AU594086B2 - Method for the manufacture of formed parts from Al2O3 and ZrO2 and formed parts manufactured by this method - Google Patents

Abstract

For producing shaped articles from mixtures of 60 to 40% by weight of Al2O3 and 40 to 60% by weight of ZrO2, the starting mixture is shaped and fired. The shaped article produced according to the invention has a particularly good temperature shock resistance and high elasticity. In the process, a) the starting materials Al2O3 and ZrO2 are used in a purity of at least 99.0%, b) the starting materials are comminuted, separately or together, by fine grinding down to a particle size of less than 90 mu m or, if these already have a particle size of less than 90 mu m, are intimately mixed with one another, c) the mixture obtained in stage b) is either granulated or made up with water to give a slip, and the desired shaped article is formed from the granules or the slip, and dried, d) the shaped article is fired at temperatures of 1400 DEG C to 1800 DEG C and e) the cooling after firing is carried out at such a low rate that ZrO2 changes completely or almost completely to the monoclinic phase.

Description

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5940 8Fo6 COMMONWEALTH OF AUSTRALIA PATENTS ACT 1952-69 COMPLETE SPECIFICATION

(ORIGINAL)

Class Int. Class Application Number: awl C Lodged: Complete Specification Lodged: Accepted: Published: Priority: oRglated Art: o 9 99.

S 6 *99 Nare of Applicant: Address of Applicant: tt 1 Actual Inventor: ,t A dress for Service:

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DIDIER-WERKE AG Lessingstrasse 16-18, 6200 Wiesbaden, Federal Republic of Germany INGO ELSTNER and HANS LEISTNER EDWD. WATERS SONS, 50 QUEEN STREET, MELBOURNE, AUSTRALIA, 3000.

A fl Complete Specification for the invention entitled: METHOD FOR THE MANUFACTURE OF FORMED PARTS FROM A1 2 0 3 AND ZrO 2 AND FORMED PARTS MANUFACTURED BY THIS METHOD The following statement is a full description of this invention, Including the best method of performing it known to i' US 1.

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-2- The present invention relates to a method for the manufacture of formed bodies from mixtures comprising or consisting of 60 to 40 percent by weight of A1 2 0 3 and 40 to percent by weight of ZrO 2 in which method the starting mixture is shaped and baked.

Formed parts of 50% Al 2 0 3 and 50% ZrO 2 used as sliding stoppers for ladles are known from STAHL UND EISEN, 100, (1980), pp. 1457 1462. However, these are formed parts of high density, in which the strong Al 2 0 3 "frame" prevents any change in volume of the ZrO 2 crystallites and impedes the modification change of ZrO 2 in use at approximately 1000 C.

The object of the present invention is to make available a method for manufacturing formed parts of the abovementioned type, by which method formed parts are produced that show very high resistance to cyclic temperature changes as well as high strength, and in which method the phase conversion achieved in the manufacturing process is utilized to increase the elasticity.

o The object is achieved by the method characterized by: the starting materials being Al 0 and ZrO 2 with a 4 purity of at least 99.0%, the starting materials being reduced in size, together or separately, to a grain size4 90pm, or if their grain size is initiallyC90 pm being thoroughly mixed, the mixture obtained in step b) being either -r granulated or made up with water to form a slurry, and the desired formed part being formed from the granulate or slurry and then being dried, S1i a i 3 the formed part being baked at temperatures between 1400 0 C and 1800 0 C, and cooling after the baking process being sufficiently slow to permit the ZrO 2 to convert completely or almost completely to the monoclinic phase.

Preferred embodiments of the present invention are described in greater detail in Claims 2 to 7.

The present invention relates, furthermore, to formed parts manufactured by the method, which show the abovementioned good properties of high resistance to cyclic temperature change, high mechanical strength, and relatively high elasticity, and which, because of their high resistance to cyclic temperature changes, can be used without any problems also at temperatures above the transformation point S 15 of ZrO 2 at 900 0 C or 1100 C, respectively.

It is assumed, that as a result of the method according to the present invention, firstly an A1 2 0 3 frame is formed in which the primary Al 2 0 3 grains are strongly sintered together by their corners, edges, and surfaces, S. 20 i.e. where crystallization by accretion has taken place, while, secondly, a ZrO 2 frame is formed of the individual S: ZrO 2 grains remaining as individual grains.

An essential requirement of the method according to the present invention is that the starting materials Al 2 0 3 and ZrO 2 have a chemical purity of at least 99%. It is advantageous to use sintered corundum or tabular alumina as A1 2 0 3 material.

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4 y YI.~*yurLI_-r 4 Similarly, the yttrium oxide being used as stabilizer, as the case may be, should be free from any appreciable quantities of impurities such as MgO, CaO, and/or SiO 2 If more than 1% by weight, referred to the total mixture, of such impurities is present, reaction with A1 2 0 3 to form spinels, calcium aluminates, or aluminium silicates takes place irrespective of the presence of impurities in certain starting materials.

The presence of the component Y 2 0 3 leads to reduced, i.e. only almost complete transition of the ZrO 2 to the monoclinic phase on cooling.

The grain size of the starting materials is likewise important in the method according to the present invention. Prior to shaping of the formed part by compression molding or casting of the slurry, the maximum grain size has to be reduced to 90 pm. This applies also in the case of yttrium oxide being added.

Accordingly, fine size reduction, i.e. fine :grinding, usually consisting in dry grinding or wet grinding 20 in a ball type mill, is required unless the starting materials themselves are of suitable maximum grain size.

If the starting materials themselves are of the I°r desired maximum grain size it is still necessary to S thoroughly mix them, i.e. to homogenize them. This is expediently achieved in a ball type mill, in which case the ";possible further reduction in grain size is harmless, or even may be of advantage in many cases.

I" It is not necessary for the two main components S A1 2 0 3 and ZrO 2 and for the Y 2 0 3 that may be present to have the same grain size or grain size distribution, .respectively. Starting mixtures are particularly preferred in which the maximum grain size of the ZrO 2 is as small as possible, in particularL25 pm.

l i It is possible, for example, to use a mixture in which the grain size of the Al 2 0 3 is L90 pm, while that of the ZrO 2 isz25 pm, with up to 30% of the ZrO 2 grains beingZ- 3 pm.

After completing the preparation of the starting mixture, i.e. after the starting materials employed have reached the desired maximum grain size, the mixture is either granulated or made into a slurry capable of being i cast. Granulation of the completed starting mixture is achieved, in a known manner, by adding small quantities, or in the case of spray gas granulation of larger quantities of a liquid, in particular of water, and, as the case may be, under addition of binding agents, preferably organic binding agents, that burn, without leaving any residues, during the later baking process. The granulate obtained in this way o can be directly used for press molding.

Further treaincnt of the prepared starting mixture Sto form a slurry capable of being cast consists, in a manner that likewise is known, in adding the required quantity of 20 water or in adding the finely ground starting mixture to the C water in a suitable stirring device or a ball type mill. It is expedient also in this case to use a binding agent, i O preferably an organic binding agent such as starch or a cellulose ether such as methyl cellulose or carboxymethyl cellulose. The slurry is then further processed, in a known Smanner,, to produce the formed parts by casting in suitable molds.

Sreace The produced formed parts are then dried at temperatures above 100 C, preferably at 110°C to 180 0 C, to prevent the formation of cracks during the subsequent baking process, caused by water that may have remained present.

I te n s i ab e st r ing- de ic or a ball type mill It -6 Baking of the dried formed parts takes place, in a known manner, in any suitable furnace at temperatures between 1400 C and 1800 0 C. The duration of baking at these temperatures depends on the size of the formed parts; it is usually between 1 and 12 hours.

After baking, cooling down from the baking temperature must take place at a sufficiently slow rate to permit the complete or almost complete transformatifn into the monoclinic phase of the ZrO 2 that exists in the tetragonal phase at high temperature. A cooling rate not exceeding 150 C per hour has been found to be suitable, with the lower limit of the cooling rate being approximately per hour.

In preparation of the starting mixture of A1 2 0 3 and 15 ZrO 2 it has been found expedient to first mix together the 'starting materials and then to pre-bake this mixture at 0 0 temperatures between 1000°C and 1500°C. During this process the phase transformation of the ZrO 2 from monoclinic to 2 I" tetragonal or vice versa takes place, which in each case is connected with a change in volume, such that particularly good homogenisation of the mixture is achieved in the subsequent grinding process.

04,04 The present invention is explained in greater detail in the following with reference to the Examples: 25 Example i: *8*t 100 Parts by weight of sintered corundum of 99.2% purity and an initial grain sizeL 250 pm and 100 parts by 9weight of ZrO 2 of 99.1% purity and of a maximum grain size of 150 pm were dry-mixed, and, as a first step, were 30 pre-baked at 1200 C in a crucible placed into an oven for one hour. Subsequently, the content of the crucible was transferred into a ball type mill and ground for 6 hours.

The resulting maximum grain size was L60 pm. The finely ground mixture of the starting materials obtained in this manner was then granulated under addition of a small i o_ _I I~ 6? r ~z" 7 quantity of water, and a formed part was produced from the granulate by press molding. The formed part was dried for 2 hours at 120°C and subsequently baked for 6 hours at 1700°C, followed by cooling at the rate of 60 0 °C per hour. The dimensions of the formed part were 250 x 125 x 65 mm. The produced formed part had the following properties: Density (g/cm 3 4.72 Raw density (g/cm 3 4.42 Total porosity 6.4 Open porosity 3.6 Bending strength (MPa) 127 Modulus of elasticity (static)(GPa): 135 Example 2 120 Parts by weight of tabular alumina of 99.8% 15 purity and 80 parts by weight of yttrium-stabilized ZrO 2 with an yttrium content of 3% by weight and less than by weight of other impurities were wet-ground under addition of water in a ball type mill for 16 hours. The resulting maximum grain size was 25 pm, with approximately 25% of the 20 grains having a size of43 irn. The wet ground material was mixed with 0.8% by weight of starch, referred to solid substance, and further water was added to obtain a slurry of c suitable consistency. This slurry was then poured into a suitable mold, and after tightening and reaching the desired wall thickness, and after the remainder of the slurry having r.' been discarded, the formed part having the shape of a hollow cylinder of 50 mm external diameter with a wall thickness of mm was pre-dried in the mold, then taken out of the mold and dried first in the open air for 48 hours and then at I 30 110°C for another 8 hours. The dried formed part was then 0 C 0 baked at 1550 C for 6 hours, and then cooled with a cooling rate of 120°C per hour. i r 8 The produced formed part had the following properties: Density (g/cm 3 4.58 Raw density (g/cm 3 4.16 Total porosity 9.2 Open porosity 6.1 Bending strength (MPa) 150 Modulus of elasticity (statis)(GPa): 190 4 @4 C wet

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Claims (5)

1. A method for the manufacture of formed bodies from mixtures comprising or consisting of 60 to 40 percent by weight of Al 2 0 3 and 40 to 60 percent by weight of ZrO 2 in which method the starting mixture is shaped and baked, characterized by: the starting materials being Al 2 0 3 and ZrO 2 with a purity of at least 99.0%. the starting materials being reduced in size, together or separately, to a grain size /-90 Vm, or if their grain size is initially<90 pm, being thoroughly mixed, the mixture obtained in step b) being either granulated or made up with water to form a slurry, and the S- desired formed part being formed from the granulate or *slurry and then being dried, the formed part being baked at temperatures between 1400 0 C and 1800 0 C, and cooling after the baking process being sufficiently slow to permit the ZrO 2 to convert completely or almost completely to the monoclinic phase. I t A method according to Claim 1, characterized by the rate of cooling being not faster than 150 C per hour.

3. A method according to Claim 1 or 2, characterized by up to 4 parts by weight of yttrium oxide Y 2 0 3 being ii I .added, as a further component, to 100 parts by weight of Al 2 0 3 and ZrO 2 to act as stabilizer.

4. A method according to any one of Claims 1 to 3, characterized by the reduction, namely by grinding, in step b) being continued until the grain size of the starting materials is reduced to /40 pm, or by starting materials of grain sizeL40 pm being used. t i r A method according to Claim 4, characterized by 4 grinding being continued to a maximum grain size of 25 um, or by starting materials of grain size L25 pii being used.

6. A method according to any one of Claims 1 to characterized by the mixture of the starting materials being first pre-baked at 1000 0Cto 1500 0 C, and then being finely ground or being further finely ground until the desired maximum grain size is reached.

7. A method according to any one of Claims 1 to 6, characterized by a starting mixture being used in which, after step b) has been carried out, up to 30% of the particles have a grain sizee '3 yin. e~ 8. A formed body manufactured by the method according *a~to any one of Claims 1 to 7. DATED this 7th day of October, 1986 DIDIER-WERKE AG. EDWD. WATERS SONS QUEEN STREET, MELBOURNE, VICTORIA AUSTRALIA 1/24:pj