Page 300 - 00. Complete Version - Progress Report IPEN 2014-2016

300 Materials and Nanotechnology | Progress Report

Mullite-based coating on silicon carbide refractory

Silicon carbide (SiC) presents low thermal expansion, high strength and thermal conductivity.
For this reason, it is used as kiln furniture for materials sintering. On the other hand, SiC de-
grades at high temperature under aggressive atmosphere. The use of protective coatings can
avoid the right exposition of SiC surface to the furnace atmosphere. Mullite can be a suitable
material as protective coating because of its high corrosion resistance and thermal expansion
coefficient matching that of SiC (4,7 x10-6/°C e 5,3 x10-6/°C, respectively). In the present work,
a mullite coating obtained from ceramic precursor polymer and aluminium powder was stud-
ied to be applied over SiC refractories. Compositions were prepared with 10, 20, 30 and 50%
(vol.) of aluminium powder added to the polymer. Aluminium powders were used with dif-
ferent distributions sizes. These compositions were heat treated at different thermal cycles to
determine a suitable condition to obtain a high mullite content. The composition with 20% of
the smaller particle size Al powder was selected and used to be applied as a suspension over
SiC refractory. The applied suspension, after dried, crosslinked and heat treated, formed a mul-
lite coating over SiC refractory. Cycles of thermal shock were performed in coated and uncoat-
ed SiC samples(Figure 23) to compare each other. They were carried out 26 cycles of thermal
shock, in the following conditions: 600°C/30 min. and air cooling to room temperature. After
each thermal shock, samples were analyzed by mean of optical and electron microscopy (Fig-
ure 24), elastic modulus was also determined. After thermal shock cycles, the coating present-
ed good adhesion and no significant damage were observed.

Figure 23: Coated (a) and uncoated (b) SiC refractory bars.

Instituto de Pesquisas Energéticas e Nucleares