than those ones grown on zirconia. Nanoindentation and nano-scratch tests Fig. were also recorded. Elemental distribution maps show that Ca and P are uniformly distributed on the film surface. Table 2 presents the measured and mean values of the hardness and Young modulus of as-deposited and ion-implanted HA films as measured by nanoindentation.4. The occurrence of frontiers proves residual stress developing in the film structure by ion bombardment. The film stoichiometry was not modified by ion bombardment. The highest values (5 GPa hardness and 130 GPa Young modulus. the ion beam destroys droplets and other defects. Light micrographs showing the surface morphology of an asdeposited (a) and implanted HAyTiN (b) and HAyZrO2 (c) films. Large formations of grains delimited by well-defined frontiers were formed after implantation. the CayP atomic ratio of films varies within the 1. As lighter than Ca. Nelea et al. Curves show that both implanted films (grown on TiN and ZrO2) were tougher.7) was evidenced.
scratch tests at a constant load of 25 mN were performed. Films grown on TiN successfully withstand loading. similar tests performed on HAy TiN samples showed that films withstand loading wear Implanted (1) The best results (2) No accidents on the load–displacement curves (1) Very good results (2) No accidents on the load–displacement curves (1) Good results (2) Rare accidents on the load–displacement curves (1) Satisfactory results (2) Sometimes accidents occur on the curves . In Fig. The extent of pores. 3a displays the profiles under load as measured in the implanted regions during a scratch test. 2. 3b the friction force (Ff) between indenter and film material as a function of the applied normal load (F) was drawn. Table 3 Films behavior under nanoindentation Films HAyTiNyTiAlFe HAyZrO2 yTiAlFe Non-implanted while films grown on ZrO2 crack and damage at loads up to 12 mN. The indenter was translated on the same typical 500-mm length distance. 4 presents the friction coefficient (m) as varying with the scratch length (a) and a light micrograph (b) showing the scratching track at the frontier of nonimplanted–implanted zones. In order to study features of non-implanted and implanted zones of HAyZrO2 films. / Thin Solid Films 453 – 454 (2004) 208–214 211 Fig. Accidents occur under loading and poor load–displacement curves are recorded. As evidenced by the data fluctuations. As recently reported. starting from a non-implanted zone to an implanted one. the best results were obtained for the implanted films grown on TiN.V.5 60–130 110 100–150 130 30–45 60–70 35 65 HAyZrO2 yTiAlFe NI 0.6 3. great instabilities of the friction coefficient immediately when the indenter reaches the implanted zone was recorded.6 value of m measured in the non-implanted region. Evolution of the maximum depth with the applied load obtained by nanoindentation for as-deposited and implanted films grown on TiN (a) and ZrO2 (b) buffer layers. The normal load was linearly increased from 5 to 25 mN. mean value. observed in static nanoindentation. Fig.5 2. droplets and other defects are reduced after ionimplantation by network structural relaxation and grains rearrangement.5–3 I 4–5. NI. Indeed.8 5 0. Curves show that until cracking the penetration depth are lower for films grown on TiN.8 I 3–4 I. Table 2 Hardness (H) and Young modulus (E) of as-deposited and implanted HA films as measured by nanoindentation Films HAyTiNyTiAlFe H (GPa) Hm (GPa) E (GPa) Em (GPa) NI 2. 4b). It means that. m. Fig. non-implanted. film grown on ZrO2 suddenly cracks when the load reaches a critical load of approximately 12 mN value. confirming better resistance of HAyTiN films. Table 3 collects films behaviors under nanoindentation in term of toughness increasing and test reproducibility performed on the as-deposited and implanted structures. Worse results of as-deposited films are due to the presence of pores and droplets into the film depth w2x. after a relatively constant 0. implanted.4–0. The implanted zone cracks and failures at the level of TiAlFe substrate by delamination (Fig. films became denser and load– displacement curves became smoother and more reproducible under loading. Nelea et al. Again.
material is transferred to the substrate following laser beam–HA target interaction. HA powder injected into the plasma is heated to several 1000 8C and propelled at high speed to a substrate w17x. Friction coefficient as varying with the scratching length (a) and the corresponding scratching trace (b) obtained during a test performed at a constant load of 25 mN starting from the non-implanted to the implanted regions of HAyZrO2 films. heating is confined to a thin surface layer during laser irradiation. despite the density increasing (as evidenced by the better mechanical characteristics obtained by indentation). while those grown on ZrO2 crack and damage at 12 mN load. They should have also sufficient toughness and acceptable mechanical properties necessary for load-bearing devices. dense and adherent HA thin films can be produced by PLD w2–5x. crystalline. for example plasma spraying that is commercially used to cover bone prosthesis with HA. Results show that. large optical absorption and poor thermal conductivity of HA. The target Fig. Moreover. This characteristic. 4. good control of densityyporosity and excellent adherence to the metallic support. In PLD. less convenient for the subsequent biomedical application. (a) Profiles under loading as drawn in the films implanted regions during a scratch test. proper stoichiometry. demonstrating film toughness increasing. the films grown on zirconia buffer became brittle after implantation. stoichiometric. (b) Friction force in function of the applied load recorded during the same test. In plasma spraying. The normal load was linearly increased from 5 to 25 mN. PLD offers decisive advantages with respect to other deposition techniques. . Nelea et al. under optimal conditions. 4. w11x. Due to short laser pulse duration. 3. Discussion HA coatings with application in bone implantology should have high crystallinity. As known. The implanted zone cracks and failure by delamination. the width of the scratch trace diminishes when the indenter arrives in the implanted region.212 V. Films grown on TiN successfully withstand loads. comes from residual stress developed in the film structure during deposition and ion beam treatment. / Thin Solid Films 453 – 454 (2004) 208–214 Fig. The obtained coatings contain anhydrous calcium phosphate phases decomposed from HA at high temperature and an important level of amorphous material due to rapid cooling of particles reaching the substrate.
J. Alford. Oliver. C. F. The best results were obtained for the implanted films grown on TiN buffer layer. It means that residual stress is expected to be greater for the HAy ZrO2 yTiAlFe structure. while films grown on TiN withstand tests. Ristoscu. T. We believe that film densification is due to combined effects of electronic and nuclear loss modes of bombarding ions. Pelletier. Phys. L. 1994. G. By passing through the film. Levine et al.N. w12x D. H. Grabowski. Arq ions diminish the number of pores. the interaction between Arq ions and droplets and their mechanism of destruction is not exhaustively clear. Harder and more difficult to scratch HA thin films were obtained by implantation with Ar.M. J. Fernandez-Pradas. Ca. A 23 (1981) 64. Ion beam implantation is a non-conventional technique used for surface modification and thin films densification. References w1x H. proving stress existence at this interface. C. J. Cotell. The densification is proven by the improved mechanical properties of films as measured by nanoindentation. 236 (1992) 429. Littmark. Some authors reported densification of thin films using ion beam implantation technique. Singh. Lopatin. Surf. Stress should be localized in both ZrO2 inter-layer and HA film. D. N. w8x L.M. w11x V. Proc. Conclusion The ion beam implantation technique revealed to be a convenient tool for mechanical properties improvement of HA films.B. Torrisi. w2x D. Tokyo. Lindhard. A. Surf. Chrisey. Schiott. Aoki. Films grown on ZrO2 delaminate under scratch. w14x U. Ions collide with the atoms of film and of the substrate. C. V. This densification way fundamentally differs from that of conventional sintering at high temperature characterized by condensation. Soc. Symp. w10x C. Methods B 145 (1998) 522. films were harder. 186 (2002) 483. respectively. Ghica.B. Sci. 1c). Sci. P. Sardin. Densification is most likely achieved by condensationycross-linking reactions and structural relaxation. atoms. Matthews. Moudgil. . Kuan. B. physically moving them so that the interface becomes integrated and blurred. Despite a density increasing. more resistant and have a higher Young modulus. Morenza. 1991. Mille. Mihailescu. respectively) residual stress into the multi-layers are expected to appear during PLD processing due to difference between the thermal expansion coefficients of the involved materials. C. structural relaxation and viscous flow w20x. film delamination occurs at the level of second interface. TiN w22x and HA w8x are (10.-C. As TiN is a related compound of the substrate (forming by insertion of N atoms within the Ti lattice). I. J. Science and Medical Application of Hydroxyapatite. I.A. 14 (1963) 33. Ziegler. F. Dan. C.D. Craciun. like absorption of energy by inverse bremsstrahlung are of high importance yielding particles with energies far higher than those of thermal regime w18x. Sci. w7x C. The HAyTiN films withstand the loading and no cracks or film damage occurred. Cleries. P. Mille. Vid.S. Sci. Our results showed that HA films grown on TiN and ZrO2 inter-layers became denser after implantation with Arq ions of 1. Scharff. ZrO2 w21x. H. Qian.L. H. As both inter-layer and HA film were proceeded on heated substrate (650 and 550 8C. Cotell. V. Vac. Technol. the films grown on zirconia buffer layers accumulate residual stress and became brittle after implantation. 7–8. Surf.K. During ion bombardment electronic and nuclear collisions occur. Gossett. Med. P. Chuang. inducing structural relaxation and redistribution of grains into the film network. Nagabushnam. in PLD other physical phenomena. A 3 (6) (1985) 2670. Smith. N and O ions of 100-keV energy w19x. Nucl.M. Pulsed Laser Deposition of Thin Films. / Thin Solid Films 453 – 454 (2004) 208–214 213 surface is melted and rapid thermal expansion inducing a massive explosion of the material occurs. C. Nelea et al.A. 3 (1992) 87. Nelea. M. ´ ` Ferrater. performed densification of zirconia sol–gel thin films by implantation with 280-keV-energy Xeq ions w20x.-C.W. K. Nelea. Its existence was proved by the surface morphology of film (Fig. R. Instrum.). w5x J. 69 (1–4) (1993) 140.M. In contrast to plasma spraying that is essentially a thermal processing technique. 54 (1992) 75. Fys. Chrisey. Appl. Thin Solid Films 317 (1998) 393. it was expected that a high-quality TiNy TiAlFe interface with greater bond strength with respect to the ZrO2 yTiAlFe interface develops. Pelletier.5-MeV energy. the stress level may increase after ion bombardment. The theoretical thermal expansion coefficients of Ti- based substrate w8x. where well-defined grain frontiers have been observed. w15x J.B. Pizziconi. P and O are uniformly distributed on the film surface for both as-deposited and implanted films. M.V. These phenomena are favored by increased network mobility due to some bond-breaking and network rupture following the ion–matter interaction.L. R. Sci. Hubler (Eds. w9x P. w4x R. The expulsed material forms a plasma plume containing ions. It is also very probably that grains frontiers suffer conformational modification that induces material compactness. Wiley. Moreover. Serra. w13x G. However. Damodaran. G. Biomater. Mat. Ristoscu. Selsk. w3x C. Res. Chiritescu. Rev. Mater. The scratch tests showed different behavior after implantation between films grown on TiN and ZrO2 inter-layers. JAAS. V. The ion beam passes through and looses its energy by deceleration into the film depth. Craciun. Chen. T.F. 168 (1–4) (2000) 127. Appl. Laursen. Appl. R. Sprague.E. After implantation. J. Sioshansi. 5. Setola.M. Moreover. D.K. particles and defects. Thin Solid Films 227 (1993) 32. N. 9. Surf. molecules. Broll. ZrO2 yTiAlFe substrate. Mihailescu.3 and 11)=10y6 Ky1. Muller. w6x V. Appl. L.N. Biomaterials 15 (7) (1994) 522.R. Appl. clusters and hot particles. Cornet.
Biomaterials 20 (1999) 1. Res. Keddie. April (1989) 27. Mater. Pharr. Appl. Kreutz. 127–129 (1998) 606.214 V. Maccauro. E. Ph. Nelea. Levine.W. J. w21x C.P. Ceram. J. G.A. MRS Bull.M. J. Res. Surf. J. Giannelis. Soc. K. . Mayer. 2002. w22x V. Nelea et al. Biomed.D. w19x H.W. Gross.C. Oliver. 76 (5) (1993) 1369.E. w18x E. / Thin Solid Films 453 – 454 (2004) 208–214 w20x T. Piconi. w17x K. Am.L. 39 (1998) 580.C. Solnick-Legg. Berndt. Thesis. Legg. w16x W. Sci. C. Mater. Revesz. 7 (6) (1992) 1564. University of Strasbourg. J. G. P.

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