Finely divided powder and process and apparatus for treating the same

A process and apparatus for treating finely divided powders of brittle materials capable of sintering or injection molding, such as ceramics or intermetallic phases in a predetermined maximum size of the powder particles which is maintained with reliability.

The present invention relates to a process and apparatus for treating 
finely divided powders. As fine a powder as possible is desired, 
especially for materials capable of sintering or injection molding such as 
ceramics or inter-metallic phases. 
However, the known manufacturing processes supply only medium grain sizes 
of the powders which, depending on the material, lie within the range of 
about 1 micrometer up to about 50 micrometers. 
If particularly high demands are made of the strength properties of parts 
made of the aforementioned materials (brittle materials), the problem 
exists that the strength is determined to a considerable extent by the 
size of the existing texture defects or flaws. 
Textural defects or flaws can be caused by 
(a) grains in the starting powder whose size exceed by a multiple the 
average grain size of the powder, 
(b) grains of foreign material whose size exceeds by a multiple the average 
grain size of the powder, 
(c) pores in the starting material which do not close during the 
processing, especially during sintering. 
The closing of pores as in the last-named case, is costly even with the use 
of pressure and heat, as during hot isostatic pressing. However, in any 
case, the grains mentioned under (a) and (b) remain limiting the strength. 
Attempts have already been made to remove undesired particles from powders 
by known separating methods such as air separation, sedimentation or the 
like. However, the known processes did not prove satisfactory, whereby 
under the term sedimentation are to be understood also those processes 
under gravity as also under centrifugal force, because undesirably large 
particles, even though of smaller concentration, are still present in the 
powder. The known processes, however, are technically expensive and 
involve large amounts of time as well as costs. 
Accordingly, it is the object of the present invention to provide a process 
and apparatus which permits, respectively, permit to obtain finely divided 
powders, especially of the aforementioned materials in which the 
aforementioned undesirably large particles are eliminated. 
The underlying problems are solved according to the present invention in 
that the powder particles are put into a suspension with high liquid and 
low solid proportion, the suspension is conducted through at least one 
roller pair which includes therebetween a predetermined fixedly adjusted 
gap that corresponds to the desired maximum size of the powder particles, 
the through-put thereof (volume per unit time) is matched to the roller 
movement and thereafter the powder particles are separated from the liquid 
in the suspension. 
The most significant advantages of the present invention are as follows: 
A separate classification by means of expensive separating processes and 
apparatus can be dispensed with. 
The apparatus according to the present invention is of simple construction 
and permits a limitation of the maximum size of finely divided powders. 
The powders manufactured in this manner are suitable in particular for 
ceramics or intermetallic phases capable of injection molding or 
sintering, whereby all types of sintering processes, such as pressureless 
sintering, reaction sintering as well as simultaneous application of high 
pressure and temperature (HIP) are to be understood by the term sintering 
process. "Injection molding" is to be understood to include also extruding 
and similar processes.

Referring now to the drawing, in the process according to the present 
invention, powder is dissolved in a liquid solvent with which it does not 
chemically react. The suspension is so adjusted that it exhibits a high 
liquid proportion and low solid proportion and the latter amounts to 25% 
and less, especially to 10% and less. It is particularly advantageous to 
select the concentration of solid particles in the liquid in such a manner 
that the average distance of the particles in the suspension lies within 
the range up to 100 times, preferably up to about 2 to 50 times the 
average particle size of the powder. 
This suspension is conducted through a roller pair with an adjustable gap 
which is readjusted according to the operating duration, i.e., to the 
carrying out of the operating process of powder manufacture. The 
suspension which has been made and/or collected in the container 1, drips 
down as droplets 2 into the gap between the rollers 3 and 4 which are 
advantageously driven in opposite directions. The feed of the suspension 
and the drive of the rollers is so adjusted that the suspension is drawn 
safely through the roller gap before it can drain off laterally. 
It is achieved thereby that the excessively large particles in the powder 
are drawn into the roller gap and are comminuted and crushed approximately 
to the width of the roller gap. 
It is advantageous if the suspension drips off from the rollers without 
reaching the surfaces, along which the rollers are pressed against one 
another. For that purpose, ring-shaped recesses may be provided in 
addition to the roller running or contact surfaces. This involves the 
circumferential grooves 5 which delimit the surfaces 6 that are located 
axially outwardly, along which the rollers run up against one another. The 
roller gap 7 can be seen between the annular grooves 5. The roller gap 7 
is smaller than 50 .mu.m, preferably smaller than 25 .mu.m (adjustable). 
In order to be able to maintain very accurately the desired gap width with 
very fine roller gaps, the rollers are pressed against one another along 
the wall end areas where are also provided the running or contact surfaces 
of the rollers. The gap which is formed by the recess in the center part 
either of one or of both rollers is predeterminable and possibly 
adjustable by way of the outwardly disposed running or contact surfaces 
(wheels) which may possibly be provided with a suitable yielding abutment 
layer or with other types of elastically yielding or springy or other 
adjusting means of mechanical type, of any known manner. Such mechanical 
adjusting means are provided already, as such, in comminuting apparatus 
such as grinding units and the like. 
As a result of the great thinning of the powder in the suspension, it is 
assured that only few of the excessively large particles--above a 
predetermined maximum size--are present simultaneously at any time in the 
gap. The forces which the particles exert on the rollers before they are 
comminuted, therefore do not suffice to enlarge the roller gap, for 
example, by pressing the rollers apart to such an extent as would 
interfere. The rollers are preferably driven in opposite direction by an 
electric motor, i.e., one roller in the clockwise direction and the other 
in the counterclockwise direction, however, with the same rotational speed 
and with a relatively high circumferential velocity in comparison to the 
feed velocity of the suspension. The control, respectively, regulation of 
the electric motor or motors, possibly of gear motors, is also of any 
conventional, known type. The person skilled in the art can select the 
same in such a manner that they are matched to the aforementioned purpose 
that is to be achieved. 
After the powder particles have been brought to the predetermined maximum 
size, i.e., again leave the rollers, they are dried and collected. The 
liquid was separated beforehand (by discharge especially centrifuging from 
the annular grooves) or subsequently. 
The process and the apparatus according to the present invention find 
particular application for the manufacture of powders for sintered parts 
or injection molded parts of materials such as ceramics (SiC, Si.sub.3 
N.sub.4), (Al.sub.2 O.sub.3, MgO, ZrO.sub.2, AlN) or intermetallic phases 
(TiAl, TiAl.sub.3, NiAl.sub.3, NiAl, Ni.sub.3 Al, Ti.sub.5 Si.sub.3, 
FeSi). The powders thereof can be obtained prior to the mixing or 
dissolution in a container or chamber or the like by atomization through a 
nozzle or rotary comminution, especially of melted droplets or by another 
chemical and/or physical approach of the fragmentation of particles such 
as fine grinding. Structural parts can be obtained with materials made 
with the process according to the present invention from powders that are 
processed such as sintered or injection molded into objects, which are of 
high strength, i.e., especially high heat-resistant, erosion-resistant and 
corrosion-resistant, respectively, chemically stable. Such structural 
parts, in addition to the aforementioned requirements, must also withstand 
mechanical loads and stresses, especially in mechanical engineering, as 
are caused by centrifugal forces, pressure and others. 
The following are typical examples of the present invention: 
APATUS FOR PROCESSING 
The apparatus for processing finely divided powders and for carrying out 
the process includes a container in which a discharge opening, especially 
having a metering valve (not shown) or at least an adjustable cross 
section is arranged accurately in the center above the gap between the 
roller pair. A conventional stirrer or mixer of known type may also be 
arranged in the container which can also be designated as mixing, 
respectively, dissolving chamber; additionally, tempering means or other 
conditioning means may be coordinated to the container if so desired. The 
container may be constructed open or adapted to be closed. The roller pair 
whose drive is not shown, may be made, for example, of the same material 
as the powder to be conditioned. However, it does not include a surface 
layer of this material. The advantage of similar material resides in the 
fact that any eventual roller wear is similar to the processed powder. 
If extremely small powder particles are required, the suspension passes 
sequentially through several rollers or roller pairs. It is assured 
thereby that in case one of the excessively large particles passes through 
the first rollers, due to local roller wear thereof, it is then comminuted 
in the following rollers. In case contamination or impurities are present 
in the powder, which cannot be comminuted in the rollers, these impurities 
must be separated beforehand by other methods (known separating methods). 
Impurities or contaminations may be, for example, metallic particles which 
can be plastically deformed in the roller gap. These metallic particles 
can be washed out wet-chemically. 
Organic particles which can be elastically or plastically deformed, can be 
converted into carbon by a heat treatment at 600.degree. C., possibly in a 
protective gas atmosphere. The carbon is comminuted reliably in the roller 
gap. In case the nature of the organic contamination is known, the same 
can be washed out with a solvent. After passing through the roller gap or 
gaps, the suspension can be atomized through a nozzle in a cooling tower 
and possibly dried. 
POWDER 
Example No. 1 Silicon carbide powder, of average grain size ( &lt;2 .mu.m) of 
specific surface .about.15 m.sup.2 /g is cleaned beforehand, especially is 
washed with hydrofluoric acid so that no metallic impurities or 
contaminants are present any longer. A heat treatment of the powder in an 
argon atmosphere at about 600.degree. C. follows thereafter, in which the 
organic contaminants are eliminated. Thereafter a suspension of the powder 
in water is made (200 gr powder in 1 liter). The suspension sequentially 
passes through three roller gaps with a width of 10 .mu., a roller 
diameter of about 50 mm, and a roller width of about 200 mm. The roller 
material: silicon carbide or materials of a similar high E-modulus and 
similar high wear strength, such as tungsten carbide, silicon nitride, 
agate, however, these materials or other roller base bodies can be coated 
with hard materials such as titanium nitride, titanium carbide and boron 
carbide. 
Example No. 2 
A TiAl.sub.3 powder with an average grain size of 5 .mu.m serves as 
starting powder. The powder is pretreated, for example, washed for the 
pretreatment in a conventional manner. A heat treatment under a protective 
gas atmosphere at about 600.degree. C. takes place subsequently for 
evaporating organic contaminants. 
Thereafter a suspension of the powder is made in ethanol at the ratio of 
100 gr powder to 1 liter liquid. 
The suspension passes sequentially through three roller gaps with a gap 
width of 25 .mu.m, a roller width of 300 mm and a roller diameter of 80 
mm. Roller material: tungsten carbide or a roller base body coated with 
titanium nitride. Thereafter, the powder is atomized in a cooling tower 
and is dried thereby. The collected powder is then subsequently further 
treated, for example, sintered, pressed, especially hot-pressed into 
objects of desired type and shape. With advantage: application for high 
strength structural parts of mechanical and chemical apparatus design, 
especially highly stressed parts of force and work machines such as 
fluid-flow engines, especially turbines. 
Various modifications are readily feasible within the scope of the present 
invention. For example, a wiper installation coordinated to the roller or 
rollers which is adjustable to a gap that amounts to about 10% of the 
roller gap and thus enables a microsedimentation. In lieu of 
sedimentation, the separation can also be carried out with the aid of a 
centrifugation. 
While I have shown and described only one embodiment in accordance with the 
present invention, it is understood that the same is not limited thereto 
but is susceptible of numerous changes and modifications as known to those 
skilled in the art. For example, the present invention is suitable also 
for applications other than those described. Additionally, the 
combinations and subcombinations described herein may be modified as known 
to a person skilled in the art, both with respect to one another as also 
with respect to known features. Accordingly, I do not wish to be limited 
to the details shown and described herein but intend to cover all such 
changes and modifications as are encompassed by the scope of the appended 
claims.