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Surface characterization and mechanical property evaluationof thermally oxidized Ti-6Al-4V

Amit Biswas, Jyotsna Dutta Majumdar⁎

Department of Metal. & Maters. Engg., I. I. T. Kharagpur, W. B. – 721302, India

A R T I C L E D A T A

⁎ Corresponding author. Tel.: +91 3222 283288E-mail address: jyotsna@metal.iitkgp.ernet

1044-5803/$ – see front matter © 2008 Publisdoi:10.1016/j.matchar.2008.12.014

A B S T R A C T

Article history:Received 26 November 2007Received in revised form30 August 2008Accepted 9 December 2008

The present study concerns development of a thin and adherent oxide film on the surface ofTi-6Al-4V by thermal oxidation. Thermal oxidation was carried out over a range oftemperature between 400 to 600 °C and a time from 25 h to 60 h. A detailed characterizationof the surface and cross section of the oxidized surface was carried out by optical/scanningelectron microscopy and X-ray diffraction techniques. Finally, the mechanical properties ofthe oxidized surface in terms of microindentation hardness and wear resistance wereevaluated as a function of oxidation parameters. Surface oxidation of Ti-6Al-4V at 600 °C for36 h offered a defect free oxide scale with improved hardness and wear resistance.

© 2008 Published by Elsevier Inc.

Keywords:Ti-6Al-4VThermal oxidationMicrostructureMicrohardnessWear

1. Introduction

Titanium and its alloys are widely used as surgical implantsbecause of their good corrosion resistance, high specific strengthandbiocompatibility [1–3]. However, a poorwear resistance of Ti-based alloys results in limitations for prolonged use of thecomponent especially as an implant for hip joint and dentalapplications. These environments reduce the life of the compo-nent. Furthermore, loss of adhesion at the interfacewas found tobe caused by the existence of a layer of porous titaniumoxide [4].

Many methods have been used to produce the oxide layeron a Ti surface, including anodizing [5,6], oxygen diffusion [7],ion implantation [8,9], thermal oxidation [10,11] and palla-dium treated thermal oxidation [12]. However, it was observedthat thermal oxidation of Ti surface shows better propertiesthan others, because it produces a thick, highly crystallinerutile oxide film [12].

Anodizing generates anatase and/or hydrated oxides of lowcrystallinity [13]. Siva Rama Krishna et al. [14] studied theinfluence of thermal oxidation on wear resistance of titanium.

; fax: +91 3222 282280..in (J. Dutta Majumdar).

hed by Elsevier Inc.

They reported the formation of a thick and well adherentrutile scale on titanium, with improved wear resistance andreduced friction coefficient by thermal oxidation at 850 °Cfollowed by furnace cooling. However the oxide layer pro-duced by prolonged thermal oxidation at high temperatures(above 800 °C) leads to oxide de-bonding [15]. On the otherhand, an oxide layer produced at low temperatures and forshort duration is not thick enough for potential tribologicalapplications. Thus, an optimum choice of oxidation para-meters is important so as to produce a well adherent surfacescale with optimum thickness.

In the present study, an attempt has been made to developa uniform oxide layer on the surface of Ti-6Al-4V by thermaloxidation between 400 to 600 °C. A detailed characterization ofthe surface and cross sectional plane of the oxidized surfacehas been characterized by light optical microscopy, scanningelectron microscopy and X-ray diffraction techniques. Finally,the hardness and wear resistance of the oxide surface wereevaluated to study the effect of thermal oxidation on themechanical properties of the oxidized surface.

Fig. 1 –Scanning electron micrographs of the cross section ofthermally oxidized Ti-6Al-4V oxidized at 600 °C for (a) 25 hand (b) 60 h.

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2. Experimental

In the present investigation, Ti-6Al-4V coupons of dimension:10 mm×10 mm×5 mm was used as substrate. The sampleswere polished with fine grade diamond paste prior to thermaloxidation for cleaning and reducing surface roughness. Iso-thermal oxidation treatments were carried out over a range oftemperature between 400 to 600 °C and time between 25 to 60 hin normal atmosphere. Following thermal oxidation, themicrostructures of the top surface and the cross section of theoxide layer are characterized by light optical microscopy andscanning electron microscopy. The phases present on theoxidized surface were analyzed by X-ray diffraction techniqueusing Cu as targetmaterial (wavelength=1.5405 A). The relativemass fractions of the individual identified phases were calcu-lated using semi-quantitative analysis based on the RIR(Reference Intensity Ratio) values (often called I/Ic values)available from the database. This method is known as thenormalized RIR method [16]. The normalization used in thismethod assumes that the sum of all identified phases is 100%and there are no unidentified crystalline phases or an amor-phous phase present. Only under these conditions can mean-ingful semi-quantitative results be obtained. The concentrationX of phase TiO2 (say rutile) is calculated from:

XTiO2 rutileð Þ =I hklð ÞTiO2 rutileð Þ

RIRaIrelhklð ÞTiO2 rutileð Þ

1Pnj = 1

IVhklð Þj=RIRjIrelhklð Þ Vj

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Themicrohardnessof the surface layer at different loadswasmeasured on the Vickers scale with applied load ranging from50g to1000g.Thekinetics ofwearwasanalyzedbya frictionandwear monitor unit based on the pin-on-disc wear testingmethod with the specimen as disc and steel ball (5 mm dia.) aspin. During wear testing, the pin was allowed to slide over thediskwith a 15 rpmwheel speedat anapplied load of 1 kg.Duringwear testing, cumulative depth of wear was measured as afunction of time. The complete software records the verticaldisplacement of the pin from the surface of the specimen as thesample rotates and plots a graph of depth of indenter penetra-tion vs. time.

3. Results and Discussions

3.1. Characteristics of the Oxide Scale

In the present section, a detailed characteristic of the oxidescale (both the top surface and cross section) of thermallyoxidized Ti-6Al-4V and its mechanical properties (hardnessand wear resistance) will be presented in detail.

Fig. 1 shows the scanning electron micrograph of the crosssection of thermally oxidized Ti-6Al-4V oxidized at 600 °C for(a) 25 h and (b) 60 h. The thickness of the oxide layer variesfrom 3 µm to 12 µm for different combinations of temperatureand time and increases with temperature and time ofoxidation. A comparison between Fig. 1(a) and (b) shows thatthe oxide layer is continuous and adherent when oxidized fora short duration (Fig. 1(a)). Furthermore, it consists of two

different layers with the inner layer formed due to the inwarddiffusion of oxygen ions and outer layer formed due to theoutward diffusion of titanium ions. Similar observation wasnoticed when oxidized at a lower temperature. On the otherhand, when oxidized for a longer duration, there is presence ofan internally oxidized layer formed due to diffusion of oxygenions along grain boundaries and formation of oxides at thegrain boundary regions [17]. Fig. 1b shows the presence ofoxides along grain boundaries well below the surface region.Oxidation at a very high temperature or for a longer durationcauses debonding of the oxide layer when the thicknessreaches a threshold value, which decreases with increase intemperature and time of oxidation. Decreased adherence ofthe oxide scale with increase in the thickness is due todevelopment of thermal stress in the oxide layer, which in-creases with increase in thickness. The adherency tends tozero at 700 °C as reported by Coddet et al. [18]. Fig. 2(a–c) showthe scanning electron micrographs of the top surface of ther-mally oxidized Ti-6Al-4V oxidized at 400 °C for (a) 25 h, (b) 36 hand (c) 60 h, respectively. FromFig. 2 it is evident that at a lowertime of oxidation, though the surface is covered with very fineoxide scale growth is not uniform. Partial growth of the oxidescale occurs along the outward direction. With increase intime, the oxide scale grows and subsequently covers thewhole surface (cf. Fig. 2b and c). Oxidation for a very long timecauses sintering of the oxide particles along with its growth.

Fig. 3 –Scanning electron micrographs of the top surface ofthermally oxidized Ti-6Al-4V oxidized at (a) 400 °C and (b)600 °C for 60 h.

Fig. 4 –X-ray diffraction profiles of as-received and thermallyoxidized Ti-6Al-4V oxidized at 600 °C for 25 h, 36 h and 60 h.

Fig. 2 –Scanning electron micrographs of the top surface ofthermally oxidized Ti-6Al-4V oxidized at 600 °C for (a) 25 h,(b) 36 h and (c) 60 h.

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Fig. 3(a,b) show scanning electronmicrographs of the oxidizedsurface isothermally oxidized at (a) 400 °C and (b) 600 °C for60 h of exposure. The surface oxide scale at a highermagnification shows that clustering of very fine nano-sizedoxide particles is responsible for its growth. Furthermore, aclose comparison between Fig. 3(a) and (b) shows that thereare a few micro-cracks present when oxidized at a hightemperature for a prolonged duration (Fig. 3b). On the otherhand, at a higher temperature, the preferential growth of a fewoxide particles occurs at a faster rate. Furthermore, thepresence of fine micro-cracks was observed when oxidizedat a higher temperature (Fig. 3b). Preferential growth alongcertain crystal directions and subsequent sintering cause theformation of very fine rod like oxide particles when oxidizedfor a longer duration or at a high temperature (Fig. 3b).

Fig. 4 shows the X-ray diffraction profiles of as-receivedand thermally oxidized Ti-6Al-4V oxidized at 600 °C for 25 h,

36 h and 60 h, respectively. From Fig. 4 it may be noted thatboth anatase and rutile peaks are present in the oxide scale.Furthermore, the intensities of rutile peaks increase whenoxidized for a longer duration (60 h).

A detailed quantitative analysis of the mass fraction ofindividual phase was undertaken by analysis of the X-raydiffraction profiles using the normalized RIR method [16].Fig. 5 shows the relativemass fraction of anatase and rutile on

Fig. 5 –Bar chart showing the effect of oxidation time on therelative mass fraction of anatase and rutile phase inthermally oxidized Ti-6Al-4V.

Fig. 7 –Bar chart showing the effect of oxidation temperatureon the relative mass fraction of Ti2O3, anatase and rutilephase in thermally oxidized Ti-6Al-4V.

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the oxide scale formed at 600 °C for different periods of oxida-tion. From Fig. 5 it may be noted that the rutile content in-creases, and theanatasecontentdecreaseswith increasing timeof oxidation. In this regard, it is relevant to mention that rutilecontains a higher hardness than anatase and hence, a desiredphase when a high hardness on the surface is required [19].

Fig. 6 compares the X-ray diffraction profiles of the surfaceof oxidized Ti-6Al-4V at 400 °C, 500 °C and 600 °C for 60 h. FromFig. 6 it is evident that at a low temperature of oxidation (400and 500 °C), oxide peak intensities are very low and comprisemainly anatase, a few rutile and Ti2O3 peaks. The broadeningof α-Ti peaks at low angles was observed. These are known tobe the peaks of distorted Ti, originating from the latticeexpansion as a result of oxygen dissolution [20]. The intensityof the distorted Ti peaks gradually decreased and the anataseand rutile peaks became more dominant with higher inten-sities at 600 °C. However, rutile peaks appeared to be moreprominent with increase in temperature.

Fig. 7 compares the mass fraction of individual oxides as afunction of temperature of oxidation after 60 h of exposure.

Fig. 6 –X-ray diffraction profiles of (a) as-received andthermally oxidized Ti-6Al-4V at (b) 400 °C, (c) 500 °C and(d) 600 °C for 60 h.

From Fig. 7 it may be noted that at lower temperature ofoxidation (400 and 500 °C), oxide scales were predominantlyanatase in nature with the presence of very little Ti2O3. On theother hand, at 600 °C, there is a substantial mass fraction ofthe rutile phase.

3.2. Mechanical Properties of the Oxidized Surface

A detailed study of the hardness of the oxide surface wasundertaken using a Vickers microindentation hardness tester,with awide range of applied loads. Fig. 8 shows thehardness ofthe oxidized surface as a function of applied load in untreatedand surface oxidized Ti-6Al-4V. In the untreated sample, thehardness value remains almost constant to about 290 HV.However, a significant enhancement of surface hardness wasachieved on the surface of oxidized Ti-6Al-4V. The hardness ofthe oxidized surface was found to decrease with load. Thesurfacemicrohardness was found to vary with oxidation para-meters. From Fig. 8 it is evident that with increasing time of

Fig. 8 –Variation of hardness profile with load for as-received(plot 1) and thermally oxidized Ti-6Al-4V at 600 °C for 25 h(plot 2), 36 h (plot 3) and 60 h (plot 4).

Fig. 10 –Cumulativewear loss (in termsofverticaldisplacement)asa functionof time for (1) as-receivedand (2) thermallyoxidizedTi-6Al-4V at 600 °C for 36 h at an applied load of 9.8 N against ahardened steel ball.

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oxidation the surface hardness increases, reaching a max-imum value of 1100 HV when oxidized at 600 °C for 60 h. Thehigh hardness value for the oxidized surface is mainly due tothe formation of an oxide phase in the microstructure.

On the oxidized surfaces, the decrease in hardness withincreasing indentation load is attributed to the response of theindenter at high penetration depths to softer regions. It shouldbe noted that, higher hardness values was obtained at highoxidation time due to formation of a deeper and uniform oxidelayer.

Fig. 9 shows the effect of oxidation temperature on thehardness distributionwith load. From Fig. 9 it is evident that at400 °C and 500 °C, even when oxidized for a long duration(60 hrs), the enhancement of surface hardness was insignif-icant when compared to the same surface oxidized at 600 °C. Asignificant improvement in hardness at 600 °C is possibly dueto of a highermass fraction of rutile at 600 °C. Rutile has higherhardness than anatase. The hardness of rutile and anatase isapproximately 1733 HV and 815 HV, respectively [17,18].Oxidation for 60 h at 600 °C causes formation of very finemicro-cracks on the oxidized surface as observed in thepresent study (cf. Fig. 3b). Hence, the optimum processparameters for the present study were 600 °C and 36 h.

Fig. 10 shows the variation of wear loss (in terms of depth ofwear) as a function of time for as-received (plot 1) andthermally oxidized samples of Ti-6Al-4V oxidized at 600 °Cfor 36 h (plot 2). A friction and wear monitor unit was usedwith an applied load of 1 kg against a hardened steel ball. FromFig. 10, the rate of wear increases with time and is very high atthe initial stage of wear (up to 400 s of interaction time) for as-received Ti-6Al-4V subsequently, the rate of wear decreases.In thermally oxidized (600 °C for 36 h) Ti-6Al-4V, themagnitude and rate of wear are significantly lower than thatof as-received Ti-6Al-4V. The wear resistance of the thermallyoxidized surface was found to increase with increasinghardness of the oxidized surface. Hence, it may be concludedthat the improved wear resistance of the oxidized surface is

Fig. 9 –Variation of hardness profile with load for samplesoxidized at different temperatures but same time periods(1) as-received and thermally oxidized Ti-6Al-4V at (2) 400 °Cfor 60 h, (3) 500 °C for 60 h, (4) 600 °C for 60 h.

due to improved hardness of the layer, because of theformation of rutile and anatase phase on the oxidized surface.

4. Conclusions

In the present study, thermal oxidation of Ti-6Al-4V wascarried out at temperatures ranging from 400 –600 °C for aperiod of 25–60 h under each temperature with an objective todevelop an adherent oxide surface to improvewear resistance.From the detailed investigations it is concluded that thermaloxidation leads to formation of defect-free oxide scale,consisting of different oxides of titanium, mainly Ti2O3, TiO2

(both rutile and anatase). The mass fraction of an individualphasewas found to varywith oxidation parameters. Oxidationat 600 °C for 60 h leads to formation of very finemicrocracks. Asignificant improvement in hardness of the oxide layer wasachieved by thermal oxidation, which was found to increasewith increase in oxidation time or oxidation temperature.Wear resistance of the oxidized surface was higher than as-received Ti-6Al-4V.

Acknowledgement

The financial support for this work was provided by theCouncil of Scientific and Industrial Research (CSIR), N. Delhiand Board of research on Nuclear Science (BRNS), Bombay.This is gratefully acknowledged.

R E F E R E N C E S

[1] Brunette DM, Tengvall P, Textor M, Thomsen P. Titanium inmedicine. Berlin: Springer; 2001.

[2] Albrektsson T, Branemark PI, Hansson HA, Kasemo B,Larsson K, Lundström I, et al. The interface zone of inorganicimplants in-vivo —titanium implants in bone. Ann BiomedEng 1983;11:1–27.

518 M A T E R I A L S C H A R A C T E R I Z A T I O N 6 0 ( 2 0 0 9 ) 5 1 3 – 5 1 8

[3] Buser D, Schenk RK, Steinemann S, Fiorellini JP, Fox CH, StichH. Influence of surface characteristics on bone integration oftitanium implants. A histomorphometric study in miniaturepigs. J Biomed Mater Res 1991;25:889–902.

[4] Wang RR, Welsch GE, Monteiro O. Silicon nitride coating ontitanium to. enable titanium-ceramic bonding. J BiomedMater Res 1999;46:262–70.

[5] De Sena LA, Rocha NCC, Andrade MC, Soares GA. Bioactivityassessment of titanium sheets electrochemically coated withthick oxide film. Surf Coat Technol 2003;166(2):254–8.

[6] Frey N, Buchillier T, Le VD, Steinemann SG. Properties ofsurface oxides on titanium and titanium alloys. Titanium'92Science and Technology; 1993. p. 1999–2006.

[7] Boettcher C. Deep case hardening of titanium alloys withoxygen. Surf Eng 2000;16(2):148–52.

[8] Li J, SunM,Ma X. Structural characterization of titanium oxidelayers prepared by plasma based ion implantation withoxygen on Ti6Al4V alloy. Appl Surf Sci 2006;252(20):7503–8.

[9] Loinaz A, Rinner M, Alonso F, Oñate JI, Ensinger W. Effects ofplasma immersion ion implantation of oxygen onmechanicalproperties and microstructure of Ti6Al4V. Surf Coat Technol1998;103–104:262–7.

[10] Dong H, Bloyce A, Morton PH, Bell T. Surface engineering oftitanium alloy with oxygen. Titanium'95 Science andTechnology; 1995. p. 1999–2006.

[11] Dong H, Bell T. Enhancedwear resistance of titanium surfacesby a new thermal oxidation treatment. Wear 2000;238:131–7.

[12] Bloyce A, Qi PY, Dong H, Bell T. Surface modification oftitanium alloys for combined improvements in corrosion andwear resistance. Surf Coat Technol 1998;107:125–32.

[13] Sul Y-T, Johansson CB, Jeong Y, Albrektsson T. Theelectrochemical oxide growth behaviour on titanium in acidand alkaline electrolytes. Med Eng Phys 2001;23:329–46.

[14] Siva Rama Krishna D, Brama YL, Sun Y. Thick rutile layer ontitanium for tribological applications. Tribol Int 2007;40:329–34.

[15] Dong H, Li XY. Oxygen boost diffusion for the deep-casehardening of titanium alloys. Mater Sci Eng A 1980;80(4):2783–91.

[16] Chung H. Quantitative interpretation of X-ray diffractionpatterns ofmixtures. I. Matrix-flushingmethod of quantitativemulticomponent analysis. J Appl Crystallogr 1974;7:513–9.

[17] Dutta Majumdar J, Mordike BL, Roy SK, Manna I.High-temperature oxidation behavior of laser-surface-alloyedTi with Si and Si + Al. Oxid Met 2002;57:473–98.

[18] Coddet C, Chaze AM, Beranger G. Measurement of theadhesion of thermal oxide films: application to the oxidationof titanium. J Mater Sci 1987;22:2969–74.

[19] LiuX,ChuPK,DingC. Surfacemodification of titanium, titaniumalloys, and related materials for biomedical applications. MaterSci Eng R Rep 2004;47:49–121.

[20] Zywitzki O, Modes T, Sahm H, Frach P, Goedicke K, Gloβ D.Structure and properties of crystalline titanium oxide layersdeposited by reactive pulse magnetron sputtering. Surf CoatTechnol 2004;180–181:538–43.