Polycrystalline aluminum oxide abrasive

Abstract

Provided are: polycrystalline alumina abrasive grains having a small alumina crystal grain size and excellent polishing and grinding performance; a method for producing the polycrystalline alumina abrasive grains at low cost; and a grindstone using the polycrystalline alumina abrasive grains. Polycrystalline alumina abrasive grains having an alumina content of 95.0% by mass or more, a silica content of 0.05-3.30% by mass, a crystal grain diameter of 0.100-0.400 [mu] m, and a Vickers hardness of 21.00 or more are obtained by a production method comprising: a step for obtaining a mixed solution that is a mixed solution of boehmite, predetermined silica, and water; a step for obtaining a sol by adding a deflocculating agent to the mixed solution; a step for adding an alumina seed crystal to the sol to obtain a seed crystal-containing sol; a step in which the seed crystal-containing sol is dried to obtain a residue, and the residue is crushed and then presintered at a temperature of 550 DEG C or more and less than 1200 DEG C; and a step for subsequently firing at a temperature of 1200-1500 DEG C.

Description

Technical Field

[0001] The present invention relates to polycrystalline alumina abrasive grains suitable for grinding and lapping of difficult-to-machine materials such as chrome molybdenum steel, a method for manufacturing the same, and a grindstone. Background Art

[0002] Polycrystalline alumina abrasive grains are used as a material for grindstones of grinding and lapping tools. It is known that: the smaller the crystal grain size of alumina in polycrystalline alumina abrasive grains, and the fewer the pores, the denser the grains, the closer the specific gravity is to the theoretical specific gravity, and the more excellent the grinding and lapping performance is shown.

[0003] As a method for manufacturing polycrystalline alumina abrasive grains, a method using the sol-gel method is generally used. In this method, after peptization and gelation of boehmite, firing is performed.

[0004] In order to obtain polycrystalline alumina abrasive grains having excellent grinding and lapping performance as described above, for example, the following method is known: adding a rare earth element to a boehmite sol or a dry gel, performing firing, and using acicular crystals of the generated rare earth composite oxide to hinder the growth of alumina crystals (for example, Patent Document 1, etc.).

[0005] In addition, a method of adding silica and zirconia to a boehmite sol or a dry gel to suppress the growth of alumina crystals has been proposed (for example, Patent Document 2); iron and silica are used as additives added to an alumina precursor (for example, Patent Document 3).

[0006] Prior Art Documents

[0007] Patent Documents

[0008] Patent Document 1: Japanese Patent Publication No. 6-92575

[0009] Patent Document 2: Japanese Patent Application Laid-Open No. 2003-510417

[0010] Patent Document 3: Japanese Patent No. 2756258 Summary of the Invention

[0011] However, in the manufacturing method described in Patent Document 1, the crystal grain size of alumina is only suppressed to about 0.5 μm, and in addition, since expensive rare earth elements are used, there is a problem that the manufacturing cost becomes high.

[0012] On the other hand, in the methods described in Patent Documents 2 and 3, the alumina purity of the abrasive grains is low, and the content of metal components such as zirconium and iron is high. There is concern about the influence of such metal components on the object to be polished or ground. In addition, the polishing and grinding performance of the abrasive grains is not always uniformly controlled. Further, zirconia is relatively expensive, and there is also a problem that the manufacturing cost becomes high when using zirconia.

[0013] The present invention has been made in view of such problems, and an object thereof is to provide polycrystalline alumina abrasive grains having a small crystal grain size of alumina and excellent polishing and grinding performance, a manufacturing method capable of manufacturing the polycrystalline alumina abrasive grains at low cost, and a grindstone using the polycrystalline alumina abrasive grains.

[0014] The present invention is based on the following finding: In the production of polycrystalline alumina abrasive grains by the sol-gel method, by adding a prescribed amount of silica, the growth of alumina crystals can be suppressed, and polycrystalline alumina abrasive grains having excellent polishing and grinding performance can be obtained at low manufacturing cost.

[0015] The present invention provides the following solutions.

[0016] [1] A polycrystalline alumina abrasive grain,

[0017] The alumina content is 95.0 mass% or more, the silica content is 0.05 to 3.30 mass%, the crystal grain size is 0.100 to 0.400 μm, and the Vickers hardness is 21.00 or more.

[0018] [2] The polycrystalline alumina abrasive grain according to [1],

[0019] The apparent specific gravity is 3.800 g / cm 3 or more.

[0020] [3] The polycrystalline alumina abrasive grain according to [1] or [2],

[0021] The alumina content is 99.0 mass% or more, the silica content is 0.10 to 0.40 mass%, the crystal grain size is 0.100 to 0.180 μm, the apparent specific gravity is 3.900 to 3.960 g / cm 3 , and the Vickers hardness is 23.10 to 24.00.

[0022] [4] A method for manufacturing polycrystalline alumina abrasive grains, which is a method for manufacturing the polycrystalline alumina abrasive grains according to any one of [1] to [3], and includes:

[0023] A step of obtaining a mixed liquid, which is a mixed liquid of pseudo-boehmite, silica having an average particle size of 100 nm or less and a content of 0.01 parts by mass or more and less than 3.0 parts by mass relative to 100 parts by mass of the solid component of the pseudo-boehmite, and water having a content of 400 to 630 parts by mass relative to 100 parts by mass of the solid component of the pseudo-boehmite;

[0024] A step of adding an anti-flocculant to the mixed liquid to obtain a sol;

[0025] A step of adding alumina seeds having a median particle size D50 of 700 nm or less and a content of 0.01 to 5.0 parts by mass relative to 100 parts by mass of the solid component of the pseudo-boehmite to the sol to obtain a seeded sol;

[0026] A step of pre-firing at a temperature of 550°C or higher and lower than 1200°C after crushing the residue obtained by drying the seeded sol to a water content of 15% by mass or less; and

[0027] Subsequently, a step of firing at a temperature of 1200 to 1500°C.

[0028] [5] The method for manufacturing polycrystalline alumina abrasive grains according to [4],

[0029] In the step of obtaining the mixed liquid, the compounding amount of the silica is 0.08 to 0.35 parts by mass relative to 100 parts by mass of the solid component of the pseudo-boehmite.

[0030] [6] The method for manufacturing polycrystalline alumina abrasive grains according to [4] or [5],

[0031] The step of performing the pre-firing includes:

[0032] A first pre-firing step at a temperature of 550°C or higher and lower than 1000°C; and

[0033] A second pre-firing step at a temperature of 800°C or higher and lower than 1200°C and higher than the first pre-firing step.

[0034] [7] The method for manufacturing polycrystalline alumina abrasive grains according to any one of [4] to [6],

[0035] The pre-firing and the firing are performed using a rotary kiln.

[0036] [8] An abrasive stone comprising the polycrystalline alumina abrasive grains according to any one of [1] to [3].

[0037] According to the present invention, it is possible to provide polycrystalline alumina abrasive grains having a small crystal grain size of alumina and excellent grinding and abrasive properties, and a grindstone using such abrasive grains.

[0038] In addition, according to the manufacturing method of the present invention, the polycrystalline alumina abrasive grains can be manufactured at low cost. Detailed Embodiments

[0039] The following shows the definitions and meanings of terms and expressions in this specification.

[0040] Regarding preferred numerical ranges, each of the preferred lower limit value and the upper limit value can be arbitrarily combined.

[0041] The crystal grain size is a value obtained by identifying grain boundaries and measuring them in an observation image of the abrasive grains observed by a scanning electron microscope (SEM). Specifically, it is obtained by the method described in the examples.

[0042] The apparent specific gravity is a value measured by a gas displacement pycnometer. Specifically, it is obtained by the method described in the examples.

[0043] The median particle size (D50) is the cumulative volume 50% particle size measured by the laser diffraction scattering method.

[0044] [Polycrystalline Alumina Abrasive Grains]

[0045] The polycrystalline alumina abrasive grains according to an embodiment of the present invention (hereinafter also referred to as this embodiment) have an alumina content of 95.0% by mass or more, a silica content of 0.05 to 3.30% by mass, a crystal grain size of 0.100 to 0.400 μm, and a Vickers hardness of 21.00 or more.

[0046] The abrasive grains of this embodiment (hereinafter also simply referred to as abrasive grains) contain a specified amount of silica, have a small crystal grain size of alumina, and have excellent grinding and abrasive properties. The abrasive grains of this embodiment can be particularly suitably applied to the grinding and abrasive machining of difficult-to-machine materials and are very suitable as a material for grindstones.

[0047] The abrasive grains of this embodiment are abrasive grains mainly composed of alumina, and the alumina content is 95.0% by mass or more, preferably 97.0% by mass or more, more preferably 98.0% by mass or more, and further preferably 99.0% by mass or more.

[0048] The upper limit of the alumina content in the abrasive grains is not particularly limited, but considering the silica content, it is 99.95% by mass or less, preferably 99.9% by mass or less.

[0049] For the abrasive grains to exhibit good grinding and abrasive performance, the silica content is 0.05 to 3.30% by mass, preferably 0.06 to 3.00% by mass, more preferably 0.07 to 2.50% by mass, still more preferably 0.08 to 1.00% by mass, and particularly preferably 0.10 to 0.40% by mass.

[0050] If the silica content is 0.05% by mass or more, the crystal grain size of the abrasive grains tends to become smaller. Additionally, if the silica content is 3.00% by mass or less, abrasive grains with high hardness can be easily obtained.

[0051] In addition to the constituent components such as alumina and silica, the abrasive grains may contain impurity metal elements that are inevitable components derived from raw materials or mixed in during the manufacturing process. Examples of impurity metal elements include sodium, calcium, iron, etc.

[0052] From the viewpoints of good grinding and abrasive performance of the abrasive grains and suppression of coloring, etc., the content of components other than alumina and silica in the abrasive grains is preferably 0.20% by mass or less, more preferably 0.18% by mass or less, and further preferably 0.16% by mass or less.

[0053] For the abrasive grains to exhibit good grinding and abrasive performance, the crystal grain size is 0.400 μm or less, preferably 0.300 μm or less, more preferably 0.250 μm or less, still more preferably 0.200 μm or less, and particularly preferably 0.180 μm or less. As high-hardness abrasive grains, the lower limit of the crystal grain size is 0.100 μm or more, preferably 0.110 μm or more, more preferably 0.120 μm or more, still more preferably 0.130 μm or more, and even more preferably 0.140 μm or more.

[0054] For the abrasive grains, from the viewpoint of good grinding and abrasive performance, the apparent specific gravity is preferably 3.800 g / cm 3 or more, more preferably 3.810 g / cm 3 or more, further preferably 3.900 g / cm 3 or more. The larger the apparent specific gravity, the more preferable, but in practice, the upper limit is preferably 3.960 g / cm 3 or less.

[0055] For the abrasive grains, from the viewpoint of good grinding and abrasive performance, they are preferably of high hardness, with a Vickers hardness of 21.00 or more, preferably 21.50 or more, more preferably 22.00 or more, still more preferably 23.00 or more, and even more preferably 23.10 or more. The higher the Vickers hardness, the more preferable, but in practice, the upper limit is preferably 24.00 or less.

[0056] Furthermore, the Vickers hardness can specifically be measured by the method described in the examples.

[0057] From the viewpoint of excellent grinding and abrasive cutting performance, the abrasive grains of the present embodiment are particularly preferably: alumina content of 99.0% by mass or more, silica content of 0.10 to 0.40% by mass, crystal grain size of 0.100 to 0.180 μm, apparent specific gravity of 3.900 to 3.960 g / cm 3 , and Vickers hardness of 23.10 to 24.00.

[0058] The grinding and abrasive cutting performance of the abrasive grains can be evaluated as follows, for example. That is, a grindstone is made using the abrasive grains, and using this grindstone, a difficult-to-machine material such as chromium molybdenum steel is used as the material to be machined, and evaluation is performed using the grinding ratio and the required grinding power in this case. Specifically, evaluation can be performed by the method described in the examples.

[0059] [Manufacturing method]

[0060] The abrasive grains of the present embodiment can be suitably manufactured by the manufacturing method according to the embodiment of the present invention. The manufacturing method according to the present embodiment includes: a step of obtaining a mixed solution (step 1), the mixed solution being a mixed solution of pseudo-boehmite, silica having an average particle size of 100 nm or less and being 0.02 parts by mass or more and less than 3.0 parts by mass with respect to 100 parts by mass of the solid content of the pseudo-boehmite, and water being 400 to 630 parts by mass with respect to 100 parts by mass of the solid content of the pseudo-boehmite; a step of adding an anti-flocculant to the mixed solution to obtain a sol (step 2); a step of adding alumina seeds having a D50 of 700 nm or less and being 0.01 to 5.0 parts by mass with respect to 100 parts by mass of the solid content of the pseudo-boehmite to the sol to obtain a sol containing seeds (step 3); a step of drying the sol containing seeds until the water content is 15% by mass or less to obtain a solid, crushing the solid, and pre-firing at a temperature of 550°C or higher and lower than 1200°C (step 4); and then, a step of firing at a temperature of 1200 to 1500°C (step 5).

[0061] According to the manufacturing method of the present embodiment, polycrystalline alumina abrasive grains having a small crystal grain size can be manufactured at low cost without using expensive rare earth elements and zirconium as in the past.

[0062] Hereinafter, each step of the manufacturing method of the present embodiment will be described.

[0063] (Step 1)

[0064] In Step 1, a mixed solution is obtained, which is a mixed solution of pseudo-boehmite, silica with an average particle size of 100 nm or less and in an amount of 0.02 parts by mass or more and less than 3.0 parts by mass relative to 100 parts by mass of the solid component of the pseudo-boehmite, and water in an amount of 300 to 800 parts by mass relative to 100 parts by mass of the solid component of the pseudo-boehmite.

[0065] Specifically, a uniform mixed solution is obtained by adding pseudo-boehmite and silica to water and stirring and mixing them.

[0066] In the present embodiment, pseudo-boehmite is used as the main raw material. Pseudo-boehmite is an aluminum oxide hydrate, and commercially available products usually contain 10 to 20% by mass of water. Regarding pseudo-boehmite, the amount of water reduced by drying it sufficiently at a temperature of 100°C is regarded as the water content, and the dried residue is regarded as the solid component amount.

[0067] As for silica, fine particles with an average particle size of 100 nm or less are used. The fine silica particles segregate at the grain boundaries of alumina in the subsequent firing process, suppressing the crystal growth of alumina, whereby polycrystalline alumina abrasive grains with a small crystal grain size can be obtained.

[0068] From the viewpoints of operability, cost, etc., the maximum particle size of silica is preferably 1 μm or less, more preferably 500 nm or less, and still more preferably 200 nm or less. From the viewpoint of suppressing the grain growth of alumina during firing, the average particle size of silica is preferably 1 to 50 nm, more preferably 2 to 30 nm. As such fine particle silica, from the viewpoints of ease of obtaining, etc., colloidal silica dispersed in water is preferably used.

[0069] Furthermore, the average particle size of silica in this specification is the Sears particle size when it is 5 nm or less, and the value calculated from the specific surface area measured by the BET method (single-point method, nitrogen adsorption) when it exceeds 5 nm. The Sears particle size is the equivalent diameter calculated by regarding the spherical particles obtained by finding the specific surface area of the silica (colloidal silica) equivalent to 1.5 g of SiO2 based on the titration amount of a 0.1 mol / L sodium hydroxide aqueous solution required to change the pH from 4 to 9 (refer to G.W. Sears, Jr., “Analytical Chemistry”, vol. 28, No. 12, pp. 1981-1983 (1956)). The average particle size [nm] calculated by the BET method is calculated using the relational expression 6000 / {(density [g / cm 3 )×(specific surface area [m 2 / g])} (density of silica: 2.2 g / cm 3 )

[0070] Sometimes, silica as an impurity derived from raw materials and components containing other elements are also included in the pseudo-boehmite as inevitable components. Therefore, in order to obtain the abrasive grains of the present embodiment with a silica content of 0.05 to 3.30% by mass, the compounding amount of silica when obtaining the mixed liquid is set to 0.01 mass part or more and less than 3.0 mass parts with respect to 100 mass parts of the solid component of the pseudo-boehmite, preferably 0.02 to 2.5 mass parts, more preferably 0.03 to 2.0 mass parts, still more preferably 0.05 to 0.4 mass parts, and particularly preferably 0.08 to 0.35 mass parts.

[0071] If the compounding amount of silica is 0.01 mass part or more, the crystal particle size of the abrasive grains is likely to become small, and if it is less than 3.0 mass parts, it is easy to obtain abrasive grains with a large apparent specific gravity and high hardness.

[0072] From the viewpoints of ease of preparation of the mixed liquid in which the pseudo-boehmite and silica are uniformly dispersed, operation efficiency, etc., the amount of water in the mixed liquid (including the water content of the pseudo-boehmite and silica (colloidal silica)) is set to 400 to 630 mass parts with respect to 100 mass parts of the solid component of the pseudo-boehmite, preferably 430 to 570 mass parts, more preferably 450 to 490 mass parts.

[0073] (Step 2)

[0074] Next, in Step 2, an anti-flocculant is added to the mixed liquid to obtain a sol.

[0075] By adding the anti-flocculant, the particles in the mixed liquid are made colloidal to form a stably dispersed sol.

[0076] As the anti-flocculant, a monobasic acid is preferred, and examples thereof include acetic acid, hydrochloric acid, formic acid, nitric acid, etc. The anti-flocculant can be used alone as 1 type, or 2 or more types can be used in combination. Among them, nitric acid is preferred.

[0077] The usage amount of the anti-flocculant can be appropriately adjusted according to the type of the anti-flocculant, the content of the particles in the mixed liquid, the particle size, etc. For example, when nitric acid is used as the anti-flocculant, the usage amount of nitric acid is preferably 0.1 to 20 mass parts with respect to 100 mass parts of the solid component of the pseudo-boehmite, more preferably 0.5 to 15 mass parts, and further preferably 1 to 10 mass parts. Furthermore, from the viewpoints of uniform mixing property, operability, etc., the anti-flocculant can also be appropriately diluted with water or the like before use.

[0078] (Step 3)

[0079] Next, in Step 3, an alumina seed crystal having a D50 of 700 nm or less and in an amount of 0.01 to 5.0 parts by mass relative to 100 parts by mass of the solid content of the boehmite is added to the sol to prepare a seeded sol. From the viewpoint of promoting the formation of alumina crystals in the subsequent sintering step, an alumina seed crystal is added to the sol. The seeded sol is preferably uniformly mixed and stirred.

[0080] In order to obtain polycrystalline alumina abrasive grains with a small crystal grain size, the alumina seed crystal used is an alumina seed crystal having a D50 of 700 nm or less, and its D50 is preferably 400 nm or less, more preferably 200 nm or less. The lower limit of the D50 of the alumina seed crystal is not particularly limited, but in practice, from the viewpoints of ease of operation, etc., it is preferably 80 nm or more, more preferably 100 nm or more, and further preferably 120 nm or more.

[0081] From the viewpoint of uniform dispersibility in the sol, the alumina seed crystal can also be added in the form of a dispersion dispersed in a liquid medium such as water.

[0082] From the viewpoints of removing lumps caused by insufficient deflocculation, foreign matter mixed in, etc., the seeded sol can be refined by means such as filtration and then supplied to the next step.

[0083] (Step 4)

[0084] Next, in Step 4, the seeded sol is dried until the water content is 15% by mass or less to obtain a residue, the residue is crushed, and then pre-sintered at a temperature of 550 °C or higher and lower than 1200 °C.

[0085] From the viewpoint of uniformizing the firing in the subsequent firing step, drying and pre-sintering are carried out.

[0086] The drying of the seeded sol is carried out until the water content of the dried residue (dried product) becomes 15% by mass or less. The drying reduction amount is regarded as the water content of the dried material. Drying can be carried out, for example, by heating at a temperature of 100 to 140 °C.

[0087] From the viewpoint of obtaining homogeneous abrasive grains, the pre-sintering is carried out after crushing the dried material. The crushing also depends on the desired particle size of the abrasive grains, but for example, in order to obtain particles equivalent to particle size F60 specified in JIS R6001-1:2017, it is preferably carried out in such a way that the particle size becomes about 1 mm or less, more preferably 900 μm or less, and further preferably 700 μm or less.

[0088] The crushing means is not particularly limited and can be carried out by known means. For example, roll crushers, hammer crushers, jaw crushers, ball mills, etc. can be cited.

[0089] For the particles obtained by crushing, depending on the desired particle size of the abrasive grains, part of them can be screened out considering the shrinkage of the particles in the firing process and then pre-fired.

[0090] The pre-firing is carried out at a temperature in the range of 550°C or higher and lower than 1200°C, preferably at a temperature of 600 - 1150°C, more preferably at a temperature of 650 - 1100°C. The pre-firing can be carried out using various heat treatment furnaces, but from the viewpoint of uniform heating of the particles, it is preferably carried out using a rotary kiln.

[0091] In the pre-firing, the particles are sufficiently dehydrated and impurities such as those derived from the deflocculant are sufficiently removed.

[0092] The pre-firing preferably includes a first pre-firing step carried out at a temperature of 550°C or higher and lower than 1000°C, and a second pre-firing step carried out at a temperature of 800°C or higher and lower than 1200°C and higher than the first pre-firing step.

[0093] By passing through multiple pre-firing steps with a gradually increasing temperature, abrasive grains with a smaller crystal grain size, a larger apparent specific gravity, and a higher hardness can be obtained.

[0094] From the viewpoint of the manufacturing efficiency of the abrasive grains, the multiple pre-firing steps can be two pre-firing steps (setting two stages of temperature), but three or more stages of temperature can also be set to carry out three or more pre-firing steps.

[0095] The temperature of the first pre-firing is preferably 550°C or higher and lower than 1000°C, more preferably 600 - 900°C, and further preferably 650 - 800°C. The time of the first pre-firing is, for example, 1 - 30 minutes.

[0096] The temperature of the second pre-firing, on the condition that it is higher than the temperature of the first pre-firing step, is preferably 800°C or higher and lower than 1200°C, more preferably 900 - 1150°C, and further preferably 950 - 1100°C. The time of the second pre-firing is, for example, 1 - 30 minutes.

[0097] (Process 5)

[0098] Next, in Process 5, firing is carried out at a temperature of 1200 - 1500°C.

[0099] Through the firing at a high temperature (final firing), starting from the alumina seed crystals, the sintering of the alumina polycrystals with a small crystal grain size is promoted.

[0100] Firing is carried out at a temperature in the range of 1200 to 1500 °C, preferably at a temperature of 1250 to 1450 °C, and more preferably at a temperature of 1300 to 1400 °C. The firing can be carried out using various heat treatment furnaces, but from the viewpoint of uniform heating of the particles, it is preferably carried out using a rotary kiln. The firing time is, for example, 1 to 30 minutes.

[0101] After the obtained fired powder is naturally cooled, it is sieved as needed, whereby abrasive grains having a desired particle size can be obtained.

[0102] The grindstone of the present embodiment is a grindstone containing the polycrystalline alumina abrasive grains of the present embodiment. The abrasive grains of the present embodiment can be applied to abrasive products and grinding products by known methods, and are particularly suitable as a material for grindstones for abrasive and grinding tools. Examples of abrasive products and grinding products include ceramic bond grinding wheels, resin bond grinding wheels, abrasive cloth and paper, etc.

[0103] The abrasive grains of the present embodiment can also be used, for example, in a formed grindstone, which is a formed grindstone formed into various shapes such as a star shape, an arrow shape, a polygon, a rod shape, and a cross shape by using an adhesive or the like through forming processes such as extrusion molding and press molding.

[0104] Examples

[0105] Hereinafter, embodiments of the present invention will be described based on examples, but the present invention is not limited to the following examples.

[0106] [Manufacture of Abrasive Grains]

[0107] The details of the raw materials used in the examples and comparative examples are as described below.

[0108] · Boehmite: water content 20% by mass

[0109] · Colloidal silica: "Snowtex (registered trademark) ST-NXS", manufactured by Nissan Chemical Industries, Ltd., water dispersion, average particle size 5 nm, solid content 15% by mass

[0110] · Deflocculant: dilute nitric acid (concentration 17.8% by mass)

[0111] · Alumina seed crystals: alumina purity 99% by mass or more, D50 150 nm

[0112] Furthermore, the alumina seed crystals are pulverized products using a ball mill, and D50 is measured using a laser diffraction scattering type particle size distribution device ("MT3300EXII", manufactured by Microtrac).

[0113] (Example 1)

[0114] 7326.15 g of pure water, 17.0 g of colloidal silica (the solid content is 0.15 parts by mass relative to 100 parts by mass of the solid content of boehmite), and 2125 g of boehmite were stirred and mixed for 30 minutes. 425 g of dilute nitric acid as an anti-flocculant was added thereto over 1 minute, and the mixture was stirred for 4 hours or more to obtain a sol.

[0115] 85.0 g of an aqueous dispersion of alumina seeds with a concentration of 20% by mass (1 part by mass of alumina seeds relative to 100 parts by mass of the solid content of boehmite) was added to the obtained sol. After stirring for 30 minutes, the content was poured into a flat-bottomed vat (thickness: 20 - 30 mm).

[0116] The flat-bottomed vat was placed in a dryer set at a temperature of 100°C, and the content was dried for 15 hours until the water content became 15% by mass or less.

[0117] The obtained dried product was crushed with a roll crusher to a particle size of about 700 μm or less, and then sieved with a vibrating screen to obtain a sized powder having a particle size exceeding 375 μm and being 475 μm or less.

[0118] The sized powder was pre-fired (first time) in a rotary kiln at a temperature of 700°C for 10 minutes, and further pre-fired (second time) at a temperature of 1050°C for 8 minutes.

[0119] Next, the obtained pre-fired powder was fired in a rotary kiln at a temperature of 1360°C for 8 minutes.

[0120] After natural cooling, the obtained fired powder was sieved with a vibrating screen to obtain abrasive grains having a particle size exceeding 180 μm and being 450 μm or less (particle size F60).

[0121] (Examples 2 - 8 and Comparative Examples 1 - 3)

[0122] Silica was set to each addition amount shown in Table 1, and abrasive grains were produced in the same manner as in Example 1 except for this. Further, in Example 8, the pre-firing (second time) at a temperature of 1050°C was not performed, and only the first pre-firing was performed.

[0123] [Measurement and Evaluation of Abrasive Grains]

[0124] For each of the abrasive grains obtained in the examples and comparative examples, the following items were measured and evaluated. The evaluation and measurement results are shown in Table 1.

[0125] Furthermore, alumina in each of the abrasive grains was confirmed to be α-alumina by X-ray diffraction analysis.

[0126] (Crystal Grain Size)

[0127] After washing approximately 0.15 g of abrasive grains with water, they were placed in an alumina boat, and 0.7 g of an aqueous sodium tetraborate solution with a concentration of 5% by mass was added dropwise. After drying at a temperature of 100 °C, the temperature was increased at a rate of 100 °C per hour, and heat treatment was performed at a temperature of 950 °C for 30 minutes. After natural cooling, it was heated to 100 °C again, and hydrochloric acid with a concentration of 17.5% by mass was added dropwise to the extent of soaking the abrasive grains and stirred. After the abrasive grains were sufficiently dispersed, they were washed with water and dried at a temperature of 100 °C to obtain a powder, and this powder was used as a sample.

[0128] The sample was observed with a scanning electron microscope (SEM) (“JSM-6510LV”, manufactured by JEOL Ltd.; magnification: 20,000 times). The number of grain boundaries on a total of 4 lines, namely vertical, horizontal, and diagonal lines passing through the center of the observed image (rectangle), was counted, and the value obtained by dividing the length of each line by the respective number of grain boundaries was calculated. The average value of the observed images for 10 fields of view was taken as the crystal grain size.

[0129] (Composition analysis)

[0130] For the abrasive grains, the content of various metal components in terms of oxide conversion was determined using a fluorescent X-ray analyzer (“ZSX (registered trademark) Primus II”, manufactured by Rigaku Corporation; fundamental parameter method).

[0131] (Apparent specific gravity)

[0132] For the abrasive grains, measurement was performed using a pycnometer (gas displacement pycnometer “AccuPyc II 1340”, manufactured by Micromeritics; helium displacement).

[0133] (Vickers hardness)

[0134] According to JIS Z 2244:2009, the Vickers hardness was measured as follows.

[0135] 0.1 g of abrasive grains was placed in a sample cup (round bottom surface with a diameter of 25 mm), and a two-component epoxy resin was poured in and cured to obtain a cured product in which the abrasive grains were embedded.

[0136] The bottom surface of the cured product taken out from the sample cup was polished with sandpaper. After surface alignment of the abrasive grain surface, mirror polishing with diamond abrasive grains to a depth of 1 μm was performed to prepare a measurement sample.

[0137] For the measurement sample, the Vickers hardness was measured using a micro-Vickers hardness tester (“HMV-G21”, manufactured by Shimadzu Corporation). Each abrasive grain was measured once, and the Vickers hardness value shown in Table 1 is the arithmetic mean of the measurement values of 15 abrasive grains.

[0138] Table 1

[0139]

[0140] (Remarks) * Mixing amount per 100 parts by mass of the solid component relative to pseudo-boehmite

[0141] As is understood from the results shown in Table 1, it was confirmed that polycrystalline alumina abrasive grains containing a specified amount of silica have a small crystal grain size, an easily increased apparent specific gravity, and a high Vickers hardness.

[0142] In addition, from the comparison between Example 1 and Example 8, it can be said that abrasive grains having a larger apparent specific gravity and a higher Vickers hardness can be obtained by pre-firing in two stages.

[0143] [Evaluation of grinding performance]

[0144] Using the respective abrasive grains obtained in Examples 1 to 4 and Comparative Example 1, ceramic bond grinding wheels were produced, and the grinding performance of the abrasive grains was evaluated.

[0145] 46 vol% of abrasive grains, 5 vol% of frit bond, and 49 vol% of binder (phenolic resin: "BRP-5417", manufactured by Aika Kogyo Co., Ltd.) were mixed and fired at a temperature of 1050 °C to produce ceramic bond grinding wheels (segment grindstones).

[0146] The following shows the grinding conditions in the evaluation of grinding performance.

[0147] (Grinding conditions)

[0148] · Workpiece material: SCM435 (chrome molybdenum steel; Rockwell hardness (HRC) 42)

[0149] · Workpiece surface: length 200 mm, width 100 mm

[0150] · Grinding machine: surface grinder "PSG-63AN", manufactured by Okamoto Machine Works, Ltd.; magnetic pole shaft motor 3.7 kW

[0151] · Grinding method: wet surface longitudinal grinding

[0152] · Grinding wheel peripheral speed: 1800 mm / min

[0153] · Worktable speed: 15 m / min

[0154] · Cross feed: 4 mm / pass

[0155] · Depth of cut: 10 μm (total cutting depth: 1 mm)

[0156] (Evaluation item)

[0157] <Grinding ratio>

[0158] Calculate the volume reduction of the material to be machined before and after grinding (200 mm × 100 mm × 1 mm = 20,000 mm 3 ) with respect to the grinding wheel wear amount [mm 3 , and use it as the grinding ratio. Furthermore, the grinding wheel wear amount is obtained by calculating the wear volume based on the reduction amount of the grinding wheel diameter before and after grinding.

[0159] The grinding ratio in Table 2 is shown as a relative value with Comparative Example 1 as the reference (reference value is 1).

[0160] <Grinding power>

[0161] Find the average electrical power during grinding as the grinding power.

[0162] The grinding power in Table 2 is shown as a relative value with Comparative Example 1 as the reference (reference value is 1). The smaller the grinding power, the smaller the grinding resistance.

[0163] Table 2

[0164] Grinding ratio (relative ratio) Grinding power (relative ratio) Example 1 2.61 0.67 Example 2 1.98 0.82 Example 3 2.61 0.75 Example 4 1.91 0.79 Comparative Example 1 1 1

[0165] In addition, as can be seen from the results shown in Table 2, it was confirmed that: compared with the alumina abrasive grains (Comparative Example 1) manufactured without adding silica, the abrasive grains of the present embodiment have a larger grinding ratio, and a smaller grinding power, and excellent grinding performance.

Claims

1. A polycrystalline alumina abrasive grain, wherein the alumina content is 95.0 mass % or more, the silicon dioxide content is 0.05 to 3.30 mass %, the crystal grain size is 0.100 to 0.400 μm, and the Vickers hardness is 21.00 or more.

2. The polycrystalline alumina abrasive grain according to claim 1, having an apparent specific gravity of 3.800 g / cm 3 above.

3. The polycrystalline alumina abrasive grain according to claim 1, wherein the alumina content is 99.0 mass % or more, the silicon dioxide content is 0.10 to 0.40 mass %, the crystal grain size is 0.100 to 0.180 μm, and the apparent specific gravity is 3.900 to 3.960 g / cm 3 , Vickers hardness is 23.10~24.

00.

4. A method for producing polycrystalline alumina abrasive grains, the method being a method for producing the polycrystalline alumina abrasive grains according to any one of claims 1 to 3, comprising: A step of obtaining a mixed solution, wherein the mixed solution is a mixed solution of pseudo-boehmite, 0.01 parts by mass or more and less than 3.0 parts by mass of silicon dioxide having an average particle size of 100 nm or less relative to 100 parts by mass of the solid content of the pseudo-boehmite, and 400 to 630 parts by mass of water relative to 100 parts by mass of the solid content of the pseudo-boehmite; A step of adding a deflocculant to the mixed solution to obtain a sol; A step of adding 0.01 to 5.0 parts by mass of alumina seed crystals having a median particle size D50 of 700 nm or less to the sol based on 100 parts by mass of the solid content of the pseudo-boehmite to obtain a seed-containing sol; A step of crushing a residue obtained by drying the seed crystal-containing sol until the water content is 15 mass % or less, and then pre-calcining the residue at a temperature of 550° C. to less than 1200° C.; and Next, a sintering step is performed at a temperature of 1200 to 1500°C.

5. The method for producing polycrystalline alumina abrasive grains according to claim 4, In the step of obtaining the mixed solution, the blending amount of the silica is 0.08 to 0.35 parts by mass based on 100 parts by mass of the solid content of the pseudo-boehmite.

6. The method for producing polycrystalline alumina abrasive grains according to claim 4, The process of pre-firing comprises: A first pre-firing step at a temperature of 550°C or higher and lower than 1000°C; and The second preliminary calcination step is performed at a temperature of 800°C or higher and lower than 1200°C and higher than the first preliminary calcination step.

7. The method for producing polycrystalline alumina abrasive grains according to claim 4, The pre-firing and the firing are performed using a rotary kiln. 8 . A grindstone comprising the polycrystalline alumina abrasive grain according to claim 1 .

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