     |
A new type of ceramic superhard material: cubic silicon nitride
From the material silicon nitride Si3N4 - which is used in cutting tools, in ball bearings, and in the
assembly of motors - only the two hexagonal modifications a- and ß-Si3N4 had been acknowledged until
recently. But now researchers have investigated the properties, deciphered the structure, and
synthesized yet another form of Si3N4.
The new Si3N4-modification is formed at temperatures circa 2000ºC, at a pressure of about 15 GPa (150
kbar), and derived from the elements silicon and nitrogen - the a- or ß-phase - or from the amorphous
preliminary stage. Results from investigations carried out with the TEM (transmission electron
microscopy) and x-ray diffraction on powder samples showed that the material possesses a cubic spinell
structure. This type of crystal is characterized by a hexahedron coordination of a part of the silicon
atoms with the nitrogen atoms, which have not been recognized as a solid up to now.
The density of the new cubic Si3N4 at 3.9 G/cm3 lies considerably above the values of known a- and
ß-phases at circa 3.2 g/cm3. Based on theoretical considerations and computer analysis, it is presumed
that the new material is 20 - 30% harder than both of the other silicon nitride forms are. This would
classify its hardness at about the same as the hardest oxide - stishovite - which, following diamond and
cubic boronitride, is the third hardest material. Cubic silicon nitride is stable under normal pressure up to 700 K.
Further information
http://www.stp-gateway.de/News/news182e.html
Summary courtesy of Prof. Dr. Ralf Riedel
Fig. 1. Crystal lattice of alpha-silicon nitride
Courtesy of Dr. Stephan Rudolph.
