EFFECT ALUMINA AND

OF

PRELIMINARY

ON T H E

STRENGTH R.S.

HEAT

TREATMENT

THERMAL-SHOCK

OF

Churakova

CORUNDUM and

E.P.

OF

RESISTANCE

CERAMICS Fedorova

UDC 666.764.32

One of the disadvantages of s i n t e r e d alumina p r o d u c t s is t h e i r low t h e r m a l - s h o c k r e s i s t a n c e . In a n u m b e r of investigations [1-4] an i n c r e a s e in the t h e r m a l - s h o c k r e s i s t a n c e and a drop in the sintering t e m p e r a t u r e w e r e achieved by adding c e r t a i n m e t a l oxides to the body compositions: ZrO2, SiO2, TiO2, and SnO2, etc. However, even s m a l l quantities of additives m a r k e d l y reduce the c h e m i c a l r e s i s t a n c e and the r e f r a c t o r i n e s s under load of such m a t e r i a l s . The p r e s e n t authors developed a technology for obtaining dense, highly t h e r m a l - s h o c k r e s i s t a n t p r o d ucts f r o m alumina without additives. The s t a r t i n g m a t e r i a l s consisted of GA-85 alumina (GOST 6912-64), and e l e c t r o f u s e d corundum ~B (standard ON-8-59). The alumina was p r e c a l c i n e d at 1400, 1600, and 1750~ The calcined alumina and the e l e c t r o f u s e d corundum w e r e f i r m l y ground in a v i b r o m i l l by the dry method to obtain an a v e r a g e g r a i n size of about 2 #. After grinding all the powders w e r e t r e a t e d with a hot solution of h y d r o c h l o r i c acid to r e m o v e the iron picked up during grinding. The p r e p a r e d m a t e r i a l s w e r e then sufficiently p u r e (Table 1). To study the effect of the p r e l i m i n a r y firing t e m p e r a t u r e of the original aiumina on the p r o p e r t i e s of the corundum c e r a m i c s made f r o m the p o w d e r s , bodies w e r e p r e p a r e d f r o m 100% e l e c t r o c o r u n d u m (body f~I0; 100% alumina calcined at different t e m p e r a t u r e s (bodies G-1750, G-1600, G-1400); and 100% unfired alumina (body G-00). In addition m i x t u r e s of e l e c t r o c o r u n d u m with calcined alumina w e r e studied (bodies f~K-G-1750, ]~K-G-1600, ]~K-G-1400, and ~K-00), and m i x t u r e s of aluminas calcined at different t e m p e r a t u r e s (bodies G-1750-1600, G--1750-00, and G-1600-00). The bodies w e r e made into tiles m e a s u r i n g 80 • 80 • 10 m m by c a s t t n g i n p l a s t e r m o l d s , a n d t h e n d r i e d a n d f i r e d in a p e r i o d i c kiln at 1750~ Specimens of the r e q u i r e d dimensions w e r e cut f r o m the f i r e d tiles with a diamond saw. The bending strength was d e t e r m i n e d on s p e c i m e n s m e a s u r i n g 70 • 10 • 10 m m , and the t h e r m a l shock r e s i s t a n c e (number of heat cycles f r o m 1300~ to air) on s p e c i m e n s m e a s u r i n g 70 • 30 • 10 m m . The open p o r o s i t y and the apparent density w e r e found by the well-known method, the s p e c i m e n s being weighed on an analytical balance. Some p r o p e r t i e s of the s p e c i m e n s a r e given in Table 2. The firing t e m p e r a t u r e of the s t a r t i n g alumina s e r i o u s l y affects the p r o p e r t i e s of the m a t e r i a l . Specimens made f r o m alumina p r e c a l c i n e d at different t e m p e r a t u r e s have an a p p a r e n t density r e a c h i n g 3.89 g / c m 3 and p r a c t i c a l l y z e r o open p o r o s i t y , while the s p e c i m e n s made f r o m e l e c t r o f u s e d corundum a r e l e s s dense. The b e n d i n g s t r e n g t h s of the s p e c i m e n s made f r o m aluminas having different p r e l i m i n a r y firing t e m p e r a t u r e s range f r o m 1210 to 2470 k g / c m 2. When alumina is added to the e l e c t r o c o r u n d u m bodies, r e g a r d l e s s of its firing t e m p e r a t u r e , t h e r e is an i n c r e a s e in the a p p a r e n t density of the m a t e r i a l (to 3.85 g /cm3), and in the bending strength (to 2150 kg/cm2). TABLE 1. Chemical Composition of Original P o w d e r s , % i

Material

slo~

AI=O=

Fe20= [ CaO MgO[ Na=O i

Alumina . . . . . . . . . . . . . . . . Electroeorundum . . . . . . . . . . All-Union Institute of R e f r a c t o r i e s .

t 0,12 99,73--99,79 [ 0,02--0,09 O, 13 99,40 O, 15 0~0

0?o2[o,o6_

T r a n s l a t e d f r o m Ogneupory, No. 2, pp. 44-49, F e b r u a r y ,

1971. 9 1971 Consultants Bureau, a division of Plenum Publishing Corporation, 227 West 17th Street, New York, N. Y. 10011. All rights reserved. This article cannot be reproduced for any purpose whatsoever without permission of the publisher. A copy of this article is available from the publisher for $15.00.

116

T A B L E 2. P r o p e r t i e s of E x p e r i m e n t a l S p e c i m e n s

Bodies

EK

Thermal-shock resistance Bending strength, kg/cm z Apparent (13oo~ air), heat cycles at temperature, ~ density, g before the ap- before /ClTl 3 pear ance of failure 20 900 1 cracks l 3.390

3.885 G-1600 3.888 G-1400 [ 8.870 8.890 G-00 8.440 iK-C-175o EK-G-1600 3.870 EK-6-1400 3. 750 ~:K-G-00 3.800 G-1750-1600" 3~ G-1750-00 3.850 i 3.670 G-1600-00 G-1750

4 2 24 14

8 10 25 44

1700 1570 1520 2470

1270 1600 1600 2485

1 3 1 2 33 1 13

17 25 6 3 33 3 25

2000 2000 2040 2153 1210 1415

1450

1500

1200

1400 1270

Numbers of heat cycles 1600~ - air equals 33.

F i g . 1. M i c r o s t r u c t u r e 2) p o r e s .

of s p e c i m e n s on b o d i e s EK.

•

F i g . 2. M i c r o s t r u c t u r e

of a f r a c t u r e s u r f a c e of b o d y f~K. •

1) c o r u n d u m ;

T h e t h e r m a l - s h o c k r e s i s t a n c e of b o d i e s l~K w a s c o m p a r a t i v e l y low (4 h e a t c y c l e s ) and d i d not r i s e w h e n a l u m i n a w a s a d d e d to t h e m . The s t r e n g t h of s u c h s p e c i m e n s is r a t h e r high. A m o n g the a l u m i n a s p e c i m e n s a high t h e r m a l - s h o c k r e s i s t a n c e (25-33 h e a t c y c l e s ) w a s p o s s e s s e d b y s p e c i m e n s whose" c o m p o s i t i o n c o n t a i n e d a l u m i n a c a l c i n e d at 1600~ The b e n d i n g s t r e n g t h of t h e s e m a t e r i a l s w a s r e l a t i v e l y low (see T a b l e 2). T h e m i c r o s t r u c t u r e of the s p e c i m e n s w a s s t u d i e d w i t h a p o l a r i z e d m i c r o s c o p e M I M - 8 on p o l i s h e d s l i d e s in r e f l e c t e d l i g h t . M o s t a t t e n t i o n w a s p a i d to d e t e r m i n i n g the d i m e n s i o n s , s h a p e , and a r r a n g e m e n t of the c r y s t a l s , and a l s o the n a t u r e of the p o r e d i s t r i b u t i o n in the s p e c i m e n s . The m i c r o s t r u c t u r e w a s a l s o s t u d i e d b y the r e p l i c a m e t h o d w i t h f r a g m e n t s of s p e c i m e n s on an e l e c t r o n m i c r o s c o p e of the transmitting type (MZ-3). F i g u r e 1 s h o w s the m i c r o s t r u c t u r e of s p e c i m e n s of b o d y I~K. It h a s a f i n e - g r a i n s t r u c t u r e , and is c o m p o s e d m a i n l y of i s o m e t r i c g r a i n s of c o r u n d u m , the a v e r a g e s i z e of w h i c h d o e s n o t e x c e e d 0.01 x 0.025 r a m . The m i c r o s t r u c t u r e of o t h e r m a t e r i a l s c o n t a i n i n g e l e c t r o f u s e d c o r u n d u m ( b o d i e s ~ K - 1 7 5 0 , ~ K - G - 0 0 , f~K-G-1600, and f~K-G-1400) a r e g e n e r a l l y s i m i l a r to the m i e r o s t r u c t u r e of b o d i e s ~ K . The f i n e - g r a i n s t r u c t u r e of s u c h m a t e r i a l s i s due to the i n e r t i a of the g r a i n s of e l e c t r o f u s e d c o r u n d u m . C o m p l e t e d e n s i f i c a t i o n did n o t o c c u r , and the o p e n p o r o s i t y of the s p e c i m e n s w a s r a t h e r high. A c c o r d i n g to the d a t a of the

117

Fig. 3. M i c r o s t r u c t u r e of s p e c i m e n s of bodies G-1750 (a), G-1600 (b), G-1400 (c), and G-00 (d), • 1) corundum, 2) p o r e s .

Fig. 4. M i c r o s t r u c t u r e of the surface fraction f r o m bodies G-1750 (a), G-1700 (b); G-1400 (c); and G-00 (d). • e l e c t r o n m i c r o s c o p i c study, the s t r u c t u r e of these s p e c i m e n s consists mainly of individual g r a i n s of c o r u n dum, which during firing undergo slight changes (Fig. 2). Despite their i n c r e a s e d p o r o s i t y , the s p e c i m e n s have a high strength (bending strength 1700-2150 kg/cm2); their t h e r m a l - s h o c k r e s i s t a n c e is low, and does not exceed 1-4 heat cycles. During the sintering of bodies made f r o m different alumina p o w d e r s , m a r k e d r e c r y s t a l l i z a t i o n of the g r a i n s of corundum o c c u r r e d . Thus, body G-1750 has a c o a r s e - g r a i n e d m i c r o s t r u c t u r e (Fig. 3a), the dim e n s i o n s of the s e p a r a t e c r y s t a l s r e a c h i n g 0.05 • 0.2 ram. The m i c r o s t r u c t u r e of the s p e c i m e n made f r o m

118

Fig. 5. M i c r o s t r u c t u r e of s p e c i m e n s of bodies G-1750-1600.

x l l 0 : 1) corundum; 2) p o r e s .

Fig. 6. M i c r o s t r u c t u r e of the s u r f a c e of a f r a g m e n t of s p e c i m e n of body G-1750-1600. tablets; b) p e r p e n d i c u l a r to them.

Xl0,000: a) p a r a l l e l

body G-1600 (Fig. 3b) c o n s i s t s of w e l l - f o r m e d e l o n g a t e d - t a b u l a r c r y s t a l s of corudum. r e a c h 0.06 x 0.16 m m .

The g r a i n sizes

The m i c r o s t r u c t u r e of the s p e c i m e n s made f r o m low f i r e d body G-1400 is f i n e - g r a i n e d (Fig. 3c). The main p a r t of the s p e c i m e n c o n s i s t s of w e l l - f o r m e d elongated tabular c r y s t a l s of corundum m e a s u r i n g 0.02-0.08 m m , and containing l a r g e r c r y s t a l s , r e a c h i n g 0.05 x 0.35 m m in size. The m i c r o s t r u c t u r e of the s p e c i m e n s of G-00 (Fig. 3d) w e r e f i n e - g r a i n e d and h e t e r o g e n e o u s . The m a i n m a s s consists of i s o m e t r i c g r a i n s of corundum m e a s u r i n g 0.01-0.05 m m . Thus, the m i c r o i n v e s t i g a t i o n o f the bodies shows that the lower the firing t e m p e r a t u r e of the s t a r t i n g alumina then the s m a l l e r the size of the newly f o r m e d corundum c r y s t a l s . Thus, the g r a i n size of the corundum in the s p e c i m e n G1750 is five t i m e s g r e a t e r than in the s p e c i m e n of G-00. E l e c t r o n m i c r o s c o p i c study of the f r a c t u r e d alumina bodies showed that in all m a t e r i a l s there is a t e r r a c e - l i k e s t r u c t u r e in the s u r f a c e of the f r a g m e n t , with a c l e a r l y e x p r e s s e d step f o r m a t i o n in the growth. H o w e v e r , the a p p e a r a n c e s of the s u r f a c e s for different alumina m a t e r i a l s a r e different. In the m a t e r i a l s G-1750 (Fig. 4a) and G-1600 (Fig. 4b), the s u r f a c e c o n s i s t s of a c l e a r a s s e m b l y of plates with c l e a r l y e x p r e s s e d growth steps. The s u r f a c e of the f r a g m e n t s of m a t e r i a l G-1400 (Fig. 4c) contains t r a c e s of s e p a r a t e , r a n d o m l y a r r a n g e d c o a r s e p r i s m a t i c g r a i n s . The surface of the f r a g m e n t of m a t e r i a l G-00 (Fig. 4d) consists of s e p a r a t e growths of plates a r r a n g e d in the f o r m of a t e r r a c e . Examining the m i c r o s t r u c t u r e of alumina bodies and their p r o p e r t i e s , it should be noted that the fineg r a i n c r y s t a l l i z a t i o n of the m a t e r i a l (body G-1400) p r o d u c e s s p e c i m e n s of high strength (bending strength 2470 kg/cm2). The c o a r s e l y c r y s t a l l i n e s t r u c t u r e of bodies G-1750, and G-1600 tend to r e d u c e the strength to 1520 k g / c m 2. The t h e r m a l - s h o c k r e s i s t a n c e of the bodies, a p p a r e n t l y , is d e t e r m i n e d not only by the s i z e s and o r i entation of the c r y s t a l s , but also by their s t r u c t u r e , which was confirmed on the bodies G-1750 and G-1600, having a p p r o x i m a t e l y the s a m e c r y s t a l s i z e s but differing t h e r m a l - s h o c k r e s i s t a n c e s (2 and 24 heat cycles). In connection with this g r e a t i n t e r e s t is attached to the m i c r o s t r u c t u r e of body G-1750-1600, composed of a m i x t u r e of a l u m i n a s f i r e d at different t e m p e r a t u r e s . The m i c r o s t r u c t u r e of such s p e c i m e n s is c o a r s e l y c r y s t a l l i n e (Fig. 5); the c r y s t a l s i z e s r e a c h 0.1 • 1.0 m m . It c o n s i s t s of r e g u l a r , e l o n g a t e d - t a b u l a r c r y s t a l s of corundum, d i s o r i e n t a t e d and in close contact. F r o m t h e e l e c t r o n m i c r o s c o p i e study of the m a t e r i a l s G-1750-1600 we note t r a c e s of smooth densely contacting plates on the s u r f a c e of the f r a g m e n t s (Fig. 6a). In s o m e p l a c e s the plates a r e a l t e r n a t e d by c r y s t a l s of corundum (Fig. 6b). Body G-1750-1600 has a h i g h - t h e r m a l - s h o c k r e s i s t a n c e . This a r r a n g e m e n t of the t a b l e t s in the c r y s t a l s , and their shape, tend to reduce the s t r e s s e s in t h e r m a l testing. This m a y explain the high t h e r m a l - s h o c k r e s i s t a n c e of the m a t e r i a l G-1750-1600. The c o a r s e c r y s t a l l i n e s t r u c t u r e also explains the r e d u c e d strength of the m a t e r i a l G-1750-1600 (strength 1200 kg/cm2).

119

The microscopic study shows that pure alumina materials undergo noticeable reerystallization during firing: in this case, the specimens have an unusual structure; the crystals of cortmdum in them are separated by numerous growth steps. This structure indicates the high mobility of alumina during sintering when each "free surface" grows in the form of a terrace [5, 6]. CONCLUSIONS The firing temperature of the starting alumina is one of the important factors influencing the formation of the microstructure of corundum materials and determines the properties of the ceramics, At low alumina firing temperatures (1400~ the structure of the specimens is finely crystalline, An increase in the firing temperature (to 1600-1750~ causes an increase in the crystal sizes. The finely crystalline structure ensures ahigh strength in corundum materials, while the coarsely crystalline structure reduces it. The high thermal-shock resistance of specimens is determined by the formation of regular, random orientation, and close contacting crystals of corundum. An important role in this case, apparently, is played by the structure of the crystals. The thermal-shock resistance of the materials is favored by a combination of starting materials having different firing temperatures, namely, aluminas fired at 1750 and 1600~ LITERATURE 1.

2. 3. 4. 5. 6.

7.

120

CITED

D.N. Poluboyarinov and N.P. Silina, Trudy MKhTI, No. 24, 155 (1957). N.V. SemLna et al., Trudy VostIO, Metallurgiya, No. 5, 49 (1964). E.S. Lukin and D.N. Poluboyarinov, Ogneupory, No. 7, 318 (1963). D.N. Poluboyarinov and E.S. Lukin, Ogneupory, No. 5, 230 (1962), S.G. Tresvyatskii and A.M. Cherepanov, Highly Refractory Materials and Products from Oxides [in Russian], Metallurgiya (1964). Catler, in: Kinetics of High-Temperature Processes [Russian translation], W.D. Kingery (editor), Metallurgizdat (1965), p. 173. W. Schatt and D. Schneze, Ber. Dtsch. Keram. Ges., 36, No. 11, 364 (1959).