Sintered, microcrystalline abrasive material on an alpha AL203 basis

(42) Date of commencement of the patent: 15. 1. 1992 (45) Date of issue: 10. 8.1992

<td>(56) Documents:</td><td>(73) Patent owner:</td>

<td>EP-A2-248788</td><td>TREIBACHER CHEMICAL WORKS AG A-9330 TREIBACH, CARINTHIA (AT).</td>

<td></td><td>(72) Inventor:</td>

<td></td><td>WINTER HERWIG DR. KLAGENFURT, CARINTHIA (AT). JANZ PETER DR. KLAGENFURT, CARINTHIA (AT). GOTTSCHAMEL GEORG DR. TREIBACH, CARINTHIA (AT).</td>

(54) SINTERED MICROCRYSTALLINE GRINDING MATERIAL BASED ON ALPHA-AL203 (57) Sintered microcrystalline abrasive material based on alpha-Al 2 θ 3 » ^wTo help achieve a

Dispersion hardening an addition of 0.05 to 40 Gew.-ϊό one or more compounds containing an SE metal from the group Ce, La, Pr and Nd is provided.

CQ

AT 394 962

TOR 0073016

The invention relates to a microcrystalline sintered abrasive material based on a-AljOß.

From EP-A 248 788 a sintered microcrystalline abrasive material made from a mixture of calcined clay and from 1 to 60% by weight of a highly dispersed a-alumina hydrate is known. The size of the a-Aluminiumoxidkristallite is below 4 pm, preferably below 3 pm. Crystallite sizes which are substantially below these values ​​are virtually unattainable with the process described in this application. Thus, the quality of the abrasive material clear limits

For the production of sintered grinding materials based on a-Al₂O₃ Both bauxite and alumina and finely disperse alumina monohydrates can be used. From EP-A-0 228 856 an a-Al based abrasive material is disclosed₂O₃ known that contains a certain proportion of yttrium. This is present in the abrasive material in the form of garnet, so it is with A1₂O₃ reacts chemically To achieve a fine grain Y contributes nothing in this case. Such Schleifmateriäl is less suitable for grinding wheels, since it is relatively brittle and tends to form cracks.

Furthermore, fine-grained abrasive materials are known. Thus, US Pat. No. 4,314,827 describes a microcrystalline abrasive material based on a-Al₂O₃However, to achieve good grinding performance, a further addition of at least one modifying component is needed. As modifying component, at least 10% by volume ZtO₂ and / or HfO₂ and / or at least 1% by volume of a spinel of A1₂O₃ proposed with oxides of Co, Ni, Zn or Mg.

The effect of adding the finest a-Al₂O₃Particles to an alumina hydrate gel before sintering is described in EP-A-0 152 768. Thereafter, the sintering temperature can be lowered by this addition and the crystallite size of the abrasive material are kept low. As M. Kugami (J.Amer.Co. Soc., 68/1985, p.500) states, the added a-Al₂Oy particles should not be larger than 0.1 pm. The difficulty of producing such a fine, coarse-grain powder constitutes a significant disadvantage of this process.

Iron oxide is therefore proposed as nucleating additive in US Pat. No. 4,744,802. If, however, the solubility of Fe ^ OO in AloOo is exceeded, it leads to the formation of aluminates, which have lower hardness.

EP-A-0 263 810 describes a high quality microcrystalline sintered abrasive material without the need for a nucleating additive during manufacture.

The aim of the invention is now a grinding material based on a-Al₂O₃, which is characterized by high density and high uniformity of the microcrystalline structure, resulting in a high grinding performance.

Such an abrasive material according to the invention is a sintered microcrystalline abrasive material based on a-Al₂O₃, characterized by an addition of 0.05 to 40 wt .-% of one or more compounds containing an SE metal from the group Ce, La, Pr and Nd, wherein the crystallite size is substantially less than 0.5 pm. This abrasive material is characterized by excellent grinding performance and can be processed into bands as well as grinding wheels.

Preferably, the compounds of the SE metals are oxides. In this way, a significant increase in grinding performance can be achieved through the mechanism of dispersion hardening. Mainly responsible for this effect is the cerium oxide, but also the oxides of La, Nd and Pr contribute to this. The use of the SEMetalle advantageously takes place on the cheap cerium chloride.

The invention further relates to a process consisting of stirring a fumed a-alumina hydrate into a dilute acid solution, subjecting the resulting suspension to vacuum deaeration to remove trapped air bubbles, then deagglomerating the suspension and reacting with compounds from the group of metals Ce, La, Pr, Nd are added, which merge into the metal oxides during sintering. The suspension is then optionally freed from coarse particles, dried, comminuted and sintered, the last stage of the sintering being carried out by a gas pressure sintering at 50-100 bar at temperatures between 1250 and 1350 ° C.

Surprisingly, it was found that in the presence of oxides from the group of metals Ce, La, Pr, Nd, the desired low porosity of the sintered product can be achieved at a significantly lower sintering temperature than without this addition. Furthermore, one achieves a fine microstructure and high strength of the abrasive grains. The crystallite size is below 0.5 pm, a part of the crystallites is even &lt;0.2 pm. The positive effect of the addition of the compounds of these rare earth metals Ce, La, Pr and Nd probably based on the formation of the oxides already during the heating phase. For example, citrate is already converted to Ce-oxide at 450 ° C, Pr-nitrate at ca. 500 ° C, La and Nd nitrate at approx. 700 ° C, the oxides formed are thus very homogeneously distributed and feinstkömig, they can therefore similar to the finest A1₂O₃ act as germs. CeO₂ also does not react with A1₂O₃That is, there is no compounding to aluminate, the finely dispersed distribution of CeO₂ is retained and thus the effect as dispersion hardening is possible.

The strength of the abrasive material achieved by the addition of the oxides of Ce, La, Pr, Nd can be further increased by gas pressure sintering. The almost dense product, which still contains closed pores, becomes

-2AT394962B sintered under pressure, whereby the closed pores are greatly reduced and sometimes completely disappear. However, since the strength of a material is known to decrease with increasing pore size, such a significant increase in strength can be achieved, although the porosity in this step only by a few vol .-% decreases.

The resulting abrasive material thus combines high density, submicrocrystalline microstructure and fineness of the residual pores, thereby providing exceptionally good removal rates and tool life during use. This material is also well suited as a cutting ceramic

As finely divided α-Aluminiumoxidhydiate the pseudoboehmite commercially available (Pural ^, Disperal ^, Versal ^) with a purity of over 99% and a spec. Surface between 180 and 290 m²/ g are used. The solids content of the suspension was between 5 and 40 wt .-%, preferably between 15 and 25 wt .-%. As Peptisatorsäure nitric acid, hydrochloric acid or acetic acid can be used.

The addition carried out during the deagglomeration in the dispersion apparatus from the group of elements Ce, La, Pr »Nd takes place in the form of their inorganic or organic salts, which are present after sintering in the form of their oxides. However, it is also possible to use coarse grain-free submicron powder composed of an oxide or hydroxide of the abovementioned elements. The additives are added in an amount of 0.05-40% by weight (referred to as oxide in the end product).

The drying of the deagglomerated sol can be carried out in various ways. It is either dried directly into irregular pieces, which are then sized and sieved, or the gel coagulated by concentration is subjected to shaping prior to final drying. The shaping can be done by rolling, extrusion or similar processes. But the sol can also be pressed into powder or Dried granules are. For example, spray dryers, spray granulators, or paddle dryers are suitable. The pressed granules are then pressed into molded parts. For this purpose, all known pressing methods such. B. Die presses, extrusion presses or roll presses.

The dried material is then sintered. The sintering takes place in three stages. First, at 500-800 ° C, the expulsion of the residual water and the acid constituents derived from the peptizator acid are carried out with concomitant conversion of the Ce, La, Pr, Nd salts to the oxides. In the second stage, the elimination of most of the open pores, while all existing modifications of the alumina in a-Al₂Oss be transferred. It takes place at 1000 -1300 ° C. In the third stage, the material is sintered under pressure. The gas pressure in the furnace is increased from 1 to max. 100 bar increased. If necessary, the temperature can be increased up to 1400 ° C. Preferably, gas pressure sintering takes place at 1250-1350 ° C. The sintering times depend on the temperature and are between a few minutes and max. 3 Hours per level. In order to preserve the crystallinity, it proves beneficial to sinter at a lower temperature for a longer period of time at a higher temperature. The three-stage sintering process can, in principle, be carried out in a suitable furnace, but it has proved to be favorable if the first stage is carried out in particular because of the attack of acidic exhaust gases in a specially lined furnace.

The material according to the invention is outstandingly suitable for grinding purposes, but it can also be used for all known uses of aluminum oxide ceramics. For use in grinding wheels, comminution of the material before sintering proved to be advantageous, for use in belts and flexible disks comminution after sintering is cheaper.

The invention will now be explained in more detail with reference to examples.

Example 1: (suspension preparation)

In a solution of 39.5 kg of water and 440 ε conc. Nitric acid was continuously added to 10 kg of Disperal brand α-alumina monohydrate¹^ introduced. The suspension was then deaerated in a laboratory vacuum deaerator at 100 mbar and then pumped through a disperser with two feed ports. The flow rate was 31 / h, the rotor speed was 15,000 rpm. A solution of 61.4 g / l citrate hexahydrate at 270 ml / h was pumped into the second feed opening of the disperser. The deagglomerated suspension was then further processed. The sintered end product from this suspension contained 1% by weight of CeO₂,

Example 2:

The suspension of Example 1 was dried in polypropylene cups in a hot air dryer at 80 ° C for 36 hours to brittle plates The dried plates were crushed into granules in a hammer mill and classified in a screening machine. Subsequently, the beads were calcined for 1 hour in a muffle furnace at 500 ° C. and then sintered in a gas pressure sintering furnace at 1300 ° C. for 2 hours at 1 bar, after which they were allowed to stand for 15 minutes. Finished at 100 bar. The material had 2 vol% porosity, the crystallite size of the a-Al₂O₃ was 0.3-0.5 pm. The mean pore size was 0.4 pm.

-3AT394962B

PeisptelJk

The suspension from Example 1 was spray-dried and pressed with a water content of 15 wt .-% to cylinders (30 mm diameter, 5 mm long). The pressing was uniaxial in a template at 400MPa. The cylinders were calcined in a muffle furnace at 500 ° C for 1 hour and then sintered in gas pressure. The sintered material had 1 vol.% Porosity, the crystallite size of a-Al₂O₃Crystals was below 0.3 pm, the pore size was &lt;0.1 pm.

Example 4:

The a-alumina monohydrate suspension was prepared as in Example 1. The dosed citrate solution contained 3.lg / l citrate hexahydrate so that in the final product O, 0.05 wt% CeO₂ were included. The deagglomerated sol was used for Examples 5 and 6

Esispisl-fc

The suspension of Example 4 was processed as in Example 2 (drying and sintering). The material thus prepared had 4% by volume of pores and a crystallite size of &lt;0.7 μm.

Example 6:

The suspension from Example 4 was processed as in Example 3 (compression, sintering). The sintered material had 3% by volume of pores and a crystallite size of 0.3-0.5 μm. The pore size was &lt;02 pm.

Example 7:

The cylinders of Example 3 were comminuted to grain before sintering and sintered as in Example 3.

Example 8:

The cylinders of Example 6 were comminuted to grain before sintering and sintered as in Example 3.

Example 9:

A suspension of Disperal^K was prepared analogously to Example 1 (without SE addition). This suspension was dried, crushed and sieved as in Example 2. The sintering was carried out at 1400 ° C in a muffle furnace. The holding time was 2 hours. The porosity of the sintered material was 2 vol.%, The crystallite size was less than 2 pm (= EU patent 0 263 810).

Example 10:

A Disperal suspension analogously to Example 1 (without addition of SE) was dried, pressed and comminuted as in Example 3. The sintering was carried out at 1400 ° C in a muffle furnace. The holding time was 2 hours. The porosity was 1.5 vol.%, The crystallite size was below 1.5 pm.

Example 11:

The compressed material analogous to Example 10 was sintered in cylindrical form (analogously to Example 10) and then comminuted to size and classified

Example 12:

However, as in Example 9, the crushing and sieving was carried out after sintering

Example 13:

However, as in Example 2, comminution and sieving were carried out after sintering.

Example 14:

However, as in Example 5, comminution and sieving were carried out after sintering.

ggigpislJJ;

The dried and pressed material of Example 3 was sintered at 1350 ° C for 2 hours in a muffle furnace and then crushed The porosity was 4.5 vol .-%, the crystallite size was below 0.7 pm. The pore size was 0.5 pm.

The dried and pressed material of Example 3 was crushed and sieved and then sintered as in Example 15

-4AT394962B

Example 17;

Analogously to Example 1, an α-alumina monohydrate suspension was prepared and deaerated. On dispersing, a solution of 472 g of citrate hexahydrate in 11 water was mixed at a flow rate of 11 / h. The suspension flowed at 31 / h. The suspension was then dried and comminuted and sintered as in Example 2. The final sintered product contained 35% by weight of CeO₂, Porosity was 5 vol .-%, the crystallite size was 0.5 - 0.7 pm.

Example 18:

The dried material of Example 17 was sintered as in Example 2 and then crushed and sieved.

The abrasive materials produced by the process of the present invention were made into abrasive belts and grinding wheels and the performance of the material was determined in a comparative test against C 45 carbon steel. The grinding performance was compared to that of Eu. 0 263 810 (= 100) determined The values ​​obtained are listed in the tables and clearly show the excellent grinding performance of the materials produced according to the invention.

Abtragsleistung of the bands = total removal by

Decrease of the removal rate to 10%

<td>material</td><td>removal</td>

<td>Example 3</td><td>320</td>

<td>Example 6</td><td>290</td>

<td>Example 11</td><td>130</td>

<td>Example 12 (- EU Note 0 263 810)</td><td>100</td>

<td>Example 13 (= 2)</td><td>240</td>

<td>Example 14 (= 5)</td><td>190</td>

<td>Example 15</td><td>180</td>

<td>Example 18</td><td>240</td>

Tüklkl

Abrasive performance of the discs - ratio material removal to disc closure

<td>material</td><td>grinding performance</td>

<td>Example 2</td><td>250</td>

<td>Example 5</td><td>200</td>

<td>Example 7 (= 3)</td><td>330</td>

<td>Example 8 (= 6)</td><td>280</td>

<td>Example 9 (= EU Note 0 263 810)</td><td>100</td>

<td>Example 10</td><td>130</td>

<td>Example 16</td><td>190</td>

<td>Example 17</td><td>220</td>