Color tone and interfacial microstructure of white oxide layer

on commercially pure Ti and Ti–Nb–Ta–Zr alloys

Eri Miura-Fujiwara1», Keisuke Mizushima1, Yoshimi Watanabe2, Toshihiro Kasuga2, and Mitsuo Niinomi3

1Graduate School of Engineering, University of Hyogo, Himeji, Hyogo 671-2280, Japan2Graduate School of Engineering, Nagoya Institute of Technology, Nagoya 466-8555, Japan3Institute for Materials Research, Tohoku University, Sendai 980-8577, JapanE-mail: emiura@eng.u-hyogo.ac.jp

Received May 14, 2014; accepted August 11, 2014; published online October 23, 2014

In this study, the relationships among oxidation condition, color tone, and the cross-sectional microstructure of the oxide layer on commerciallypure (CP) Ti and Ti–36Nb–2Ta–3Zr–0.3O were investigated. “White metals” are ideal metallic materials having a white color with sufficient strengthand ductility like a metal. Such materials have long been sought for in dentistry. We have found that the specific biomedical Ti alloys, such asCP Ti, Ti–36Nb–2Ta–3Zr–0.3O, and Ti–29Nb–13Ta–4.6Zr, form a bright yellowish-white oxide layer after a particular oxidation heat treatment. Thebrightness L» and yellowness +b» of the oxide layer on CP Ti and Ti–36Nb–2Ta–3Zr–0.3O increased with heating time and temperature.Microstructural observations indicated that the oxide layer on Ti–29Nb–13Ta–4.6Zr and Ti–36Nb–2Ta–3Zr–0.3O was dense and firm, whereas apiecrust-like layer was formed on CP Ti. The results obtained in this study suggest that oxide layer coating on Ti–36Nb–2Ta–3Zr–0.3O is anexcellent technique for dental applications. © 2014 The Japan Society of Applied Physics

1. Introduction

Numerous kinds of materials made of metals, ceramics(porcelain), and polymers (resins) are produced and used fordental devices to reconstruct certain oral structures in patients.The recovery of not only oral function but also oral appear-ance has recently become very important in terms of reductionof the patients’ psychological burden. It is mentioned in ourprevious reports1,2) that metallic materials have been espe-cially used as the principal components of prostheses andorthodontic devices such as dental implants, crowns, dentures,brackets, and arch wires. For example, Ag–Pd–Au, Au–Pt,Co–Cr, Ni–Cr, and Ti-based alloys have been commonlyused as dental materials because of their excellent mechanicalproperties, corrosion resistance, usability, and durability.

Recently, Ti-based alloys have become popular, becauseTi has high specific strength, low Young’s modulus, andexcellent corrosion resistance, in addition to excellentbiocompatibility.3–6) However, the metallic devices made ofthese alloys are not “white”, that is, they are inferior toceramics and polymers in terms of esthetic properties. Forthese reasons, white “metallic” materials, that is, materialswith high ductility, high strength, and excellent estheticproperties, are sought for in dentistry. Unfortunately, Ti isgrayish silver, which is not esthetically suitable as a dentalprosthestic material because it is conspicuous among naturalteeth. Thus, coating Ti alloys with a polymer or a ceramicon metallic dental devices is one of the possible techniques toconceal its metallic color or improve osseoconnectivity. Toimprove corrosion resistance7,8) or to enhance osseointegra-tion and regeneration of the bone tissue on a surface,9,10)

surface coating of Ti with an oxide layer, calcium phosphatessuch as hydroxyapatite11–13) or other ceramics9,14,15) has beeninvestigated.4,16) In addition, as mentioned above, the shadeof color on dental devices such as an artificial tooth ororthodontic devices is as important as corrosion resistanceand mechanical properties for improving the patients’ qualityof life (QOL).17) Therefore, hybrid materials with metal suchas a metal-bond porcelain crown or a resin-facing crown havebeen developed and are commonly used in dentistry toconceal the metallic abutment tooth’s color.

TiO2 powder is white and is used as a pigment i.e.,“titanium white”. We have reported that a thick white oxidelayer is formed on a Ti substrate after heat treatment at morethan 1123K.1,2,18) The oxide layer formed on Ti consisted ofstacked rutile TiO2 layers and gaps of 1–3 µm thickness.1) Inaddition, we have also reported that a dense and firmyellowish-white oxide layer is formed on Ti–29Nb–13Ta–4.6Zr (TNTZ) alloy following heat treatment.1,2,18)

Ti–29Nb–13Ta–4.6Zr and Ti–36Nb–2Ta–3Zr–0.3O alloysare both ¢-type Ti alloys with low Young’s modulusdeveloped using the molecular orbital calculation of elec-tronic structures (DV-Xa cluster method).19,20) Ti–29Nb–13Ta–4.6Zr was developed by Kuroda et al.,21) and Ti–36Nb–2Ta–3Zr–0.3O was one of the series of Gummetalμ

developed by Toyota research laboratory. Gummetalμ

exhibits a very interesting mechanical behavior withoutdislocations following the specific heat treatment5,22–25) andis already sold as a low-Young’s modulus bracing wire.26)

Coating Ti–36Nb–2Ta–3Zr–0.3O or Ti–29Nb–13Ta–4.6Zrwith a white oxide layer will expand the boundaries of theirbiomedical applications.

Therefore, in this study, we investigated the relationshipsamong oxide layer growth, heat treatment condition, andcolor change of the oxide surface on Ti–36Nb–2Ta–3Zr–0.3O, commercially pure (CP) Ti, and Ti–29Nb–13Ta–4.6Zr.Cross-sectional microstructural observations were also per-formed to investigate oxide formation behaviors. Certainparts of this research on Ti–29Nb–13Ta–4.6Zr were alreadyreported.1,2,18) In this study, Ti–36Nb–2Ta–3Zr–0.3O, whichhas a similar composition to and the same alloy system asTi–29Nb–13Ta–4.6Zr, was mainly investigated. Furthermore,new additional data on Ti–29Nb–13Ta–4.6Zr and CP Ti willbe shown and discussed in addition to those previouslyreported.1,2,18)

2. Experimental procedures

Hot-rolled Ti–36Nb–2Ta–3Zr–0.3O (Toyotsu MaterialGummetalμ), Ti–29Nb–13Ta–4.6Zr (TNTZ), and CP Tigrade 2 (Selec) bars with a diameter of º10–15mm wereused as substrates. The bars were sliced at a thickness ofabout t = 1mm using a wire electron discharge machine

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(EDM). The slices were then polished using emery paper upto #4000. The polished samples were oxidized in an airfurnace at 1273, 1298, 1323, and 1348K from 0.3 to 5.4 ks.The samples were furnace-cooled after heat treatment.

To investigate the oxidation behavior of the substrate,thermal analysis was conducted using a thermogravimetric-differential thermal analyzer (TG-DTA) from room temper-ature to 1473K in Ar gas flow. The heating rate was 0.33K/s. Although the TG-DTA measurements were performedin Ar gas flow, the partial pressure of O2 gas in the chamber issupposed to be sufficient for oxidizing the Ti alloys since thechamber is not sufficiently sealed during the analysis. Inaddition, residual O2 in the chamber and gas release from thechamber and specimen are assumed to exist.

Color tone measurement of the oxidized surface wasperformed using a spectrophotometric colorimeter. N = 5,and which is the average value, was plotted. The measure-ment was conducted in specular-component-included (SCI)mode, which is suitable for measuring the color of an object.This measurement includes any specular reflected light,making it more sensitive to surface conditions.27) Colortone was quantified using a CIE 1976 L+a+b+ uniform colorspace (CIELAB color space, CIE S 014-4/E, ISO 11664-4).Figure 1 shows a schematic of the CIELAB color space. Inthis system, the color space was expressed by the brightnessL+ and the chromaticity coordinates a+ and b+. L+ indicates thebrightness from 0 to 100, and the brightness increases withL+. The chromaticity coordinates a+ and b+ express redness(+) to greenness (¹) and blueness (+) to yellowness (¹),respectively. The chromaticity, that is, hue and chroma isdefined as a composite vector of a+ and b+. The L+a+b+ colorsystem is defined as the device-free parameters, so that thecolor tone of a sample can be qualified regardless of the typeof readout device and ambient conditions. The principle ofspectrophotometry and the calculation formulas of L+a+b+ canbe obtained in the literature, e.g., Ref. 28.

Oxide phases were identified by X-ray diffractionometry(XRD). Microstructural observations were performed usingan optical microscope (OM). Cross-sectional microstructuralobservations were performed by scanning electron microsco-py and energy-dispersive X-ray spectroscopy with energy-dispersed spectroscopy (SEM–EDS). The chemical compo-sition of the oxide layer was investigated by electron probemicroanalysis (EPMA). Cross-sectional samples for SEMwere prepared as follows: an oxidized specimen embeddedin resin was cut perpendicular to a surface using a diamondwheel saw. Then, a cross-sectional specimen was polished up

to colloidal silica. Oxide layer thickness was measured fromthe obtained SEM images.

3. Results

3.1 Thermal analysis of CP Ti and Ti–36Nb–2Ta–3Zr–0.3OFigure 2 shows the TG-DTA curves of CP Ti and Ti–36Nb–2Ta–3Zr–0.3O. In these curves, weight gain started slowlyat the beginning of heating, and a marked weight gain dueto oxidation starts at 1034K for CP Ti and at 1043K forTi–36Nb–2Ta–3Zr–0.3O. The TG-DTA curve of Ti–29Nb–13Ta–4.6Zr was also presented in our previous paper.2) SinceTi–36Nb–2Ta–3Zr–0.3O is a ¢ alloy, ¢-transus temperaturedisappeared. Therefore, heat treatment should be performedat least above these temperatures to obtain an oxidation layerwith sufficient thickness to cover a metallic surface.

3.2 Color tone of oxide layer of Ti alloysFigure 3 shows optical images of CP Ti and Ti–36Nb–2Ta–3Zr–0.3O covered with a white oxide layer that formedfollowing the heat treatment. From visual observation, ahomogeneously matte and bright, slightly yellowish-whitelayer was formed on the metallic substrate after oxidation.

Figure 4 shows the typical spectral reflectance, R, againstthe visible light wavelength, ­, of the oxide surface on CP Tiand Ti–36Nb–2Ta–3Zr–0.3O oxidized at 1273K. R is theratio of the reflected spectrum to the incident spectrum. The

Fig. 1. CIELAB color space.

(a)

(b)

Fig. 2. TG-DTA curves of CP Ti and Ti–36Nb–2Ta–3Zr–0.3O(Gummetal): (a) CP Ti and (b) Ti–36Nb–2Ta–3Zr–0.3O.

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higher the R is, the more visible the color at the visiblewavelength is. When the obtained R is adequately high andplateaus in the range from 380 to 780 nm, that is, R is flatwithin the entire range of the visible light wavelength, thecolor of the surface appears to be a mixture of all thewavelengths, i.e., white. In this graph, the R of the metallicsurface is less than 30% and is relatively constant in theentire visible light wavelength range. After oxidation, in bothalloys, R drastically increased with ­ from 400 to 430 nm,where R was in the range of 40–60%. After its drasticincrease in Ti–36Nb–2Ta–3Zr–0.3O, R increased slightlywith increasing ­, and then reached the maximum at ­ ofapproximately 550 nm. On the other hand, in CP Ti, thecurve plateaued from 540 ¯ ­ ¯ 750 nm, and the R curve ofthe CP Ti oxide surface increased further in the range of

600 < ­ < 750 nm. The constant R in the visible lightwavelength range indicates that the surface color tone isfrom gray to white, and the slight increase in R in the long-wavelength range for CP Ti suggests that the oxide sur-face color, especially yellowness and redness, is slightlydifferent from the color of the oxide layer on Ti–36Nb–2Ta–3Zr–0.3O.

Our previous studies1,2) indicated that the thickness of theoxide layer on Ti–29Nb–13Ta–4.6Zr and CP Ti exhibitedheating time dependence up to 10.8 ks, and the dependenceof L+ showed good agreement with the layer growth. In thisstudy, the relationship within a short heating time range wasfocused on. Results of L+ and layer thickness l against heatingtime up to 3.6 ks at 1273 and 1323K are shown in Fig. 5.The L values in all the alloys were in the L+ range of70 < L+ ¯ 85, and a positive time dependence was observedat 1273K. For example, L+ increased from L+ = 73.7 at 0.3 ksto L+ = 81.7 at 3.6 ks in Ti–36Nb–2Ta–3Zr–0.3O. At 1323K,taken altogether, L+ was slightly higher than that at 1273K(L+ = 78.2 to L+ 85.0 in Ti–36Nb–2Ta–3Zr–0.3O). A slighttime dependence of L+ was also observed in Ti–36Nb–2Ta–3Zr–0.3O; however, CP Ti did not show a clear dependence.As a matter of fact, an inversed time dependence wasobserved in CP Ti oxidized at 1323K.

(a) (b)

(c) (d)

Fig. 3. Optical surface images of polished surface and oxide layer formedon CP Ti and Ti–36Nb–2Ta–3Zr–0.3O: (a) Mirror polished surface of CP Ti,(b) surface after oxidation on CP Ti, (c) mirror polished surface ofTi–36Nb–2Ta–3Zr–0.3O, and (d) surface after oxidation onTi–36Nb–2Ta–3Zr–0.3O.

Fig. 4. (Color online) Reflectance (R) curves against visible lightwavelength (­) on surfaces of CP Ti and Ti–36Nb–2Ta–3Zr–0.3O afteroxidation measured using spectrophotometric colorimeter. Inset: surfaceimages before and after oxidation of Ti–36Nb–2Ta–3Zr–0.3O (the sameimages in Fig. 3).

(a)

(b)

Fig. 5. Time dependences of brightness (L+) and thickness (l) of oxidelayer on CP Ti, Ti–36Nb–2Ta–3Zr–0.3O, and Ti–29Nb–13Ta–4.6Zr(TNTZ). Oxidized at (a) 1273 and (b) 1323K. HT TNTZ: Ti–29Nb–13Ta–4.6Zr annealed at 973K for 3.6 ks in vacuum before oxidation. HT TNTZAC: Ti–29Nb–13Ta–4.6Zr annealed at 973K for 3.6 ks in vacuum beforeoxidation, and air cooled after oxidation.

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According to the color range of the Japanese natural teethreported by Hasegawa et al.,29) the average brightness of aJapanese incisor tooth is approximately L+ ² 75. In addition,it is reported by Shiba et al.30) that the maximum brightnessof an opaque resin for a facing metal crown is L+ Ù 80. Thusalthough the reason for this unusual dependence has not yetbeen understood, the results at least suggest that the oxidelayer of a few tens micron meter thickness might be sufficientfor obtaining appropriate brightness. Therefore, the suitablethickness of the oxide layer on Ti–36Nb–2Ta–3Zr–0.3Oand Ti–29Nb–13Ta–4.6Zr for obtaining a sufficiently whiteopaque coating would be in the range from 15 to 40 µm.

Figure 6 shows a scatter diagram of a+ and b+ of the oxidesformed at 1273 and 1323K at various heating times. Thechromaticity a+ and b+ ranges at a central site of a Japaneseincisor tooth reported by Hasegawa et al.29) are also describ-ed in this figure. From the results, generally, the a+ ofTi–36Nb–2Ta–3Zr–0.3O was smaller than those of CP Tiand Ti–29Nb–13Ta–4.6Zr, although their difference is verysmall (¦a+ ¯ 3). All the oxide layers formed on the alloysdo not have the redness factor even in CP Ti since a+ isalways < 0. In contrast, yellowness (+b+) varied widely, andthe average b+ values of CP Ti, Ti–36Nb–2Ta–3Zr–0.3O,and Ti–29Nb–13Ta–4.6Zr were generally similar. As a trend,b+ and a+, especially b+, increased with time as indicated bythe arrow. Thus, the longer time, the larger b+ became so thatb+ was within the incisor tooth range, as previously reportedin Ref. 1.

We also performed a preliminary experiment on the effectof surface roughness on the color tone of the oxide layer. Thedifferences in a+, b+, and L+ between a rough surface polishedusing emery paper up to #1500, and surface polished up to#4000, were negligible.

3.3 Thickness of oxide layer on Ti alloysRegarding the oxide layer thickness l shown in Fig. 5, theheating time dependence on layer thickness was observed inTi–36Nb–2Ta–3Zr–0.3O, as well as in CP Ti and Ti–29Nb–13Ta–4.6Zr, as previously reported.1) In this figures, theoxide layer thickness curves increased parabolically withheating time. The layer thickness of the oxidation layer on

Ti–36Nb–2Ta–3Zr–0.3O is always less than that of CP Ti;the maximum l on Ti–36Nb–2Ta–3Zr–0.3O at 1273K wasless than 50 µm, whereas l was almost 100 µm on CP Ti. At1273K, the oxide layer thickness saturated at 3 ks in CP Ti.At 1323K, the maximum oxide layer thickness of CP Tiincreased up to 123 µm, and saturation starts from approx-imately 2 ks; therefore, the thickness more rapidly increasedat 1323K than at 1273K. In Ti–36Nb–2Ta–3Zr–0.3O, theoxide thickness moderately increased as compared withCP Ti at either oxidation temperature. The maximum layerthickness became larger and the heating time at which thethickness became maximum became shorter as temperatureincreased. These results suggest that a high temperatureenhances oxide formation. Furthermore, from the relationshipbetween l and L+ of the oxide layer on CP Ti, l ¯ 90 µmmight be suitable for obtaining a sufficient L+.

Although the sample preparation and oxidation conditionsare different from the other two materials, the oxide layergrowth on Ti–29Nb–13Ta–4.6Zr (heat treated, air cooledafter oxidation1)) exhibited a similar tendency and was evenmoderate compared with that on Ti–36Nb–2Ta–3Zr–0.3O.On the other hand, it is natural to presume that the oxidationbehavior of CP Ti and Ti–36Nb–2Ta–3Zr–0.3O differbecause of differences in their microstructures. Their cross-sectional microstructures will be shown in Sect. 3.5.

To discuss oxide growth during heat treatment, oxide layerthickness increment during furnace cooling should be takeninto account. However, focusing on both the thickness of theoxide layer on the annealed Ti–29Nb–13Ta–4.6Zr [“HTTNTZ” in Fig. 5(a)] and that on the as-wrought Ti–29Nb–13Ta–4.6Zr (TNTZ), which were furnace-cooled afteroxidation for 2.7 ks, they are almost on the fitting line ofl of the annealed and air-cooled Ti–29Nb–13Ta–4.6Zr (HTTNTZ AC). In addition, their thicknesses were very similar.Therefore, oxide growth on Ti–29Nb–13Ta–4.6Zr is presum-ably slower than that on Ti–36Nb–2Ta–3Zr–0.3O.

3.4 XRD profile of oxide layer on Ti–36Nb–2Ta–3Zr–0.3OThe white oxide layer on CP Ti consisted of rutile TiO2,and that on Ti–29Nb–13Ta–4.6Zr consisted of TiO2 andTiNb2O7.2,18,31) Thus, it is expected that the Ti–36Nb–2Ta–3Zr–0.3O oxide layer consists of TiO2 and TiNb2O7. TheXRD profile of an oxide surface on Ti–36Nb–2Ta–3Zr–0.3Oheat-treated at 1273K for 1.8 ks is shown in Fig. 7. It seemsthat TiNb2O7 (and, presumably TiTa2O7) in addition to TiO2

existed in the oxide layer, as well as Ti–29Nb–13Ta–4.6Zr.1,2,18) TiTa2O7 may also exist since the alloys containTa to some extent; however, indexation by XRD is difficultsince most of the diffraction peaks of both TiNb2O7 andTiTa2O7 overlapped and that volume fraction of TiNb2O7 wassmall, if any. Otherwise, Ti(Nb,Ta)2O7 may have precipitatedsince Nb and Ta have similar atomic radii and they formcomplete solid solutions.

3.5 Microstructural observation and composition mappingof oxide layerCross-sectional SEM images are shown in Fig. 8. From theimages of CP Ti in Fig. 8(a), a porous microstructure, with astratified “piecrust”-like morphology, an oxide particle layerand a gap stacked one by one, was observed in the oxidelayer. 1–3µm TiO2 grains form a layer network. Note that the

0

5

10

15

20

25

-4 -3 -2 -1 0 1 2

CP Ti 1273 KCP Ti 1323 KTi-36Nb-2Ta-3Zr-0.3O 1273 KTi-36Nb-2Ta-3Zr-0.3O 1323 K

HT Ti-29Nb-13Ta-4.6Zr AC 1273 K(1)

b*

a*

b* r

ange

of i

ncis

or to

oth

a* range of incisor tooth

RedGreen

Yellow

Blue

Duration time

Fig. 6. Relationship between a+ and b+ of CP Ti, Ti–36Nb–2Ta–3Zr–0.3Oand Ti–29Nb–13Ta–4.6Zr oxidized at 1273 and 1323K. The shaded areashows the a+ and b+ ranges of the incisor tooth reported by Hasegawa et al.29)

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layer thickness of CP Ti in Fig. 5 is therefore the bulkthickness; thus, the true oxide thickness should be smallerthan it appeared to be. In contrast, it seems that a dense oxidelayer was formed on the Ti–36Nb–2Ta–3Zr–0.3O substrate,as seen in Fig. 8(b). This microstructure is very similar to thatof Ti–29Nb–13Ta–4.6Zr, as already reported in Ref. 1. Inaddition, morphologies such like the lamellar structure andthe grain boundary in the substrate were extended to theoxide layer.

EPMA composition mapping of oxidized Ti–36Nb–2Ta–3Zr–0.3O is shown in Fig. 9. The substrate microstructurebeneath the oxide layer exhibited a lamellar structure, whichconsisted of the ¡ + ¢ phase, as previously reported inRefs. 1 and 2. The separation into the Ti–O-rich and Nb–Ta-rich phases was clearly observed and extended to theinterface, as seen in Fig. 9. This phase separation wasattributed to O diffusion into the substrate, since O and Tiare ¡ phase stabilizers and Nb and Ta are ¢ stabilizers. Byexaming the substrate, the Ti-rich ¡ phase was frequentlyobserved beneath the oxide layer and along the grainboundary. This observation indicates that O diffused deepdown into the substrate not only through lattice defects but

also through grain boundaries, where the ¢ phase matrixconsequently transformed into the ¡ phase along theboundaries. In addition, the O-rich transition layer wasobserved at the interface.

4. Discussion

4.1 Relationship between color tone and oxide layerthicknessConsidering the relationships among color tone, layerthickness, and oxide phases from the results shown inFigs. 5 and 6, it seems that brightness (L+) and yellowness(+b+) increased with oxide layer thickness. In addition,Fig. 6 shows that there are slight differences in a+ betweenTi–36Nb–2Ta–3Zr–0.3O and CP Ti. Basically, the oxidelayer in both CP Ti and Ti–36Nb–2Ta–3Zr–0.3O reflectedthe entire visible light wavelength range, as shown in Fig. 4,which means that the oxide layer appears white. Therefore,oxide microstructures consisting of only TiO2 and, consistingof both TiO2 and TiNb2O7 appear white color. However, at­ ² 600 nm, R increased in TiO2 (oxide on CP Ti), indicatingthat the redness wavelength component for TiO2 was higherthan that for TiO2 + TiNb2O7 (oxide on Ti–36Nb–2Ta–3Zr–

(a) (b)

Fig. 8. Back-scattered electron images (upper) of cross-sections of(a) CP Ti and (b) Ti–36Nb–2Ta–3Zr–0.3O after oxidation. Lower images aremagnified secondary electron images of the oxide layers.

Fig. 9. (Color online) Chemical composition mapping at the interface of Ti–36Nb–2Ta–3Zr–0.3O by EPMA.

Fig. 7. XRD profile of oxide layer on Ti–36Nb–2Ta–3Zr–0.3O heat-treated at 1273K for 1.8 ks.

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0.3O). As a consequence of this difference shown in Fig. 4,the +a+ of the oxide layer on Ti–36Nb–2Ta–3Zr–0.3Owas always less than that of the oxide layer on CP Ti.Furthermore, the exfoliated surface of the oxide layerappeared yellowish and the counterface seemed dark gray,as previously reported in Ref. 2. The gray color might beattributed to ¡-case (O-containing hcp phase) formation atthe interface on the substrate side. Therefore, it is suggestedthat the oxide layer is originally semitransparent, and thatthe ¡-case is formed and its impermeability to light is dueto the interfacial phases formed during the first oxidation.Subsequently, TiO2 and TiNb2O7 layer thickness increasedduring oxidation, and then the gray color due to the ¡-caseunderneath the oxides should eventually be covered by lightscattered by the fine grain in the oxide layer.

The difference in layer thickness shown in this study doesnot necessarily mean that it is due to the bulk thickness ofoxide since the Ti oxide layer was not dense, although thegrowth rate of the oxide layer on CP Ti was much higherthan those on Ti–29Nb–13Ta–4.6Zr and Ti–36Nb–2Ta–3Zr–0.3O, as mentioned in Sect. 3.5. In this study, layerthickness was measured directly from cross-sectional SEMimages; thus, the layer thickness of CP Ti is an “apparent”thickness. The difference in the apparent growth rate of theoxide layer means that alloying elements such as Nb, Ta, andZr affected oxide formation. Consequently, L+ and b+ werealso affected by alloying elements.

4.2 Cross-sectional microstructure and oxidation curvesfor Ti alloysOn the basis of the kinetic theory, oxidation curves generallyobey the linear or parabolic rule, such as the 1/2 power law,1/3 power law, or reciprocal natural logarithm;32–34) however,the regression curve fitting in Fig. 5 suggests that theobtained curves seem to obey the polynomial or linear fit.In particular, the curve fitting suggests that the oxide layergrowth curves for Ti–29Nb–13Ta–4.6Zr and Ti–36Nb–2Ta–3Zr–0.3O at 1273K are almost linear against time.Generally, oxide growth rate obeys the linear rule when oxideforms a weak protective barrier or the adhesiveness betweenmetal and oxide is low.33) This maybe because the coolingprocess also affected layer thickness, although this oxidationbehavior should be investigated under more precise oxygen-gas-pressure- and temperature-controlled conditions.

Regarding the interfacial microstructure of oxidized CP Tiin Fig. 8(a), the morphology of the oxide layer on CP Tiwas completely different from that of the substrate. On thebasis of the investigation of the interfacial reaction duringporcelain fusion on CP Ti using a Au marker,34) it issuggested that oxidation is controlled by the out-diffusion ofTi. Taking this into account, oxide layer formation on CP Tican be illustrated in Fig. 10 and described as follows: the Tiatom diffuses to the surface during heat treatment, andthe O2¹ ion decomposes from O2 which diffuses into thesubstrate. At the initial stage of heating, the O ion diffusesinto the substrate, which would turn into the ¡-case aftercooling. Since the reactivity of Ti–O is stronger and fasterthan Ti diffusion, TiO2 is eventually nucleated at the surface(out-diffusion). Once nucleation of TiO2 occurs, the Ti atomaround the oxide grain contributes to grain growth untilsaturation. Therefore, the Ti-poor region is generated around

the grain, i.e., Kirkendall void layer formation. Nextnucleation occurs underneath the Ti-poor region, resultingin gap generation between them. Therefore, Ti diffusion is therate-controlling process in this mechanism.

On the other hand, the cross-sectional microstructures andcomposition mapping of Ti–36Nb–2Ta–3Zr–0.3O shown inFigs. 8(b) and 9, respectively, indicate that O had diffuseddeep into the substrate not only interstitially but also throughthe grain boundaries, where consequently the ¢ phase matrixtransformed into the ¡ phase along the boundaries.Furthermore, the continuous morphology from the substrateto the oxide layer through the interface, and the O-richtransition layer at the interface observed in the O map suggestthat oxidation proceeded without drastic microstructurereconstructions such as the formation of large oxideprecipitates in the substrate. These observations indicate thatdiffusion of Ti atoms was suppressed during oxidation atthe interface. The following may be the reasons for thesuppression: namely, the effect of alloying elements, thedifference in crystal structure (hcp or bcc), and the reactionand formation of TiNb2O7 in addition to TiO2. Accordingto reported diffusion parameters,35) the self-diffusion coef-ficient of the 44Ti tracer is higher in ¢ Ti than in ¡ Ti. On theother hand, the report also suggests that alloying Zr to Tisuppressed Ti diffusion in the alloy. The diffusion coefficientof Ti in Ti–Zr is comparatively smaller than the self-diffusion

TiO2 nucleation

1st TiO2 layer (grain growth saturated)

Ti diffusionβ phase

2nd TiO2 nucleationTi = Ti4+ + 4e- Ti4+ + 2O2- = TiO2

Ti diffusionβ phase

Ti Ti Ti

Ti Ti Ti

O diffusion

Ti4+ + 2O2- = TiO2

Ti deficient layer (Kirkendall boids layer)O2 + 4e- = 2O2-

TiO2 layer

Ti diffusionα phase

Gap (Kirkendall boids layer)O diffused layer

O diffusion

Ti deficient layer

α case

α case

α case

α case

Ti = Ti4+ + 4e-

O2 + 4e- = 2O2-

High temperature oxdation model on CP TiOut-diffusion of Ti, (Reactivity of TiO2, RO ) >> (Ti Diffusion, DTi )

Ti4+ + 2O2- = TiO2

Ti diffusion

O diffusion

β phase

Ti Ti Ti

TiO2 grain growth

After cooling

Fig. 10. Schematic illustrations of the oxidation mechanism on CP Tibased on obtained results.

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coefficient.35) Finally, when it comes to interdiffusion, theinterdiffusion coefficient of Ti–Nb decreased by about twoorders of magnitude as compared with the self-diffusioncoefficient of 44Ti.35) Therefore, the effect of alloying mightbe one of the reasons for diffusion suppression, whichconsequently contributed to the difference in oxidation be-havior between Ti–Nb–Ta–Zr alloys and CP Ti. Thus, thisis probably one of the reasons that the oxide layers onTi–36Nb–2Ta–3Zr–0.3O and Ti–29Nb–13Ta–4.6Zr becamedense and thinner than that on CP Ti.

Okazaki et al.36) reported that on electrochemically robustoxide layer consisted of TiO2 with small amounts of ZrO2,Nb2O5, and Ta2O5 on Ti–15Zr–4Nb–4Ta. Although thisphase is different from the oxide layer on Ti–29Nb–13Ta–4.6Zr or Ti–36Nb–2Ta–3Zr–0.3O, oxide formation on Ti–Nb–Ta–Zr alloys could be a promising techunique forbiomedical applications.

Further investigation of the exfoliation resistance of theoxide layer and the mechanical property of the substrate isalso needed for biomedical applications. In addition, a moredetailed study is expected in order to clarify the oxideformation mechanism of these Ti-based alloys. These topicswill be dealt with in further studies by the authors.Exfoliation resistance and further microstructural investiga-tion will be reported in our future paper.

5. Conclusions

To improve the esthetic properties of Ti–36Nb–2Ta–3Zr–0.3O, Ti–29Nb–13Ta–4.6Zr, and CP Ti, Ti oxide layercoating by heat treatment was investigated. The relationshipsamong color tone and the thickness of the oxide layer, andcross-sectional microstructures were studied. The conclu-sions obtained from the results of this study are as follows:

(1) Drastic weight gain started above 1034 and 1043K inCP Ti and Ti–36Nb–2Ta–3Zr–0.3O, respectively. Therefore,the heat treatment temperature should be above thesetemperatures to obtain sufficient brightness by covering withan oxide layer of sufficient thickness.

(2) The brightness L+ increased with oxidation tem-perature. The dependence of L+ on heating time was alsoobserved in Ti–36Nb–2Ta–3Zr–0.3O but not in CP Ti at1323K. Oxide layer thickness also depended on heatingtime; therefore, the L+ of the oxide layer on Ti–36Nb–2Ta–3Zr–0.3O depended on layer thickness. In addition, theL+ values of the oxide layer on Ti–36Nb–2Ta–3Zr–0.3Oobtained in this study were mostly within the range of L+

values of the Japanese’s incisor teeth, which is L+ ² 75.(3) The chromaticity coordinates, a+ and b+, were

basically outside this range; however, b+ was rapidlyincreased by controlling heat treatment conditions. Theresults at least suggest that the oxide layer with a few tensmicron thickness might be sufficient for obtaining theappropriate b+.

(4) According to the relationship between the l and L+ ofthe oxide layer on Ti–36Nb–2Ta–3Zr–0.3O, l = 15–40 µmwill be the suitable thickness range for obtaining a suffi-ciently high L+. For CP Ti, l ¯ 90 µm might be suitable forobtaining a sufficient L+.

(5) The obtained white layer on Ti–36Nb–2Ta–3Zr–0.3O consisted of the same multiphase oxides of TiO2 andTiNb2O7 as Ti–29Nb–13Ta–4.6Zr, which was previously

reported by Miura-Fujiwara et al.2) Furthermore, a densemicrostructure of the oxide layer on Ti–36Nb–2Ta–3Zr–0.3Owas also similar to that of Ti–29Nb–13Ta–4.6Zr.1,18) There-fore, it is considered that the oxide formation mechanismon Ti–36Nb–2Ta–3Zr–0.3O is also similar to that onTi–29Nb–13Ta–4.6Zr.

(6) From the results of cross-sectional microstructuralobservation of Ti–36Nb–2Ta–3Zr–0.3O, the oxide layer wasdense, whereas a piecrust-like structure was observed in theoxide layer on CP Ti. The O-rich transition layer wasobserved at the interface between the oxide layer and themetal substrate. The ¡ + ¢ lamellar structure was formedon the substrate beneath the oxide layer, as well as Ti–29Nb–13Ta–4.6Zr. These observations indicate that the oxidationmechanism of Ti–36Nb–2Ta–3Zr–0.3O is different from thatof CP Ti but similar to that of Ti–29Nb–13Ta–4.6Zr.

Acknowledgements

This work was partly supported by JSPS KAKENHI GrantNumber 24560857-51. This work was also partly performedunder the Inter-university Cooperative Research Programof the Institute for Materials Research (IMR), TohokuUniversity (No. 13K0103). The authors are grateful to Mr.Yoshihiro Murakami and Mr. Issei Narita of IMR, TohokuUniversity for their help in EPMA.

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