KR920007018B1 - Abrasive material and method for preparing the same - Google Patents

Abstract

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Description

Ceramic body and its manufacturing method

The present invention relates to a method for producing shaped articles containing such alumina with alumina abrasive grit or high density polycrystalline alpha alumina or other additives.

Robust hard abrasive grit for grinding wheels, bendable coated abrasives ("sand papers"), or loose abrasives, melts alumina-containing raw materials in an electric furnace or forms molded articles containing finely divided alumina. It is commercially produced by firing at temperatures well below the melting point of the material.

The low temperature method described above is called a sintering method, and the present invention relates to an alumina abrasive produced by the sintering method.

Sintered abrasives were first commercially produced on a large scale by the method described in Mr. Weltz's US Patent No. 3,072,243, which milled the calcined bauxite as a raw material of a fine particle size, followed by grinding grit particle size. It is then formed into a hard and strong polycrystalline alumina pellet by baking at about 1500 ° C.

Recently, abrasive materials consisting of alumina and magnesia spinel composition grit, which have been prepared by the method described in U.S. Patent No. 4,314,827 and the method described in British Patent Application No. 2,009,012A, issued December 1, 1982, have been commercially available. These materials are prepared by sintering dry alumina gel particles at about 1400 ° C. Bugosh's U.S. Pat. The manufacturing method of the high density alumina (or alumina containing) product which carries out hot pressing is described.

Alumina / magnesia-spinel commercial abrasives prepared from gels have cell-type alumina with a diameter of 5 to 15 microns, and the cell (or “sunburst”) has numerous elongations with a diameter of 0.2 to 0.4 micrometers. Consisting of alumina arms, however, some of which are roughly spherical droplets of 1 micrometer in diameter, and the arms within each cell are generally from the center of the cell. It forms a radiating form. All of the arms in a given cell clearly show crystallographically identical orientations, the same orientation as described above is due to the fact that all regions of a given cell are simultaneously lost in sample rotation when observed with transmitted light microscopy between crossed polarizers. Able to know.

Although the quality of commercial abrasives prepared from sintered gels containing alumina and magnesia belong to high quality, it was not possible to produce high purity alumina grit by gel method, which is a "comparative" example in the specification of US Pat. No. 4,314,827. This is evidenced by the lack of relative softness and polishing efficiency described in 13 (prepared from alumina gel without addition of metal oxides or metal salts).

The present invention improves the manufacturing method of the conventional reinforced abrasive, and according to the present invention, a useful abrasive product can be prepared with or without addition of zirconia or spinel forming material (eg magnesia) to the alumina gel.

The present invention is based on the fact that controlling the microstructure of a fired product, such as eliminating the cell structure of alumina in abrasives according to the prior art, improves the performance of the product.

Products produced by the present invention have alpha alumina particles (microcrystals) of submicron size (0.2 to 0.4 micrometers) instead of cell regions having diameters of 5 to 10 micrometers.

When the amount of added magnesium oxide (MgO) is increased (such as 5%), these alumina particles are surrounded by the spinel matrix.

Control of the gel to achieve the above-mentioned effects can be carried out by vibratory milling a mixture of sol or dilute gel-like mixtures and at the same time using alumina bodies as grinding media inside the mill, which is the main effect of milling. Is thought to introduce into the alumina gel from the alumina grinding media. In addition, impurities such as zinc and iron are introduced from pipes and combination facilities, for example, when milling together with zirconia, it becomes very difficult to obtain a desired non-cell structure.

By vibrating milling the gel with the alumina body using the most effective and successful method found by the inventors, the above-mentioned materials in the gel can be produced. Suitable vibration mills are known from US Pat. No. 3,100,088, and typically media can be used that are generally 1/2 inch in diameter and 1/2 to 3/4 inch in length. The tub containing the mixture with the medium is vibrated in a horizontal plane by an unbalance weight connected to the shaft of a coaxially assembled motor mounted on a spring. An unbalance weight is installed adjacent to the bottom of the cylinder, and a second weight is installed below it. The typical rotational speed of the motor is 1200 rpm and the contents are milled by the grinding media means by a combination of vibrations. The inner surface of the pulverizer is preferably embedded with rubber to prevent contamination by corrosion of the metal wall.

Alumina can be added with various additives described in US Pat. No. 4,314,827 and British Patent Application No. 2,099,021A before or after gelation, and the most effective additives currently available are all compatible precursors of MgO with MgO. Material, and it is therefore desirable for the final product to contain about 5% MgO. MgO exists in the product as spinel (magnesium aluminate: MgAl ₂ O ₄ ), but is calculated as MgO in the analysis. However, even when no additives are added to the alumina, when the abrasive is prepared by the method according to the present invention, an excellent abrasive is produced, so that MgO may be used in an amount less than the amount described above. The ground gel according to the present invention can be used as a matrix of various additives or abrasive particles.

The ground mixture is crushed to a suitable size by a crushing operation with a small material which is simply injected or introduced into the container, dried and then recycled back to the initial operating stage of the process, or alternatively formed into shaped particles as by extrusion means. It can be molded or extruded. In the case of extrusion, the forming rods can later be cut or crushed to the appropriate size. The lowest effective firing temperature is usually well below the transition point to be converted to alpha alumina or so called 1200 ° C., and no upper temperature is necessary unless the melting point is reached. If the firing time is too long or the temperature is too high, excessive crystal growth may occur. If the higher temperature is used, the process cost is increased, so the firing temperature is preferably 1200 to 1500 ° C.

Example 1

In a large polymeric plastic mixing vessel, 30 pounds (13.6 kg) of Condea SB Pural alumina produced by Condea is mixed with 30 gallons of water (British, 136 L). Then 4.1 L of 14% by weight nitric acid is added to gel the mixture. Next, a solution of 7.5 pounds (3.4 kg) of magnesium nitrate hydrate dissolved in 3 gallons (13.7 L) of water was added to the alumina gel, which contained 5% by weight of MgO in the final product. Mix for 15 minutes, transfer to a M451 Sweco mill, mill 1 hour with 1700 pounds of alumina medium, and then mix the mill at a rate of about 4 gallons per minute for 1 hour milling time. Recycle me After the milling operation it is pumped into an aluminum tray at a thickness of about 3 inches (7.6 cm) and dried on an electric strip dryer.

The alumina medium consists of about 90% alpha alumina containing silica whose composition is the main impurity.

A series of batches are constructed and combined by the method described above for shredding and firing operations.

The dried gel is then roll crushed prior to the firing operation and passed through a 14 mesh sorter to screen the desired final grit size. It is then prebaked at 400 ° C. for 16 hours and calcined at 1400 ° C. for 30 minutes in a rotary kiln.

All products after the firing operation had a hardness of 19 kPa (Beakers hardness tester, 500g load), very fine microstructure without cell microstructure, and alpha alumina was almost all 5 microns in diameter. Except for flaky granulated crystals with a cut surface, they have the form of equiaxed particles (microcrystals) having a diameter of 0.2 to 0.4 microns. The site containing the granulated crystals was contaminated with the above-mentioned contaminants, and the scanning electron microscope showed that the product consists of a spinel matrix and discontinuous alpha alumina.

Compared to some coated abrasive grinding, the product was superior to molten alumina-zirconia and superior to commercially available sintered gel type abrasives with alumina-spinel composition.

Example 2

22.7 kg of fural microcrystalline boehmite alumina is mixed with 225 L of water and 13.5 L of 14% nitric acid for 10 to 15 minutes.

Half of the gel mixture is commercially available from Coors Co., Ltd., 1/2 × 1/2 inch ceramic bond alumina, 88Al ₂ O ₃ (main impurities are MgO 1.74%, SiO ₂ 8.9%, Fe ₂ O ₃ 0.18). %, TiO ₂ 0.2%, CaO 0.8%, Na ₂ O 0.34%), and milled for 2 hours in a Sweko mill. This is the same medium as used in Example 1. On the other hand half of the gel mixture is simply dried without milling, the dry gel is crushed to the desired size and then prebaked at 450 ° C. for 16 hours and calcined at 1400 ° C. for 1 hour.

The milled product has a hardness of 19.1 kPa and the unmilled product has a hardness of 11.0 kPa.

50 grit abrasives are screened for each batch of product and used to make vulcanized fiber back coated abrasive discs. The milled product performed at least 10% better than alumina zirconia abrasives commercially available at 1020 steel grinding (more than 14% more metal removed as a result of the test).

The unmilled product was inferior to the molten abrasive in all the tests of the grinding, as can be expected from its low hardness.

Example 3

In an example similar to the Example section of the milling article of Example 1, the gel is milled for 0.2 hours. The product fired at 1400 ° C. for 1 hour is mainly irregular and fine 0.2-0.3 mm crystal structure, but shows some cell appearance.

[Examples 4 to 9]

Several examples are carried out at 1400 ° C. to study and study the effect of firing time. All samples are manufactured by the general process of Example 1. Condea microcrystalline boehmite alumina is used and milling is carried out for 2 hours, but after drying operation the gel is prebaked at 750 ° C. for 30 minutes. As the firing time increases, crystallization of coarse lath-type alumina begins to appear irregularly in fine 0.2-0.4 micrometer alumina particles in the product.

The results are as follows.

Since it is not desirable to have coarse particles in the product, if the material is precombusted at 750 ° C. for 30 minutes, the combustion time under 1400 ° C. should be within 5 minutes to obtain the desired product.

In all cases no cell structure is observed. The microstructures consist of submicron-sized particles with no cutting surface and flaky assembly crystals with cutting surface, except in the case of 1 minute firing period where no lath is found.

The term "no cutting plane" means that there is no regular cutting plane of microcrystals on the crushed surface when viewed at 5000 times magnification with a scanning electron microscope. However, the particles of alpha alumina are amorphous but generally equiaxed into a curved outline (almost no linear outline). When observed at 20,000 times magnification, the cut surface structure began to be clearly seen.

The abrasive grit according to the invention has a hardness value of at least 16 kPa (90% density) for alumina without additives by a beakers hardness tester with a 500 g load and is modified in the presence of at least 2% spinel formations. Or a hardness value of 14 kPa or more for the modified grit by addition of other additives. Pure high-density alpha alumina has a hardness value of about 20 to 21 kPa, and some pores may be used in certain applications of low hardness. When the hardness value of the alumina is 13 kPa or less, it is too porous to achieve the object of the present invention. Therefore, the hardness value measured by the beaker hardness tester under 500 g load is preferably 18 kPa or more.

Example 10

A series of abrasives are prepared with varying magnesia content. The general procedure of Example 1 is used, including milling with alumina media (milling for 2 hours). In all cases the gel is dried at 200 ° C. for about 30 hours, then crushed and screened and then calcined at 450 ° C. for 16 hours. The resulting grit-sized particles were fired in a rotary kiln at 1400 ° C., whereby the heating time until reaching 1400 ° C. was about 15 minutes and the combustion time at 1400 ° C. was about 15 minutes.

Prior to the gelling operation, the amount of magnesium nitrate is added with varying amounts (no magnesium nitrate is added to one of the experiments). The MgO content and the hardness of the abrasive are as follows:

In the test of a series of vitrified (glass-bonded) grinding wheels using 54 grit (combination of 46 grit and 60 grit) sizes, the grinding wheels made of the above-mentioned grit were subjected to the best known quality of the molten alumina abrasive. Compare.

These tests are carried out by grinding of slots in tool steel D3 with varying feeds. For dry grinding at 0.5 mil (0.0005 in.) Downfeed, the grinding ratio of the abrasive without magnesium oxide (0.14% MgO) is 16.18 times the grinding ratio of the molten abrasive (grinding ratio, G ratio to wheel wear) Volume fraction of material removed). According to the dry grinding test, all the abrasives added with magnesium oxide are superior in performance to the molten abrasive. In addition, wet grinding tests show that MgO-added experimental abrasives perform poorly or similarly to molten abrasives. The abrasive without magnesia outperforms the molten abrasive under 2 mils.

According to the coated abrasive test (CAMI standard) using a 50-grit size abrasive, the abrasive prepared according to Example 10 and mixed with 0.6% MgO in a bendable abrasive disc performed better than the abrasive in which alumina zirconia was melted together for 1020 steel. This is 136% good and almost the same performance as molten alumina-zirconia for stainless steel. In addition, the abrasive containing 2.5% MgO and 7.59% MgO is excellent for 1020 steel, and the larger the amount of MgO added, the less effective it is for stainless steel.

0.14% MgO abrasive contains impurities such as 0.25% SiO ₂ , 0.18 Fe ₂ O ₃ , 0.28% TiO ₂ , 0.05% CaO and 0.04% Na ₂ O in addition to alumina, which are mainly introduced during milling operations. I can think of it. The aforementioned impurities are included among other abrasives in amounts similar to the above.

While not wishing to be bound by any particular theory of the invention, it is understood that the introduction from the alumina medium, which is a particulate material, may affect the crystallization seed of alpha alumina during firing operation, in addition to other impurities introduced during milling operations. They may inhibit the crystal growth of the final product if these impurities are present at the grain boundaries between the alpha alumina particles.

Wear from the milling media is effective in the adjustment of the gel, and without milling the gel as evidence of the obvious fact that the ideal density, microcrystals and non-cell-type alpha alumina are obtained upon firing at around 1400 ° C. Milled water is added to the alumina monohydrate with acid.

Except for 6 batches, boehmite of water, nitric acid and microcrystals were mixed as in Example 2, where water (no other material was added to the water) was mixed with alumina grinding media for several hours of milling. Changes in the amount of water in the abrasions that are worn out from

[Milling Water Addition of Alumina Monohydrate (Condea)]

* Note: 30% of the average weight loss during firing (assumed value).

In the above, hardness is measured about the baked product which baked for about 10 minutes at 1400 degreeC +/- 20 degreeC, and burns using the electric baking furnace in air atmosphere.

Investigation of the milling abrasives showed that most of the alpha alumina had a surface area of about 39 m 2 / g.

High purity alumina prepared by recovering the suspended alumina microparticles remaining in the suspension when the fine alumina powder is mixed with water and then settled is effective when used in an amount of about 0.1% or more as a calcined gel solid.

Experiments with commercially available fine alpha alumina powders and very high purity molten alumina as milling media using fine alumina obtained by milling using such alumina itself are very effective for producing the dense microcrystalline products according to the present invention.

According to the results of the differential thermal analysis, the conversion of gel alumina from gamma type to alpha type in the presence of alpha alumina seed particles occurs at about 1090 ° C., while the absence of the above-mentioned seed material occurs at about 1190 ° C. The theoretical lowest firing temperature of the product according to the present invention may be a temperature below the known transition temperature.

According to the present invention, a high purity alpha alumina body having a particle size of submicron size and a density of 95% or more can be sintered at low temperature to produce a product having a hardness of 18 kPa or more, and the coating, Products other than abrasives such as thin films, fibers, rods or small parts can be made.

The present inventors conducted tests by adding crystal growth inhibitors such as SiO ₂ , Cr ₂ O ₃ , MgO, and ZrO ₂ to the control gel. As a result, when MgO was added, spinel was formed by reacting with alpha alumina. It surrounds alpha alumina and, in the case of other additives, minimizes compound formation with alpha alumina and these remain in the crystal boundary. It is clear from the above experimental results that the crystal growth by diffusion and recrystallization is suppressed by the additives, and the above-mentioned experiment can be more flexible in the time-temperature relationship of the sintered product. Has The use of growth inhibitors is well known in the ceramic art and is not an essential component in the present invention, but is very useful in maintaining the ideal microstructure over a wide range of sintering time-temperatures without the need for high purity alpha alumina. .

Claims (12)

It is essentially a dense polycrystalline phase of alpha alumina particles of submicron size (diameter less than 1 micron), most of which have no cross section, as measured by Vickers hardness tester with a hardness of at least 16 kPa and a density of at least 90% of the theoretical density of alpha alumina. Ceramic body in the form of one or more abrasive grit composed. The ceramic body according to claim 1, wherein said grit has a hardness of 18 kPa or more. The ceramic body of claim 1, wherein the ceramic body comprises an additive that is compatible with alpha alumina particles. The ceramic body according to any one of claims 1-3, wherein the particles have a diameter of 0.4 micron or less. The ceramic body according to any one of claims 1 to 3, wherein the ceramic body comprises flake-shaped alumina granulated crystals with a cut face irregularly dispersed in the submicron particles. The ceramic body according to any one of claims 1-3, wherein the ceramic body contains impurities. The ceramic body according to any one of claims 1-3, wherein the density is 95% or more of the theoretical density of alpha alumina. A method for producing a ceramic body containing a high density polycrystalline alpha alumina, comprising sintering a dry alumina gel or a dried alumina gel treated by adding submicron alpha alumina particles to a precursor of the gel. The method of claim 8, wherein the gel or sol used to prepare the gel is milled in a mill using alpha alumina bodies as grinding media. The method of claim 8, wherein the gel is treated by adding grinding water suspended in submicron alpha alumina particles formed by milling the grinding alpha alumina body and water. The method of claim 8, wherein the gel consists of an aqueous dispersion of submicron hydrated alumina particles. The method of claim 8, comprising crushing the dry gel before or after sintering to form abrasive grit.