JPH044103B2 - - Google Patents

Description

[Detailed description of the invention]

FIELD OF THE INVENTION This invention relates to alumina abrasive grains containing dense polycrystalline alumina or such alumina and other additives, and abrasive products using the abrasive grains. BACKGROUND OF THE INVENTION Hard, high-strength abrasive grains for grinding wheels, flexible coated abrasive papers ("sandpaper"), or loose abrasive grains are commercially produced using alumina-containing raw materials, melted in electric furnaces or It is manufactured by firing a compact containing finely pulverized alumina at a temperature sufficiently lower than the melting point of the raw material. This lower temperature treatment is called sintering, and the present invention relates to an alumina abrasive material made by the sintering method. The first commercially produced sintered abrasive on a large scale was made by the method taught in Ueltz, US Pat. No. 3,079,243. According to the teachings of this patent, calcined bauxite is crushed to create a fine particle size raw material, which is then reduced to abrasive grain size particles and fired at approximately 1500°C to create a hard, strong and tough material. Create polycrystalline alumina pellets. Recently, abrasive materials with abrasive grains consisting of alumina and magnesia spinel have appeared on the market, possibly in accordance with the teachings of U.S. Pat. did. These materials are manufactured by sintering dry alumina gel particles at (approximately 1400°C).
Bugosh, U.S. Pat. No. 3,108,888, also discloses the production of dense alumina (or alumina-containing material) by calcining a dry alumina gel made from alpha alumina monohydrate (boehmite) or by hot pressing a dry powder made from such a gel. ) teaches the manufacture of products. Commercial alumina/magnesia-spinel abrasives made from gels contain alumina crystals in the form of cells 5 to 15 micrometers in diameter. Cells or "sunbursts" are made of long alumina arms with a diameter of 0.2 to 0.4 micrometers.
(However, some of these are approximately spherical "blots" with a size of 1 micrometer.
), the arms of each cell appear to extend generally radially from the center of the cell. The arms of a given cell are all apparently crystallographically oriented in the same direction. That is, each cell is a single crystallite. Such orientation is indicated by the fact that when viewed between perpendicular polarizers in a transmitted light microscope, all areas of a given cell disappear simultaneously when the sample is rotated. As seen in the scanning electron micrograph of FIG. 1 showing a commercially available abrasive made from alumina gel, the generally star-shaped alumina crystal cells have a substantial amount of porosity. Pores appear as dark spots within alumina crystals in electron micrographs. The microcrystals filling between the arms of the alumina crystals and between adjacent alumina crystal cells are magnesia spinel. Although commercially available abrasives made from sintered gels containing alumina and magnesia are high quality abrasives, it has heretofore been impossible to produce high purity alumina abrasive grains through gels. This is US Patent No.
Control Example 13 of No. 4,314,827, made of alumina gel without the addition of metal oxides or metal salts, exhibits relative softness and lack of utility as an abrasive. The present invention provides improvements in the field of high-strength abrasive manufacturing technology that allow useful abrasive products to be made from alumina gel with or without the addition of spinel formers such as zirconia or magnesia. It is something. DISCLOSURE OF THE INVENTION In accordance with the present invention, the performance of the fired product is improved by controlling the microstructure of the fired product to eliminate the cellular structure of alumina in the abrasive grains of the prior art. The resulting product contains alpha alumina particles (crystallites) of submicron dimensions (0.2-0.4 micrometers) instead of cell areas of 5-10 micrometers in diameter. Thus, according to the invention, α-Al 2 O 3 particles are composed of non-cells consisting of groups of arms radiating from a center with substantially the same crystallographic orientation and are substantially entirely composed of α-Al ₂ O ₃ particles. Abrasive grains consisting essentially of a dense polycrystalline phase in which the particle size of α-Al ₂ O ₃ particles is 1 μm or less and having a hardness of at least 18 GPa, as well as vitrified grinding wheels and flexible abrasive wheels using this abrasive grains. Discs, coated abrasive papers, and vulcanized fiber-based coated abrasive papers are provided. Although the abrasive grains of the present invention are composed of submicron microcrystals, they have a very dense structure, and as a result, they have high hardness and toughness of 18 GPa or more, and even 20 to 21 GPa or more when measured with a Vickers indenter. has. If the hardness is lower than 18 GPa, it is not useful as an abrasive grain. On the contrary, the abrasive grains of the present invention have a high hardness of 18 GPa or more, which is useful as an abrasive grain, but they also exhibit unique crushing characteristics due to their microstructure consisting of submicron microcrystals. (Toughness) and crushing properties ensure that the cutting edge is always sharp and the cutting quality is improved. Although the addition of MgO is not necessary, if high concentrations (eg, 5% in some cases) of MgO are added, the alumina particles will be surrounded by a matrix of spinel. The useful alumina abrasive grains of the present invention are obtained by drying a sol of alpha alumina monohydrate (boehmite), firing the gel to obtain alumina, and pulverizing this into abrasive grains. It is produced by seeding the gel or gel precursor with submicron particles before drying. To explain based on the progress of the present invention, the formation of gel conditions for obtaining abrasive grains exhibiting the effects of the present invention involves using an alumina body as a grinding medium in a mill, and forming a sol or dilute gel. This was accomplished by vibratory milling of the mixture. It is believed that the main effect of this milling is the introduction of material from the alumina grinding media into the alumina gel. Similarly, impurities such as zinc and iron are believed to have been introduced from pipes and related equipment. On the other hand, milling with zirconia bodies, for example, has not been effective in forming the desired essentially non-cellular structure. The first effective and reproducible method we found was to generate the above-mentioned substances in the gel by vibration milling of the gel with an alumina body. A suitable vibratory mill is shown in US Pat. No. 3,100,088. Typically, the grinding media can be 1/2 inch in diameter and 1/2 to 3/4 inch long.
An unbalanced weight connected to the shaft of a motor mounted concentrically with the tab mounted on a spring causes the tab containing the grinding media and mixture to vibrate in a horizontal plane. The unbalanced weight is mounted near the bottom surface of the tub and the second weight is mounted below it. The motor is typically
Rotates at 1200rpm. The combined vibration causes the grinding media to perform a grinding action on the contents. The inner surface of the mill is preferably lined with rubber or the like to prevent corrosion of the metal walls and contamination. US Patent No. 4,314,827 and UK Application Publication No.
Various additives, such as those taught in US Pat. No. 2,099,012, may, but are not required, be added to the alumina before or after gelling. The most useful additives known today are any of the compatible MgO precursors, which allow the final product to contain as much as 5%
Preferably, it contains MgO. MgO exists in the product as spinel (magnesium aluminate MgAl ₂ O ₄ ), but is calculated as MgO in the analysis.
Since the additive-free alumina produced according to the invention is itself an excellent abrasive, less
It is clear that MgO may also be added. The milled gel of the present invention can serve as a matrix for various additive materials or abrasive particles. The milled mixture can be simply poured or placed into a container to dry and then crushed to the appropriate size and any material that is too small can be recycled to the beginning of the process. Optionally, the material can be shaped or molded into shaped particles, such as by extrusion. In the case of extrusion molding, the formed rods are later broken into suitable sizes. The lowest useful calcination temperature is 1200, which is usually considered the transformation temperature to alpha alumina.
substantially lower than °C. The upper limit of firing temperature is not critical as long as the melting temperature is not reached. If the firing time is too long or the firing temperature is too high, excessive crystal growth may occur. Also, higher temperatures increase the cost of processing, so the preferred firing temperature range is 1200
~1500℃. The scanning electron micrograph of FIG. 2 shows the crystal structure of the alpha alumina abrasive grain of the present invention. In this case, the material is 100% alpha alumina. As is evident from this photograph, such abrasive grains are essentially composed of a polycrystalline phase of alpha alumina with a relatively narrow particle size distribution. This is another important difference between the crystal structure of the alpha alumina abrasive grains of the present invention and the crystal structures of other fine grain ceramics of the prior art. This is an important factor in the abrasive grains of the present invention, as it particularly contributes to the shell-like fracture properties of the abrasive grains.
When abrasive grains have a wide particle size distribution, the strength of the grain boundary phase decreases, and the ceramic body tends to collapse in the same way as sugar cubes. In this case, a new cutting edge is not regenerated, and the abrasive grains lose their cutting ability and are no longer useful as abrasive grains. The unusually high density of the abrasive grains of the present invention is also evident in FIG. Even at 20,000x magnification, virtually no pores are visible. Example 1 Condea in a large polymer plastic mixing container
30 pounds (13.6Kg) of SB Pural Alumina (Condea) and 13 gallons (136 liters) of water were mixed. The material was then gelled by adding 4.11% of 14 wt.% _HNO3 . Next, 7.5 pounds (3.4 Kg) of magnesium nitrate hydrate dissolved in 3 gallons (13.7 liters) of water was added to the alumina gel to provide 5% by weight MgO in the final product. It was mixed for 15 minutes, transferred to a Model M451 Sweco mill, and milled with 1700 pounds of alumina media for 1 hour. The mixture is milled at a rate of approximately 4 gallons per minute.
Recirculated with a grinding time of hours. After grinding, it is pumped into aluminum trays to a thickness of approximately 3 inches (7.6 cm) and dried in an electric strip dryer. The composition of the alumina medium was about 90% alpha alumina and silica as the main impurity. A series of batches are made from the above preparation, mixed and subjected to crushing and baking. The dried gel was crushed with a roller and sized to fit through a 14 mesh prior to firing to give the desired final particle size. Then pre-baked at 400℃ for 16 hours,
Baked at 1400°C for 30 minutes (rotary kiln). After firing, all products have a hardness of 19GPa (Vitkers indenter, load 500g) and a very fine microstructure, with no cellular microstructure at all, and almost all of the alpha alumina is almost equiaxed grains. (microcrystal), with a diameter of 0.2 to 0.4 μm, and in rare cases, irregularly shaped crystals with a diameter of about 5 μm were only observed. Rugged shapes were shown to be contaminants. When examined by scanning electron microscopy, the product consisted of a matrix of spinel and a discontinuous phase of alpha alumina. In certain coated abrasive applications in abrasive applications, this material was superior to fused alumina zirconia and superior to commercially available alumina-spinel composition sintered gel type abrasives. Example 2 22.7 kg of Pural microcrystalline boehmite alumina was mixed with 225 liters of water and _13.5 liters of 14%-HNO3 for 10-15 minutes. Half of the gel mixture was mixed with 1/2 x 1/2 inch ceramic bound alumina, 88Al ₂ O ₃ (major impurities were 1.74% MgO, 8.9% SiO ₂ from Coors Porelain).
%, _{Fe2O3 0.18} %, _TiO2 0.2%, CaO0.8%,
2 with Sweco mill (product name) containing Ma ₂ O0.34%)
Grind for hours and dry. This was the same medium used in Example 1. The other half was simply dried without grinding. Crush these dried gels and 16
Preliminary firing was carried out at 450°C for 1 hour, followed by firing at 1400°C for 1 hour. Materials that have undergone pulverization (milling) processing are
The material had a hardness of 19.1 GPa, and the material without milling had a hardness of 11.0 GPa. The material from each batch was graded to create 50 grit abrasive grains, which were used to create vulcanized fiber-based abrasive cloth discs. The milled material is superior in grinding 1020 steel compared to commercially available alumina-zirconia abrasives.
It showed more than 10% better performance (metal grinding amount was 14% higher in the test). The unmilled material was inferior to the fused abrasive in all grinding tests, which was thought to be due to its lower hardness. Example 3 A gel was milled for 0.2 hours in a similar example to the milled product of Example 1. 1 hour 1400
The products fired at ℃ are mainly fine and irregular 0.2
Although the crystal structure was ~0.3 mm, some cell-like appearance was observed. Examples 4-9 Further experiments were conducted to examine the effect of firing time at 1400°C. All samples were made according to the procedure in Schematic Example 1. Grinding was carried out for 2 hours using Condea microcrystalline boehmite alumina, and after drying the gel was pre-calcined at 750°C for 30 minutes.
As the baking time increases, the product becomes finer than 0.2
Coarse lath-shaped (rod-shaped) crystals of alumina irregularly dispersed among the ~0.4 μm alumina particles began to appear. The results are tabulated as shown below.

[Table] Since the presence of rough parts is considered undesirable, in order to obtain a desirable product, the material should be heated at 750℃30
The firing time at 1400° C. should not exceed 5 minutes if pre-calcined for 1 minute. That is, the alumina particles and rod-shaped alumina crystals have substantially no pores therein. No cellular structure was seen in all cases. The microstructure consisted of submicron particles without small planes and lath-shaped (rod-shaped) coarse crystals with small planes, however, no lath-shaped crystals were observed only when the firing time was 1 minute. "Non-faceted" means that the microcrystals are not in a flat shape, such as a prismatic shape, and therefore, when observed under a scanning electron microscope at 5000x magnification, the microcrystals have a regular structure on the fracture surface. This means that there are no visible facetings.
The particles of alpha alumina were rather amorphous, but equiaxed, with approximately curvilinear contours, and very rarely straight contours. magnification
At 20,000x magnification, the facet structure began to become clearly visible. The abrasive grains according to the invention have a hardness measured by a Bitkers indenter at a load of 500 grams of at least 18 GPa on alumina without additives and have been modified with the presence of 2 mol % or more of spinel precursors or with other additives. The abrasive grain has a hardness of 14 GPa or more, but for the purpose of the present invention, it is limited to a hardness of 18 GPa or more. Purely dense alpha alumina is about 20
It has a hardness of ˜21 GPa, although certain porosity may be desirable for certain applications, such as having a lower hardness. Although alumina itself may have a hardness of less than 13 GPa, it is too porous for the purposes of this invention. Hardness of 18 GPa or higher is required as measured with a Bitkers indenter with a load of 500 g. Example 10 A series of abrasives with varying magnesia contents were created. The procedure of General Example 1 was adopted, including milling with alumina media (but for 2 hours). In all cases, the gels were dried at 200 °C for approximately 30 h and then crushed.
It was classified and then calcined at 450°C for 16 hours. The particles of the obtained abrasive size are heated at 1400℃ in a rotary kiln.
It was fired at. The heating time to 1400°C was about 15 minutes, and the holding time at 1400°C was about 15 minutes. Various amounts of magnesium nitrate were added before gelation. In one experiment no magnesium nitrate was added. The MgO content and hardness of the abrasives were as follows.

[Table] In a series of tests on vitrified (glass-bonded) grinding wheels using abrasive grains of 54 grit size (mixture of 46 grit size and 60 grit size), wheels made with the above abrasive grains were compared with known fused alumina abrasives (sulfide method abrasives). compared with the highest quality products (wood). Tests were carried out by grinding tool steel (D3) bars at various feed rates. 0.5 mil (0.0005 inch)
The abrasive material without additives (MgO 0.14%) is 16.1−
It had a grinding ratio of 8 times (G ratio is the volume ratio of the amount of grinding of the object to be ground to the amount of wear of the grindstone). In dry grinding tests, all MgO-added abrasives showed better performance than fused abrasives. In the wet grinding test
Experimental abrasives with added MgO were inferior to or comparable to fused abrasives. At 2 mil, the abrasive without added magnesia was superior to the fused abrasive. For coated abrasive tests using 50 grit size abrasive grains (CMI standard), 0.6%
of MgO-containing abrasive grains attached to a flexible abrasive disk outperformed co-fused alumina-zirconia grains for 1020 steel (136%) and was nearly equivalent to fused alumina-zirconia for stainless steel. . 2.5% MgO and 7.59% MgO
The abrasive grain containing was also better against 1020 steel. Addition of even larger amounts of MgO reduced the effectiveness against stainless steel. In addition to alumina, SiO ₂ 0.25%, Fe ₂ O ₃ 0.18%,
Abrasive particles of 0.14% MgO containing 0.28% TiO ₂ , 0.05% CaO and 0.04% Na _{2 O} were probably mainly introduced in the grinding operation. These impurities are present in other abrasive grains at similar levels. Example 11 The grinding ratio of a vitrified abrasive wheel made with a seeded gel abrasive of the present invention was compared to a prior art unseeded sol-gel abrasive and expensive fused alumina. The grinding test was a plunge grinding test in which a quarter inch of slotted D3 tool steel was ground at a metal removal rate of 0.14 in ³ /min.

[Table] Thus, in this application, seeded gel wheels perform nearly five times better than high-grade fused alumina, more than four times as well as unseeded sol-gel wheels, and require significantly lower pressures than either of them. It is. Although we do not wish to associate our invention with any particular theory, we believe that the introduction of particulate matter from the alumina medium may act as a seed for crystallization of the alpha alumina during firing. Therefore, other impurities introduced during the milling process will inhibit crystal growth in the final product due to their presence at grain boundaries between alpha alumina particles. As evidence that it is the debris powder from the grinding media that is effective in conditioning the gel to produce the desired dense, microcrystalline, noncellular alpha alumina when calcined at approximately 1400 °C. The milled water was added to the alumina monohydrate and acid without milling the gel. Water, nitric acid, and microcrystalline boehmite were mixed as in Example 2. However, six batches were made with various water additions as described below, including debris powder that was abraded from the alumina grinding media during several hours of water milling (without any additives other than water). Addition of “milled water” to alumina monohydrate (Condea):

[Table] The hardness of fired products fired at 1400°C ± 20°C for approximately 10 minutes was measured. The furnace was electrically heated and the atmosphere was air. Examination of the milled debris powder showed that it was mostly alpha alumina with a surface area of approximately 39 square meters per gram (corresponding to a particle size of approximately 0.04 μm). In general, finer seed (nucleation) material is preferred because it yields a higher number of nuclei per given weight of seed material. Depending on the type of alumina grinding media used, the seeds are approximately
It can be as fine as 125 square meters/gram (approximately 0.01 micron). As shown in the table above, as little as 0.1% by weight seed (based on the final product) can provide the desired high degree of abrasive grain, but about 1% by weight seed material is preferred as it provides the best results.
The use of more than about 5% by weight of seed material may compromise the properties of the final product. Similarly, high-purity alumina obtained by recovering the fine suspended alumina particles that remain in suspension when very fine alumina powder is mixed with water and then allowed to settle.
It is useful when used in an amount of at least about 0.1% of the calcined gel solids. Testing with commercially available fine alpha alumina powder,
Tests with fine alumina obtained by grinding very pure molten alumina and very pure molten alumina using such alumina itself as grinding media have shown that the production of dense microcrystalline products according to the present invention It was shown to be very effective. Example 12 Using a Waring industrial mixer, with 16N nitric acid
Add 1.5 kg of alpha alumina powder (AKP- manufactured by Sumitomo Chemical) to 3 liters of water acidified to PH16.
50) was dispersed at high speed for 2 minutes to prepare a suspension of alpha alumina seed particles. This suspension was centrifuged at 1000 times the force of gravity for 10, 20 and 30 minutes, leaving only the supernatant from each operation and used in the next centrifugation. To prepare the sol, add 800 grams of water to 2400 milliliters of water acidified with 48 grams of nitric acid.
Condea SB Pural Alumina (manufactured by Condea) was mixed to disperse the alumina monohydrate powder. To this was added a centrifuged suspension containing 12 grams of alumina powder. The particle size of alpha alumina particles is
It was 120 nanometers as measured with a Nicomp model 200 Laser Particle Sizer. This mixture was mixed at high speed for 2 minutes in a 6 liter stainless steel container in a Warning mixer. The sol was then poured into a glass tray and kept at 100°C to gel and dry. The dried product was then crushed with a roll and passed through a 14-mesh screen to give the desired particle size during firing. Calcinate the sieved materials at 400℃ for 1 hour,
It was baked at 1350°C for 5 minutes. After firing, the particles all had densities greater than 95% of the theoretical density of alpha alumina. The alpha alumina crystallites that make up the particles were found to be 20,000 using a scanning electron microscope.
When observed under magnification, it had a maximum particle size of about 0.4 microns or less, and did not have a cellular structure. Differential thermal analysis shows that the transformation of gelled alumina from the probable gamma form to the alpha form occurs at about 1090 °C in the presence of alpha alumina seed particles, but at about 1190 °C in the absence of such seed material. It has been shown. Thus, the theoretical calcination temperature of products according to the invention can be lower than the commonly reported transformation temperatures. With this invention, for the first time, submicron particle size and
It becomes possible to produce high purity alpha alumina bodies with a density greater than 95% and a hardness greater than 18 GPa at low temperature firings. With the method of the invention, it is also possible to produce products other than abrasives, such as coatings, films, fibers, rods, or small molded parts. Grain growth inhibitors such as SiO ₂ , Cr ₂ O ₃ , MgO and ZrO ₂ were added to the conditioned gel. MgO
In experiments in which spinel was added, spinel was observed to react with alpha alumina and surround the remaining unreacted alpha alumina. For other additives, the formation of compounds with alpha alumina was minimal, and it was thought that they remained at the grain boundaries. The experiments conducted clearly show that crystal growth due to diffusion and recrystallization was suppressed by the additive. This is valuable in that it gives greater flexibility to the time-temperature relationship of the sintered product. The use of grain growth inhibitors is well known in the ceramics field and is not an essential part of the present invention, but can be used to achieve the desired microstructure over a wide range of sintering times and temperatures when high purity alpha alumina is not required. Very useful for holding.

[Brief explanation of drawings]

FIG. 1 is a scanning electron micrograph showing a commercially available abrasive made from alumina gel, and FIG. 2 is a scanning electron micrograph showing the crystal structure of alpha alumina abrasive grains of the present invention.

Claims (1)

[Scope of Claims] 1. Consisting of α-Al ₂ O ₃ particles that do not constitute cells consisting of a group of arms radiating from a center having substantially the same crystallographic orientation and containing substantially all of the particles. Consisting essentially of _a dense polycrystalline phase in which the particle size of α- _Al2O3 particles is less than 1 μm, and at least 18 GPa
Abrasive grains with a hardness of 2. The abrasive grain according to claim 1, wherein the abrasive grain contains rod-shaped coarse alumina crystals composed of planes irregularly distributed between submicron particles. 3. The abrasive grain according to claim 1 or 2, wherein a part of the α-Al ₂ O ₃ is substituted with MgO or zirconia existing as spinel. 4. The abrasive grain according to claim 1, 2 or 3, wherein submicron α-Al ₂ O ₃ species particles are dispersed. 5. The abrasive grain according to claim 1, 2, 3 or 4 _, wherein the submicron α- _Al2O3 particles have a particle size of 0.4 μm or less. 6 consisting of α-Al ₂ O ₃ particles that do not constitute cells consisting of groups of arms radiating from a center having substantially the same crystallographic orientation and containing substantially all α-Al ₂ O ₃ Consisting essentially of a dense polycrystalline phase with a particle size of 1 μm or less, and at least 18 GPa
A vitrified whetstone containing alumina abrasive grains with a hardness of . 7. The bitrified grindstone according to claim 6, wherein the alumina abrasive grains include rod-shaped coarse alumina crystals composed of planes irregularly dispersed between submicron particles. 8. The bitrified grindstone according to claim 6 or 7, wherein a part of the α-Al ₂ O ₃ is substituted with MgO or zirconia existing as spinel. 9 Consisting of α-Al ₂ O ₃ particles that do not constitute cells consisting of groups of arms radiating from a center with substantially the same crystallographic orientation and that contain substantially all α-Al ₂ O ₃ Consisting essentially of a dense polycrystalline phase with a particle size of 1 μm or less, and at least 18 GPa
A flexible abrasive disc containing alumina abrasive grains with a hardness of . 10. The flexible abrasive disk of claim 9, wherein the alumina abrasive grains include rod-shaped coarse alumina crystals comprised of irregularly distributed flat surfaces between submicron particles. 11. A flexible abrasive disk according to claim 9 or 10, wherein a part of _the α- _Al2O3 is replaced by MgO or zirconia present as spinel. 12 consisting of α-Al ₂ O ₃ particles that do not constitute cells consisting of groups of arms radiating from a center having substantially the same crystallographic orientation and containing substantially all α-Al ₂ O ₃ consisting essentially of a dense polycrystalline phase in which the grain size of the particles is 1 μm or less, and at least
A coated abrasive paper containing alumina abrasive grains with a hardness of 18GPa. 13. The coated abrasive paper according to claim 12, wherein the alumina abrasive grains include rod-shaped coarse alumina crystals composed of planes irregularly dispersed between submicron particles. 14. The coated abrasive paper according to claim 12 or 13, wherein a part of the α-Al ₂ O ₃ is substituted with MgO or zirconia present as spinel. 15 consisting of α-Al ₂ O ₃ particles that do not constitute cells consisting of groups of arms radiating from a center having substantially the same crystallographic orientation and containing substantially all α-Al ₂ O ₃ consisting essentially of a dense polycrystalline phase in which the grain size of the particles is 1 μm or less, and at least
Vulcanized fiber-based coated abrasive paper containing alumina abrasive grains with a hardness of 18GPa. 16. The vulcanized fiber-based coated abrasive paper according to claim 15, wherein the alumina abrasive grains include rod-shaped coarse alumina crystals composed of planes irregularly dispersed between submicron particles. 17. The vulcanized fiber-based coated abrasive paper according to claim 15 or 16, wherein a part of the α-Al ₂ O ₃ is substituted with MgO or zirconia present as spinel.