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

99
compared with the dimensions acquired using the ocular scale of the
indentor.
A summary of the calculated interfacial frictional stresses is
given in Table 10.2.
10.3 Fib er Tensile Streneth
The tensile strengths of the coated and heat-treated Nicalon fiber
yarns are listed in Table 10.3.
The listed results include data
retrieved from the literature (119-123). Nicalon tows coated with either
silicon from silane or silicon carbide from methylsilane could not be
tested. The fibers failed during heat treatment, and therefore, the
strength of these samples could not be evaluated.
10.4 Fracture Behavior and Interf acial Stresses
10.4.1 Silicon carbide coatina
The sample composed of uncoated fibers and the composites prepared
from preforms treated with thin coatings of SIC from methylsilane
exhibited low flexure strength.
The specimens displayed completely
brittle failure, with no signs of fiber pull-out. Load-deflection curves
and photomicrographs of the fracture surface of these composites are
shown in Figure 10.3.
Auger analysis of fracture surfaces of composite specimens with
silicon carbide coatings, either from methylsilane or MTS, provided
evidence of an oxygen-rich layer on the fiber surfaces and in the pull-
out grooves (Figures 10.4 and 10.5).
A silica layer forms on the surface
of the fibers during heating, and this layer adheres well to the fiber
and the subsequent Sic coating.
SEM photographs of fracture surfaces of