.alpha.-Al.sub.2 O.sub.3 abrasive material and method of preparing the same

A sintered, microcrystalline abrasive material consists of at least 99.5%, by weight, of .alpha.-Al.sub.2 O.sub.3 crystallites having a size of less than 2 .mu. and the material having a density of at least 97% of the theoretical density. This abrasive material has a very high abrasive property and can be used in grinding bands or discs.

The present invention relates to a sintered, microcrystalline abrasive 
material consisting essentially of .alpha.-Al.sub.2 O.sub.3, and to a 
method of preparing the same. 
In addition to a method of preparing corundum abrasive materials by melting 
aluminum oxide in an electric arc furnace, other methods have been 
proposed for the manufacture of such materials on the basis of 
.alpha.-Al.sub.2 O.sub.3 by sintering. 
Bauxite and clay as well as finely dispersed aluminum oxide monohydrate may 
be used as raw materials for the preparation of sintered abrasive 
materials. 
For example, U.S. Pat. No. 4,314,827 discloses a microcrystalline abrasive 
material consisting essentially of .alpha.-Al.sub.2 O.sub.3, which has 
been produced by drying and sintering an aluminum oxide hydrate gel at a 
temperature of about 1400.degree. C. To obtain a good abrasive quality, it 
is necessary, however, to add at least one modifying component, such as at 
least 10% of ZrO.sub.2 and/or HfO.sub.2 and/or at least 1% of a spinel of 
Al.sub.2 O.sub.3 with oxides of Co, Ni, Nz or Mg. 
Published European patent application No. 0 152 768 discloses an abrasive 
material prepared by sintering an aluminum oxide hydrate gel, very fine 
.alpha.-Al.sub.2 O.sub.3 particles being present as seed crystals to 
reduce the conversion temperature of .alpha.-Al.sub.2 O.sub.3 in the gel 
to the .alpha.-Al.sub.2 O.sub.3 form. It is possible to add oxides of Si, 
Cr, Mg and Zr as inhibitors of the crystal growth to the gel. 
Nevertheless, crystallites up to a magnitude of 8.mu. (depending on the 
sintering conditions) are found in this material, in addition to the 
crystallites of sub-micron size. These large crystallites are presumably 
due to the presence of the .alpha.-Al.sub.2 O.sub.3 particles (seed 
crystals) in the dried product because, even with the greatest care in the 
preparation, the larger particles are present in addition to the 
sub-micron crystallites. But even the smallest .alpha.-Al.sub.2 O.sub.3 
particles are larger than the .alpha.-Al.sub.2 O.sub.3 seed crystals 
coming from the gel, which favors an uneven crystallite growth. 
It is the primary object of this invention to produce an .alpha.-Al.sub.2 
O.sub.3 abrasive material which is characterized by a high density, 
outstanding purity and high uniformity of its microcrystalline structure. 
This object is accomplished according to one aspect of the invention with a 
sintered, microcrystalline abrasive material consisting essentially of 
.alpha.-Al.sub.2 O.sub.3, said abrasive material consisting of at least 
99.5%, by weight, of .alpha.-Al.sub.2 O.sub.3 crystallites having a size 
of less than 2.mu. and the material having a density of at least 97% of 
the theoretical density. 
According to another aspect of the present invention, there is provided a 
method of preparing such an abrasive material, which comprises the steps 
of stirring a highly dispersed .alpha.-aluminum oxide hydrate into a 
dilute acidic solution until a suspension of the .alpha.-aluminum oxide 
hydrate has been formed, subjecting the suspension to a vacuum venting to 
remove occluded air therefrom, dispersing the vented suspension to 
disagglomerate the same, removing any residual coarse particles from the 
dispersed suspension, drying the dispersed suspension to obtain a dry 
material, comminuting the dried material, and sintering the dried material 
at a temperature between 1000.degree. C. and 1500.degree. C. 
The abrasive material of this invention has excellent abrasive properties 
and may be used in grinding bands as well as discs. It has been found 
unexpectedly that the microcrystalline structure combined with a high 
density and purity of the alumina may be obtained by suitably 
disagglomerating the finely dispersed .alpha.-aluminum oxide hydrate 
without the need for a high spinel content or the presence of 
.alpha.-Al.sub.2 O.sub.3 in the dried product. The conversion in the 
.alpha.-Al.sub.2 O.sub.3 begins at a temperature below 1000.degree. C. 
With a set sintering temperature, the difference between it and the 
conversion temperature is larger, which leads to the formation of a larger 
number of .alpha.-Al.sub.2 O.sub.3 seed crystals, causing a higher 
uniformity of the microcrystalline structure even at extended sintering 
times. 
A small addition of a salt of an element from the group consisting of Mg, 
Ca, Co, Ni, Cr, Fe, Ti, Si, Zn, Mn and Zr has been found advantageous, 
such salts being converted to the corresponding oxides during sintering. 
This additive is added to the dispersed suspension in the dispersing 
vessel and comprises no more than 0.2%, preferably less than 0.1%, by 
weight, based on the oxide in the end product. Such additives are known to 
reduce the non-uniform and high crystal growth during sintering. 
The disagglomeration of the suspended particles passes through a maximum in 
dependence on the ratio of shearing velocity:throughput. If the shearing 
velocity is too low, the energy is insufficient and, if it is too high, 
reagglomeration occurs. The optimum of the shearing velocity must be 
established experimentally for the disagglomeration apparatus. 
Any residual agglomerates remaining after the vented suspension has been 
disagglomerated are removed, preferably by centrifuging. 
It has further been found unexpectedly that the removal of occluded air 
from the suspension before the same is subjected to disagglomeration 
produces a further improvement of the abrasive material. This is due to 
the fact that finely dispersed air bubbles increase the porosity of the 
final product and thus unfavorably influence the density and hardness 
thereof. Pores having a size exceeding 0.4.mu. cannot be eliminated by 
sintering because the larger pores grow during sintering at the expense of 
the smaller pores. Such large pores could be removed only at extremely 
high sintering temperatures. Therefore, to enable a lower sintering 
temperature to be used, the suspension is vacuum vented to remove occluded 
air which may leave pores or cavities in the product after drying and 
sintering. Air bubbles of small diameter rise slowly, and the velocity of 
rising of the air bubbles increases with the square of the diameter of the 
bubbles in the dispersion. For example, with a suspension having a height 
of 150 mm and using a pressure of 0.4 bar, it takes more than two hours to 
obtain a suspension substantially free of occluded air and ready to be 
sintered. From this, it can be calculated that all air occlusions having a 
diameter exceeding 7.mu. must be removed. To do this in as short a time as 
possible, a small thickness of the suspension layer is required. The 
experiments were effected with a vacuum venting apparatus of the type 
VE/FRYMA. The suspension containing the occluded air is distributed by a 
special device in the vacuum chamber in a thin layer so that the air 
occlusions expand and burst. A dwell time of less than one minute at a 
pressure of 0.1 bar was sufficient to remove the occluded air from a 
suspension layer of 1 mm thickness. 
The highly dispersible .alpha.-aluminum oxide hydrates commercially 
available as pseudobohmites under the trademarks Pural, Dispersal and 
Versal may be used in the method of the present invention. The solids 
content of the suspension was between 5% and 40%, preferably 15-25%, by 
weight. The acidic solution may be nitric, hydrochloric or acetic acid. 
The optional additives added during the disagglomeration are inorganic or 
organic salts of elements of the group consisting of Mg, Ca, Co, Ni, Cr, 
Fe, Ti, Si, Zn, Mn and Zr, which are converted to the corresponding oxides 
during sintering. They are added in a maximum amount of 0.2%, by weight, 
based on the oxide in the end product, preferably less than 0.1%. The 
salts of the corresponding acidic solution were found to be most 
advantageous. 
The suspension coming from the dispersing apparatus is dried, for example 
in a hot air box at temperatures between 80.degree. C. and 120.degree. C. 
for several hours. 
The dried material was comminuted and then sintered in a furnace at a 
temperature of 1000.degree. C. to 1500.degree. C. The sintering time 
decreases with an increase in temperature, and is between a few minutes to 
two hours. 
The abrasive material produced by the hereinabove described method was used 
in the manufacture of grinding bands and grinding discs, and the 
effectiveness of the material was determined in a comparative test series 
against carbon steel C 45 by comparing its abrasive capacity with that of 
electromelt corundum (=100%). The resultant values are indicated in the 
Table hereinbelow and clearly show the excellent abrasive quality of the 
abrasive material of the present invention.

The invention will be further elucidated in the following Examples: 
EXAMPLE 1 
Ten kilograms of .alpha.-aluminum oxide monohydrate powder sold under the 
trademark Dispersal were continuously stirred into a solution of 39.5 kg 
water and 440 g concentrated nitric acid. The resultant suspension was 
then vented at a pressure of 100 mbar in a laboratory vacuum venting 
apparatus of the type LVE/A/FRYMA and subsequently pumped through a 
fast-running dispersing apparatus with two inlets. The throughput velocity 
was 3 l/h and the dispersing rotor was run at a speed of about 15,000 rpm. 
The second inlet of the dispersing apparatus remained closed. The 
disagglomerated suspension was placed into polyethylene saucers in layers 
of about 5 cm thickness and dried in a hot air box at an air temperature 
of 80.degree. C. for 36 hours until the layers on the saucers became hard, 
brittle plates. The dried plates were milled to produce transparent grains 
and the grains were classified in a screening machine. The sub-sized 
grains were recycled for use in preparing the suspension. The green grains 
were charged into an electric arc furnace which was heated for seven hours 
to a temperature of 1400.degree. C. and then sintered at that temperature 
for about two hours. The density of the sintered material was 98.1% of the 
theoretical density and the crystallite size was below 2.mu.. 
EXAMPLE 2 
The preparation and the venting of the dispersion was the same as in 
Example 1. However, a solution of 6.3 g/l of magnesium nitrate was 
introduced into the dispersing apparatus through the second inlet at a 
velocity of 270 ml/h. The suspension was then treated in the same manner 
as in Example 1, i.e. the only difference in this method as compared to 
that of Example 1 was the addition of the magnesium nitrate. The sintered 
end product contained 0.05%, by weight, of MgO and had a density of 99.0% 
of the theoretical density. The crystallite size was less than 1.mu.. 
COMATIVE EXAMPLE 
The preparation of the abrasive material followed the procedure of Example 
2, with the single exception that the suspension was not subjected to 
vacuum venting. The density of the resultant sintered material was 75% of 
the theoretical density. 
TABLE 
______________________________________ 
Grinding Test Results 
Abrasive Efficiency (%) 
Material Fiber Disc Band 
______________________________________ 
Electromelt corundum 
100 100 
Zr--melt corundum 
220 310 
Example 1 290 360 
Example 2 330 400 
Comparative Example 
90 95 
______________________________________