PREPARATION OF DENSE CERAMICS
BASED ON SILICON NITRIDE NANOPOWDERS
V. P. Paranosenkov,
I. Yu. Kelina,
L. A. Plyasunkova,
and Yu. V. Bykov
Translated from Ogneupory i Tekhnicheskaya Keramika, No. 3, pp.2–6,March, 2003.
Synthesis of ceramics from silicon nitride nanopowders using hot-pressing and pressure-assisted and micro
wave (MW) sintering techniques has been studied. Dense ceramics with improved physicomechanical charac
teristics can be prepared on condition that most of the oxygen in the form of silicon monoxide (contained at
5 – 8 wt.% in the precursor powders) has been removed from the material. In ceramics treated by the three
sintering technologies, an extensive grain growth is observed. The microstructure of these materials was more
disperse in comparison with that of ceramic prepared from conventional a-Si
powders synthesized in a
furnace. This provides a route towards preparing nanopowder-based ceramics with a 25 – 30% increase in
strength in comparison to ceramics of the same density prepared from conventional powders.
Synthesis of dense ceramics based on silicon nitride
nanopowders is a promising but still little-studied route to-
wards ceramic engineering materials [1 – 9]. A benefit of
such powders, apart from their ultradispersity, is the high
oxygen concentration (5 – 8 wt.%), which exerts some effect
on the sintering mechanism and ceramic properties.
In this work, the microstructure, phase composition, and
strength characteristics of ceramics prepared from silicon
carbide nanopowders using hot-pressing and pressure-as
sisted and microwave (MW) sintering technologies have
been studied. The precursor powders were synthesized by
plasma CVD technology from a semiconductor-purity silicon
and had an average particle size of 25 nm and a specific sur
face of 80 m
/g. The sintering aids were oxides of yttrium
and aluminum that were added at concentrations of 9 and
1 wt.%, respectively, in the pressure and MW technologies;
in the hot-pressing technique, 9 wt.% yttrium oxide was
used. All sintering processes were carried out using a pack
compositionally close to the original specimens.
For pressure and MW sintering technologies, the pre
forms were prepared by a uniaxial pressing technique fol
lowed by hydrostatic repressing under a pressure of
4 – 5 kbar; latex molds were used. The repressed preforms
had a density of 1.6 g/cm
(46% of the theoretical value) and
showed a uniform microstructure composed of nanosize par
ticles clustered into conglomerates of 0.2 – 1.0 mm (Fig. 1).
The pressure-assisted sintering was conducted using two
temperature regimes. In one regime, specimens were pre-
pressed for 1 h at 1850°C under nitrogen (0.5 atm); the final
sintering was conducted at the same temperature for 2 h un-
der a gas pressure of 20 atm. This pre-pressing technique
was employed to make the high-pressure step more effective.
This route produced good results for preforms sintered from
powders synthesized in a furnace . In sintering
step 1, the powdered specimens were consolidated to a struc
ture with predominantly closed porosity, which consequently
made it possible, under the higher nitrogen pressure condi
tions, to use a static gas effect for further compaction.
In the second pressure sintering regime, the process was
conducted at a pressure of 20 atm.
Refractories and Industrial Ceramics Vol. 44, No. 4, 2003
1083-4877/03/4404-0223$25.00 © 2003 Plenum Publishing Corporation
Tekhnologiya Research and Production Association Federal State
Unitary Enterprise, Obninsk, Kaluga Region, Russia; Institute of
Applied Physics, Russian Academy of Sciences, Nizhny
Fig. 1. Photomicrograph of a hydrostatically re-pressed ceramic