Covered electrofused alumina particles and its production

The present invention relates to coated particles of fused alumina, a method of making the same, an abrasive sand, an abrasive disc and a coated abrasive.

EP 161 869 A2 discloses coated abrasive products which comprise a carrier layer on which abrasive grains having no coating layer are applied, such as, for example, in the case of sandpaper.

US Pat. No. 5,143,522 A discloses alumina-zirconium dioxide abrasives which are improved by adding reduced titanium dioxide (titanium oxide). The abrasives have a composition comprising clays, zirconia and titanium dioxide, the titanium dioxide being one of the constituents of the solid solution of the abrasives.

As explained in JIS R6111-1987 for Artificial Abrasives, alumina abrasives made by melting Baeyer alumina in an electric furnace include white alumina abrasives (WA), pink alumina abrasives (PA), monocrystalline alumina abrasives! (HA) etc. The toughness of the abrasive increases from WA to PA to HA (HA> PA> WA), but they do not yet have suitable grinding properties for tool steel or other hard materials. Therefore, attempts have been made to coat the surface of the molten alumina with various additives to improve its abrasive properties.

For example, JP-B-29-25620 discloses that an abrasive sand made of molten alumina is coated with 0.5 to 2.0% of an iron compound and heated to 700 to 950 ° C to form a thin iron oxide film on the abrasive sand. JP-B-40-16594 discloses that an abrasive sand made of molten alumina is coated with 0.01 to 1.0% of a chromium compound and heated to 800 to 1,400 ° C. to form a thin chromium oxide film on the surface of the abrasive sand. JP-B-44-637 discloses that an abrasive sand made of molten alumina is coated with 0.2 to 2.0% of a copper compound and heated to 900 to 1,250 ° C to form a thin copper oxide film on the surface of the abrasive sand. JP-B-44 638 discloses that an abrasive sand made of molten alumina is coated with 0.2 to 2.0% of a nickel compound and heated to 800 to 1,000 ° C to form a thin film of nickel oxide on the surface of the abrasive sand.

These coating layers are intended to compensate for micro-defects in the particles of molten alumina which occur in the grinding of the particles from the ingot and do not significantly improve the abrasive properties of the abrasive sand.

Furthermore, these coating layers physically cover the surface of the abrasive sand and can be easily peeled off. As a result, the abrasive properties of the abrasive sand are not improved.

JP-B-39-11196 discloses that a molten clay is coated with colloidal clay. JP-B-40-14357 discloses that a molten alumina is wetted with an organic adhesive solution, powder of refractory materials such as Fe₂O₃-ZnO-MnO mixture, a TiO₂-SiO₂-ZnO mixture or TiO₂ are applied to the wetted molten alumina and the molten alumina is heated with the refractory powders to melt the refractory powders onto the surface of the molten alumina.

These coating layers are intended to improve the wetting of the grinding sand with a binder in the manufacture of a grinding wheel, so that the toughness of the grinding wheel is improved. The abrasive sand itself is thus not significantly improved and an improvement in the abrasive properties of the abrasive sand is not achieved.

It is an object of the present invention to provide coated particles of fused alumina which can be used as an abrasive material having excellent abrasive properties or a refractory material having excellent thermal shock resistance. The present invention particularly aims to improve the bonding of the coating layer to the molten alumina by chemically bonding the coating layer and the molten alumina to prevent the coating layer from peeling and falling off, and to improve the mechanical strength and toughness of the alumina particles.

According to the present invention, the coated particles are of fused alumina characterized in that the particles are coated with a coating containing aluminum titanate as a main component. According to the invention, the alumina particles are characterized in that the coating consists of aluminum titanate and an alumina compound

AT 407 749 B

Another feature of the invention is that the alumina compound is selected from the group consisting of spinels formed from alumina and an oxide of either Fe, Mg, Co, Zn, Mn or Ni. According to the invention, the coating of the alumina particles is free of titanium oxide; but it can also consist essentially of alumina titanate.

According to a further feature of the invention, the coating layer has a thickness of 10 to 20 µm. According to the invention, the particles have a Vickers hardness of> 2,100 kg / mm².

One embodiment of the invention is that the aluminum titanate of the particle coating is β-aluminum titanate. According to the invention, the particles have a particle size of 3,000 to 20 μm.

The invention also relates to a process for the production of coated particles of molten alumina which is characterized in that it comprises the following steps, namely that a titanium-containing compound is applied to the surface of the particles, that the particles are at a temperature of 1200 ° C or higher are heated to form a coating layer containing aluminum titanate as a main component on the surface of the particles.

According to the invention, the method is further characterized in that the titanium-containing compound is selected from the group consisting of titanium oxides, titanates, water-soluble titanium compounds and titanium alkoxides. According to a further embodiment of the method according to the invention, titanium-containing compound is applied on the surface of the particles in an amount of 0.03 to 3.0 parts by weight as TiO₂ applied to 100 parts by weight of alumina. The method is further characterized according to the invention in that the particles are heated to a temperature between 1,400 to 1,700.degree.

The invention further relates to an abrasive sand which is characterized in that it consists of the coated particles of molten alumina as indicated above according to the invention.

The invention also relates to a grinding wheel which, according to the invention, has such an abrasive sand, as well as to a coated abrasive which, according to the invention, is characterized in that it has such an abrasive sand.

The fused alumina particles used in the present invention are electrically fused alumina particles as set out among alumina particles in JIS R6111-1987 for artificial abrasives as brown alumina (A), white alumina (WA), pink alumina (PA) or monocrystalline abrasives Alumina (HA) is defined. The particle size, properties, etc. are not limited to those in JIS R6111-1987.

The following describes the preparation of the coated particles of fused alumina.

First, a titanium-containing compound is deposited on the surface of the molten alumina particles, such as molten alumina abrasive sand.

The titanium-containing compound can be powders of titanium oxides such as rutile and anatase, titanates such as iron titanate, magnesium titanate, cobalt titanate, zinc titanate, manganese titanate and nickel titanate, water-soluble titanium-containing compounds such as titanium tetrachloride, metatitanate and titanium acid, titanium alkoxides such as titanium isopropoxide.

The amount of the titanium-containing compound applied to the particle of fused alumina is preferably 0.03 to 3.0 parts by weight of TiO 2₂ per 100 parts by weight of particles of fused alumina. If the amount of the titanium-containing compound applied is less than 0.03 part by weight, the aluminum titanate to be formed is too small to coat the entire surface of the molten alumina particles. If the amount of the titanium-containing compound applied is greater than 3.0 parts by weight, the titanium content is too high, so that aluminum titanate is formed, but unreacted TiO₂ and the like remain, thereby deteriorating the properties of the coating layer.

The applied titanium-containing compound essentially reacts with the alumina to form aluminum titanate and form a coating layer on the surface of the particles of molten alumina.

The coating layer on the surface of the particles of molten alumina according to the present invention contains aluminum titanate as at least a main component. The coating layer may contain other alumina compounds formed by a reaction between the alumina of the particles of molten alumina and the titanium-containing compound

AT 407 749 Β can be. Preferred alumina compounds include various spinels formed from alumina and oxides of divalent metals such as Fe, Mg, Co, Zn, Mn and Ni. However, it is preferable that the surface coating layer be made of aluminum titanate.

The method of applying the titanium-containing compound to the particles of molten alumina is described below.

In the event that powder of titanium oxide, titanate or the like. are suitable for application to the surface of the alumina particles, the mixture of these powders with the alumina particles may be sufficient. If such simple mixing is not sufficient and the powders are not applied and separate from the alumina particles, the alumina particles can first be wetted with a liquid such as water in an amount of 5 to 20% by weight, after which the powders are applied to the Alumina particles are applied.

In the case where a water-soluble or organic solvent-soluble titanium compound is coated on the surface of the alumina particles, a solvent is preferably used in such an amount that the surface of the alumina particles is just wetted, for example 5 to 20% by weight, and the Alumina particles are mixed with the titanium compound solution.

After the titanium-containing compound has been applied to the alumina particles, the particles are dried to remove the water or solvent, if any. After drying, the solvent is evaporated and titanate, titanium hydroxide or the like. remains on the surfaces of the alumina particles. If the amount of the solvent is large, the solute may migrate during the drying process, resulting in no uniform distribution of the compound on the surface of the alumina particles, or the alumina particles may aggregate with each other with the compound therebetween, in this case, if the amount of the compound is not large, aggregation is relatively rare, but if the amount of compound is large, The disintegration of the alumina particle aggregates can lead to the removal of the compound from the alumina particles or to a non-uniform distribution of the compound on the surfaces of the alumina particles.

For this reason, if a relatively large amount of solvent is required, a rotary or fluidized bed dryer is preferably used in order to prevent agglomeration or migration of the solute during drying.

The drying after application of the compound is preferably carried out at a temperature of 40 to 90 ° C., more preferably at a relatively low temperature in this range for a longer period of time. If the temperature is above 90 ° C, the water is completely removed after the drying step and the applied compound is disadvantageously separated from the alumina particles . The drying time is preferably 10 to 20 hours.

The dried sample is then placed in a crucible, such as an alumina crucible, and heated, for example, in an electric furnace, in order to react the titanium-containing compound with the alumina. As a result, the surfaces of the particles of molten alumina are coated with a coating layer of aluminum titanate.

The heating is preferably carried out at a temperature in the range from 1,200 ° C to the melting point of aluminum titanate, i.e. 1,860 ° C, more preferably 1,400 to 1,700 ° C. The aluminum titanate forms at a temperature above about 1200 ° C, but a temperature of 1400 ° C or higher is preferable because a lower temperature requires a longer heating time. A temperature of 1,700 ° C or less is preferable because a temperature near the melting point of aluminum titanate can cause the particles to coalesce. The time of heating at 1,400 ° C is preferably 30 minutes or more, and for heating at 1,700 ° C it can be as short as 5 minutes.

In this way, particles of fused alumina coated with a coating layer of aluminum titanate are obtained.

The molten alumina particles coated in this way have a coating layer of aluminum titanate with a thickness of about 10 to 20 µm. The titanium compound was completely converted to aluminum titanate and no other titanium compound was detected by X-ray diffraction. No titanium was found at a depth greater than 25 µm from the particle surface. The aluminum titanate which coats the surfaces of the alumina particles is a low temperature aluminum titanate, β-aluminum titanate.

The coated particles of fused alumina of the present invention are harder than

AT 407 749 B the corresponding uncoated particles of molten alumina and have a Vickers hardness of 2,100 kg / mm² or more, whereas uncoated fused alumina has a Vickers hardness of less than 2,100 kg / mm² Has.

After the heat treatment at 1,200 ° C or more, the coated particles of molten alumina are cooled to room temperature and sieved with a sieve or the like to obtain a predetermined grain size of the abrasive sand. In this way, the abrasive sand with a desired grain size is obtained.

The abrasive sand generally has a grain size as prescribed in JIS R6111-1987 for Artificial Abrasives and JIS R6001-1987 for Abrasive Grain Sizes, but is not limited to these in the present invention. For example, the abrasive sand according to the invention has a grain size of 3,000 to 20 μm.

The abrasive sand of the invention can be free abrasive sand or used to make abrasive wheels and coated abrasives as below.

An abrasive wheel is made by shaping and binding (or hardening) the abrasive sand with a binder such as a glazed binder, a metal binder, or a resin binder. A grinding wheel with a glazed binder is preferable. The binder used for a grinding wheel with a glazed binder is a so-called frit and is made by mixing feldspar, pottery stone (Toseki), borax, clay and the like and consists of SiO₂, B₂O_3l AI₂O_3i Fe₂O₃, CaO, MgO, Na₂OK₂O, etc. A glazed binder grinding wheel is made by adding a small amount of glue such as dextrin to the above binder, mixing the sand with the binder, pressing it into a mold, drying it, and firing the molding. The firing temperature is preferably 950 to 1,150 ° C.

A coated abrasive material is produced by bonding the abrasive sand to a base with an adhesive. The adhesive is preferably a phenolic resin-based adhesive from the viewpoints of excellent grinding performance and water resistance, and can be used in combination with resorcinol or a derivative thereof so that the setting conditions are favored.

The substrate for the coated abrasive can be, for example, paper, woven fabric and non-woven fabric. A polyester fiber cloth is also used in an abrasive belt, etc. for heavy-duty work. Synthetic fiber non-woven fabrics, such as nylon fiber, are also used as a substrate for non-woven fabric abrasive materials.

Among other coated abrasives, JIS (Japanese Industrial Standard) gives abrasive cloths (JIS R6251), abrasive papers (JIS R6252), waterproof abrasive papers (JIS R6253), endless abrasive belts (JIS R6254), abrasive wheels (JIS R6225), abrasive belts (JIS R6256), and cylindrical ones Abrasive drums (JIS R6257), but the coated abrasives of the present invention are not limited thereto.

An important coated abrasive, which is not specified in JIS, comprises an abrasive made of non-woven fabric, which is a flexible abrasive material (abrasive cloth) in the form of a non-woven fabric, composed of three components, an abrasive sand, a fiber (e.g. nylon or polyester fiber) and an adhesive. This non-woven fabric abrasive has a three-dimensional network structure of randomly oriented and crossed fibers and has a large volume of contiguous spaces and a thickness of about 2 to 8 mm, so that it is excellent in flexibility and pressure recovery.

EXAMPLES

example 1

500 g of white fused alumina abrasive (WA, manufactured by Showa Denko KK) with a grain size of # 60 was placed in a bowl made of SUS304. The grain size of # 60 is given as 350 to 210 μm. 1.25 g anatase TiO₂Powder (Ishihara Sangyo KK, A-100; mean particle size 0.2 μm, 0.25% by weight TiO₂ 100% by weight of clay) was dispersed in 50 ml of distilled water. The resulting 2.44 weight percent dispersion was added to and mixed with the abrasive in the cup with stirring to thereby obtain the TiO₂-Apply powder to the abrasive particles.

AT 407 749 Β

The particles were then dried in a blow dryer at 60 ° C. for 16 hours to reduce the remaining water content to 2.3% by weight.

The powder coated abrasive was placed in an aluminum pan and heated up to 1,500 ° C in a muffle furnace for 7 hours and held at 1,500 ° C for 10 hours after which it was allowed to cool in the furnace. When the temperature in the furnace reached room temperature, the abrasive was sieved with a sieve of 350 to 210 μm to remove aggregated raw grain and unreacted TiO₂- Remove powder. In this way, abrasive particles having a grain size of # 60 were obtained.

This abrasive had a density of 3.98 g / cm³ and a Vickers hardness of 2,130 kg / mm²under a load of 500 g.

The abrasive was analyzed by X-ray diffraction and it was found that only aluminum titanate was formed on the surface of the abrasive.

The toughness of the abrasive was measured by a method designated as the C coefficient in JIS R1628-1975 (Ball Mill Test for Toughness of Artificial Abrasive). About 250 g of a sample are sieved with sets of standard sieves as outlined in JIS R6001-1987 in a Ro-Tap shaker for 10 minutes. The abrasive remaining on the third sieve is sieved again with the standard sieve for 10 minutes. The abrasive then left on the third sieve in an amount of 100 g is collected as a sample to be tested. This sample is ground in a ball mill in the manner explained in JIS R6128-1975. The ground sample is sieved for 5 minutes with sets of standard sieves and the abrasive remaining on the fourth sieve is weighed and denoted by R (X). The same procedure is repeated using # 60 black silicon carbide abrasive as a standard sample as explained in JIS R6128-1975, and the weight of the sample remaining on the fourth sieve after ball milling is indicated as R (S). The C coefficient is calculated using the following formula:

C coefficient = log [100 / R (X)] log [100 / R (S)]

The toughness is higher as the above value of the C coefficient is lower.

The C-coefficient of the abrasive in Example 1 was 0.77.

Example 2

Example 1 was repeated except that the amount of TiO₂Powder 5.1 g (1.0% by weight TiO₂ 100% by weight of alumina).

The abrasive thus obtained had a density of 3.98 g / cm³ and a Vickers hardness of 2.160 kg / mm² at a load of 500 g. The C coefficient was 0.79. X-ray diffraction showed that only aluminum titanate had formed on the abrasive surface.

Example 3

8.9 g of titanium tetraisopropoxide (manufactured by Wako Junyaku, 0.5% by weight of TiO₂ 100% by weight of alumina) were placed in a tray containing the same abrasive in the same amount as in Example 1, followed by stirring, and then adding and mixing 50 ml of isopropyl alcohol thereto to thereby apply the titanium tetraisopropoxide to the abrasive surface. The mixture in which the titanium tetraisopropoxide is applied to the abrasive was heated on a hot plate to evaporate all of the alcohol while stirring.

After drying, the abrasive was placed in an alumina crucible and heated to 1400 ° C in a muffle furnace over a period of 7 hours and held at 1400 ° C over 2 hours and then allowed to cool in the furnace. After the abrasive was cooled to room temperature, the abrasive was sieved through 350-210 µm screens to obtain the # 60 abrasive as in Example 1.

The resulting abrasive had a density of 3.98 g / cm³ and a Vickers hardness of 2,180 kg / mm² under a load of 500 g. The C coefficient was 0.76.

AT 407 749 B

The abrasive surface was qualitatively analyzed by X-ray diffraction and it was determined that only aluminum titanate had formed.

Example 4

Example 3 was repeated, but 9.8 g of aqueous titanium tetrachloride solution (manufactured by Showa Titanium KK; Ti content 15.4% by weight, 0.5% by weight of TiO₂ to 100 wt .-% clay) dissolved in 50 ml of water.

The abrasive thus obtained had a density of 3.98 g / cm³, a Vickers hardness of 2,130 kg / mm² and a C-coefficient of 0.75.

X-ray diffraction showed that only aluminum titanate had formed on the abrasive surface.

Example 5

Example 1 was repeated, but 83.1 g of a 3.02% by weight aqueous dispersion of the NiTiO₃Powder (0.26 wt% TiO₂ to 100 wt .-% alumina) mixed with the abrasive. The TiNiO₃-Powder was made using the coprecipitation method. Titanium tetrachloride and nickel tetrachloride were precipitated together by neutralization with sodium hydroxide. The coprecipitate was heated to 800 ° C. in the presence of sodium chloride for desalting, followed by grinding and centrifugation treatment to obtain a fine NiTiO₃- or NiO.TiO₂-Powder (mean particle size 0.058 pm). 83.1 g of a 3.02% by weight aqueous dispersion of the NiTiO₃Powder (0.26 wt% TiO₂ 100% by weight of alumina) were mixed with the abrasive.

The abrasive thus obtained had a density of 3.99 g / cm³, a Vickers hardness of 2,190 kg / mm² and a C-coefficient of 0.75.

The abrasive surface was qualitatively analyzed by X-ray diffraction and found to be composed of aluminum titanate and nickel oxide-alumina spinel.

Comparative Examples 1 to 2 # 60 White fused alumina (WA) and fused single crystal alumina (SA) abrasive particles, both manufactured by Showa Denko KK, were tested for density, hardness and C coefficient.

Table 1

<td>sample (# 60)</td><td>density (g / cm²)</td><td>Vickers hardness below it. Load of 500 g (kg / mm²)</td><td>C coefficient</td>

<td>WA</td><td> 3,94</td><td> 2.050</td><td> 1,10</td>

<td>SA</td><td> 3,95</td><td> 2.020</td><td> 0,92</td>

Comparative example 3

This was a control study of JP-B-40-16594.

500 g # 60 WA abrasives, as in Example 1, were treated with a 5% by weight aqueous solution of chromic anhydride (CrO₃) in an amount of 0.5 wt .-% CrO₃ mixed to apply the chromium trioxide to the surface of the abrasive, which was then heated for 2 hours at 1,200 ° C to form a coating layer of chromium oxide on the abrasive surface.

This abrasive was tested and found to have a density of 3.96 g / cm³, a Vickers hardness of 2,070 kg / mm² and had a C-coefficient of 1.05.

AT 407 749 B

Comparative example 4

This was a control examination of JP-B-44-638.

# 60 WA as in Comparative Example 3 was mixed with an aqueous nickel nitrate solution in an amount of 0.5% by weight of nickel nitrate, dried and heated at 900 ° C. for 2 hours to obtain an abrasive having a nickel oxide layer on the surface thereof .

The resulting abrasive had a density of 3.95 g / cm³, a Vickers hardness of 2,040 kg / mm² and a C-coefficient of 1.09.

Examples 6 to 10 and Comparative Examples 5 to 8

100 Parts by weight of each of the # 60 abrasives from Examples 1 to 5 and Comparative Examples 1 to 4 were mixed with 13 parts by weight borosilicate frit, 2 parts by weight dextrin, and 2.5 parts by weight of water as a binder for a vitrified grinding wheel. The borosilicate frit contains 70% by weight SiO₂, 7% by weight Al₂O₃, 18 wt% B₂O₃, 4% by weight (Na₂O + K₂O) and 0.5% by weight (CaO + MgO).

The mixture was pressed into the mold, the resulting molded article containing 45% of the abrasive. The molded body was dried at 110 ° C. for 20 hours and then heated to 1,050 ° C. for 20 hours. It was slowly cooled, the temperature falling by 1 ° C / min or less, in particular between 600 ° C to 500 ° C. In this way, a glazed grinding wheel having a hardness K as set out in JIS R6210 was obtained.

All grinding wheels had an outside diameter of 200 mm, an inside diameter of 50.8 mm and a thickness of 19 mm.

Examples 11-15 and Comparative Examples 9-12

The grinding properties of the glazed grinding wheels from Examples 6 to 10 and Comparative Examples 5 to 8 were examined under the following conditions.

Machine: Okamoto surface grinder PSG-52DX (3.7 kW)

Type of grinding: Plunge grinding, manual plunge cut

Work piece: SUJ-2 (H_RC 60),

100 mm L x 50 mm H x 10 mm D

Disk peripheral speed: 2,000 m / min

Face plate circulation: 20 m / min

Piercing speed: AR 20 pm / gear

Total depth adjustment: 5 mm

Grinding width: 10 mm

Firing out: 1

Grinding oil: Noritake Cool K-82B (water-soluble grinding oil)

Dressing conditions: monolithic diamond dresser,

Grooving: AR 20 pm / gear pitch: 0.2 mm / disk revolution Sparking out: none

The results of the grinding performance, maximum power consumption (value after deducting the no-load power (0.4 kW)) and surface roughness of the work pieces are shown in Table 2.

AT 407 749 B

Table 2

<td>sample</td><td>Grinding middle</td><td>Grinding dwindles. (mm³/ mm³)</td><td>Max. Power recording (kW / cm)</td><td>Surface roughness (gmRz)</td>

<td>Bp. 11</td><td>Bp. 1</td><td> 55</td><td> 1,6</td><td> 9</td>

<td>Bp. 12</td><td>Bp. 2</td><td> 60</td><td> 1,6</td><td> 8</td>

<td>Bp. 13</td><td>Bp. 3</td><td> 62</td><td> 1,5</td><td> 10</td>

<td>Bp. 14</td><td>Bp. 4</td><td> 68</td><td> 1,7</td><td> 8</td>

<td>Bp. 15</td><td>Bp. 5</td><td> 61</td><td> 1,5</td><td> 9</td>

<td>V-bp. 9</td><td>V-bp. 1</td><td> 30</td><td> 2,1</td><td> 11</td>

<td>V-bp. 10</td><td>V-bp. 2</td><td> 36</td><td> 1,9</td><td> 10</td>

<td>V-bp. 11</td><td>V-bp. 3</td><td> 39</td><td> 1,9</td><td> 9</td>

<td>V-bp. 12th</td><td>V-bp. 4th</td><td> 33</td><td> 2,0</td><td> 11</td>

As shown in Table 2, when the abrasives according to the invention were used in the grinding wheels, the grinding wheels had significantly higher grinding speeds than the grinding wheels with commercial WA abrasives (about 2 times), the grinding wheels with S abrasives (about 1.7 times) Grinding wheels with abrasives coated with chromic acid from Comparative Example 3, (about 1.5 times) and the grinding wheels with the abrasives coated with nickel from Comparative Example 4 (about 1.8 times). Although the grinding speeds of Examples 11 to 13 are excellent compared with Comparative Examples 9 to 12, the maximum power consumptions of the Examples are lower than those of the Comparative Examples and the value of the surface roughness is excellent with respect to the Comparative Examples.

Example 16

An abrasive was made by the same procedures as in Example 1, except that the starting abrasive was a decomposed alumina abrasive containing 0.30% by weight of TiO 2₂ (Showa Den ko KK, SA) was.

The resulting abrasive had a density of 3.98 g / cm³, a Vickers hardness of 2,210 kg / mm² and a C-coefficient of 0.69. When X-ray diffraction was performed, only aluminum titanate was found on the abrasive surface.

Using this abrasive, a grinding wheel was manufactured in the same manner as in Examples 6 to 10 and tested for grinding performance in the manner as in Examples 11 to 15. ,,

As a result, the grinding speed was 78 mm / mm, the maximum power consumption was 1.5 kW / cm, and the surface roughness was 8 gmRz.

Example 17

An abrasive was prepared in the same procedure as in Example 1 except that Bayern's alumina was mixed with 0.7 wt% titanium oxide, melted and solidified in an electric arc furnace, and ground and sieved to obtain # 60 abrasive. After TiO₂Powder was applied to the surface of this abrasive, the particles were heated to 1,400 ° C for 5 hours.

The abrasive thus obtained had a density of 3.97 g / cm³, a Vickers hardness of 2,150 kg / mm² and a C-coefficient of 0.70. X-ray diffraction indicated that only aluminum titanate had formed on the abrasive surface.

Comparative example 13

500 g # 60 brown fused clay abrasive (Showa Denko KK, A-40)

AT 407 749 B were with a dispersion of 13 g of anatase TiO₂-Powder (Ishihara Sangyo KK, A-100, mean particle size 0.2 μΐυ) mixed in 100 ml of distilled water and mixed with stirring to make the TiO₂-Apply powder to the surface of the abrasive. After application, the abrasive was dried in a blow dryer at 60 ° C. for 20 hours to lose 2.8% by weight of water.

The abrasive was placed in an alumina crucible and heated in a muffle furnace to 1,400 ° C in 6 hours and held at 1,400 ° C for 3 hours.

The abrasive thus obtained had a density of 3.98 g / cm³, a Vickers hardness of 2,020 kg / mm² under a load of 500 g and a C coefficient of 1.10. These values ​​did not differ from those obtained before treatment, i.e. a density of 3.98 g / cm³, a hardness of 2,010 kg / mm² and a C-coefficient of 1.10.

X-ray diffraction showed that the abrasive had a coating of predominantly titanium oxide with a minor amount of aluminum titanate. The reason why the abrasive did not have improved properties even after titanium oxide was applied to the surface thereof and heated is believed to be that the surface of the abrasive was coated with a vitreous phase composed in addition to aluminum titanate a significant amount of impurities due to a significant amount of impurities such as titanium oxide, Silica gel and iron oxide that has escaped from inside the abrasive.

According to the present invention, the particle of molten alumina coated with a coating of aluminum titanate contains a chemical bond between the coating layer and the alumina particle so that the coating layer is not peeled off. Therefore, when this particle is used as an abrasive material, the grinding performance is improved as compared with conventional abrasives.