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

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failure and extensive fiber pull-out. The same behavior was noted
in the LAS composites.
The force required to displace the fiber within the matrix is
derived from the dimensions of the impression on the fiber end and the
hardness of the fiber. The results are, therefore, strongly dependent on
the measured hardness of the fibers. The hardness of 22.0 k 0.7 GPa was
calculated from indent impressions made at 1-N loads on longitudinally
oriented fibers. Ten indents were made on fibers in each sample. This
value is high compared with a reported value of 13 GPa (88) for Nicalon
in a glass matrix. Crystallization of Nicalon, which occurs upon
heating above 1475 K, would explain the lncrease in the hardness
values (130-132).
The limitations of the indentation methods become apparent when high
shear strengths are measured. The application of large compressive loads
on the fiber ends as delivered by the indentor split or crushed the
fiber. Matrix cracking and fracture at the edges of the fiber cavities
made accurate measurement of impression dimensions difficult. This is
reflected in the large errors in the interfacial frictional stresses.
Others applying this type of test to the well-bonded, glass-epoxy and
carbon-epoxy systems have avoided fiber damage by utilizing the roundedtip
diamond indentor. Interfacial debonding stresses in excess of 88 MPa
have been measured with no mention of splitting or crushing.
The
pyramidal Vickers diamond may accentuate the damage because of its shape.
Unfortunately, the rounded tip does not leave an indent that can be
measured easily after loading.
Using a ground diamond or a flattened
point requires the instrument be equipped to measure load and
displacement, thus complicating the necessary instrumentation.