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Thin Solid Films 453 454 (2004) 208214

0040-6090/04/$ - see front matter 2003 Elsevier B.V. All rights reserved.doi:10.1016/j.tsf.2003.11.119

High-energy ion beam implantation of hydroxyapatite thin films grownon TiN and ZrO inter-layers by pulsed laser deposition2

V. Nelea *, H. Pelletier , P. Mille , D. Mullera, a a b

Laboratoire dIngenierie des Surfaces, Institut National des Sciences Appliquees, 24, Bld. de la Victoire, 67084 Strasbourg, Francea

Laboratoire PHASE, CNRS, 23 rue de Loess, BP 20CR, 67037 Strasbourg, Franceb

Abstract

Current drawbacks in the hydroxyapatite (HA) thin film production for applications in bone surgery are their poor mechanicalstrength and limited adherence. This paper presents the ion beam implantation technique as an efficient method to improve themechanical characteristics of HA films. Crystalline films of HA were grown by pulsed laser deposition, using a KrF* excimerlaser (ls248 nm, t020 ns). The depositions were performed from pure HA targets on Ti5Al2.5Fe alloys coated with TiN orZrO buffer layers. Samples were then implanted with Ar ions of high-energy (1.5 MeV) at a dose of 10 cm . The as-q 16 y22deposited and implanted films were characterized by light microscopy and energy dispersive X-ray spectrometry. The mechanicalproperties of films were studied by nanoindentation and nano-scratch techniques using a Berkovich indenter tip. Films becomeharder 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 mechanicalbehavior of films was observed. Films grown on ZrO are brittle and crack at moderate load (;12 mN) during scratch while2these ones deposited on TiN successfully withstand loading. Residual stresses occur into the HAyZrO structure during processing2and ion bombardment. 2003 Elsevier B.V. All rights reserved.

Keywords: Hydroxyapatite thin films; Pulsed laser deposition; Ion beam implantation; Nanoindentation

1. Introduction

Hydroxyapatite (HA, Ca (PO ) OH) ceramics are5 4 3widely used as substitute of bone w1x. As coated onmetallic prostheses, HA offers the biocompatibility nec-essary for bone cell apposition. Current drawbacks inthe HA thin film production are their poor mechanicalstrength and limited adherence.

A feasible and modern technique for thin film pro-

duction is the pulsed laser deposition (PLD) w2x. Dif-ferent growth procedures of HA thin films using PLDwere reported w3 6x. Since depositions were performedin oxidizing environment on heated Ti-based substrates,precautions against substrate surface deterioration byoxidation during equilibration of deposition conditionsand film growing should be taken. The quality of thefilmsubstrate interface influences the film properties

*Corresponding author. Tel.: q33-3-88-14-47-00; fax: q33-3-88-14-47-39.

E-mail addresses: [email protected] (V. Nelea),[email protected] (V. Nelea).

and its subsequent functionality. Ti oxidation inducesstructural modifications of HA layers in the region nearthe substrate w6x and considerably reduces its adherencew7x.

As previously reported w6x, high quality HA filmswere growing by PLD inserting different buffer layers(TiN, ZrO or Al O ) at the filmsubstrate interface.2 2 3

Since HA starts to decompose at temperatures up to900 8C w8x, the densification of HA films during growing

or by subsequent annealing was limited. An efficientway to increase the density and adherence of a coatingat low temperature is the ion beam implantation tech-nique w9x. Ions accelerated at convenient energies (MeVrange) are passed through the film and come to stop inthe substrate. The film density and adherence enhancesby multiple in depth collisions and mixing of atoms atthe interface.

Lopatin et al. w10x reported densification of HAcoatings by high-energy ion beam implantation. Thefilms were created on Si substrates by solgel routesand implanted therefore with Si ions. Recently weqq

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209V. Nelea et al. / Thin Solid Films 453 454 (2004) 208214

Table 1Mean projected range (R ) and its standard deviation (DR ) of Arqp Pions implanted at 1.5-MeV energy and 10 cm dose, as calculated16 y2

by the TRIM code

Multi-layer Density of buffer Density of HA Rp DRpsystem layers (g cm )y3 films (g cm )y3 (mm) (nm)

HAyTiNyTiAlFe 2.8 2.5 1.33 2074 2.5 1.18 153

HAyZrO yTiAlFe2 3.5 2.5 1.24 1694.5 2.5 1.07 162

have extended the ion implantation technique to HAfilms grown on metallic substrates covered with TiNinter-layers using Ar ions w11x. The hardness andq

mechanical resistance of films increase after implanta-tion while the crystalline structure of HA was preserved.

This paper presents comparative results of the ion

beam implantation method applied to HA thin filmsgrown on TiN and ZrO buffer layers by PLD in order2to improve the mechanical properties and adherence ofHA films.

2. Experimental details

The HA films were deposited by a KrF* excimerlaser (ls248 nm, t 020 ns). The laser beam wasFWHMfocused at 458 on a hot-pressing pellet obtained from ahigh-purity (99.98%) polycrystalline HA powder. Pol-ished disks cut from Ti5Al2.5Fe (TiAlFe) alloy bars

were used as collectors. Before deposition the substrateswere coated with a TiN or ZrO buffer layer. Buffer2layers were also grown by PLD from stoichiometric TiNand ZrO targets in low-pressure of nitrogen and oxygen,2respectively, on substrates heated at 650 8C w12,13x. HAfilms were grown in vacuum (;10 Pa residualy4

pressure) at room temperature of substrates and subse-quently annealed in ambient air at 5508 C for 1 h. HAfilms have a thickness of;1 mm, while the TiN andZrO buffer layers are approximately 500 nm thick.2

After deposition the samples were implanted withAr ions using a Van de Graaff-type implanter. The ionq

beam was 1.5 MeV energy and 10 cm dose. The16 y2

current density was held at 1 mA cm .y2

In order to choose the proper implantation energy,simulations of ions distributions as implanted in thefilms depth were performed using the TRIM code w14x.Calculations were run for multi-layer systems of HAyTiNyTiAlFe and HAyZrO yTiAlFe using different val-2ues of thickness and density of layers. According to theLindhardScharffSchiott (LSS) model w15x, the theo-retical projection can be described by the mean projectedrange (R ), its standard deviation (DR ) and the maxi-p Pmal concentration (C ) of the implanted atoms.max

The as-deposited and implanted films were character-

ized by light microscopy and energy dispersive X-rayspectrometry (EDS). EDS microanalyses were carriedout in order to determine the CayP atomic ratio in thefilms. We used a Kevex EDX analyser operated on aPhilips XL 30 environmental scanning electron micro-scope. To study the distribution of Ca, P and O elementson the film surface, cartography analyses (elementaldistribution maps) were performed.

The mechanical properties of films were studied bynanoindentation and nano-scratch techniques. The hard-ness (H) and Young modulus (E) were determinedusing an ultra-low load indentation system, the NanoIndenter XP (MTS, Corp.), equipped with a Berkovich

indenter tip. This highly accurate microprobe of mechan-ical properties analysis has the capability of sensingcontinuously the load (F) and displacement (h) asindents are being made in the sample. Multiple inden-tations were made with load ranging from 0.5 to 15mN. E and H were calculated starting from the loaddisplacement curves using the OliverPharr model w16x.

Friction behavior was studied on the same apparatusequipped with a nano-scratch attachment. A well-definedtrigonal diamond Berkovich indenter (tip radius lessthan 40 nm) is drawing over the film surface with aconstant translating speed of 10 mm s . The normaly1

applied load was linearly ramped between two imposedvalues, Fs5 mN and Fs25 mN, respectively. The1 2length of the sliding track was approximately 500 mm.The normal load (F), indentation depth (h), frictionforce (F) and friction coefficient (m) are simultaneouslyfmonitored. In order to show possible conformationaldifferences between the implanted and non-implanted

regions of films, tests at a constant normal load of 25mN were also performed.

3. Results

3.1. TRIM simulations

Table 1 presents the values of R and DR character-p Pistic to a HA (1 mm)ybuffer layer (500 nm)yTiAlFemulti-layer system obtained by implantation simulationswith Ar ions of 1.5-MeV energy and 10 cm dose.q 16 y2

Two values of the buffer layer density were used foreach system, 2.8 and 4 g cm as estimated for TiNy3

and 3.5 and 4.5 g cm as supposed for ZrO , respec-y3 2tively. The density of HA films was assumed to be 2.5g cm , i.e. 79% of theoretical density of HA bulk.y3

Calculated R values are within the (11.5 mm) rangepwith values ofDR down to 210 nm. These results showPthat Ar ions were generally centered at the middle ofq

the buffer layer. This avoids gaseous argon bubbles toappear into the HA coating, often observed for ion-implanted films.

3.2. Surface morphology

Fig. 1 presents light micrographs showing the surfacemorphology of an as-deposited (a) and implanted HAy

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Fig. 1. Light micrographs showing the surface morphology of an as-deposited (a) and implanted HAyTiN (b) and HAyZrO (c) films.2

TiN (b) and HAyZrO (c) films. An irregular granular2morphology suggesting an important surface roughnessof non-implanted films was observed. The surface isconstituted by agglomerations of grain-shape particles(droplets) that are typical for pulsed laser depositedfilms w2x.

An important modification of the surface aspect afterimplantation was observed. The number of droplets was

significantly reduced and films exhibited practicallyparticles-free surfaces. It was supposed that passingthrough the film, the ion beam destroys droplets andother defects. However, the interaction between Arq

ions and droplets and their mechanism of destruction isnot very clear. As shown in Fig. 1c, the implanted

surface of HAyZrO film has a particular morphology,2in contrast to the implanted one of HAyTiN (Fig. 1b).Large formations of grains delimited by well-definedfrontiers were formed after implantation. The occurrenceof frontiers proves residual stress developing in the filmstructure by ion bombardment.

3.3. Chemical microanalysis

As measured by EDS, the CayP atomic ratio of filmsvaries within the 1.8 2 range, independently of thebuffer layer nature and ion treatment. A depletion in P

of the as-deposited films with respect to the HA targetstoichiometry (CayPs1.7) was evidenced. As lighterthan Ca, P can be lost during film deposition or re-evaporate from the substrate under bombardment withenergetic species of plasma. The film stoichiometry wasnot modified by ion bombardment.

Elemental distribution maps show that Ca and P areuniformly distributed on the film surface. Oxygen wasfound as homogenous located on the entire surface. Itssignal should been coming from both HA structure andmolecular oxygen trapped in the film surface from air.No particular features between the implanted and non-

implanted regions were observed. Signals of Ti, Al andFe, characteristic to the Ti alloy substrate, were alsorecorded.

3.4. Nanoindentation and nano-scratch tests

Fig. 2 presents the evolution of the maximum depthwith the applied load obtained by nanoindentation forthe as-deposited and implanted films grown on TiN (a)and ZrO (b) buffer layers. The curves give qualitative2information on the films resistance at the indenterpenetration, i.e. films toughness. Curves show that both

implanted films (grown on TiN and ZrO ) were tougher.2Comparing the penetration depth, it concludes that filmsgrown on TiN became more resistant after implantationthan those ones grown on zirconia. Results demonstratethat a more compact structure of HA was formed byimplantation.

Table 2 presents the measured and mean values ofthe hardness and Young modulus of as-deposited andion-implanted HA films as measured by nanoindenta-tion. Implanted films were harder and have a higherYoung modulus. The highest values (5 GPa hardnessand 130 GPa Young modulus, respectively) wereobtained for films grown on TiN.

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Fig. 2. Evolution of the maximum depth with the applied load obtained by nanoindentation for as-deposited and implanted films grown on TiN(a) and ZrO (b) buffer layers.2

Table 2Hardness (H) and Young modulus (E) of as-deposited and implantedHA films as measured by nanoindentation

Films H (GPa) H (GPa)m E (GPa) E (GPa)m

HAyTiNyTiAlFe NI 2.53 2.8 60130 110I 45.5 5 100150 130

HAyZrO yTiAlFe2 NI 0.40.8 0.6 3045 35I 34 3.5 6070 65

I, implanted; NI, non-implanted; m, mean value.

Table 3Films behavior under nanoindentation

Films Non-implanted Implanted

HAyTiNyTiAlFe (1) Good results (1) The best results(2) Rare accidents on the load displacement curves (2) No accidents on the loaddisplacement curves

HAyZrO yTiAlFe2 (1) Satisfactory results (1) Very good results(2) Sometimes accidents occur on the curves (2) No accidents on the loaddisplacement curves

Table 3 collects films behaviors under nanoindentation

in term of toughness increasing and test reproducibilityperformed on the as-deposited and implanted structures.Again, the best results were obtained for the implantedfilms grown on TiN. Worse results of as-deposited filmsare due to the presence of pores and droplets into thefilm depth w2x. Accidents occur under loading and poorload displacement curves are recorded. The extent ofpores, droplets and other defects are reduced after ion-implantation by network structural relaxation and grainsrearrangement. Indeed, films became denser and loaddisplacement curves became smoother and more repro-ducible under loading.

Fig. 3a displays the profiles under load as measuredin the implanted regions during a scratch test. Thenormal load was linearly increased from 5 to 25 mN.Films grown on TiN successfully withstand loading,

while films grown on ZrO crack and damage at loads2up to 12 mN. Curves show that until cracking thepenetration depth are lower for films grown on TiN,confirming better resistance of HAyTiN films, observedin static nanoindentation. In Fig. 3b the friction force(F) between indenter and film material as a functionfof the applied normal load (F) was drawn. As evidencedby the data fluctuations, film grown on ZrO suddenly2cracks when the load reaches a critical load of approx-imately 12 mN value.

In order to study features of non-implanted andimplanted zones of HAyZrO films, scratch tests at a2

constant load of 25 mN were performed. The indenterwas translated on the same typical 500-mm lengthdistance, starting from a non-implanted zone to animplanted one.

Fig. 4 presents the friction coefficient (m) as varyingwith the scratch length (a) and a light micrograph (b)showing the scratching track at the frontier of non-implantedimplanted zones. It means that, after a rela-tively constant 0.6 value of m measured in thenon-implanted region, great instabilities of the frictioncoefficient immediately when the indenter reaches theimplanted zone was recorded. The implanted zone cracks

and failures at the level of TiAlFe substrate by delami-nation (Fig. 4b).As recently reported, similar tests performed on HAy

TiN samples showed that films withstand loading wear

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Fig. 3. (a) Profiles under loading as drawn in the films implanted regions during a scratch test. The normal load was linearly increased from 5to 25 mN. (b) Friction force in function of the applied load recorded during the same test. Films grown on TiN successfully withstand loads,

while those grown on ZrO crack and damage at 12 mN load.2

Fig. 4. Friction coefficient as varying with the scratching length (a)and the corresponding scratching trace (b) obtained during a test per-formed at a constant load of 25 mN starting from the non-implantedto the implanted regions of HAyZrO films. The implanted zone2cracks and failure by delamination.

w11x. Moreover, the width of the scratch trace diminisheswhen the indenter arrives in the implanted region,demonstrating film toughness increasing.

Results show that, despite the density increasing (asevidenced by the better mechanical characteristicsobtained by indentation), the films grown on zirconiabuffer became brittle after implantation. This character-istic, less convenient for the subsequent biomedicalapplication, comes from residual stress developed in thefilm structure during deposition and ion beam treatment.

4. Discussion

HA coatings with application in bone implantologyshould have high crystallinity, proper stoichiometry,good control of densityyporosity and excellent adher-ence to the metallic support. They should have alsosufficient toughness and acceptable mechanical proper-ties necessary for load-bearing devices. As known, underoptimal conditions, crystalline, stoichiometric, dense andadherent HA thin films can be produced by PLD w2 5x.PLD offers decisive advantages with respect to other

deposition techniques, for example plasma spraying thatis commercially used to cover bone prosthesis with HA.In plasma spraying, HA powder injected into the plasmais heated to several 1000 8C and propelled at high speedto a substrate w17x. The obtained coatings contain anhy-drous calcium phosphate phases decomposed from HAat high temperature and an important level of amorphousmaterial due to rapid cooling of particles reaching thesubstrate. In PLD, material is transferred to the substratefollowing laser beamHA target interaction. Due toshort laser pulse duration, large optical absorption andpoor thermal conductivity of HA, heating is confined toa thin surface layer during laser irradiation. The target

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surface is melted and rapid thermal expansion inducinga massive explosion of the material occurs. The expulsedmaterial forms a plasma plume containing ions, atoms,molecules, clusters and hot particles. In contrast toplasma spraying that is essentially a thermal processingtechnique, in PLD other physical phenomena, like

absorption of energy by inverse bremsstrahlung are ofhigh importance yielding particles with energies farhigher than those of thermal regime w18x.

Ion beam implantation is a non-conventional tech-nique used for surface modification and thin filmsdensification. The ion beam passes through and loosesits energy by deceleration into the film depth. Duringion bombardment electronic and nuclear collisions occur.Ions collide with the atoms of film and of the substrate,physically moving them so that the interface becomesintegrated and blurred. Some authors reported densifi-cation of thin films using ion beam implantation tech-

nique. Harder and more difficult to scratch HA thinfilms were obtained by implantation with Ar, N and Oions of 100-keV energy w19x. Levine et al. performeddensification of zirconia solgel thin films by implan-tation with 280-keV-energy Xe ions w20x.q

Our results showed that HA films grown on TiN andZrO inter-layers became denser after implantation with2Ar ions of 1.5-MeV energy. The densification is provenq

by the improved mechanical properties of films asmeasured by nanoindentation. By passing through thefilm, Ar ions diminish the number of pores, particlesq

and defects, inducing structural relaxation and redistri-bution of grains into the film network. It is also very

probably that grains frontiers suffer conformational mod-ification that induces material compactness. However,the interaction between Ar ions and droplets and theirq

mechanism of destruction is not exhaustively clear. Webelieve that film densification is due to combined effectsof electronic and nuclear loss modes of bombardingions. Densification is most likely achieved by conden-sationycross-linking reactions and structural relaxation.These phenomena are favored by increased networkmobility due to some bond-breaking and network rupturefollowing the ionmatter interaction. This densificationway fundamentally differs from that of conventional

sintering at high temperature characterized by conden-sation, structural relaxation and viscous flow w20x.The scratch tests showed different behavior after

implantation between films grown on TiN and ZrO2inter-layers. Films grown on ZrO delaminate under2scratch, while films grown on TiN withstand tests. Asboth inter-layer and HA film were proceeded on heatedsubstrate (650 and 550 8C, respectively) residual stressinto the multi-layers are expected to appear during PLDprocessing due to difference between the thermal expan-sion coefficients of the involved materials. Moreover,the stress level may increase after ion bombardment.The theoretical thermal expansion coefficients of Ti-

based substrate w8x, ZrO w21x, TiN w22x and HA w8x are2(10, 78, 9.3 and 11)=10 K , respectively. It meansy6 y1

that residual stress is expected to be greater for the HAyZrO yTiAlFe structure. Stress should be localized in2both ZrO inter-layer and HA film. Its existence was2proved by the surface morphology of film (Fig. 1c),

where well-defined grain frontiers have been observed.Moreover, film delamination occurs at the level ofsecond interface, ZrO yTiAlFe substrate, proving stress2existence at this interface. As TiN is a related compoundof the substrate (forming by insertion of N atoms withinthe Ti lattice), it was expected that a high-quality TiNyTiAlFe interface with greater bond strength with respectto the ZrO yTiAlFe interface develops.2

5. Conclusion

The ion beam implantation technique revealed to be

a convenient tool for mechanical properties improvementof HA films. After implantation, films were harder, moreresistant and have a higher Young modulus. The bestresults were obtained for the implanted films grown onTiN buffer layer. Ca, P and O are uniformly distributedon the film surface for both as-deposited and implantedfilms. Despite a density increasing, the films grown onzirconia buffer layers accumulate residual stress andbecame brittle after implantation. The HAyTiN filmswithstand the loading and no cracks or film damageoccurred.

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