AT395317B - MICROCRYSTALLINE SINTERED ABRASIVE MATERIAL BASED ON ALPHA-Al2O3 - Google Patents

Abstract

Sintered, microcrystalline abrasive material based on alpha-Al2O3, with an addition of a cerium compound, preferably a cerium oxide. A particularly tough and wear- resistant abrasive material is obtained by virtue of the fact that it consists of crystallites of size < 0.5 micrometres and needles of length 1-10 micrometres and thickness 0.2-1 micrometre, the proportion of the needles being within the range 1-30% by volume, and also by virtue of the fact that the proportion of the cerium compound is 0.01% by weight - 0.5% by weight.

Description

AT 395 317 B

The present invention relates to a sintered, microcrystalline, ceramic material based on a-A1203 with the addition of a cerium compound, preferably a cerium oxide.

From DE-C 24 50 385 an abrasive material is known which contains various additives, including cerium oxide. The fact is found that the addition of cerium oxide is advantageous for abrasive materials if they contain a relatively high proportion of zirconium oxide. However, zirconium oxide is relatively expensive and the processing of these materials is complex.

Furthermore, EP-A 293 164 discloses a ceramic abrasive material which contains at least 0.5% by weight of a rare earth compound. The finished abrasive material has a phase that consists of the reaction product from the oxide of a rare earth with the aluminum oxide. Good abrasive performance is only achieved if the rare earth compound contains at least 0.5% by weight but preferably 1% by weight. is present.

Furthermore, fine-grained abrasive materials are known. EP-A 0 24 099 describes a microcrystalline abrasive material based on a-A ^ O ^, which is produced by sintering an alumina hydrate gel at approx. 1400 ° C. At least 10% by volume of ZrO 2 and / or HfO 2 and / or at least 1% by volume of a spinel made of A1203 with oxides of Co, Ni, Zn or Mg are proposed as modifying components.

The effect of adding the finest a-Al203 to an alumina hydrate gel before sintering is described in EP-A 0 152 768. The additive can then be used to lower the sintering temperature and keep the crystallite size of the abrasive material small. If sintering times are too long, undesirable, coarse, elongated (“lath-like”) crystallines are formed.

WieM. Kugami (J. Amer. Cer. Soc., 68/1985, p. 500) found that the added a-Al203 particles should not be larger than 0.1 pm. The difficulty of producing such a fine, coarse-grain powder is a major disadvantage of this process.

In US Pat. No. 4,744,802 iron oxide is therefore proposed as a nucleating additive. However, if the solubility of Fe203 in A1203 is exceeded, aluminates with a lower hardness are formed.

EP-A0 263 810 (TCW) describes a microcrystalline, sintered abrasive material of high quality without the need for a nucleating additive during manufacture

In AT note. 2695/88 (TCW) describes a microcrystalline abrasive material that contains fine Ce02 particles to increase toughness, which cause dispersion hardening. The gas pressure sintering at 1000-1400 ° C is used for the sintering, the effect of the gas pressure only taking place after all the pores have been closed.

The object of the invention is to provide a ceramic material which is suitable as an abrasive material and which, due to its special structure, has a particular toughness with a high abrasive capacity which is simultaneously present.

This goal is achieved in that the ceramic material is made of a &lt; 0.5 .mu.m and needles with a length of 1 to 10 .mu.m exist, the proportion of the needles being in the range of 1-30 vol.% And the proportion of the cerium compound being 0.01% by weight to 0.5% by weight .

In contrast to EU-A 0 152 768, which considers the presence of the elongated crystallites caused by the too high sintering temperature to be undesirable, it was surprisingly found that the whisker-like needles significantly improve the mechanical properties of the abrasive grains when ceria in the formation of the needles reactive form is present. The diameter of the rods is between 0.2 and 1 pm. The length is 1 to 10 pm. The volume fraction of needles is between 1 and 30% depending on the manufacturing conditions.

The Al203 matrix preferably has a crystallite size of less than 0.5 pm and a very uniform structure.

According to the invention, such an abrasive material is a sintered microcrystalline abrasive material based on a-A12C &gt; 3 with the addition of 0.01 x 0.5% by weight of cerium oxide, which is sintered from a sol-gel material with the addition of small amounts of a cerium compound under gas pressure becomes. The sintering process is carried out such that needle-shaped crystals form in addition to a sub-microcrystalline Al203 matrix. According to the AT note. 2695/88 the pressure is only increased when most of the pores are closed, whereby a material is formed from a sub-microcrystalline A1203 matrix with embedded Ce02 particles of 50 to 500 nm (dispersion hardening). Surprisingly, however, it was found that when the pressure was increased, the elongated crystals mentioned already formed during the conversion into a-Al2C> 3.

The application of pressure during the conversion of δ over Θ to 01-AI2O3 presumably causes internal stresses between the crystals of the various modifications of A1203, which cause different growth rates at the different crystallographic levels. This leads to anisotropic growth of the a-Al203 crystals and their elongated shape. These tensions presumably only occur occasionally at inhomogeneity points of the structure, which results in the characteristic structure with -2-

AT 395 317 B

Needles in a fine-grained matrix. The needles contain a small amount of CeC &gt; 2 in solid solution. (The X-ray diagrams only show the lines of α-AI2O3 and CeC ^). The excess of CeC ^ is as in the above. Patent application in the form of very fine particles. The properties of the needles differ somewhat from the matrix in terms of their cerium content. The tension between the matrix and the needles that occurs during temperature changes causes « a slight detachment of the needles from the matrix (micro crack). Therefore, cracks hitting a needle are deflected. The tension that breaks the needles is reduced because the needle is not fully attached to the matrix. However, the adhesive forces are sufficient to hold the matrix together. The grinding performance is probably also increased by a self-sharpening effect. The deflection of the tears on the needles can contribute to the formation of new, sharp edges. The size of the broken fragments is probably mainly determined by the distance between the needles. The wear behavior could therefore be adapted and optimized for certain applications by varying the needle concentration (number per unit volume). This grinding material is characterized by excellent grinding performance and can be processed into belts as well as grinding wheels. It achieves exceptionally good stock removal rates and tool life during use. The material is also well suited as a cutting ceramic if it is shaped accordingly by pressing.

The best results in terms of grinding performance (material removal rate) are achieved with a CeC ^ content of 0.4% by weight and approximately 25% by volume of needles.

The invention further relates to a process which consists in that a highly disperse α-aluminum oxide hydrate is stirred into a dilute acid solution, the suspension formed is removed for removal, including air bubbles, and then subjected to vacuum deaeration, then the suspension is deagglomerated and with a Ce -The compound is added, which passes into the metal oxide during sintering. Then the suspension is optionally freed from coarse particles, dried, crushed and sintered, a pressure of 50-100 bar being exerted on the material during the conversion of δ to 6 into (X-Al2O3) via the sintering atmosphere Gas pressure sintering takes place after the pores have been closed without pressure at approx. 1200 · 1400 ° C.

The commercially available pseudoboehmites (PuralR, DisperalR, VersalR) with a degree of purity of over 99% and a specific surface area between 180 and 290 m2 / g can be used as highly disperse α-aluminum oxide hydrates. The solids content of the suspension was between 5 and 40% by weight, preferably between 15 and 25% by weight. Nitric acid, hydrochloric acid or acetic acid can be used as the peptizer acid.

The Ce element added during disagglomeration in the dispersing apparatus is in the form of an inorganic or organic salt, which is present in the form of the oxide after sintering. However, coarse-grained submicron powder made from an oxide or hydroxide of Ce can also be used. The additives follow in an amount of 0.01-0.5% by weight (based on the oxide in the end product).

Drying is carried out in a thin-film evaporator with a connected belt dryer. However, any other drying apparatus can be used which allows the material to be mixed thoroughly in the "drying phase" so that separation is no longer possible. This is "necessary to achieve the desired homogeneous distribution of the" needles to achieve in the end product. The pore structure of the dried «! Materials appear to be conducive to the formation of needles, since there are far fewer needles when drying at rest. The dried material is crushed and sieved. The «targeted grain can either be sintered directly to Schleifkom or it can be pressed into a correspondingly fine form as a powder with organic binders to give molded parts. B. die presses, extrusion presses or «roller presses.

The dried or pressed material is then sintered. Sinting takes place in four stages. First, the residual water and the acidic constituents derived from the peptisic acid are expelled at 500-800 ° C. while the Ce salt is simultaneously converted into the oxide. In the second stage, the pressure is increased to 50-100 bar at approximately 800 ° C. It takes approx. 1 h until the conversion to OC-AI2O3 is completed, whereby the temperature is limited to max. 1100 ° C may rise. Before closed pores are formed, the pressure must be reduced again, otherwise the compressed gas will be trapped in the pores, which would impede the sealing system.

In the third stage, the majority of the open pores are eliminated. It follows without pressure at 1000 -1300 ° C. In the fourth stage, the material is sintered under pressure. The gas pressure in the furnace is increased from 1 to max. 100 bar increased The temperature can, if necessary, be maintained up to 1400 ° C. The second gas pressure sintering stage is preferably carried out at 1250 ° -1350 ° C. The sintering times depend on the temperature and are between a few minutes and max. 3 hours per level. To maintain the fine crystallinity, it proves to be more favorable to sinter a little longer at a lower temperature than a shorter time at a higher temperature. In principle, the four-stage Sint process can be carried out in a suitable furnace, but it has proven to be advantageous if the first stage, particularly because of the formation of acidic exhaust gases,

AT395317B for this lined oven. The material according to the invention is outstandingly suitable for grinding purposes, but can also be used for all known uses of aluminum oxide ceramics. For use in grinding wheels, reducing the size of the material before sintering has proven to be advantageous; for use in belts and flexible wheels, size reduction after sintering is more advantageous. S The invention will now be explained in more detail with the aid of examples.

Example 1 (suspension preparation)

10 kg of aluminum oxide monohydrate of the Disperal® brand were continuously introduced into a solution of 39.5 kg of water and 440 g of concentrated nitric acid. The suspension was then deaerated in a laboratory vacuum ventilation system 10 at 100 mbar and then pumped through a dispersing device with two inflow openings The flow rate was 3] / h, the rotor speed was 15,000 rpm. A solution of 61.4 g / l cemitate hexahydrate with 270 ml / he was pumped into the second inlet opening of the dispersing device. The agglomerated Sus pension was then processed further. The sintered end product from this suspension contained 1% by weight CeC ^. as in Example 2

The suspension from Example 1 was evaporated with stirring and, after coagulation in polypropylene cups, dried to brittle plates in a hot air dryer at 80 ° C. for 36 hours. The dried plates were crushed into grains in a Hammermill and classified in a sieving machine (fine grain fraction <0.1 mm - see example 3). The grains were then calcined in a muffle furnace at 600 ° C for one hour. Then the material was heated to 900 ° C in the gas pressure sintering furnace. After increasing the pressure to about 100 bar, the temperature was raised to 100 ° C within an hour, then the pressure was reduced to 1 bar and the temperature was high "Increased to 1300 ° C. After a further 1.5 hours at 1300 ° C, the material was sintered at 100 bar for 15 minutes. The material had 3% by volume porosity and contained about 20% by volume of CeC ^ particles of size 100 - 500 nm .-% needles with diameters of 0.2 - 0.5 pm and a length between 3 and 5 pm. 25th

Bgigpigi 3

Dried fine grain (<0.1 mm) from Example 2 was mixed with approx. 20-25% by weight of H2O and pressed into cylinders (30 mm 0.5 mm in length). The pressing was carried out uniaxially in a die at 400 MPa. The cylinders were sintered as in Example 2. The sintered material had 2% by volume porosity and showed practically the same 30 microstructure with needles as the material of Example 2. The sintered cylinders were crushed to Kam and sieved

Example 4

The cylinders pressed as in Example 3 were crushed and sieved before sintering. The screened grain 35 was sintered as in Example 2.

Basoisli

The α-alumina monohydrate suspension was prepared as in Example 1. The metered citrate solution contained 6.2 g / l citrate hexahydrate, so that the end product contained 0.1% by weight CeC ^. The deagglomerated suspension was used for Examples 6-8.

Example 6

The suspension of Example 5 was processed as in Example 2 (drying and sintering). The material thus produced had 2% by volume of pores and, in addition to a few CeC ^ particles, contained 15% by volume of needles with 45 diameters between 0.4 and 0.7 μm and lengths between 6 and 10 μm.

Example 7

The dried fine grain of example 6 was pressed into cylinders as in example 3, these sintered, crushed and sieved 50

Example 8

The cylinders pressed in Example 7 were crushed and sieved before sintering. The sieved grain was sintered as in Example 2. 55 Example 9

The α-alumina monohydrate suspension was prepared as in Example 1. The metered cemitra solution contained 24.8 g / l cemitate hexahydrate, so that 0.4% by weight CeC ^ was contained in the end product. -4-

AT395 317B

Example JQ

The suspension of Example 9 was dried as in Example 2, ground to size, sieved and calcined at 600 ° C. The grain was then heated to 800 ° C. in a gas pressure sintering furnace. After increasing the pressure to approx. 90 bar, the temperature was raised to 1150 ° C. in the course of an hour, then the pressure was reduced to 1 bar and the temperature was raised to 1350 ° C. After a further hour at 1350 ° C., 30 minutes. Sintered at 100 bar. The material had 2% by volume of pores and, in addition to CeC ^ particles, contained 25% by volume of needles with diameters between 0.2 and 0.5 pm and lengths between 5 and 9 pm.

Example 11

The dried fine grain from Example 10 was pressed into cylinders as in Example 3 and sintered as in Example 10. After sintering, the cylinders were comminuted and sieved

Example 12

The cylinders pressed as in Example 11 were crushed and screened before sintering. The screened grain was sintered as in Example 10

Example 13

The suspension of Example 9 was dried in polypropylene cups at 80 ° C. at rest. The dried plates were processed further as in Example 10. The sintered product contained 10% by volume of needles. The porosity is 2.5 vol%. The chopsticks were unevenly distributed.

Example 14 (without Ce addition)

The suspension from Example 1 was dormant in polypropylene cups in a hot air dryer at 80 ° C. for 36 hours. The dried plates were crushed into grains in a hammer mill and classified in a sieving machine. The grains were then stirred in a muffle furnace at 500 ° C. for one hour calcined and then sintered in a gas pressure sintering furnace at 1300 ° C. for 2 hours at 1 bar, after which it was sintered for 15 minutes at 100 bar. The material had 2% by volume porosity, the crystallite size of the a-A ^ Og was 0.3-0.5 pm. No needles were found.

Example,!?

The suspension from Example 1 was spray-dried and pressed into cylinders (30 mm in diameter, 5 mm in length). The pressing was carried out uniaxially in a die at 400 MPa. The cylinders were calcined in the muffle furnace at 500 ° C. for 1 hour and then sintered in gas pressure as in Example 14. The sintered material had 1 vol.% Porosity, the crystallite size of the a-A ^ Og crystals was less than 0.3 pm. The sintered cylinders were comminuted and sieved

Example 16

The pressed cylinders from Example 15 were comminuted and sieved before sintering. The crane was then sintered as in Example 14.

Example 17

Analogous to example 2, but the comminution and sieving was carried out after the sintering.

Example 18

Analogous to example 6, but the comminution and sieving was carried out after the sintering.

Example 19

Analogous to example 10, but the comminution and sieving was carried out after the sintering

Example 20

Analogous to example 13, but the comminution and sieving was carried out after the sintering

The abrasive materials produced by the process according to the invention were processed into abrasive materials and grinding wheels and the performance of the material was determined in a comparative test series against carbon steel C 45. The grinding performance was compared to material according to AT note. No. 2695/88 (= 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. -5-

AT395 317B laMkl

Belt removal rate = total removal until the removal rate drops to 10%

Matena!

Stock removal rate

Example 3 220 tt 7 170 tt 11 250 tt 15 100 tt 17 210 tt 18 150 tt 19 240 tt 20 180

Table 2

Grinding performance of the discs - ratio of material removal to disc wear

Material grinding performance example 2 220 tt 4 225 6 150 8 185 10 240 12 260 13 180 14 76 16 100

Table 3

Properties of the abrasive material Ce pore needles

Example% by weight VoL% 0x1 (pm)% by volume 2 1 3 03x4 -20 3 1 2 03x4 -20 4 1 2 03x4 -20 6 0.1 2 03 x 8 -15 7 0.1 1 0 , 5x8 -15 8 0.1 1 0.5 x 8 -15 10 0.4 2 0.3 x 6 -25 -6-

Claims (8)

AT395317B needles Ce pores example wt% vol% 0x1 (pm) vol% 11 0.4 1 03x6 ~ 25 12 0.4 1 03x6 ~ 25 13 0.4 2.5 0.4x6 ~ 10 ( dried at rest) 14 - 2 - ATAnm. 2695/88 B. 2 15 - 1 - AT 2695/88 B. 3 16 - 1 - AT 2695/88 B. 7 17 1 3 03x4 -20 18 0.1 2 03x8 -10 19 0.4 2 03x6 -25 20 0.4 2.5 0.4x6 -10 (dry dried) PATENT CLAIMS 1. Sintered, microcrystalline Abrasive material based on aA ^ Og, with the addition of a cerium compound, preferably a cerium oxide, characterized in that it consists of crystallites of size &lt; 0.5 µm and needles with a length of 1 to 10 Jim and a thickness of 0.2 to 1 µm, the proportion of the needles being in the range of 1 to 30% by volume, and the proportion of the cervical binding being 0.01% by weight. -% to 0.5 wt .-% 2. A process for producing an abrasive material according to claim 1, characterized in that a highly disperse α-alumina hydrate is stirred into a dilute acid solution, the resulting suspension is subjected to vacuum ventilation to remove trapped air bubbles, then the suspension is deagglomerated and a cerium additive is added , then the suspension is optionally freed from coarse particles, dried, crushed and subjected to a 4-stage sintering process. 3. The method according to claim 2, characterized in that the solids content of the suspension is between 5 and 40 wt .-%, preferably between 15 and 25 wt .-% 4. The method according to any one of claims 2 and 3, characterized in that the dilute acid solution consists of nitric acid, hydrochloric acid or acetic acid 5. The method according to claim 2 to 4, characterized in that the cerium addition in the form of an inorganic or organic salt or as the finest, coarsely submicron powder from the oxide or hydroxide 6. The method according to any one of claims 2 to 5, characterized in that the dried product is subjected to a 4-stage heat treatment process at temperatures between 500 and 1400 ° C, the sintering in stages 2 and 4 taking place under gas pressure -7- AT 395 317 B 7. The method according to any one of claims 2 to 6, characterized in that the first pressure sintering at 800 to 1150 ° C and the second pressure sintering at temperatures between 1250 and 1400 ° C at pressures from 1 to 1000 bar, preferably 50 to 100 bar, for 5 minutes to max. three hours. -8th-