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

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infiltration by the preferred production of Ho5Si3.
This also can lead
to a carbon enrichment of the fiber surface-intermediate coating
boundary.
The reactions exacerbate fiber-degradation mechanisms.
Molybdenum was not as evident as expected in the Auger spectral
analyses (Figure 10.24).
The three Mo peaks that occur at 160, 186, and
217 eV were extremely weak compared with those for carbon, silicon, and
oxygen (Appendix A).
Molybdenum could be found in some areas of the
matrix and fibers of a specimen but not in all regions of the same
specimen. As predicted by the thermochemical calculations, carbon-
enrichment was observed at fiber surfaces and in pull-out grooves.
10.5 Framented CoatinPs on Individual Filaments
10.5.1 -re e-diameter filaments
Initial tests were performed on AVCO SCS-6 filaments with only a
silicon carbide coating and with a modifying interlayer of pyrolytic
carbon. The critical lengths were measured to be 1.67 f 0.58 mm and
7.46 2 0.61 mm, respectively.
Using a coating fracture strength of
500 MPa, extrapolated from matrix fracture results recorded during
flexure testing, the interfacial shear strengths were calculated to be
3.98 f 0.37 MPa and 0.89 f 0.04 MPa.
Figure 10.25 shows fractures as
observed through a stereoscopic microscope at 70X.
10.5.2 Nicalon fibers
Tensile testing of the Nicalon fibers to determine interfacial
frictional stresses was more complex and tedious than for the AVCO
filaments.
Segmented fracturing was observed on a limited portion of
the filaments in each fiber bundle.
The lengths of the fragments for