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

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the cracks. In addition, if the segments slipped during stressing,
sections of the coating buckled and spalled upon load release.
The procedure was repeated for filaments precoated with a 4.0-pm-
thick film of pyrolytic carbon.
The pyrocarbon layer was deposited on
the filaments from a mixture of 25-cm3/min propylene gas in 500-cm3/min
argon at 1375 K and a pressure of 3 KPa.
A 10-pm Sic coating was then
applied employing the previously described fixturing and procedures.
The
coated filaments were tested, and critical lengths were recorded.
The same methods were applied to Nicalon fiber tows. Acetone-washed
yarns were placed in the same graphite fixtures and coated with Sic.
Additional fiber lengths were coated with pyrolytic carbon and then
silicon carbide. In this series of experiments, the Sic deposition
pressure was reduced from 13.3 KPa to 3 KPa to produce uniform coatings
throughout the fiber bundles.
The fiber tows were tested, and segment
lengths were measured from photographs at a magnification of 320X.
Uniform loading of each individual filament within the entangled yarns
was impossible, and fusion of filaments at contact points during coating
compounded the difficulty.
Only small areas exhibited the desired
segmental fracturing of the coating.
It was recognized that the procedure could only be easily and
routinely implemented using individual filaments.
The average diameter
of Nicalon filaments is in the range of 10-20 pm; therefore, the graphite
tensile fixtures were miniaturized. At the same time, the gage length of
the fixtures was shortened to fit into the chamber of a JEOL 35CF
scanning electron microscope.
The stage of the microscope is capable of
applying a tensile load to small specimens; thus, deformation and