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

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stress fields along the fiber interface (85,86).
To alleviate the
questions and problems associated with the aforementioned factor and also
with the general aspects of elastic recovery in the impressions,
alternative indentor geometries have been investigated.
Some work has
been completed using a rounded-tip diamond indentor and a flat tungsten
rod to apply loads to fibers,
In the first case, standard Vicker's
indentors are carefully rounded using a flat diamond-impregnated grinding
wheel. This method has been successfully applied to carbon and glass-
fiber-reinforced plastics. The second method utilizes a fine tungsten
point flattened to produce a 50-pm-diam face (161).
The indentor has
been used to apply loads to the relatively large 140-prn diam AVCQ CVD Sic
filaments embedded in a glass matrix. The major disadvantage in this
case is that the surface of the indentor and fiber must be perfectly
parallel during push-out. The test is in its earliest stages of
development; thus, results are inconclusive.
Questions arise concerning the preparation of test specimens.
Cutting, grinding, and polishing probably disturb the interface. The
tested fiber must also be perpendicular to the surface, which poses a
number of potential problems in multidimensional reinforced systems.
The
load must also be placed as close to the center of the fiber as possible.
Off-center loading complicates the distribution of forces, leading to
erroneous results.
The low values of frictional stress for the composite samples con,-
taining carbon-coated Eibers agrees well with the measured shear of 2 MPa
for the Nicalon reinforced LAS glass-ceramic composites (88). The
specimens having the lowest frictional forces exhibited gradual