Toughened glass-ceramics containing ZrO2 and Al2O3 prepared by the sol-gel process from metal alkoxides

298

Journal of Non-Crystalline Solids 100 (1988) 298-302 North-Holland, Amsterdam

TOUGHENED GLASS-CERAMICS CONTAINING Z r O 2 AND BY THE SOL-GEL PROCESS FROM METAL ALKOXIDES

AI203 P R E P A R E D

M. N O G A M I and K. N A G A S A K A Aichi Institute of Technology, 1247 Yakusa Toyota, Aichi, Japan

K. K A D O N O Government Industrial Research Institute of Osaka, 1-8-31 Midorigaoka Ikeda, Osaka, Japan

T. K I S H I M O T O Osaka Yogyo Co. Ltd., Wakmohama Kaizuka, Osaka, Japan

Glasses of compositions 5ZrO2-5SiO2(ZS), 5ZrO2. A1203.4SiO2(ZAS) and 5ZrO2.0.5A1203•0.5Na2° . 4SiO2(ZANS) were prepared by the sol-gel process from metal alkoxides and sintered to make glass-ceramics. Tetragonal ZrO2was precipitated by heat treatment at 900 to 1300o C. The activation energy for tetragonal ZrO2 crystal growth was extremely high in A1203 containing glasses. ZAS and ZS were sintered to the near theoretical densities above 1200o C, at which the predominant phase was tetragonal ZrO2. On the other hand, for ZANS, high densification was not attained owing to the large pores enclosed by the glass phase. Strength and fracture toughness increased with the densification and the crystal growth of tetragonal ZrO2, reaching 450 MPa and 9 MN/m1"5,respectively.

1. Introduction ZrO2-containing ceramics have recently become of interest because of their high strength and fracture toughness, which have been attributed to the stress induced transformation of tetragonal (t-) ZrO 2 [1-4]• These same ZrO2-transformation toughening should be applicable to an amorphous matrix. Tetragonal ZrO2 precipitation has been observed during the crystallization of the glasses, in which the ZrO 2 content is limited to 20 to 30 wt% since it requires a high temperature [5,6]. Recently, using the metal alkoxide sol-gel process, one of the authors prepared the glass-ceramics of the ZrO2-SiO 2 system containing up to 60 mol% ZrO 2 in which t-ZrO 2 crystal was precipitated, and measured the crystal growth and fracture toughness [7-9]. ZrO 2 crystal grew according to the coarsing mechanism [7] and t-ZrO2 crystals larger than a critical size transformed into monoclinic (m-) ZrO 2 during cooling [8]. The fracture toughness of these glass-ceramics increased with increasing the size of t-ZrO2, reaching about 5 M N / m xs [9]. 0022-3093/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

In the present work, the glass powders of 5ZrO 2 • A120 3 • 4SiO2(ZAS), 5ZRO2- 0.5A120 3 • 0.5Na20 • 4SiO2(ZANS ) and 5ZrO2.5SiO2(ZS ) in mole ratio were sintered and their strength and fracture toughness were measured• The effect of A1203 and N a 20 on them was investigated.

2. Experimental procedure Si(OC2H5)4, Zr(OC3HT)4, AI(OCaH9) 3 and N a O C H 3 were used as the starting materials. Si(OC2Hs) 4 was at first partially hydrolyzed by dropping it into a mixed solution of 0.15 mol/1 HC1 aqueous solution and C 2 H s O H such that the resulting mixture consists of 1 to 1 mole ratio of H 2 0 and Si(OC2Hs) 4 and 15 vol% C2HsOH. After stirring this solution for 3 h, Zr(OC3H7)4, AI(OC4H9) 3 and N a O C H 3 were added in this order drop by drop during stirring. After adding all the metal alkoxides, the transparent solution was stirred for 1 day at room temperature. The prepared metal alkoxide solution was hydrolyzed by dropping it into the mixed solution of 0.15

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mol/1 HC1 aqueous solution and C 2 H s O H such that the resulting mixture consists of 1 to 1 mole ratio of H 2 0 and metal alkoxide and 2 1 CEHsOH. Hydrolyzed metal alkoxide solution was filtrated, followed by heating at 150 ° C. Crushed powders, calcined at 800 ° C for 2 h, were cold die-pressed into 5 cm diameter and 4 mm high, followed by cold isostatic pressing at pressure of 200 MPa. The compacts were heated at 85 ° C / h to a fixed temperature and kept at that temperature for 2 h in an electric furnace in air. Densities of sintered bodies were calculated from measured weights, thicknesses and diameters. Sintered bodies were cut into the test bars with about 3 x 4 × 3 0 mm 3. Four faces were ground using diamond paste. The fracture strength was measured by a three-point bending method at a constant cross-head speed of 0.4 ram/rain. The fracture toughness was determined by the indentation method using the equation of Niihara et al. [10]. To analyze the t - Z r O 2 crystallization, differential thermal analysis (DTA) was carried out using a heating rate of 2.5 to 25 ° C / m i n , a static air atmosphere and a-A1203 as the reference material. Precipitated crystals were also characterized by a powder X-ray diffractometer. The crystallite size of t-ZrO 2 was calculated by Scherrer's formula using the line broadening of the (111) diffraction line [11]. The relative tetragonal/monoclinic crystal of Z r O 2 precipitated was also calculated using the equation F t = I t ( 1 1 1 ) / ( I t ( 1 1 1 ) + Im(111)+Im(111)) where I ( h k l ) was the diffracted beam intensity from the ( h k l ) phases [12].

peak temperature reaches the same specific value, 0.7, irrespective of the heating rate. Therefore, the activation energy for crystal growth, if the nucleation mechanism is predetermined, can be directly obtained from the slope of the straight line between the heating rate and the reciprocal peak temperature. For three different samples, the exothermic peak was observed at 850 to 950 ° C, which could be attributed to the precipitation of t-ZrO 2 from the X-ray diffraction studies. The exothermic peak temperature was seen to become lower by replacing SiO 2 with A1203 or N a 2 0 . Fig. 1 shows the relations between the heating rate and the peak temperature, which hold a good linearity. It has already been reported that the nuclei of ZrO 2 crystals in the Z r O : - S i O 2 system are formed sufficiently in the gel and the crystals grow three-dimensionally [7]. Therefore, from the slope of the straight lines, the activation energies were calculated as 690, 970 and 1020 k J / m o l for ZS, ZANS and ZAS, respectively. The activation energies of A1203 containing systems are very much higher than that of A1203 free sample (ZS). Generally, the activation energy for crystal growth is considered to be controlled by the viscous flow of the amorphous matrix surrounding the crystals. The high activation energy obtained in this study would indicate that A1202 made the high viscous glass matrix by incorporating with SiO:. This A1203

Peak temperature (°C) 900 875 850

925 i

3. Results and discussion

.~ 2o E ,,z-

10

3.1. Crystallization

DTA can be used for studying the crystallization kinetics of glass-ceramics [13-16]. The crystals precipitated in the glass increase their volume and size with increasing temperature, which causes the exothermic peak in the DTA trace. And the value of activation energy for the crystal growth can also be evaluated from DTA curves. It is known that the crystallization degree at the exothermic

i

"5 '-

5

"6 -i!

8.4 816 Reciprocal peak

818

91o

temperature (K-I)

Fig. 1. Relation between In (heating rate) and reciprocal peak temperature.

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M. Nogami et aL / Glass ceramics containing ZrO 2 and Al203

(o)

(b)

t1200oC

200°C

~

t

1300oC

__X__.

1;00o0

m

1400°C

_k_J~ 2b

2'5 20

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Z

mm

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z

I ~

z

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2b

45'

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(Degrees)

3b

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(Degrees)

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m

Js

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3's

rrl

L0

4's

(Degrees)

Fig. 2. X-ray diffraction patterns o f samples, ZS(a), ZAS(b) and ZANS(c), heated under various conditions as shown in the figure t = t-ZrO 2, m = m-ZrO2, Z = zircon.

M. Nogami et aL / Glass ceramics containing ZrO 2 and Al20 J

10C

~10 ±

ZS

"6 0.5 G

'~ 80

u

"tD

.o It

301

0~

o

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I000 1100 1200 1300 IZO0 Temperature

(°C)

5 60 r,r

' 800 ' 1000 ' 12'00 ' 1/-,'00 Temperature (°C)

Fig. 3. Fraction of t-ZrO2 precipitated as a function of heating temperature.

Fig. 4. Change in bulk density with increasing temperature. behavior in the glass structure is same as that in the conventional melted glasses. Fig. 2 shows the X R D patterns of ZS, ZAS and ZANS heated in the range of 1100 to 1400°C. With increasing temperature, it is seen that the predominant crystalline phase for each sample is changed from t-ZrO2, precipitated above about 900 o C, to m-ZrO 2 and then especially to zircon for ZAS (fig. 2b). It has been reported that the growth of t-ZrO 2 is controlled by the Ostwald ripening coasening process and crystals larger than a critical size transform into m-ZrO 2 during cooling [8]. The transformation temperature of t- to m-ZrO 2 is strongly affected by the glass matrix. The relative fraction of t-ZrO 2 is shown in fig. 3 as a function of heating temperature. It can be seen that the relative t-ZrO 2 content of ZAS is almost similar to that of ZS, suggesting that A1203 has less effect on the transformation of t-ZrO 2. On the other hand, for ZANS the temperature for m-ZrO 2 precipitation is markedly decreased by N a 2 0 . Zircon crystals precipitated above 1300°C in ZAS (fig. 2(b)) would be due to the reaction of ZrO2 with SiO 2 under consisting A1203.

3.2. Sintering

other hand, the densification degree of ZANS is only 90% contrary to the expectation from the easy viscous flow as a result of introducing Na 2O. SEM observation for ZANS revealed that the liquidus glass phase was formed and large pores were enclosed by this glass phase, which would hinder increased densification. On the other hand, these pores were scarcely observed in ZAS and ZS compositions.

3.3. Strength and fracture toughness Strength and fracture toughness of ZS, ZAS and ZANS heated at various temperatures for 2 h are shown in figs. 5 and 6, respectively. With increasing temperature, both values increase. And

500 400

Z A ~

300 c-

200

G

Fig. 4 shows the sintering behaviors of three samples. The relative density of moulded powders increases sigmoidally and reaches a constant value above the temperature, which is 1200, 1200 and 1100 ° C for ZS, ZAS and ZANS, respectively. The moulded powders of ZS and ZAS are sintered up to above 98% of the theoretical density. On the

P

i

i

I

i

1000 1100 1200 1300 Temperature (°C) Fig. 5. Relation between strength and heat treatment temperature.

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M. Nogami et al. / Glass ceramics containing ZrO 2 and Al203

powders of 5 Z r O 2 - A 1 2 0 3 - 4 S i O 2 composition, prepared by the s o l - g e l process f r o m metal alkoxides, high strength (450 MPa) and high fracture toughness (9 M N / m 15) could be obtained by densification and precipitation of t-ZrO 2. Introduction of A1203 was effective in increasing the strength of ceramics because of its high Y o u n g ' s modulus.

% lO z58 m r.-

Ne 0

t_

~6

82

1.1_

References

li00

1300

Temperature

13'00 (°C)

Fig. 6. Relation between fracture toughness and heat treatment temperature. after reaching m a x i m u m values, they decrease by further heating. The change of the strength and fracture toughness can be discussed on basis of the densification and the appearance of crystal phases. Namely, the densification and the growth of t-ZrO 2 increase the strength and fracture toughness. The appearance of m - Z r O 2, however, results in a marked decrease in both values. The low m a x i m u m values of strength and fracture toughness for Z A N S could be due to the formation of the glass phase and large closed pores. O n the other hand, ZAS, sintered to near theoretical density without forming the pores, shows the high strength and fracture toughness, that is, 450 M P a and 9.0 M N / m 1"5, respectively. It could be considered that the i m p r o v e m e n t of these properties was carried out by the introduction of A120 3 with high Y o u n g ' s modulus. In conclusion, it was f o u n d that for glass

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