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

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In four of the samples, the fibers were exposed to a silicon-rich
environment. These samples exhibited the highest bonds, and fiber tows
coated using similar conditions failed before they could be tested.
Silicon can diffuse relatively quickly through the silica and react with
free carbon to form Sic. If carbon is present as a thin coating around
the Sic crystallite, the reaction will accelerate grain growth in the
fibers. The Nicalon fibers depend on their amorphous nature for
strength, and it has been shown that crystallization and grain growth
lead to reductions in fiber strength.
The grain growth will also lead to a reduction of grain-boundary
volume, forcing the silica to move outward toward the surface of the
fibers. This is useful in explaining the high oxygen content a% the
fiber-matrix boundary in these samples.
The silica layer would strongly
bond the fibers to the matrix.
These observations contradict the results of the analysis of
composites composed of Nicalon fibers in glass or glass-ceramic matrices.
Kesearchers have shown that a carbon layer forms at the surface of the
fibers when they are hot pressed into an aluminosilicate-based matrix
(106-108,150,151). The carbon layer allows for fiber debonding and slip,
and the samples exhibit high strength and toughness. Samples without the
carbon-rich surface fail in a brittle manner.
It has been hypothesized
that the carbon diffuses to the surface as CO gas, which reacts with the
glass, which is most likely in a reduced state, to produce carbon. An
exact explanation for the formation of this layer is unknown.
The observed phenomena in the CVI composites may be useful in
explaining the events that proceed in the glass-matrix composites. The