E L S E V I E R Tlun Sohd Films 295 (1997) 101-103

Alumina strengthening by silica sol-gel coating

Ljubica Nikolic, Ljiljana Radonjic Faculty of Technology, Department of Materials Sc,ence, But. Cara Lazara 1, 21000 Novi Sad, Yugoslavia

Received 15 Apn11996; accepted 22 July 1996

Abstract

The Sol-gel coating to stengthen an alumina substrate has been studied.Increase the strength for two times the strength of uncoated substrate has been achieved.The strengthening depends on coating microstructure and the degree of bonding to the substrate. The coating microstructure has been changed by changing the processing parameters of silica sol preparation. The coating microstucture has been related to the pH value and water content of silica sol. © 1997 Elsevier Science S.A.

Keywords: Sol-gel; Strength; Coatings; Cerarmc

1. Introduction

Sol-gel coating is one of the important techniques for the substrate surface modification and the improvement of mate- rial properties which are effected by the surface conditions. Today, sol-gel coatings have very diverse applications [ 1]. The strength of the brittle materials is bulk property which is largely controlled by surface conditions. Sol-gel coating could improve the strength of brittle materials. In currently available literature [2] however, there is only limited infor- mation on this subject. Strengthening of the glass substrate by sol-gel coating was recently studied by Ulhmann's group [ 3,4]. According to Ulhmann's results, sol-gel coating could improve strengthening of the brittle materials by following mechanisms: (1) by filling in surface flaws, (2) blunting crack tips, (3) adding a compressive stress to the surface which is caused by a mismatch in the coefficient of thermal expansion (CTE) between substrate and coating and (4) protecting the material from the fatigue, or by any combina- tion of the above [4].

This paper is concerned with ceramic substrate strength- ening by sol-gel silica coating. According to the authors knowledge, only Noda's work [2] could be taken as a similar subject of study (but not sol-gel method). The aim of this paper is to present the results of the effect of silica sol-gel coating on the strength of ceramics (96% alumina). Since the resulting silica sol-gel coating structure is strongly affected by solution processing parameters [5], different sil- ica solutions are prepared by changing processing parameters (quantity of water and pH). As it is well known [ 1], a small quantity of water and pH of coating solution produces rather

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linear polymer-type oligomers, which are more coatable then the particulate-like species, produced by a high quantity of water or high pH values. Besides, strengthening of the ceramic substrate had to be affected by the heat-treatment conditions (temperature and time) too.

2. Experimental

As a substrate in the experiments, 96% alumina plates (produced by Tectronix) were coated with silica sol. Silica sols were separately prepared using thetraethoxysilane (TEOS) as a precursor, ethanol, water and catalyst (HC1 or NH3). Samples were coated by withdrawing the alumina plates (at a rate of 6 cm min- 1 ) from coating solution. After coating all samples were dried and heated up to 600 °C for 10 min. Multilayer depositions (10 and 15 times) were car- ried out to increase the film thickness, and then heat-treated at 1100 °C for different times of holding. The samples are identified as Ca pH, where index R represents a moll ratio of water per moll of TEOS, sometimes called "the degree of hydrolysis" [3] (being from 1 to 8), pH represents the pH coating sol values (being from 1 to 9).

Samples were examined using X-ray diffraction, XRD (Philips 70) and scanning electron microscopy (SEM) (JEOL 35). Modules of the rupture (MOR) were calculated using measured bending test values [6].

3. Results and discussion

The XRD patterns of uncoated alumina before and after heat treatment at 600 °C and 1100 °C, are shown in Fig. 1. It

102 L Nikohc, L Radonjzc I Thm Sohd Films 295 (1997) 101-103

$

z

z S

I 20

A

A

A

i A A '

A A A HA i

5 A

A J 60

AS A AI ~A i

I I I t I I 30 Z,0 50

2 '~ (degree)

Fig. 1 X-ray dlffractograms of the alurmna substrate' (a) before and after heating; (b) at 600 °C, (c) at 1100 °C for 10 roan. and (d) at 1100 °C for 2 5 h, A. alumina; S, spinel (Cu Ka radiation).

was known that electronic grade alumina (96%) which was used as a substrate in these experiments, contains a continu- ous glassy phase along the grain boundary [7-9]. XRD dia- grams in Fig. 1 show the presence of a spinel phase, S, in addition to the peaks typical for a-alumina, A. It is obvious that there is no considerable phase change after heat treatment at 600 °C and 1100 °C, for short time. In the same time, under these conditions, MOR values are not changed and remained at a level of 354 MPa. After a longer holding time at 1100 °C (about 5 h) there is a change (this result is not shown in Fig. 1) and MOR values decrease to 255 MPa [ 10]. This result could be explained by the changing of the crystalline phase along the alumina grain boundary [7].

If the strengthening involves penetration of the coating into the flaws (this was one of our assumption in this study) then chemical interaction between the silica sol and alumina sub- strate must be important in optimizing strengthening. Since mechanism and rates of the hydrolysis and condensation reac- tions of TEOS and resulting structural development of coat- ing are strongly affected by sol processing parameters [5], one expected that changes in the reaction conclitlons of the TEOS solution could significantly affect sol-gel strengthen- ing. The importance of two such parameters--the degree of hydrolysis, R, and solution pH, were examined and results are presented in Figs. 2 and 3. The amount of water added for hydrolysis was varied from 1 tool of H20 per mol of TEOS to 8 mol of HaO per tool of TEOS. It is presumed [3] that silanol groups are necessary for condensatton between the silica coating and alumina substrate.

From the results presented in Fig. 2, it is apparent that the degree of hydrolysis, R, has a strong affect on the strength- ening for 1 < R < 5. After coating the alumina by CR 2 silica sol, and heating to 600 °C (Fig. 2) the sample MOR values

did not change considerable (which could be explained mainly by filling of flaws), and samples heated at 1100 °C had increasing strength by R. For R > 5 the increasing dimin- ished; there appeared to be a critical concentration of water, above which the degree of hydrolysis was unimportant. It was not surprising that some degree of hydrolysis was nec- essary to achieve strengthening; and increasing the silanol content (up to R > 5) enhances strengthening. A remarkable strengthening increase (Fig. 2) after increasing heat-treat- ment temperature at 1100 °C and increasing time from 1 h to 2.5 h could be correlated to the densification of the coating. The highest value of MOR (about 590 MPa) was reached after heating at 1100 °C for 2.5 h when densification of coat- ing was probably completed. It seems that the main strength- ening mechanism in this case could be attributed to the compressive stress on the substrate surface, as a result of the CTE mismatch of alumina (CTE = 8.8 × 10- 6 K - 1) and sil- ica (approximately C T E = 0 . 5 × 10 .6 K - l ) . This mecha- nism seems to dominate only after coating reaches the high density (Fig. 2).

As expected, the silica sol pH has a strong effect on the strengthening (Fig. 3) too. By increasing pH values from 1 to 4.5, the MOR values increase from 354 MPa for uncoated alumina to a maximal value of 590 MPa (for R = 8) for the

600

50O

MOR (NPat

40O

CR.2 ~"

. ~ J" At 2 O:3

~0C I ! I I I I 1 ~ I 1 2 3 t, 5 6 7 8 9

R = H 2 0 / T E O S

Fig. 2. MOR values of the samples CR.2 heated at different temperatures and times.

600

500

H0R (MPa)

400

30C I I P I I I 2 ~ t, 5

pH-val.ues Fig. 3. MOR values of the samples as a functton of pH coaung sols, after heat-treatment at 1100 °C for 2.5 h.

L. Ntkohc, L Radonjic / Thm Sohd Fibns 295 (1997) 101-103 103

Fig. 4 SEM images of the microstructure of the samples' (a) C8 :, (b) Cn s, (S, substrate; C, coating).

coated one. It has been known that the ol igomer size tends to decrease with increasing pH for TEOS solutions; and smaller ol igomers would tend to enhance penetration into the flaws, but only up to some pH values. With regard to future increase in pH values, the ol igomer size does not play any more impor- tant roles in the strengthening process. Changes in the silanol content of the coating silica solutions represents another pos- sible explanation [5] for the effect silica sol pH on the strength. Keefer [5] showed that silanol content of TEOS solutions decreases as pH increases. Certainly the silica sol

pH affects more than just by the silanol content [ 11 ] (for example, according to Pantano [ 11 ] by elastic module) .

The main affect of the silica sol pH on the strengthening is rather related to the CTE mismatch of the sihca and alumina

after heating at high temperature when coating has a high density. It has a similar strengthening mechanism to the one explained for the effect of R on the strengthening (Fig. 2).

The different coating microstructures as a function of the processing parameters are shown in Fig. 4. By changing val- ues of R from 4 to 8 (Fig. 4 (a) and ( b ) ) and pH values from 2 to 8 (Fig. 4 ( a ) and ( b ) ) the coating micros tructure changes from a very fine scale microstructure to a coarse one.

SEM images in Fig. 4 show only the types of coatings which are very well bonded to the substrate and rather even without breaking. These microstructural features are very important for strengthening.

References

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[2] S. Noda, H. Do1, T. Hloki, J. Kawamoto and O. Kamigaito, J Am. Ceram Soc, 69 (1986) C-2103

[3] B D Fobes, W.F. Doyle, B.J.J. Zehnski, L.A. Sllverman and D.R Uhlmann, J. Non-Cryst. Solids, 82 (1986) 349

[4] B D Fobes and D.R Ulhmann, 3. Am Ceram. Soc, 73 (4) (1990) 978

[5] R D Keefer, Mater. ~Res Soc. Syrup. Proc , 32 (1984) 15. [6] J.F. Shackelford, Introducnon to Materials Science for Engineers,

Macrmllan, New York, 1988, p. 390 [7] C A. Powell-Dogan and A.H. Heuer, J. Am. Ceram. Soc., 73 (12)

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(1990) 3677. [9] C.A. Powell-Dogan and A.H Heuer, J. Am Cerarn. Soc., 73 (12)

(1990) 3684 [ 10] L. Ntkohc and L. Radonjlc, in B.S. Tranctuna and A. Bellost (eds),

Fourth Euro-Ceramics, Vol 9, C.N R.-IRTEC, Faenza, Italy, 1995, pp 315.

[ I 1 ] M J Murtagh, E K Graham and C G. Pantano, J Am Ceram Soc, 69 (11) (1968) 775.