Sinterable Si.sub.3 N.sub.4 powder and a process for its preparation

DETAILED DESCRIPTION OF THE INVENTION The present invention thus relates to a sinterable Si.
sub.
3 N.
sub.
4 powder in which the agglomerates have an average particle size of less than 1 .
mu.
m and which contains no particles having a diameter greater than 100 .
mu.
m and in which the total content in metallic impurities is less than 1000 ppm and in particular the iron content is less than 200 ppm.
A Si.
sub.
3 N.
sub.
4 powder according to the invention in which the total content in metallic impurities is less than 200 ppm is particularly preferred.
This may be prepared particularly advantageously from a Si.
sub.
3 N.
sub.
4 starting powder which has been obtained by the reaction of SiCl.
sub.
4 with NH.
sub.
3.
The Si.
sub.
3 N.
sub.
4 powders according to the invention have excellent sintering characteristics and the ceramic parts obtained from them have a very high mechanical strength.
Although the powders known from the state of the art generally fulfill the requirement for having an average particle size of less than 1 .
mu.
m, they do not have comparably advantageous properties.
It is evident from investigations carried out on commercial powders that in the powders known in the art no great significance was attached to their freedom or otherwise from oversized particles.
When only a small number of oversized grains are present in a powder, they cannot be detected by laser diffraction methods, sedimentation analyses or similar conventional methods of determining the particle size distribution.
When a Hegman grindometer is used, however, it is possible to detect even a small number of oversized grains in the powder.
The assessment of graininess of pigments by means of a grindometer has been described in German Industrial Standard ("DIN") 53,203.
Although there is no corresponding standard for ceramic powders, the method described in DIN 53,203 may be used equally well for ceramic materials, in particular for determining the size and quantity of oversized grains.
Grindometer measurements carried out on commercial powders show that even though some of these powders have particle diameters of less than 1