FIELD OF THE INVENTION
The present invention relates to zirconia alumina composites. In particular, the invention relates to zirconia alumina composites where the alumina crystals are dispersed in the matrix of zirconia matrix. The composites have fine-grained micro structure with high toughness and moderate hardness. The invention also relates to a process for the preparation of sintered zirconia alumina composites for various structural and functional applications.
BACKGROUND OF THE INVENTION
Zirconia ceramics are well known for its polymorphic phase transformation, high fracture toughness, high thermal expansion, low thermal conductivity, moderate toughness etc. The zirconia is often partially stabilized to metastable tetragonal phase using various metal oxides where the transformation toughening is being used to arrest the crack propagation thus increasing the fracture toughness.
Zirconia by itself has advantageous property of enhanced toughness; however it does not have ideal hardness for some applications. While composites comprising alumina combined with zirconia to form alumina rich composites are preferred where high impact is concerned such composites do not form a good choice for applications where high pressure is concerned. Further some of the specific compositions comprising alumina and zirconia are processed through electro fusion methods such as eutectic compositions of alumina zirconia (zirconia -40% or -25% zirconia). The high melting point and high fracture toughness makes the electro fusion and subsequent process very expensive.
Accordingly it is an objective of the present invention to provide zirconia alumina composites for applications that require enhanced toughness and where high pressure is concerned. It is also an objective to provide an economical process for preparing such composites.
SUMMARY OF THE INVENTION
In one aspect, the present invention relates to zirconia alumina composite comprising 60 to 99 wt. % of zirconia, 1 to 40 wt. % of alumina, more particularly 70 to 90 wt. % of zirconia, ) 0 lo 30 wt. % of alumina, a zirconia stabilizer, and additives.
Another aspect of the present invention relates to a process for the preparation of sintered zirconia alumina composites comprising 60 to 99 wt. % of zirconia, 1 to 40 wt. % of alumina,

more particularly 70 to 90 wt. % of zirconia, 10 to 30 wl. % of alumina, a zirconia stabilizer, and additives comprising the steps of: (a) milling of zirconia powder and alumina powder to obtain a homogenous mixture; (b) adding additives and stabilizer to the mixture obtained in step (a) and milling together for homogeneity to obtain a slurry; (c) drying the slurry followed by grinding and screening to obtain powder of desired particle size; (d) compacting the powder from step (c) followed by extruding and drying the compacted mass; (e) sintering the dry compacted mass to obtain the sintered zirconia alumina composite.
Yet another aspect of the present invention provides a zirconia alumina composite product(s) suitable for wide spectrum of applications such as abrasives, metal matrix composites, ware ceramics, cutting tools etc.
An advantageous aspect of the present invention resides in the processing of zirconia rich zirconia alumina composite wherein the alumina is dispersed in the zirconia matrix. The added advantage of fine grained alumina ceramics to the zirconia ceramics results in superior composites with high toughness, moderate hardness, high thermal expansion, high thermal conductivity etc. The zirconia alumina composites are generally processed through sintering route where the flexibility of selecting wide range of composition are explored economically. The present invention discloses the simplified process of making sintered zirconia alumina composites from the economical precursors. The products made out of this novel process can be suitable for wide spectrum of applications such as abrasives, metal matrix composites, wear ceramics, cutting tools etc.
DETAILED DESCRIPTION
For the purposes of the following detailed description, it is to be understood that the invention may assume various alternative variations and step sequences, except where explicitly specified to the contrary. Moreover, other than in any operating examples, or where otherwise indicated, all numbers expressing, for example, quantities of ingredients used in the specification are to be understood as being modified in all instances by the term "about". It is noted that, unless otherwise slated, all percentages given in this specification and appended claims refer to percentages by weight of the total composition.
Thus, before describing the present invention in detail, it is to be understood that this invention is not limited to particularly exemplified systems or process parameters that may of

course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments of the invention only, and is not intended to limit the scope of the invention in any manner.
Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary person skilled in the art to which this invention pertains. In the case of conflict, the present document, including definitions will control.
It must be noted that, as used in this specification and the appended claims, the singular forms "a," "an" and "the" include plural referents unless the content clearly dictates otherwise. Thus, for example, reference to a "solvent" may include two or more such solvents.
The terms "preferred" and "preferably" refer to embodiments of the invention that may afford certain benefits, under certain circumstances. However, other embodiments may also be preferred, under the same or other circumstances. Furthermore, the recitation of one or more preferred embodiments does not imply that other embodiments are not useful, and is not intended to exclude other embodiments from the scope of the invention.
As used herein, the terms "comprising" "including," "having," "containing," "involving," and the like arc to be understood to be open-ended, i.e., to mean including but not limited to.
As will be described herein the present invention relates to a zirconia alumina composite comprising 60 to 99 wt. % of zirconia, 1 to 40 wt. % of alumina, more particularly 70 to 90 wt. % of zirconia, 10 to 30 wt. % of alumina, a zirconia stabilizer, and additives. The source of alumina is selected from aluminium salt, aluminium hydroxide, boehmite calcined alumina from Bayer's process, reactive alumina, or combination thereof. In a preferred embodiment, boehmite may be used as a source of alumina. Boehmite is one of the aluminium hydrates having formula of AJOOH. During heat treatment il forms aluminium oxide and releases the structural water. The volatile content of the boehmite can vary from 15 to 35 % which comprise of the structural water and free moisture. It is commercially available from several manufacturers for example Sasol Germany GmbH with trade name Disperal®.

The source of zirconia is selected from monoclinic zirconia powder, zirconium hydroxide, zirconium nitrates, zirconium oxy chloride or combination thereof. In a preferred embodiment the source of zirconia is monoclinic zirconia powder. The monoclinic zirconia is one of the polymorphs of zirconium oxide. It has other forms like tetragonal zirconia, cubic zirconia and the polymorphic transformation takes place with the increment of temperature.
Monoclinic zirconia is stable at room and the stabilizing agents are required in order to retain other forms at room temperature. For example partially stabilized zirconia contains mctastable tetragonal phase at room temperature and stabilizing agents do the same. The monoclinic zirconia is commercially available from several manufacturers for example Foskor Zirconia (Pty) Ltd. In some embodiments, the alumina and zirconia powders may be in the range of 5 micron (dso).
In accordance to one aspect of the present invention zirconia can be added as partially stabilized zirconia. In some embodiments, zirconia can be stabilized in-silu. The amount of the stabilizer is based on the total weight of the zirconia content present in the zirconia alumina composite. The amount of the stabilizer may vary from 1 wt. % to 8 wl. %, and more preferably from about 2 wt. % to about 6 wt. % of the zirconia content. The zirconia stabilizer is selected from MgO, ¥203, Ce2C>j, SC2O3, or combination thereof. In a preferred embodiment, MgO and Y2O3 may be used as zirconia stabilizer.
In one aspect of the present invention, additives are added to the mix as powder and/or their respective salts. The amount of these additives can be range from 0.001 to 20 wt.% of the alumina content of the composite. The main purpose of this set of additives is to enhance the hardness of alumina when added as small percentage i.e., up to 5 wt.% whereas form secondary phase known as rare earth hexaaluminate when used higher amount. The rare earth hexaaluminatc has platelet morphology, which reinforces the matrix i.e. toughening the matrix. The additives may be selected from rare earth oxides such as lanthanum oxide, neodymium oxides, cerium oxide etc., and combinations thereof.
Another aspect of the invention relates to a process for the preparation of sintered zirconia alumina composites having superior mechanical properties. In some embodiments the sintered zirconia alumina composites obtained from the process disclosed herein has high toughness and moderate hardness characteristics.

In accordance to the present invention, the process for the preparation of sintered zirconia alumina composites comprising 60 to 99 wt. % of zirconia, 1 to 40 wt. % of alumina, more particularly 70 to 90 wt. % of zirconia, 10 to 30 wt. % of alumina, a zirconia stabilizer, and additives' comprising the steps of: (a) milling of zirconia powder and alumina powder to obtain a homogenous mixture; (b) adding additives and stabilizer to the mixture obtained in step (a) and milling together for homogeneity to obtain a slurry; (c) drying the slurry followed by grinding and screening to obtain powder of desired particle size; (d) compacting the powder from step (c) followed by extruding and drying the compacted mass; (e) sintering the dry compacted mass to obtain the sintered zirconia alumina composite.
In one aspect of the present invention, the process of making zirconia alumina composites starts with blending of alumina powder and zirconia powder thoroughly using a suitable mixer as described in the following example(s). The zirconia during the process of preparation of the composite is added as partially stabilized zirconia or the zirconia is stabilized in-situ.
In one aspect, the blending which is a homogeneous blending of the precursors i.e. zirconia powder and alumina powder can be achieved using a plurality o( mixer or blenders. In the case of powder precursors, the particle size of the powder can be in the range of 5-25 pm (djo)- Zirconia and alumina proddcr can be milled together to bring down the particle size further below 1 pm, more preferably below 0.8 pm. Milling can be done in different type of mill such as bail mill, colloidal mill, attrition mill etc. either in dry or in wet conditions. In case of wet milling, water is used. The powder to water ratio can be varied from 3:1 to 1:3. The required amount of additives of their oxide form is added to the mixed and milled together for homogeneity. In the case of metal salt/hydroxide is the precursors, the water based solution/slurry is prepared where water content can vary from 40 to 70 wt%. The salt solution of the desired additives is added to the mix and dispersed thoroughly. In case of dry mixing, small amount water is added to the mix to achieve sufficient plasticity for compaction.
The homogenous slurry so obtained is dried where drying can be done in tray dryer, spray dryer etc. in case of tray drying, the dry cake is ground further and screened to obtain desired size particle. The temperature of the dryer can be at 75 to 150°C, more preferably 90 to

120°C for 1 to 24 hours On the other hand, spray drying of the slurry provides fine spherical powder.
The powder so obtained is later subjected to compaction. Compaction is the process to consolidate powder mass into a solid body by applying load. The voids between powders get almost eliminated during this process. In one aspect, the compaction of the mixed material can be performed by compactors such as hydraulic press, extruder, roll and the like. In one embodiment the compaction of the mixed material is achieved by an extruder. In a preferred embodiment, the piston type as well as screw type extruder can be used for obtaining the compacted material. The piston type or screw type extruder used for the compaction of the mixed material is a continuous extruder.
In a further aspect of the invention certain amount of organic binder such as po)y vinyl alcohol, poly ethylene glycol etc., is added as a binder prior to extrusion/compaction. This acts as a binder and provides sufficient green strength. Binder acts as a bridge between the ceramic particles and holds them together which in turns aid the easy handling of the compacted mass prior to calcination process. Sufficient green strength can prevent the unwanted breakage/ crumbling of the compacted mass. The amount of the binder solution can be 0.01 to 3 vvt.% of the total mix, more preferably 0.05 to 2 wt.% . The compacted mass is then dried in an oven at 75 to 150°C, more preferably 90 to 120°C for 1 to 24 hours. The dried material is subsequently crushed and graded to obtain desired particle size.
In some embodiments the extruded body or material can be obtained in shapes such as, but not limited to, cylindrical, square rod, triangular rod including other shapes. In a preferred embodiment, the extruded body is cylindrical which can further be broken into the desired length to maintain the aspect ratio. In one embodiment, the ratio of diameter to length can be from 1:4 (o 1:10, In a preferred embodiment, the ratio of diameter to length is 1:5 to 1:8.
The moisture content of the dried product could be in the range of 1 to 10 wt. %, more preferably 2-6 wt.%. Shape of the material highly influences the application. Therefore, the suitable crushing method and the additional shaping can be selected to meet the application requirement. The crusher can be roll crusher, plate crusher, comb mill etc., and the shaping can be done by tumbling mill in case spherical particles are preferred.

]n one other aspect of the invention, instead of the milled slurry subjecting to drying and compaction, the shaping can be done through well-known drip casting, gel casting etc., where the mill slurry is gellified using suitable gelling agent such as sodium alginate and the slurry is casted as drops to the gelling medium which is basically a divalent, trivalent metal salt solution and a combination thereof. The shape of the particle can be varied from spherical to the distorted shape depending on the application requirement. The casted zirconia alumina composites can be taken out from the gelling medium and dried, preferably by spray drying subsequently.
The dried mass is then sintered at elevated temperature in the range of 1350°C to 1650°C with a dwelling time of 20 minutes to 5 hours. Sintering of the dried particle can be done in muffle furnace, shaft kiln, tunnel kiln, rotary furnace. The sintering schedule highly depends on the desired end properties such as density, microstructure, phases etc. of the composites. The dried material can be calcined prior to sintering at 400 to 700°C for 1-4 hours if it contains large amount of volatiles. In some embodiments the in-situ stabilization of zirconia during sintering of the composite is preferred as it is more economical and provides certain benefits. The additives such as Y2O3 react with monoclinic zirconia during sintering and it stabilizes the zirconia.
In order to illustrate the invention more clearly, the following examples are given explaining the preferred modes of carrying it into effect and the advantageous results obtained thereby. The use of examples in no way limits the scope and meaning of the invention or of any exemplified term. Likewise, the invention is not limited to various embodiments given in this specification.
EXAMPLES
Various samples of zirconia alumina composites were prepared by the process as outlined
above and tested for its properties.
1. True density measurement was done by Helium Gas Pycnometer
2. Micro hardness was determined by ASTM CI327 standard test method for Vickers indentation hardness of advanced ceramics

3. Fracture toughness was determined by indentation technique by Vickers hardness tested. Anstis formula is used for calculating the fracture toughness (Kic)
/ E \2 P KIC = 0.016 (—) -3
\*V C2
Where, E is elastic modulus (- 244 GPa used for AZ composites), Hv is the vickers hardness (GPa), P is the load in N, C is the half crack length in m.
[Anstis G. R., Chantikul P., Lawn B. R., and Marshall D. B. (1981). A Critical Evaluation of Indentation Techniques for Measuring Fracture Toughness: I, Direct Crack Measurements, Journal of the American Ceramic Society, Vol. 64, No. 9, (September 1983) pp. 533-538]
Example 1
1528 gram (76.40 wt.%) of monoclinic zirconia powder was mixed with 400 gram (20.00 wt.%) of reactive alumina powder and 72 gram (3.60 wt.%) yttrium oxide was added to it. The precursors were put into a ball mill along with water and milled for 48hours where particle size reduced to less than 1 micron. The powder to water ratio was maintained at 2:1. Milled material was discharged from the ball mill and dried in a hot oven at 110°C for 5 hours. Dry mass formed a cake which was then ground and screened to obtain desired powder. The dried powder is extruded to cylindrical mass of 6 mm using 0.06 wt.% polyvinyl ethylene glycol as binder. The extruded mass was then dried in a hot oven at 110°C for 5 hours and crushed and graded subsequently. 1-3 mm size particles were used for sintering which was conducted in a muffle furnace at 1550 °C for 2 hours. The heating rate was maintained at 3°C/minute. The properties of the sintered product are summarized in Table 1.
Example 2
1500 gram (75.00 wt.%) of monoclinic zirconia powder was mixed with 400 gram (20.00 wt.%) of reactive alumina powder and 100 gram (5.00 wt.%) yttrium oxide was added to it. The precursors were put into a ball mill along with water and milled for 48hours where particle size reduced to less than 1 micron. The powder to water ratio was maintained at 2:1. Milled material was discharged from the ball mill and dried in a hot oven at 110°C for 5 hours. Dry mass formed a cake which was then ground and screened to obtain desired powder. The dried powder is extruded to cylindrical mass of 6 mm using 0.06 wt.% polyvinyl ethylene glycol as binder. The extruded mass was then dried in a hot oven at 110°C for 5 hours and crushed and graded subsequently. 1-3 mm size particles were used for sintering

which was conducted in a muffle furnace at 1550 °C for 2 hours. The heating rate was maintained at 3°C/minute. The properties of the sintered product are summarized in Table 1.
Example 3
1528 gram (76.40 wt.%) of monoclinic zirconia powder was mixed with 396 gram (19.80 wt.%) of reactive alumina powder. 72 gram (3,60 wt.%) of yttrium oxide and 4 gram (0.20 wt.%) of lanthanum oxide were added to it. The precursors were put into a ball mill along with water and milled for 48hours where particle size reduced to less than 1 micron. The powder to water ratio was maintained at 2:1. Milled material was discharged from the ball mill and dried in a hot oven at 110°C for 5 hours. Dry mass formed a cake which was then ground and screened to obtain desired powder. The dried powder is extruded to cylindrical mass of 6 mm using 0.06 wt.% polyvinyl ethylene glycol as binder. The extruded mass was then dried in a hot oven at 110°C for 5 hours and crushed and graded subsequently. 1-3 mm si/e particles were used for sintering which was conducted in a muffle furnace at 1550 °C for 2 hours. The heating rate was maintained at 3°C/minute. The properties of the sintered product are summarized in Table 1.
Example 4
572 gram (20 wt.%) of alumina) of boehmite powder (LOI —30%) was dispersed in hot water and peptised using dilute nitric acid to convert into alumina hydrosol. The pH of the sol was maintained at 2.5. 1528 gram (76.40 wt.%) slurry of ball milled monoclinic zirconia powders was added to the sol. 122 gram (3.60 wt.% of Yttria) yttrium nitrate hexahydrate was dissolved in water and added to the sol. The sol was homogenised for 15 minutes in a high speed stirrer and spray dried into fine powder. The spray dried powder is extruded to cylindrical mass of 6 mm diameter and dried in a hot oven at 110°C for 5 hours. Dry mass was crushed and graded to 1-3 mm size particles and sintered in a muffle furnace at 1550 °C for 2 hours. The heating rate was maintained at 3°C/minute. The properties of the sintered product are summarized in Table 1.
Example 5
1688 gram (84.40 wt.%) of monoclinic zirconia powder was mixed with 200 gram (10.00 wt%) of reactive alumina powder and 112 gram (5.60 wt.%) of yttrium oxide was added to it. The precursors were put into a bail mill along with water and milled for 48hours where

particle size reduced to less than 1 micron. The powder to water ratio was maintained at 2:1. Milled material was discharged from the ball mill and dried in a hot oven at 110°C for 5 hours. Dry mass formed a cake which was then ground and screened to obtain desired powder. The dried powder is extruded to cylindrical mass of 6 mm using 0.06 wt.% polyvinyl ethylene glycol as binder. The extruded mass was then dried in a hot oven at 110°C for 5 hours and crushed and graded subsequently. 1-3 mm size particles were used for sintering which was conducted in a muffle furnace at 1550 °C for 2 hours. The heating rate was maintained at 3°C/minule. The properties of the sintered product are summarized in Table 1.
Comparative Example 1
1225 gram (85.75 wt% as oxide) of boehmite powder was added to the high shear mixer and 5 liter water added to it. The LOI of boehmite was around 30%. The dilute nitric acid was added to peptize the slurry and the pH of the slurry was maintained at 2.5. 140 gram (14 wt%) of monoclinic zirconia powder and 10 gram (1 wt%) yttrium oxide was added to it. The mix was homogenized for another 15 minutes and dried in an oven at 110°C for 10 hours to bring down the moisture content to 15 wt. %. The mixture was then extruded in screw extruder where the diameter of the extruded mass was 6 mm. The extruded mass was then dried in a hot oven at 110°C for 5 hours. The dried mass was then crushed and graded where 1-3 mm size particles were taken for subsequent processing. The material is then sintered in muffle furnace at 1600 °C for 5 hours. The properties of the sintered product are summarized in Table 1.

From the above table, it is clearly evident that zirconia alumina composites comprising more than 70 wt % of zirconia content exhibits enhanced fracture toughness over composites with lesser content of zirconia.
While only few specific descriptions have been made in this present disclosure, it is understood that various modifications will be apparent to those skilled in the art and such modifications and changes can be made without departing from the scope and sprit of the present invention.

Wc Claim:
1. A zirconia alumina composite comprising 70 to 90 wt. % of zirconia, 10 to 30 wt. % of alumina, a zirconia stabilizer, and additives.
2. The composite as claimed in claim 1 wherein the alumina is selected from aluminium salt, aluminium hydroxide, boehmite calcined alumina from Bayer's process, reactive alumina, or combination thereof.
3. The composite as claimed in claim 1 wherein the zirconia is selected from monoclinic zirconia powder, zirconium hydroxide, zirconium nitrates, zirconium oxy chloride or combination thereof.
4. The composite as claimed in claim 1 wherein the amount of zirconia stabilizer is from 1 wt. % to 8 wt. % of the total weight of the zirconia content present in the composite.
5. The composite as claimed in claim 1 wherein the zirconia stabilizer is selected from yttrium oxide, cerium oxide, scandium oxide, magnesium oxide or combinations thereof.
6. The composite as claimed in claim 1 wherein the amount of additive is from 0.001 wt. % to 20 wt. % of the total weight of the alumina content present in the composite.
7. The composite as claimed in claim 1 wherein the additive is selected from rare earth oxides such as lanthanum oxide, neodymium oxides, cerium oxides, or combination thereof.
8. A process for the preparation of sintered zirconia alumina composites comprising 70 to 90 wt. % of zirconia, 10 to 30 wt. % of alumina, a zirconia stabilizer, and additives comprising the steps of:
(a) milling of zirconia powder and alumina powder to obtain a homogenous mixture;
(b) adding additives and stabilizer to the mixture obtained in step (a) and milling
together for homogeneity to obtain a slurry;

(c) drying the slurry followed by grinding and screening to obtain powder of desired particle size;
(d) compacting the powder from step (c) followed by extruding and drying the compacted mass;
(e) sintering the dry compacted mass to obtain the sintered zirconia alumina composite.

9. The process as claimed in claim 8, wherein the milling is done either in dry or wet conditions.
10. The process as claimed in claim 9. wherein the milling is carried out under wet conditions with a powder to water in a ratio of 3:1 to 1:3.
11. The process as claimed in claim 8, wherein prior to step (d), 0.01 to 3.0 wt % of an organic binder is added.
12. The composite as claimed in claim 11 wherein the organic binder is selected from polyvinyl alcohol (PVA), polyethylene glycol, or combinations thereof.
13. The process as claimed in claim 8, wherein the dried and compacted mass obtained in step (d) of the process has a moisture content of 2.0 to 6.0 wt%.
14. The process as claimed in claim 8, wherein the sintering is done at temperature ranging from 1350 °C to 1650 °C with a dwelling time of 20 minutes to 5 hours.
15. The process as claimed in claim 8, wherein the prior to step (e), the dried material is calcined at temperature ranging from 400 °C to 700°C for 1 to 4 hours.