THE EFFECT OF Si CONTENT, FIRING TEMPERATURE,
AND UREA ADDITIVE ON THE PROPERTIES
OF NITRIDE-BONDED SiC REFRACTORIES
A. A. Nourbakhsh,
and H. R. Rezaie
Translated from Novye Ogneupory, No. 10, pp. 68 – 72, October 2007.
Original article submitted February 17, 2007.
The microstructure/properties relationship of nitride-bonded SiC refractories has been studied in the presence
of Si and urea additives. The samples were prepared by mixing, shaping, and firing at 1450 to 1600
trolled atmosphere. Physical properties were evaluated by measuring density, porosity, and cold crushing
strength (CCS). XRD, SEM, and TEM were employed to study the phase and microstructural evolution.
The increase of Si enhanced the bond formation, which contributed to density and strength improvement.
The firing temperature had the same effect up to 1550
C. At 1600
C, however, the density and CCS de
creased, which were attributed to nitride phase decomposition. The urea addition improved the mechanical
properties and increased the depth of nitridation.
Silicon Carbide based refractories are valuable for their
excellent thermomechanical and thermochemical properties
, and are widely used in the ferrous, non-ferrous, and ce-
ramic industries . Their properties are highly dependent
on microstructure and bonding system . It has been well
established that a nitride bond could provide improved
high-temperature properties .The behavior of the nitride-
bonded refractories, however, is efficiently related to the
amount of silicon and additives . Albano has studied the
effect of the Si amount on the mechanical strength of Si
bonded SiC refractories and has shown a linear relation
between the bending strength of the composite and its den
It has been claimed that the most effective method to im-
prove densification of Si
bodies is the addition of oxide
and non-oxide additives. Single and mixed additives such as
are useful to
enhance the mechanical properties at high temperatures
[7 – 9].
The evolution of the bond system in the presence of dif
ferent amounts of Si and urea additives is the aim of this
study. Also the diffusion depth of nitrogen in the presence of
urea additive in nitride-bonded SiC refractories has been in
The a-SiC powder with particle size below 1.25 mm was
supplied by ESK-GmbH. The Si powder was metallurgical
grade with average particle size of 15 µm (Elkem Co.). Urea
fine powder was analytical grade (Merck 12007). The com
Refractories and Industrial Ceramics Vol. 48, No. 5, 2007
1083-4877/07/4805-0383 © 2007 Springer Science+Business Media, Inc.
Research Center of Azar Refractories Company, Iran.
Department of materials and metallurgical engineering, Iran Uni
versity of science and technology, Iran.
TABLE 1. Batch Composition of Samples (wt.%)
Samples A B C D E F G H I J
Si 10 15 17.5 20 25 25 25 25 25 25