FIELD OF THE INVENTION:

The present invention relates to the process for preparing sintered aluminium oxide powder possessing superior properties such as well-developed sub-micron crystals in the matrix, micro fracturing characteristics, high hardness and high toughness that render it suitable for employment as super abrasive products. The instant invention also relates to sintered aluminium oxide powder obtained by said process and its use.

BACKGROUND OF THE INVENTION:

Hard and tough abrasive grits are employed in grinding wheels. Sintered materials based on aluminium oxide are suitable for use as an abrasive. Particularly, for this purpose a special grade of aluminium oxide is processed to have controlled crystal size and having high hardness and toughness in the required particle size are found ,to be useful in enhancing the performance of the super abrasive products like diamond and CBN wheel. These super abrasive wheels have major application for grinding and polishing of diamond, tungsten carbide, cemented carbide, glass, hard ceramics etc. In the process of manufacturing diamond for jewelry applications, raw diamond (natural as well as synthetic) is cut into facets and polished to get the desired product. Polishing is one of the critical steps in the manufacturing process. Generally diamond bruiting wheel is used to grindle the diamond into a conical shape. Since grinding of diamond is very heat sensitive. Temperature of diamond should not cross 700°C during grinding. This makes quality of diamond bruiting wheel very critical to the application.

Different filler materials mainly ceramics and metal fillers have been used in diamond wheel. Ceramic fillers like calcined alumina, silicon carbide etc. and metallic filler like copper, silver etc. are extensively used with respective bond systems. Super abrasives wheel are mainly made of ceramic, metallic and resinoid bond systems for different types of application. A special grade of sintered alumina needs to be incorporated in super abrasives wheel matrix along with parent super abrasive grains to further enhance and improve the performance as well as grinding efficiency of the wheel. Addition of certain percentage of sintered alumina powder can directly replace the equivalent amount of diamond/CBN powder with added advantages as mentioned earlier. The first of the sintered aluminium oxide products were described in US 2,278,442 which were produced from pure aluminium oxide or aluminium hydroxide or from bauxite by a fine grinding, optionally adding sintering auxiliary agents, compacting and sintering or hot pressing. However, it was seen that the materials thus obtained did not exhibit optimal grinding properties owing to inappropriate crystallite sizes.

The first large scale sintered abrasives were produced by the method taught in US Patent No. 3,079,243. This patent application teaches the milling of calcined bauxite to produce a fine particle size raw material which is then formed into abrasive grit sized particles and fired at about 1500°C to form hard, strong, tough pellets of poly crystalline alumina. US Patent No. 3,108,888 also teaches making high density alumina or alumina containing products by firing dried alumina gel made from alpha alumina monohydrate (boehmite) or by hot pressing dried powders made from such gels. Although there are numerous conventional methodologies of sintering aluminium oxide powder, these methods are laden with difficulties or inadequacies that render them unfeasible and unviable. The sintered alumina produced by the prior art methods of normal sintering are defective in one way or the other. There is still a need in the art to arrive at a commercially viable, cost-effective and efficient process of making sintered aluminium oxide with superior abrasive properties. Further, it is desirable to be able to influence the properties of the sintered product, especially its fracture, toughness and hardness in relatively simple way.

The special grade sintered alumina produced by the process of the present invention has high hardness along with high toughness. The combination of super abrasives and this sintered alumina impart the superior properties than the parent super abrasives wheel. This composite wheel has high hardness along with high toughness which helps to perform better during grinding and polishing applications. The special sintered aluminium oxide of the instant invention has unique properties like well developed submicron crystals in the matrix, micro fracturing characteristics, high hardness and high toughness. Present invention discloses the method of manufacturing this special grade sintered aluminium oxide, its superior properties and a method of employing it in the conventional super abrasive product.

OBJECT OF THE INVENTION

In accordance with the purposes of the present invention, as embodied and broadly described herein, the first object of the present invention is to provide for a process of making sintered aluminium oxide powder suitable as a super abrasive. The second object of the present invention is provide a sintered aluminium oxide powder that has superior properties such as micro fracture characteristics, higher hardness and higher toughness. The sintered aluminium oxide in accordance with the present invention is produced by a process comprising the following steps: (i) preparing a homogenous mixture of alumina gel and other additives (ii) drying the said mixture to a temperature not exceeding 120°C (iii) compacting the dried powder (iv) the compacted mass is subjected to drying at a temperature below 150°C (v) Sintering the dried compacted mass at a temperature of about 1300°C to 1600°C with soaking. The hardness of the sintered alumina produced according to the process of the present invention has hardness in the range of 2400HV to 2800HV, more preferably 2500HV to 2600HV and toughness in the range from about 3.5 to 7 MPa.m1/2 and more preferably about 5 to 6 MPa.m1/2.

Thus, the present invention comprises a combination of features and advantages which enable it to overcome various problems of prior art methods. Other features and advantages of the present invention will become apparent as the following detailed description proceeds or may be learned by practice of the invention. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention.

BRIEF DESCRIPTION OF DRAWINGS

The foregoing summary, as well as 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. In the drawings: Figure 1: Image of sintered aluminium oxide at 100X and 300X magnification.

DETAILED DESCRIPTION OF THE INVENTION

In describing and claiming the invention, the following terminology will be used in accordance with the definitions set forth below. Unless defined otherwise, 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 belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods and materials are described herein. As used herein, each of the following terms has the meaning associated with it in this section. Specific and preferred values listed below for individual process parameters, substituents, and ranges are for illustration only; they do not exclude other defined values or other values falling within the preferred defined ranges. As used herein, the singular forms "a," "an," and "the" include plural reference unless the context clearly dictates otherwise.

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. When the term "about" is used in describing a value or an endpoint of a range, the disclosure should be understood to include both the specific value or end-point referred to. As used herein the terms "comprises", "comprising", "includes", "including", "containing", "characterized by", "having" or any other variation thereof, are intended to cover a non-exclusive inclusion. The process start with alumina gel prepared from various precursors like various salt of aluminium, aluminium hydroxide, aluminium oxy hydroxide etc. In case of salt precursors aluminium nitrate is preferred over other aluminium salt for easy handling of subsequent process. The processes of making alumina gel from various precursors are conventionally known.

Different additives are added to the alumina gel and mix homogeneously by high share mixer. In order to attain a chemically homogenous mixture, the mixing may be done by conventional methods or by employing an ultrasonic disperser or a combination of both. Additives are mainly water soluble compound preferably salt solution. These additives influence immensely to achieve the desired properties of final sintered product. The amount of additives can be upto 20% of the final product composition, preferably 10%. The additives used for sintering of this special alumina can any of salt or hydroxide of magnesium, niobium, strontium, lanthanum, cerium, neodymium, yttrium, iron, titanium and the like. These additives are added to the mix in their respective water solution. Then the mixture is dried in powder forms in a drier. It is preferred that the drying of alumina gel be done in a fast instantaneous drier to form a free flowing powder. It is thus preferred that the drier used for the present process be instantaneous drier like spin flash drier, agitated thin film drier, drum drier, spray drier etc. materials temperature should not cross 120°C in any case more preferably not more than 100°C. The particle size of the dry powder is below 200 micron, more preferably 120 micron.

Dry powder is compacted prior to sintering. Compacting is the process of consolidation where loose particle are pressed together to obtain a denser mass. Compaction can be done in various types of equipments like dry roll compactor, hydraulic press, extruder etc. Slip casting, cold isostatic pressing, uniaxial press compaction may also be cited as examples without this being seen as a limitation. The dry powder is to be preconditioned depending on the type of compactor. Here preconditioning means the percentage of moisture content of dry powder before compaction. The percentage of moisture requirement varies from 10 to 50%. More preferably moisture content 20 to 40 percentages provides better compaction. This compacted mass is then dried to remove the moisture added during compaction. Drying can be done in tray drier; belt mesh drier, rotary drier etc. the source of heating could be electrical, microwave, radiant heating or combination of them. The drying temperature could be 80°C to 150°C, more preferably 90°C to 120°C and the moisture content of the dry product should be less than 8%, more preferably less than 5%.

This dry compacted mass is then sintered in conventional furnaces, advance furnaces or combination of them. Conventional furnace mentioned here can be muffle furnace, batch type high temperature furnace, rotary furnace roller hearth furnace, pusher type furnace etc. where source of heating is electrical or fuel burnt. Advance sintering furnace represents the microwave heated muffle furnace, microwave heated vertical furnace, microwave heated rotary furnace. Some hybrid heating system can also be used which contains electrical heating as well as microwave heating system. Initial heating may be done by electrically and later on heating by microwave. "Sintering" refers to densification or consolidation of a powder compact during firing. Sintering is the process of densification. The bonding of particles takes place at high temperature which intern improves the strength and density. Sintering can be done two stages, initial heating is done up to intermediate temperature and final sintering is at desired temperature. Intermediate stage could be at 400°C to 700°C with 30 minutes to 150 minutes soaking. More preferable range is 500°C to 600°C with 60 to 120 rninutes soaking. This step helps to gradual removal of volatiles which are embedded in the compacted mass. Final stage sintering can be done at 1300°C to 1600°C with 10 to 60 minutes soaking.

More preferably, 1400°C to 1500°C with 20 minutes to 45 minutes soaking can be used. Final stage sintering schedule is very critical, because it controls the properties of the product. Depending on sintering schedule final microstructure can be tailored to get uniformly distributed well developed sub micron crystals in the matrix. The kind of microstructure of the product is developed during sintering determines the properties like high hardness, high toughness, micro fracturing characteristics etc. Single stage sintering can also do the job but the stringent sintering schedule is to be maintained. Here stringent sintering schedule means initial heating to be done very slowly and then faster heating to the final temperature. The slower heating at in initial period helps to avoid unwanted crack in the final product. The range of slower heating can be 2°C/minutes to 4°C/minutes and faster heating could be 5°C/minutes to 10°C/minutes. In case of rotary furnace sintering compacted mass to be pass through preheating heating and cooling zone in controlled manner. The residence at each zone can dictates the final microstructure as well as properties of the product. Hot zone temperature can be in the range form 1300°C-1600°C and residence time could be 10 minutes to 60 minutes.

Crushing and grading of dry compacted mass can be done prior to sintering or can be done after sintering. Grading is the process of classifying a wide range of particle sizes into different size groups. This compacted mass either dry or sintered is ground to fine powders and classify into desired size ranges. The size reduction of compacted mass into required product range could be done by various crushers and mills. The communition can be used here are roll crusher, pulveriser, ball mill, jet mill etc. the classification of ground powder can be done in grader with stainless steel mesh, vibro screener, air classifications etc. Depending on the required size ranges grading system can be chosen. For example, if average particle is greater than 45 micron normal grader, vibro screen can be used. On the other hand air classification to be done to get the desired size range. This classified final product is having properties like micro fracture characteristics, higher hardness, higher toughness, provides cooler cutting. The hardness of this sintered alumina is in the range of 2400 HV to 2800HV, more preferably 2500HV to 2600HV. And the toughness is in the range from 3.5 to 7 MPa.m1/2 more preferably 5 to 6 MPa.m1/2.

On the other hand convention aluminium oxide either fused or sintered, the hardness is varies between 1700HV to 2100HV and toughness is around 3 to 4.2 MPa.mI/2. The addition of this special grade sintered aluminium oxide in the matrix of super abrasive wheel provides several advantages. The major benefits are superior microstructure with well developed submicron crystal provides controlled micro fracture of the grain. This phenomena repeats though out the wheel life. When micro fracture takes place, fresh cutting edges exposed to perform the grinding. And when this tiny particle is going out of the system is carrying small amount of heat which provides cooler cutting. As mentioned earlier most of the gems especially diamond cutting/bruiting is very heat sensitive and required cooled cutting. Superior hardness of super abrasive is synchronized with the higher toughness of sintered alumina provides faster and cooler grinding operation than the conventional super abrasive wheels. This combination extends the life of the new super abrasive wheel. The cost of this powder is less than the diamond powder as a result the final cost of the super abrasive wheel with this sintered aluminium oxide powder is less compare to conventional super abrasive wheel. For example, it is seen that the life increased by 20% when 20% of super abrasive replaced by this sintered alumina. And the cost will also decreased by 20%.

The amount of this sintered aluminium oxide can be added to the matrix is varies from 10 to 40%, more preferably form 15 to 30%. The new composite wheel can be made using standard vitrified, metal and resinoid bonding system for different end applications. The following examples are provided to better illustrate the claimed invention and are not to be interpreted in any way as limiting the scope of the invention. All specific compositions, materials, and methods described below, in whole or in part, fall within the scope of the invention. These specific compositions, materials, and methods are not intended to limit the invention, but merely to illustrate specific embodiments falling within the scope of the invention. One skilled in the art may develop equivalent compositions, materials, and methods without the exercise of inventive capacity and without departing from the scope of the invention. It will be understood that many variations can be made in the procedures herein described while still remaining within the bounds of the invention. It is the intention of the inventors that such variations are included within the scope of the invention.

EXAMPLES

Example 1: 1 liter alumina gel is taken in beaker and place under high share mixer. 5 gram of lanthanum nitrate and 11 gram of magnesium nitrate is dissolved in water separately. This additives solution is added to the alumina gel slowly and stirs vigorously. After homogeneous mixing for 30 minutes, alumina gel is spray dried. Free flowing dry powder of size ranging from 50 to 150 micron is the compacted in an extruder by adding 30% moisture. Compacted mass is then dried at 120°C in a chamber drier for 4 hours. Then this dry compacted mass is sintered in two stages, initial heating is done at 700°C for 40 minutes and final sintering is performed at 1500°C for 40 minutes. Sintered mass is the primary crushed and then mill into fine powders. Fine powders are classified into different groups. Sintered mass is characterized and results are follows Micro-hardness: 2512 HV, Fracture toughness 5.53 MPa.ml/2.

Example 2: 1 liter alumina gel is taken in beaker and place under high share mixer. 10 gram of lanthanum nitrate and 8 gram of magnesium nitrate is dissolved in water separately. This additives solution is added to the alumina gel slowly and stirs vigorously. After homogeneous mixing for 30 minutes, alumina gel is spray dried. Free flowing dry powder of size ranging from 50 to 150 microns is the compacted in an extruder by adding 30% moisture. Compacted mass is then dried at 120°C in a chamber drier for 4 hours. Then this dry compacted mass is sintered in two stages, initial heating is done at 550°C for 60 minutes and final sintering is performed at 1550°C for 30 minutes. Sintered mass is the primary crushed and then mill into fine powders. Fine powders are classified into different groups. Sintered mass is characterized and results are follows Micro-hardness: 2583 HV, Fracture toughness 5.36 MPa.ml/2.

Example 3: 30 gram sintered aluminium oxide (prepared as per example 2, size range: 36 to 35 micron), 200 gram diamond powder (D 46) and 360 gm standard vitrified bond mix thoroughly. Then the mix is molded into required dimension to get fired dimension of D: 150mm x T:10mm x H:32mm x X:10mm. The wheel is fired at 850°C for lhour. Later on metal part is fixed using organic adhesive. Diamond bruiting wheel made through this process is tested on a natural diamond point 0.5ct cone. Results are reported in table 1. Table 1: Test conditions and results of example 3.

Example 4: 40 gram sintered aluminium oxide (prepared as per example 2, size range: 36 to 35 micron), 200 gram diamond powder (D 46) and 360 gm standard vitrified bond mix thoroughly. Then the mix is molded into required dimension to get fired dimension of D: 150mm x T:10mm x H:32mm x X:10mm. The wheel is fired at 850°C for lhour. Later on metal part is fixed using organic adhesive. Diamond bruiting wheel made through this process is tested on a natural diamond point 0.5ct cone. Results are reported in table 2. Table 2: Test conditions and results of example 4.

Example 5: 50 gram sintered aluminium oxide (prepared as per example 2, size range: 36 to 35 micron), 200 gram diamond powder (D 46) and 360 gm standard vitrified bond mix thoroughly. Then the mix is molded into required dimension to get fired dimension of D: 150mm x T:10mm x H:32mm x X:10mm. The wheel is fired at 850oC for lhour. Later on metal part is fixed using organic adhesive. Diamond bruiting wheel made through this process is tested on a natural diamond point 0.5ct cone. Results are reported in table 3. Table 3: Test conditions and results of example 5. The foregoing description of the exemplary embodiments of the disclosure has been presented only for the purposes of illustration and description and is not intended to be exhaustive or to limit the disclosure to the precise forms disclosed. Many modifications and variations are possible in light of the above everything. The embodiments were chosen and described in order to explain the principles of the disclosure and their practical application so as to enable others of ordinary skill in the art to utilize the disclosure and various embodiments and with various modifications as are suited to the particular use contemplated. Alternate embodiments will become apparent to those of ordinary skill in the art to which the present disclosure pertains without departing from its spirit and scope. Accordingly, the scope of the present disclosure is defined by the appended claims rather than the foregoing description and exemplary embodiments described therein.

We claim,

1. A process for manufacturing sintering aluminium oxide powder comprising the steps of:

(a) Preparing a homogenous mixed solution alumina gel and additives

(b) Drying the mixture of step (a) at a temperature not exceeding 120°C to form a dry powder

(c) Compressing the dry powder of step (b) into a compact

(d) Sintering the compact at temperature of about 1300°C to 1600°C with a soaking of about 10 to 60 minutes.

2. The process as claimed in claim 1, wherein the additives are selected from the group comprising salt or hydroxide of magnesium, niobium, strontium, lanthanum, cerium, neodymium, yttrium, iron, titanium and the like

3. The process as claimed in claim 1, wherein the additives constitute about 20% of the mixture of step (a).

4. The process as claimed in claim 1, wherein the alumina gel is prepared from precursors such as salt of aluminium, aluminium nitrate, aluminium hydroxide, aluminium oxy hydroxide and the like.

5. The process as claimed in claim 1, wherein the alumina gel constitutes about 80% of the mixture of step (a).

6. The process as claimed in claim 1, wherein the additives are added to the mixture in their respective water solution.

7. The process as claimed in claim 1, wherein the drying is undertaken in a fast instantaneous drier to form a free flowing powder.

8. The process as claimed in claim 7, wherein the particle size of the dried powder is below 200 microns.

9. The process as claimed in claim 1, wherein the compaction of step (c) is undertaken in dry roll compactor, hydraulic press, extruder and the like.

10. The process as claimed in claim 1, wherein the dried powder of step (b) is optionally pre-conditioned prior to compaction.

11. The process as claimed in claim 1, wherein the compacted mass of step (c) is dried at a temperature of about 80°C to 150°C.

12. The process as claim in claim 1, wherein the sintering is undertaken as a single stage sintering or two stage sintering.

13. The process as claimed in claim 12, wherein the sintering when undertaken as a two stage sintering process comprises the following steps

(i) Initial heating is undertaken at an intermediate temperature of about 400oC to 700oC with 30 minutes to 150 minutes soaking, (ii) Final sintering is undertaken at a desired temperature of about 1300oC to 16OO0C with 10 to 60 minutes soaking

14. The process as claimed in claim 1, wherein the dried compacted mass of step (c) can be optionally crushed and graded prior to the sintering step.

15. The process as claimed in claims 1-14, wherein the sintered aluminium oxide has a hardness of about 2400 HV to 2800HV.

16. The process as claimed in any of the preceding claims, wherein the sintered aluminium oxide has a toughness of about 3.5 to 7 MPa.m1/2. ;

17. The process as claimed in any of the preceding claims, wherein the sintered aluminium oxide obtained is incorporated in the matrix of super abrasive wheel.

18. The process as claimed in claim 17, wherein the amount of this sintered aluminium oxide added to the matrix varies from 10 to 40%.

19. A sintered aluminium oxide produced by any of claims 1-18, wherein said sintered aluminium oxide has a hardness of 2400 HV to 2800HV and toughness of about 3,5 to 7 MPa.mI/2.