CA1326688C - Abrasive material and method - Google Patents

Abstract

ABSTRACT
The hardness and microstructure of aluminous abrasives produced from alumina gels are enhanced by introduction of seed material as by wet vibratory milling of the material with alumina media, or by the direct addition of very fine alpha alumina particles in the amount of 1% or less.

Description

:~326~8 ABR~SIVE ~ATERIAL AND MET~OD
FIELD OF THE INVENTION
The invention relates to the production of aluminous abrasive grits or shaped bodies containing high density poly-crystalline alpha alu~ina, or such alumina --:
with other additives.
B~CKGROUND OF THE I~VENTION
Hard and strong abrasive ~rits, for use in grinding wheels, flexi~le coated abrasive products ("sandpaper"), or as loose abrasive, are produced commercially from alumina containing raw materials either by fusion in an electric furnace or by the firing of shaped bodies containing finely divided alumina at temperatures well below the fusion points of the material. Such lower temperature process is called sinteringO Thi5 invention relat~s to aluminous abrasives ~-made by the sintering process.
The first large-scale commercially produced sintered abrasives were produced by the method taught in U.S. Paten~ 3,079,243 to Ueltz. This patent teaches the milling of calcined bauxite to produce a fine particle qi~e raw material which is then formed into abrasive grit sized particles and fired at about 1500C to ~orm hard, strong, tough, pellets of poly-crystalline alumina. ~;
Recently, abrasive ~aterials consisting of grits made up of alumina and magnesia spinel, presumably made according to the teachings of U.S. Patent 4,314,827, and made according to the teaching of ~ublished British Application 2,0~9,012A, published l December 1982, have 30~ been commercially introduced. These materials are produced by the sintering (at about 1400C) of dried alumina gel particles. U.S. Patent 3,108,888 to Bugosh aIso teaches making high density alumina (or alumina containing) products by firing a dried alumina gel made ~from alpha alumina monohydrate (boehmite) or by hot presslng dried powders made from such qels. ~

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Alumina/magnesia-spinel commercial abrasives made from gels contain alumina in the form of cells fro~ 5 to ~ -15 microns in diameter. The cells, or "sunbursts", are made up of many elongated alumina ar~s 0.2 to 0.4 micrometers in diameter (but some of which can be as large as 1 micrometer in the form of very roughly spherical "blobs"), the arms in each cell appearing generally to radiate from the center of the cell. All of the arms in a given cell are apparently crystallographically identically oriented. Such orientation is shown by the fact all the area of a given cell extinguishes simultaneously upon sample rotation when viewed between crossed polarizers ~y -transmitted light microscopy.
While the commercial a~rasives made from sintered gels containing alumina and magnesia are high quality abrasives, it has not been possible to produce high purity alumina grits by the gel route. This is shown hy the relative softness and lack of abrasive utility for the "control" example 13 in U~S. Patent 4,314,827 which was made from an alumina qel without metal oxide or metal salt additions. -The present invention is an improvement in the art of making strong abrasive bodies whereby useful abrasive products can be made fro~ alumina gel with or .:
without the addition of zirconia or spinel formers such as magnesia~, DISCLOSURE OF THE INVENTION
The invention resides in the discovery t~at control of the micros~ructure of the fired product, such 3;0 ~that the celiular structure of the alumina in the prior art abrasives is avoided, results in improved product performance. The resulting product, instead of cell areas of~5 to 10 micrometers in fliameter, contains alpha alumina particles (crystallites) of submicron size (0.2 to 0.4 ,i 35 ~ micrometers). . -~

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In the case of the higher (e.g. in some cases at 5%) MgO additions these alumina particles are surrounded by a matrix of spinel.
Conditioning of the gel to achieve the descrihed effect can be achieved by vibratory milling of the mix in the sol or diluted gel form while employing alumina bodies as the grinding medium in the mill. It is believed that the main effect of the milling is to introduce material from the alumina grinding media into the alumina gel.
Also impurities such as zinc and iron are intro~uced from the piping and associated equipment. Milling with -~
zirconia bodies, for example, is ineffective to produce the desired essentially non-cellular structur~.
The first effective and reproducable method found by us was to generate such material in the gel by vibratory milling of the gel with alumina bodies. A
suitable vibra'ory mill is shown in U.S. Patent 3,100,088~ Typically the media may be 1/2 inch in diameter and 1/2 to 3/4 inches long. The tub which contains the media and the mix is vibrate~ in the horizontal plane by an off-balance weight connected to the shaft of a motor mounted coaxially with the tub which is mounted on springs. The off-balance weight is mounted a~acent the plane of the bottom of the tub end a second weight is mountsd below it. The motor typically rotates at 1200 rpm. The combined oscillation subjects the contents to a milling action by the grinding media. The interior surface of the mill is preferably lined, ~s with rubber, to prevent contamination by erosion of the metal 30 ~ walls-Various additives such as taught in U.S. Patent ; 4,314,827 and British Application 2,0g9,012A can be added to the alumina before or after gelling. The most useful additive presently known IS any compatible precursor of : :, ~ 3 ~ 8 MgO, whereby the final product contains preferably around5~ MgO. The MgO is present, however, in the product as spinel (magnesium aluminate: MgA1204) but calculated as MgO in the analysis. Obviously lesser amounts of MgO
may be included since the alumina with no addition is an excellent abrasive in its own right when produced according to the present invention. The milled gel of the present invention may serve as a matrix for various added materials or abrasive particles.
The milled MiX may be simply poured or placed into containers to dry and then broXen up into a~propriate sized pieces by crushing, with undersized material recycled to the beginning of the process. Alternatively, the material may be formed or molded into shaped particles as hy extrusion. In the case of extrusion, the rods formed would later be cut or broken into appropriately sized pieces. The minimum useful firing temperature is significantly below 1200C, usually considered the transformation point to convert to alpha alumina. The upper limit is not critical so long as the fusion temperature is not reached. Too long a firing or too high a temperature can cause exce~sive crystal growth. Higher temperatures increase the cost of the process also, so the ~ preferred firing range is 1200 to 1500C.
; 25 EXAMPLE I
In a large polymeric plastic mixing ~essel 30 pounds (13.6 Kg) of ~ondea SB Pural Alumina (supplied by Condea) and 30 Imperial gallons (136 liters) of water were mixedO This material was then gelled by adding 4.1 liters of 14 weight ~ HN03. Magnesium nitrate hydrate (7.5 pounds, or 3.4 kg~ dissolved in 3 gallons (13.7 liters) of water was then added to the alumina gel to give 5~ by weight of MgO in the final product. It was mixed for 15 minutes and transferred to a Model M451 ~ weco mill and ~35 *
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~ 3 ~ 8 milled for 1 hour with 1700 pounds of alumina media. The mix was recirculated through the mill for the one hour milling time at a rate of about four gallons per minute.
After milling it was pumped into aluminum trays to a thickness of about 3 inches (7.6 cm) for drying on electric strip dryers.
The alumina media composition was about 90~ alpha alumina with silica as the main impurity.
A series of batches according to the above formulation were made up and combined for crushing and firing.
The dried gel was then roll crushed and screened to a through 14 mesh sizing before firing, to yield the ~esired final grit sizes. It was t~en prefired at 4~0C
for 16 hours and fired at 1400C for 30 minutes in a rotary kiln.
After firing all of the products had a hardness of 19 GPa (~ickers indenter, 500g load) and a very fine microstructure in which there was no cellular microstructure and, almost all of the alpha alumina was in the form of generally equiaxe~ particles (crystallites), 0.2 to 0.4 microns in diameter, except Por rare square blocky shapes about 5 microns in diameter. The blocky ~shapes may have indicated contamination. The product, upon examination by the scanning electron microscope, was seen to be comprised of a spinel matrix and a ~ .:
discontinuous phase of alpha alumina.
In some specific coated abrasive grinding applications the material was superior to fused 30~ ~alumina~zirconia and superior to commercially available sintered gel type abrasive of the alumina-spinel compositionO
EXAMPL~
j Pural microcrystalline boehmite alumina, 22.7 ~ ~

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kilograms, was mixed with 225 liters of water and 13.5 , liters of 14% HNO3 for 10 to 15 minutes.
i One half of the gel mixture was milled for 2 hours in the Sweco mill containing 1/2 x 1/2 inch ceramic ' 5 bonded alumina, 88 A12O3 (the main impurit~es being ¦ MgO 1.74~, sio2 8.9%, Fe2O3 0.18%, Tio2, 0.2%, CaO
0.8~, Na2O 0.34%), available from Coors Porcelain Co., and dried. This was the sa~e media as used in ~xample I.
The other half was simply dried without milling. The 10 dried gels were crushed to size, prefired at 450C for 16 hours and fired at 1400C for 1 hour.
The milled material had a hardness of 19.1 GPa, the unmilled material had a hardness of ll.O GPa.
Material from each batch was screened to produce 50 grit abrasive grains which were then used to produce vulcanized fiber ~acke~ coated a~rasive discs. The mille~
material outperformed commercial alumina zirconia abrasive by better than 10% in grinding 1020 steel (the test showed a 14~ higher metal removal). -The un~illed product was inferior to fused abrasive in all grinding tests, which was to be expected -in view of its low hardness~
EXAMPLE III -- - -In an example similar to that of the milled ; 25 product of Example I, the gel was milled for 0.2 hours.
The pro~uct, fired at 1400C for one hour, was mainly of -~ ;the fine random O.2 t-o- 0.3 mm crystal structure, but ; ~ ~ showed some cellular appearance, .
~ EXAMPLES IV TO IX -:-..:.
~ Further examples were performed in a study of the effect of firing time at 1400C. All samples were made by the general procedure of Example I. Condea microcrystalline boehmite alumina was employed, milling --:: ~ :: : .
was for two hours, but a~ter drying, the gels were ~ ~
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prefired at 750C for 30 minutes. As the firing time was lncreased, there began to appear in the product a coarse lath shaped crystallization of alumina, randomly dispersed among the fine 0.2 to 0.4 ~icrometer alumina particles.
The results are tabulated as follows:
Particle Size Firing Time (Micrometers~Ratio _(minutes)_ Coarse Fine~ Coarse/% Fine l ~one 0.2-0.3 0 lO 3 1.0-2.0 0.2-0.3 5 ~-5 0.2-0.320 4-8 0.2-0.350 Up to 8 0.2-0.380 ~0 Up to ~ 0.2-0.395 Since the presence of the coarse fraction is believed to be less desirable, the firing time at 1400C
should not be more than 5 minutes for the preferred product when the material is pref;red at 750C for 30 minutes~
In all cases, no cellular structure was observed. Th2 microstructures consisted of the non-faceted submicron particles and the faceted lath-like ~`
~coarse crystals, excPpt in the ca~;e of the l minute firing ~-where no laths were found. -i .
By "non-aceted" we mean no regular faceting of the crystallites was observed in a fractured surface at ~5,000X magnification~y the scanni~ng electron microscope.
The particles of alpha alumina were, instead, rather formless, but equiaxed, with generally curved outlines, 30~ ~and very~few apparen~traight-~utlines. At 20,000X
magni~ication faceted structure begins to be clearly apparent.
The abrasive grits o~ t~is invention have a hardne~s measured ~y~the ~ickers indenter with a 500 gram ~ ~ ~
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load of at least 16 GPa (90% density) for alumina without additions and at least 14 GPa for the grits which are modified by the presence of 2~ or more of spinel formers or are modified by other additives. I~ile pure dense alpha alumina has a hardness of about 20-21 GPa, some porosity may be desirable for certain applications which would lower the hardness. When the alumina has a hardness of 13 GPa or less, it is too porous for the purposes of this invention. Preferred are hardnesses of 18 GPa or higher as measured by the Vickers indenter ~t a 500g load.
EXAMPLE X
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A series of abrasives of varying magnesia content were made.
The general procedures of Example I were employed, including the milling (but for 2 hours) with alumina media. In all cases the gels, after drying at 200C for about 30 hours, were crushed and screened and then calcined at 450C for 16 hours. The resulting grit sized particles were fired in a rotary kiln at 1400C.
The heat-up time to 1400C was about 15 rninutes, and the time at 1400C was about 15 minutes.
Various amounts of magnesium nitrate were added prior to the gelling. In one run no magnesium nitrate was added. The MgO content and hardness of the abrasives were as follows: `
Hardness Run No. MgO Content % by Wgt. (Vickers 500 g load) 9498 0.14 19.9 ~ -949g 2.50 19 9500 7.g5 19 , :. .
9502 12.71 19 -In a series of tests of vitrified (glass bon~ed~ ~
grinding wheels employing 54 grit (a combination of 46 -grit and 60 grit sizes) sized abrasive wheels made wit~
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g the above grits were compared with the highest quality known fused alumina abrasive (sulfide process abrasive).
The tests were carried out by grinding slots in tool steel (D3) at various controlled in feeds. In dry grinding, at 0.5 mils (.0005 inches) downfeed, the abrasive containing no added MgO (0.14~ MgO) had a grinding ratio 16.18 times the grinding ratio of the fused abrasive (G ratio is the volumetric ratio of material removed to wheel wear). All of the MgO additions resulted in superior performance over the fuised abrasive in the dry grinding tests. In the wet grinding tests the experim~ntal abrasives with MgO added were poorer than or equal to the fused abrasive. At 2 mils the -~
no-magnesia-addition abrasive was superior to the fused.
In coated abrasive tests employing 50 grit size abrasive (CAMI standard) an abrasive made according to Example X, and containing 0.6~ MgO, incorporated into flexihle abrasive discs performea better (136%) than co-fused alumina zirconia abrasive on 1020 steel and almost equivalent to fused alumina-zirconia on stainless steel. The abrasives containing 2.5% MgO and 7.59% MgO
were also superior on 1020 steelO The higher MgO a~dition was leis effective on stainless.
The 0.14~i MgO abrasive contained, in addition to the alumina 0~25% Si02, 0.18% Fe203, 0.28% Ti02, 0.0S% CaO, and 0.04% Na20, presumably mainly introduced in the milling operation. Similar levels of these impurities were present in the other abrasives.
While applicants do not wish to be bound by any particular theory of the invention, it is believed that the introduction from the alu~ina media of particulate --matter may effeat seeding of the crystallization of alpha alumina during the firing. Additlonally, the other impurities introduced in the milling step may inhibit ~35 ` ~"

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As evidence of the fact that it is the debris from the ~illing media ~hich is effective to condition the gel so that it produces the desired high density, finely crystalline, non-cellular alpha alumin~ upon firing at about 1400C, additions of milled water were made to alumina monohydrate together with acid, without milling of *he gel.
Water, nitric acid, and microcrystalline boehmite were mixed, as in Example lI, except that 6 batches were --made, with varying additions of water containing the debris worn from alumina grinding media, when milled for several hours with water (no other addition to the water), as follows:
"Milled water" additions to alumina monoh~arate ~Condea~:
Wt.Ratio of milling debris Wt. % Debris in Hardness Trial to alumina monohydrate fired product* GPa 20 1. 0.0074 1.07 20+

2. 0,0037 0.53 20 3. 0.0019 0.27 19+
. 0.00075 0.11 17 0.00038 0.05 15 , ^
25 ~~ 6_ 0 0 12.S
*Note: Assuming an average loss of weight in firing of 3~%~
The hardness was determined on the fired product, fired at 1400C~+ 20C, for about 10 minutes. The furnace 30 i~was elect~rically fired, the atmosphere was air.
Examination of the milled debris showed it to be ;mostl~y~alpha alu~in~a with a surface area of about 39 square meters/gram.
H~igh purity~alumina produced by recovery of the 35~
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fine suspended alumina particles left in suspension when very fine alumina powders are permitted to settle after being mixed with water is also effective, when used in an amount of at least about 0.1~ of the fired gel solids.
Tests with commercial fine alpha alumina powders, and tests with fine alumina generated by milling very high purity fused alumina, using such alumina itself as a milling medium, were very effective in producing the dense finely crystalline product of the invention.
Differential thermal analysis has shown that, when the alpha alumina seed particles are present the transition of the gel alumina from presumably the gamma form to the alpha form takes place at about 1090C, while, in the absence of such seed material the transition takes place at about 1190C. Thus, the theoretical minimum firing temperature of the products of the present invention can be below the usual reported transformation temperature.
This invention, for the first time, permits the manufacture by low temperature sintering of high purity alpha alumina bodies having a submicron particle size and a density of greater than 95%, resulting in a hardness .
greater than 18 GPa. Products other than abrasive, such as coatings, thin films, fibers, rods, or small shaped parts, can be made by the process of the present invention.
Grain growth inhibitors such as SiO2 Cr203, MgO, and ~rO2, have been added to the conditioned gel.
In ~he experiments in which MgO was added there was -reaction with the alpha alumina and spinel was formed and ~-~30 was observed as surrounding the remaining unreacted alpha alumina. It was assumed that with the other additives compound formation with the alpha alumina was minimal and -they remained in the crystal boundaries. The experiments which have been run clearly show that the crystal growth , . ~ ~ , . .
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by diffusion and recrystallization was suppressed by the additives. This has value in allowing more flexibility in the time-temperature relationships for the sintered products. The use of the growth inhibitors ~re well known 5 in ceramic technology and are not a necessary part of the invention but are very useful to maintain the desired microstructure over a broad range of sintering time-temperature and where the high purity of alpha alumina is not a requirement.

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Claims (59)

1. A method of making ceramic abrasive grits containing alpha alumina, said grits having a hardness of greater than 16 GPa, a density greater than 90% of theoretical, and an alpha alumina particle size below one micron, comprising providing a gelled dispersion of submicron hydrated alumina particles, said gelled dispersion including an effective amount of submicron alpha alumina crystals, whereby upon drying and firing said hydrated alumina particles are converted to alpha alumina at a temperature below 1100° C., drying said gelled dispersion to form a body and firing said body to below 1500° C.

2. A method of making an aluminous abrasive body comprising providing a gelled aqueous dispersion of hydrated alumina particles of submicron size, said gelled dispersion including submicron alpha alumina seed particles, drying and firing said dispersion for converting the hydrated aluminaparticles to alpha alumina particles of a submicron size, said body having a density of at least 90% of theoretical and a hardness of at least 16 GPa.

3. Ceramic abrasive grits having a matrix consisting of equiaxed alpha alumina crystallites, said clystallites having a maximum size of 0.4 microns, said grits having a hardness of at least 16 GPa, and a density of 90%
or greater.

4. Ceramic grits as in claim 3 in which the alumina is a matrix for ceramic particles of different composition.

5. Abrasive grits as in claim 3 containing a spinel matrix surrounding the alumina particles.

6. Abrasive grits consisting of an alumina matrix and a secondary particulate material, said alumina matrix consisting essentially of submicron equiaxed alpha alumina particles having a density of at least 95% of theoretical and a hardness of at least 18 GPa.

7. Abrasive grits consisting of a matrix of spinel and a discontinuous phase of submicron alpha alumina crystals, said alpha alumina crystals being equiaxed and non-faceted, said grits having a density of at least95% of theoretical and a hardness of at least 18 GPa.

8. Alpha alumina polycrystalline abrasive grits consisting essentially of alpha alumina crystals having a size no greater than about one micron or a mixture of such alpha alumina crystal and alumina containing spinel crystals, said grits having a hardness of at least 18 GPa.

9. An alumina gel containing dispersed alpha alumina seed particles in an amount and size such that drying said gel and firing it at a selected temperature between 1200° and 1500°C and for a selected time of 15 minutes or less which are sufficient to convert the alumina of the gel to alpha alumina having a hardness of at least 16 GPa, will transform any unreacted portion of said alumina to alpha alumina having a crystal size less than 1 micron.

10. A gel as in Claim 9 further containing MgO or a precursor thereof for reacting with alumina in said gel during firing to form a spinel.

11. A polycrystalline ceramic body having a hardness of at least 18 GPa, including at least 87 weight per cent stoichiometric equivalent of alumina, and consisting essentially of alpha alumina crystals or of a mixture of alpha alumina crystals and crystals of an alumina containing spinel, characterized in that, when examined with a scanning electron microscope at a magnification of 5000 times, at least a substantial portion of said body appears as generally equiaxed crystals with a size of not more than about 0.4 microns and in that any portion of said alpha alumina crystals that are individually larger than about 1 micron consists essentially of lath shaped crystals.

12. A polycrystalline ceramic body in accordance with claim 11 wherein at least 80% of said body appears as generally equiaxed crystals having a size not more than about 0.4 microns.

13. A plurality of polycrystalline ceramic bodies each in accordance with claim 12 wherein said bodies are polycrystalline abrasive grain having a density of at least 95% of theoretical.

14 14. Polycrystalline abrasive grain in accordance with claim 13 including alumina-magnesia spinel crystals.

15. Abrasive grain comprising polycrystalline abrasive grits, each of said grits consisting essentially of seeded alpha alumina crystals having a size of less than about 1 micron and having a hardness of at least 18 GPa.

16. Abrasive grain in accordance with claim 15 wherein said grits further include alumina containing spinel crystals.

17. An abrasive tool containing abrasive grain as defined in claims 13, 14 and 15.

18. A abrasive tool containing abrasive grain as defined in claim 16.

19. A grinding wheel containing abrasive grain as defined in any one of claims 13,14 and 15.

20. A grinding wheel containing abrasive grain as defined in claim 16.

21. A flexible abrasive backing having bonded thereto abrasive grain as defined in any one of claims 13, 14 and 15.

22. A flexible abrasive backing having bonded thereto abrasive grain as defined in claim 16.

23. A polycrystalline ceramic body consisting essentially of alpha alumina crystals having a size no greater than about 0.4 microns and a hardness of at least 18 GPa, said body being substantially free from crystalline cells.

24. A method of making polycrystalline ceramic bodies containing alpha alumina, comprising the steps of:
providing a gelled dispersion of sub-micron hydrated alumina particles, said dispersion including an effective amount of sub-micron alpha alumina seed particles for facilitating the conversion of said hydrated alumina to alpha alumina upon sintering;

drying said gelled dispersion; and firing the dried gel to a temperature of between 1090°C and 1500°C for a time sufficient to convert said dried gel to a strong body having a matrix of sub-micron alpha alumina crystals said matrix having a density of at least 90% of theoretical.

25. A method in accordance with claim 24 wherein said step of providing a gelled dispersion includes the step of milling an aqueous phase with alpha alumina containing milling media for dispersing sub-micron alpha alumina seed particles in said phase.

26. The method of claim 24 wherein said dispersion further includes up to about 13% by weight of MgO or a precursor thereof.

27. The method of claim 24 further including the step of crushing the dried gel prior to firing to form abrasive grains upon firing.

28. A method of making a coated abrasive from grain produced by the method of claim 26 comprising the step of bonding said grain to a flexible backing.

29. The coated abrasive produced by the method of Claim 28.

30. The method of claim 24 further including the step of forming said gelled dispersion into a shaped body.

31. A method of making polycrystalline ceramic bodies containing alpha alumina having an alpha alumina particles size of about 0.4 micron or below, comprising the steps of:
providing a gelled dispersion of sub-micron hydrated alumina particles, said dispersion including an effective amount of sub-micron seed particles for facilitating the conversion of said hydrated alumina to alpha alumina upon sintering;
drying said gelled dispersion; and firing the dried gel to a temperature between 1090°C and 1500°C for a time selected to convert said dried gel to a ceramic body, the major portion of which is made up of alpha alumina crystals of about 0.4 micron or below in size.

32. A method of making ceramic bodies containing alpha alumina, comprising the steps of:
drying a gelled dispersion of sub-micron hydrated alumina particles, said dispersion including dispersed alpha alumina seed particles in an amount and size such that firing said dispersion at a selected temperature between 1200° and 1500°C for a selected time of 15 minutes or less sufficient to convert the hydrated alumina particles to alpha alumina having a hardness of at least 16 GPA, will transform said particles to alpha alumina having a crystal size no larger than 0.4 microns, and firing the dried gel to a temperature below 1500°C for a time sufficient to convert said hydrated alumina particles to alpha alumina, said bodies having a density of at least 90% theoretical.

33. A method of making a body containing alpha alumina comprising the steps of:
providing a hydrated alumina gel containing about 0 to 13% of an alumina spinel former or a precursor thereof, and dispersed sub-micron alpha alumina seed particles in an amount such that upon firing at a temperature below 1500°C the alumina of the gel will be converted to polycrystalline alpha alumina or to a mixture of such polycrystalline alpha alumina and a spinel said body having a density of at least 90% of theoretical, and drying said gel and firing it at a temperature between 1090°C
and 1500°C.

34. A method of making polycrystalline abrasive grits comprising:
(a) providing a hydrated alumina sol including dispersed sub-micron alpha alumina particles formed by milling a water containing fluid phase with milling media that contain alpha alumina;
(b) gelling and drying said hydrated alumina sol;
(c) firing said dried gel to convert at least part of the alumina in said dried gel to alpha alumina; and (d) crushing said dried gel either before or after said firing step.

35. A process for making alpha alumina based abrasive grits comprising the steps of:
(a) preparing a dispersion of hydrated alumina particles;
(b) gelling the dispersion;
(c) seeding said dispersion prior to the drying step by adding a nucleating agent thereto;
(d) drying the gel to form a solid;
(e) calcining the solid; and (f) sintering the calcined solid.

36. A process for making alpha alumina based abrasive grits in accordance with claim 35 further comprising the step of forming said solid into abrasive grits.

37. A process for making alpha alumina based abrasive grits in accordance with claim 36 wherein said forming step includes crushing said solid to form abrasive grits.

38. A highly dense sintered alumina body comprising an intimate mixture of finely dispersed stable solid alumina particles and finely dispersed solid nucleation particles, said body having a density in excess of 93%

39. The sintered body defined in Claim 38, having a density in excess of 95%.

40. In the sol-gel process for forming alumina-based ceramic, the process comprising:
a. preparing a dispersion of alpha alumina monohydrate particles;
b. gelling the dispersion;
c. drying the gel dispersion to form a solid;
d. calcining the solid; and e. sintering the calcined solid, the improvement comprising introducing nucleating sites into the dispersion before the drying step.

41. The process of claim 40 wherein the gel also contains a precursor of a modifying additive.

42. The process of claim 41 where the modifying additive is a metal-containing compound.

43. The process of claim 42 where the metal-containing compound is at least one precursor of the oxide of magnesium.

44. The process of claim 40 where the nucleating agent is alpha alumina.

45. The process of claim 44 where the amount by weight of alpha alumina particles is less than the amount by weight of alpha alumina monohydrate particles.

46. The process of claim 40 where the alumina-based ceramic is an abrasive grain.

47. The process of claim 40 including the step of crushing the calcined solid.

48. Alumina-based sol-gel ceramic comprising alpha alumina having nucleating material dispersed therein.

49. The alumina-based sol-gel ceramic of claim 48 including a modifying additive.

50. The alumina-based sol-gel ceramic of claim 48 consisting essentially of alpha alumina having nucleating material dispersed therein.

51. Abrasive grain comprising alumina-based sol-gel ceramic comprising alpha alumina having nucleating material dispersed therein.

52. The abrasive grain of claim 51 including a modifying additive.

53. The abrasive grain of claim 51 consisting essentially of alpha alumina having nucleating material dispersed therein.

54. An abrasive article comprising abrasive grain at least a portion of which comprises sol-gel abrasive grain comprising alpha alumina having nucleating material dispersed therein.

55. An abrasive article of claim 54 where said sol-gel abrasive grain also includes a modifying additive.

56. An abrasive article comprising abrasive grain at least a portion of which comprises sol-gel abrasive grain consisting essentially of alpha alumina having nucleating material dispersed therein.

57. The abrasive article of any one of claims 54, 55 or 56 in the form of a coated abrasive product.

58. The abrasive article of any one of claims 54, 55 or 56 in the form of a bonded abrasive product.

59. The abrasive article of any one of claims 54, 55 or 56 in the form of a bonded abrasive wheel.