Ceramics hlternationa112 (1986) 19-24

Sintering of Compositions in the System AI203-Cr=03-SiO =

Akira Yamaguchi

Department of Materials Science, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466, Japan

Abstract: The sintering behaviour of refractory compositions in the system A120342r203-SiO z was studied. The conditions for densification, the mechanism of densification and the reactions during sintering were investigated. Powders of AI2Oj, Cr203, mullite and SiO 2 were used as a starting material and were sintered in compacted form under different conditions at 1300 to 1550 °C. A dense body was obtained by heating in a carbon powder bed at temperatures around 1500°C. The microstructure of the body was skeleton-like and contained rectangular crystals of (Cr, Al)203 s.s., which appeared to be desirable for an improvement of the thermal shock resistance.

1. INTRODUCTION

In general the resistance against slag attack of AIzO3-SiO 2 refractories is improved by addition of Cr203 .1 Therefore, it can be expected that com- positions in the AI203-Cr203-SiO 2 system can be used as refractories in the steel industry. 2 However, the behaviour of such materials has been un- satisfactory, since it is difficult to sinter them to a high density.

The author has investigated the densification behaviour of powder compacts and obtained a dense body by heating in a strong reducing atmosphere, i.e in a carbon powder bed at temperatures around 1500°C. In this paper, the conditions for densi- fication of the A1203-Cr203-SiO 2 system by sintering are outlined, and the sintering mechanism is discussed.

2. EXPERIMENTAL PROCEDURE

Cr203, AI20 3 and SiO2 were reagent grade powders. A sintered mullite powder with a particle size below 150 llm was obtained from a commercial source; its typical analysis was 7 1 . 0 w t ~ AI20 3, 28.0 ~o SiO2, 0-2 ~ Fe203, 0.2 ~ N a 2 0 , 0-1 ~ TiO 2 and 0 . 1 ~ K 20. The powder mixtures of

Cr20 3 --]- mullite, C r20 3 --I-- AI20 3 -Jr- mullite and AI20 3 q- Cr20 3 -{- SiO 2 were pressed uniaxially with 78.5 MPa to compacts of 20 x 20 x (10-15)mm. The compacts were placed in alumina crucibles. Some of the crucibles were filled up with carbon powder, and the others were without carbon powder. Each crucible was closed with an alumina lid, and heated in an electric furnace to various temperatures for 2 h. A reaction layer, composed of Cr2(C,N ) and Cr3C 2, with a thickness below 0-5 mm formed on the surfaces of the bodies heated in carbon powder above a sintering temperature of 1400°C and was removed before testing.

3. RESULTS

3. 1. Sintering of Cr20 a +mull i te

19

Figure 1 shows the relative density of the CrzO 3 + mullite compacts after heating with and without a carbon powder bed at 1500°C for 2h. Without carbon no shrinkage occurred, and Cr20 3 did not react with mullite. On the other hand, by heating in a carbon powder bed, the relative density of the sintered bodies increased with an increasing amount o f C r 2 0 3 and attained 90 ~o in the bodies containing more than 4 0 w t ~ Cr203. Cr203 reacted with

Ceramics International 0272-8842/86/$03.50 © Elsevier Applied Science Publishers Ltd, England, 1986. Printed in Great Britain

20 Akira Yanzaguchi

lOO

9O

8O

7l] ..~

Mullite 20 40 60 80 CraOa (wt %]

Fig. 1. Relative density of the bodies sintered by heating in a carbon powder bed (curve A) and without a carbon powder bed (curve B) at ! 500 °C for 2 h versus ratio of mullite and Cr 203 in

starting materials.

mullite to form a Cr203-AI20 a solid solution (s.s.) ((Cr,AI)203 s.s.) and eristobalite; mullite was not detected in the bodies containing more than 20 wt Cr20 3.

Figure 2 shows the relative density and the compounds detected in the bodies which were prepared from a 50wtTo Cr203 + 50wt~/o mullite mixture after heating in a carbon powder bed at

100

a~ 90

~ a e IIU

~ 70

n ¢

6O

E aJ

t-I

x 0

i

i I t I

/ 80 (Cr, AI)203/

/ //

60 ...~r=O= " ~ . / I

40 Mullite ~ . Cristobalit~ 2~-

1300 1400 1500 Temperature (°C)

Fig. 2. Relative density and compounds detected by X-ray analysis in a sintered sample made from 50wt Cr203+50wt ~ mullite compact after heating in a carbon

powder bed at various temperatures for 2 h.

various temperatures for 2h. Cr203 reacted with mullite above about 1350°C to (Cr,Al)203 s.s. and cristobalite, and then densification of the compact occurred. Cr20 3 and mullite disappeared above 1450 °C; the relative density of the bodies was higher than 9670 after sintering at 1550°C. Figure 3 shows SEM photographs of the fracture surfaces of the bodies after sintering in a carbon powder bed at various temperatures. The microstructures of the samples sintered below 1300°C were not different from the original compact except for the crystal growth of Cr203 as shown in Fig. 3(A). Drastic changes occurred in the body which had been sintered at 1500°C. Many rectangular crystals of (Cr,Al)203 s.s., 10 to 501tm in length, grew in various directions, and cristobalite and a silicate glass filled up the gaps between these crystals (Figs 3(C) and 3(D)). Figure 3(D) shows the microstruc- ture of the body after removal of the cristobalite and the silicate glass by a treatment with hydrofluoric acid. The (Cr,A1)203 s.s. crystals had a skeleton-like structure.

3.2. Effect of silica on the densification in the AI2Oa-Cr20=-SiO = system

Cr203 powder compacts sintered to above 96 % of theoretical density in a carbon powder bed at 1500°C after 2 h (Fig. 4). Under the same condi- tions, however, the densification of Cr20 a + A1203 powder compacts became difficult with increasing amounts of A1203, and stopped almost completely in compacts containing more than 1 0 w t ~ A1203. When quartz or a compound containing SiO 2, such as mullite, was added to the CrzO3+AI203 mixture, the densification was improved with an increasing amount of SiO2 (Fig. 4(B)). A Cr203 -I- A1203 compact with 10wt ~ SiO2 sintered to about 98 ~o of theoretical density at 1550°C after 2h.

The sintered compacts were transformed into a powder, and the weight change of the powder was measured during heat-treatment in air (Fig. 5). Almost no weight change occurred in the SiO2-free sample whereas an increase in weight was observed in the other samples with increasing silica content.

4. D I S C U S S I O N

4.1. Conditions for densification in the AI=Oa-Cr2Oa-Si02 system

The densification of the compacts was influenced by the sintering atmosphere and by the amount of the

Shttering of compositions hz the system AI203-Cr2Oa-SiO 2 21

Fig. 3. SEM photographs of a sample made from 50wt% Cr20 z + 50wt % mullite by heating in a carbon powder bed for

2h at 1300°C (A), 1400°C (B) and 1500°C (C, D).

Fig. 3 . - - contd.

SiO2 componen, t in the compacts as shown in Figs 1 and 4. When the" compacts contained more than 10wt% SiO 2 and were heated in a carbon powder bed above 1450°C, densification was enhanced as shown in Figs 2 and 4. On the other hand, in the case of heating without carbon, densification could not be achieved.

The influence of the atmosphere on densification is considered to be as follows. Carbon reacts with oxygen to form CO and/or CO2 gas at temperatures above 600°C. When Pco is I atm, the equilibrium partial pressures of 0 2 and CO2 gas in equilibrium with carbon are very small as shown in Table 1. Since Pco2 and Po2 are almost negligible as compared with Pco, itCan be assumed that the atmosphere in the

Table 1. Equi l ibr ium partial pressures of CO 2 and O= gas over C(s) at P c o = l a t m

Temperature Pco (atm) Pco= (atm) Po, (atm)

1500K(1227°C) 1 6 -78x10 -4 1 . 0 7 x 1 0 -1~ 1600K(1327°C) 1 2 . 9 6 x 1 0 -4 3 . 4 0 x 1 0 - i v 1700K(1427"C) 1 1 . 4 4 x 1 0 -4 9 . 5 1 x 1 0 -1~' 1800K(1527"C) 1 7 -62x10 -5 2 . 3 9 x 1 0 -16 1900K(1627"C) 1 4 - 3 3 x 1 0 -s 5 . 4 7 x 1 0 -16

22 Akira Yamaguchi

- J o o

8C r -

"10

60

~: 4o

(m) . ~

1500 *C

Cr203 20 40 0 2 4 6 8 10 12 AI,o~ (wt~) sio~ (wt~)

interior a n d around the compacts in the carbon powder bed at high temperature is mainly CO.

When SiO'2, Cr203, A1203 and 3A1203 . 2SiO 2 as a condensed phase coexist with carbon, various gaseous species can be formed as shown in Table 2.

Table 2. Composi t ion for compounds and species in the Si-O-C, AI-O-C and Cr-O-C systems

Compound Gaseous species (condensed phase)

SiO=(s), SiC(s)

Al=O3(s), AI4C3(s)

Cr=Os(s), Cr3C=(s)

SiO=(g), SiO(g), Si(g), SiO=(g), Si3(g ), SiC(g), Si=C(g) AlO=(g), AlO(g), Al(g), Al=Oz(g), AI20(g ), AIC(g) CrOz(g), CrOz(g), CrO(g), Cr(g)

As Po, decreases, however, the condensed phases are SiC(s), AI4C3(s ) and Cr3C2(s ) in the Si-O-C, AI-O-C, and Cr-O-C systems, respectively. Equilibrium constants between compounds, ele- ments and gaseous species are obtained for the reactions listed in Table 3 on the basis of JANAF- thermodynamic data. 3

Starting materials

~,,~ Crl+O, 1

, , : o , J Mullite/

\sio2 ) Fig. 4. Relative density of the sintered samples made from Cr203-AI203 compacts (A) and Cr203 4 AI203 + SiO2 compacts (B) by heating in a carbon powder bed at 1500 °C for 2 h. (In the case of(B), Cr203 and AI203 are equivalent in weight.)

As an example, the equilibrium partial pressure of the gaseous species is shown as a function of Po2 for the Si-O-C system in Fig. 6. At Pco = I atm, SiO has the highest pressure, and the partial pressures of other compounds are considerably lower and, in effect, negligible. In the AI-O-C and the Cr -O-C systems the partial pressures of Al(g) or Cr(g) are the highest at la t in of Pco. Figure 7 shows the equilibrium partial pressures of SiO(g), Cr(g) and Al(g) as a function of temperature at Pco = I atm. Psio has the highest partial pressure of these gaseous species. This phenomenon appears to influence the densification of the compact. The generation of SiO vapour induces the formation of a liquid such as Si-SiO2, whicll plays an important role in the densification as mentioned below. In air, the liquid phase is not formed at 1575°C as shown in the A12 O3-CrzO3-SiO2-phase diagram. 4

4.2. Sintering mechanism in the system AIzO3-Cr=03-SiO = in carbon powder

The powders, which were obtained by grinding samples after sintering in a carbon powder bed, increased in weight when heated in air as shown in

o~ 1.2

1/) 0 . 9

u e-

"- 0.6

._~ 4J 0.3

- !14wt% SiO2 - - =

- / _ I - -

- / / - ! - tg~.A_ _

- i o I i I I I I I I I I I I i I I 1 I I

3 0 0 6 0 0 9 0 0 2 0 4 0 e o

Temperature (°cl t2so T ime (min)

Fig. 5. Weight change of the powder of the bodies shown in Fig. 4(B) during heating in air. The SiO2 concentrations are the amounts of SiO 2 in the original

compact.

Shztering of compositions in the system A 1203-Cr 203-Si02 23

° I "i

I

-5

.E,~ .ic _ _ dE

_o -15

.20

Fig. 6.

S iC (s)

- 1 4 2 7 °C

-

. . . .

- si~(g} /

- ! l t v l v v t t l v v -25 -20

--!< S i 0 2 (s)

' \ \,',.\ \

3 \ "V',\ \ ,! \ \ \ \ ' \

*T I i\ I ~lXw w P'~ I I I f wl -15 -I0 -5

l o g Po2 Corm)

Vapour pressure of compounds in the Si-O-C system at 1427°C.

Fig. 5. It was considered that the weight increase was caused by oxygen which was picked up by the samples, i.e. by the oxidation of the samples, especially of SiO2_x (0 < x < 2), to SiO 2. Because the oxidat ion hardly occurred in the SiO2-free sample (Fig. 5), SiO2_xappeared to be silicon and /or

- - . 0 E 4_a cO v

-1

2 o 2

O..< - 3

o3 o

~ - 4

0

~-5 03 0

a t P J o : l a t r~

- 6

- 7

/

/

8

I I 1200

I I I I I I I I I I I I I l I

1400 I§00

Temperature CC)

t I !

a eutectic composition of Si-SiOz, which were liquid at sintering temperature. In the Si-SiO2 phase diagram, 5 the liquid's temperature is 1405 °C, and if AI203 and/or Cr203 coexist, it is assumed that this temperature is lowered due to their dissolution in the liquid phase.

Kr6nert and Buhl 6 reported that mullite was reduced by carbon monoxide at temperatures above 1000°C to form corundum and silicon monoxide according to the equation

3A1203 . 2SiOz(s ) + 2CO(g) = 3A1203(s ) + 2SiO(g) + 2COz(g)

Mullite is also inferred to be reduced by carbon according to the following equation

3A1203 . 2SiO2(s ) + 2C(s)

= 3AlzO3(s ) + 2SiO(g) + CO(g)

The AI203 reacts with Cr203 to form a solid solution. It is considered that the format ion of SiO(g) simultaneously gives rise to a liquid of Si-SiO2 which promotes a densification of the compact and growth of rectangular crystals of (Cr,Al)203 s.s. as shown in Fig. 3.

SiO2(s) is also considered to react with CO(g) or

, , l lw 1800

Fig. 7. Equilibrium partial pressures of SiO(g), Cr(g) and Al(g) in the Si-O--C, Cr-O-C and AI-O-C systems, respectively, in equilibrium with

C(s) at Pco = I atm.

24 Akira YamaguchL

Table 3. Equilibrium reaction equations in the Si-O-C, A I -O-C and Cr -O-C systems

SiO2(s ) +C(s) =SiO(g) + CO (g) SiO=(s) =SiO=(g) SiO=(s) +2C(s) =Si(g) +2CO(g) 2SiO=(s) +4C(s) =Si=(g) +4CO(g) 3SiO=(s) +6C(s) =Sia(g ) +6CO(g) SiO=(s) +3C(s) =SiC(g) +2CO(g) 2SiO=(s) +SC(s) =Si=C(g) +4CO(g) SiC(s) +2CO(g) =SiO=(s) +3C(s) SiC(s) +CO(g) =SiO(g) +2C(s) SiC(s) +2CO(g) =SiO=(g) +3C(s) SiC(s) =Si(g) + C(s) 2SiC(s) =Si=(g) +2C(s) 3SiC(s) =Sia(g ) +3C(s) SiC(s) =SiC(g) 2SiC(s) =Si=C(g) +C(s)

CrzOa(s ) +3C(s) =2Cr(g) +3CO(g) Cr=Oa(s ) +C(s) =2CrO(g) +CO(g) Cr=Oa(s) +CO(g) =CrO=(g) +C(s) Cr=Oa(s ) +3CO(g) =CrOa(g ) +3C(s) 2CraC=(s ) +gCO(g) =13Cr=O3(s)l + 13C(s) CraCz(s ) =3Cr(g) +2C(s) CraC=(s ) +3CO(g) =3CrO(g) +5C(s) CraC=(s) +6CO(g) =3CrO=(g) +7C(s) CraC=(s ) +9CO(g) =3CrO3(g ) +11C(s)

3AI=O 3 .2SiO=(s) +2C(s) =3Al=Oa(s) +2SiO(g)

Al=O3(s) Al=Oa(s) Al=O3(s) Al=Oa(s) Al=Oa(s) AI=Oa(s) AI4Ca(s) AI4Ca(s) Al=Ca(s) AI4C 3 (s) AI=C 3 (s) AI=Ca(s) AI=Ca(s) +2CO(g)

+C(s) =2AlO(g) +CO(g) +C(s) =Al=O=(g) +CO(g) +CO(g) =2AlO=(g) +C(s) +2C(s) =Al=O(g) +2CO(g) +3C(s) =Al(g) +3CO(g) +5C(s) =2AIC(s) +3CO(g) +6CO(g) =2Al=Oa(s ) +9C(s) +4CO(g) =4AlO(g) +7C(s) +4CO(g) =2Al=O3(g ) +7C(s) +8CO(g) =4AlO=(g) + 11C(s) +2CO(g) =2Al=O(g) +SC(s) =4Al(g) +3C(s) + C(s) =4AIC(g)

C(s) to form SiO(g) as indicated by the following equation

SiO2(s) + CO(g) -- SiO(g) + CO2(g)

SiO2(s) + C(s) = SiO(g) + CO(g)

The skeleton-like structure composed of the rect- angular crystals of (Cr, AI)203 s.s. appeared to be desirable to enhance the thermal shock resistance.

On the other hand, if the samples were heated without carbon the compacts barely shrunk even at 1500 °C as shown in Fig. 1, and mullite did not react with Cr203. It is assumed that sintering occurs mainly by an evaporation-condensation mechanism of C r 2 0 3 .7 That is, in the compacts the smaller Cr203 particles evaporate as CrOa(g ) and disap- pear, while the larger Cr203 particles grow by condensation of Cr203(s), as indicated by the following equation

Cr203(s ) -t- (3/2) O2(g) ~-2CrO3(g)

As a result, crystal growth of C r 2 0 3 and no shrinkage of the compacts occur.

5. C O N C L U S I O N S

The densification of compositions in the A l 2 O a - C r 2 0 3 - S i O 2 system, the mechanism and the reactions during sintering were investigated in the case of A12Oa+Cr203+mul l i t e and AI203 -t- Cr203 "1- SiO 2 as starting materials, and by heating with and without a carbon powder bed between I300 to 1550°C.

1. A dense body with more than 9570 of

theoretical density was obtained from a compact which contained more than 10wt ~ SiO 2 by heating in carbon powder at temperatures around 1500 °C.

2. Mullite or SiO 2 in the compact is reduced by carbon and then a liquid such as Si-SiO 2 is formed. The liquid promotes densification of the compact and the development of a skeleton-like structure composed of rectangular crystals of (Cr,Al)20 a s.s.

3. Mullite and SiO 2 play almost the same role for sintering in carbon powder, but the body obtained in the case of mullite is denser than that in the case of SiO 2.

4. In the absence of carbon, densification does not proceed, and mullite does not react with Cr203.

REFERENCES

I. SOMIYA, S., TANI, T. and HAYASHI, T., Chromium oxide impregnated silica-alumina refractories, Refractories (Japan), 21 (1969), 259.

2. KOLTERMANN, M., Refractories from the European view point--Development and trend of refractories in the steel industry, First hzt. Col~ on Refractories, The Technical Association of Refractories, Tokyo, Japan, 1983, p. 63.

3. STULL, D. R.,. PROPHET, H. et al., J A N A F Thermo- chemicalData Second Edition, Dow Chemical Co., Midland, Michigan, USA, June 1971.

4. ROEDER, P. L., GLASSER, F. P. and OSBORN, E. F., The system AI2Oa-Cr203-SiO2, J. American Ceram. Soc., 51 (1968), 585.

5. JOHNSON, R. E. and MUAN, A., Phase diagrams for the systems Si-O and Cr-O, J. American Ceram. Soc., 51 (1968), 43O.

6. KRONERT, W. and BUHL, H., The influence of various gaseous atmospheres on refractories of the AI2Oa-SiO 2- system, Part II, hlterceram., 4 (1978), 140.

7. YAMAGUCHI, A., Grain growth during sintering of Cr203, J. Ceram. Soc. Japan, 87 (1979), 253.