DK175226B1 - Fine grained ceramic body or abrasive grit - Google Patents

Abstract

Body sintered below 1400 deg.C or abrasive grit, each with hardness over 16 GPa and 90% density, comprises mainly submicron equiaxed alpha-Al2O3 grains. Body is pref. matrix of Al2O3 contg. grains of different compsn., or of spinel contg. Al2O3 grains. Prepn. is from dispersion of submicron (mono)hydrated Al2O3 particles and effective amt. of submicron alpha-Al2O3 particles which is dried and fired below 1500 deg.C; hydrate converts to alpha-Al2O3 below 1100 deg.C. Grit comprises Al2O3 matrix with density 95% theoretical contg. secondary particles or spinel matrix contg. Al2O3 phase; hardness is at least 18 GPa. Polycrystalline body is obtd. by sintering mixt. of fine non-alpha and alpha-Al2O3 particles above 1090 deg.C.

Description

DK 175226 B1

The present invention relates to a ceramic material in the form of at least one abrasive grain, a process for preparing the ceramic material containing high density polycrystalline α-alumina by sintering a dried alumina gel and a grinding tool comprising the abrasive grain of the ceramic material.

Hard and strong abrasives were used in abrasive disks, flexible, coated abrasive products ("sandpaper") or as a loose abrasive manufactured industrially from alumina-containing raw materials, either by melting in an electric furnace or by burning shaped bodies containing comminuted alumina 10 by temperatures somewhat below the melting points of the material. Such a process at lower temperature is called sintering. The present invention relates to aluminum-containing abrasives made by the sintering process.

The first sintered abrasives made on a large industrial scale were prepared by the method described in U.S. Patent No. 3,079,243.

This patent describes milling of calcined bauxite to produce a fine particle-size feedstock which is then formed into abrasive grain size particles and burned at approx. 1500 ° C to form hard, strong, cool beads of polycrystalline alumina.

Recently, abrasive materials consisting of 20 grains made of alumina and magnesium spinel have been commercially introduced, presumably manufactured in accordance with the disclosure of U.S. Patent No. 4,314,827 and prepared as disclosed in British Patent Application Laid-Open No. 2 099 012A, published December 1, 1982. These materials are prepared by sintering (at about 1400 ° C) dried alumina gel particles. U.S. Pat.

25 3 108 888 discloses and manufacture high-density alumina (or alumina-containing products) by burning a dried alumina gel made from α-alumina monohydrate (boehmite) or by hot pressing dried powders made from such gels.

Industrial abrasives of aluminum / magnesium spinel made of gels 30 contain alumina in the form of cells of 5 to 15 microns in diameter.

I DK 175226 B1 I

I 2 I

The cells consist of many long alumina arms having a diameter I

In 0.2-0.4 microns (but some may also be as large as 1 micron)

In chromium in the form of very coarse spherical lumps), and the arms of each cell I

You generally seem to radiate from the center of the cell. All the arms in a given cell are you

In 5 apparently crystallographically identical oriented. Such an orientation appears

I by the fact that the entire area of a given cell is extinguished simultaneously by I

In rotation of a sample when considered between crossed polarizers in an I

In a microscope with transmitted light. IN

I The industrial abrasives made from sintered gels containing aluminum I

In 10 umoxide and magnesium oxide are high quality abrasives, but it has not been

It is possible to make high purity alumina grains by the gel method. This I

You are shown by the relative softness and lack of abrasiveness of the control in Eq

In Example 13 of U.S. Patent No. 4,314,827, which was manufactured from an alu-I

In mini oxide gel without the addition of metal oxide or metal salt. IN

U.S. Patent No. 3,909,991 discloses a process for making slides

In bending grains of high density by a hot pressing method. This forward

In the process, the starting material which may be selected from I

In bauxite, α-alumina and zirconia, are ground into a powder with a gene

In average particle size of less than 1 µm. This powder is then preformed

For abrasive grains which are mixed with powdered graphite in such an amount,

In that each grain is completely surrounded by graphite. The mixture of cereals and I.

Graphite powder is then subjected to a pressure of at least 2000 psi (about 138 bar) I

I at a temperature of 1200-1600 ° C to eliminate porosity. IN

After a certain amount of time, the mixture is cooled and the product of hot pressed sludge

In 25 bones grains are separated from the graphite powder. IN

However, the product obtained by this hot pressing method has been used

In stretchable abrasive properties, in particular the resistance of the grains to abrasion I

You are too low for demanding grinding tasks. Furthermore, it is both expensive and long-lasting

In producing only small quantities of the product. IN

3 DK 175226 B1

The object of the present invention is thus to provide a ceramic material in the form of at least one abrasive grain which has improved abrasive properties and high abrasion resistance and which can simultaneously be manufactured on an industrial scale in an economically advantageous way.

Surprisingly, it has been found that this object can be achieved with a material of the kind set forth in the preamble of claim 1 which has a phase of polycrystalline α-alumina having a particle size not exceeding 0.4 µm. The reason for the good abrasive properties of the product according to the invention is probably that, due to the low particle size, the abrasive grains wear out in a manner that is markedly different from that of corresponding products with larger particles. Where abrasive grains with particles greater than 0.4 µm are generally worn by cleavage of the particles along their crystal lattice planes to form blunt and blunt particles, abrasive grains of the invention with particles smaller than 0.4 µm are likely worn by tearing off single particles, leaving sharp 15 edges.

Thus, the present invention relates to a ceramic material in the form of at least one abrasive grain containing a high density polycrystalline phase of submicron alpha alumina particles of non-cellular structure as defined herein and optionally an additive having a hardness of 20 at least 14 GPa, with the proviso that the hardness must be at least 16 GPa when the material does not contain spinel as an additive, which is characterized in that the submicron α-alumina particles have a size not exceeding 0.4 µm.

The invention further relates to a process for preparing the high density polycrystalline alumina ceramic material by sintering a dried alumina gel, characterized in that the gel has been treated with the addition of submicron α-alumina particles to the gel or to a gel. precursor to the gel.

The invention further relates to an abrasive tool made of the ceramic material in the form of abrasive grains in a bonded form.

In the invention, the invention lies in the fact that control of the microstructure of the burnt product so that the cellular microstructure of aluminum oxide in the known abrasives is avoided results in improved product characteristics. Instead of cell areas of 5-10 microns in diameter, the product obtained in 5 contains α-alumina particles (crystallites) of a size,

not exceeding 0.4 μιτι (preferably 0.2-0.4 micrometers). IN

In the case of the higher (eg in some cases 5%) MgO-I

additives, these alumina particles are surrounded by a matrix of I

In spinel. IN

Adaptation of the gel to achieve the effect described can be achieved by vibration

In running milling of the mixture in sol form or diluted gel form while I

In alumina bodies are used as milling medium in the mill. It assumes I

It is given that the main effect of the milling is to introduce material from aluminum I

In the oxide grinding medium in the alumina gel. Also impurities such as zinc and I

H 15 iron is introduced from pipelines and associated equipment. Grinding with zirconia

In xid bodies, e.g. ineffective in producing the desired substantially I

non-cellular structure, i.e. essentially free of cells. By "cells" is meant I

here such that have the same crystallographic orientation and are the result I

of growth of the same crystalline structure. One can thus to a certain extent I

In 20 we say that cells mean single crystals. IN

The first effective and reproducible method found in accordance with the invention

In the process, such material was developed in the gel by vibrating grinding of I

the gel with alumina bodies. A suitable vibrating mill is shown in America

In U.S. Patent No. 3,100,088. Typically, the medium may be 1.2 cm in diameter and I

In 1.2 to 1.8 cm long. The vessel containing the medium and the mixture is vibrated in it

In a horizontal plane with an unbalanced weight connected to the shaft of an engine I

In mounted coaxially with the tub, which is mounted on springs. The unbalanced I

weight is mounted in the immediate vicinity of the base of the tub, and another I

weight is mounted under it. The motor typically rotates at 1200 rpm

30 pr. minute. The combined oscillation subject the contents to a grinding I

In effect with the grinding medium. The inner surface of the mill is preferably 1

L1, e.g. with rubber, to prevent contamination by erosion of the metal walls.

Various additives as disclosed in U.S. Patent No. 4,314,827 and British Patent Application No. 2,099,012A may be added to the aluminum oxide before or after gelation. The most useful additive known at the moment is any compatible precursor of MgO, the final product preferably containing about 10 mg. 5%

MgO. However, MgO is found in the product as spinel (magnesium aluminate: MgAI2O4), but is calculated as MgO in the assay. It is to be understood that smaller amounts of MgO may be included, since alumina is, without any addition, an excellent abrasive in itself when produced in accordance with the present invention. The ground gel according to the invention can serve as a matrix for various added materials or abrasive particles.

The milled mixture can simply be poured or placed in containers for drying and then broken up into pieces of appropriate size by crushing, with 15 material of too small size being recycled for the beginning of the process. Alternatively, the material may be formed or molded into shaped particles such as by extrusion. In the case of extrusion, the formed rods will later be cut or broken into pieces of appropriate size. The minimum useful burning temperature is considerably below 1200 ° C and is generally considered the point of conversion for the conversion of α-alumina. The upper limit is not essential when the melting temperature is not reached. Excessive burning or too high a temperature can cause too much crystal growth. Higher temperatures increase the cost of the process; the preferred firing range is 1200-1500 ° C.

EXAMPLE 1 In a large polymer plastic mixing vessel, 13.6 kg of Condea SB Pural alumina (supplied by Condea) and 136 liters of water were mixed. This material was then gelled by the addition of 4.1 liters of 14 wt% HNO3. Magnesium nitrate hydrate (3.4 kg) dissolved in 13.7 liters of water was then added to the aluminum oxide gel 30 to form 5% by weight MgO in the final product. It was mixed for 15 minutes in DK 175226 B1 I 6

and transferred to a Model M451 Sweco mill and ground for one hour with 765 kg I

aluminum oxide medium. The mixture was recycled through the mill in a single I

For hours of grinding time at a rate of approx. 18 liters per minute. After grinding I

In it it was pumped into aluminum trays to a thickness of approx. 7.6 cm with I

5 for drying on electric distillation dryers. IN

H The composition of the alumina medium was approx. 90% α-alumina I

with silica as the major impurity. IN

A number of portions of the above composition, I, were prepared

and they were united for the purpose of crushing and burning. IN

The dried gel was then rolled and sieved through a * 14 mesh sieve before I

In firing to form the desired final grain sizes. It then became you

I burned at 400 ° C for 16 hours and burned at 1400 ° C for 30 minutes in an I

rotary kiln. IN

After firing, all the product had a hardness of 19 GPa (Vickers impression- I

15 g apparatus, 500 g load) and a very fine microstructure in which I do not

You had some cellular microstructure and almost all of the α-alumina was in I

In the form of generally equal-axis particles (crystallites), 0.2-0.4 microns in diameter, I

with the exception of rare square, block-like shapes with a diameter I

I in approx. 5 microns. The blocky forms may have shown contamination. In case of under-

In a scanning electron microscope search, the product was seen to be together

set of a spinel matrix and a discontinuous phase of α-alumina.

I For some special grinding purposes with coated abrasive, the material was better than molten alumina-zirconia and better than industrially available.

In a sintered gel-type abrasive with alumina spinel I

25 composition.

In Example 2, Pural microcrystalline boehmite alumina, 22.7 kg, was mixed with I 225 liters of water and 13.5 liters of 14% HNO3 for 10-15 minutes.

7 DK 175226 B1

Half of the gel mixture was ground for 2 hours in the Sweco mill containing 1.2 x 1.2 cm of ceramic bonded alumina, 88 Al 2 O 3 (where the main impurities are MgO 1.74%, SiO 2 8.9%, Fe 2 C> 3 0.18 %, TiO2 0.2%, CaO 0.8%, Na2 0.34%) available from Coors Porcelain Co., and dried. This was the same medium as used in Example 1. The other half was simply dried without grinding. The dried gels were crushed to the desired size, burned at 450 ° C for 16 hours and burned at 1400 ° C for one hour.

The milled material had a hardness of 19.1 GPa and the non-milled material had a hardness of 11.0 GPa.

10 Material from each batch was screened to form 50 abrasive grains, which were used to make coated abrasive discs with a vulcanized fiber backing. The milled material was 10% better than industrial alumina / zirconia abrasive for grinding 1020 steel (the sample showed a 14% greater metal removal).

The non-milled product was inferior to molten abrasive in all abrasion tests, which is to be expected given its lower hardness.

The physical difference between the milled and non-milled product is such that the milled product has a microcrystalline non-cellular structure, while the non-milled has a cellular structure. EXAMPLE 3 In an example similar to that of the milled product in Example 1, the gel was milled for 0.2 hours. The product burned at 1400 ° C for one hour was mainly of the fine, random 0.2-0.3 mm crystal structure, but exhibited some cellular appearance.

The result is thus a mixture of both cellular and non-cellular structure. I DK 175226 B1 I 8 IN EXAMPLES 4-9

Further examples were carried out by examining the effect of firing time at 1400 ° C. All samples were prepared by the general method of Example 1. "Condea" microcrystalline boehmite H 5 alumina was used, which was ground for 2 hours, but after drying the gels were pre-H burned at 750 ° C for 30 minutes. . As the burning time was increased, a coarse rod-shaped crystallization of alumina began to occur in the product, randomly distributed among the fine 0.2-0.4 microns of alumina particles.

I 10 The results are given in the table below: H Particle size (micrometers)

Burning time Ratio, I (minutes) Rough_ Fine_% Rough /% Fine 1 None 0.2-0.3 0 3 1.0-2.0 0.2-0.3 5 I 5 2-5 0.2-0 , 3 20 10 4-8 0.2-0.3 50 I 30 Up to 8 0.2-0.3 80 60_Op to 8 0.2-0.3_95_

Since the presence of the coarse fraction is assumed to be less desirable, the burning time at 1400 ° C should not be more than 5 minutes to the preferred product when the material is burned at 750 ° C for 30 minutes.

In all cases, no cellular structure was observed. The microstructures consisted of the non-faceted submicron particles and the faceted, rod-like, coarse crystals, except in the case of one minute firing where no rods were found.

By "non-faceted" is meant that no regular faceting of the crystallites was observed in a fracture surface at 5,000 times magnification with 9 scanning electron microscope. Instead, the particles of α-alumina were rather formless, but equal to axes with generally curved outlines and very few seemingly straight contours. At 20,000 times magnification, a faceted structure begins to become visible.

The abrasive grains of the invention have a hardness, measured with Vicker's pressing apparatus, with a 500 g load of at least 16 GPa (90% density) for alumina without additives and at least 14 GPa for grains modified by the presence of 2% or more by spinel or modified by other additives. Pure, heavy α-alumina has a hardness 10 of approx. 20-21 GPa, but some porosity may be desirable for certain applications, which will reduce the hardness. When the alumina has a hardness of 13 GPa or less, it is too porous for the purposes of the present invention. Preferences are hardnesses of 18 GPa or more, as measured by Vicker's indentation device and a load of 500 g.

EXAMPLE 10

A number of abrasives with varying levels of magnesium oxide were prepared.

The general procedure of Example 1 was used including the milling (but for 2 hours) with alumina medium. In all cases, the gels 20 after drying at 200 ° G for approx. 30 hours crushed and sieved and then calcined at 450 ° C for 16 hours. The resulting particles of abrasive grain size were burnt in a rotary furnace at 1400 ° C. The heating time to 1400 ° C was approx.

15 minutes and the time at 1400 ° C was approx. 15 minutes.

Various amounts of magnesium nitrate were added before the gelling. In one experiment, no magnesium nitrate was added. The MgO content and hardness of the abrasives were as follows:

I DK 175226 B1 I

I 10 I

In Hardness (Vickers, 500 g)

In Experiment No. MgO content (wt%) loading) _ I

I 9498 0.14 19.9 I

I 9499 2.50 19. IN

I 9500 7.95 19 I

I 9502_1271_19_ - I

I In a series of tests with vitrified (glass-bottomed) grinding wheels during use I

In view of 54 grains (a combination of grain sizes 46 and 60), abrasive washers I

I made with the above grains compared to the best known I

5 molten alumina abrasive (sulphide process abrasive). IN

The experiments were performed by grinding grooves in tool steel (D3) at different I

In regulated grinding. For dry grinding and grinding of 0.0013 cm sea-I

In those abrasives containing no added MgO (0.14% MgO), a form I

In gelation ratios that were 16.18 times the milling ratio of the molten abrasive

In medium (the grinding ratio is the volumetric ratio of removed matter

In the rial and the wear on the grinding wheel). All the MgO additions resulted in better I

In effect compared to the molten abrasive in dry grinding tests. IN

In the wet grinding tests, the experimental abrasives with MgO I were added

In worse than or equal to the molten abrasive. At 0.005 cm it was slippery

In 15 mediated without magnesium oxide addition better than it melted. IN

For coated abrasive samples using the abrasive with I

In the grain size 50 (CAMI standard), an abrasive made according to I worked

In Example 10 and containing 0.6% MgO, incorporated in flexible grinding wheels, I

In better (136%) than a fused alumina / zirconia abrasive I

In 20 on 1020 steels and almost equivalent to molten alumina / zirconia I

In stainless steel. The abrasives containing 2.5% MgO and 7.59% MgO were I

also better on 1020 steel. The higher MgO addition was less effective on I

stainless steel. IN

The abrasive with 0.14% MgO contained in addition to the alumina: 0.25% SiO2, I

0.18% Fe 2 O 3, 0.28% TiO 2, 0.05% CaO and 0.04% Na 2 O, presumably

DK 175226 B1 11 attachment introduced during grinding. Similar amounts of these impurities were present in the other abrasives.

Thus, the above indicates a correlation between the MgO content's significance for the abrasive properties in relation to the selected steel.

The invention is not tied to any particular theory, but it is believed that the introduction from the alumina medium of particulate matter may cause grafting of the crystallization of α-alumina during firing. In addition, the other impurities introduced in the milling step may inhibit crystal growth in the finished product by their presence at grain boundaries between the α-10 alumina particles.

As evidence that it is the waste from the grinding medium that is effective in adapting the gel to produce the desired finely crystalline, non-cellular, high density α-alumina by firing at ca.

At 1400 ° C, addition of ground water to alumina monohydrate was made with acid without grinding the gel.

Water, nitric acid and microcrystalline boehmite were mixed as in Example 2, except that six batches of varying addition of water containing waste separated from the alumina forming medium were prepared and then milled for several hours with water (no other addition). 20 to the water).

25

I DK 175226 B1 I

I 12 I

Addition of "ground water" to alumina monohydrate ("Condea"): I

I Weight ratio of milling waste- Weight% of waste in I

In Experiment Fall and Aluminum Monohydrate Burned Product x) Hardness I

I _nr_ (GPa) I

I 1 0.0074 1.07 20+ I

I 2 0.0037 0.53 20 I

I 3 0.0019 0.27 19+ I

I 4 0.00075 0.11 17 I

I 5 xx) 0.00038 0.05 15 I

I 6 xx) _0_0_12.5 I

I Notes I

x) Assuming a 30% average loss on burning. IN

In accordance with the invention. IN

In Hardness was determined on the burnt product burned at 1400 I

I ° C ± for approx. 10 minutes. The oven was electrically heated and the atmosphere was air. IN

In Examination of the milled waste showed that it was mostly α-I

In alumina with a surface area of approx. 39 m2 per g

In 10 high purity alumina, produced by the recovery of the fine suspension

In the alumina particles left in the suspension, very fine I

In alumina powders are allowed to precipitate after mixing with water, you are

Also effective when used in an amount of approx. 0.1% of burnt solid I

In substances from the gel. IN

I 15 Experiments with industrial fine α-alumina powders and experiments with fine aluminum I

In minic oxide, developed by grinding molten alumina of very high I

In purity using alumina even as grinding medium vi- I

They were very effective in producing the heavy fine crystalline process

In cloth according to the invention. IN

13 DK 175226 B1

Differential thermal analysis has shown that when the α-alumina graft particles are present, the transition of the gel alumina from the presumably γ-form to the α-form occurs at ca. 1090 ° C while the transition in the absence of such seed material takes place at ca. 1190 ° C. Thus, the theoretical minimum firing temperature for the products of the invention may be below the usual mentioned transition temperature.

The present invention enables, for the first time, the preparation of low temperature sintering of high purity ot alumina bodies having a submicron particle size and a density greater than 95% resulting in a hardness greater than 18 GPa. Non-abrasive products, such as coatings, thin films, fibers, rods or small shaped parts, can be made by the process of the invention.

Grain growth inhibitors such as SiO 2, Cr 2 O 3, MgO and ZrO 2 have been added to the conditioned gel. In these experiments where MgO was added, there was reaction with the 15 α-alumina and spinel was formed and observed as surrounding the remaining unreacted α-alumina. It was believed that with the other additives, mixing formation with the α-alumina was minimal and remained at the crystal limits. The experiments performed clearly show that the crystal growth by diffusion and recrystallization was suppressed by the additives. This has 20 value by enabling greater flexibility in the time-temperature relationship to the sintered products. The use of the growth inhibitors is well known in the ceramic technology and is not a necessary part of the invention, but is very useful for maintaining the desired microstructure within a wide range of sintering time and temperature and where the high purity of α-25 alumina does not occur. is a requirement.

EXAMPLE 11

A ceramic material consisting of fine-grained α-alumina particles was prepared using the hot pressing method described in US 3,909,991.

I DK 175226 B1 I

I 14 I

In 30 g alumina powder of the brand Ucar A from Union Carbide (99.98% I

I Al2O3; grain size: 0.3 µm) was subjected to 20 minutes of ultrasound treatment

Dissolve in 75 ml of anhydrous isopropanol. Then the isopropanol was completely I

Evaporated under vacuum over 40 hours to form a dry aluminum I

In 5 oxide powder. The dried alumina powder remained under helium atmosphere I

I filled in a graphite press, which was closed at both ends with graphite slabs. · I

In the Aluminum powder, a pressure of 2000 psi was first applied, while for 24 hours

More was evacuated to a vacuum of 100 mTorr. Then the temperature was I

I raised to 800 ° C at a rate of 12 ° C / min and kept at this temperature for 1 hour

For 10 12 minutes. Over three minutes, the pressure was then increased to 6140 psi, I

After which the temperature was raised to 1300 ° C at a rate of 36 L

In ° C / min and kept at this temperature for 5 hours. Eventually the pressure was taken off in I

Over three minutes, the product cooled to 1100 ° C at a rate of 12 L

In ° C / min, followed by natural cooling to ambient temperature. IN

The resulting hot pressed alumina product had a density I

I of 3.81 g / cm3, a mean particle size of 0.84 μιτι and a Vickers hardness I

At 18.6 GPa. IN

In Figure 1, the particle size distribution of the obtained product (referred to as I

In HP AI2O3) shown together with the particle size distribution of a commercial I

In an alumina product according to the invention (referred to as SG AI2O3) prepared at I

In the sol-gel method described in the specification. It can be seen from the figure that under I

In 15% of the particles in the HP AI2O3 product has a particle size of 0.4 µm I

I or less, while all the particles in the SG AI2O3 product have a size, I

In which does not exceed 0.4 pm. IN

In order to compare the abrasive properties of the present invention, I

Sample material manufactured with the commercial product of the invention I

In late, both materials were subjected to the so-called "scratch disc" test. IN

This is accomplished by applying a single abrasive grain to a grinding wheel, and brining

In 30 g of abrasive grain in contact with a flat workpiece in the form of a plate of I

15 DK 175226 B1 52100 steel (cf. the working arrangement in figure 2). The grinding wheel is rotated so that the grain moves at a linear speed of 30 m / s and the workpiece is advanced under the grinding wheel at a speed of 16.5 mm / s, whereby the grinding grain cuts a scratch in the workpiece. By comparing the cross-sectional area of the scratch resulting from the first cut with the cross-sectional area of the scratch after a fixed number of cuts, a determination of the amount of abrasive grain is worn away, and thus a determination of the abrasion resistance of abrasion.

The result of this test performed on a grinding grain of a commercial material according to the invention and the material prepared according to the present example, respectively, is shown in Figure 3. It is immediately apparent from this figure that the material according to the invention has considerably better resistance to abrasion than the hot pressed alumina prepared according to the method described in US 3,909,991.

15

Claims (17)

A ceramic material in the form of at least one abrasive grain, containing a polycrystalline high density phase of submicron α-alumina particles of non-cellular structure as defined in the specification, and optionally an additive, II and a hardness of at least 14 GPa, with the proviso that the hardness II must be at least 16 GPa when the material does not contain spinel as an additive, II characterized in that the submicron α-alumina particles have a II 10 size not exceeding 0.4 microns. IN Ceramic material according to claim 1, characterized in that the I grains have a hardness of at least 18 GPa. IN Ceramic material according to claim 1 or 2, characterized in that it contains an additive compatible with it. I I 15 Ceramic material according to claim 3, characterized in that the I additive is magnesium oxide present as spinel. IN 5. Ceramic material according to one of the preceding claims, characterized in that it contains faceted, rod-like coarse-grained aluminum oxide casings randomly distributed among the submicron particles. I 20 6. Ceramic material according to one of the preceding claims, characterized in that it contains impurities. IN Ceramic material according to claim 6, characterized in that the impurities include zinc and / or iron. IN 8. Ceramic material according to any one of the preceding claims, characterized in that it has a density of at least 90% of the theoretical density of α-alumina. I 17 DK 175226 B1 Ceramic material according to claim 8, characterized in that the density is at least 95% of the theoretical density of α-alumina. Process for the preparation of a ceramic material according to claims 1-9, containing high density polycrystalline α-alumina by sintering a dried alumina gel, characterized in that the gel has been treated by the addition of submicron α-alumina particles. to the gel or to a precursor to the gel. Process according to claim 10, characterized in that the gel or a sol used for the preparation of the gel is ground using 10-alumina bodies as milling medium in the mill. Process according to claim 10, characterized in that the gel is treated by adding water milled with α-alumina bodies as grinding medium. Process according to claims 10-12, characterized in that the gel comprises an aqueous dispersion of submicron hydrated alumina particles. A process according to any one of claims 10-12, characterized in that the dried gel is crushed before or after sintering to form abrasive grains. 15. Abrasive tools made of bonded ceramic materials in the form of abrasive. grain according to any one of claims 1-9. Abrasive preparedness according to claim 15 in the form of a flexible backing with ceramic materials in the form of abrasive grain according to any one of claims 1-9. Abrasive preparation according to claim 16 in the form of a grinding wheel containing ceramic materials in the form of abrasive grains according to any one of claims 1-9.