FIELD OF THE INVENTION:
The present invention relates to a novel method for synthesis of abrasive grains with improved hardness and toughness or more particularly a method of fabricating a high resistant abrasive grains with improved interfacial strength.
BACKGROUND OF THE INVENTION:
Wear is one of the most commonly encountered industrial problems in coal fired power stations, where coal is ground to powder in impact mills by grinding rolls. The major concern of the grinding rolls during the course of coal pulverization is its "harsh abrasive action with the coal,, due to higher ash and silica (Quartz) content present in the coal. Silica has the Vickers hardness of > 1100 Hv, (due to this, surface of the grinding rolls (which has hardness of around 600-700 Hv) gets worn out faster and it needs to be replaced frequently.
In addition to this, wear resistance components are also subjected to high mechanical stresses. It is therefore, desirable that, these components should exhibit a high abrasion resistance as well as some ductility, to withstand against the mechanical stresses such as impacts. Hence, it is critical to develop a composite material showing a good balance between the resistance to the abrasion and the impacts.
The major concern of the grinding rolls during the course of coal pulverization is its “harsh abrasive action with the coal” due to higher ash and silica (Quartz) content present in the coal. Silica has the Vickers hardness of > 1100 Hv, due to this, surface of the grinding rolls (which has hardness of around 600-700 Hv) gets worn out faster and it needs to be replaced frequently
The life of grinding rolls can be enhanced by reinforcement of hard particles having some ductility in the metal matrix. once the hardness of particles is higher than the quartz, then it will not be adversely affected by the quartz content of the ash as compared with those of grinding elements composed of materials softer than quartz.

It is known that, ceramic materials have high hardness, wear resistance, and toughness (i.e., fracture resistance) and can be employed for improving the abrasion resistance of wear components. However, for the grinding media, it is desirable to develop the ceramic material having a good balance of hardness and toughness. Traditional materials generally meet one or the other of these types of requirement but are very rarely resistant to both impact and abrasion indeed, ductile materials offer enhanced resistance to impact but have very little resistance to abrasion. On the other hand, hard abrasion-resistant materials have very little resistance to violent impact.
In this regard, document (U.S. Patent No RE39, 998 E issued Jan'2008) discloses a method where a combination of Al2O3 and ZrO2 is proposed by a judicious choice to adjust the hardness and the toughness of ceramic composites. Al2O3 contributes to good hardness however ZrO2 present in the alumina make it possible to increase the resistance of the latter to cracking and thus to obtain a toughness greater than that of each of the components considered in isolation.
Document (US. Pat. No. 3,181,939 issued May 1965) also discloses a method for manufacturing of fused Al2O3 and ZrO2 abrasives, which combine good wear resistance characteristics of Al2O3: and the toughness of ZrO2 and can be considered as a suitable candidate for metal matrix ceramic composites with improved wear resistant. Robert A. Rowse et. al, (US. Pat. No. 3,891,408 issued June, 1975) also discloses Al2O3 and ZrO2 type electro-molten abrasives with ZrO2 content of between 35 and 50%. They have reported that, by modifying the cooling conditions for these abrasives, and by employing a near-eutectic composition of Al2O3 and ZrO2, an entirely new family of abrasives can be produced having desirable hardness.
The document U.S. Pat. No 201310126649 A1 issued in May, 2013 also discloses a method where working face of the segment is reinforced by granules (size 0.5 to 5 mm) of a ceramic composite of 57 Wt % Al2O3 and 43 Wt % ZrO2. To fabricate the wear surface, compacted insert made of granules is placed in

the mould before the segment is cast at about 1500°C. During the cast operation, the ceramic insert is impregnated by the liquid cast metal and the ceramic particles are completely embedded in the metal matrix.
To fabricate the abrasive grains having higher hardness and toughness ceramic co-fused method is one of the attractive solution. The document U.S. Pat. No 4,3l4,827issued in Fab, 1982 discloses a process where Co-fused blends are typically prepared by heating a mixture of Al2O3 and ZrO2 precursor above 1800°C, where mixture is fused to a molten state in an arc tapping furnace and the fused product is quickly cooled. The fabricated abrasive grains has a Knoop hardness value (500 gram load) of about 1800 kg per /mm2.
Another attractive method to fabricate the abrasive grains is sol-gel. The document U.S. pat. No 4,744,802 issued in May, 1988 discloses the formation of an alumina-based ceramic particularly useful as abrasive grain, by a sol-gel process. However, the process of fabricating abrasive grains by sol-gel is very tedious and involved lot of steps like, preparing a dispersion, gelling the dispersion, drying the gelled dispersion to form a solid, calcining of the solid and finally sintering of the solid.
There are many other processing routes for producing abrasive ceramic grains, including tape casting, slip casting, uniaxial pressing, isostatic pressing, injection molding and extrusion. The selection of sintering method depends on many factors, such as the characteristics of the raw materials, economic considerations and the desired properties of the final product.
Even though aforesaid varieties for fabrication of abrasive grains are available, still there is a need to develop abrasive grain in a relatively inexpensive way with improved hardness and toughness. In addition to this, a method is required to enhance the interface strength (improved bonding) between abrasive ceramic grains and high chromium cast iron' According to the literature, during pouring of molten metal a crack always exists at the interface, because poor wetting between ceramic grains and liquid iron makes it easier for gas to segregate the surface of Al2O3 / ZrO2 composite grains. Also the difference in

heat expansion coefficient of ceramic grains and molten metal cause gaps on the interface, which leads to easy cleavage of abrasive grains during grinding process.
But the present invention meets the long-felt need by overcoming the drawbacks of the prior arts.
SUMMARY OF THE INVENTION:
A method for fabrication of the abrasive grains comprises the steps of: -preparing of slurry by mixing of Aluminum oxide (Al2O3), Yttria stabilized (Y2O3)-zirconium oxide (ZrO2) or [YSZ] and titanium diboride (TiB2) in deionized water (DI) water along with binder and dispersants; - mixing of small amount (provide the amount) of TiB2 for improving hardness; - stirring of the mixture for 10 minutes and the suspension is stirred for a suitable period for producing a homogenous dispersion and ceramic powder were added along with binders with magnetic stirring at a suitable speed for suitable time; - degassing for 10 minutes and sedimentation test was performed to have a desired pH value and viscosity measurement; -pouring of stabilized slurry in mould and a casted green specimen was dried in open environment after absorption of water for a suitable duration at a suitable temperature; -sintering was carried out at a preferred range of temperature; -crushing the grains for obtaining a suitable size and substantial hardness coating of abrasive grains with cobalt tungsten carbide alloy and poly-vinyl alcohol (PVA); -placing the abrasive grains into pop mould for achieving desired patterns.
OBJECTS OF THE INVENTION:
It is therefore, the primary object of the present invention to provide a method for synthesis of abrasive grain with substantially enhanced hardness, toughness and high wear resistance.
Another object of the present invention to provide a method for synthesis of abrasive grain, where the final hardness of the product depends on solid loading, dispersed concentration, viscosity, pH value, particle size and other

processing parameters and the parameters are optimized to get the higher hardness i.e., 1600Hv alongwith desired toughness.
Yet another object of the present invention to provide a process which enhances the interface strength between abrasive grains and high chromium cast iron.
A further object of the present invention is to fabricate the high resistant component, using developed abrasive grains to enhance the life of grinding rolls.
Another object of the present invention to provide a method for synthesis of abrasive grain where the slurry is prepared in DI water (give full form) and have 70% loading.
Further object of the present invention to provide a method for synthesis of abrasive grain, where eliminates the frequent replacement of the grinding rolls in coal industry.
Yet another object of the present invention to provide a method for synthesis of abrasive grain, where fabrications takes place with the principle of slip casting method.
Another object of the present invention to provide a method for synthesis of abrasive grain, where the abrasive grains exhibits the hardness in the range of 1550-1650 Hv and the amount of TiB2 is preferably between about 0.2 to about 1.5% of the.
Further, object of the present invention to provide a method for synthesis of abrasive grain, where the wear resistant of abrasive grains based high resistant component is three-four time higher than the simple high chromium cast iron component.
Another object of the present invention to provide a method for synthesis of abrasive grain, which is simple yet effective economic.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING:

It is to be noted, however, that the appended drawings illustrate only typical embodiments of the present subject matter and are therefore not to be considered for limiting of its scope, for the invention may admit to other equally effective embodiments. The detailed description is described with reference to the accompanying figures. Some embodiments of system or methods in accordance with embodiments of the present subject matter are now described, by way of example, and with reference to the accompanying figures, in which:
Figure 1 illustrates flowchart for the preparation of A1203, YSZ, and TiB2 composite slurry and abrasive grains.
Figure 2 illustrates particle size distribution of A12O3 powder with respect to number %. It is evident that most of particles (~95%) are finer than 1 µm.
Figure 3 illustrates the viscosity of 70% loading (A12O3 + YSZ) slip as a function of pH at a shear rate of 100 s-l and the Lower viscosity (~ 0.2Pa.s) i.e, observed in the pH range of 2-4 and 9-10.
Figure 4 illustrates optical micrograph of Vickers indentation on the surface of abrasive grains, fabricated by slip casting process.
Figure 5 illustrates Cross-section of the high resistant component, showing the distribution of Co-WC coated abrasive grains inside the metal matrix.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS:
The present subject matter relates to a method for synthesis of abrasive grains with superior hardness and toughness.
The present invention utilizes Aluminium Oxide (Al2O3), Yttria Stabilized (Y2O3) – Zirconium Oxide (ZrO2) [YSZ] and Titanium diboride (TiB2) as abrasive grains where the desired hardness is around 1650HV and toughness of around 5.0MPam1/2.
The present invention, discloses slip casting process to fabricate the abrasive grains (A12O3 + YSZ + TiB2) with hardness of around 1650 Hv and toughness of around 5.0 MPam l/2 The desire hardness and toughness of abrasive grains is achieved at 70% loading, dispersant percentage of 0.2-0.6 wt%, viscosity

0.2Pa.s and pH of around 2-4. For the improved bonding of abrasive grains inside the high chromium cast iron matrix, the abrasive grains are coated with 12% Co-WC. The Co-WC alloy improve the wetting as well as accommodate difference in heat expansion coefficient between abrasive grains and high chromium cast iron, which minimize cleavage during grinding process. The wear resistant of abrasive grains based high resistant component is three-four time higher than the simple high chromium cast iron component, which can enhance the life of grinding object.
In accordance with one embodiment of the present invention, the process is relatively inexpensive fabrication for preparing the abrasive grains with improved hardness and toughness.
A method for fabrication of the abrasive grains comprises the steps of:
-preparing of slurry by mixing of Aluminum oxide (Al2O3), Yttria stabilized (Y2O3)-zirconium oxide (ZrO2) or [YSZ] and titanium diboride (TiB2) in DI water alongwith binder and dispersants;
- mixing of small amount (provide the amount) of TiB2 for improving
hardness;
-stirring of the mixture for 10 minutes and the suspension is stirred for a suitable period for producing a homogenous dispersion and ceramic powder were added alongwith binders (give details) with magnetic stirring at a speed of 1000rpm for 24 hours;
- degassing for 10 minutes and sedimentation test was performed to have
a desired pH value and viscosity measurement;
-pouring of stabilized slurry in mould and a casted green specimen was dried in open environment after absorption of water for a 6 hours at a temperature of 110°C;
-sintering was carried out at a preferred range of temperature;

-crushing the grains for obtaining a suitable size and substantial hardness coating of abrasive grains with cobalt tungsten carbide alloy and poly-vinyl alcohol (PVA);
-placing the abrasive grains into pop mould for achieving desired patterns.
The percentage of Al2O3 , YSZ amd TiB2 in the slurry are given below:
Al2O3 – 40 to 50%
YSZ – 15 to 20%
TiB2 – 0.2 to 1.5%
The Fig 1 illustrates the fabrication process of the abrasive grains.
The novel method is for synthesis of abrasive grains and the final hardness of the products depends on solid loading dispersant concentration, viscosity, pH value, particle size and other processing parameters. In the present invention the parameters are optimized to get the higher hardness > 1600 Hv along with the desired toughness.
The slurry was prepared by mixing of Aluminium Oxide (Al2O3), Yttria Stabilized (Y2O3) – Zirconium Oxide (ZrO2) and Titanium diboride in DI water alongwith a binder or dispersant.
Though Alumimum oxide contributes to good hardness, but it is very brittle in nature. Hence, to overcome the the brittleness, reinforcement of YSZ is introduced into Al2O3 matrix, producing zirconia toughened alumina (ZTA). YSZ increases the toughness by transforming the phase from tetragonal ZrO2 to monocline ZrO2 [ZrO2 (t) to ZrO2 (m)]. In addition to this, the incorporation of Y2O3 also lower the sintering temperature and enhance the sintering density, which leads to enhancement in hardness. After judicious choice of Al2O3 and ZrO2, small amount of TiB2 also added in the mixture, because of its inherent high hardness (~ 34 GPa).

However for improved hardness and toughness, the aqueous suspensions (slurries) should have homogenous dispersion and should fulfil following two important requirements. One is the particles should not be too large otherwise gravity will produce rapid sedimentation. Other important factor is the attractive intra-particle Van der Waals forces among the particles, which should be eliminated. If the attractive force is large enough, the particles will collide and stick together, leading to rapid sedimentation of particle clusters (i.e., to flocculation or coagulation).
Each slip was prepared by first mixing dispersant /binder and water. Then, while stirring, the powders ((Al2O3 + YSZ) were added and the resulting suspension was stirred for 8 hrs. The percentage of YSZ in each loading, range from 15-22 %, more preferably 20%.
To eliminate the attractive intra-particle Van der Waals forces among the particles, initially dispersant is dissolved in DI water properly. In the present invention, dispersant is alkali-free anionic polyelectrolyte (which do not foam) which contained 30 wt% of the ammonium salt of polymethacrylic acid (PMAA-NH4). The percentage of dispersant depend on the loading percentage.
To maintain the homogenous dispersion of slurries, the average particle size of all powders are in the range of 250-300 nm. For example, the average particle size of dispersed alumina powder particles with respect to number percentage is shown in Fig. 2. It can be observed that the maximum number of particles (~ 95%) are finer than 1 μm. In addition to this, 40 % of particles are below 200 nm. In the present invention, the mixture of coarse and fine powders are used to achieve higher density.
Sedimentation test was done to check the dispersion and stability of the slurries. The suspension with 70 % loading was well dispersed (compared to other slurries) and stable at highly acidic pH of 3 in the presence of polymethacrylic acid based dispersant. A slight increase in pH destabilizes the suspension and this is observed up to pH 8.5. Further increase in pH reverses the trend and the suspension is again stable in the alkaline pH range. The pH

was adjusted in the range of 2-3 using 0.25-0.5 ml HCl. The percentage of dispersant in slurry range between 0.2-0.6 wt %, more preferably 0.5 wt%.
Thereafter, the stabilized slurry was poured in the rectangular shape mould and left for some time. The plaster of paris mould absorb the water from the slip by capillary action and then refilled with the slip, till the mould is completely filled. The casted green specimen was dried in open environment for 6 hrs at temperature 110°C. After drying, sintering was carried out between 1350°C-1500°C, preferably at 1400°C. Sintered material is crushed in order to obtain the grains, the size of which varies from 1 to 10 mm, preferably from 1 to 5mm, and more preferably from 1.4 to 2.5 mm.
Further, TiB2 in slurry is added for substantial enhancement of hardness.
The abrasive grains exhibits the average hardness in the range of 1300-1450 Hv. The amount of TiB2 is preferably between about 0.2 to about 1.5% of the total loading, more preferably between about 0.6 to about 1%. The toughness of abrasive grains is around 5.0MPam1/2, which is calculated by measuring the crack length at corners.
In order to enhance the interface strength (or to accommodate difference in heat expansion coefficient) between abrasive grains and high chromium cast iron, the grains were first coated with cobalt tungsten carbide (12 % Co-WC) alloy to improve the wetting. In the present invention, poly-vinyl alcohol (PVA) is used as an adhesive /binder.
The viscosity of 70 % loading (Al2O3 + YSZ) slip measured as pH as a function at a shear rate of 100 s-1 is presented in Fig. 3. Lower viscosities (~ 0.2Pa.s) are observed in the pH range of 2-4 and 9-10. This shows that the powders are fairly well dispersed in this pH range. Very high viscosities, which is not suitable for castings are observed in the pH range of 6-8.
In general, viscosity and instability of slurry increase with increasing solids content. In the present invention, better dispersion (minimization of attractive van der Waals) and lower viscosity at higher loading content is achieved using

polymethacrylic acid based dispersant. During the slurry dispersion methacrylate poly-ions tend to be adsorbed on the surface particles, which create negative charge around the particles. The excess of the positive ions (NH4+ and H+) in the solution then surround the diffuse layer due to electrostatic attraction. The particles being negatively charged, repel each other, which reduce the viscosity and stabilize the slurry, which leads to homogenous density in cast product.
In accordance to the another embodiment of the present invention, for enhancing the interface strength between abrasive grains and high chromium cast iron, the grains were first coated with cobalt tungsten carbide (12 % Co-WC) alloy to improve the wetting. In the present invention, poly-vinyl alcohol (PVA) is used as an adhesive. The tungsten carbide (WC) particles are selected for reinforcement, not only because of their high hardness, but also because of their ability to be entirely wetted by molten iron. The presence of Co further improve the wettability (as shown in Fig 5). It is observed that, the thin spinel layer of 12 % Co-WC has positive effect on bonding.
In the present invention, working face of high resistant component is reinforced in depth by abrasive grains of hardness 1650 Hv. The thickness of the reinforced layer is generally higher than 25 mm, preferably higher than 30 mm and generally situated between 40 and 45mm. To see the infiltration of metal into ceramic phase, the fabricated segment is cut into many parts. The cross section of the segment is shown in Fig.5. From the figure, it is evident that, ceramic grains are completely embedded in the metal matrix with improved bonding. The wear resistant of abrasive grains based high resistant component is three time higher than the simple high chromium cast iron component. The high resistant component embedded with abrasive grains can improve the life of grinding rolls.
The non-limiting advantages are given below:
1. YSZ increases the toughness by transforming the phase from tetragonal ZrO2 to monocline ZrO2 [ZrO2 (t) to ZrO2 (m)].

2. Average particle size of powders range from 250-400 nm.
3. Stabilized by a dispersant which contained 30 wt% of the ammonium salt of polymethacrylic acid (PMAA-NH4).
4. Crushed in order to obtain the grains, the size of which varies from 1 to 10 mm, preferably from 1 to 5 mm, and more preferably from 1.4 to 2.5 mm.
5. Exhibits the hardness in the range of 1550-1650 Hv.
6. Exhibits the toughness of around 5.0 MPam1/2.
7. Used to fabricate the high resistant component, where abrasive grains are embedded in high chromium cast iron.
8. The working face of high resistant component is reinforced in depth by
abrasive grains of hardness 1650 Hv. The thickness of the reinforced layer is
generally higher than 25 mm, preferably higher than 30 mm and generally
situated between 40 and 45mm.
9. the wear resistant of abrasive grains based high resistant component is three
–four time higher than the simple high chromium cast iron component. The
high resistant component embedded with abrasive grains can improve the life of
grinding rolls.
Although embodiments for the present subject matter have been described in language specific to structural features, it is to be understood that the present subject matter is not necessarily limited to the specific features described. Rather, the specific features and methods are disclosed as embodiments for the present subject matter. Numerous modifications and adaptations of the method of the present invention will be apparent to those skilled in the art, and thus it is intended by the appended claims to cover all such modifications and adaptations which fall within the scope of the present subject matter.

WE CLAIM:
1. A method for fabrication of the abrasive grains comprises the steps of:
-preparing of slurry by mixing of Aluminum oxide (Al2O3), Yttria stabilized (Y2O3)-zirconium oxide (ZrO2) or [YSZ] and titanium diboride (TiB2) in DI water alongwith binder and dispersants (please let us known the what are used in binder and dispersants);
- mixing of small amount (provide the amount) of TiB2 for improving
hardness;
-stirring of the mixture for 10 minutes and the suspension is stirred for a suitable period for producing a homogenous dispersion and ceramic powder (please give the details that what is used as ceramic powder) were added alongwith binders (give details) with magnetic stirring at a suitable speed for suitable time;
- degassing for 10 minutes and sedimentation test was performed to
have a desired pH value and viscosity measurement;
-pouring of stabilized slurry in mould and a casted green specimen was dried in open environment after absorption of water for a suitable duration at a suitable temperature;
-sintering was carried out at a preferred range of temperature;
-crushing the grains for obtaining a suitable size and substantial hardness coating of abrasive grains with cobalt tungsten carbide alloy and poly-vinyl alcohol (PVA);
-placing the abrasive grains into pop mould for achieving desired patterns.
2. The method for fabrication of abrasive grain as claimed in claim 1, wherein the carbide used is the alloy of 12% Co-WC.
3. The method for fabrication of abrasive grain as claimed in claim 1, wherein the magnetic stirring occurs at 1000rpm for 24 hours.

4. The method for fabrication of abrasive grain as claimed in claim 1, wherein the stirring of suspension continuous for 30 to 60 minutes.
5. The method for fabrication of abrasive grain as claimed in claim 1, wherein the substrate sizes are 1 to 5mm or more preferably from 1.4 to 2.5mm.
6. The method for fabrication of abrasive grain as claimed in claim 1, wherein the quantities of Al2O3, YSZ and TiB2 in slurry are given below:
Al2O3- 40 to 50%
YSZ – 15 to 20%
TiB2 – 0.2 to 1.5%
7. The method for fabrication of abrasive grain as claimed in claim 1, wherein the weight percentage of Al2O3 and YSZ is 15 to 20% or more preferably 20%.
8. The method for fabrication of abrasive grain as claimed in claim 1, wherein the duration of stirring of the resultant mixture is 8 hours.
9. The method for fabrication of abrasive grain as claimed in claim 1, wherein the pH of the suspension was maintained at 2 to 3 by adding of polymethylacrylic acid alongwith 0.25 to 0.5 ml Hcl.

10. The method for fabrication of abrasive grain as claimed in claim 1, wherein average particle size of all powders are in the range of 250 to 300nm.
11. The method for fabrication of abrasive grain as claimed in claim 1, wherein the dispersant is alkali-free anionic polyelectrolyte which incorporates 30 wt % of ammonium salt of polymethylacrylic acid (PMAA-NH4).
12. The method for fabrication of abrasive grain as claimed in claim 1, wherein the drying process takes place at 110°C for a duration of 6 hours.
13. The method for fabrication of abrasive grain as claimed in claim 1, wherein sintering process was carried out 1350°C to 1500°C or more preferably at 1400°C.

14. The method for fabrication of abrasive grain with improved hardness and toughness as claimed in claim 1, wherein TiB2 is used between about 0.2 to 1.5% of the total loading.