A devitrifying cordierite based glass-ceramic
(2MgO Æ 2Al2O3 Æ 5SiO2) was observed to bubble in contact
with SiC [11] when fired in air at temperatures
between 1723 and 1823K. Glasses in the system MgO–
Li2O–Al2O3–SiO2 showed no chemical interaction with
SiC at 1523K [2,25]. This is consistent with the thermodynamic
predictions that show that the reactions are
dependent on temperature. These glasses may be used
as coatings for low temperature applications [26]. Barium
aluminosilicate glass-ceramics have also been
shown to be stable with SiC [10].
Surface modification of SiC is one approach that has
been used in SiC fiber reinforced composites to reduce
reactions with the glass matrix. For example, Nicalon
SiC fibers consist of microcrystalline SiC grains surrounded
by a silicon oxycarbide amorphous phase [3].
The reactivity of the SiC fibers is then controlled by
the presence of the oxycarbide phase. A carbon interphase
interface can be created between the glass and
SiC [27,28]. The layer not only allows accommodation
of the interfacial stresses, acting as a compliant layer
but also minimizes reaction between fiber and matrix.
Glasses based on the SiO2–Na2O system (Na2O would
react with SiC according to Figs. 4 and 5) did not show evidence of a reaction in contact with coated SiC [29].
The presence of a SiOx rich layer surrounding an inner
carbon rich region on the surface of the SiC fibers, effectively
prevents the redox reactions that would be predicted
based on the glass composition.