REACTION SINTERED ZIRCONIA - MULLITE COMPOSITES WITH CaO

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REACTION SINTERED ZIRCONIA - MULLITE COMPOSITES WITH CaO

J. Rincon, G. Thomas, P. Pena, S. de Aza, J. Moya

To cite this version:

J. Rincon, G. Thomas, P. Pena, S. de Aza, J. Moya. REACTION SINTERED ZIRCONIA - MUL- LITE COMPOSITES WITH CaO. Journal de Physique Colloques, 1986, 47 (C1), pp.C1-423-C1-427.

�10.1051/jphyscol:1986163�. �jpa-00225594�

JOURNAL DE PHYSIQUE

Colloque C1, suppl6ment au n02, Tome 47, fbvrier 1986 page

REACTION SINTERED ZIRCONIA - MULLITE COMPOSITES WITH CaO

J. RINCON'~), G. THOMAS, P. PENA*, S. DE

and J.S. MOYA'

Dept. Material Science and Min. Eng. and National Center for Electron Microscopy, University of California, Berkeley, CA 94720, U.S.A.

* m s t i t u t o de ~ e r h i c a y Vidrio, C.S.I.C., Arganda del Reg, Madrid, Spain

Re'swne' -

a e'tudie'par diffraction de rayons-X, microscopic e'lectronique et microanalyse, le de'veloppement de la microstructure et des phases qui condui- sent, par frittage re'actif avec

Za chaw,

un compose' final dense de zip- cone - ^mullite.

Abstract - The development of microstructure and phases leading to a final tough composite of zirconia-mullite using reaction sintering with CaO has been studied by X-ray diffraction, electron microscopy and microanalysis.

I - INTRODUCTION

The reaction sintering of zircon based powders has proved to be a simple and unexpen- sive route to produce mullite-ZrO, tough composites (1-2).

A previous work

has shown that additions of CaO increase the reaction sintering process and decrease the final sintering temperature, by the appearance of transito- ry Ziquid phase.

The purpose of the present investigation is to amZYze the microstructura2 evolution as well as the appearance and microchemistry of the different phases during the reac- tion sintering process by transmission electron microscopy (TEM) and analytical elec- tron microscopy IAEM) in

selected CaO-containing composite.

II - MATERIALS AND METHODS

Taking into account the compatibility rela8ionships in the quaternary system Zr0,- AZ,O,-Si0,-CaO a composition with 1.0 mo2 %CaO has been formulated according to the fo Zlowing equation:

4ZrSi0, + 4AZ,O, + CaO

4Zr0,

3~l,Si,O,,

c ~ A ~ , s ~ , O ,

zircon [1 I

mullite anorthite

The formulated composition is located in the compatibility plane Zr0,-mullite-anor- thite being the invariant poip&at 1440°C.

(1)

On leave from the Institute de Cerhica y Vidrio, CSIC, Arganda del Rey, Madrid, Espafia.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986163

C 1 - 4 2 4 JOURNAL

DE

The mullite-Zr0,-anorthite conposite used in the present work was obtained by the fo- llowing method: a very fine (L1.3 pm average grain size) zircon powder, alumina (Flu- ka low-NaO-content) and CaCO, (Merck product) were homogenxzed by attrition milling in isopropyl aleoh02 medium for 1 h. Subsequently, the powders were isostaticaZZy pressed at 200 MPa and fired for 6, 17 and 65 h. at 1425, and 2 hours at 2450°C.

The advance of reaction was followed by XRD quantitative ana2ysis.

Observation by transmission electron microscopy has been performed in a TEM/STEM equipped with an energy dispersive X-rag (EDXI detector. Th'hin foils were prepared by

wafering, grinding and ion bombardment. Spectra at 100 kv were acquired in TEMproce- dure (40

probe diameter) and quantitative analysis

performed according to the ClifS-Lorimer method (31. Standards specimens to obtain calibration factors were used viz: pure muZlite (BaikaZoxl, Zircon (0pazir) and a synthetic gehlenite, being these factors: KAISi= 1.04f0.05; KZrSi= 2.30k7.10 and IfCaSi= 1.00W. 10

III - RESULTS AND DISCUSSION

The evolution of the reaction

11 has been folZowed by quantitative XRD analysis of the different phases, in terns of the relationships:

whrec and czs are the concentrdtions of zirconia and zircon and cM and cA

the concedrations of alumina and mullite respectively.

Figure

shows the variation versus time of

and

parameters at 142.S°C and 1450°C.

Fig.1.-

and Bparameters vs. time at 2425'6' and 1450°C.

Pig.2.- TEM micrographs corresponding to specimen heat treated at 1425OC, 6h.

Figs. 2 a ) and blr show t h e general microstructure corresponding t o

-mullite-anor- t h i t e composite heat t r e a t e d a t 142S°Cfor 6 hours. Alumina and zirconia grains i n a large proportion are observed between amorphous areasand a n o r t h i t e c r y s t a l s . Mullite c r y s t a l s were not detected by TEM i n good agreement w i t h t h e

a n a l y s i s (Fig.1).

Some zircon grains appear as larger and square shape c r y s t a l s (Fig.2bl. Two kinds o f glassy or amorphous phaseswhere detected v i z : a ) glassy phase i n t h e grain bounda- r i e s o r a s pockets i n t r i p Z e points w i t h t h e following composition: ( w t % ) 3.2 CaO-

72.3 Al,O,-21.2 Si0,-3.3

which i s c l o s e t o a n o r t h i t e areas with a composition (wt%l 11.5 Ca0-43.1 Al,O,-41.8 Si0,-3.6

Both glassy phases show frequently so- me radiation damagesproduced by e l e c t r o n beam [Figs. 2al and b ) ] .

Figs. 3a) and b l correspond t o bright and dark f i e l d transmission e l e c t r o n microsco- py images r e s p e c t i v e l y i n t h e specimen sintered a t 1425OCfor 17 hours. Mullite and alumina grains of approx. 0,5

are present.

Fig.3.- Brigh ( a ) and dark (bl f i e l d TEM micrographs of specimen t r e a t e d a t 142A°C, 17 h. showing amorphous phase a t t r i p l e points.

At t r i p l e points pockets o f amorphous phase are observed. Anorthite c r y s t a l s [Fig.4 ( a ) ] appear frequently showing i n a l l cases twins a s has been observed i n feldspars c r y s t a l s ( 4 ) . Different m i e n t a t i o n s with r e p e e 2 t o electron beam are observed main-

l y , 1 0 1 ,

1 0 1 ,

0 11 , i 3 1 11 , { I 1 1 ) , { 3 1 3 ) .

Fig. 4. -

micrographs showing twinned anorthite c r y s t a l s ( a ) and glassy phase ( b l , Amorphous or glassy phase closer t o t h e s e c r y s t a l s [Fig. 4 ( b l ] has a composition of

( w t % ) 14.8 CaO; 33.4 Al,O,; 48.4 SiO,; 3.4

Anorthite c r y s t a l s w i t h a composi- t i o n similar t o t h e s t o i c h ~ k m e t r i c one (Ca A l , , S i ,

are a l s o detected.

Figure 5 shows t h e general microstructure of t h i s chnpobite, a f t e r 65 hours o f treat-

C1-426 JOURNAL DE PHYSIQUE

ment, with well formed mullite crystals, intragranular ZrO, and small A2 ,03 crystals otc luded, and larger and twinned crysta 2s of intergranu lar ZrO, . ^{Para 2 Ze 2 white and}

dark diffuse bands in the upper corner corresponding to anorthite crystals, are also observed. No glassy phase was detected.

Both Zr02 (inter and intragranular) show solid-solutions of AZ,O, and SiO,

wt%

Al,03 and

wt% SiO,). Some rounded intragranular grains bigger than the critical size

0.5 pm) appear twinned as is shown in Fig. 6. Twins in this case are perpendi- cular to the equator of the grains. It seems like the transformation starts inside this kind of intragranular zirconia.

Fig. 5. -

micrographs corresponding to Fig. 6. - TEM micrograph showing an intra- sample treated at 1450°C, 65

granuh-Zr02grain with perpen- M-mllite; A=AZ,O,; 2 =intergra- dicular twins to equator.

nular-Zr02; Z -intrag$anular-~r02;

An=anorthite. b-

Fig.

shows a general microstructure of the sample sintered at 2450°C for 2 hours.

Acoording to the corresponding equilibrium diagram a very small amount of glassy pha- se was detected at triple points with an average composition of (wt%) 15.5Cao; 36.6 AZ,O,; 43.0 SiO, and 6.0 ZrO, which is close to the corresponding peritectic composi- tion (5) at 1490°C. Dislocations of low angle grain-boundaries in the mullite crys- tals were frequently detected.

No significant solid solution of CaO in ZrO, and mullite was detected, which is in agreement with the previms. results of Pena et al.

Fig.7.-

micrograph corresponding to sample treated at 1450°C,

h. showing the different phases and a low angle grain boundary disZocations(arrows).

?"he

results obtained showed that the reaction sintering starts with the formation of

a n o r t h i t e (Fig.21 and decomposition o f zircon. The ?,ce2~2>1 coexistence o f a n o r t h i t e , zircon and z i r c o n i a produces a Liquid phase (Figs. 2-41 corresponding t o t h e lowest invariant point of t h e Zr0,-Al,O,-Si0,-CaO system 151, t h i s z i r c o n phase

which d i f f u s e s by c a p i l l a r i t y between t h e d i f f e r e n t grains enhances t h e mass-tram- port and consequently increases t h e s i n t e r i n g and r e a c t i o n r a t e s up t o i t s f i n a l

s

The m u l l i t e grains grow trapping Z r O , p a r t i c l e s and some aLwnina grains (Fig. 5-71.

The intergranular and twined Z r O , grains have bigger average s i z e due t o t h e fact t h a t t h e mass-transport t a k e s place through a l i q u i d phase, a s observed i n [Figs. 4

( b 1 , 51.

As a summary i n Fig. 8

a

Fig. 8.- Scheme of t h e Reaction S i n t e r i n g process. Z = Z r O , , C = CaO, A

=

,

,

=

REFERENCES

/ 1 / ANSEAU, M.R., LEBLUD, C. and CAMBIER, F . , J . Muter. S c i . L e t t .

2

/ 2 / PENA, P., MIRANZO, P., MOYA, J.S. and de AZA, S.,J.MaDer.Sci.z (1985)2011.

/ 3 / CLIFF, G. and LORIMER, G. W., J . of Microscopy 103 f1975), 2 , 203.

/ 4 / WENK,

X.

,

/ 5 / PENA, P. and de AZA, S . , J . Amer. Ceram. Soc.

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