Volume 7. number 5,6 MATERIALS LETTERS November I988

THE MECHANICAL BEHAVIOUR OF THE Ti-Ti02 SYSTEM

Chr. PANAGOPOULOS and Hel. BADEKAS Lulwatory qfPhysica/ Metallurgy. National Technrcal C:niwrsity ofAthens.

%o,wu/ixc Campus. 15 773 .Athens. Greece

Received 2 August I988

The strengthening effect produced by anodic titanium dioxides grown on titanium was studied. Anodic titanium dioxides of

1 b- I35 nm thickness were found to increase the strength of anodized titanium metal. The failure strain of titanium dioxides was

observed to pass through a maximum of 0.3% at 95 nm film thickness.

1. Introduction

The mechanical properties of oxide films on var-

ious metals and alloys have been considered to be very important in conditions in which chemical and mechanical factors play a critical role such as the

corrosion fatigue and the stress corrosion cracking. The reported investigations dealing with the me- chanical behaviour of the anodic oxide fitilms grown on valve metals, i.e. aluminium, tantalum, titanium,

etc., had given values for the various mechanical pa- rameters which show very large variations.

Edeleanu and Law [ 1 ] strained thick porous an- odic oxide films on aluminium and observed them to be elastic with failure occurring at l-2% strain.

Bradhurst and Leach [ 21 deformed aluminium spec- imens with their barrier-type anodic oxide films and found the oxide films to fail at l-3% strain. Bubar

and Vermilyea [3] also observed thin anodic alu- minium oxides to show some ductility, while thick anodic oxides were brittle. Using a bulge test tech- nique, Brandon and Eliezer [4] found the biaxial

modulus of barrier-type aluminium oxides to be not a function of their thickness but the fracture strain and stress to increase with increasing film thickness. Grosskreutz [ 51 observed anodic aluminium oxides to be elastic up to fracture with the failure strain oc- curring at O.l-0.3%. In these anodic films, fracture occurred at slip steps and normal to the tensile axis. Choo and Deveroux [ 61 found barrier-type alumin- ium oxides with thickness greater than ~40 nm to

fail at 0.92% strain normal to the tensile axis and to

suppress substrate slip emergence. Eliezer and Brandon [ 71 examined anodic tan-

talum oxides and found a direct relation to exist be- tween the incidence of crystalline defects and the reduction in mechanical properties of the anodic films. Anodic tantalum oxides grown on mechani- cally polished substrates failed at 0.28% strain, in- dependent of thickness, whereas oxide films formed

on chemically polished substrates failed at 0.14% strain for thicknesses greater than 68 nm and 0.2% strain for thicknesses less than that value [6]. Fi- nally, Propp and Young [ 8 ] strained anodic tan-

talum oxides grown on chemically polished substrates and found after a certain degree of deformation the oxides crack and widening areas of bare metals

develop. The following study deals with the mechanical be-

haviour of the titanium-anodic titanium oxide sys- tem. which had never been investigated.

2. Experimental method

Titanium wire specimens, of 10 cm length and 1 cm diameter, used in this work were 99.8% pure, with oxygen, iron and carbon as the main impurities. The specimens were mechanically polished with emery paper, of increasing fineness. After this treatment, the specimens were chemically etched in a fresh mix- ture of 40 vol.% HNOs, 20 vol.Yo HZS04, 2 vol.% HF

0167-577x/88/$ 03.50 0 Elsevier Science Publishers B.V. ( North-Holland Physics Publishing Division )

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Volume 7, number 5,6 MATERIALS LETTERS November 1988

and 38 vol.% Hz0 for 60 s. They were then de- greased with methanol, washed in doubly distilled water and then dried.

Potentiostatic anodization of the titanium wire

specimens was carried out using a stabilized variable voltage supply. A 0.01 M sodium phosphate solution

with pH= 9 and T= 17°C was used for titanium anodization. These anodizations were performed

under various applied voltages but at a constant an- odizing time of 5 min. The titanium anodized spec-

imens were washed in doubly distilled water and

methanol and then dried. The impedance of the an- odized specimens was measured in a 0.0 1 M sodium

phosphate solution with pH=9 at f= 1 kHz and T= 17 “C with a platinized platinum counter electrode.

Using the impedance technique, the thickness of the anodic dioxides grown on the titanium wires was calculated with the aid of the equation

d=ccors/C, (1)

where d is the dioxide thickness (m), co is the die- lectric constant of the vacuum ( 1 /~KX 9 x lo9 C/V m), E is the dielectric constant of the titanium an- odic dioxide (40 [ 9 ] ), s is the geometric area of the dioxide (3.14~ lop4 m’), C is the dioxide capaci-

tance given by the impedance technique and r is the roughness factor of the dioxide surface ( 1.1).

Tension tests of the anodized wires were carried out in an Istron machine. All the tension tests were performed in laboratory air at 17 “C. Crosshead speed was always 0.5 mm/min. The surface morphology of

several deformed specimens was examined with the help of a JEOL 733 scanning electron microscope. The failure strain of the dioxide layers was deter- mined by a travelling optical microscope.

3. Results and discussion

Fig. 1 shows the stress-strain curves for a bare ti- tanium wire and a titanium wire carrying an anodic titanium dioxide of 106 nm thickness. The figure shows clearly that the titanium dioxide strengthens the bare titanium wire. We can also observe that the titanium-titanium dioxide system fractures at higher strain than the bare titanium wire. Several titanium wires carrying titanium dioxides in the thickness

202

lo010 &+, I%)

Fig. 1. Stress versus strain curves for titanium and titanium-an- odic titanium dioxide respectively. The anodic film had 106 nm thickness.

range 16-l 35 nm were examined and were always found to strengthen the bare titanium wire. The grown titanium dioxides, 16- 135 nm thickness, were

always found to be very adherent and were never de- tached from their substrate up to their fracture stress.

Fig. 2 shows the failure strain of the dioxide films as a function of their thickness. From the last figure,

we observe that the failure strain passes through a maximum of 0.3% at the dioxide film thickness of

= 95 nm. The division of experimental results given in fig. 2 in two parts, a first one in which the failure

0.L -

. . .

.

3 0.2 - . . l .

UC .

o- 50 100 150

d,, lnml

Fig. 2. Failure strain versus film thickness for anodic films formed on titanium.

Volume 7. number 5,6 MATERIALS LETTERS November I988

strain increases with increasing dioxide thickness and a second in which the failure strain decreases for dioxide thickness higher than ~95 nm, can mainly be attributed to the changing crystal structure of the anodic dioxide films. It is well known [ lo], that these films are mainly amorphous for thicknesses lower

than E 100 nm, and the same films become mainly

crystalline for thicknesses higher than z 100 nm.

The increase of failure strain with increasing diox-

ide thickness in the amorphous region can mainly be

explained by the assumption that the number of de- fects in the anodic film decreases with increasing film

thickness which leads to the higher values of failure strain of the films. On the contrary, in the crystalline

region of the oxide films, it seems that the number of various defects (microcracks, dielectric break- downs, etc.) increases with increasing film thick-

ness, leading to lower values of failure strain of the anodic films.

Generally, several mechanisms have been sug- gested for explaining the tensile strengthening of var- ious metals due to an oxide or surface layer: (a) blocking egress of dislocations to the surface [ 111, (b) elastic repulsion of dislocations from the surface

when the surface layer has a higher shear modulus than the substrate [ 121, (c) repressing multiplica- tion of dislocations from sources of the surface layer

[ 13 ] and (d) impeding approach of generated dis- locations to the surface or oxide layer by areas of higher concentration of dislocations which were gen- erated at the oxide layer by cracking of the oxide film during deformation [ 14 1.

The simultaneous occurrence of one or more of the

above mechanisms can provide an adequate expla- nation for the strengthening effect of the crystalline titanium dioxides on the titanium substrates. On the other hand, the influence of non-crystalline titanium

dioxides on the strength of the titanium substrate can possibly be explained by one of the above mecha- nisms which does not presuppose the dioxide surface layer to be crystalline for the strengthening phenom- enon, e.g. mechanism (a) and/or (b).

Anodic titanium dioxides were formed with 20 V applied voltage at different anodizing times. For the grown dioxides, stress-strain curves were obtained up to 15% strain. Fig. 3 shows the stress at 15% strain as a function of the anodizing time. As we note, the stress decreases for oxides grown at anodizing times

0 10 7.0 30

Anodizing timebin)

Fig. 3. Stress versus anodizing time of titanium. The strain was

15% and the titanium was anodized potentiostatically.

longer than 5 min. This phenomenon can mainly be

attributed to the experimental observation that in- creasing anodizing time (under 20 V applied volt- age) leads to increased cracking of the growing dioxides [ 15 1. This experimental observation re- sults in the titanium-titanium dioxide system to re-

ceive lower values of the stress for a constant value of the true strain.

A titanium-titanium dioxide system was strained up to 5% true strain. Then the surface dioxide was

removed by dissolving it with dilute hydrofluoric acid, leaving the bare titanium wire. This wire was

strained again, fig. 4. The reloaded stress-strain curve

25

Orlde removed . . . . : C _- ’ /- . .,

, : .;. ’ ‘C

/ I..:: ; ‘I

I ’ / ’ I ,‘/ I

/

.’ / I

! I I

’ I I

1

-.- Ti +TiO2

-- Ti

.“. Ti after the removal of the oxide film

i 20 i , 1

0 5 10 Etr 1%)

Fig. 4. Stress versus strain curves for titanium, titanium-tita-

nium dioxide (46 nm thickness) up to 5% strain and titanium

after the removal of the oxide film at 5% strain.

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Volume 7, number 5,6 MATERIALS LETTERS November 1988

Fig. 5. Surface morphology of a titanium dioxide ( 119 nm thickness) grown on a titanium wire subsequent to 18% strain.

does not coincide with that of the bare titanium wire, indicating a much higher strengthening effect. This

experimental observation may mainly be attributed to the familiar phenomenon of the strain strength- ening mechanism.

Finally, fig. 5 shows the surface of a specimen sub- sequent to 18% strain. The specimen consisted of a titanium wire carrying a dioxide of 119 nm thick-

ness. As we observe several cracks appear at the ti- tanium dioxide surface.

Acknowledgement

The authors wish to thank Dr. G. Spathis of the Strength of Materials Laboratory, National Techni- cal University of Athens, and C. Tolias of the Tech- nical Center, Greek Ministry of Culture, for their experimental assistance.

References

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[4] D.G. Brandon and D. Eliezer, Mater. Res. Bull. 6 ( 197 I ) 153.

[ 51 J.C. Grosskreutz, J. Electrochem. Sot. 117 ( 1970) 940.

[6] Y.H. Choo and O.F. Deveroux, J. Electrochem. Sot. 123

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[7] D. Eliezer and D.G. Brandon, Thin Solid Films 12 ( 1972)

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[8] M. Propp and L. Young, J. Electrochem. Sot. 126 (1979)

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[ 131 A.K. Head, Phil. Mag. 44 (1953) 92.

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