Specialized metallurgical processes – compositions for use therei – Processes – Producing or purifying free metal powder or producing or...
Reexamination Certificate
1998-08-28
2002-02-12
Sheehan, John (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Producing or purifying free metal powder or producing or...
C075S365000, C075S369000
Reexamination Certificate
active
06346137
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to ultrafine cobalt metal powder consisting of fine crystallites, wherein the crystallites exhibit a habit ranging from rice-grain shaped to spherical and more than 90% of the crystallites have a diameter in the range of from 0.5 &mgr;m to 2 &mgr;m, a process for the production of the cobalt metal powder via the intermediate stage of the cobalt carbonate, and methods for the use of the cobalt metal powder and of the cobalt carbonate.
The main fields of application of ultrafine cobalt metal powder are the production of hard metals and of diamond tools. The two applications place different demands on the cobalt metal powder. For use in hard metals, a very low content of impurities such as sodium, calcium and sulphur is particularly important. It is also important that the content of oxygen and carbon is not too high. The particle size and particle shape are of secondary importance.
To produce hard metals, mainly mixtures of tungsten carbide and of about 6 to 15% of cobalt metal powder are sintered at temperatures of about 1350 to 1450° C. This is a liquid phase sintering of the cobalt metal powder, during which part of the tungsten carbide dissolves in the cobalt. On cooling, recrystallization processes take place in the course of which small quantities of impurities such as sodium (Na), calcium (Ca) and sulphur (S) contained in the starting materials are already deposited preferentially at the grain boundaries of the tungsten carbide crystals. This can lead to a local reduction in strength and hence to a decrease in bending strength (12th International Plansee Seminar '89, Vol. 2 (pages 421-428)). In the case of very fine hard metal parts such as, for example, microbores, this effect results in the tools readily breaking at the positions of decreased strength.
It is also important that the content of oxygen and carbon is not too high, with values to a total of up to 0.9 wt % being acceptable. Both an increased oxygen content and an increased carbon content can influence the carbon balance during the sintering process, so that the development of embrittlement through etaphasis or through formation of C-porosity owing to carbon esters may possibly result. The two effects also distinctly impair the quality of the hard metal.
In the production of diamond tools based on cobalt metal powder, which tools consist mainly of cobalt metal powder, synthetic diamonds and other powdered substances, for example, copper, tin, iron, nickel, etc. the influence of the physical properties such as particle size and particle shape quite definitely dominates. Although the chemical impurities in the above-mentioned elements can give rise to a microporosity, this is only a minor factor. The reason for this lies in the temperature range of 700 to 950° C. conventionally employed in the production of diamond tools. In contrast to the production of hard metals, at these temperatures solid phase sintering occurs, so that the properties of the initial powders are predominantly preserved.
Experimentally it is observed that a decrease in the particle size gives rise to an increase in the hardness of hot-pressed cobalt segments. In general the Hall-Petch equation states that the hardness is reciprocal to the square root of the medium particle diameter.
This relation can be explained theoretically by the fact that the hardness is influenced by the specific proportion of grain boundaries per unit of volume, since the grain boundaries impede the propagation of dislocations. As the hardness correlates with the cutting properties of the segments, an increase in hardness frequently results in tools having a longer useful life and is therefore of great importance. In order to increase the specific proportion of grain boundaries per unit of volume, the primary particle size of the powders can be decreased (J. Konstanty and A. Busch in PMI, Vol. 23, No. 6, (1991)). Another possible method of increasing the specific proportion of grain boundaries per unit of volume consists however, at an identical or similar particle size, in altering the particle shape in such a way that the primary crystals have more of a rounded habit.
There are a number of different ultrafine cobalt metal powders which to varying degrees fulfil the requirements of manufacturers of hard metals or of diamond tools.
EP-A 0 113 281, owned by the firm Eurotungstene, Grenoble, France, describes the production of cobalt metal powder by the polyol process, whereby different cobalt compounds are reduced by polyols at 85° C. This cobalt metal powder may contain up to 3 wt. % of carbon and oxygen, so that at the otherwise high chemical purity and the high specific proportion of grain boundaries per unit of volume, effected by the particle shape, an adverse influence on the hard metal properties cannot be excluded.
The commercial cobalt metal powder product Co UF from the firm Eurotungstene, according to technical information supplied by the company, is manufactured from cobalt hydroxides. This product is distinguished by a relatively high specific proportion of grain boundaries per unit of volume. However, the increased content of sodium and sulphur can be disadvantageous.
A completely different technical procedure is disclosed in U.S. Pat. No. 5,246,481, owned by the firm Sherritt Gordon, Alberta, Canada. Here the production of this powder is carried out through the reduction of cobaltammine sulphate solutions, to which have been added soluble silver salts as nucleating agents. The doping with silver salts can lead to exceptionally high contents of silver, typically of up to 3,600 ppm, in the cobalt metal powder. Furthermore the carbon content, which according to information from the company is about 1,750 ppm, is remarkable.
The principal object of the present invention is to provide a cobalt metal powder which does not possess the disadvantages of the powders described above.
SUMMARY OF THE INVENTION
There has now been found a cobalt metal powder which possesses the required properties. The present invention provides an ultrafine cobalt metal powder consisting of fine crystallites, wherein the crystallites exhibit a habit ranging from rice-grain shaped to spherical and more than 90% of the crystallites have a diameter in the range of from 0.5 &mgr;m to 2 &mgr;m, characterised in that it has a sodium content of less than 100 ppm and a carbon content of less than 500 ppm.
Preferably the content of sodium is less than 50 ppm and that of calcium and sulphur respectively is less than 30 ppm.
In an additional preferred embodiment, more than 90% of the crystallites have a length to width ratio in the range of from 1:1 to 5:1, while the diameter of the crystallites is preferably from 0.7 &mgr;m to 1.1 &mgr;m. The particle size of the crystallites, measured in accordance with ASTM B 330, is preferably from 0.7 &mgr;m to 0.95 &mgr;m.
Table 1 below provides a survey of the ultrafine cobalt metal powder according to the invention compared with various commercial products.
TABLE 1
Survey of various ultrafine cobalt metal powders
H. C. Starck
Euro-
Euro-
Sherritt
Manufacturer
HCST
GmbH
tungstene
tungstene
Gordon
Sumitomo
Product
Product
Co IV C
Co UF
Co ex
Co UF
according
Commercial
Polyol
to the
product
invention
Particle
0.7-0.95
>0.95
0.9
0.5
0.7-0.9
0.8-1.9
size
FSSS/&mgr;m
Habit
rice-grain
ellipsoidat
spheroidal
spherical
spherical
oblong
to spherical
habit
with
crystals
habit
rounded
crystal
surfaces
Typical inpurities
Na (ppm)
50
60
240
5
20-90
130-150
Ca (ppm)
30
40
8
6
6
70-80
S (ppm)
30
35
140
20
50
**
C (ppm)
<500
<500
<500
2000
1750
**
C + O
2
0.8
0.8
0.9
3-4
0.9
**
(art %)
*Due to the production process, this grade of Co has Ag contents of up to 3,600 ppm, which is exceptionally high for Co metal powders.
**The relevant information is not given in the patent specification.
Compared with the commercial product Co IV C from HCST (Hermann C. Starck, GmbH & Co. KG, Goslar) the new product according to the invention exhibits a further increase in purity and again an increased speci
Eikemeyer Knut
Höhne Matthias
Mende Bernd
Eyl Diderico van
Gil Joseph C.
H. C. Starck GmbH & Co. KG
Sheehan John
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