Specialized metallurgical processes – compositions for use therei – Compositions – Loose particulate mixture containing metal particles
Patent
1998-04-24
2000-02-01
Mai, Ngoclan
Specialized metallurgical processes, compositions for use therei
Compositions
Loose particulate mixture containing metal particles
75369, 75374, 75430, 75365, 419 13, 419 14, 419 18, 419 19, B22F 900
Patent
active
060198133
DESCRIPTION:
BRIEF SUMMARY
FIELD AND BACKGROUND OF THE INVENTION
The present invention relates to cobalt metal agglomerates consisting of peanut-shaped primary particles with average particle sizes in the range from 0.5 to 2 .mu.m, to a process for the production thereof and to the use thereof.
Finely divided cobalt metal is mainly used as a binder in the production of hard metal and cutting tools based on various hard materials, such as for example WC, diamond, SiC and CBN. The cobalt metals used, for example, in the production of diamond tools must fulfil specific requirements. These include, in the first instance, that impurities such a Al, Ca, Mg, S and Si should be avoided as these elements readily form stabile oxides with the residual oxygen of the cobalt metal powder, so causing unwanted porosity in the segments.
It is also necessary, especially when producing segments with synthetic diamonds, to use only cobalt metal powders with very active sintering properties, as minimum densities of 8.5 g/cm.sup.3 are required in this case. These densities should be achieved at a sintering temperatures of as low as <900.degree. C. because the diamond may be converted into graphite at higher temperatures. If the sintering activity of the cobalt metal is inadequate, sufficient hardness is not achieved. Under the extreme stresses to which annular drilling bits or cutting tools are exposed, the abrasive action of stone dust leads to deep erosion and unwanted detachment of the diamonds or other hard materials and consequently a loss of cutting performance.
According to the prior art, cobalt metals are used, on the one hand, in the form of mixtures of atomised cobalt metal powders with hydrogen-reduced powders, as are disclosed in DE-A 4 343 594, on the other hand as ultra-fine and extra-fine grade cobalt metal powders.
Ultra-fine powders are differentiated by their FSSS value of <1.0 .mu.m from extra-fine powders which have FSSS values of between 1.2 and 1.4 .mu.m.
The small particle size and the resultant large surface areas of the described cobalt metal powders promote the absorption of atmospheric oxygen and moisture, which frequently leads to degradation of the flowability of the powders.
The object of the present invention was thus to provide a sintering active cobalt metal which does not exhibit the stated disadvantages, but does allow the production of segments with elevated density and hardness.
SUMMARY OF THE INVENTION
It has now proved possible to provide a cobalt metal powder which exhibits these required properties. These are cobalt metal agglomerates consisting of peanut-shaped primary particles with average particle sizes in the range from 0.5 to 2 .mu.m, characterised in that they have a spherical secondary structure with average agglomerate diameters of 3 to 50 .mu.m. These cobalt agglomerates are the subject matter of this invention. The agglomerate diameter of the cobalt metal agglomerates according to the invention is preferably 5 to 20 .mu.m. By virtue of their spherical secondary structure they are distinguished by good flow properties.
The irregularly shaped elongated primary particles preferably have an average particle length of 0.5 to 2 .mu.m and, generally, a diameter of <0.5 .mu.m.
FIGS. 3A and 3B show 1000 and 5000 times magnification scanning electron micrographs of the cobalt metal powder agglomerates of the invention produced according to example 3.
The specific surface areas of the cobalt metal agglomerates according to the invention (determined using the nitrogen single point method to DIN 66 131) are preferably 2 to 3.5 m.sup.2 /g. These surface areas and the small particle sizes of the primary particles are responsible for the elevated sintering activity of the cobalt metal agglomerates according to the invention, from which sintered articles having densities of 8.5 g/cm.sup.3 may be produced at temperatures of as low as 700.degree. C.
The present invention also provides a process for the production of the cobalt metal agglomerates according to the invention. This process is characterised in tha
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Gorge Astrid
Gutknecht Wilfried
Naumann Dirk
Olbrich Armin
Plaga Katrin
Cohen Jerry
H.C. Starck GmbH & Co. KG
Mai Ngoclan
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