Surface & Coatings Technology 204 (2010) 1755–1763

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Surface & Coatings Technology

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In situ composite coating of titania–hydroxyapatite on commercially pure titaniumby microwave processing

A. Siddharthan 1, T.S. Sampath Kumar ⁎, S.K. SeshadriDepartment of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai – 600036, India

⁎ Corresponding author. Tel.: +91 44 2257477; fax: +E-mail address: tssk@iitm.ac.in (T.S.S. Kumar).

1 Present address: Department of Mechanical EnginGuindy Campus, Anna University Chennai, Chennai – 60

0257-8972/$ – see front matter © 2009 Elsevier B.V. Aldoi:10.1016/j.surfcoat.2009.11.003

a b s t r a c t

a r t i c l e i n f o

Article history:Received 1 April 2009Accepted in revised form 3 November 2009Available online 10 November 2009

Keywords:[C] Microwave[D] Titanium oxide[D] Titanium[X] Hydroxyapatite[B] Scanning electron microscopy[B] Transmission electron microscopy

Microwave (MW) processing has been studied as an alternative method of hydroxyapatite (HA) basedcomposite coatings on commercially pure titanium (CPTi) to enhance the bioactivity for orthopaedic anddental implant applications. The coating was formed by processing CPTi metal packed in HA and at 800 Wmicrowave power for 22 min. The composition of the coating was found to be TiO2 (rutile) as major phasealong with HA as minor phase. The MW absorption of non-stoichiometric TiO2 layer, which was grownduring the initial hybrid heating, resulted in sintering of apatite particles interfacing them. The non-stoichiometric nature of TiO2 was evident from the observed mid-gap bands in ultraviolet–visible diffusivereflectance (UV–VIS-DR) spectrum. The lamellar α structure of the substrate suggests that the processingtemperature was above β transus of CPTi (1155 K). The oxygen stabilized α phase whose thickness increasedwith microwave processing time, was likely to be the reason for the increase in Young's Modulus andhardness of the substrate. The coating induced apatite precipitation in bioactivity test. The osteoblast celladhesion test demonstrated cell spreading which is considered favourable for cell proliferation anddifferentiation. Thus, in situ composite coating of titania and HA on CPTi was obtained by a simple one-stepprocess.

91 44 22570545.

eering, College of Engineering0025, India.

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© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Titanium and its alloys have been successful for implantapplication because of their excellent biocompatibility and corrosionresistance arising from the formation of a self-healing passive oxidelayer on their surface [1]. Though they satisfy the requiredmechanicalproperties and bio-inertness, surface modification has becomeessential to enhance their osteo-integration [2]. Such surfacemodifications could alter the physical, chemical and biologicalproperties of the surface [3–7]. Coating these titanium based materialwith a layer of hydroxyapatite (HA) has received considerableattention [2]. A number of techniques to coat HA on metals areavailable by either high temperature process (thermal spray, pulsedlaser deposition etc) or low temperature process (electrophoretic,electrolytic, biomimetic etc) [8]. However, failure at the metal–coating interface leading to loosening of implants is a matter of majorconcern. With a view to improve the strength of the coating, postcoatheat treatment has been attempted, for both categories of coatings[8,9], which add to cost and time. Not only the mechanical property ofthe coatings has been increased by incorporating other phases like

titania [10], zirconia [11], alumina [12], carbon nanotube [13], silica[14] and titanium [15] along with HA, but also the bioactivity andosteo-integration.

Microwave processing of materials has the advantage of reducingprocessing time and saving energy. Energy transfer is by theinteraction of material with electromagnetic field and is influencedby its dielectric property [16]. Bulkmetals were considered unsuitablefor microwave processing, until remedies such as pre-heating themprior to microwave processing [17] and/or covering them with highlymicrowave absorbent powders were thought of [18]. In our earlierwork, HA coating on Ti–6Al–4V by microwave processing has beenreported [19]. This paper reports the formation of HA coating on CPTiand attempts to understand the mechanism of coating and in vitrotesting.

2. Materials and methods

The HA powder was prepared using eggshell as calcium source bymicrowave synthesis following a procedure reported earlier [20]. TheCPTi metal samples (of dimensions 15×15×2 mm) were grit blastedwith silicon carbideparticles. A cup shaped cruciblemadeoutof aluminaand silicon carbide (70:30 mixture) was used. At room temperaturesilicon carbide absorbsmicrowave and generate heat, whereas aluminais transparent. Alumina would absorb microwave at high temperatureand enhance heating. The dimensions of the crucible were: diameter of65 mm, height of 75 mm with wall thickness of 15 mm. The metal

Fig. 2. XRD patterns of (a) CPTi prior to MWprocessing (b) MW20 sample and (c) MW22sample.

Fig. 1. Schematic diagram of experimental setup.

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sample was placed inside the crucible in amanner (Fig. 1) such that theitwas covered byHApowder and aluminapowder as twodistinct layersand the thickness of HA layer to alumina layer was 1:5. This assemblywas enclosed with the microwave transparent aluminium oxide basedinsulation board to avoid loss of heat and was placed inside a glasscontainer as shown in Fig. 1. The crucible and alumina powder shieldsmetal fromsparking. The container (borosilicate glassware)wasflushedwithnitrogen gas initially and later evacuated till the pressure inside thechamber was 3 Pa. The microwave (MW) processing was carried outusing a domestic microwave oven (BPL India, 2.45 GHz, 800W power).Nitrogen gas was employed as use of argon gas resulted in sparkingduring processing. The MW processing was carried out for 20 min atlower microwave power for initial heating and equilibrium. Subse-quently, processing at 800W MW power was carried out for differentdurations and the codes are listed in Table 1.

The coating was analyzed for phase identification by X-raydiffraction (XRD) study with Cu Kα radiation (Shimadzu, XD-D1,Japan). Scanning electron microscope (SEM) (JEOL JSM — 840 A) wasused to study the morphology and microstructure of the coatings.Energy dispersive X-ray analysis (EDAX) was used to identify theelements present and to map their distribution. UV–visible diffusivereflectance (UV–VIS-DR) spectroscopy (Varian Cary 5E) was done tostudy the nature of titanium oxide in the coatings.

The coatings were ground to expose the metal surface in order tostudy microstructure, hardness and Young's Modulus of the substrate.After polishing, the samples were etched using Krolls reagent. Themicrostructure was analyzed using optical microscope (Leitz Laborlux12ME) while hardness was measured using Vickers micro indentationmethod with 200 g load (MMT-7 Matsuzawa). The Young's Moduli ofsubstrate were measured by ultrasonic pulse echo method at 5 MHz(Panametrics, Model 500 PR, USA). A sample of the ground coatingwas analysed in Transmission electron microscope (TEM, PhilipsCM12wSTEM, Netherlands).

Table 1Nomenclature of coatings.

Code MW18 MW20 MW22 MW25 MW27

Initial heating 20 min at lower microwave powerMicrowave processing time at800 W (min)

18 20 22 25 27

Investigation of in vitro bioactivity was conducted by immersingcoated samples in simulated bodyfluids (SBF), of composition similar tohuman blood plasma solution, maintained at 37 °C for one week. TheSBF solution was prepared as reported in literature [21]. In vitro cellculture test was done using Human osteosarcoma (HOS) cell line inminimum essential medium (MEM) supplemented with Foetal BovineSerum. Uniform number of cells were seeded onto test materials and

Fig. 3. XRD pattern of MW22 sample with slow scan to identify apatite peaks.

Fig. 4. SEM images showing top surface of (a) MW20 and (b) MW 22 sample.

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control (glass) and incubated at 37±2 °C for 24±1 h under humidifiedatmosphere in presence of 5% CO2. Cell seeded samples were fixed ingluataraldehyde, dehydrated and processed for SEM analysis (S2400Hitachi, Japan). The cell culture test was carried out at the Sri ChitraTirunal Institute for Medical Science and Technology, Thiruvanantha-puram, India.

Table 2EDAX- Composition of top surface of MW20 and MW22 samples.

Elements MW20 MW22

wt.% at.% wt.% at.%

N 04.93 10.58 04.42 09.82O 23.25 43.65 21.74 42.23Mg 00.64 00.79 00.39 00.50P 01.11 01.08 00.16 00.16Ca 01.53 01.15 00.73 00.57Ti 67.68 42.45 71.31 46.28Sr 00.85 00.29 01.25 00.44

3. Results and discussion

The XRD pattern of the CPTi sample prior to microwave processing(Fig. 2(a)) indicates the presence of α-Ti phase. The XRD pattern ofMW20 sample (Fig. 2(b)) showed more intense peaks of titania (TiO2:rutile) as major phase apart from the α-Ti phase. It was also observedthat the diffraction peaks of α-Ti were shifted slightly towards lowerangle. The MW22 sample (Fig. 2(c)) showed peaks corresponding torutile and less intense peaks of HA (JCPDS: 9-432). The peaksrepresentative of α-Ti were not observed, which suggest that thethickness of the coating was greater than the depth of penetration ofX-rays. Pattern obtained by slow scan around the main peaks of HA fortheMW22 sample (Fig. 3) shows also the presence of β-TCP in additiontoHA. HA, in presence of TiO2, would decompose to β-TCP and CaO [22].CaO was not detected in XRD as it would be less than detection limit. A

mixture of HA and β-TCP was observed as the processing time was lessthan that required for complete transformation [19].

TheMW processing for 25 min was reported to produce satisfactoryHA coating on Ti–6Al–4V by a similar process [19], whereas in this studyprocessing for 22 min has resulted in HA coating on CPTi without anyindication of delamination. The shorter duration required to produceoptimum coating on CPTi was probably due to the faster rate of oxygendissolution in theCPTi aswell asoxide formation compared to Ti–6Al–4V[23]. The MW processing had resulted in a composite coating of titania

Fig. 5. UV–VIS-DR spectra of (a) MW18 (b) MW20 (c) and (d) MW 22 and (e) CPTisample heated in muffle furnace.

Table 3Microhardness and Young's Modulus of CPTi alloy after removing the coating from thesubstrate.

Samples Hardness (Hv) Young's Modulus (MPa)

MW20 147 110.7MW 22 206 125.7MW25 290 126MW27 327 126.6

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and apatite as seen from XRD studies. The XRD patterns showeddistinctive sharp peaks and does not indicate any amorphous phases.Hence the MW processed coatings may not require any post coattreatment. The titania–HA coatings obtained have one added advantageover pure HA coating in that titania has been shown to enhanceosteoblast adhesion and induce cell growth [24].

Fig. 6. Optical micrograph of CPTi substrate after removing the c

The SEMmicrograph shows growth of oxide layer around particlesfor MW22 (Fig. 4(b)) sample than MW20 sample (Fig. 4(a)). Theparticles seen were confirmed to be HA based on EDAX data (shownas insert). The composition of the coating (Table 2) showed thepresence of elements like Mg and Sr, these being present in the HApowder used for coating. The SEM study also indicates the compositenature of coatings comprising of rutile and apatite. The change incomposition is due to increase in thickness of the coating due toformation of greater proportion of titania.

The growth of oxide scales and sintering of HA particleswas presumably due to the microwave absorbing property of non-stoichiometric titaniumdioxide resulting in situ composite coating [19]. Astudyonbandgapenergy of titaniumdioxidewas carriedout to shed lighton thedefect structuredue tonon-stoichiometry. Todifferentiate thenon-stoichiometric titanium dioxide from stoichiometric titanium oxide, CPTiwas heated in a conventional muffle furnace at 1223 K for 30 min. Theheated CPTi sample showed (Fig. 5(e)) clear absorption edge at thresholdwavelength of 392 nm. On the other hand the microwave processedcoating samples exhibited a staggeredmid-gapbandwhichwas similar toreports on titaniumdioxidewith dopants [25] (Fig. 5(a)–(d)). The oxygenvacancies of titaniumdioxide and defect structurewould have resulted inlocalized states in the band gap, allowing low energy excitation pathways[26]. This result confirmed that the titanium dioxide was non-stoichio-metric in nature.

oatings of (a) MW18 (b) MW20 (c) MW22 and (d) MW25.

Fig. 7. TEM bright field image of (a) chip out of coating from MW22 sample; (b), (c) and (d) SAED patterns from regions indicated by arrows.

Table 4TEM-EDAX composition of chip out of coating from MW22 sample.

Elements wt.% at.%

O 09.20 22.80Mg 00.40 00.70Sr 01.20 00.50P 03.30 04.20Ca 08.00 07.80Ti 77.90 64.00

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The optical micrograph of all the CPTi samples showed lamellarα phase (Fig. 6) which indicates that the β transus temperature of1155 K was reached during processing [27]. Since, both the dissolu-tion of oxygen in the substrate and growth of the oxide layer takesplace simultaneously, this has resulted in the formation of oxygenstabilizedα phase. Also, themicrographs indicate that the thickness oflamella increase with microwave processing time. The appearance ofblack colour between α lamella structures may be due to possible TiHand retained beta phase [27]. Themicrostructure seems to support theobserved mechanical properties, like microhardness and Young'sModulus, given in Table 3. The increase in thickness of the α lamellacould be due to higher amount of oxygen diffusion and retainedβ phase with increase in MW processing time.

The TEM studies of chips broken from MW22 sample was done tosubstantiate the composite nature of coating. Fig. 7(a) showsminuteHA

particles at the outer periphery of the chip. The selected area electrondiffraction (SAED) pattern obtained from this region (Fig. 7(b)) showedring pattern corresponding to HA (similar to pattern reported for HA

Fig. 8. SEM images of samples after bioactivity test onMW22sample. (a) apatite formed throughout the coating surface and the insert show chemical composition fromEDAX(b) showingapatite formation over coated apatite particles and (c) apatite formation in grain boundary regions.

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[20]). The SAED pattern from a region, free from apatite particles,corresponds to rutile phase (Fig. 7(c)). The pattern was indexed basedon the camera constant (19.61 mm Å) and the zoneaxiswas found to be[101̄]. The SAED of rutile phase did not show regular repetition of latticepoints, which might be because of non-stoichiometry of titaniumdioxide. The SAEDpattern from region,where bothminuteparticles andscale were present (Fig. 7(d)), was found to be combination of Fig. 7(b)and (c). This again supports the composite nature of coatingscomprising of apatite and titanium dioxide (rutile) obtained bymicrowave processing of CPTi. The chemical composition obtainedusing EDAX attachment (Table 4) show the presence of Mg elementsimilar to SEM-EDAX studies.

Bioactivity test was conducted to study the apatite forming ability ofcoated samples on immersion in SBF. It can be seen that the MW22sample was bioactive based on apatite precipitation throughout thesurface and from the chemical composition (Fig. 8) [21]. The presence ofglobular apatites, precipitated preferentially in grain boundaries ofoxides or over the HAparticleswas observed. TheMW22 coated sample

also showed appreciable osteoblast cell adhesion (Fig. 9). Normally,osteoblast needs to attach and spread, ormore commonly adhere, to thesurface in order to deposit bonematrix in a controlledway at an implantinterface [28]. Osteoblast cell spreading was considered a prerequisitefor cell proliferation and differentiation [29,30]. The MW22 sampleshowed good cell spreading with filopodia extension compared to thecontrol sample, even though thenumber of cellswas comparatively less.The factors that contribute to filopodia extension might be due to thechemical composition (titania and apatite) as well as roughness.

Before processing, the container was purged with high puritynitrogen andwas evacuated to a pressure of 3 Pa. The volume fraction ofoxygen in nitrogenwas assumed to be 1% even though it was claimed tobe 99.9% pure. The partial pressure of oxygen within the container wascalculated to be 0.03 Pa assuming themole fraction of oxygen as 0.1. Thenumber of moles of oxygen inside the container would be 2.032,assuming 50% of volume of chamber (3.39×10−3 m3). Further, therewould be some amount of seepage of atmospheric air into the chamber.Such a situation was favourable for oxidation of titanium in a gradual

Fig. 9. SEM images of osteoblast cell culture test on (a) control-glass and (b) MW22 sample.

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manner. The initial 20 min of heating at low microwave power wouldhave resulted in heating of the crucible due to absorption by siliconcarbide. The CPTi sample would have been heated by conduction fromthe crucible through the HA and alumina powders. The dissolution ofoxygen in CPTi and oxide scale formation occurred simultaneously [23]as represented by Fig. 10(b) and (c). However, the process involved useof nitrogen atmosphere and the titanium alloy samples were packed inHA powders and alumina powders, the oxides of titanium formedduring the MW processing might be non-stoichiometric due to limitedoxygen supply. The enthalpy of formation of rutile (TiO2), anatase(TiO2), Ti2O3 and TiO were reported to be 945 [31], 939 [31], 1518.37[32] and 518.4 kJ/mol [32] respectively. The enthalpy of formation ofnon-stoichiometric titanium dioxide ranged from 88.07 to 122.46 kJ/mol in temperature range of 873–1573 K under oxygen pressure of0.001–1013.25 Pa [33]. Based on enthalpy formation of various oxidesand literatures on non-stoichiometric titanium dioxide [33,34], theprobability of formation of non-stoichiometric titanium dioxide washigh. These non-stoichiometric titanium dioxides being very strongmicrowave susceptors [35]wouldhave generated theheat necessary for

sintering of the HA powder on to the metal substrate with MWprocessing for 22 min. The SEM images of coated samples (Fig. 11)showed that the oxide layers were enclosing the HA particles and thatsintering had taken place in accordance with the proposed mechanism.However, with 25 min of MW processing, delamination of oxides scaleoccurred. Thismight be due tomicrowave absorption by titaniumoxideand titaniummetal was so rapid that an excessive growth of oxide layerhad taken place resulting in loss of adherence with the CPTi substrate.

4. Conclusions

A composite coating of HA, β-TCP and titania phaseswere formed onCPTi by microwave processing and the rate of coating was found to befaster than that reported for Ti–6Al–4V alloy due to its relatively higheroxidation rate. Adherent coating was obtained with 22 min ofprocessing at 800W microwave power. The microwave absorption bynon-stoichiometric titanium dioxide formed duringmicrowave proces-singwas instrumental in sintering HA alongwith oxide layers. The non-stoichiometry of titanium dioxide was confirmed from UV–VIS-DR

Fig. 10. Pictorial representation of possible mechanism of HA–TiO2 coatings bymicrowave processing. (a) Initial metal–HA setup (b) dissolution of oxygen in titanium(c) formation of titanium dioxide layer on titanium substrate and (d) sintering of HAwith TiO2 due to microwave absorption by non-stoichiometric titanium dioxide.

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spectrum by the presence of midgap bands. The composite nature ofcoatingwith titania asmajor phase andHA asminor phase was inferredfrom XRD, SEM and TEM results. The partial decomposition of HA wasobserved due to lesser processing time. The property of the metalsubstrate like hardness and Young'sModulus increasedwith increase in

processing time due to formation of oxygen stabilized α phase andretained β phase. The bioactivity of coated samplewas evident from theobserved apatite precipitation all over the surface at end of thebioactivity test. It also exhibited osteoblast cell adhesion with longfilopodia extension which was prerequisite for good osteoblastproliferation and differentiation.

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Fig. 11. SEM images showing the smaller particles engulfed prior to larger size particles in (a) MW20 sample (with insert on composition of apatite particles) and (b) MW22 sample.

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