DE19629690C2 - Process for the production of sintered alpha-AL¶2¶0¶3¶ bodies and their use - Google Patents

Description

The present invention relates to a method for producing sintered, microcrystalline bodies based on α-Al ₂ O ₃ and the use thereof.

A preferred application of α-Al ₂ O ₃ sintered bodies is their use as an abrasive. In addition to molten corundum abrasives, those made of sintered material have been known for more than 50 years.

US Pat. No. 3,909,991 describes polycrystalline α-Al ₂ O ₃ bodies whose crystallite size is in the submicron range and whose density is above 95% of the theoretical density. The production takes place by hot pressing from a mixture of carbon black and granulated α-Al ₂ O ₃ bodies, which are obtained according to US Pat. No. 3,079,243 by crushing cold-pressed α-Al ₂ O ₃ shaped bodies.

More recently, similar sintered abrasives based on α-Al ₂ O _{3 have been} known, which have advantages over the known corundum abrasives due to their microcrystalline structure. For example, EP-B 0 152 768 discloses an abrasive which is produced using the sol-gel technique at sintering temperatures of approximately 1,400 ° C. Crystallization nuclei are added as a sintering aid. Similar processes and substances are known from EP-A 0 024 099, DE-A 32 19 607, US-A 4 518 397, US-A 4 574 003, US-A 4 623 364, EP-A 0 168 606, EP -A 0 200 487, EP-A 0 228 856, EP-A 0 209 084 and EP-A 0 263 810.

All of the latter methods have in common that they have a sol-gel method with finely dispersed aluminum oxide monohydrate of the boehmite type become. The relatively expensive raw materials that are involved in the hydrolysis of Aluminum-organic compounds are obtained, and the energy expenditure process engineering, the costs of sol-gel corundum are many times over of conventional corundum. Another disadvantage of these methods is that the colloidal solutions are usually more volatile with relatively large amounts inorganic acids are stabilized, which is procedural and environmental Brings problems.  

DE-C 36 04 848 describes a method of dispersing alumina-containing raw materials, silicic acid-containing compounds and other additives add (e.g. compounds of the metals Co, Ni, Mg, Cr, Zr, Zn, Si or Ti) to grind a sinterable slurry, from which by gradual drying and sintering at temperatures up to 1,700 ° C can, whose primary corundum crystallites have a diameter of less than 5 microns to have. The product obtained in this way has a crystallite size of less than 5 μm not yet the fine structure of a substance using the sol-gel method corresponding sintering additives is produced.

In addition, the method disclosed in DE-C 36 04 848 silicic acid compounds are added, which act as a sintering aid. During sintering, mullite is formed from this by reaction with the aluminum oxide. It is known that due to the presence of silicate phases the performance of an abrasive grain is reduced.

Numerous grinding tests on sintered corundums in recent years Years have shown that grinding performance is inversely proportional to size is the primary crystals. That is, the finer the structure, the higher the Rule grinding performance.

EP-A 0 524 436 discloses a method whereby instead of the expensive boehmite other inexpensive precursors of aluminum oxide (e.g. hydrargillite) are used become. By grinding and subsequent deagglomeration Suspen Sions with a solids content between 10 and 40 wt .-% obtained, the analog can be further processed into the sol-gel process. Even with this ver the remaining water has to be removed with a lot of energy. With the exception of the expensive raw material, this is demonstrated in EP-A 0 524 436 Process described all procedural disadvantages of the sol-gel method on.

Compared to the sol-gel process, however, all previously known alternative processes for the production of microcrystalline sintered corundum, which are based on inexpensive raw materials, have the disadvantage that after sintering there are significantly coarser structures than in the case of the sol-gel processes. Processed products. The reasons for this are obvious. The sol-gel processes are based on particularly finely divided starting materials that can be densely sintered with the addition of sintering aids due to the process at very low temperatures. As a result, crystal growth is suppressed. The alternative processes generally require higher sintering temperatures, which lead to increased crystal growth. In addition, a homogeneous and uncontrolled crystal growth is initiated by the phase conversions to α-Al ₂ O ₃ which take place during the sintering. The performance of the sintered corundum obtained via the alternative process is therefore significantly lower than that of the sol-gel corundum.

EP-A 0 402 G86 shows a process for extracting microcrystalline corundum from an α-Al ₂ O ₃ -containing organic suspension by electrophoretic deposition. It is thus possible to maintain a relatively dense green body, which - despite the dense and homogeneous packing - can only be densely sintered at temperatures of 1,600 ° C, so that there is increased crystal growth again. The structure of a sintered corundum obtained by means of electrophoretic deposition is significantly coarser than that of the sintered layer produced using the sol-gel process. The performance of the sintered corundum obtained by electrophoresis is correspondingly lower.

It is therefore the object of this invention to provide a method for producing a microcrystalline α-Al ₂ O ₃ sintered body which does not have the disadvantages of the prior art described, and to indicate the use thereof.

Surprisingly, it was found that these requirements are met by a process for producing sintered, microcrystalline bodies based on α-Al ₂ O ₃ , characterized in that an α-Al ₂ O ₃ powder with an average grain size ≦ 3 µm is used as the starting material is ground or deagglomerated into a slurry with a particle size of <1 µm, the slurry is granulated with the aid of a fluidized bed spray granulator into green bodies with a density ≧ 40% of the theoretical density and a residual moisture content of ≦ 6% by weight and then a shock sintering at temperatures in the range from 1300 to 1550 ° C.

The first process step consists of wet grinding or disagglomeration of an α-Al ₂ O ₃ powder that is already as finely divided as possible. The aim of wet grinding or deagglomeration is to obtain an even finer, very homogeneous α-Al ₂ O ₃ suspension with an extremely fine particle size distribution curve. The wet grinding and deagglomeration is advantageously carried out in an aqueous medium. In the second step, the aqueous suspension obtained during grinding or de-agglomeration is granulated in a fluidized bed granulator into spherical granules with a diameter between 1 and 10 mm.

DE-A 35 07 376 describes a method and an apparatus for the manufacture development of granules with a narrow particle size distribution, in which the granulate to be granulated Product in liquid form sprayed into a fluidized bed and there on correspond germs are applied. The granule size is determined by the strength of the Visible gas flow of a zigzag classifier set. Similar procedures or Further developments of the so-called fluidized bed spray granulation are in DE-A 38 08 277 and DE-A 43 04 405.

In the third step, the granules are subjected to shock sintering.

The granules can be cut to the desired grain shape before or after sintering size and size. The preparation and classification for the finished Abrasive grains are made using the usual methods.

Surprisingly, sintered corundum can be obtained from the method according to the invention, starting from inexpensive raw materials, and its properties are comparable to that of sol-gel corundum. Since the crystallite size in the sintered end product essentially depends on the average grain size and the grain distribution in the starting material, it is expedient to use α-Al ₂ O ₃ types which are as finely divided as possible and which are then ground or deagglomerated to the desired grain size.

The solids are preferably ground down and / or deagglomerated to an average particle size of less than 1 μm, particularly preferably less than 0.4 μm, in order to obtain the desired starting grain size or to break up the agglomerates present into individual crystallites. The grinding or deagglomeration is preferably carried out wet in vibration mills, attritors or ball mills. The grinding time depends on the starting grain size and the type of mill used. Although it should be obvious to choose a starting material that is as fine as possible, economic considerations often speak against it; because the superfine α-aluminum oxides are often so expensive that one of the essential advantages of the process according to the invention, namely the use of an inexpensive raw material, is lost. In the method according to the invention, however, coarser α-Al ₂ O ₃ starting powder can advantageously also be used.

Since the grinding power is inversely proportional to the primary crystal size in Abrasive grain behaves, it may be advantageous to use the starting material additional components, sintering aids and / or crystal growth inhibitors offset. Suitable additional components, sintering aids and crystal wax inhibitors are the oxides of the elements Mg, Co, Ni, Zn, Hf, Ti, Zr, Cu, Li, Sr, Ba, K, Nb, Si, B and / or rare earths.

The amount of liquid during grinding is preferably chosen so that the resulting suspension has a solids content of 15 to 80% by weight, preferably 30 up to 70% by weight. Water is preferably used as the solvent. Against other solvents such as. B. alcohols, acetone, which are also used ecological considerations speak above all.

Since a sintered body with an average primary crystal size is particularly preferred less than 0.4 µm is desired and if the sintering temperature is selected correctly In the case of the method according to the invention, crystal growth can be suppressed , it is often sufficient to continue grinding until the middle one Grain size in the suspension flies below 0.4 µm.

Suspensions whose solid particles have a bimodal grain can be advantageous have distribution curve, are used.

The best results can be achieved with slip whose bimodal grain size distribution has a maximum in the range between 0.1 and 0.3 µm and a further maximum in the range between 0.2 and 1.0 µm. The particle size distribution is measured using a laser dispersion method (Microtrac Type MIC 2, Micromeritics) in an aqueous solution and Na ₄ P ₂ O ₇ as a dispersing aid.

The suspension can be steric or electrostatic by inorganic or organic compounds are stabilized. In the case of steric stabilization all known dispersing aids can be used. Suitable as such especially polyacrylic acids, polyglycolic acids, polymethacrylic acids, organi  bases such as triethylamine or carboxylic acids such as acetic acid or propion acid. The suspension preferably contains between 0.5 and 5% by weight organic stabilizers. In the case of electrostatic stabilization can advantageously volatile inorganic acids such as nitric acid or hydrochloric acid and ammonia can be used as the base.

The suspension is stabilized either during or after grinding with the aid of a disperser, which ensures that the stabilizer is distributed quickly and evenly. Sintering additives and binders can preferably be added to the suspension before, but also during or after grinding or stabilization. All sintering aids known for Al ₂ O ₃ or their precursors are suitable as sintering additives.

The suspension according to the invention preferably contains 0.5 to 10% by weight of one or several binders from the group alginates, dextrin, glycols, gum arabic, Lignin sulfonate, methyl cellulose, polyvinyl acetate, polyvinyl alcohol, polyvinyl pyrro lidon, starch and sugar, based on the solids content of the suspension. As well The suspension can advantageously be 0.5 to 10% by weight of one or more binders from the group boehmite sol, silica sol and water glass.

The suspension is dried and the solid is compacted in one Step in a fluidized bed granulator. The granulation is preferably on Air is carried out and can be started in a fluidized bed apparatus that already contains starting granules. However, it is also possible to do the granulation in one empty apparatus to start, the fluidized bed granulation as spray drying begins and germs are generated in situ.

The suspension to be granulated is sprayed into the fluidized bed brings. The use of two-component nozzles is particularly advantageous. As Zer Dust gas can be any gas that is inert under the working conditions the. The amount of atomizing gas can vary within a wide range are and generally depends on the dimensions of the apparatus and the type and quantity of the product to be sprayed. The temperature of the atomization Bergas current or the air inlet temperature is also within a gr variable range. Generally one works at temperatures between 20 and 350 ° C. The sight gas temperatures are also in a larger range variable. Here, too, it is preferred to work in a range between 20 and  350 ° C. The amount and speed of the sight gas depends on the Density and the desired grain size of the granulate.

The grain size is primarily determined by the gas flow and velocity of the classifying gas controlled. With the zigzag sifter described in DE-A 35 07 376 it is possible, a narrow grain band in the grain size range between 0.01 and 10 mm with a bandwidth of ≦ 1 mm. The finished granulate can be used directly - or after an intermediate calcining step at temperatures between 300 and 600 ° C - to be sintered.

Sintering takes place at temperatures between 1300 ° C and 1550 ° C. Thus, the necessary sintering temperature is significantly lower than the usual temperatures for the sintering of conventional α-Al ₂ 0 ₂ ceramics of approx. 1600 ° C, but at the same time it is still significantly higher than that required for the sol-gel process Temperature, which is preferably below 1300 ° C. It is all the more surprising that the process according to the invention succeeds in almost completely suppressing crystal growth. In addition to the fine particle size and very narrow particle size distribution of the starting powder and the high density of the green body, a further requirement is that the required sintering temperature be reached very quickly. This means that the green body should get into the hottest zone of the sintering furnace as quickly as possible. This shock sintering enables the sintering process to be completed before crystal growth begins. At the same time, the good pre-compression speeds up the sintering process. The homogeneous grain distribution and the fineness of the starting material favors a uniform dense sintering, whereby a coalescence of the primary crystals into larger crystallites can be avoided.

All types of furnaces or sinting processes are suitable for sintering allow sudden heating of the green body. Can be beneficial directly or indirectly heated rotary kilns, pendulum furnaces, push-through furnaces, fluidized beds sintering ovens or microwave sintering ovens are used. This is advantageous Shock sintering so that the green body in ≦ 60 seconds, preferably ≦ 10 Seconds, particularly preferably ≦ 3 seconds, on the required sintering temperature is brought. The holding time during sintering is ≦ 60 minutes, preferably ≦ 30 minutes, particularly preferably ≦ 15 minutes.  

The granules are crushed to the desired grain size from energeti reasons immediately after granulation. Depending on the application however, it may also be advantageous that after the calcining step or after the sine The crushing of the granules is carried out to make them particularly sharp Get cutting edges.

The method according to the invention has the advantage over electrophoresis that that with comparable grain sizes of the starting materials finer crystallite structure in sintered end product can be obtained.

Since the fineness of the crystallite structure is directly related to the performance of the Abrasive grain stands, can be an abrasive grain according to the inventive method with a higher stock removal rate. Another An advantage from an ecological point of view is that it works in an aqueous medium and environmental pollution from organic solvents is avoided.

Compared to the sol-gel process, there are also - in addition to the economic advantage through the use of inexpensive raw materials - not too underestimated environmental benefits. So in the sol-gel process Stabilization of the suspension of relatively large amounts of volatile acids used during drying and especially during calcining need to be evaporated. The acids are preferably Nitric or hydrochloric acid.

There are environmental impacts that, despite the complex technology, are not yet can be completely avoided. Another advantage over the Sol-gel method consists in the simplicity of the method according to the invention, whereby the production in the continuous production operation is facilitated, what ultimately brings economic benefits again.

The process according to the invention enables high-density to be produced sintered, microcrystalline bodies with a high hardness, their crystallite size is adjustable between 0.1 and 10 µm. Because of these properties are suitable these sintered bodies are excellent as abrasives, with the crystallite sizes here be between 0.1 and 3 µm.  

This invention therefore also relates to the use of the invention produced sintered microcrystalline body as an abrasive and for the Manufacture of grinding and cutting tools.

Because the grinding properties differ significantly from the crystallite structure of each Depend on abrasive grain, it succeeds in the inventive method for the Various applications with one grit for the To provide optimal crystallite structure for each application.

In the following the invention will be explained by way of example without one Restriction can be seen.  

example

α-Alumina with an average grain size (d ₅₀ ) of 1.5 μm was ground in an agitator ball mill (type PMC 25 TEX, Drais) to an average grain size (d ₅₀ ) of 320 nm in an aqueous medium. The resulting slurry with a solids content of 30 wt .-% and ad ₉₀ of 630 nm was mixed with a 10% aqueous suspension of a polyvinyl alcohol as a binder (Mowiol 8/88, Hoechst AG, Germany), so that the proportion of Binder was approximately 4% by weight, based on the solids content of the slurry.

The suspension was then placed in a fluidized bed Spray granulator (AGT 400, Glatt Germany) at one Air inlet temperature of 320 ° C, a layer temperature of 75 ° C and a spray rate of 2.4 kg / min to green bodies with a density of ≧ 40% of the theoretical density tet. A fine granulate fraction was used to nucleate an average granule size between 0.5 and 1 mm, which at a previous fluidized bed spray granulation in situ nucleation was used.

The desired granules were separated off using a Zigzag sifter. 60% by weight of the granules obtained in this way had a diameter between 3 and 5 mm, approx. 30% by weight the granules had a diameter between 1 and 3 mm approx. 10% by weight of the granules had a diameter of <5 mm. The residual moisture of the granules was less than 1%.

The granules were crushed and then at 1480 ° C shock sintering in a heated rotary tube Subjected to temperatures from 1300 to 1550 ° C.

The product obtained has a Vickers hardness (HV 0.2) of 20.1 GPa and an average primary crystallite size of 0.38 µm. The Density is 98.7% of the theoretical density.  

Grinding test (grinding belt)

Claims (16)

1. Process for the production of sintered, microcrystalline bodies based on α-Al ₂ O ₃ , characterized in that an α-Al ₂ O ₃ powder with a mean grain size ≦ 3 µm is used as the starting material to form a slick with a particle size <1 µm is ground or desagg lomerated, the slip is granulated with the help of a fluidized bed spray granulator to form green bodies with a density of ≧ 40% of the theoretical density and a residual moisture content of ≦ 6% by weight and then shock sintering at temperatures in the range from 1300 to 1550 ° C. 2. The method according to claim 1, characterized in that Green bodies with a density of ≧ 50% of the theoretical Density can be used. 3. The method according to one or more of claims 1 and 2, characterized in that green body with a Residual moisture of ≦ 1 wt .-% can be used. 4. The method according to one or more of claims 1 to 3, characterized in that the green body in the Shock sintering in ≦ 60 seconds, preferably ≦ 10 seconds, particularly preferred in ≦ 3 seconds, to the required be brought to the sintering temperature.   5. The method according to one or more of claims 1 to 4, characterized in that a slip with wide ren components, sinter additives and / or crystal wax inhibitors, preferably oxides from the group of Elements Mg, Zn, Ni, Co, Hf, Zr, Si, Ti, Cu, Sr, Ba, K, Nb, B and / or rare earths is used. 6. The method according to one or more of claims 1 to 5, characterized in that a slip with a average particle size ≦ 0.4 µm is used. 7. The method according to one or more of claims 1 to 6, characterized in that the particles in the slip have a bimodal distribution curve. 8. The method according to claim 7, characterized in that a bimodal distribution curve with the first maximum in Range between 0.1 and 0.3 microns, preferably 0.1 and 0.2 µm, and the second maximum in the range between 0.2 and 1.0 µm, preferably 0.3 and 0.7 µm, is present. 9. The method according to claim 8, characterized in that at 20 to 70 wt .-%, preferably 30 to 50 wt .-% of Solids content of the slip a particle size between 0.1 and 0.3 µm and at 80 to 30% by weight before, adds 70 to 50 wt .-%, the solids content of the Schlic kers use a particle size between 0.3 and 1.0 microns det. 10. The method according to one or more of claims 1 to 9, characterized in that a slip with a solids content, calculated as α-Al ₂ O ₃ , of 5 to 80 wt .-%, preferably 15 to 50 wt .-%, and with 0.5 to 5% by weight of inorganic or organic stabilizers, based on the solids content, is used as dispersion aid. 11. The method according to one or more of claims 1 to 10, characterized in that a suspension is used det, the 0.5 to 10 wt .-% of one or more Binders from the group methyl cellulose, dextrin, Sugar, starch, alginates, glycols, polyvinylpyrrolidone, Lignin sulfonate, gum arabic, polyvinyl alcohol and Polyvinyl acetate, based on the solids content of the Suspension. 12. The method according to one or more of claims 1 to 11, characterized in that a suspension is used det, the 0.5 to 10 wt .-% of one or more Binder from the group of water glass, silica sol and Boehmite sol contains. 13. The method according to one or more of claims 1 to 12, characterized in that a spray granulate with a residual moisture content of ≦ 6% by weight, preferably ≦ 1% by weight, based on the solids content, is used. 14. The method according to one or more of claims 1 to 13, characterized in that the granules before Sintering at temperatures between 300 and 600 ° C kalzi be kidneyed. 15. The method according to one or more of claims 1 to 14, characterized in that the granules before Sintering is crushed to the desired grain shape. 16. Use of the method according to one or  several of claims 1 to 15 produced micro crystalline body as an abrasive and / or for the Manufacture of grinding and cutting tools.