Field of Invention:
The present invention relates to a process for the development of water impervious ceramic particulate composite composed of zirconia toughened alumina (ZTA). The invention also relates to ZTA ceramic particulate composite wherein alumina is present as a ceramic matrix and zirconia is dispersed therein. Said ceramic particulate composite possesses good mechanical properties such as high fracture toughness and flexural strength and is useful in high impact applications.
Background of the Invention:
Dense sintered alumina ceramic is one of the widely used materials. It has varied structural applications but not limited, as cutting tools, and in many medical applications. When compared with metal, it demonstrates excellent properties such as hardness, wear resistance, heat resistance and corrosion resistance. Fine grained alumina ceramic is found to have high strength though the fracture toughness is limited due to the crack propagation. Fracture toughness, or crack initiation toughness, describes the resistance of a material against the onset of crack propagation. Bending strength is the maximum mechanical stress a material resists without breaking while bending. Impact resistance is the ability of a material to withstand a high force of shock applied to it over a short period of time.
In order to use the alumina ceramic in severe conditions, it is required to improve the fracture toughness, the bending strength and impact resistance of the alumina ceramic. Addition of zirconia into alumina matrix has been found to increase the mechanical strength and fracture toughness of the alumina ceramic material sufficiently because of the following properties of zirconia.
Zirconia (Zr02) is the oxide of zirconium which exists in three phases such as monoclinic at a temperature <1170°C, tetragonal at a temperature between 1,170-2,370°C and cubic at a temperature >2,370°C. The trend is for higher symmetry at

higher temperatures. Zirconia adopts a monoclinic crystal structure at room temperature and transitions to tetragonal and cubic at higher temperatures. The change of volume caused by the structure transitions from tetragonal to monoclinic induces large stresses, causing it to crack open upon cooling from high temperatures. Cooling to the monoclinic phase after sintering causes a large volume change, which often causes stress fracture in pure zirconia. Additives such as magnesia, calcia, yttria are used to stabilize the high-temperature phases and minimize this volume change. Small cracks allow phase transformations to occur, which essentially close the cracks and prevent catastrophic failure, resulting in a relatively tough ceramic material.
Zirconia is often more useful in its phase 'stabilized' state. Upon heating, zirconia undergoes disruptive phase changes. By adding small percentages of yttria, these phase changes are eliminated, and the resulting material has superior thermal, mechanical, and electrical properties. In some cases, the tetragonal phase can be metastable. If sufficient quantities of the metastable tetragonal phase is present, then an applied stress, magnified by the stress concentration at a crack tip, can cause the tetragonal phase to convert to monoclinic, with the associated volume expansion. This phase transformation can then put the crack into compression, retarding its growth, and enhancing the fracture toughness. This mechanism is known as transformation toughening, and significantly extends the reliability and lifetime of products made with stabilized zirconia.
Thus the dispersed meta-stabilized tetragonal zirconia while dispersing in the alumina ceramic matrix transforms into monoclinic zirconia with a volume expansion of 3- 4% and stops the crack propagation by stress induction mechanism.
The inventors of the present invention have identified a commercially viable cost-effective process for the production of such ceramic particulate composite using simple ceramic processing methods which includes die pressing, iso pressing, injection moulding, extrusion etc.

Process for the production of ZTA ceramic composite employing costly processes such as co-precipitation for production of ceramic powder granules and hot-pressing for densification are disclosed in prior art. Some of them are listed as follows:
US4218253 illustrates development of a toughened ceramic product by wet milling and hand granulation, followed by hot pressing and/ or hot isostatic pressing. The matrix consisting of at least one member of the group consisting of silicon carbide, titanium carbide, niobium carbide, tungsten carbide, silicon nitride, titanium nitride and alumina, wherein the individual particles of monoclinic zirconia are being uniformly distributed wherein the particle size of zirconia particles vary from 0.5 urn to 2 urn wherein said material contains 2.5 to 20 vol%.
US4298385 discloses a ceramic body with high toughness and improved resistance to temperature, wherein the un- stabilized zirconia particle of 0.3 urn- 1.25 urn size at a volume percentage of 4 to 25 is being dispersed in alumina matrix. The final sintered body is prepared by hot pressing of the blended powder.
US4316194 uses the principle of martensitic- type transformation of tetragonal zirconia in order to increase the toughness of alumina- zirconia composite. A second phase dopant of Y203/ Ce02/ Er203/ La203 is being added to the batch for achieving the metastability of the tetragonal phase of zirconia grains. The diameter of zirconia grain size is typically around 2 urn to maintain the metastable tetragonal phase of zirconia. The second phase dopant is being introduced as the nitrate precursor of the said oxide. The final powder is being prepared by calcining the mixture at 400°C. Final sintered body has been achieved by hot pressing the powder compact.
US4666467 discloses the use of A1203 and/or MgAl204 spinel to improve the sintered properties of the alumina- zirconia based particulate composite. The initial powder is being prepared with hand granulation, followed by hot pressing the same into a desired shape with a relative density of not less than 90%. The product said to have a good strength and can be deployed for using as cutting tools and ceramic dies.
US503255 describes a co- precipitation method where in the tetragonal stabilization of zirconia is being achieved at powder stage. Oxychloride or nitrate precursors of the

alumina, zirconia and yttria salt has been used in the process using ammonia and/or isopropane as precipitation base.
US5183610 discloses an invention of manufacturing an alumina- zirconia body where in the powder has been prepared by regular comminution process. In order to increase the toughness of the batch, Cr203 has been added as a solid solution agent in alumina. A composite material of alumina- zirconia has been prepared using 10%- 12% ceria as the tetragonal stabilizing agent. The sub-micron sized alumina grains are dispersed in the Zr02 matrix at 40 to 70 vol%. The said zirconia and alumina powder is prepared from an aqueous solution of zirconium and aluminium salt. The alumina powder comprised of high surface area 9 and y being the phase constituents of Alumina.
In addition to the high cost involved in the processes disclosed above, the yield of ceramic powder granules produced by conventional known processes such as co-precipitation and hand granulation is also comparatively very low.
Therefore there exists a need in the industry for a suitable process which generates ceramic composite in a commercially viable cost-effective manner without compromising the mechanical strength and fracture toughness of the generated ceramic composite. There is still a requirement in the industry for a process which employs simple straight forward method for producing the zirconia toughened alumina powder, which is free flowing in nature. There is also a requirement for a commercially viable process which yields a homogeneous micro structure without any presence of glass phase which can be used for producing high volume zirconia toughened sintered product.
Objective of the Invention:
Accordingly the objective of the present invention is to develop a commercially viable, cost effective process for making free flowing zirconia toughened alumina powder which can be processed by all the standard ceramic processing methods. Another objective of the present invention is to stabilize the zirconia at metastable tetragonal phase by simple mechano - chemical reaction and not involving any co -

precipitation or chemical route and without using any organic or inorganic precursors of the said oxides or stabilizing dopant element.
Yet another objective of the present invention is to develop a compact, water impervious ZTA ceramic composite having a homogeneous micro structure with a good distribution of zirconia grains in alumina matrix without the presence of any glass phase in the micro structure. Said homogeneous micro structure also possesses a better crack pinning mechanism conferred to it by a simple sintering mechanism.
Summary of the Invention:
In one aspect of the invention, there is provided a cost effective and a commercially viable process for production of ceramic composite of ZTA which employs simple milling and powder atomization process for the generation of free flowing zirconia toughened alumina powder.
In another aspect, the present invention provides a process for stabilizing the zirconia at metastable tetragonal phase by simple mechano - chemical reaction and not involving any co - precipitation or chemical route and without using any organic or inorganic precursors of the said oxides or stabilizing dopant element
In yet another aspect, the present invention provides a compact, water impervious ZTA ceramic composite having a homogeneous micro structure with a good distribution of zirconia grains in alumina matrix without the presence of any glass phase in the micro structure. Said homogeneous microstructure wherein the zirconia grain size is higher than that of alumina grain size possesses a better crack pinning mechanism conferred to it by a simple sintering process. Said homogeneous microstructure also possesses higher fracture toughness conferred to it by a larger zirconia grain size.
The ZTA product prepared by the process of present invention can be applied for wear-resistance, corrosion-resistance & impact-resistant application such as abrasive material conveying, making of pump components, ceramic armour etc.

In one embodiment, the present invention discloses a process for the production of composite of ZTA comprising the steps of
a) preparing zirconia powder partially stabilized at tetragonal phase
b) mixing of partially stabilized zirconia powder, alumina and water to form a slurry
c) drying the slurry to yield free flowing granules
d) forming the green ZTA ceramic bodies by pressing free flowing granules and
e) sintering the green ZTA ceramic bodies at a temperature ranging from 1250°Cto 1750°C.
Zirconia in said process may be stabilized in tetragonal phase with a stabilizing agent
o o
by means of a mechano-chemical reaction at a temperature of 25 C to 300 C.
In an embodiment, this invention discloses a process for the production of composite of ZTA, comprising the steps of
a) mixing of monoclinic fused zirconia powder, alumina, stabilizing agent and water to form a slurry
c) drying the slurry to yield free flowing granules
d) forming the green ZTA ceramic bodies by pressing free flowing granules and
e) sintering the green ZTA ceramic bodies at a temperature ranging from
1250°Cto 1750°C
wherein zirconia is stabilized at tetragonal phase in -situ during sintering.
The stabilizing agent as per the invention may comprise Yttria (Y203), Magnesia (MgO), Calcia (CaO), Ceria (Ce02/Ce203), Lanthanum oxide (La203), Scandium oxide (Sc203) and Europium oxide (Eu203).
The above said process may further involve addition of organic/inorganic deflocculants, defoaming agents & organic temporary binders to make the slurry.

The organic/inorganic deflocculants may comprise Darvan (Ammonium polymeric monomer), Dolapix (Synthetic polyelectrolyte), Tetron, Na-hexametaphosphate and Borresperse (Lignosulphonate) or combination of these materials.
The defoaming agent as per the invention may be selected from Contraspum, Nalco etc.
The organic temporary binder may be selected from polyacrylate, polymethacrylate, polyvinyl alcohol, polyethylene glycol, polyethylene oxide and a mixture thereof.
The drying step of the process may involve any known granulation process such as filter pressing followed by granulation through sieving, pan granulation, freeze drying etc.
The forming step may involve one or more conventional ceramic processing method such as uniaxial pressing, biaxial pressing, iso-static pressing, ceramic injection moulding, extrusion, tape casting and slurry casting.
As per the invention, 80 to 100% of zirconia may be present as tetragonal form of crystals upon sintering and 0 to 20% zirconia is present as monoclinic and/or cubic phases.
The stabilizing agent for stabilizing zirconia may be present in the range of
Y203: 0.5 to 8 mol%
MgO: 0.5 to 15 mol%
Ce02: 0.5 to 20 mol%
La203: 0.5 to 20 mol%
Sc203: 0.5 to 8 mol%
Eu203: 0.5 to 9.5 mol%.
In an alternate embodiment the stabilizing agent for stabilizing zirconia is present in the range of

Y203:2to8mol%
MgO: 5 to 15 mol%
Ce02: 5 to 20 mol%
La203:2 to 20 mol%
Sc203:lto8mol%
Eu203:2 to 9.5 mol%.
In another embodiment the present invention relates to a composite of ZTA, wherein alumina is present as a matrix and zirconia is dispersed therein, wherein zirconia is present in the form of crystalline grains in alumina matrix in the range of 0.5 wt% to 65 wt% and alumina in the range of 35 wt% to 99.5%
In one embodiment the grain size of zirconia may be larger than that of alumina grains.
Said composite may have a flexural strength of more than 400 MPa and a fracture toughness of more than 4.5 MPa.m0 5
Brief description of Figures:
The following detailed description of the invention will be better understood when read in conjunction with the appended drawings. For the purpose of assisting in the explanation of the invention, there are shown in the drawings embodiments which are presently preferred and considered illustrative. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown therein.
Figure 1: Micro structure of ZTA composite wherein the grain size of zirconia is higher than that of alumina.
Figure 2a: Increment of fracture toughness with addition of zirconia.

Figure 2b: Increment of strength with addition of zirconia.
Figure 3: c- DTA graph of zirconia- yttria mixture signifying the transformation of monoclinic to tetragonal phase and crystallographic orientation of the formed tetragonal phase.
Figure 4: Sintering Curve of ZTA demonstrating the sintering (shrinkage with temperature) behaviour of Alumina - Zr02 mixture.
Figure 5: XRD of yttria stabilized zirconia powder prepared with mechano-chemical reaction.
Detailed description of the invention:
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 expressly specified to the contrary.
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 skill in the art to which this invention pertains. In the case of conflict, the present document, including definitions will control.
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 are to be understood to be open-ended, i.e., to mean including but not limited to.
The following definitions are used in connection with the present application unless the context indicates otherwise.
Fracture Toughness: It is a property which describes the ability of a material to resist fracture. It is a quantitative way of expressing a material's resistance to brittle fracture when a crack is present.
Composite: A composite material is a material made from two or more constituent materials with significantly different physical or chemical properties that when combined, produce a material with characteristics different from the individual components.
The present invention relates to a process of production of ZTA ceramic composite wherein the ceramic granules are being formed from the fine grained alumina and fused/ chemical grade of monoclinic zirconia
In one of the embodiment of the present invention, in the process of production of ZTA ceramic composite, the stabilization of zirconia at metastable tetragonal phase is carried out by a mechano- chemical reaction wherein fused zirconia is mixed with a stabilizing agent and the mechano-chemical reaction is performed at a temperature of 25°C to 300°C.
In another embodiment of the above mentioned process of production of ZTA ceramic composite the stabilization of zirconia at metastable tetragonal phase is carried out in-situ during the sintering process. Said process comprises, mixing of the constituents or raw materials such as alumina, fused monoclinic zirconia, yttria and magnesia in a ball or tube mill together with water as solvent to form slurry. Subsequently the slurry is dried using any known granulation process such as spray-drying, filter pressing

followed by granulation through sieving, pan granulation, freeze drying etc, to achieve free flowing powder granules. The powder granules are then consolidated into green ZTA bodies by any conventional ceramic processing method such as uniaxial pressing, biaxial pressing or iso-static pressing or other ceramic forming process such as ceramic injection moulding, extrusion, tape casting or slurry casting. The consolidated green ZTA bodies can then be sintered at a temperature ranging from 1250°C to 1750°C to achieve highly densified ceramic bodies with sub- micron sized alumina grains and zirconia grains with size ranging from 1 urn - 5 urn. A small amount of MgO may be added in the form of precipitated MgO or Mg(OH)2 or MgC03, in order to decrease the vapour pressure for sintering and also to inhibit the grain growth of alumina.
The stabilizing agent employed for the stabilization of zirconia at metastable tetragonal phase is selected from Yttria (Y203), Magnesia (MgO), Calcia (CaO), Ceria (Ce02/Ce203), Lanthanum oxide (La203), Scandium oxide (Sc203), Europium oxide (Eu203).
The stabilizing agent employed for the stabilization of zirconia at metastable tetragonal phase is also selected from an admixture of two or more compounds selected from oxides such as Yttria (Y203), Magnesia (MgO), Calcia (CaO), Ceria (Ce02/Ce203), Lanthanum oxide (La203), Scandium oxide (Sc203) and Europium oxide (Eu203).
The above said process may involve further addition of organic/inorganic deflocculants, defoaming agents and organic temporary binders to form the slurry.
The organic/inorganic deflocculants employed in the above mentioned process of production of ZTA ceramic composite may be selected from Darvan (Ammonium polymeric monomer), Dolapix (Synthetic polyelectrolyte), Tetron, Na-hexametaphosphate, Borresperse (Lignosulphonate) etc.
The defoaming agents employed in the above mentioned process of production of ZTA ceramic composite may be selected from Contraspum, Nalco etc.

The organic temporary binders employed in the above mentioned process of production of ZTA ceramic composite may be selected from polyacrylate, polymethacrylate, polyvinyl alcohol, polyethylene glycol, polyethylene oxide and a mixture thereof.
The present invention is also related to a composite of ZTA, wherein alumina may be present as a matrix constituting 35 wt% to 99.5 wt% of the total content and zirconia may be dispersed therein in the form of crystalline grains in the range of 0.5 wt% to 65 wt%.
The composite of ZTA comprising 80 to 100% of zirconia present as tetragonal form of crystals upon sintering while the remaining 0 to 20% may be present as monoclinic and/or cubic phases.
The composite of ZTA wherein the stabilizing agent for stabilizing zirconia is present in the range of Y203: 0.5 to 8 mol% preferably in the range of 2 to 8 mol%, MgO: 0.5 to 15 mol% preferably in the range of 5 to 15 mol%, Ce02: 0.5 to 20 mol%, preferably in the range of 5 to 20 mol% , La203: 0.5 to 20 mol% preferably in the range of 2 to 20 mol%, Sc203: 0.5 to 8 mol% preferably in the range of 1 to 8 mol% , Eu203: 0.5 to 9.5 mol%, preferably in the range of 2 to 9.5 mol%.
The composite of ZTA as described above has a flexural strength of more than 400 MPa and a fracture toughness of more than 4.5 MPa.m0 5
In another embodiment, the present invention also discloses a ZTA composite comprising zirconia present in amounts of 1 to 20 volume percent and alumina in amounts of 80 to 99 volume percent along with the stabilizing agent, yttria and ceria which is present at 1 to 6 volume percent of zirconia.
In yet another embodiment, the ZTA composite contains the grain size of zirconia which is larger than that of grain size of alumina.
For purpose of further illustrations, the process of invention and product obtained thereby are illustrated in the following specific examples. These examples are

considered to be illustrative only and are not intended to limit the scope and content of the invention or obvious variations there over.
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:
Example 1:
94.65 wt% of monoclinic fused zirconia powder with primary particle size of 5 urn was ball milled with 5.35 wt% of Yttria using water as a solution. The resultant slurry was dried in microwave. The powder was prepared through granulation subjected to passing through #30 mesh. The resultant powder was calcined at a temperature 1300°C. The XRD confirms the resultant phase as tetragonal stabilized zirconia. 2 vol% of the calcined zirconia- yttria powder was milled with 98 vol% submicron grained alumina powder having a primary particle size of 0.5 urn. MgO in a very low amount was being used as a sintering aid. The milling was done using water as a solvent and dispersing agent. Polyethylene based binder was being added upon achieving the particle size of d50 not more than 0.7 urn. The slurry was prepared with high solid content of 70%. The slurry has been spray dried by a counter current spray drier. The process yields free flowing spherical granules with a PBD of 1.18 gm/cc. The resultant spray dried granules were pressed at a GBD of 2.30 gm/cc and the same was fired at a temperature of 1565°C. The sintered article thus produced yields a micro structure wherein the zirconia grains were homogeneously distributed in the sub-micron grained alumina matrix. The zirconia grain size was around 2 urn- 6 urn. The final sintered product was having a fracture toughness of 6.20 MPa.m0 5 and strength of480MPa.
Example 2:

The tetragonal zirconia as formed by mechano-chemical reaction as per example 1 has been used here. 7 vol% of the same was being added with 93 vol% of sub-micron alumina with a primary particle size of 0.5 urn and a small amount of MgO as sintering aid. The further processing step was same as example 1. The sintered articles thus produce a similar micro structure as per example 1 with a higher amount of zirconia distribution in matrix. The final sintered article was having a fracture toughness of 6.70 MPa.m0 5 and flexural strength of 540 MPa.
Example 3:
The tetragonal zirconia as formed by mechano-chemical reaction as per example 1 and 2 has been used here. 10 vol% of the same was being added with 90 vol% of sub-micron alumina with a primary particle size of 0.5 urn and a small amount of MgO as sintering aid. The further processing step was same as example 1 and example 2. The sintered articles thus produce a similar micro structure as per example 1 and 2 with a higher amount of zirconia distribution in matrix. The final sintered article was having a fracture toughness of 7.10 MPa.m0 5 and flexural strength of 590 MPa.
Example 4:
The tetragonal zirconia as formed by mechano-chemical reaction as per example 1, 2 and 3 has been used here. 7 vol% of the same was being added with 93 vol% of alumina with primary particle size of 5 urn and a small amount of MgO as sintering aid. The slurry has been milled in order to achieve particle size of 2.10 urn. Further processing step was same as example 1, 2 and 3. The product has been fired at a temperature of 1650°C. The sintered article thus produces a microstructure with homogeneous distribution of zirconia in alumina matrix. The zirconia and alumina were having almost the same grain size of 6.5 um in this case. The resultant sintered article was having a fracture toughness of 6.00 MPa.m0 5 and flexural strength of 400 MPa.
Example 5:

The tetragonal zirconia as formed by mechano-chemical reaction as per example 1, 2 and 3 has been used here. 14 vol% of the same was being added with 86 vol% of alumina with primary particle size of 5 urn and a small amount of MgO as sintering aid. The slurry has been milled in order to achieve particle size of 2.10 urn. Further processing step was same as example 1, 2, 3 and 4. The product has been fired at a temperature of 1650°C. The sintered article thus produces a micro structure with homogeneous distribution of zirconia in alumina matrix. The zirconia and alumina were having almost the same grain size of 6.5 urn in this case. The resultant sintered article was having a fracture toughness of 6.40 MPa.m0 5 and flexural strength of 450 MPa.
Example 6:
Here the Fused monoclinic zirconia of primary particle size of 5 urn, Yttria, Alumina with primary particle size of 0.7 urn and MgO as sintering aid has been added and ball milled using water as solvent. The Alumina content in the same was 90 vol% and zirconia content was around 10 vol%. Yttria has been added at 6 vol% of total zirconia present in the matrix. The further processing step of making granulated free flowing ceramic powder and consolidating the same into a sintered body was similar to the example of 1, 2 and 3. The resultant sintered product yields a micro structure with homogeneous distribution of zirconia grains of 2- 6 urn in size in alumina matrix. The fracture toughness and mechanical properties were similar to that of example 3.

Table 1 discloses the properties of ZTA composite generated by the process of the present invention. The fracture toughness and strength of the ZTA wherein the grain size of zirconia is greater than that of grain size of alumina is comparatively higher than those of ZTA composite ZTA wherein the grain size of zirconia is same as that of grain size of alumina. YPSZ: Yttria Partially stabilized Zr02

Table 2 exemplifies the cost effectiveness of the process of production of ZTA composite when compared with the process involving co-precipitation in terms of requirement of raw materials for the production. The process of the subject invention employs minimum quantity of raw material to produce the same output (100 kg of ZTA) as compared to co-precipitation process.

WE CLAIM:
1. A process for the production of composite of zirconia toughened alumina (ZTA)
comprising the steps of
a) preparing zirconia powder partially stabilized at tetragonal phase
b) mixing of partially stabilized zirconia powder, alumina and water to form a slurry
c) drying the slurry to yield free flowing granules
d) forming the green ZTA ceramic bodies by pressing free flowing granules and
e) sintering the green ZTA ceramic bodies at a temperature ranging from 1250°Cto 1750°C.

2. The process as claimed in claim 1, wherein zirconia is stabilized in tetragonal phase with a stabilizing agent by means of a mechano-chemical reaction at a temperature of 25°C to 300°C.
3. A process for the production of composite of ZTA, comprising the steps of

a) mixing of monoclinic fused zirconia powder, alumina, stabilizing agent and water to form a slurry
b) drying the slurry to yield free flowing granules

c) forming the green ZTA ceramic bodies by pressing free flowing granules and
d) sintering the green ZTA ceramic bodies at a temperature ranging from 1250°C to 1750°C, wherein zirconia is stabilized at tetragonal phase in -situ during sintering.

4. The process as claimed in claims 1-3, wherein the stabilizing agent is selected from Yttria (Y203), Magnesia (MgO), Calcia (CaO), Ceria (Ce02/Ce203), Lanthanum oxide (La203), Scandium oxide (Sc203) and Europium oxide (Eu203).
5. The process as claimed in claims 1-4, wherein formation of slurry involves addition of organic/inorganic deflocculants, defoaming agents & organic temporary binders.

6. The process as claimed in claims 1-5, wherein the organic/inorganic deflocculant is selected from Darvan, Dolapix, Tetron, Na-hexametaphosphate and Borresperse.
7. The process as claimed in claims 1-6, wherein the defoaming agent is selected from Contraspum and Nalco.
8. The process as claimed in claims 1-7, wherein the organic temporary binder is selected from polyacrylate, polymethacrylate, polyvinyl alcohol, polyethylene glycol, polyethylene oxide and a mixture thereof.
9. The process as claimed in claim 1 -8, wherein the drying step involves spray drying or any known granulation process such as filter pressing followed by granulation through sieving, pan granulation, freeze drying.

10. The process as claimed in claim 1-9, wherein the forming step involves one or more conventional ceramic processing method such as uniaxial pressing, biaxial pressing, iso-static pressing, ceramic injection moulding, extrusion, tape casting and slurry casting.
11. The process as claimed in claims 1-10, wherein 80 to 100% of zirconia is present as tetragonal form of crystals upon sintering and 0 to 20% zirconia is present as monoclinic and/or cubic phases.
12. The process as claimed in claims 1-11, wherein the stabilizing agent for stabilizing zirconia is present in the range of
Y203: 0.5 to 8 mol%
MgO: 0.5 to 15 mol%
Ce02: 0.5 to 20 mol%
La2O3:0.5to20mol%
Sc203: 0.5 to 8 mol%
Eu203: 0.5 to 9.5 mol%.

13. The process as claimed in claims 1-12, wherein the stabilizing agent for stabilizing
zirconia is present in the range of
Y203: 2to8mol%
MgO: 5 to 15 mol%
Ce02: 5to20mol%
La203: 2to20mol%
Sc203: lto8mol%
Eu203: 2to9.5mol%.
14. A composite of ZTA, wherein alumina is present as a matrix and zirconia is dispersed therein, wherein zirconia is present in the form of crystalline grains in alumina matrix in the range of 0.5 wt% to 65 wt%.
15. The composite as claimed in claim 14 wherein alumina constitutes 35 wt% to 99.5 wt% of the total content while the zirconia constitutes 0.5 wt% to 65wt%.
16. The composite as claimed in claims 14-15, wherein the grain size of zirconia may be larger than that of alumina grains.
17. The composite as claimed in claims 14-16, having a flexural strength of more than 400 MPa and a fracture toughness of more than 4.5 MPa.m0 5