Fig. 2 shows how the microstructure of the samples containing
SiC (d50 = 9.3m) changes with increasing firing
temperature. The microstructure for the STD composition at
1200 ◦C, which is its industrial firing temperature, is included
for comparison purposes. The evolution of the microstructure
of the SiC-containing composition is very similar to that of
the STD composition until 1180 ◦C (figures of the STD composition
not reported here for the sake of brevity), and is
characterised by a porous texture made up of a network of
highly interconnected, small pores at low temperature (1140 ◦C).
When the temperature rises (1180 ◦C), the liquid-phase content
increases and the liquid-phase viscosity decreases. This
allows the smallest pores to be removed, thus reducing the total
and open porosity, while the remaining pores increase slightly
in size due to coalescence and expansion. The initially interconnected
pore system progressively loses its connectivity, and
the pores begin to close (see the reduction in open porosity,Fig. 1).
The STD composition reaches its minimum porosity at
1200 ◦C, while the samples containing SiC have already begun to
swell, as highlighted by the greater number of pores visible in the
sample. The fact that the size of the pores at this temperature is no
larger than that of the STD composition pores is because the gas
pressure (Pg) is not very high yet. This means that the SiC oxidation
is just beginning, or that part of the evolved gas has escaped
from the piece through the remaining pore interconnections as
other authors pointed out.