Ceramic sintered body and method of producing it

Ceramic sintered body, in particular abrasive grain, based on .alpha.-Al.sub.2 O.sub.3, said body being arranged in such a manner that improved abrasive results are achieved, said body being bound into a structure by means of a glass-like ceramic binder and showing a drop in hardness from its center to its edge of less than 1800 N/mm.sup.2, preferably less than 1000 N/mm.sup.2, and, in particular, less than 500 N/mm.sup.2.

TECHNICAL FIELD 
The invention refers to a ceramic, sintered body, in particular abrasive 
grain based on .alpha.-Al.sub.2 O.sub.3. 
The production of formed bodies based on .alpha.-alumina by sintering is 
known. The forming may take place by drying by pressing, extruding ductile 
masses, slip casting or drying of a thickened suspension etc. Crushing of 
the moulded masses before or after the sintering leads to sharp-edged 
grains which may also be used as abrasive grains. 
Oxides or hydroxides of aluminum are mostly used as basic materials for 
sintered bodies based on .alpha.-Al.sub.2 O.sub.3. 
BACKGROUND ART 
The U.S. Pat. No. 3 377 176 discloses the production of a ceramic formed 
body made of Al.sub.2 O.sub.3 with a minute admixture of foreign oxides, 
whereby alum earth, the basic material, is pressed, said alumia having a 
grain size of &lt;1 .mu.m. 
The U.S. Pat. No. 4 786 292 discloses the production of a sintered abrasive 
grain, said grain being derived from a mixture of alumina and highly 
dispersable .alpha.-Al-oxide monohydrate, whereby admixtures of oxides of 
various metals up to 1 weight percent are stated therein. 
The European Patent Application EP-A 0 304 616 refers to surface treated 
abrasive grain based on alumina. The treatment of the surface may take 
place, amongst others, by means of highly disperse Al.sub.2 O.sub.3. The 
sole object of such treatment is, however, the achievement of high surface 
conductivity by applying hygroscopic and hydrophile layers. This 
publication does not disclose any items taking influence on the hardness. 
The European Patent Application EP-A 0 263 810 refers to the production of 
a microcrystalline abrasive grain based on .alpha.-Al.sub.2 O.sub.3. The 
basic material used this case is highly dispersable .alpha.-Al-oxide 
monohydrate. 
The European Patent Application EP-A 0 293 163 describes the production of 
an abrasive grain from an Al-oxide-sol, said grain, after having been 
calcined, being subjected to impregnation by means of a saline solution 
for improving its properties. The salts used in said saline solution are 
metal compounds which, during the subsequent heating, transform into metal 
oxides and react with the matrix. 
The U.S. Pat. No. 4 007 020 discloses a method in order to bring about 
higher hardness in a porous, ceramic body. This takes place mainly by 
impregnating the porous structure with a solution of an anorganic chromium 
compound, said compound transforming into an oxide during heating. The 
impregnation is repeated several times, whereby, as a last step, said 
impregnation may be performed with phosphoric acid. 
All above mentioned ceramic bodies, when used as abrasive grains in an 
abrasive disc, show high reactivity with the glass-like, ceramic binding 
material which is used in the production of discs. The surface of the 
grains reacts with the binder during the firing and said surface is 
"softened". Thus a decrease in the hardness on the surface of the grains 
occurs and, consequently, a decrease in the abrasive performance. 
Disclosure of Invention 
It is the object of the present invention to create an abrasive grain, the 
properties of which are not essentially impaired when said grain is bound 
by means of a ceramic binder. 
A further object of the invention is to prevent a decrease in hardness on 
the surface of said grain and thus improved abrasive properties. 
These tasks are solved in that the body, said body being bound into a 
structure by means of a glass-like, ceramic binder, shows a decrease in 
hardness from its center to its edge of less than 1800 N/mm.sup.2, 
preferably less than 1000 N/mm.sup.2, and in particular less than 500 
N/mm.sup.2. Specifically, the decrease in hardness from the center of the 
grain to its edge was determined as being particularly adverse for the 
abrasive properties. The ceramic bodies according to the invention are 
capable of achieving extremely high abrasive results. Grain sizes between 
0.05 and 1.5 mm and, in particular, between 0.2 and 1 mm have proved to be 
especially preferable. 
The invention further refers to the use of a ceramic, sintered body for the 
production of abrasive discs and similar items. 
In accordance with the invention, the grains are bound by a ceramic, 
glass-like binder. Abrasive discs produced in such a manner achieve the 
highest abrasive results. 
A further aspect of the invention is a method for producing a ceramic body, 
said body being based on .alpha.-Al.sub.2 O.sub.3, by means of sintering. 
This method is characterized in that the presintered body is deaerated in 
a vacuum and said body is subsequently treated with an aqueous suspension, 
whereby the grain sizes of the solid matter in said suspension are less 
than 0.5 .mu.m, preferably less than 0.05 .mu.m, and even more preferably 
less than 0.02 .mu.m, whereinafter excessive suspension is taken away and 
the bodies treated in such a manner are dried and sintered again. 
Quite surprisingly it was noticed that by treating near densely sintered 
bodies with a suspension of highly dispersable .alpha.-Al-oxide 
monohydrate, the decrease in the hardness on the grain surface, which was 
to be expected after the burning with the binder, was considerably 
reduced. 
The known sintered, ceramic formed bodies comprise a more or less 
considerable residual porosity. When such grains are used in ceramic 
abrasive discs, said residual porosity has a negative effect on the 
abrasive properties. The glass binder comprises oxides with a low melting 
point and said oxides, during burning of the discs, enter into a liquid 
phase and penetrate the pores of the abrasive grain. This leads to a 
considerable chemical attack against the aluminum oxide located in the 
outer layers and along the grain boundaries. The consequence thereof is a 
strong decrease in the hardness of the grain surfaces which engage in the 
abrasive process. 
The infiltration of the pores by the suspension, said suspension hardening 
in the pores during drying and sintering, brings about the effect that the 
liquid phase of the ceramic binder is no longer in the position to 
penetrate the structure of the grain and "soften" it. 
In principle the infiltration may be performed in any sintered body still 
having open pores. 
Any suspension is suitable the viscosity of which is low enough to 
penetrate pores whose sizes are between 0.5 and 1 .mu.m and where said 
suspension has solid matter consisting of .alpha.-Al oxide monohydrate 
and/or Al-oxide and/or Al-hydroxide. The grain size of the suspended 
particles should be less than 0.05 .mu.m in order to guarantee complete 
filling of the pores. The addition of acid to the suspension has proved to 
be advantageous. The content of solid matter in the suspension lies 
between 5-30 weight percent, preferably between 10 and 25 weight percent. 
The infiltration itself can be performed by immersing the grains into the 
suspension and, subsequently, by filtering through a wide-meshed screen. 
During this process the grains are preferably deaerated, so that the pores 
can be well filled with suspension. 
After getting rid of excessive suspension, the grains are dried. The 
increase in weight, depending on the porosity and grain size, may reach up 
to 15 weight percent. The dried product may also be carefully comminuted, 
in order to separate grains which are stuck together due to suspension 
remnants. Subsequently the final sintering of the filtered and dried 
grains takes place at temperatures between 1100.degree. C. and 
1400.degree. C. 
Like abrasive grains, said surface treatment may also be applied to 
sintered formed parts, e.g. cutting discs or expendable parts for 
improving the surface. 
EXAMPLES 
The following examples are intended to outline the invention without, 
however, limiting it in any way.

EXAMPLE 1 
An abrasive grain, said grain having a grain size of 0.25 mm, having been 
produced in accordance with the U.S. Pat. No. 4 786 292, and having been 
sintered at 1500.degree. C. to a theoretical density of 90 percent, was 
deaerated in an air-tight pot. Subsequently layers of a hydrous nitric 
acid suspension, said suspension containing 10 weight percent alum earth 
and 10 weight percent of dispersable .alpha.-Al-oxide monohydrate are 
placed on the grains under a vacuum and aired. After approx. 30 minutes 
the suspension is poured through a screen and the grains are dried at 
80.degree. C. in a dry box. The weight increase was 6 percent after the 
drying. The dried grains were then sintered at 1400.degree. C. for one 
hour. 
EXAMPLE 2 
An abrasive grain produced in accordance with the European Application 0 
263 810 (grain size 1 mm) is sintered at 1300.degree. C. to 95 percent 
theoretical density. The grains were deaerated in a tightly closed vessel 
and layers of a suspension, said suspension containing 20 weight percent 
.alpha.-Al-oxide monohydrate and 1 weight percent nitric acid 
(concentrated;. were placed on to the grains in a vacuum. The grain sizes 
of the suspended particles were less than 0.02 .mu.m. The suspension was 
poured through a screen after 30 minutes and the grain was then dried in a 
dry box at 80.degree. C. The weight increase amounted to 1.5 percent. 
Final sintering of the grains took place at 1300.degree. C. for two hours. 
EXAMPLE 3 
An abrasive grain produced in accordance with European Application 0 263 
810, said grain having a grain size of 0.4 mm, was sintered at 
1200.degree. C. to a theoretical density of 88 percent. The grains were 
subsequently deaerated in a tightly closed vessel and layers of a 
suspension, said suspension containing 20 weight percent .alpha.-Al-oxide 
monohydrate and 1 weight percent nitric acid (conc.) were placed on to the 
grains in a vacuum. The grain sizes of the suspended particles were less 
than 0.02 .mu.m. After 30 minutes the suspension was poured through a 
screen and the grain was then dried in a dry box at 80.degree. C. The 
weight increase amounted to 5 percent. Final sintering took place at 
1300.degree. C. for 1.5 hours. 
The table below shows the drop in hardness of the bound bodies from the 
center to the edge of the grains in, N/mm.sup.2 and the abrasive 
performance (i.e. ratio of amount of abrased material to wear of discs) of 
grains in ceramically bound abrasive discs, said grains having been 
treated in accordance with the invention. 
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Schleifleistung 
Drop in hardn. 
(referring to material 
Material N/mm.sup.2 without treatment) 
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U.S. Pat. No. 4,786,292 
2500 100 
Example 1 500 140 
EU-App. 0 263 810 
2000 100 
Example 2 1500 120 
Example 3 300 190 
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