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

as-received fibers have a slightly oxygen-rich surface layer.
After
processing, a carbon layer is found at the fiber-matrix boundary. The
composition of the fibers changes; the amount of Sic increases and the
Si02 concentration decreases correspondingly.
Traces of aluminum are
also found in the fibers embedded in the aluminosilicate glass matrices.
It appears that aluminum diffuses into the silica to form mullite, and
silica diffuses into the aluiiinosilicate matrix. The mullite reaction,
combined with grain growth, will degrade the fiber properties. As the
grain-boundary volume decreases, the silica and the free carbon are
forced to the surface of the fiber. The silica can diffuse into the
matrix; the carbon, on the other hand, remains at the fiber-matrix
interface because it has nothing with which to react.
The mention of the mullite formation in the fiber leads to a
discussion of the boron-containing interlayers.
Boron can react with the
silica in the fiber to form boria (B2O3). Boria diffuses rather rapidly
through silica and could possibly react with the free carbon or Sic to
form boron carbide (BqC), Even if these reactions do not occur, boron
additions reduce the strength of a glass.
Table 11.4 lists the strengths
of various glasses and shows that trace amounts of boria result in large
decreases in strength (7,152,153). The boron appears to react to form
B4C during the matrix deposition process, as seen in the highermagnification
photos in Figure 10.19.
Molybdenum proved to be chemically reactive with every element and
compound in the system, which demonstrates the need for a chemically
inert interfacial coating. The molybdenum layer not only disturbed the
composition of the fibers but was found in regions of the matrix