MICROMECHANICAL PROPERTIES, PHASE COMPOSITION,
AND MICROSTRUCTURE OF CERAMICS PREPARED
FROM COMPOSITE POWDER (SiC – C) – Si
TECHNIQUES FOR INTRODUCTION OF SINTERING ACTIVATORS
N. K. Davidchuk,
N. F. Gadzyra,
and G. G. Gnesin
Translated from Ogneupory i Tekhnicheskaya Keramika, No. 5, pp.2–6,May,2003.
Mixtures based on a nanocomposite powder (SiC – C) for hot pressure molding of ceramics are prepared.
Micromechanical properties of the sintered ceramics are shown to depend on the technique by which the
sintering activator is introduced to the mixture. Results of a study of the structure and phase composition of
the sintered ceramics by transmission electron microscopy and x-ray diffractometry are reported.
The nanocomposite powder (SiC – C) we are concerned
with in this study is a nonequilibrium solid solution of super-
stoichiometric carbon in the b-SiC lattice. Detailed structural
studies of this powder were reported in [1, 2]. As shown in a
previous x-ray diffraction study, the precursor powder con-
tains 85% (SiC – C), 15% Si
, and, in minor amounts, free
O, and Si. A specific feature of the synthetic
powder is its propensity to aggregation. This is due to the
powder’s high specific surface; in size, its particles range
from 0.03 to 0.5 mm across . As known, the structure of
sintered materials shows different degrees of homogeneity
depending on the method of preparation of the mixture for
activated sintering. Our goal in this study was to see how the
conditions of preparation of a raw mixture may affect the
hardness, density, fracture toughness, phase composition,
and structure of materials based on (SiC – C) – Si
and thus to find a route for obtaining superior micromecha
nical properties for the system in question.
The starting material was a highly dispersed composite
powder (SiC – C) – Si
synthesized as recommended in
[2 – 4]. As activators for the sintering process, aluminum
oxide (MRTU 6-09 No. 2046–64 Specifications), yttrium ox
ide (TU 48-4-191–72, “Its”-2 Specifications), and aqueous
aluminum nitrate solution Al(NO
O (State Standard
GOST 3757–75) were used.
Two techniques — mechanical mixing and solvent disso-
lution — were used to introduce the activation additives.
The mechanical mixing was carried out in a plastic drum
and an attritor; the grinding bodies were metal rag bolts and
silicon carbide balls. The grinding medium were air, water,
water + acetone, and isopropanol. The grinding time was 3, 4
and 14 h.
In the solvent dissolution method, Al(NO
was introduced and then thermally decomposed to yield
. First, the precursor powder was soaked with an aqu
eous salt solution (with surface-active substances added) and
dried in a cabinet drier at 200°C; next, it was transferred to a
closed graphite crucible and heated in a furnace at 700°C in
air. The heating time did not exceed 60 min.
Preforms for sintering were pressed in a steel pressure
mold under a load of 200 MPa.
The preforms were sintered by hot pressing technique
using an inductively heated graphite pressure mold; the pres
sure was less than 20 MPa, the temperature was 1800°C, and
the sintering time was 30 min.
The ceramic microstructure was studied by transmission
electron microscopy. To determine the average particle size
and particle size distribution, a variant of the planimetric
method was used in which the particle size was measured
from photomicrographs .
The precursor powder and sintered ceramic were ana
lyzed for phase composition using a DRON-UM1 diffrac
tometer (monochromatic CuK
radiation, in scan steps of
0.05°, exposure time at each point, 120 sec). The microhard
Refractories and Industrial Ceramics Vol. 44, No. 4, 2003
1083-4877/03/4404-0245$25.00 © 2003 Plenum Publishing Corporation
Institute for Problems of Materials Science, National Academy of
Sciences, Kiev, Ukraine.