Metastable zoning in cordierite-based glass-ceramics

Metastable zoning in cordierite-based glass-ceramics

Mat. Res. BuU. Vol. 8, pp. 1073-I078, P r i n t e d in the United States. 1973. Pergamon Press, Inc. M E T A S T A B L E ZONING IN C O R D I E R I...

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Mat. Res. BuU. Vol. 8, pp. 1073-I078, P r i n t e d in the United States.

1973.

Pergamon Press,

Inc.

M E T A S T A B L E ZONING IN C O R D I E R I T E - B A S E D GLASS-CERAMICS R. C. De V e k e y and A. J. M a j u m d a r Building R e s e a r c h E s t a b l i s h m e n t W a r l o r d , England

( R e c e i v e d J u l y 5, 1973; C o m m u n i c a t e d by R. Roy)

ABSTRACT Nhen phase-separated TiO~-bearing glasses having compositions in the cordierite primary phase volume of the CaO-Al~O~-SiOo-MaO system are crystallised at 1000°-1100°C, a zoned mic~o~truc~ure is often seen in the glass-ceramic products by scanning electron microscopy. X-ray microanalysis by the e n e r ~ dispersive method has produced evidence of large-scale atomic segregation in these zones, particularly involving calcium and magnesium atoms. It is suggested that zoning has a markedly deleterious effect on some of the physical properties of these materials,particularly their strength.

Ceramic materials produced from glass by controlled heat-treatment have been the subject of numerous investigations in the recent past (I~

It is now

becoming clear that~although from the point of view of theoretical examinations of the processes of nucleation and crystallisation, inorganic glasses provide many admirable examples,

in practice, except for special applications,

glass ceramics do not have many advantages over traditional ceramics.

Glass

ceramics are inherently more expensive and when proper precautions are not taken, they also show extensive variations in properties.

While studying the e f f e c t of f a b r i c a t i o n v a r i a b l e s such as glass comp o s i t i o n and temperatures of ' n u c l e a t i o n ' and c r y s t a l l i s a t i o n regimes on the p r o p e r t i e s of c o r d i e r i t e - b a s e d glass ceramics (2)~ i t has r e c e n t l y been o b s e r ved t h a t the s t r e n g t h of these ceramics i s s t r o n g l y dependent on the d u r a t i o n 1073

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CORDIERITE-BASED GLASS-CERAMICS

Vol. 8, No. 9

of preheat-treatment given to the glass prior to crystallisation.

For

~stance a glass (cao 4.4 4%, AI20 3 22.~, M ~ 13.3~, SiO2 48.%, Ti02 11¢), which had not received any preheat-treatment, gave, on crystallisation at 1100°C for 3 hours, a bending strength of 185 MN/m 2, whereas the same glas8 when 'nucleated' at 740°C (which corresponds to the glass transition temperature) for 8 hours prior to the same crystallisation sequence, yielded a value of 140 ~N/m z for strength.

Examination with scanning electron microscope has

revealed the existence of extensive zoned microstructures in the weaker materials and elemental analysis by the energy dispersive x-ray microanalyser attached to the S ~

(3)suggests mutual segregation of calcium and magnesium

atoms in these zones.

It is thought that this phenomenon is, to a large

extent, responsible for the variation in strength observed for this type of glass ceramic.

FIG I

FIG 2

SEM photograph o£ the glass-ceramic

SEM photograph of the glass-ceramic

crystallised at 1100°C without any nucleation preheat-treatment.

crystallised at 1100°C after receiving a ~reheat-treatment Of 4 hour~ at 740~C.

In Fig I is shown the microstructure of the ceramic which did not receive any nucleation preheat-treatment but was crystallieed by heating to 1100°C from the ambient at a rate of 3°C per minute and holding it at this temperature for a period of 3 hours.

There is no evidence of zoning and the

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CORDIERITE-BASED GLASS-CERAMICS

material is strong.

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Fig 2 shows a micrograph with extensive zoning.

The

glass here had received a preheat-treatment of 4 hours at 740°C prior to crystallisation.

Fig 3 shows a typical area of bright contrast (marked by

a small white square in Fig 2) at a much higher magnification.

The crystal-

lites embedded in the matrix are easily discernible as is the change in their average size and population density.

FIG 3 SEM p h o t o g r a p h o f a small a r e a i n FIG 2 a t a h i g h e r .u~ifioat ion.

The microstructure in the zones with poorer electron contrast (ie dark areas in Fig 2) is similar to that seen in Fig 3 but the crystallites in these zones are smaller and not so well developed.

The contrast between the

crystallites and the matrix brought about by the etching process used in the sample preparation is also less sharp in this case.

X-ray microanalysis results are given in Table I as weight percentages. The parent glass was used as the standard in these analyses.

A ceramic pre-

pared at IOOO°C, well below the temperature at which zoned structures are visible in the SEM photograph of etched specimens, when analysed by scanning the electron beam over a large area at a low magnification yielded results (No I in the table) very close to the theoretical composition of the bulk material.

When the analyses of six randomly selected small areas (~ 2~un x

2~m) of the specimen were averaged (No 2), only minor differences from the bulk analysis were noticed.

Since the coefficients of variation in the in-

dividual analyses were less than 5% for both methods except for Ca (where it was 10% in No 2), it was concluded that the glass and the ceramics prepared from it at low temperatures (~ I000°C) did not show any evidence of elemental

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CORDIERITE-BASED GLASS-CERAMICS

Vol. 8, No. 9

segregation on a scale greater than the resolution ( 2 am) of the analytical method.

Average analyses of high-contrast and low-contrast zones in materials

crystallised at I050°C (Nos 3 and 4 respectively in Table 1) carried out on several 2 am x 2 mn areas of the specimen clearly show that zones are formed primarily due to mutual segregation of Mg and Ca ions.

TABLE 1 Results of Microanalysis of Preheat-treated Glass-ceramics i

No l

Area of study i

i

I

1000 1000

i

,

.,

m

.

.

.

.

Oxide concentration wt%

Crystn Temp oC

.

i

i

,

i

.

|

,

(Theoretical composition) Overall scan at low magni fi cat i on Randomly selected small areas

3

I050

High contrast zones

4

1050

Low contrast zones

5 6

1100

High contrast zones

1100

Low contrast zones

.

u,

i,, •

MgO

AI203

SiO 2

CaD

TiO 2

13.36

22.20

48.90

4.44

11.00

13.99

22.83

47.63

3.87

11.65

.

14.35 22.34 47.97

3 . 6 7 11.64

9.94

20.22

47.o9

lO.48

12.25

18.04

19.49

48.57

2.00

11.87

5.20

15.oo

52.53

9.94

17.30

19.12 23.98 44.58

1.60 10.70

L

When the glass is crystallised at 1100°C (Figs I - 3), in addition to a relative enrichment of 5 to I for Ca and a depletion of 4 to 1 for Mg between high and low contrast zones, there is also an indication of minor differentiation in A1 and Ti distributions as well (Nos 5 and 6 in the Table).

Although

the zones in these samples were found to be somewhat variable in composition and the coefficient of variation occasionally exceeded 40% for some elements present in low concentrations,

there is ample evidence to suggest that pre-

ferential migration of Ca ions to the high contrast zone and Mg ions to the low contrast zone had taken place in these samples.

These results lead to the hypothesis that the development of the zoned structure in these ceramics is a definite stage in a series of metastable changes which occur when TiO2-bearing glass compositions in the primary phase of cordierite in the CaO-A1203-MgO-SiO 2 system are crystallised following a nucleation regime.

The zones are thought to be a result of glass-in-glass

phase separation of the parent glass (2) in which the metastable crystallites

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CORDIERITE-BASED GLASS-CERAMICS

have formed before the appearance of cordierite crystals.

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This phase separation

is on a larger scale than is usually observed because of the high temperatures and relatively high viscosities involved in the present instance.

The zoned

structure in these ceramics disappears if the material is heated at a higher temperature (1200°C) when a ceramic with a more uniform but coarser microstructure obtains.

~G 4 DTA traces of (I) unpreheated glass (2) preheat ed-treat ed glass

"-.. t2) .......................

;\ ] ',

,.

;?

E

0

I

I

200

I

4;0

I

6;0

I

8;0

I

10100

I

1 2 0I 0

Temperature oC

The appearance of the zones can be correlated with an endotherm (E) on the differential thermal analysis trace (Fig 4) obtained with the preheattreated sample.

The DTA exotherm (D) is attributable to the formation of

cordierite (2) .

On the DTA trace of the glass which has not received any

preheat-treatment prior to crystallisation, the cordierite peak (D) appears at a lower temperature (Fig 4).

A possible explanation of this and other differ-

ences between the two samples as seen on the DTA traces has been given previously

(2).

Examination of the microstructure of a large number of glass-ceramics of the type described here indicates that the c~icium-rich matrix of the highcontrast zones has a higher expansion coefficient than the rest of the body and it cracks (Fig 3) due to tensile stresses developed during cooling from the process temperature or under additional stress during testing.

In cases where

it forms a connective network, cracks may propagate through the material easily, giving a bending strength as low as 40 MN/m 2 •

Internal cracking also

produces additional void space in the material which, in turn, reduces the density and the elastic modulus of the product.

There is also evidence to

suggest that interconnective calcium-rich zones are associated with high

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Vol. 8, No. 9

dlelec~rio losses probably ~ec~use %hey provide a conductive oon%inuum in %hess ms~erials.

Acknowledgment The work described has been carrLed out as part of the Research Programme of the Building Research Establishment of the Department of the Environment and thLs paper Ls here publtshed by permLssLon of the Director. References 1.

P . W . McMillan, Glass Ceramics.

Academic P r e s s , London (1964).

2.

R. C. de Vekey and A. J. Majumdar, MLn. Mag. 37, 291, 772 (1970).

3.

J. C. Russ, Energy DispersLon X - r a y Analysis of the ScannLng Electron Microscope Ln Energy Dispersion X-ray Analysis (J. C. Russ CoordLnator), ASTM Special TechnLcal PublLcatLon, 485, Philadelphia (1970).