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Adhesion of Titania Films on Alumina

Kailash .C. Jain

Delphi Research Labs, 51786 Shelby Parkway, Shelby

Township MI 48315

Semiconducting oxides such as SnO2, WO3, ZnO,TiO2 are especially useful for sensing residuals of

combustion components from automobile exhausts, diesel

engines, and home heating systems due to their chemical

and thermal stability. Lambda sensors using TiO2 are

simpler in construction to zirconia sensors that require

closed-tube technology. The rutile titania film is also the

material of choice for photochemical hydrogen production

from water [1]. The durability of such devices however

relies upon the adhesive strength of the titania films to the

substrates [2]. This paper focuses on assessing and

improving the adhesive strength of titania films onto

alumina substrates.

Three types of Al2O3 substrates measuring57x26x1mm were obtained by co-firing two pieces of

green alumina tapes at 1500C with or without screen-

printed Pt or Pt- TiO2 [2]. The Pt and Pt- TiO2 inks were

prepared using Pt powder, an organic binder, a solvent,

and TiO2 powder (grain size: 1-5m) 10-20 wt% such that

the inks had a solid content of roughly 65% by weight. On

these substrates, TiO2 islands were screen-printed using an

oil slip made from a 2m grain size nominal TiO2 powder.

Two pre-fired titania film thicknesses, H =15m and H

=65m and two slip mediums were selected for screen

printing. The printed test pattern consisted of seven

blocks of titania islands on alumina, measuring L =14mm

and W =5mm. These samples were dried at 150C,sintered for 1 hr in the 900C-1300C range.

A Revetest Automatic Scratch Tester equipped with

a Rockwell C diamond (conical angle: 120;

hemispherical tip: 200m radius) was used for the scratch

tests. Six to twelve separate parallel scratches were

performed under linearly increasing load for each of the

sintering temperatures. A loading rate of 100N/min and a

table speed of 10mm/min were used. These parameters

yielded a load-distance rate, dL/dx, of 10 N/mm as

recommended in the literature [3]. The acoustic emission,

tangential frictional force, and optical observations were

used to determine the critical load shown in Figure 1.

For the pull-tests tapered aluminum pins pre-coatedwith an epoxy were bonded to the film by curing the

epoxy adhesive at 150C for one hour. The adhesive

strength was calculated as the force required for pulling a

pin from the specimen divided by the area of the 3.58mm

diameter pin. Typically 5-10 pulls were made on each

sample at a given sintering temperature. Adequate

adhesion tests were obtained by incorporating two

improvements to a Comten Industries pull-tester. First, a

universal joint was designed and installed on the pull-test

fixture that allowed the fixture to rotate in the x-y plane.

Next, a redesign of the clamps using a concentric washer

with the pull-stud in the test machine resulted in

reproducible pull-strength values over the range ofsintering temperature as shown in Figure 2.

As shown in Figure 2 there is a large range in pull

strength of 15-20m films obtained from either the

aqueous or oil slip mediums. The SEM and optical

microscopy of the fracture surfaces revealed that purely

mechanical interlocks and a lack of oxide on the surface

of platinum were most likely the cause of very low to

practically no adhesion with the TiO2 film on the Al2O3

surface.

Figure 3 shows the pull strength of 65m titania

films over Pt/TiO2 coated substrate and compares with

pull strengths over Pt/Al2O3 and Al2O3 substrates. As

seen from the figure the 65m films over Pt/TiO2 have

narrow distribution and an order of magnitude higher pullstrength as compared to Pt/Al2O3 or Al2O3 substrates. The

chemical-mechanical interlocks provided 65m titania

films sintered at 1300C that were tightly bound and had

pull strength of 25 MPa. Recent work on toughening

composites [4] may be important in further optimizing the

adhesive strength of TiO2 films.

REFERENCES

1. R. Schaub et. al., Physical Review Letters, Vol. 87,

No. 26, 2001.

2. R.G. Fournier, K.C. Jain, & C.A. Valdes, U.S. Patent

# 5,776,601, 1998.

3. P.A. Steinmann, Y. Tardy, H.E. Hintermann, ThinSolid Films, 154, pp.333-349, (1987).

4. I.J. Merchant, H.W. Chandler, R.J. Henderson, T.

Stebbings, & D.E. Macphee, Mat. Res. Soc. Symp.

Proc. Vol. 702 (2002).

Fig.1 Step increase in acoustic emission vs. sintering

temperature. Thick films (62 m) require higher normal

force and overlap of both of these bands with 15-90m

films suggests that both slip mediums behave similarly.

Fig.2 Pull strength of titania films vs. firing temperature.

Fig.3. Pull strengths of TiO2 films on alumina surfaces

with or without Pt or Pt- TiO2 coatings.

800 1000 1200 1400

0

5

10

15

20

Sintering Temperature, o C

PullS

trength,

(MPa)

Squeegee o il slip

Aqueous slip

15-20 m films

800 1000 1200 1400

0

5

10

15

20

Sintering Temperature, o C

PullS

trength,

(MPa)

Squeegee o il slipSqueegee o il slip

Aqueous slipAqueous slip

15-20 m films

0

5

10

15

20

25

30

850 1050 1250

15-90um aq. Slip

15um sq. oil slip62 um sq. oil slip

Sintering Temperature, (oC)

NormalForce,

N

@s

uddenincr.Inacousticsignal

0

5

10

15

20

25

30

850 1050 1250

15-90um aq. Slip

15um sq. oil slip62 um sq. oil slip

Sintering Temperature, (oC)

NormalForce,

N

@s

uddenincr.Inacousticsignal

TiO2

on Pt- TiO2

coated Al2O

3

TiO2 over Pt coated Al2O3

TiO2 on Al2O3

P

ullStrength,

(MPa)

0

10

20

30

Temperature, oC1200 1250 1300

TiO2

on Pt- TiO2

coated Al2O

3TiO

2on Pt- TiO

2coated Al

2O

3

TiO2 over Pt coated Al2O3TiO2 over Pt coated Al2O3

TiO2 on Al2O3TiO2 on Al2O3

P

ullStrength,

(MPa)

0

10

20

30

Temperature, oC1200 1250 1300

Abs. 670, 204th Meeting, 2003 The Electrochemical Society, Inc.