Note: Descriptions are shown in the official language in which they were submitted.
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Title: Process for the preparation of sin~ered micro-
crystalline a-A1203 bodies and their use
_
BACKGROUND OF THE INVENTION
The present invention relates to a process for the pre-
paration of sintered microcrystalline bodies based on
~-A1203 and ~heir use.
A preferred use of o~-A120 sintered bodies is their use
as abrasives.
In addition ~o fused corundum abrasivés, those of sin~e-
red material have already been known for about 50 years.
Sin~ered abrasives based on ~-Al203, the microcrystal-
line structure of which has provided par~icular abrasiveadvantages, have recently become known.
US Patent 4,314,827 describes a microcrystalline abra-
sive material which is prepared via the sol-gel tech-
nique at sintering tempera~ures of about 1400C. A
modifying component, such as HfO2, ZrO2, MgO, ZnO, CoO,
Nio and the like, is added as a sintering aid.
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An abrasive material which is likewise prepared by
sintering an aluminum oxide hydrate gel is disclosed
in European Patent 0,152,768. To establish the transi-
tion temperature of ~-A12O3 into ~-Al2O3, very fine ~-
Al2O3 particles are added here as seeding agents. Other
additions of crystal growth inhibitors, such as MgO,
10 SiO2, Cr2O3, Fe2O3 or ZrO2, can be used similarly.
Similar processes and substances are to be found in
European Patent 0,024,099, ~. German Patent Application
3,219,607; US Patents 4,518,397, 4,574,003 and
15 US 4,623,364 and European patent Applications
0,168,606, 0,200,487; 0,228,856; 0,209,084 and
0,263,810.
All the abovementioned processes have the common feature
that they are carried out via a sol-gel process with one
or more very finely dispersed aluminium oxide monohy-
drate of the bohemite type. The relatively expensive raw
materials, which are obtained, for example, via hydro-
lysis of organoaluminium compounds, and the expensive
process technology increase the costs of the sol-gel
corundum to several times that of conventional corun-
dums.
W, German Patent Application 3,604,848 describes a
process for grinding a dispersion of raw materials
containing alumina, compounds containing silicic acid
and fur~her additives (compounds of the metals, Co, Ni,
Cr, Zr, Zn, Si, Ti or Ni) to a slip which can be
s;ntered, from which an abrasive can be prepared by
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stepwise drying and sintering at temperatures of up
~o 1700C, the corundum crystals of the abrasive having
a diameter of less than 5 microns. Although it is
possible ~o bypass the expensive raw material in this
manner, very expensive grinding processes and high
sintering temperatures are still required in order to
obtain the desired microcrystalline abrasive.
The product thus obtained also contains about 2 % sili-
cates which are of no value in terms of abrasive capa-
city, and with a crystallite size of less than 5 microns
still does not have the fine structure of a substance
which has been prepared via the sol-gel method (crystal-
lite size less than 0.4 microns).
A reason for the particular abrasive advantages of these
new abrasives is seen, however, in the submicron struc-
ture.
European Patent Application 0,404,721 describes a pro-
cess for achieving, via hydrothermal treatment of raw
materials containing Al(OH)3, a microcrystalline bohe-
mite which is suitable for use in the sol-gel process.
Although it was possible here to replace ~ome of the
starting material by an inexpensive raw material, very
finley disperse bohemite is still required, depending
on the intended use of the desired product, as a seed
material in amounts of between 7.5 % and 60 %, based on
the Al203 end product. The very expensive starting
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substance obtained via Al alkoxides is again used as the
very finley dispersed bohemite.
Another disadvantaRe of the abovementioned sol-gel pro-
cess is that considerable amounts of nitric acid are
used, which is then bonded in the gel and liberated
again as nitric oxide during drying and calcining. This
is accompanied by major corrosion and environmental
problems. To avoid damage here, corresponding protective
measures must be resorted to, which additionally make
the processes expensive.
DE-A 3,334,098 describes a process in which aluminium
oxide monohydrate is deposited electrophoretically from
an aqueous dispersion. The producL is dried, calcined
and sintered by a method analogous to the well-known
sol-gel technique and it can be used, among other
things, as an abrasive. The energy saving permitted by
this method is named as the main advantage of the
method over the sol-gel technique. A disadvantage of the
method is the fact that it is again necessary to use the
relatively expensive finely dispersed aluminium oxide
monohydrate as the starting material. The dispersion
described contains aluminium oxide monohydrate up to a
maximum level of 40 %. This small solids content, to-
gether with the large volumes of liquid associated with
3~ it, calls for very laborious and energy-intensive pro-
cess technology.
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The electrophoresis in the aqueous medium has the addi-
tional disadvantage ~hat electrolytic decomposition of
the water frequently occurs at the electrodes. This
causes difficultly controllable porosity in the depo-
sited sol;d, with the result that the hardness and
grinding performance of the abrasive prepared therefrom
may be impaired.
The principal obJect of this invention is thus to pro-
vide a process for ~he preparation of a microcrystalline
a-A1203 sintered body which does not have the disadvan-
tages described for the prior art.
SUMMARY OF THE INVENTION
2~It has now been found that these requirements are met
by a process which is based on electrophoretic removal
of -A1203 from a corresponding organic ~-A1203 sus-
pension.
The invention thus relates to a process for the prepa-
ration of sintered microcrystalline bodies based on
K-A1203, in which A1203 is deposited on an electrode
from an organic ~-A1203-containing suspension, removed
from the electrode, dried, sintered and comminuted. By
appropria~e choice of the particle size of the starting
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substance, the crystallite size of the end product can
be influenced here in a simple manner, Particularly good
successes are achieved if a-Al2O3 particles having a
submicron crystallite size of are employed as the star-
ting material for the K-Al2O3-containing suspension.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
From economic considerations, is is advantageous to
keep the ~-Al2O3 content high. In a particularly pre-
ferred embodiment of the process according to the in-
vention, the a-Al2O3-containing suspension has an Al2O3
content of under 50 wt.%. In order to obtain a parti-
cularly stable a-Al2O3-containing suspension, it is
necessary to add suitable organic suspending agents to
this. Solvents such as methyl isobutyl ketone, acetone,
ethers or alcohols are preferably used as the organic
suspending agents.
Particularlyl suitable dispersing aids are polyelectro-
lytes such as 3,6,9-trioxaundecanedioic acid, polyglycol
dioic acid, polyvinyl pyrrolidone, nahp~halenesulphonir
acid condensate, maleic acid copolymers, polymethacry-
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lic, polyacrylic, acid, partially esterified polyacrylic
arid, ni~rocellulose, polyvinylbutyral, or also wax and
resins or organic bases such as triethylamine, tetra-
butylammonium hydroxide or carboxylic acids such as
acetic acid or p-hydroxybenzoic acid.
Particularly good successes are achieved here by addi-
tion of polyacrylic acid. In a preferred embodiment of
the process according to the invention, the ~-Al2O3-
containing suspension thus contains 0,5 ~o 3.0 wt.%,
polyacrylic acid, based on the Al203 content.
The preferred voltage in the electrophoretic process
according to the invention is between 10 and 300 volt.
Under these condiLions, it is possible to deposit Al203
sheets several mm thick.
In cases where complete removal can be achieved only
with problems, these can be avoided by applying a
graphite film to the electrodes. Graphite as a film
material provides ~he advantage here that it burns
wi~hout residue during the sintering operatior~. A pre-
ferred embodiment of the process according to the in-
vention thus comprises coating the electrodes with a
graphite film. The graphite film servoes ~he purpose of
facilitating the removal of the layer from ~he elec-
trode.
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These green substances thus obtained are advantageously
first dried. This is preferably effected at temperatures
abvoe 50~C. A sintering operation then follows, at tem-
peratures between 1300 and 1700C, preferably at 1550
to 1650~. This sintering operation has in general ended
after 15 minutes. The sintering temperature and the pro-
duct quality can be influenced by addition of sinteringaids to the ~-Al203-containing su6pension. Preferably
0,2 to 10 % by weight of sintering aids, most preferred
0.5 to 2 % by weight, are used.
The comminution is in general carried out after the sin-
tering, because edges which have a particular cutting
capacity are in this case formed. However, in addition
to the advantages of saving energy, comminution before
sintering can also produce good results. Porferably the
communication is carried out with the aid of roller
; mills.
Another advantageous variant of the process according
to the invenLion thus lies in adding one or more sin-
tering aids to the ~-Al203-containing suspension, which
are then deposited together with the Al203. It is in
this way possible, in particular, to lower the sinLering
tempersture. This preferred embodiment of the procass
according to the invention is characterized in that
sintering aids from
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~he group comprising CoO, NiO, MgO, ZnO, Cr203, Fe203,
Si2' Ti2~ Y203~ Nb25~ HfO2 and ZrO2 are added to the
~-Al203-containing suspension.
The preparation of sintered microcrystallir,e bodies
having a density of over 98 % theoretical and a hardness
lG of 20 GPa, the crystallite size being betweer, 0,1 and
10 microns, preferably below 0,4 microns, is made
possible by the process according to the invention.
On the basis of these properties, these sintered bodies
are outstandingly suitable as abrasives and in related
products.
This invention thus also relates to the use of the sin-
tered microcrystalline bodies prepared according to the
invention as abrasives and for the produc~ion of grin-
ding cutting tools.
Since the abrasive properties largely depend on the
crystallite structure of the particular abrasive grain,
it is possible to provide abrasive grains having a
crystallite structure, which is optimum for the prati-
cular use for widley varying intended purposes, by the
process according to the invention.
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Exam~le 1:
1.5 % of polyacrylic acid (based on the quantity of Al20
3 employed) i5 dissolved in ethanol with stirring, 60 %
by weight of Al203 powder (based on the total starting
quantity) having an average particle size of 0,6 microns
are added to this solution. The suspension is homogeni-
sed by ultrasonic treatment with simultaneous stirring.
The stable suspension is centrifuged at 5000 r.p.m.
until the average particle size of the Al203 particles
in the suspension is 0.2 microns.
The suspension is filled into the electrophoretic cell
and a green body of a thickness of about 3 mm is
deposited at al voltage of 50 volts over a period of 15
minutes at 28 mA.
The green body is dried first i air an then in a drying
cabinet at 50C. It is then sintered at 1600C.
A compact Al203 body with a crystallite size of ~ 1
micron is obtained.
The (Vickers) hardness of the abrasive grain thus ob-
tained ist 21 GPa.
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Grindina tests
Grinding material Abrasive powder in %
Fibre wheel Grinding bel~
Normal corundum 10D 100
(Electromelted corundum)
Zirconium corundum 260 340
EZ 347 ex ~orton
Sol-gel corundum 350 440
Example 1 290 ~60
The material ground was C-45 steel.
ExamDle 2:
Preparation of the suspension as in Example 1).
Prior to the ultrasonic treatment 5 % of MgO are added
to the suspension which is then treated for 24 hours in
a ball mill.
Then ultrasonic treatment is carried out. The suspension
i 5 ~hen centrifuged until the average particle size is
0,2 microns.
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Elec~rophoresis, drying and sintering are carried ou~
as in Example 1).
A compact Al203 body with a spinel content of about
2 % and a crystallite size of ~ 0,5 microns is ob-
tained.
The hardness of the material is 10 GPa.
Grindina test
15 Grinding material Abrasive powder in %
Fibre wheel Grinding bel~
Normal corundum 100 100
Example 2) 330 410
Exam~le 3
The same procedure is used as in Example 2)
3 % of Mg spinel are added instead of MgO.
An abrasive grain bases on Al203 is obtained with a
hardness of 20 GPa, a density of 97 % of theory and a
crystallite size of below 0,7 microns.
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Grindinq ~est
Grinding material Abrasive powder in %
Fibre wheel Grinding belt
10 Normal ~orundum 100 100
Example 3) 310 350
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