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

115
Analysis
conversion of
of the matrix-deposition process reveals that the
the silicon layer to silicon carbide is the favored
reaction (Table 10.5).
The intermediate coating becomes the silicon
source in the deposition process, and the silicon-containing species in
the reactant methyltrichlorosilane reacts with the excess hydrogen in the
system to form various chlorosilanes.
Earlier calculations have shown
that a silicon-rich coating is favored at the given infiltration
conditions (42).
The result is methane in the gas phase, and when
elemental silicon is present, methane in the gas mixture will react to
form silicon carbide.
This reaction may explain the extreme strength of
the bonding between the matrix and the elemental silicon precoat.
Auger analysis of the fracture surface of a specimen having silicon
as an intermediate layer was complicated by the smooth fracture surfaces
and the lack of fiber pull-out.
Fiber surfaces and pull-out grooves were
elusive features; however, with diligent searching, useful areas were
found.
The majority of the observed surfaces of the fibers and pull-out
grooves were rough in texture, most likely because of the formation of
reaction zones at the boundary and the subsequent intense bonding. High
silicon concentrations were recorded for both the fiber surfaces and the
pull-out grooves (Figure 10.11).
10.4.3 Carbon interlavers
Carbon coatings were produced by the cracking of propylene gas at
1375 K and reduced pressure.
The concentration of the propylene was
varied to produce carbon coatings that ranged in thickness from 0.10 to
0.28 pm. The thickness of the interfacial layer was measured from the
weight gained during interlayer deposition. The JEOL 733 electron