Characterization and control of the fiber-matrix interface in ceramic ...

methane as the reactant and is, thus, chlorine-free and requires
relatively low temperatures, limiting fiber degradation that may be
caused by environmental factors. The carbon-coating parameters can be
adjusted to produce laminar structures in the films that lie parallel to
the fibers, enhancing slip (111,112)* However, the usefulness of
pyrolytic carbon is limited by its low resistance to oxidation. The
carbon layer may also provide for excessive slip, thus reducing the
ultimate strength of the composite. Therefore, other coatings with
different bonding and oxidation characteristics were assessed for
application to the fiber-matrix interface to alter the bonding and the
mechanical behavior of the resulting composite body.
Elemental silicon was chosen as an interface layer because it was
thought that by diffusing silicon into the fiber the stability of the
Nicalon fibers would improve.
Thermodynamic calculations have been used
to show that elemental silicon could react with the excess carbon present
in the fiber (113), preventing the formation and subsequent degassing of
carbon monoxide upon heating of the fibers.
A thin silicon film could
also oxidize at high temperatures to form a viscous glass that would heal
cracks in the composite structure and hinder further oxygen penetration.
Another approach is to deposit a silicon carbide coating from a
chlorine-free precursor to protect the fibers from chemical corrosion by
HC1 during densification.
Silicon carbide can be deposited from a
mixture of methylsilane and argon in the temperature range of 975 to
1275 K.
The coating is physically and chemically compatible with the
fibers and matrix.
The density of the deposit is generally low, and the
mechanical properties are poor. This procedure may be beneficial in