BOL.SOC.ESP.CERAM.VlDR. 28 (1989) i , 5-9

Microstructural aspects in manufacturing high-alumina refractories

WOLFGANG SCHULLE

Bergakademie Freiberg. Department of Process Engineering and Silicate Technology. German Democratic Republic

ABSTRACT.-Microstructural aspects in manufacturing high-alumina refractories.

In the worldwide growing interest for high alumina refractories it is commonly accepted, that the manufac­ture of corundum-materials with high strengths at elevated temperatures and good resistance against ther­mal shock is possible thanks to the optimization of the microstructure. Good performances for these materials can be achieved by the well oriented design of the microstructural characteristics, in special the features of the binding matrix. Binding matrices obtained from clay raw materials produce good results when the matrix is almost free of glassy phase and containing needle shaped muUite crystals which have been grown jointly with the basic grains. Otherwise, a recrystallized binding matrix of a-A1203 gives rise to corundum-materials depicting good thermal stability and resistance against thermal shock by using a sufficiently pure sintering pure alumina. This high grade binding can be obtained technically and economically by adding alumina as dopant agent at fir­ing temperatures around 1600® C.

RESUMEN.—Aspectos microestructurales de la obten­ción de materiales refractarios de alta-alúmina.

En el contexto mundial del creciente interés por los ma­teriales refractarios de alta alúmina está generalmente aceptada la idea de que se pueden lograr materiales de corindón con altas resistencias a altas temperaturas y bue­na resistencia al choque térmico por un control adecua­do de la microestructura. Las elevadas demandas que se requieren de este tipo de materiales se pueden lograr con un desarrollo bien dirigido de la microestructura. La fa­se matriz aglomerante obtenida a partir de materiales ar­cillosos conduce a buenos resultados si se logra una matriz libre de fase vitrea, conteniendo cristales de muUita en forma de agujas que crecen conjuntamente con los gra­nos principales de material. Una fase aglomerante de o:-A1203 recristalizada da lugar, además, a buena esta­bilidad térmica y buena resistencia al choque térmico si se usa alúmina activa pura sinterizada. Este alto grado de aglomeración se puede conseguir satisfactoriamen­te desde el punto de vista tecnológico y económico con adiciones de alúmina y a temperaturas cercanas a los 1600« C.

1. INTRODUCTION

High-alumina refractories have gained importance throughout the world in recent years. This is due to their remarkable hot strength allowing application in furnace con­struction above 1600° C on one hand and to their com­paratively good resistance to ageing under permanent load within the temperature range above 1600° C on the other. Nowadays, structural corundum refractories with an aluminium oxide content which is partially higher than 90% are of interest because of the crystallized high-temperature modification of aluminium oxide (a-corundum), a very stable phase under termal stress which moreover has a low vapour pressure at temperatures above 1500° C so that it can be ap­plied without problems in case of fiimace operation at reduc­ed pressure. Up to the moment the application of these materials is restricted by a thermal shock resistance wich is not sufficient always. This disadvantage could be minimiz­ed by specific microstructural optimization.

2. GENERAL MICROSTRUCTURAL ASPECTS

The concept of microstructure in this case undergo the type and amount, as well as the size, shape, orientation and distribution of the phases which make up the material. With a few exceptions, refractories consist of three major struc­tural components:

Recibido el 1-4-88 y aceptado el 1-12-88.

ENERO-FEBRERO, 1989

— Framework structure from the coarse grain. — Binding matrix from the fine grain. — Pores.

In most cases the framework structure and binding matrix can consist of more than one phase. With structural corun­dum refractories, structural properties are essentially deter­mined by the binding matrix. This binding matrix between the corundum framework structure infiuences particularly on the following materials properties:

— The hot strength, since the good hot strength of the framework structure cannot become fully effective in case of premature softening of the binding matrix.

— The thermal resistance; since the fracture behaviour is influenced by the microcrack structure and crack pro­pagation within the binding matrix.

However, it has to be taken into account that differences on material properties must also be expected if different a-corundum materials are applied as framework structure. In this context, it is possible tiie a-corundum application as fus­ed corundum of pure or normal quality and as sintered cor­undum in form of tabular alumina. The initial properties of the a-corundum framework may also influence on the pro-pierties of the newly forming binding matrix.

The required properties may only be guaranteed if the materials have a well-defined specific microstructure. In prin­ciple, this includes the following requirements:

WOLFGANG SCHULLE

— The textural stress forming during operation should be as low as possible. The stress formation in the microstruc­ture is inevitable in view of the temperature gradients, which always exist due to the heating and cooling processes in the furnace. According to the theories of crack formation the permitted temperature change per time during heating and cooling processes is described by (1) and (2):

(dT/dt)^áx. - - b2

where a E /* X a b

aE — tensile strength — linear heat expansion coefficient — modulus of elasticity — poisson ratio — thermal conductivity — temperature conductivity — linear sample dimension un the direction

of temperature gradient.

Low a-values in combination with high and/or X-values allow maximun values of a^ for a more pronounced change of temperature per time. Thus, one can assume that the com­paratively high heat expansion coefficient of the a-corundum (ca. 80x 10-^K~0 in corundum materials is a disadvantage in terms of crack formation and thermal shock resistance.

— The above mentioned crack formation does not necessarily lead to catastrophic crack propagation. In the case of a fracture propagation behaviour will not take place in­stantly. Therefore, the microstructure will have to show a good fracture behaviour. This aim can be achieved by specific microstructural optimization with special attention to the bin­ding matrix (3) and (4). The resistance to crack propagation (Rst) must have a high value defined by the expression:

Rct ^ ^st aE • vr

ap — critical fracture stress 1^ — L length of microcracks to beginning.

So, with more detail, the following microstructural pro­perties are required:

— The binding matrix will have to consist of elastically stretchable crystalline solid bridges. The binding by needle like or long prismatic crystals growing in the framework which should be felted, if possible, is particularly well suited.

— The glassy phase content of the binding matrix must be low. The crystalline binding matrix should consist in terms of materials and mineralogy of high-quality refractory phases such as mullite, a-corundum, etc. The figure 1 shows a low value glassy binding matrix. The figure 2 shows a high value cristalline binding matrix in comparison.

— The objective must be a sufficient strength which simultaneously provides a stretchable framework with a specific microcrack density giving rise to a high resistance to crack propagation. This would assure good hermal shock resistance as well. Naturally, an increased microcrack den­sity restricts the initial strength. Thus, some compromise is needed in microstructural design.

3. POSSIBILITIES FOR STRUCTURAL DESIGN

As has been discussed above, it is possible to achieve a well defined microstructure within a certain range on struc­tural corundum refractories by modification of the binding matrix. A binding matrix which conforms to the requirements may be achieved on the basis of the following technological possibilities:

— Binding by a high-quality natural raw material contain­ing clay minerals which partially may be added in form of slips.

— Binding by recry stallization of aluminium oxide which is added in form of corundum fines or calcined technical alumina at sintering temperatures above 1700° C.

Fig. 1.—Scanning electron micrograph of a low-value glassy binding matrix.

Fig. 2.—Scanning electron micrograph of a high-value crystalline binding matrix.

BOL.SOC.ESP.CERAM.VIDR. VOL. 28 - NUM. 1

Microstructural aspects in manufacturing high-alumina refractories

— Binding by recrystallization of aluminium oxide at sintering temperatures which are significantly lower than 1700° C by using doped calcined alumina products.

— Binding by hydrothermal boehmite synthesis in the sub-critical hydrothermal state with subsequent transformation of this boehmite into aluminium oxide.

In the following, some possibilities of corundum binding shall be discussed and compared. The binding by hydrother­mal boehmite synthesis is not described in this paper.

4. NATURAL MATEMAL BINDING

The application of natural high-quality raw materials con­taining clay is the old «classical» method of binding struc­tural corundum materials. Clays with high AI2O3 content (Al2O3>40%) are hardly ever used anymore since normal­ly they do not allow the formation of satisfactory hot strength. The flux components of natural clay minerals lead to premature formation of liquid phase. This liquid phase nor­mally solidifies in a glassy state and thus results in a brittle glass matrix which in case of thermal shock tends to a frac­ture behaviour with catastrophic crack propagation. Therefore, it is better to produce a crystalline binding and to use clay minerals or mixtures which lead to the formation of mullite. In our case it has been produced binding with a low content of glassy phase mostly by crystalline bridges containing needle-like mullite crystallites which are in-tergrown with the framework structure (5) and (6). Micro- Fig. 4.—Scanning electron micrograph of a Mg-Al-mixed crystal in the

binding matrix.

crack density in the matrix must be adjusted in such a way that thermal shock resistance can be improved as a conse­quence of the optimum microstructural strength and elasticity and a favourable fracture behaviour (7). The formation of such a matrix may be effected by kaolin or a mixture of kaolin and alumina. From the point of view of the low binder con-

Fig. 2>.—Scanning electron micrograph of well grown mullite crystals in the binding matrix from high-alumina refractories.

Fig. 5.—Scanning electron micrograph of recrystallized cx-corundum crystals growing on the ot-corundum

framework from coarse grains.

ENERO-FEBRERO, 1989

WOLFGANG SCHULLE

TABLE I

SOME PROPERTIES OF HIGH-ALUMINA REFRACTORIES

Fig. 6.—Scanning electron micrograph of microcrack formation at the edge of the ci-corundum framework from coarse grains.

tent, in order to guarantee a homogeneous distribution of the binding phase these materials are partly added as slips.

Coarse, medium or fine grain of pure or normal corun­dum were mixed with kaoHn, pressed and fired at 1600° C for three hours.

The properties of these samples are show in the table I. A good thermal shock resistance can be achieved as well as good cold binding strength and hot strength. The figure 3 shows well grown muUite crystals in the binding matrix which grow on the corundum framework.

5. BINDING MATRIX ON THE BASIS OF RECRYSTALLIZED a-AljOa

The binding of the framework structure by a related cor­undum binding has also been employed for years. The crystalline corundum binding may be achieved by recrystallization of corundum in the binding matrix. High firing temperatures above 1700° C are necessary (8) and use of fine-ground or sinteractive alumina as pure as possible. The main disadvantage of this method is the high energy con­sumption at firing temperatures > 1700° C. Therefore, it is of high technological and economic interest to achieve the high-quality Q:-A1203 recrystallization binding at tempera­tures below 1700° C.

The firing temperature can be reduced by using doped alumina, with the properties either being conserved or even being improved. MgO in a percentage below 3% gives good

Binding Binding with Properties with recrystalized Q!-A1203

Mullit of doped alumina

Flexural strength after 35 40 manufacturing (MPa)

Flexural strength by 1400° C (MPa) 10 12 Flexural strength after 5 x thermal 30 37

shock from 950° C to 20° C (MPa)

Flexural strength after 5 x thermal 29 34 shock from 1400° C to 20° C (MPa)

results when is employed as doping agent. During heat treat­ment above 1500° C high-temperature resistant Mg-Al mixed crystals are formed in the area with high AI2O3 content. These crystals are stable with the aluminium oxide crystals. At temperatures above 1500° C, the microstructure is characterized by an increased growth of AI2O3 grains. The formation of mixed crystals may observed in the figure 4. In accordance with the phase diagram, no liquid phase for­mation was observed. As can be seen from the figure 5, the starting recrystallization process which in supported by the formation of mixed crystals gives rise to an excellent bin­ding of the framework structure. As can be seen in the figure 6 the recrystallized a-corundum crystals growing on the a-corundum framework form a temperature-resistant binding phase which allows (for a certain degree of structural elastici­ty) the formation of a rigid microstructure with unfavourable fracture behaviour by microcrack formation.

This microstructure may serve as a basis which guarantees the thermal shock resistance together with good cold and hot strength (table I). Thus, in principle it is possible to produce a high-quality temperature- resistant corundum material with a recrystallized a-corundum binding at a temperature as low as 1600° C, a remarkable possibility both in terms of technology and economy.

6. CONCLUSIONS

The discussion of fundamental possibilities for producing and, particularly, binding structural corundum materials with good thermal shock resistance and hot strength enables us to make the following conclusions:

— The properties required may be guaranteed only by specific microstructural optimization. This applies particular­ly to the properties of the binding matrix.

— Binding with natural clay mineral materials may guarantee good hot strength together with sufficient thermal shock resistance only if an almost glass-free binding matrix is formed, which consists of needle-like mullite crystals growing on the framework structure.

— Binding with recrystallized Q:-A1203 may lead to cor­undum materials with high hot strength and good thermal

BOL.SOC.ESP.CERAM.VIDR. VOL. 28 - NUM. 1

Microstructural aspects in manufacturing high-alumina refractories

shock resistance by using sufficiently pure fine-grained, sinter-active alumina. By binding with recrystallized a-AI2O3 consisting of doped alumina products a high-quality structural corundum material can be produced under favourable technological and economical conditions requir­ing firing temperatures of 1600° C or even below.

7. REFERENCES

1. HASSELMANN, D . P. H.: Unified Theorie of Thermal Shock Fracture Initation and Crack Propagation in Brittle Ceramics. J. Amer. Ceram. Soc, 52 (1969), 11, 600-604.

2. HASSELMANN, D . P. H.: Thermal Stress Resistance Parameters for Brittle Refractory Ceramics. Amer. Ceram. Soc. Bull., 49 (1970), 2, 1033-1037.'

3. GARDU, A . N . , SCHAPIRO, J. S. y SCHAPTALA, W . I.:

Feuerfeste Korundmaterialien mit erhöhter Tempe­raturwechselbeständigkeit. Ogneupory, 36 (1971), 6, 42-45.

4. SCHULLE, W., ULLRICH, B . y UEBEL, J . : Possibilities of Bonding in Hihg-Alumina, Heavy Ceramic, Thermal Shock Resistant Corundum Materials. Silikattechnik, 36 (1985), 5, 142-144.

5. LORENZ, H.: Beitrag zur Bewertung muUitgebundener Korundsteine. Dissertation A - Bergakademie Freiberg 1972; ref. Silikattechnik, 24 (1973), 10, 355.

6. ZSCHIEDRICH, W.: Untersuchungen zur Temperatur­wechselbeständigkeit hochfeuerfester Materialien mit Mullitmatrix. Dissertation A - Bergakademie Freiberg 1975; ref. Silikattechnik, 27 (1976), 135.

7. ScHULLE, W. y UEBEL, J. : New Knowledge about Tex­ture Evaluation of Firing Auxilaries free in SiC. Silikat­technik, 34 (1983), 4, 99-102.

8. BARTUSKA, M . : Corundum Refractarles with Alumina Bond Proceedings der 6. Siliconf im Oktober 1961 in Budapest; Akademie-Verlag Budapest 1963, 29-44.

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